Recent Advances in Nano-Tailored Multi-Functional Cementitious Composites 0323852297, 9780323852296

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Recent Advances in Nano-Tailored Multi-Functional Cementitious Composites

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Woodhead Publishing Series in Civil and Structural Engineering

Recent Advances in Nano-Tailored Multi-Functional Cementitious Composites Edited by

Mustafa Sahmaran ¸ Faiz Shaikh Gu¨rkan Yıldırım

Woodhead Publishing is an imprint of Elsevier 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States The Boulevard, Langford Lane, Kidlington, OX5 1GB, United Kingdom Copyright © 2022 Elsevier Ltd. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. ISBN: 978-0-323-85229-6 (print) ISBN: 978-0-323-85230-2 (online) For information on all Woodhead Publishing publications visit our website at https://www.elsevier.com/books-and-journals

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Contents

List of contributors About the editors Foreword Preface 1

Overview of tailoring cementitious composites with various nanomaterials Linwei Li, Xinyue Wang, Ashraf Ashour, and Baoguo Han 1.1 Introduction 1.2 Basic principles of tailoring cementitious composites with nanomaterials 1.2.1 Brief introduction of nanomaterials 1.2.2 Nano-core effect in bulk cement paste phase 1.2.3 Nano-core effect in interfacial transition zone 1.2.4 Nano-core effect zone 1.2.5 Factors affecting the nano-core effect 1.3 Dispersion of nanomaterials 1.3.1 Traditional methods 1.3.2 Functional modification 1.3.3 In-situ growing method 1.3.4 Assembled methods 1.3.5 Surface coating method 1.4 Tailoring cementitious composites with 0D nanomaterials 1.4.1 Nano-SiO2 1.4.2 Nano-TiO2 1.4.3 Nano-ZrO2 1.4.4 Functional properties 1.5 Tailoring cementitious composites with 1D nanomaterials 1.5.1 Carbon nano-tubes 1.5.2 Carbon nano-fibers 1.6 Tailoring cementitious composites with 2D nanomaterials 1.6.1 Graphene 1.6.2 Nano-BN 1.7 Applications of cementitious composites with nanomaterials 1.7.1 Structural health monitoring 1.7.2 Traffic detection 1.7.3 Pollutants purifying

xiii xvii xix xxi

1 1 5 5 5 10 12 12 14 14 17 18 18 19 19 19 24 29 31 31 31 39 41 41 46 48 48 50 50

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1.7.4 Other applications 1.8 Prospects of cementitious composites with nanomaterials Acknowledgments References 2

3

Nano-tailored high-performance fiber-reinforced cementitious composites Ismail Ozgur Yaman and Burhan Alam 2.1 Introduction 2.2 High-performance fiber-reinforced cementitious composites 2.2.1 Production and design parameters of high-performance fiber-reinforced cementitious composites 2.2.2 Evaluation of the mechanical properties of high-performance fiber-reinforced cementitious composites 2.2.3 Evaluation of the other properties of high-performance fiber-reinforced cementitious composites 2.2.4 Field applications of high-performance fiber-reinforced cementitious composites 2.3 Nanomaterials in high-performance fiber-reinforced cementitious composites 2.3.1 Types of nanomaterials used in high-performance fiber-reinforced cementitious composites 2.3.2 Advantages of using nanomaterials in high-performance fiber-reinforced cementitious composites 2.3.3 Application challenges of using nanomaterials in highperformance fiber-reinforced cementitious composites 2.4 Influence of using nanomaterials in high-performance fiber-reinforced cementitious composites 2.4.1 Early and the hardening stages of nanotailored high-performance fiber-reinforced cementitious composites 2.4.2 Mechanical and durability properties of nanotailored high-performance fiber-reinforced cementitious composites 2.4.3 Strain-hardening and crack propagation of nanotailored high-performance fiber-reinforced cementitious composites 2.4.4 Structural applications of nanotailored high-performance fiber-reinforced cementitious composites 2.5 Challenges and future perspectives References Nano-tailored cementitious composites with self-sensing capability Hocine Siad, Mohamed Lachemi, and Mustafa S¸ ahmaran 3.1 Introduction 3.2 Nano-piezoresistive materials in cementitious composites: Recent advancements

50 51 52 52

67 67 68 70 74 78 79 83 84 85 86 88 89 89 90 91 93 94 103 103 105

Contents

Parameters influencing the sensing ability of nano-tailored cementitious composites 3.3.1 Intrinsic properties 3.3.2 Concentration of nanomaterials 3.3.3 Dispersion 3.3.4 Cementitious matrix properties 3.3.5 Surrounding conditions 3.4 Use of nanomaterials in self-sensing cementitious composites 3.4.1 Sensing of deformation and cracking under mechanical loading 3.4.2 Sensing of dynamic actions for traffic monitoring 3.4.3 Special self-sensing applications 3.5 Perspectives and conclusions References

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3.3

4

5

Nanomaterials in self-healing cementitious composites Gerlinde Lefever, Dimitrios G. Aggelis, Nele De Belie, Danny Van Hemelrijck, and Didier Snoeck 4.1 Introduction 4.2 Toward self-healing concrete 4.2.1 Autogenous healing 4.2.2 Autonomous healing 4.3 Nanomaterials for self-healing purposes 4.3.1 Nano-sized superabsorbent polymers 4.3.2 Nano-clays 4.3.3 Nano-silica 4.3.4 Carbon nano-tubes 4.3.5 Nano-iron 4.3.6 Nano-alumina 4.3.7 Nano-titania 4.3.8 Nano-fibers 4.4 Conclusions and future perspectives References Nano-tailored TiO2-based photocatalytic cementitious systems for NOx reduction and air cleaning ˘ Oguzhan S¸ ahin, Emrah Bah¸si, Gu¨rkan Yıldırım, and Mustafa S¸ ahmaran 5.1 Introduction 5.2 TiO2 as a photocatalyst 5.2.1 Structure of TiO2 5.2.2 Utilization of TiO2 for air purification 5.2.3 Photocatalytic property of TiO2 5.3 Utilization of TiO2 in cementitious systems for air purification purposes

108 108 111 113 116 120 121 121 127 129 129 131 141

141 142 142 143 145 146 146 147 149 149 150 151 151 151 153

161 161 163 163 164 165 173

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5.3.1

Relationship between the quality of distribution of TiO2 particles in cement-based systems and NOx degradation capability 174 5.3.2 Relationship between the particle size of TiO2 in the cement-based systems and NOx degradation capability 177 5.3.3 Relationship between the amount of TiO2 in the cement-based systems and NOx degradation capability 178 5.3.4 Relationship between the type of TiO2 in the cement-based systems and NOx degradation capability 179 5.3.5 Relationship between the combined presence of metal/non-metals and TiO2 in the cement-based systems and NOx degradation capability 180 5.3.6 Relationship between the mixture composition of cement-based systems and NOx degradation capability 182 5.3.7 Relationship between the abrasion/wearing/weathering of the surface of cement-based systems and NOx degradation capability 185 5.3.8 Relationship between the curing age/condition of cement-based systems and NOx degradation capability 186 5.3.9 Relationship between the final surface texture of cement-based systems and NOx degradation capability 187 5.3.10 Relationship between operation-related parameters and NOx degradation capability 187 5.4 Conclusions 189 Acknowledgment 190 References 190 6

Nano-modification of the rheological properties of cementitious composites Ayoub Dehghani and Farhad Aslani 6.1 Introduction 6.2 Theoretical background 6.2.1 Suspension rheology and rheological models for cement-based systems 6.3 Test methods 6.3.1 Rheometer test 6.3.2 One-factor tests 6.4 Rheology of nano-modified cementitious composites 6.4.1 Nanoscale particles 6.4.2 Nano-tubes and fibers 6.4.3 Nano-plates 6.5 Conclusions References Further reading

209 209 209 209 218 218 223 223 223 233 234 239 240 249

Contents

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8

Nano-modification in digital manufacturing of cementitious composites ˘ ˘ Fernando Franc¸a de Mendonc¸a Filho, Yu Chen, and Oguzhan C ¸ opuroglu 7.1 Introduction 7.2 Implementation of nanomaterials in extrusion-based 3D concrete printing 7.2.1 Printing processes and required material behaviors 7.2.2 Nanomaterials as thixotropic agents 7.2.3 Comparison between polymeric viscosity modifying admixtures and nanomaterials 7.3 Effects of nano-additions on fresh and hardened state of concrete 7.3.1 Nano-silica 7.3.2 Nano-titania 7.3.3 Nano-clay 7.3.4 Nano-alumina 7.3.5 Other mineral additions 7.3.6 Carbon nano-tubes 7.3.7 Carbon nano-fibers, graphene oxide, and carbon black 7.4 Challenges with using nanomaterials as additives 7.4.1 Dispersion of nanomaterials 7.4.2 Safety issues 7.5 Conclusions and future prospects References Thermal insulation of buildings through classical materials and nanomaterials Anwar Khitab, Zain Ul Abdin, Imtiaz Ahmed, and Taimur Karim 8.1 Introduction 8.2 Fundamentals of building physics 8.2.1 Heat transmission 8.2.2 Resistance (R-value) 8.2.3 Thermal conductivity, λ 8.2.4 Thermal transmittance (U-value) 8.2.5 Thermal capacity (C-value) 8.3 Energy-efficient buildings 8.4 Conventional insulation materials and methods 8.4.1 Mineral wool 8.4.2 Expanded polystyrene 8.4.3 Extruded polystyrene 8.4.4 Cellulose 8.4.5 Cork 8.4.6 Polyurethane 8.4.7 Non-zero energy buildings (nZEB) 8.5 Role of nanotechnology for building insulation 8.5.1 Nanotechnology and the construction industry

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251 251 253 253 256 257 258 258 259 259 260 260 261 262 263 263 264 265 266

277 277 277 277 278 280 281 281 282 283 283 284 284 284 284 285 285 286 286

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Contents

8.5.2 Nanotechnology applied to thermal insulation 8.5.3 Aerogels 8.5.4 Vacuum insulation panels 8.5.5 Gas-filled panel 8.5.6 Phase change materials 8.5.7 Nano-coatings for buildings 8.5.8 Types of nano-coatings 8.6 Energy-efficient coatings 8.6.1 Phase change materials 8.6.2 Electrochromic materials 8.6.3 Photovoltaic coatings 8.6.4 Nano-coating categorization 8.7 Conclusions References Further reading

286 288 288 289 290 290 290 293 293 294 294 296 296 296 302

Nano-modified green cementitious composites Salmabanu Luhar, Ismail Luhar, and Faiz Shaikh 9.1 Introduction 9.2 Types of nanomaterials used for modification of green cementitious composites 9.2.1 Nano-silica 9.2.2 Nano-titania 9.2.3 Carbon nano-tubes 9.2.4 Carbon nano-fibers 9.2.5 Carbon black nanoparticles 9.2.6 Most relevant lines of study using other nanoparticles 9.3 Properties 9.3.1 Shrinkage 9.3.2 Freeze-thaw damage 9.3.3 Abrasion or erosion 9.3.4 Nanotechnology for cementitious composites to triumph over their chemical deteriorations 9.3.5 Thermal degradation 9.3.6 Compressive strength 9.3.7 Tensile strength 9.3.8 Water sorpitivity 9.3.9 Water absorption 9.3.10 Chloride ion penetration 9.3.11 Permeability 9.3.12 Drying shrinkage 9.3.13 Chloride diffusion 9.3.14 Corrosion 9.3.15 Microstructure

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Contents

10

11

12

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9.4 Conclusions and discussion References

335 337

Nano-modified geopolymer and alkali-activated systems Partha Sarathi Deb, Jhutan Chandra Kuri, and Prabir Kumar Sarker 10.1 Introduction 10.2 Use of nanomaterials in cementitious binders 10.3 Properties of geopolymers and alkali-activated systems incorporating nanomaterials 10.3.1 Fresh properties 10.3.2 Mechanical properties 10.3.3 Microstructure development 10.4 Durability of geopolymers containing nano-SiO2 10.4.1 Carbonation of geopolymers 10.4.2 Sulfate resistance of geopolymers 10.5 Concluding remarks Acknowledgments References

347

Nanoscale characterization of cementitious composites ˙ Emircan O¨zc¸elikci, Hu¨seyin Ilcan, Gu¨rkan Yıldırım, and Mustafa S¸ ahmaran 11.1 Introduction 11.2 Nanoscale characterization techniques 11.2.1 Nano-indentation 11.2.2 Atomic force microscopy 11.2.3 Transmission electron microscopy 11.2.4 Nuclear magnetic resonance 11.2.5 Small-angle neutron scattering 11.2.6 X-ray computed nano-tomography 11.2.7 Other characterization techniques 11.3 Challenges and future perspectives References

375

Low CO2 reactive magnesia cements and their applications via nano-modification Tien-Dung Nguyen, Cise Unluer, and En-Hua Yang 12.1 Introduction 12.2 Production of reactive magnesia cements 12.2.1 Dry route 12.2.2 Wet route 12.3 Hydration and carbonation of reactive magnesia cements 12.3.1 Improving the hydration mechanism and mechanical performance of reactive magnesia cement-based materials via nano-modification

347 348 349 349 353 359 364 364 365 368 368 368

375 377 377 380 384 387 390 392 395 397 398

407 407 409 409 410 411

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12.3.2

Improving the carbonation mechanism and mechanical performance of reactive magnesia cement-based materials via nano-modification 12.3.3 Limitations of carbonation diffusion 12.4 Durability of reactive magnesia cements 12.4.1 Nitric acid resistance of reactive magnesia cement-based concretes 12.4.2 Chloride, sulfate, freeze-thaw, and seawater resistance of reactive magnesia cement-based concretes 12.4.3 Corrosion resistance of reactive magnesia cement-based pastes 12.5 Nano-tailored strain-hardening reactive magnesia cementitious composites 12.5.1 Mechanical properties 12.5.2 Self-healing performance 12.6 Other applications 12.6.1 Reactive magnesia as alkali activator 12.6.2 Reactive magnesia to accelerate the activator and hydrated magnesium carbonates as nano-seeding materials 12.6.3 Hydraulic binders of MgO
hydromagnesite 12.6.4 Reactive magnesia cement for 3D printing 12.7 Future outlook Acknowledgments References 13

Future developments and challenges of nano-tailored cementitious composites Arslan Akbar and K.M. Liew 13.1 Background 13.2 Introduction 13.3 Future developments 13.3.1 Factors affecting the design of nano-tailored cement composites 13.3.2 Production of nano-tailored cementitious composites 13.3.3 Experimental techniques for characterization of nano-tailored cementitious composites 13.3.4 Multi-functional properties of nano-tailored cementitious composites 13.3.5 Enhancement mechanism of nano-tailored cementitious composites 13.3.6 Potential use of nano-tailored cementitious composites 13.4 Challenges 13.5 Summary References

Index

414 415 418 418 422 423 424 424 430 436 436 440 444 449 450 452 452 459 459 460 461 461 461 463 464 465 466 467 469 470 473

List of contributors

Zain Ul Abdin Department of Civil Engineering, Mirpur University of Science and Technology (MUST), Mirpur, Pakistan Dimitrios G. Aggelis Department of Mechanics of Materials and Constructions, Vrije Universiteit Brussel (VUB), Brussels, Belgium Imtiaz Ahmed Department of Civil Engineering, Mirpur University of Science and Technology (MUST), Mirpur, Pakistan Arslan Akbar Department of Architecture and Civil Engineering, City University of Hong Kong, Kowloon, Hong Kong, China Burhan Alam Department of Civil Engineering, Middle East Technical University, Ankara, Turkey Ashraf Ashour Faculty of Engineering and Informatics, University of Bradford, Bradford, United Kingdom Farhad Aslani Materials and Structures Innovation Group, School of Engineering, The University of Western Australia, Crawley, WA, Australia Emrah Bahs¸i Hacettepe University, Institute of Science, Beytepe, Ankara, Turkey Yu Chen Faculty of Civil Engineering and Geosciences, Section Materials and Environment, Delft University of Technology, Delft, The Netherlands Og˘uzhan C ¸ opurog˘lu Faculty of Civil Engineering and Geosciences, Section Materials and Environment, Delft University of Technology, Delft, The Netherlands Nele De Belie Magnel-Vandepitte Laboratory for Structural Engineering and Building Materials, Department of Structural Engineering and Building Materials, Faculty of Engineering and Architecture, Ghent University, Tech Lane Ghent Science Park, Ghent, Belgium

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List of contributors

Partha Sarathi Deb School of Civil and Mechanical Engineering, Curtin University, Perth, WA, Australia Ayoub Dehghani Materials and Structures Innovation Group, School of Engineering, The University of Western Australia, Crawley, WA, Australia Fernando Franc¸a de Mendonc¸a Filho Faculty of Civil Engineering and Geosciences, Section Materials and Environment, Delft University of Technology, Delft, The Netherlands Baoguo Han School of Civil Engineering, Dalian University of Technology, Dalian, P.R. China Hu¨seyin I˙lcan Hacettepe University, Institute of Science, Beytepe, Ankara, Turkey; Department of Civil Engineering, Hacettepe University, Ankara, Turkey Taimur Karim Department of Civil Engineering, Mirpur University of Science and Technology (MUST), Mirpur, Pakistan Anwar Khitab Department of Civil Engineering, Mirpur University of Science and Technology (MUST), Mirpur, Pakistan Jhutan Chandra Kuri School of Civil and Mechanical Engineering, Curtin University, Perth, WA, Australia Mohamed Lachemi Department of Civil Engineering, Ryerson University, Toronto, ON, Canada Gerlinde Lefever Magnel-Vandepitte Laboratory for Structural Engineering and Building Materials, Department of Structural Engineering and Building Materials, Faculty of Engineering and Architecture, Ghent University, Tech Lane Ghent Science Park, Ghent, Belgium; Department of Mechanics of Materials and Constructions, Vrije Universiteit Brussel (VUB), Brussels, Belgium Linwei Li School of Civil Engineering, Dalian University of Technology, Dalian, P.R. China K.M. Liew Department of Architecture and Civil Engineering, City University of Hong Kong, Kowloon, Hong Kong, China Ismail Luhar Shri Jagdishprasad Jhabarmal Tibrewala University, Jhunjhunu, India Salmabanu Luhar Frederick Research Center, Nicosia, Cyprus

List of contributors

xv

Tien-Dung Nguyen School of Civil and Environmental Engineering, Nanyang Technological University, Singapore, Singapore; Department of Mechanical, Aerospace and Civil Engineering, University of Manchester, Manchester, United Kingdom ¨ zc¸elikci Hacettepe University, Institute of Science, Beytepe, Ankara, Emircan O Turkey; Department of Civil Engineering, Hacettepe University, Ankara, Turkey Prabir Kumar Sarker School of Civil and Mechanical Engineering, Curtin University, Perth, WA, Australia Faiz Shaikh School of Civil and Mechanical Engineering, Curtin University, Perth, WA, Australia Hocine Siad Department of Civil Engineering, Ryerson University, Toronto, ON, Canada Didier Snoeck Magnel-Vandepitte Laboratory for Structural Engineering and Building Materials, Department of Structural Engineering and Building Materials, Faculty of Engineering and Architecture, Ghent University, Tech Lane Ghent Science Park, Ghent, Belgium; Department of Mechanics of Materials and Constructions, Vrije Universiteit Brussel (VUB), Brussels, Belgium; BATir Department, Universite´ Libre de Bruxelles (ULB), 50 F.D. Roosevelt Ave., Brussels, Belgium Cise Unluer School of Engineering, University of Glasgow, Glasgow, United Kingdom Danny Van Hemelrijck Department of Mechanics of Materials and Constructions, Vrije Universiteit Brussel (VUB), Brussels, Belgium Xinyue Wang School of Civil Engineering, Dalian University of Technology, Dalian, P.R. China Ismail Ozgur Yaman Department of Civil Engineering, Middle East Technical University, Ankara, Turkey En-Hua Yang School of Civil and Environmental Engineering, Nanyang Technological University, Singapore, Singapore Gu¨rkan Yıldırım Department of Civil Engineering, Hacettepe University, Ankara, Turkey

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List of contributors

Og˘uzhan S¸ahin Hacettepe University, Institute of Science, Beytepe, Ankara, Turkey; Department of Civil Engineering, Kırs¸ehir Ahi Evran University, Kırs¸ehir, Turkey Mustafa S¸ahmaran Department of Civil Engineering, Hacettepe University, Ankara, Turkey

About the editors

Mustafa Sahmaran ¸ is currently working as a Full Professor in the Department of Civil Engineering of Hacettepe University, Turkey. His research interests include micromechanical design and durability of high-performance and ultra-ductile cementitious composites, recycling industrial and natural waste products into useful construction materials, concrete durability under mechanical and environmental loadings, new-generation multi-functional cementitious composites with selfhealing and self-sensing functionalities and nano-modification in cementitious systems. Prof. Sahmaran ¸ has authored or co-authored over 200 technical publications in his career, including more than 100 refereed journal papers. He is currently the Director of the Advanced Building Materials Laboratory at Hacettepe University. Over the past few years, 13 PhD and 30 master students have completed their degrees under his supervision/co-supervision. He has also received several prestigious young scientist awards from various national and internationals organizations, e.g., the Province of Ontario Fellowship in 2007, the Eser Tu¨men Research and Young Scientist Award in 2012 and 2014, the Turkish Academy of Sciences Distinguished Young Scientist Award in 2012, the Prof. Dr. Mustafa N. Parlar Education and Research Foundation Young Scientist Research Incentive Award in 2013, the Scientific and Technological Research Council of Turkey’s Young Scientist Incentive Award in Engineering in 2014, and the Society of Science Heroes’ Scientist of the Year Award in 2015. In 2018, Prof. Sahmaran ¸ was selected to receive the American Concrete Institute (ACI) Wason Medal for Materials Research for his co-authored paper on the proposition of new supplementary preconditioning procedure that can help accelerate the degradation process of concrete specimens when exposed to sulfate attack, which was published in the July/August 2016 issue of the ACI Materials Journal. Prof. Sahmaran ¸ is a member of several technical committees of the ACI, the American Society of Civil Engineering, the Turkish Society of Civil Engineering, and the US Transportation Research Board. Faiz Shaikh is an Associate Professor in the School of Civil and Mechanical Engineering of Curtin University, Australia. His research focus is on the development of sustainable binders by incorporating high-volume fractions of industrial byproducts as partial replacement of OPC and nano- and ultrafine materials, use of recycled aggregates in sustainable concrete, mechanical characterization of ductile fiber-reinforced cement and geopolymer composites, behavior of geopolymer composite in fire and natural fiber-reinforced composites. He is a Chartered Professional Engineer (CPEng.) in Australia, a fellow of Engineers Australia (FIEAust), and a member of the Concrete Institute of Australia. He has supervised

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About the editors

six PhD and two MPhil students and is currently supervising five PhD students. Prof. Shaikh has authored and co-authored 182 technical publications, including one book by Springer, six book chapters, 128 reviewed journal papers, and 47 reviewed conference papers. He also holds an h-index of 38 and 3,970 total citations according to Google Scholar. He has also been awarded an A$2.17M competitive grant by various research organizations and industries, including the Australian Research Council (ARC), the Cooperative Research Centre (CRC), the Japan Society for the Promotion of Science (JSPS), the Waste Authority of Western Australia, and the Tyre Stewardship Australia. Gu¨rkan Yıldırım is currently working as a full-time Associate Professor of Materials Science and Construction Materials in the Department of Civil Engineering of Hacettepe University, Turkey, and is affiliated with the Advanced Building Materials Laboratory of the same department. Prof. Yıldırım’s research area of expertise encompasses a wide range of concrete technology, in particular, very ductile fiber-reinforced concretes known as strain-hardening cementitious composites (SHCCs) or engineered cementitious composites (ECCs). His work has mainly focused on the inherent ability of ECCs to recover their own damage (i.e., cracks) without any need of outside assistance through the mechanism known as autogenous self-healing. He has also engaged in studies related to detailed fresh/ mechanical/durability property characterization of ECCs, the utilization of nanomaterials in conventional and/or ECC-like composites for performance enhancement and improved self-sensing capability, and ductile composites for repair/maintenance applications. His latest research emphasis is placed on the effective recycling/utilization of construction and demolition waste (CDW) in alkali activation and development of engineered geopolymer composites (EGC) based on CDW. Prof. Yıldırım is the author/co-author of two book chapters and more than 80 journal and conference papers, including 42 journal articles indexed by SCI/SCI-Expanded. He has acted as a reviewer for more than 25 scientific journals and is a Review Editor for the Journal of Frontiers in Built Environment—Construction Materials. He was also involved in several national and international research and governmental projects funded by several authorities (i.e., the Ministry of Environment and Urbanization, TUBITAK, the British Council, and the European Commission). Most recently, he was awarded the prestigious Individual Fellowship of Marie Skłodowska-Curie Actions (MSCA-IF-2019) funded by the European Commission to perform research at the University of Bradford, UK.

Foreword Civil infrastructure utilizing traditional concrete serves as one of the key foundations of the society. However, aging of infrastructures requires expensive and often repetitive repair and maintenance, creating economic and environmental burdens. It is desired to have a cleaner, more sustainable, and longer-lasting infrastructure built with eco-friendly construction materials. Traditional concrete, although proven to perform in a satisfactory manner, is brittle with almost no control over cracking. Cracking increases the transport of water and aggressive agents through the concrete cover, leading to corrosion of steel reinforcement. Moreover, the traditional concrete production industry is heavily dependent on Portland cement as a binding material, the production of which is responsible for up to 7% of global CO2 emissions. The concrete industry, therefore, needs to undergo a paradigm shift to lower its negative impact on the environment, people, and economies. One plausible means is to equip concrete materials and structures with smart capabilities, enabling them to adapt to their service environment and to attain a longer service life. Toward this goal, the use of nanomaterials is potent, as these tiny particles can modify cementitious materials at the nanoscale and enable multi-functional capabilities in addition to enhanced mechanical and durability properties. This comprehensive book summarizes the latest findings from around the world on the interrelationships between nanomaterials and multi-functional characteristics such as self-compaction/self-healing/self-sensing/self-cleaning. In addition to traditional cement pastes, mortars, and concretes, the range of materials covered includes geopolymers and high-performance fiber-reinforced cementitious composites. This book is unique in that it focuses on the effects of nanomaterials on the multifunctional properties of concrete-like materials, which is often overlooked in favor of general properties such as fresh-state, strength, and durability. Starting from small- to large-scale systems, this book allows the reader to see every step from initial preparation of the nanomaterials to structural-scale applications. In this regard, this book will be of interest to a wider audience, from students and researchers at the beginning of their careers to experienced professionals and faculty members, policy makers, practitioners, and engineers looking for information on processing, production, testing, and application of advanced construction materials with multi-functional capabilities. Victor Li, FASCE, FASME, FWIF, FACI James R. Rice Distinguished University Professor of Engineering, E. Benjamin Wylie Collegiate Professor of Civil Engineering, Professor of Civil and Environmental Engineering, Professor of Materials Science and Engineering, Professor of Macromolecular Science and Engineering, Director, Center for Low Carbon Built Environment (CLCBE), Rm 2132, GGB Building, University of Michigan, Ann Arbor, MI, United States

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Preface

Since its introduction to the construction industry, concrete has been the main building block of civil infrastructure, and continuous developments have been made in the field of concrete materials with the necessity of new requirements for different applications ever since. The production and transformation of concrete are expected to grow with the increments in the world’s population and urbanization. The popularity of concrete arises mainly from its versatility, ease of producibility, low cost, ability to resist mechanical loads, and durability against weather actions. However, the material is not perfect in all senses and has one major drawback: low tensile strength coupled with a high cracking tendency. In the past, achieving highstrength levels was desirable from the standpoint of constructing larger structures in a faster way. However, higher strength increases the chance of cracking, which is not favorable for durability (i.e., interaction with service environment and influence mainly by the water tightness of the material) and service life of concrete elements, as it eases the ingress of water and harmful agents into the concrete. In this regard, improvement of strength of concrete mixtures was found inadequate to assure higher quality infrastructure; therefore, a broader concept of “high performance” came about by the end of 1980s aiming to improve both the strength and durability of concrete materials simultaneously. Despite the emergence of the concept, infrastructure built before 1990s occupy a very large share and are crumbling at the moment. This has necessitated repetitive repair/retrofit/rehabilitation applications, which are expensive in regard to the environment and economy and may not even be suitable under structural restrictions that entail structural demolishment and reconstruction. It is therefore very appealing to design and build our infrastructure to be high-performance to tackle these issues. The demand for concrete has always been tremendous, and it is the second most utilized resource in the world preceded only by water, although concrete-making may not always be a low-cost procedure. Traditionally, concrete incorporates Portland cement (PC), aggregates of different sorts/sizes, mineral/chemical admixtures, and water. Among these constituents, PC is of special importance as it is mainly responsible for the acquirement of strength and works as a glue to hold aggregates together. Cement is also important as its production is a very energyintensive and carbon-emissive process requiring huge amounts of natural raw materials together with fossil fuels to be burned. To exemplify, production of 1 ton cement requires nearly 2 tons of raw materials (e.g., limestone, clay, gypsum), consumes about 4 GJ of electrical, heat, and transport energy, equivalent to 131 m3 of natural gas, releases CO2 with a ratio higher than 900 kg, nearly 3 kg of contaminant NOx contributing to ground-level smog formation and 0.4 kg of airborne

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Preface

particulate matter (PM10), which is harmful for the respiratory tract (Han et al., 2019; Josa et al., 2007). PC manufacture alone is held responsible for up to 9% of man-made CO2 emissions globally, and the demand for PC is expected to increase in the future. In line with the projections, annual PC production is expected to increase by 50% by 2050 (Monteiro et al., 2017). Considering current emission factors and energy consumption, up to an extra 105 Gt CO2eq emissions and 505 TJ energy demand are expected within the next 33 years (Monteiro et al., 2017). Burden of PC production grows heavy by the aggregate quarrying as this also has a direct and conspicuously negative influence on the society, environment, and economy. Taking these negative effects into account, excessive production of concrete needs to be limited, and one feasible way to do that is to make our newly built infrastructure more durable, sustainable, and resilient so that they will last longer and will not necessitate additional concrete-like materials for repetitive repair/retrofit/rehabilitation. This concept has gained significant importance considering the day-by-day growing awareness of climate change, and since the beginning of new millennium, impacts of sustainable design/development on the suppliers of construction materials and industry have become more visible. Construction industry has taken numerous measures such as improvements in the production process of cement kilns, replacement of PC clinker with mineral admixtures originating from different waste streams, development of green cement/concrete mixtures, utilization of recycled aggregates in new concretes, and many more for reducing its carbon emissions and energy requirement, which are prevalent in today’s world and are anticipated to be more pronounced in the future (Meyer, 2009). In accordance with the above-mentioned statements, the future of concrete technology should be centered on the development of concrete mixtures capable of combining the attributes of high performance (i.e., high strength and durability) and improved materials’ greenness altogether to ensure that infrastructural development is resilient, durable, and sustainable. Regardless of their possible sole performance, concrete mixtures having only one of these attributes will not be satisfactory for the achievement of desirable infrastructural needs. The broad concept of infrastructural sustainability can be even furthered by equipping otherwise conventional construction materials with smart multi-functional properties. Multi-functionality is the ability of a material to adapt itself to its surrounding environment without necessitating outside interference for achieving a useful purpose. Self-sensing, self-healing, self-cleaning, self-leveling, and self-compacting are some of multi-functional attributes that can be related to the concrete materials and structures. For example, self-sensing is a material’s ability to show changes in its electrical resistivity to the generation of strain/damage as a result of the applied stress (Al-Dahawi et al., 2016; Yıldırım et al., 2018), which can be very handy to selfmonitor the condition of a certain concrete element/structure and take necessary measures before the damage gets critical and risky to be treated. Multi-functional properties are nonstructural attributes and are not the ultimate goals that are aimed to be reached on their own, although their availability makes significant contributions to the structural capability of concrete elements/structures, which can very well serve for the enhanced infrastructural resilience, durability, and sustainability.

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xxiii

Concrete is a multi-component material in nature having multi-phases ranging from nano- to macro-scales. The material’s microstructure and properties change over time as PC reactions progress, which are complex processes that are not fully understood even now, thereby limiting the utilization and predictability of concrete materials, especially in critical infrastructural components. This complexity, on the other hand, offers opportunities for tailoring the properties of ultimate concrete material for a specific use and the properties largely change starting from nano- to macro-scale depending on the material’s composition, design, and production process. It is therefore logical to have a behavioral understanding of concrete at nanoscale since the microstructure mainly forms by formation and development of PC gel structure at nanoscale and the mechanical, durability, dimensional stability, sustainability aspects, together with multi-functional attributes, which are visible at the micro-/macro-scale and are all affected by the things happening at the nanometer range, and even the smallest changes made at the nanoscale can be vividly reflected at the macro-scale. In this regard, nanoscience and nanotechnology can be considered appealing branches that material scientists and construction material researchers need to look into for better linking and comprehending the properties of concrete material starting from nano- to macro-scale. The introduction of term “nanotechnology” to the literature dates back to the 1970s (Taniguchi, 1974), although foundations of nanotechnology were laid earlier in 1959 by famous physicist Richard Feynman in his speech entitled “There is plenty of space at the bottom” (Feynman, 1960). However, nanotechnology in cement-based materials did not receive adequate attention for many years. Only after the early 2000s, the subject started to gain importance with the slow recognition of advancements made in other fields of use/application and observation of crumbling infrastructure requiring dramatic changes to be made in our understanding of “traditional concrete.” To incorporate nanotechnology into the sector of construction materials and structures, much scientific work has been done, especially in the last two decades, which focused on the enhancement of rheological, mechanical, durability, and microstructural properties, achievement of multi-functional attributes, and material’s greenness by using a wide range of nanomaterials such as nano-silica, nano-iron, nano-alumina, nano-titania, nano-clays, different types of carbon-based materials, and many more. In an effort to combine the related state-of-the-art data available in the current literature, latest advancements made in the field of use of nanomaterials in construction materials and structures to be equipped with multi-functional attributes are documented in this book along with the expected future prospects. Although special emphasis is placed on the interrelationship between the use of nanomaterials and achievement of multi-functionality, effects of nanomaterials on the construction materials’ fresh state, easy producibility, mechanical, durability, and greenness aspects are also discussed, where relevant. In this book, there are 13 chapters compiled by expert researchers along with the editors from all around the world. Chapter 1 provides a comprehensive discussion on the tailoring of the fresh and hardened (mechanical and durability) properties of cementitious composites in the presence of various nanomaterials and recent

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developments in the structural applications together with the future prospects in the building sector with nano-modified cementitious composites. In Chapter 2, a detailed evaluation has been made on the nano-tailored high-performance fiber-reinforced cementitious composites, which have recently garnered massive popularity in the construction industry owing to their advanced mechanical and durability properties. Analysis of the use of nanomaterials in such composites has been made by providing case studies from the actual field of application with their advantages and challenges. Chapter 3 offers an advanced analysis of the selfsensing ability of cementitious composites incorporating various electrically conductive nanomaterials. In addition, the latest advancements in the application of nano-sensing attribute and associated features/challenges in cementitious composites are discussed in depth. In Chapter 4, various processes that contribute to the self-healing ability of cementitious composites are discussed, as well as the effect of various nanomaterials to achieve crack sealing and water tightness for the promotion of self-healing. Furthermore, some future perspectives on the use of nanomaterials in cementitious composites are outlined. Chapter 5 presents a comprehensive analysis on the efficiency of NOx reduction and self-cleaning ability of nano-TiO2-based photocatalytic cementitious composites. Utilization of nanoTiO2 in the cementitious composites is discussed in terms of air purification performance and future perspectives. In Chapter 6, a review of the theoretical background and test methods utilized in investigating the rheological properties of nano-tailored cementitious composites are presented. In addition, recent studies on the effect of nanoscale particles such as nanotubes, nanofibers, and nanoplatelets on the rheological properties of cementitious composites are thoroughly discussed. Chapter 7 focuses on the recent developments in the area of digital manufacturing of nanomodified cementitious composites with special emphasis on the challenges of implementation, execution at large-scale, and the effect of use of various nanomaterials on the fresh and hardened states. Chapter 8 focuses on the advances made in the use of nanotechnology in tailoring the thermal insulation properties of cementitious composites. The role of nanotechnology in building insulation, nanomaterialbased thermally insulated building materials, and nano-coatings is thoroughly discussed. Chapter 9 looks into the development of green cementitious composites by lowering the amount of PC with the addition of pozzolanic materials such as fly ash, ground granulated blast furnace slag, silica fume, rice husk ask, metakaolin, and volcanic ash. Moreover, the effect of use of nanomaterials in green cementitious composites is taken into consideration in terms of plastic state, mechanical, durability, and microstructural properties. In Chapter 10, PC-free geopolymer systems, which are extensively researched nowadays for the sake of developing ecofriendly construction materials, are considered for use in combination with different nanomaterials. Nano-tailored geopolymer systems are deeply evaluated in terms of their fresh-state, mechanical, durability, and microstructural properties. Chapter 11 furnishes a critical review on works on the nanoscale characterization of mechanical and microstructural properties of cementitious composites. Sophisticated methods of characterization at nanoscale such as nano-indentation, atomic force microscopy, transmission electron microscopy, and their recent applications are discussed in detail. In Chapter 12, reactive-magnesia cements and their applications, which are

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xxv

topics recently trending for lowering CO2 emissions from the production of cementitious composites, will be discussed in relation to the utilization of nano-modification in their composition and production. Chapter 13 describes the methods commonly used for the nano-modified cementitious composites, gives an insight into possible trends, and discusses future advancements from numerous fields of research to explain the actions of nano-modified cementitious composites. This book is useful to advanced students, researchers, university professors, governmental bodies, practitioners, and contractors alike who are in desire of building knowledge for the design and production of new-generation nano-tailored multifunctional construction materials and structures for truly resilient and sustainable infrastructure. We believe that with the fast advancements made in nanotechnology and recognition of infrastructural sustainability in the coming years, the importance of equipping conventional concrete-like materials with multi-functional attributes will be realized even more, so as the importance of the current book. Mustafa Sahmaran ¸ Faiz Shaikh Gu¨rkan Yıldırım

References ¨ ztu¨rk, O., Yıldırım, G., Akın, A., Sahmaran, Al-Dahawi, A., Sarwary, M. H., O ¸ M., & Lachemi, M. (2016). Electrical percolation threshold of cementitious composites possessing self-sensing functionality incorporating different carbon-based materials. Smart Materials and Structures, 25(10), 105005. Feynman, R. P. (1960). There’s plenty of room at the bottom. California Institute of Technology, Engineering and Science magazine. Han, B., Ding, S., Wang, J., & Ou, J. (2019). Nano-engineered cementitious composites: principles and practices. Springer. Josa, A., Aguado, A., Cardim, A., & Byars, E. (2007). Comparative analysis of the life cycle impact assessment of available cement inventories in the EU. Cement and Concrete Research, 37, 781
788. Meyer, C. (2009). The greening of the concrete industry. Cement and Concrete Composites, 31(8), 601
605. Monteiro, P. J. M., Miller, S. A., & Horvath, A. (2017). Towards sustainable concrete. Nature Materials, 16, 698
699. Taniguchi, N. (1974). On the basic concept of nanotechnology. Proceeding of the ICPE. ¨ ztu¨rk, O., Anıl, O ¨ ., & Sahmaran, Yıldırım, G., Sarwary, M. H., Al-Dahawi, A., O ¸ M. (2018). Piezoresistive behavior of CF-and CNT-based reinforced concrete beams subjected to static flexural loading: shear failure investigation. Construction and Building Materials, 168, 266
279.

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Overview of tailoring cementitious composites with various nanomaterials

1

Linwei Li1, Xinyue Wang1, Ashraf Ashour2, and Baoguo Han1 1 School of Civil Engineering, Dalian University of Technology, Dalian, P.R. China, 2 Faculty of Engineering and Informatics, University of Bradford, Bradford, United Kingdom

1.1

Introduction

Cementitious composites are acknowledged as the most commonly used construction and building materials owing to their excellent mechanical properties, durability, accessibility, easy shaping, and low cost. It can be expected that cementitious composites will continue to serve as the primary materials in constructions in the next few decades. However, there is a significant potential for improved cementitious composites’ properties, including brittleness, low tensile strength, poor deformation performance, and high cracking tendency. In addition, traditional cementitious composites only serve as load-bearing materials without functional properties. Nanomaterials have the potential to modify the structure of cementitious composites by following a bottom-up approach at four elementary scale levels, as shown in Fig. 1.1, namely, (1) Level I (1028B1026 m): different types of calcium silicate hydrate (C
S
H); (2) Level II (1025B10
4 m): C
S
H gels together with calcium hydroxide (CH) crystals, unhydrated binder particles, and micrometer pores; (3) Level III (1023B1022 m): cement paste phase, aggregate phase, and interfacial transition zone (ITZ), and (4) Level IV ( . 1021 m): structural element/structures/ infrastructures (Constantinides & Ulm, 2004). Usually, performance of each level is influenced and predicted by the next smaller scale. Small changes at nanoscale are likely to exert big impacts on the four levels of cementitious composites, owing to the excellent mechanical, thermal, acoustic, optical, magnetic, and electrical performances of nanomaterials. Therefore using nanomaterials provides a promising approach to develop cementitious composites with superior mechanical properties, durability, and smart/multi-functional properties (e.g., ultra-high performance, smart/multi-functional, and resilient). Since the beginning of this century, nanomaterials as the smallest gradation composition bring significant changes to cementitious composites through the nano-core effect, including modifying C
S
H structures at level I, reducing CH crystal size and filling pores at level II, modifying bulk cement paste phase and ITZ at level III, and realizing superior mechanical properties, durability, and smart/multi-functional Recent Advances in Nano-Tailored Multi-Functional Cementitious Composites. DOI: https://doi.org/10.1016/B978-0-323-85229-6.00012-3 © 2022 Elsevier Ltd. All rights reserved.

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Recent Advances in Nano-Tailored Multi-Functional Cementitious Composites

Figure 1.1 The illustration of four elementary scale levels of the structure in cementitious composites.

properties at level IV. Tailoring cementitious composites with nanomaterials can improve the integrity and uniformity of cementitious composites, such as matrix compactness and water-retaining ability. Therefore as shown in Fig. 1.2, nanomaterials gradually show the potential to become the indispensable seventh component of cementitious composites, besides cement, water, fine aggregates, coarse aggregates, chemical additives, and mineral additives. The study of cementitious composites with various nanomaterials started in 1989 by utilizing nano-ZrO2 to enhance the strength and fracture toughness (Chyad, 1989). The past two decades have witnessed a dramatic growth of research interest in the use of various nanomaterials to tailor properties and functionality of cementitious composites as summarized in Table 1.1 (Han et al., 2019). Besides, some hybrid nanomaterials, such as self-assembled carbon nano-tubes (CNTs) and nanocarbon black (NCB), are used for tailoring cementitious composites (Zhang, Ding, et al., 2018; Zhang, Han, et al., 2018). Massive attention has been paid to some representative materials for tailoring cementitious composites, such as nano-SiO2, nano-TiO2, CNTs, and graphene. Fig. 1.3 shows the approximate appearance of cementitious composites with these typical nanomaterials (Chyad, 1989; Huang, 2012; Makar et al., 2005; Rafiee et al., 2013; Ye, 2001; Zhang, Ding, et al., 2018; Zhang, Han, et al., 2018). Nano-SiO2 can consume CH crystals because of their pozzolanic activity. Cementitious composites with nano-SiO2 show improvement of

Overview of tailoring cementitious composites with various nanomaterials

3

Figure 1.2 Seven components of cementitious composites with nanomaterials.

45.6% in compressive strength and 16.0% in flexural strength (Zhang et al., 2016). Tailoring ultra-high performance concrete (UHPC) with nano-SiO2 shows a superior self-healing behavior with the enhancement of 39.4% in compressive strength, and 33.7% in flexural strength after a 28-day recovery (Wang, Zhang, et al., 2018). Nano-TiO2 is a representative of nanomaterials to endow cementitious composites with purifying ability. He et al. reported that incorporating iron coating with nanoTiO2 particles is efficient for degraded vehicle exhausts (He et al., 2019). Nanocarbon materials such as NCB, CNTs, carbon nano-fibers (CNFs), and graphene are featured by high modulus, strength, toughness, and excellent electrical conductivity, thermal conductivity, electromagnetic wave absorbing, and shielding properties. CNTs can improve dynamic impact toughness by 100.8% and dissipation energy of UHPC by 93.8% (Wang, Dong, Ashour, et al., 2020). Tailoring cementitious composites with CNTs is efficient to detect stain, stress, and damage of structures, and is conductive to shield electromagnetic wave radiation (Ding et al., 2019). Cementitious composites with nanomaterials have gained significant attention because of applications of structural health monitoring, traffic detection, pollutant purifying, and energy saving. This chapter presents a systematic review of tailoring cementitious composites with various nanomaterials. It first introduces basic principles of tailoring cementitious composites with various nanomaterials, that is, the nano-core effect. It then introduces dispersion of nanomaterials in cementitious composites, which is the key technique for tailoring cementitious composites with nanomaterials. It also provides basic properties and performances of typical cementitious composites with nanomaterials including hydration, rheology, workability, mechanical properties, durability, and functional properties, and concludes with the evolution of the applications of cementitious composites with nanomaterials. Prospects of cementitious composites with nanomaterials is presented in the last section.

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Recent Advances in Nano-Tailored Multi-Functional Cementitious Composites

Table 1.1 Classification of various nanomaterials applied in cementitious composites. Classification of nanomaterials Nano-binders

Dimensions

Nano-cement

0D

Nano-silica fume Nano-fly ash Nanomaterials added in cementitious composites

Nano-oxide

Nano-SiO2

Nano-salt Nano-carbon materials

Nano-TiO2 Nano-ZrO2 Nano-Al2O3 Nano-MgO Nano-ZnO Nano-Zn2O Nano-CuO Nano-Fe2O3 Nano-Fe3O4 Nano-Cr2O3 Nano-CaCO3 Nano-BaSO4 Nano-carbon black

Nano C
S
H seed Nano-clay Nano-perovskite Nanocellulose

1D

Nano-carbonized bagasse fibers Al2SiO5 nano-tubes Nano-SiC Nano-carbon materials

CNTs CNFs Graphene Oxidized graphene Nano-BN

2D

Nano-Ti3C2 Note: 0D,1D, and 2D denote nanomaterials with nanosize in one dimension, two dimensions, and three dimensions, respectively.

Overview of tailoring cementitious composites with various nanomaterials

5

Figure 1.3 The approximate appearance of cementitious composites with these typical nanomaterials.

1.2

Basic principles of tailoring cementitious composites with nanomaterials

1.2.1 Brief introduction of nanomaterials Nanomaterials are characterized by their tiny sizes of 1
100 nm in at least one dimension, amounting to the scale of 10
100 atoms closely arranged together. According to the morphology, nanomaterials are usually classified by 0D nanoparticles with nanometer size at three dimensions, 1D nanometer tube with nanometer size at two dimension, and 2D nanoplate with nanometer size at 1D. Owing to the tiny size, nanomaterials have large specific surface area, surface energy, and large fraction of surface atoms. The appearance of large fraction of surface atoms makes nanomaterials easily form physical adsorption or chemical reaction with other particles, providing kinetic feasibility for tailoring cementitious composites. Besides, the sharply tiny size makes nanomaterials have significantly different properties from micro-/macromaterials with regard to various aspects of mechanics, thermology, acoustics, optics, magnetics, and electrics.

1.2.2 Nano-core effect in bulk cement paste phase The nano-core effect is the combination of the nano-effect and core effect of nanomaterials in cementitious composites. The small size makes nanomaterials at a low content extensively distributed in the cementitious composites. Owing to the ultrahigh specific surface area and surface energy, the widely distributed nanomaterials can adsorb ions and hydration products, thus forming numerous nano-core shell structures. The brief description of the nano-core effect is illustrated in Fig. 1.4.

1.2.2.1 Nano-effect The nano-effect of nanomaterials in cementitious composites includes small size and high surface area.

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Recent Advances in Nano-Tailored Multi-Functional Cementitious Composites

Figure 1.4 Brief description of nano-core effect (A) nano-effect, (B) nano-core and nanocore zone, and (C) core effect in cementitious composites.

Small size The number of typical nanomaterials at the content of 0.1 vol.% in 1 cm3 cementitious composites along with their size is illustrated in Figs. 1.5 and 1.6 (Han et al., 2019). It can be seen that the particle number ranges from 3.2 3 10,108 to 8.0 3 1016, demonstrating that numerous nanomaterials at a low content can extensively distribute in cementitious composites, achieving huge impact on cementitious composites. In addition, the small size endows nanomaterials with unique physical and chemical properties. For example, nano-SiO2 has higher pozzolanic activity than micro-SiO2 (Ye, 2001).

Large surface area The specific surface area of typical nanomaterials and raw materials of cementitious composites are shown in Fig. 1.7 (Han, Ding, & Yu, 2015; Han, Sun, et al., 2015; Han, Zhang, et al., 2015). It can be seen that the surface area of nanomaterials is about 35
3571 times of that of cement particles. Large surface of nanomaterials provides area for nanomaterials to interact and synergize with other components in cementitious composites, usually with the disappearance, formation, and transformation of hydration products in cementitious composites matrix (such as CH crystals, C
S
H gel, and ettringite). Therefore the large surface area is an important fundamental for the whole hydration process.

Overview of tailoring cementitious composites with various nanomaterials

7

Figure 1.5 Approximate estimate about numbers of particles in 1 cm3 of cementitious composites with 0.1 vol.% 1D and 2D nanomaterials.

Figure 1.6 Approximate estimate about numbers of particles in 1 cm3 of cementitious composites with 0.1 vol.% 0D nanomaterials.

1.2.2.2 Core effect Nanomaterials exist in cementitious composites as a core surrounded by hydration products (core shell), as depicted in Fig. 2.4B. Thanks to their extremely tiny size,

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Recent Advances in Nano-Tailored Multi-Functional Cementitious Composites

Figure 1.7 Specific surface area of typical nanomaterials and raw materials.

nanomaterials exhibit great mechanical properties. The core effect includes the intrinsic, nucleating, filling or bonding, and pining effects as explained below.

Intrinsic effect Table 1.2 shows the classification of intrinsic characteristic of nanomaterials and their benefits to cementitious composites, which can be classified into the following four aspects. G

G

G

G

Adsorption of ions, water, and hydration products: Adsorption of ions can significantly cut down the transportation of ion, resulting in less corrosion. On the other hand, adsorption of water at early stage decreases the water to binder (w/b) ratio of matrix around nanomaterials, thereby enhancing the early strength and adsorption of hydration products and producing a more compact matrix. Furthermore, adsorption of water at early stage and release of further hydration products are beneficial for self-curing; Reducing inherent defects as some nanomaterials can transfer heat of hydration and reduce thermal stresses accordingly; Modification of hydration process: The active or pozzolanic nanomaterials can participate in hydration reaction or secondary hydration, thereby consuming CH crystal and transferring C
S
H gel to the needle-like or the columnar, decreasing the orientation of CH crystals and reducing CH crystal size; Functional effects: Electrical, thermal, electromagnetic, sensing, and photocatalytic behaviors will endow cementitious composites the corresponding functional properties.

Nucleating effect Addition of nanomaterials accelerates the hydration process due to nucleation and growth of hydration products on nanomaterials with large surface area and high surface energy. Nanomaterials as the “nucleus” wrapped by hydration products act as

Overview of tailoring cementitious composites with various nanomaterials

Table 1.2 Classification of intrinsic characteristics of nanomaterials and their benefits to cementitious composites. Classification of intrinsic characteristics of nanomaterials Physical properties

Morphology

Dimension

Benefits Strengthen bond of nanomaterial with matrix; Enhance nucleating effect.

Size Roughness Density Surface features

Hydrophilic/ hydrophobic With functional group

Reduce dosage of nanomaterials with low density Beneficial to the dispersion of nanomaterials. Strengthen bond of nanomaterial with matrix; Beneficial to the dispersion of nanomaterials.

Charged or absorbable Mechanical properties

Strength

Improve mechanical properties of cementitious composites with nanomaterials.

Modulus Hardness Toughness Damping Wear resistance Hydration activity

Active

Accelerate hydration and participate in secondary hydration

Pozzolanic Multifunctional properties

Electrical properties

Electrically conductive

Improve electrical conductivity of cementitious composites.

Electromechanical

Endow self-sensing property with cementitious composites. Endow self-heating property with cementitious composites. Increase thermal conductivity

Electrothermal Thermal conductivity Electromagnetic shielding and absorbing Photocatalytic properties

Endow cementitious composites with electromagnetic shielding and absorbing properties. Endow cementitious composites with photocatalytic capability.

9

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Recent Advances in Nano-Tailored Multi-Functional Cementitious Composites

Figure 1.8 CNT bridges cement matrix and pulls out form cement matrix.

nano-core in matrix, allowing the hydration products uniformly distribute throughout the cementitious composites.

Filling or bonding effect Nanomaterials with tiny size are the smallest raw materials to fill pores in cementitious composites, thus decreasing the porosity and increasing the compactness. Along with the filling effect in cementitious composites, nanomaterials tightly bond with matrix, allowing stress transfer with bridge effect and pull-out effect, as shown in Fig. 1.8.

Pinning effect Hard inactive nanomaterials (such as nano-TiO2 and nano-ZrO2) form nano-coreshell structures by adhering hydration products during the hydration process. The nano-core-shell structures with hard core can inhibit crack growth at the initial stage and deflect cracks when the crack tip reaches the hard core, improving the fracture toughness of cementitious composites. In addition, nano-ZrO2 with tetragonal structure would transform to nano-ZrO2 with monoclinic structure because of stresses. The transformation will consume the energy of damage and inhibit the extension of cracks.

1.2.3 Nano-core effect in interfacial transition zone Cementitious composites can be considered to be a three-phase material, composed of aggregate phase, bulk cement paste phase, and ITZ between them. The ITZ has the characteristics of low strength, high porosity, high CH content, ettringite content, high metal cation content, and low molar ratio of CaO to SiO2 (Ca/Si ratio), acting as “the weakest link in the chain.” Nanomaterials can tailor microstructures in ITZ, which is not only governed by the nano-core effect but also dependent on the nanomaterial enrichment effect in ITZ. Few studies have demonstrated that the presence of different types of nanomaterials can modify ITZ between aggregate and bulk cement paste, enhancing interfacial bond strength, reducing ITZ thickness by optimizing intrinsic compositions, and microstructures of ITZ between aggregate and bulk cement paste (Gao et al., 2019; Szymanowski & Sadowski, 2020a, 2020b; Wang et al., 2018).

Overview of tailoring cementitious composites with various nanomaterials

11

Figure 1.9 The formation process of nanomaterial enrichment layer in ITZ. Source: From Wang, X. Y., Dong, S. F., Ashour, A., Zhang, W., & Han, B. G. (2019). Effect and mechanisms of nanomaterials on interface between aggregates and cement mortars. Construction and Building Materials, 240.

The modification of nanomaterials on ITZ through nano-core effect is attributed to a nanofiller enrichment layer formed near the aggregate surface because of the wall effect and nanomaterial migration (Wang, Dong, et al., 2019). First, the smaller particles in fresh cementitious composites migrate to the surface of aggregate because of the wall effect, leading to the increase of local content of nanomaterials in ITZ. Then, nanomaterials move with water in the voids among binder particles because water is absorbed by hydrophilic aggregate. Therefore a nanomaterial enrichment layer forms in ITZ between aggregate and bulk cement paste. The formation process of nanomaterial enrichment layer in ITZ is illustrated in Fig. 1.9 (Wang, Dong, et al., 2019).

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Recent Advances in Nano-Tailored Multi-Functional Cementitious Composites

The presence of nanomaterial enrichment layer in ITZ provides numerous nucleation sites for hydration products. Nanomaterials can adsorb metal cations (such as Ca21 and Al31) at the early hydration stage, thus reducing the enrichment of metal cations in ITZ. The adsorbed Ca21 cannot form CH crystals because of the coordination number limitation, the size and content of CH crystals in ITZ. Meanwhile, the adsorbed Al31 is beneficial for modifying ITZ in cementitious composites with nanomaterials. It will inhibit the formation of calcium aluminate hydrate (C
A
H), thereby reducing the content of ettringite (reaction product of C
A
H and sulfate). Also, low aluminum doping in C
S
H gels is propitious to increase the ordered degree of C
S
H and decrease the distance between C
S
H layers (Wang, Zheng, et al., 2020). In addition, the modifying mechanism of nanomaterials on ITZ is the same as that on bulk cement paste. The enriched nanomaterials in ITZ form numerous nanoshell-core structures, thereby densifying interfacial microstructures, restricting cracks due to pinning effect, and deflecting cracks due to bridge effect. The effect of nanomaterials on ITZ between aggregate and bulk cement paste is shown in Fig. 1.10.

1.2.4 Nano-core effect zone Nano-core is extremely tiny, with the result that intrinsic and nucleating effects only influence hydration products in short range. The hydration products adsorbed by nanomaterials act as the shell of nano-core. The formation of a nano-core-shell element can represent nano-core effect zone. Nano-core effect zone is a transition zone that differs from cementitious composites in long range. Cementitious composites with nanomaterials depend on properties, distribution, and numbers of nanocore-shell element to a certain extent. The properties and performances of nanocore-shell element depend on the nano-core effect.

1.2.5 Factors affecting the nano-core effect The nano-core effect is dominated by intrinsic properties of nanomaterials, dispersion of nanomaterials, and composition of cementitious composites. Intrinsic properties of nanomaterials are the primary factors governing the nanocore effect. Uniform dispersion of nanomaterials can effectively enhance the properties of cementitious composites. Because aggregation of nanomaterials will reduce numbers of nano-core-shell elements and weaken the nano-core effect, or even damage cementitious composites because of the appearance of weak areas in matrix. The composition of cementitious composites affects the nano-core effect as well. The w/b ratio determines the dispersion of nanomaterials to a certain extent. Different physical properties and chemical compositions of various cement, silica fume, and fly ash can also affect the nucleating effect by influencing hydration.

Figure 1.10 Effect of nanomaterials on ITZ between aggregate and bulk cement paste. Source: From Wang, X., Zheng, Q., Dong, S., Ashour, A., & Han, B. (2020). Interfacial characteristics of nano-engineered concrete composites. Construction and Building Materials, 259. https://doi.org/10.1016/j.conbuildmat.2020.119803.

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1.3

Recent Advances in Nano-Tailored Multi-Functional Cementitious Composites

Dispersion of nanomaterials

Nanomaterials are liable to aggregate in cementitious composites or reunite during the hydration process because of their large specific surface area and high surface energy. This aggregation weakens the nano-core effect, which consequently makes reinforcing mechanism a failure or may even damage the strength of cementitious composites by producing weak areas in matrix. Therefore uniform dispersion is the key for tailoring cementitious composites with nanomaterials. Different strategies regarding the dispersion of nanomaterials have been proposed for seeking out a technology with simplicity, economy, and efficiency. Fig. 1.11 (Han, Ding, & Yu, 2015; Han, Sun, et al., 2015; Han, Zhang, et al., 2015) shows typical measures to disperse nanomaterials, and the typical dispersion methods for each nanomaterial used in cementitious composites are summarized in Table 1.3 (Cai et al., 2017; Cui et al., 2017; Cwirzen et al., 2008; D’alessandro et al., 2016; Gu et al., 2017; Han et al., 2009; Han, Li, et al., 2017; Han, Wang, et al., 2017; Han, Zhang, et al., 2017; Han, Zheng, et al., 2017; Hanif et al., 2017; Jiang, Zhou, et al., 2018; Jin et al., 2017; Jo et al., 2007; Li, Ding, et al., 2020; Li, Dong, et al., 2020; Li, Corr, et al., 2020; Li, Dong, Wang, et al., 2020; Lichao et al., 2013; Ltifi et al., 2011; Luo et al., 2009; Ma et al., 2015; Nasibulin et al., 2013; Nazari & Riahi, 2013; Oltulu & Sahin, ¸ 2013; Ouyang et al., 2018; Rafiee et al., 2013; Rafieipour et al., 2012; Salman et al., 2016; Senff et al., 2009; Sun et al., 2013; Wang, Dong, et al., 2019; Wang, Dong, et al., 2019; Zhang, Ding, et al., 2018; Zhang, Han, et al., 2018; Zhang, Li, et al., 2020; Zhang, Zheng, et al., 2020; Zhang, Wu, et al., 2020).

1.3.1 Traditional methods Traditional methods are the basic measures to disperse nanomaterials in cementitious composites, including mechanical methods and physical surface modification utilizing dispersing agents. The specific classification and their advantages and disadvantages are generalized in Table 1.4 (Han, Ding, & Yu, 2015; Han, Sun, et al., 2015; Han, Zhang, et al., 2015).

Figure 1.11 Diagrammatic sketch of typical measures for dispersing nanomaterial.

Overview of tailoring cementitious composites with various nanomaterials

15

Table 1.3 The typical dispersing method for nanomaterials used in the cementitious composites. Dimension Nanomaterials Dispersing methods

References

0D

Nano-ZrO2

Shear mixing

Nazari and Riahi (2013)

Nano-TiO2

Shear mixing

Rafieipour et al. (2012) Ma et al. (2015)

Ultrasonic treatment

Feng et al. (2013) Zhang, Li, et al. (2020), Zhang, Zheng, et al. (2020), Zhang, Wu, et al. (2020) SiO2 coating Han, Li, et al. (2017), Surface coating nano-TiO2 Han, Wang, et al. (2017), method Han, Zhang, et al. (2017), Han, Zheng, et al. (2017) Al2O3 coating Li, Ding, et al. (2020), nano-TiO2 Li, Dong, et al. (2020); Li, Corr, et al. (2020), Li, Dong, Wang, et al. (2020) Ultrasonic Han, Li, et al. (2017), treatment 1 SP Han, Wang, et al. (2017), Han, Zhang, et al. (2017), Han, Zheng, et al. (2017) Shear mixing Oltulu and Sahin ¸ (2013) Assembled methods

Nano-SiO2

CNT/microTiO2

Dispersing agent

SRA Ultrasonic treatment

Shear mixing 1 SP 1D

Salemi et al. (2014) Salman et al. (2016)

CNT

Ultrasonic treatment 1 SP In situ growing on cement, clicker, and sand Assembled methods

Functional modification Dispersing agent

Jo et al. (2007) Senff et al. (2009) Guefrech et al. (2011) Gu et al. (2017) Hanif et al. (2017) Cai et al. (2017) Ouyang et al. (2018) Wang, Dong, et al. (2019) Sun et al. (2013)

Nasibulin et al. (2013) Zhang, Li, et al. (2020), Zhang, Zheng, et al. (2020), Zhang, Wu, et al. (2020) CNT/NCB Zhang et al. (2018) -COOH, -OH Cui et al. (2017) CNT/microTiO2,

SDS, SDBS SDC PAC

Han et al. (2009) Luo et al. (2009) Cwirzen et al. (2008)

(Continued)

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Recent Advances in Nano-Tailored Multi-Functional Cementitious Composites

Table 1.3 (Continued) Dimension Nanomaterials Dispersing methods CNFs

References

Shear mixing Ultrasonic treatment 1 SP

2D

Graphene

Shear mixing

Baeza et al. (2013) Jiang, Zhou, et al. (2018) Jin et al. (2017)

Ultrasonic treatment 1 shear mixing D’alessandro et al. (2016) Nano-BN

Ball milling Stirring

Rafiee et al. (2013) Zhang, Han, et al. (2018), Zhang, Ding, et al. (2018) Zhang, Han, et al. (2018), Zhang, Ding, et al. (2018)

Note: SP, SDS, SDBS, SDC, SRA, and PAC denote superplasticizer, sodium dodecyl sulfonate, sodium dodecylbenzene sulfonate, sodium deoxycholate, shrinkage reducing admixture and polyacrylic acid polymers, respectively.

Table 1.4 Specific classification of traditional dispersing methods and their advantages and disadvantages. Traditional methods Mechanical method

Classification Shear mixing

Stirring Ultrasonic dispersion

Dispersing agent

Surfactants

Advantages

Disadvantages

Beneficial for manufacturing

Large energy consumption Complicated operation Lake of efficiency

Beneficial for manufacturing Realization of well dispersion

Ionic

Easy control

Nonionic

Low energy consumption

Polymers

Strengthen bond between nanomaterials and hydration products

Inconvenience for manufacture Large energy consumption Possibly restrain hydration, setting, and hardening Possibly exert negatively impact on mechanical and durability performance

Overview of tailoring cementitious composites with various nanomaterials

17

Mechanical methods include ordinary stirring, shear mixing with high speed, and ultrasonic dispersion (Bastos et al., 2012). Ordinary stirring and shear mixing with high speed are suitable for manufacturing. However, the two methods consume large energy and lack efficiency. Owing to the apparent effectiveness and simplicity of processing, along with low cost, ultrasonic dispersion has been widely applied to get a solution where well-dispersed nanomaterials suspend. Ultrasonic energy has been proposed to accurately describe the dispersing effect and decide the ultrasonic time under a certain treatment power (Zou et al., 2015). Dispersing agents (including surfactants and polymers) for physical surface modification work by wrapping nanomaterials tightly to form core shell particles and making them repel from each other because of the van der Waals interactions between nanomaterial particles. Dispersing agents give rise to a pure physical process that neither alters structural features of nanomaterials nor affects their properties. A number of works have been done on dispersing agents, such as sodium dodecyl sulfonate (SDS) (Han et al., 2009), sodium dodecylbenzene sulfonate (SDBS) (Luo et al., 2009), sodium deoxycholate (SDC) (Cwirzen et al., 2008), polyacrylic acid polymers (Cwirzen et al., 2008), superplasticizer (SP) (Feng et al., 2020; Gu et al., 2016), and shrinkage reducing admixture (Gu et al., 2017). These dispersing agents are effective indeed, but the application of some of them alone in cementitious composites may affect the latter’s strength and durability. Among these agents, SP is the most popular as a bifunctional agent without damaging properties of cementitious composites, because it can disperse nanomaterials and increase the workability of cementitious composites as water-reducing admixture. The utilization of SP in dispersing nanomaterials such as nano-SiO2 (Feng et al., 2020; Gu et al., 2016), nano-TiO2 (Li, Jia, et al., 2018; Li, Ding, et al., 2018), nano-ZrO2 (Ruan, Han, Yu, Li, et al., 2018; Ruan, Han, Yu, Zhang, et al., 2018), and nano-carbon materials (Zheng et al., 2016) has been certified valid. The combination of dispensing agents and traditional methods is more effective than their application alone, especially the combination of SP and ultrasonic dispersion. The utilization of dispersion agent such as SP is more complex and time-consuming because they should be deposited on nanomaterials before fabrication of cementitious composites, which is expected to last for a longer duration (Heinz et al., 2017). Therefore to seek a technology with simplicity, economy, and efficiency, SP and nanomaterial are directly added in the water for ultrasonic treatment, instead of ultrasonic treatment after pretreatment of nanomaterials by dispersing agent.

1.3.2 Functional modification Functional modification aims to alter surface structures of nanomaterials, thereby increasing their wettability and solubility. Measures for oxidizing nanomaterials and adding functional group have been certified effective by the utilization of strong acids (Saito et al., 2002), ozone (Mawhinney et al., 2000), strong oxidant, and plasma excitation (Wang et al., 2009). Functional modification has the ability to alter the structure and properties of nanomaterials; therefore it is usually applied on inert material, such as carbon-based nanomaterials (e.g., CNTs and graphene).

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Recent Advances in Nano-Tailored Multi-Functional Cementitious Composites

Numerous studies have been conducted on cementitious composites with nanomaterials through the treatment of functional modification, such as hydroxyl functionalized CNTs (CNTs
OH) (Cui et al., 2017), carboxyl-functionalized CNTs (CNTs
COOH) (Azeem & Azhar Saleem, 2020; Musso et al., 2009) and oxidized graphene (Khan et al., 2019; Qureshi & Panesar, 2019).

1.3.3 In-situ growing method In-situ growing method refers to the act of making nanomaterials grow on raw materials to form combined particles with larger size. The dispersion of nanomaterials is improved because they can extensively distribute in cementitious composites with raw material particles. Nano-carbon materials are the most commonly utilized to grow on surface of raw materials, such as CNTs or CNFs on mineral admixtures (silica fume and fly ash) (Sun et al., 2013), cement or clinker (Warakulwit et al., 2015), sand (Nasibulin et al., 2013), and aggregates (Gupta et al., 2017). CNTs or CNFs also can grow on the surface of reinforced fibers such as carbon fiber (Bekyarova et al., 2007). In-situ growing method can also strengthen bond of nanomaterials with hydration products. However, the growth is inefficient and complicated.

1.3.4 Assembled methods Assembled methods include electrostatic self-assembly and chemical grafting. Electrostatic self-assembly is a novel technology for dispersing nanomaterials by utilizing the electrostatic adsorption between nanomaterials and macro-, micro-, or nanoparticles with opposite electrical charges. For example, CNTs and carbon nano black (CNB) have been self-assembled to disperse each other (Zhang et al., 2017). TiO2 with size 450 nm has been utilized to disperse CNTs through electrostatic self-assembled method to form CNTs/microTiO2 composite fillers. It has been reported that self-assembled materials may be more liable to disperse because size of self-assembled unit is larger (Zhang, Li, et al., 2020; Zhang, Zheng, et al., 2020; Zhang, Wu, et al., 2020) (as shown in Fig. 1.12). With the incorporation of larger size of micro-TiO2, the effective content of CNTs to form the conductive network in cementitious composites largely increases (Zhang, Li, et al., 2020; Zhang, Zheng, et al., 2020; Zhang, Wu, et al., 2020). Zhang et al., (2020) generalized it as the excluded volume effect and proposed two formulas to calculate the effective CNT content in cementitious composites. Their results showed the effective CNT content increases by 4.85% after adding CNT/microTiO2 at the content of 5.78%. Electrostatic self-assembly method is characterized by its stability and simple process without chemical reaction and pollution. This method integrates properties of each component of composite fillers, which is beneficial for multi-functional properties. Chemical grafting aims to attach nanomaterials to micro-/macromaterials through chemical reactions. Massive attention of researchers have been paid to fibers grafted by nanomaterials, because attaching nanomaterials to fibers increases roughness of the surface of fibers, which strengthens bond between fibers/nanocomposite and cementitious matrix. The typical representatives are hydrophobic polymer (Xia et al., 2016), carbon fibers (Li, Ding, et al., 2020; Li, Dong, et al., 2020;

Overview of tailoring cementitious composites with various nanomaterials

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Figure 1.12 Diagrammatic sketch of CNT, electrostatic self-assembled unit of CNT/NCB, and CNT/microTiO2. Zone wrapped by yellow color represents the size of electrostatic selfassembled unit. Source: Modified from Zhang, L., Li, L., Wang, Y., Yu, X., & Han, B. (2020). Multifunctional cement-based materials modified with electrostatic self-assembled CNT/ TiO2 composite filler. Construction and Building Materials, 238. https://doi.org/10.1016/j. conbuildmat.2019.117787.

Li, Corr, et al., 2020; Li, Dong, Wang, et al., 2020), CNTs (Cui et al., 2017), and polyvinyl alcohol (PVA) fibers grafted by nano-SiO2 (Cui et al., 2018), and polyethylene fibers grafted by CNFs (He et al., 2019). There also exist binders grafted by nanomaterials, such as silica fume grafted by nano-SiO2 (Cai et al., 2017). Nanomaterials are extensively distributed in matrix along with fibers, thus realizing the dispersion of nanomaterials. This method also benefits multi-functional properties but may cause pollution during chemical reaction.

1.3.5 Surface coating method Surface coating method refers to wrapping nanomaterials by utilizing another type of materials. This method is found not only beneficial to multi-functional properties but also effective for dispersion of nanomaterials. The typical representative is SiO2 coating nano-TiO2, where a number of Ti
O
Si bonds are observed in the coated interface. The appearance of Ti
O
Si bonds makes nano-TiO2 more negatively charged and easily dispersed in water owing to the existence of more electrostatic repulsion between nano-TiO2 particles (Chen et al., 2017). However, this method has disadvantages of operational complexity and high cost.

1.4

Tailoring cementitious composites with 0D nanomaterials

1.4.1 Nano-SiO2 Nano-SiO2 is a kind of 0D nanoparticle with pozzolanic activity. In addition to filling pores in cementitious composites, it can participate in the secondary hydration reaction, thus making cementitious composites more compact. Tailoring cementitious composites with nano-SiO2 has effects on hydration, rheology, workability, mechanical properties, and functional properties of cementitious composites.

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Recent Advances in Nano-Tailored Multi-Functional Cementitious Composites

1.4.1.1 Hydration Usually, nano-SiO2 accelerates hydration process and improves hydration degree of cementitious composites, because nano-SiO2 with pozzolanic activity can participate in hydration and secondary hydration process. Senff et al. (2009) observed the time reaching maximum temperature is shifted earlier by 51% when nano-SiO2 at the content of 2% is added in cement mortar. The hydration heat of cement paste also increases by 6.2% in 72 h (Jo et al., 2007). Researchers (Hou et al., 2013; Oltulu & Sahin, ¸ 2014; Zhang & Islam, 2012) have reported initial and final setting times are shortened after incorporating nano-SiO2 into cementitious composites, and the reduction becomes more obvious as the nano-SiO2 content increases.

1.4.1.2 Rheology Factors such as SP content and w/b ratio affect cementitious composites with nanoSiO2. Jiang, Zhou, et al. (2018) reported that the plastic viscosity of cement paste decreases as the nano-SiO2 increases. Moreover, the saturated point of SP content in cement paste with nano-SiO2 is 0.75%, and above this value, the rheology parameters change slowly. Ouyang et al. (2018) developed a viscosity prediction model of cement paste with nano-SiO2, with the assumption that cement paste with nanoSiO2 is a suspension. The model can accurately predict the viscosity of cementitious composites from viscosity of nano-SiO2 suspension and relative viscosity of pure cement paste when nano-SiO2 is less than 5.5%. This model can also calculate the absorbed water layer of thickness around nano-SiO2 particles.

1.4.1.3 Workability So far, there are two viewpoints in workability of cementitious composites with nanoSiO2. The first view believes that nano-SiO2 brings obstacle to workability because of the deterioration of flowability. References (Ghafari et al., 2014; Givi et al., 2011; Hosseini et al., 2011) are the strong support for this view. The other viewpoint is that nano-SiO2 can improve the workability by increasing integrity and uniformity of fresh cementitious composites, which is consequently beneficial for mechanical behaviors. Collepardi et al. (2015) reported that nano-SiO2 at the content of 1% can decrease the bleeding water content of self-compacting concrete by 50%, while the deterioration of flowability is only 9.7%. This is to say, the positive effect of nano-SiO2 on water retainment far exceeds its deterioration on flowability in self-compacting concrete.

1.4.1.4 Mechanical properties Static mechanical properties Researchers (Alireza et al., 2010; Li, 2004; Najigivi et al., 2013; Shih et al., 2006; Xiao, 2002) have investigated the effect of nano-SiO2 on strength performances of cementitious composites. It can be concluded that there is an optimal content of nanoSiO2 in making cementitious composites reach the highest strengths. The observed best modification includes 168.8% in compressive strength (Jo et al., 2007), 31.8% in

Overview of tailoring cementitious composites with various nanomaterials

21

flexural strength, and 83.3% in tensile strength (Ahmadreza et al., 2014; Alafogianni et al., 2019; Alireza et al., 2010). Besides, Taherkhani and Tajdini (2019) suggested that nano-SiO2 at content of 6% can significantly enhance the fatigue life of concrete. Szymanowski and Sadowski (2020a, 2020b) reported that incorporating amorphous nano-SiO2 into cement mortar possibly improves the bond strength between the cement mortar overlay and the concrete substrate up to 20%. The results reflect the modification of nano-SiO2 to ITZ between the two types of cementitious composites, which is the evidence behind the improved strength.

Dynamic mechanical properties The main benefit of tailoring cementitious composites with nano-SiO2 the increase in dynamic strength behaviors, especially the ability to absorb dynamic energy. Wang, Dong, et al. (2019) incorporated nano-SiO2 into UHPC and reported that the dynamic compressive strength, impact toughness, and specific energy absorption are improved by 25.0%, 13.6%, and 211.2%, respectively. Li et al. (2017) reported that the average peak stress and peak strain of UHPC are improved by 21.7% and 31.1% because of the presence of nano-SiO2.

1.4.1.5 Durability The modification of nano-SiO2 in transportation of cementitious composites consists of two main parts: (1) ion and water adsorption and (2) pore filling. Results in Table 1.5 (Behfarnia & Salemi, 2013; Ghafari et al., 2014; He & Shi, 2008; Heidari & Tavakoli, 2013; Said et al., 2012; Wang, Zhang, et al., 2018; Zhang, 2007; Zhang & Islam, 2012) demonstrate that the reduction of transportation first increases with the increase in nano-SiO2 and then decreases when nano-SiO2 exceeds the optimal limitation. This is to say, pore filling effect plays a more important role than ion adsorption in terms of transportation of cementitious composites. Apart from the improvement for chloride anticorrosion, nano-SiO2 can reduce the occurrence of alkali
silica reaction, consequently decreasing the expansion of cement mortar. Incorporating nano-SiO2 can reduce mass loss, strength loss, water absorption, and increase the dynamic elastic modulus of specimen after freeze-thaw circles (Behfarnia & Salemi, 2013; Quercia et al., 2012; Wang et al., 2008). Tailored concrete (Zhang, 2007) and UHPC (Wang, Zhang, et al., 2018) have better abrasion performance because of the presence of nano-SiO2. Moreover, limewater curing is more preferable than the water curing (Nazari & Riahi, 2011a, 2011b, 2011c; Riahi & Nazari, 2011). Additionally, Ibrahim et al. (2012) indicated that cement mortar with nano-SiO2 exhibits better antifire ability under the temperature of 26 C, 400 C, and 700 C.

1.4.1.6 Functional properties Self-sensing properties Nano-SiO2 allows cementitious composites’ self-sensing by modifying their electrical properties. Table 1.6 shows research results on the electrical resistivity of cementitious composites with nano-SiO2. In most cases, nano-SiO2 increases the electrical resistivity. ¨ ztu¨rk et al., 2020; Zhang et al., 2016), However, from Table 1.6 (Jalal et al., 2012; O

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Recent Advances in Nano-Tailored Multi-Functional Cementitious Composites

Table 1.5 Transportation performance of UHPC and ordinary cementitious composites with nano-SiO2 after 28 days. Matrix

UHPC

Ordinary cementitious composites

NanoSiO2 content

Decrease/increase (relative values)

1%/

260.00%

—

10.04%

3% 1%

278.60% —

— —

10.49% 7.5%

2% 3% 4% 1%

— — — 218.04%/

— — — —

24.20% 31.90% 25.70% —

3% 3%

210.49% 248.88%

— —

— —

6% 3%

284.00% —

— 219.93%

— —

5% 7% 1%

— — —

228.27% 220.01% 254.80%

— — —

2/% 3% 4% 5% 0.50%

— — — — —

260.40% 267.10% 272.70% 269.60% 211.10%

— — — — —

1% 1%

2 61.70%

214.70% —

— —

References

Chloride Water Gas penetration absorption permeability Wang, Zhang, et al. (2018) Ghafari et al. (2014)

Zhang (2007)

Said et al. (2012) Behfarnia and Salemi (2013)

Zhang and Islam (2012)

Heidari and Tavakoli (2013) He and Shi (2008)

the effect of nano-SiO2 on the electrical properties of cementitious composites is still controversial. Cementitious composites with nano-SiO2 can sense damage, strain, and stress. ¨ ztu¨rk et al. (2020) reported that the increase in electrical resistance in damaged O UHPC containing nano-SiO2 is 18.5% higher than the control group. Vipulanandan and Mohammed (2019) showed that the sensitivity of cement paste consisting SiO2 is over

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Table 1.6 The effect of nano-SiO2 on resistivity of UHPC and ordinary cementitious composites. Matrix type

Ordinary cementitious composites

UHPC

Nanocontent

Increase/decrease of resistivity (relative values) 3 days

7 days

28 days

90 days

0.50%

6.70%

—

11.90%

—

1.00% 1.50% 2.00% 2.0% (C40/ 45/50) 3.00%

15.90% 17.50% 19.40% —

— — — 0%/ 8.2%/ 42.6% 245.30%

21.10% 33.00% 47.00% 77.8%/ 82.0%/ 134.7% —

— — — 270.9%/ 252.5%/ 231.9% 261.00%

—

References

Zhang et al. (2016)

Jalal et al. (2012) ¨ ztu¨rk O et al. (2020)

500 times higher than the referenced group. Meanwhile, they indicated the sensitivity largely depends on the curing ages and nano-SiO2 content. The sensing ability of cementitious composites with nano-SiO2 is in the incipient state of development.

Self-healing properties The presence of nano-SiO2 can provide more nucleation sites at crack surface, which consequently accelerates the self-healing efficiency of cementitious composites. Wang, Ding, et al. (2018) investigated the self-healing of UHPC with nanoSiO2, and found that compared to the control group, the compressive strength increases by 39.4% after a recovery of 28 days, and flexural strength increases by ¨ ztu¨rk et al. (2020) reported that compared to the control group, the 33.7%. O increase of 12.8% in electrical resistance of engineered cementitious composites (ECC) with nano-SiO2 after recovering for 7 days and 21.2% after 90 days. Moreover, initial individual microcracks close to or less than 100 μm are completely healed at the surface of with nano-SiO2.

Thermal properties The incorporation of nano-SiO2 has shown to enhance the thermal conductivity of cementitious composites (Aeem et al., 2014; Zaidi et al., 2019). Zaidi et al. (2019) reported that the thermal conductivity of cement mortar with nano-SiO2 (1%
3%) is located in the range from 0.762 to 1.051 W/(mK). Aeem et al. (2014) showed that the incorporation of nano-SiO2 is beneficial in reducing thermal stress inside cement paste by observing the microchange of specimen length and calculating thermal expansion coefficient under high temperature.

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Recent Advances in Nano-Tailored Multi-Functional Cementitious Composites

1.4.2 Nano-TiO2 Nano-TiO2 has various types with different crystal structures including rutile, anatase, brookite, and rutile and anatase hybrid phases. Besides, surface coating with nanoTiO2, such as SiO2/Al2O3 coating and SiO2 coating, has been attempted to tailor cementitious composites. The presence of nano-TiO2 causes an effect on hydration, rheology, workability, durability, and mechanical and functional properties of cementitious composites. More importantly, nano-TiO2 inherent with photocatalytic properties can release free radicals to react with some harmful gases (NOX and SO2) under UV radiation, thereby endowing cementitious composites with pollutant-purifying ability.

1.4.2.1 Hydration Owing to the large surface area of nano-TiO2, much water is absorbed by nanoTiO2 during the fabrication process, thereby causing an effect on the hydration rate and degree. However, there is no consistent conclusion temporarily. The common views believe that nano-TiO2 can accelerate hydration rates and improve hydration degree (Jun et al., 2012; Mohseni et al., 2016; Soleymani, 2012a, 2012b, 2012c; Zhang et al., 2015). Zhang et al. (2015) reported that the initial/final setting time is shortened by 37.9%/15.7%, 63.4%/37.4%, and 6.5%/46.2% with the incorporation of nano-TiO2 at the content of 1%, 3%, and 5%, respectively. Besides, the small particle size (15 2 25 nm) of nano-TiO2 is more effective than the large nanoparticle size (20 2 30 nm) (Jayapalan et al., 2009). However, Folli et al. (2010) believed that nano-TiO2 is inert and will not improve cement-hydration degree.

1.4.2.2 Rheology Jiang, Shan, et al. (2018) and Li, Ding, et al. (2020) provided a comprehensive assessment of rheology behaviors of cementitious composites on the basis of various parameters of nano-TiO2, including content, crystal phase, diameter, and nanomaterial modification type. The shear stress increases with nano-TiO2 content at any shear rate (Jiang, Shan, et al., 2018). The nano-TiO2 with a smaller diameter of 20 nm has the largest yield stress and minimum viscosity (Li, Ding, et al., 2020). Moreover, the cement paste with the rutile nano-TiO2 exhibits the largest yield stress and minimum viscosity among the three crystal types. The cement paste with SiO2/Al2O3 coating nano-TiO2 shows larger yield stress and smaller minimum viscosity than the SiO2 coating form, because the electrostatic repulsion between SiO2/ Al2O3 coating nano-TiO2 particles is smaller (Li et al., 2020).

1.4.2.3 Workbality Similar to nano-SiO2, there are two arguments about the effects of nano-TiO2 on flowability of cementitious composites. From Table 1.7 (Jalal et al., 2013; Meng et al., 2012; Mohseni et al., 2015; Zhang et al., 2015), it can be concluded that the fluidity of cement paste and cement mortar presents a decreasing trend with the increase in nanoTiO2 content. However, in self-compacting cement mortar, nano-TiO2 at the content of

Overview of tailoring cementitious composites with various nanomaterials

25

Table 1.7 Flowability of UHPC and ordinary cementitious composites with nano-TiO2. Matrix type

Ordinary cementitious composites

UHPC

NanoTiO2 content

Increase/decrease (relative values) Fluidity

Slump flow diameter

V-tunnel flow time

1%

22.80%

—

—

3% 5% 5% (with/ without SP) 10% (with/ without SP) 1%

219.80% 220.80% 0.815

— — —

— — —

0.8563922942

—

—

—

—

25.50%

3% 5% 1%

— — —

3.3% 4.90% 21.30%

212.70% 225.50% 4.00%

2% 3% 4% 5%

— — — —

22.50% 25.0% 27.50% 28.80%

10.00% 16.00% 20.00% 24.00%

References

Zhang et al. (2015)

Meng et al. (2012)

Mohseni et al. (2015)

Jalal et al. (2013)

1% and 3% can improve the flowability (Mohseni et al., 2015). The possible reason is that nano-TiO2 wrapped by water-reducing agent can act as rollers for lubricating cement particles. It can be further inferred that the addition of nano-TiO2 can also enhance the water retainment and cohesion of fresh self-compacting concrete because of the adsorption of water.

1.4.2.4 Mechanical properties Static mechanical properties The presence of nano-TiO2 can improve the static strength behaviors of cementitious composites. Table 1.8 (Han, Zhang, et al., 2017; Li et al., 2017; Nazari & Riahi, 2010a, 2010b, 2011a; Noorvand et al., 2013; Rahim & Nair, 2016; Salman et al., 2016) shows the static strength variation of cementitious composites with nano-TiO2 after 28 days, indicating that the improvement of strength is largely content-dependent on nano-TiO2. Szymanowski and Sadowski (2020a) incorporated

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Recent Advances in Nano-Tailored Multi-Functional Cementitious Composites

Table 1.8 Static strength variation of UHPC and ordinary cementitious composites with nano-TiO2 after 28 days. Matrix type

Ordinary cementitious composites

UHPC

NanoTiO2 content

Increase/decrease (relative values) Flexural strength

Compressive strength

Tensile strength

0.10%

—

13.20%

—

0.50% 0.90% 1% 0.25%/

— — — 9.40%

20.80% 14.15% 10.90% 10.50%

— — — —

0.75% 1.25% 1.75% 0.50%

15.10% 13.20% 7.60% —

19.30% 15.10% 4.30% 6.70%

— — — —

1% 1.50% 1%

— — 5.56%

10.20% 19.10% 14.2%

— — 9.50%

2% 3% 4% 0.50%

14.81% 27.78% 16.67% —

26.5% 36.4% 26.4% —

23.80% 33.30% 23.80% 44.40%

1% 1.20% 2% 0.78% 2.32% 3.88% 2%

— — — 17.30% 47.10% 20.80% 22%/

— — — 8.20% 18.50% 16.23% 14.80%

66.70% 50.00% 5.60% — — — 21.30%

3% 4% 5% 6%

34.10% 40.20% 34.10% 31.10%

25% 29% 24.40% 17%

30% 34.10% 25.90% 21.90%

References

Han, Li, et al. (2017), Han, Wang, et al. (2017), Han, Zhang, et al. (2017), Han, Zheng, et al. (2017)

Salman et al. (2016)

Noorvand et al. (2013)

Nazari and Riahi (2011a, 2011b, 2011c)

Nazari and Riahi (2010a, 2010b)

Li et al. (2017)

Rahim and Nair (2016)

Overview of tailoring cementitious composites with various nanomaterials

27

tetragonal crystal nano-TiO2 into cement mortar as the overlayer placing on ordinary concrete substrate. The highest value of pull-off bond strength between the overlayer and the concrete substrate is obtained when nano-TiO2 at the content of 0.5% is incorporated into the overlayer. This can be considered to be a province for the modified ITZ between two cementitious composites.

Dynamic mechanical properties The most noteworthy benefit of tailoring cementitious composites with nano-TiO2 is to increase the ability of absorbing dynamic energy. Wang, Dong, et al. (2019) found that the incorporation of nano-TiO2 improves dynamic compressive strength, impact toughness, and specific energy absorption of UHPC up to 59.7%, 39.9%, and 246.9%, respectively.

1.4.2.5 Durability Table 1.9 (Jalal et al., 2013; Soleymani, 2012a, 2012b, 2012c; Wang, Zhang, et al., 2018) summarizes the main outcome of research investigations on the transportation performances of cementitious composites with nano-TiO2. It can be observed that nano-TiO2 can significantly decrease the transportation of cementitious composites, and the transportation performances are also related to the curing method. Incorporating nano-TiO2 can improve the corrosion resistance of cementitious composites. From parameters such as electrical resistivity, charge pass, and ultrasonic pulse velocity, references (Mohseni et al., 2015, 2016; Rahim & Nair, 2016) indicated cementitious composites with nano-TiO2 exhibit a superior antichloride corrosion ability to the composites without nano-TiO2. Besides, nano-TiO2 also can help cementitious composites resisting the sulfate attack (Salemi et al., 2014). Additionally, Szymanowski and Sadowski (2020b) and Wang, Zhang, et al. (2018) reported the addition of nano-TiO2 can increase abrasion resistance of cementitious composites. The resistance ability also slightly depends on the curing methods.

1.4.2.6 Functional properties Photocatalysis properties Nano-TiO2 inherent with photocatalytic properties can release free radicals to react with some harmful gases (NOX and SO2) under UV radiation, thereby endowing cementitious composites with pollutant-purifying ability (Sanchez & Sobolev, 2010; Senff et al., 2016; Tung & Daoud, 2011). He et al. (2019) reported iron coating of nano-TiO2 is more efficient for degrading vehicle exhausts than the ordinary TiO2. Chen et al. (2017) replaced cement by the hybrid nano-TiO2 of rutile and anatase crystal (30%, 10%, and 3%) to fabricate cementitious composites. They observed that tailoring cementitious composites with nano-TiO2 can effectively eliminate the stormwater pollutants after 48 h, with enhancement efficiency for purifying NO32, H3PO4, and chemical oxygen demand (COD) of 16%, 25%, and 18%, respectively. Furthermore, the overall removal rates of the tailored group are averagely 33%, 84%, and 67% after 3 weeks, while those of reference group are only 6%, 26%, and 20%, respectively.

28

Recent Advances in Nano-Tailored Multi-Functional Cementitious Composites

Table 1.9 Transportation performance of UHPC and ordinary cementitious composites with nano-TiO2 after 28 days. Matrix

Nano-TiO2 content

Decrease (relative values) Water absorption coefficient

Ordinary cementitious composites

UHPC

0.5% (water/ limewater curing)

22.0%/15.4%

1% (water/ limewater curing) 1.5% (water/ limewater curing) 2% (water/ limewater curing) 1% (7.5/17.5/ 25 mm) (90 days) 4% (7.5/17.5/ 25 mm) (90 days) 1% (room temperature/ heating curing) 3% (room temperature/ heating curing)

35.3%/12.9%

References

Chloride diffusion coefficient Soleymani, 2012a, 2012b, 2012c

46.5%/3.3%

55.9%/4.7%

—

5.8%/15.6%/ 13.0%

—

40.3%/ 53.3%/ 60.9% 26.3%/0%

Jalal et al. (2013)

Wang, Zhang, et al. (2018)

47.4%/


Electromagnetic shielding and absorption properties Electromagnetic properties of cementitious composites are highly dependent on the content of nano-TiO2 Various types of nano-TiO2 can modify the electromagnetic properties of cementitious composites. Li et al. (2020) fabricated cement paste with 10 types of nano-TiO2 (anatase crystal with diameters of 5/10/15 nm, rutile crystal with diameters of 50/500/1500 nm, hybrid rutile and anatase crystal, SiO2-coated rutile TiO2, and Al2O3/SiO2-coated rutile TiO2). They reported that SiO2-coated rutile TiO2 and Al2O3/SiO2-coated rutile TiO2 are efficient for enhancing the electromagnetic wave absorption performance.

Overview of tailoring cementitious composites with various nanomaterials

29

Self-sensing properties The self-sensing properties of cementitious composites with nano-TiO2 under monotonic and cyclic loads have been investigated. Xiao (2002) reported that the electrical resistivity of the composites with nano-TiO2 is linearly related to the pressure under monotonic load. Besides, the fatigue damage of composites under cyclic load can be reflected because of the presence of nano-TiO2, which provides feasibility for tailoring cementitious composites with nanomaterials to sense the fatigue behaviors.

Self-healing properties Wang, Ding, et al. (2018) employed different curing methods (water curing and air curing) to investigate the self-healing behavior of UHPC with nano-TiO2 by observing compressive and flexural strength. The results showed UHPC with nano-TiO2 cured in water has the largest healing efficiency, which is 34% higher than that of UHPC without nano-TiO2.

1.4.3 Nano-ZrO2 Nano-ZrO2 is featured by its high strength and toughness, good wear resistance, and anticorrosion performance. The phase transition of nano-ZrO2 usually appears with volume changes under pressure and high temperature, having the potential to restrain the prolongation of cracks in cementitious composites.

1.4.3.1 Workability Incorporating nano-ZrO2 into cementitious composites usually makes an impact on workability because much water is absorbed by nano-ZrO2 particles during the fabrication process. The workability is largely dependent on nano-ZrO2 content and curing method. Umarajyadav and Vahini (2017) indicated that the flowability of concrete with 0.2% of nano-ZrO2 is acceptable, but above this value, it will decrease sharply. Nazari and Riahi (2013) observed that curing in saturated limewater accelerates the C
S
H gel formation, which consequently makes fresh composites more viscous.

1.4.3.2 Durability Research results in Table 1.10 (Jaishankar & Saravana Raja Mohan, 2015; Nazari & Riahi, 2010a, 2011c; Wang, Zhang, et al., 2018) demonstrate that the durability of cementitious composites with nano-ZrO2 largely depends on nano-ZrO2 content and curing method. The reduction of water absorption is more obvious for concrete with nano-ZrO2 cured in saturated limewater as a result of more formation of C
S
H gel.

Table 1.10 Durability of UHPC and ordinary cementitious composites with nano-ZrO2. Matrix type

Ordinary cementitious composites

UHPC

Curing method

Water/ limewater curing

Water curing

Water/heating curing

Nano-ZrO2 content

References

Abrasion loss

Chloride penetration

Water absorption

0.50%

—

—

3.3%/4.2%

1.00% 1.50% 2.00% 1%

— — — —

— — — —

3.0%/4.0% 2.8%/3.8% 2.7%/3.7% 49.60%

2% 3% 4% 5% 1%

— — — — 33.3%/ 22.8% 11.1%/ 48.7% —

— — — —
/47.37%

56.60% 64.00% 70.10% 66.80% —


/5.26%

—

32.40%

27.50%

3% Water curing

Decrease (relative values)

1.50%

Nazari and Riahi (2011a, 2011b, 2011c)

Nazari and Riahi (2010a, 2010b)

Wang, Zhang, et al. (2018)

Jaishankar and Saravana Raja Mohan (2015)

Overview of tailoring cementitious composites with various nanomaterials

31

1.4.3.3 Mechanical properties Static mechanical properties Table 1.11 (Han, Wang, et al., 2017; Nazari & Riahi, 2010b, 2011b, 2011b, 2013; Soleymani, 2012a, 2012b) shows the static strength variation of cementitious composites with nano-ZrO2, confirming that optimal content of nano-ZrO2 can effectively enhance the strengths, possibly because excessive nano-ZrO2 can aggregate and produce weak areas in matrix (Soleymani, 2012b). It can be also concluded that curing in saturated limewater is superior to curing in water for improving strength. Jiang, Zhou, et al. (2018) indicated that cement paste with nano-ZrO2 exhibits the maximum improvement under temperature of 105 C. This may be attributed to the phase transition of nano-ZrO2 with volume changes under pressure and high temperature. Additionally, Wang, Dong, et al. (2019) reported the interfacial strength between aggregate and cement mortar is improved by 35.1% with incorporation of nano-ZrO2 at content of 2%. This is an explanation for the compressive, flexural, and tensile strength improvement.

Dynamic mechanical properties Wang, Dong, et al. (2019) investigated the dynamic mechanical behaviors of UHPC with nano-ZrO2. The dynamic compressive strength, impact toughness, and specific energy absorption are improved by 61.3%, 23.2%, and 201.3%, respectively. The main benefit of tailoring UHPC with nano-ZrO2 is increasing the ability to absorb dynamic energy. Umarajyadav and Vahini (2017) found that the improvement of impact resistance is up to 549.9% when nano-ZrO2 is 0.2%.

1.4.4 Functional properties 1.4.4.1 Self-healing properties Wang, Ding, et al. (2018) tailored a type of self-healing UHPC with nano-ZrO2 and measured the compressive and flexural strength loss of specimen. The results showed that the recovering efficiency is improved by the addition of nano-ZrO2 and the air curing is more effective than the water curing.

1.5

Tailoring cementitious composites with 1D nanomaterials

1.5.1 Carbon nano-tubes CNTs, a representative of 1D nano-carbon materials, include two main basic forms, single-walled CNTs (SWCNTs) and multiwalled CNTs (MWCNTs). SWCNTs are formed by a single sheet of graphite shaped in to a hollow cylinder, while MWCNTs are shaped by several layers of graphite. Additionally, CNTs have been modified for well dispersion or multi-function, such as functionalized and surface coating SWCNTs or MWCNTs (such as MWCNTs
COOH, MWCNTs
OH, and

Table 1.11 Static strength variation of UHPC and ordinary cementitious composites with nano-ZrO2 after 28 days. Matrix type

Ordinary cementitious composites

Curing method

Water/saturated limewater curing

Water/saturated lime water curing

Water/saturated limewater curing

Nano-ZrO2 content

Flexural strength

Compressive strength

Tensile strength

0.50%

—

16.0%/28.5%

—

1.00% 1.50% 2.00% 0.50%

— — — —

18.5%/36.6% 16.6%/42.7% 7.6%/52.4% 40.5%/45.6%

— — — —

1.00% 1.50% 2.00% 0.50%

41.4%/47.8% 40.7%/51.1% 37.7%/53.4% —

— — — —

— —

— —

—

—

1%

— — — 13.6%/ 36.6% 25%/48.8% 20.5%/ 56.1% 6.8%/ 65.8% 0%

5.70%

26.30%

2% 3% 4% 5% 0.50% 1.00% 3.00% 5.00%

27.40% 40.50% 51.70% 50.00% 16.40% 22.10% 36.60% 26.30%

15.20% 33.90% 49.70% 46.50% 16.30% 15.30% 14.50% 14.10%

18.80% 50% 75% 43.80% 25.80% 23.30% 34.00% 22.30%

1.00% 1.50% 2.00% UHPC

Water curing

Water curing

Increase/decrease (relative values)

References

Cwirzen et al. (2008), Ruan et al. (2017)

Dalla et al. (2015)

Saafi, 2009, Saito et al. (2002)

Ruan, Han, Yu, Li, et al. (2018)

Salemi et al. (2014)

Overview of tailoring cementitious composites with various nanomaterials

33

Ni-coated MWCNTs). Studies on cementitious composites with CNTs focus on their hydration, rheology, workability, mechanical properties, durability, and functional properties.

1.5.1.1 Hydration Incorporating CNTs can accelerate hydration process of cementitious composites. Li et al. (2020) showed that CNTs at the content of 0.1%, 0.3%, and 0.5% can decrease the time to reach maximum temperature by 3.06%, 8.69%, and 4.65%, respectively. Researchers (Li, Jia, et al., 2018; Li, Ding, et al., 2018) have indicated that CNTs contribute to peak hydration heat and the hydration reaction rate of cement paste at each stage and enhance the early hydration degree.

1.5.1.2 Workability The workability of cementitious composites is decided by various factors, such as the type and content of CNTs, content of SP, and w/c ratio. Kang et al. (2015) reported that the decrease in flow diameter caused by CNTs and acid-treated CNTs is 4.2% and 16.7%, respectively. It can be concluded that functionalized MWCNTs combine or absorb more water than ordinary CNTs and cause more obvious effect on flowability of cementitious composites accordingly. Thiyagarajan et al. (2018) incorporated MWCNTs into self-compacting concrete and found that the flowability is not affected by MWCNTs. If more SP is added to wrap CNTs, CNTs will not absorb too much water. Therefore, the flowability of self-compacting concrete does not show deterioration trend. CNTs may be beneficial to water retaining ability, thus improving integrity and uniformity of cementitious composites.

1.5.1.3 Rheology Jiang, Shan, et al. (2018) investigated the effect of w/c ratio, CNTs content, SP content and fabrication process on the rheology of cement paste. The results showed that MWCNTs lead to high yield stress, because the tubular bar makes them more liable to wrap together, and the hollow structure makes them absorb much water. Moreover, the flowability of cement paste is sensitive with w/c ratio at the range from 0.2 to 0.22, and beyond this range, flowability changes slowly. Alatawna et al. (2020) investigated the effect of various CNTs on rheology. The results showed that the relative flow of oxidized-MWCNTs is lower than that of ordinary MWCNTs. And the flowability shows a decreasing trend with increase of MWCNT content.

1.5.1.4 Mechanical properties Static mechanical properties Table 1.12 (Al-Rub et al., 2012; Cui et al., 2017; Han, Wang, et al., 2017; Kang et al., 2015; Ruan, Han, Yu, Li, et al., 2018) summarizes different trends of strengths of cementitious composites with different CNT contents, diameters and

Table 1.12 The mechanical performances of UHPC and ordinary cementitious composites with carbon nano-tubes (CNTs) after 28 days. Matrix type

Ordinary cementitious composites

Type of CNTs

MWCNTs

MWCNTsOH MWCNTsCOOH

Inner/outer diameter (nm)

Length (nm)

Dosage Dispersing method

Increase/decrease (relative values) Flexural strength

Compressive strength

Tensile strength

4.60%

7.80%

—

2
5/ , 8

0.5
2

0.10%

5
10/20
30 2
5/ , 8 5
10/20
30 2
5/ , 8

0.5
2 10
30 10
30 0.5
2

0.10% 0.10% 0.10% 0.50%

55.20% 48.30% 44.80% 21.80%

37.60% 23.70% 32.70% 61.90%

— — — —

2
5/ , 8 2
5/ , 8

10
30 0.5
2

0.50% 0.50%

24.10% 54%

64.60% 43.50%

— —

2
5/ , 8 5
10/20
30

10
30 10
30

0.50% 0.50%

49.40% 64.40%

52% 78.80%

— —

28.70%

65.30%

—

—

269%

—

10.3

20.90%

—

— 9.50%

10.20% 21.00%

— 14.10%

25.2 2.7

22.80% 9.70%

22.00% 12.60%

Ni coating MWCNTs Helical MWCNTs MWCNTs


/100
200

1
10

0.50%

9.5

1500.00

0.20%

SWCNT

0.5
3.0

100.00

0.06%

MWCNTs MWCNTs

5
100 5
15/50

100.00 10
30

0.12% 0.10%

MWCNTs MWCNTs

5
10/20
40 5
10/20
40

10
30 10
30

0.05 0.05%

Ultrasonic dispersion 1 SP

Ultrasonic treatment 1 SP Dispersing agent: TX10 Dispersing agent: TNWDIS

References

Cui et al. (2017)

Alrub et al. (2012) Kang et al. (2020) Hawreen et al. (2019)

UHPC

MWCNTsCOOH MWCNTsOH MWCNTs

5
10/20
40

10
30

0.05%

3.7

8.70%

4.1%%

5
10/10
30

10
30

0.05

6.7

6.70%

1.60%

5
10/20
30

10
30

0.25%

3.60%

15.30%

—

MWCNTsOH

2
5/ , 8

0.5
2

0.50% 0.25%

20.20% 27.20%

8.20% 30.40%

— —

MWCNTsCOOH

2
5/ , 8

0.5
2

0.50% 0.25%

23.50% 25.80%

16.80% 8.40%

— —

0.50%

12.20%

8.80%

—

Ultrasonic treatment 1 SP

Note: SP and PAP denote superplasticizer and polyacrylic acid polymer. TNWDIS is a type of polyethylene glycol aromatic imidazole agent.

Ruan, Han, Yu, Li, et al. (2018); Ruan, Han, Yu, Zhang et al. (2018)

36

Recent Advances in Nano-Tailored Multi-Functional Cementitious Composites

lengths, aspect ratios, functional groups, morphologies (helical or tubular), and dispersing methods. It can be observed, various combinations of these parameters give rise to different strength performances, justified by several arguments. Cui et al. (2017) indicated that CNTs with larger diameters, longer lengths, and larger aspect ratios can bond with matrix tightly, which is beneficial to load transferring. Moreover, CNTs containing functional groups also appear the superior positive effects on strength of cement paste. In contrast to the above common view, Hawreen et al. (2019) indicated that CNTs with lower aspect ratio act better, and functionalized CNTs do not show significant difference from the ordinary types.

Dynamic mechanical properties Wang, Dong, Ashour, et al. (2020) studied the dynamic mechanical behavior of cementitious composites with various CNTs (ordinary MWCNTs, MWCNTsCOOH, MWCNTs-OH, Ni coating MWCNTs, and graphitized MWCNTs). CNTs with larger diameters and longer lengths have the most favorable performance on improving dynamic compressive strength. Moreover, incorporating graphitized MWCNTs is effective than Ni coating MWCNTs in terms of dynamic strength behaviors including strength, impact toughness and energy dissipation.

1.5.1.5 Durability Researches on the durability of cementitious composites with CNTs are listed in Table 1.13 (Alafogianni et al., 2019; Han et al., 2013; Lee et al., 2018). It can be found that, cement mortar with CNTs presents lower chloride ion penetration, less water absorption, and higher water permeability and gas permeability, and less corrosion. Incorporating CNTs can also increase the prevention of steel rebars in concrete against chloride corrosion. Additionally, CNTs can also increase the freezethaw resistance of cementitious composites. Cwirzen (2010) found that concrete with 0.3% of functionalized CNTs performs well after 180 freeze-thaw cycles, while the control group damages after 56 cycles.

1.5.1.6 Functional properties Self-sensing property Tailoring cementitious composites with CNTs can detect strain, stress, and damage through the signals of electrical resistivity. The electrical resistivity values vary with the changing contact between CNTs and matrix. Therefore the straindependent self-sensing property is related to the distribution of CNTs inside matrix and matrix attributes (such as elastic modulus). Dispersion strategies proposed to disperse CNTs are closely related to the selfsensing properties of cementitious composites. Usually, functionalized CNTs have a stronger bond with matrix than ordinary CNTs, thereby improving the sensitivity and accuracy. The carboxyl-functionalized MWCNTs (Dalla et al., 2015) and oxygen-functionalized CNTs (Moral et al., 2020) have been certified more effective than the nonfunctionalized CNTs, in terms of sensing ability.

Table 1.13 Durability of cementitious composites with carbon nano-tubes (CNTs). Dispersing methods

Dispersing agent: Viscocrete ultra 300

Dispersing agent: SP

Dispersing agent: SDBS Dispersing agent: SDS

CNT content

Increase/decrease (relative values)

References

Water absorption

Water permeability

Gas permeability

Rapid chloride penetration

Chloride corrosion rate

0.20%

223.50%

—

28.60%

—

—

0.40% 0.60% 0.80% 0.01%

235.30% 235.30% 241.20% 245.30%

— — — —

54.70% 62.20% 48.50% —

— — — 251.20%

— — — 4.20%

0.03% 0.05% 0.07% 0.20%

262.10% 247.40% 246.70% 250.10%

— — — 65.20%

— — — 15.60%

265.70% 256.90% 249.10% —

255.60% 466.50% 811.30% —

0.20%

259.20%

42.80%

20.00%

—

—

Note: SP, SDBS, and SDS denote superplasticizer, sodium dodecylbenzene sulfonate, and sodium dodecyl sulfate, respectively.

Alafogianni et al. (2019)

Lee et al. (2018)

Han, Zhang et al. (2013)

38

Recent Advances in Nano-Tailored Multi-Functional Cementitious Composites

Usually, a “soft” matrix is more sensitive to the same stress than the hardened. Researchers (Li et al., 2007; Yoo et al., 2018) have shown that the gauge factor of cement paste with MWCNTs is higher than that of strain gauge and UHPC with MWCNTs. The result certifies the strain-dependent attribute of sensing ability of cementitious composites. The w/b ratio would also affect the sensing ability. Han et al. (2012) indicated that the electrical resistance with w/b ratio of 0.6 is more sensitive to compressive stress, as compared to those with w/b ratio of 0.45. The possible reason is that high w/b ratio will make cement paste “softer,” meanwhile much water is easier to disperse CNTs during fabrication process.

Electromagnetic properties Factors including CNT content, functional group, size, and morphology (tubular or helical), the sample thickness, and wave frequency, can govern electromagnetic performances of cementitious composites. Zhang, Zheng, et al. (2019) reported that cement paste with 0.8% carboxylate MWCNTs achieves the best absorption performance, and the ordinary MWCNTs with larger diameter and longer length realizes the maximum shielding efficiency. Li, Ding, et al. (2020) studied electromagnetic wave shielding and absorption performances of cement paste with CNTs with chirality. They reported that the electromagnetic wave shielding efficiency of cement paste with 7.5% hydroxylated CNTs (HCNTs) is 1.39 times of that of control group, and the minimum reflectivity in 20 mm-thickness cement paste with 4.5% HCNTs is 2.7 times of that of control group.

Damping properties There are two views about the modification of CNTs to damping properties of cementitious composites. Luo et al. (2015) believed that the interface slippages between MWCNTs and matrix when bearing vibration lead to the improvement of damping ratio. They reported that the damping ratio of cementitious composites is approximately increased by 60% when 2.0% MWCNTs is added. If the improved damping property results from slippages between MWCNTs and matrix, uniform dispersion will bring more enhancement to cementitious composites. Liew et al. (2017) reported CNTs at content of 0.1% dispersed by TNWDIS (an aromatic modified polyethylene glycol ether) can improve the loss factor of cementitious composites by 25.9% at 1 Hz. It is also noted that cementitious composites consisting of MWCNTs
COOH perform better than a plain group in terms of damping behaviors (Ping et al., 2019). These findings strongly support the first view. However, Zhang, Wu, et al. (2020) believed that the enhancement is mainly attributed to the slippages between the inner tubes of the MWCNTs rather than the interface slippages between the CNTs and cement matrix.

Thermal properties CNTs with excellent thermal conductivity widely distribute inside cementitious composites matrix, thus forming heat conduction network. Veedu (2011) reported that the thermal conductivity of cementitious composites with CNTs is 1.85 times of that of control group. Hassanzadeh-Aghdam et al. (2018) obtained similar conclusion based on the effective medium micromechanics-based method.

Overview of tailoring cementitious composites with various nanomaterials

39

Self-healing properties CNTs can provide more nucleus sites in crack surface, thereby accelerating the ¨ ztu¨rk et al. (2020) investigated selfself-healing of cementitious composites. O healing efficiency of ECC with CNTs by observing the increase in electrical resistivity. The results showed electrical resistivity of specimen after 7 and 90 days increases by 27.1% and 39.3%, respectively. Through macroscopic observation after self-healing for 7 days, initial microcracks close to or less than 100 μm shows complete closure at surface of ECC with CNTs.

1.5.2 Carbon nano-fibers CNFs are promising for tailoring cementitious composites owing to their high mechanical properties, thermal stability, and high electrical conductivity. Moreover, CNFs have a cost advantage compared to other nano-carbon materials and are thus more likely to achieve the large-scale application of cementitious composites.

1.5.2.1 Hydration Researchers (Meng et al., 2012; Mohseni et al., 2015) have conducted the isothermal calorimetry test to investigate the effect of s on hydration heat of UHPC. They observed an increased cumulative heat of concrete and an accelerated hydration process with the presence of CNFs, thereby leading to a faster setting. Nur and Vinoth (2016) obtained similar conclusions that incorporating CNFs into cement mortar is effective for shortening the setting time by about 7.4%.

1.5.2.2 Rheology Table 1.14 (Jiang, Shan, et al., 2018) shows the yield stress and plastic viscosity of cement paste containing 0.1% of CNFs are about three times of those of plain paste. Furthermore, CNFs higher than 0.5% cause a dramatic increase of yield stress and plastic viscosity.

1.5.2.3 Workability Sbia et al. (2014) indicated that flowability of fresh UHPC is not strongly influenced by the addition of CNFs. Aslani et al. (2019) believed that a small content of Table 1.14 Rheology parameters of cement paste with arbon nano-fibers (CNFs). Matrix type

CNF content (%)

Yield stress (Pa)

Plastic viscosity (Pa 3 s)

Reference

Cement paste

0

6.01

1.2

Kang et al. (2020)

0.10 0.50 1.00

18.39 292.93 Over range

3.96 7.55 Over range

40

Recent Advances in Nano-Tailored Multi-Functional Cementitious Composites

CNFs can improve the packing density of ECC and thus produce extra water to lubricate cement particle, thereby contributing to the flowability. However, CNFs exceeding a certain limit still cause a deterioration on flowability as too much water is absorbed by CNFs (Aslani et al., 2019).

1.5.2.4 Mechanical properties Table 1.15 (Galao et al., 2014; Gao et al., 2019; Gao et al., 2019; Gay & Sanchez, 2010; Jiang, Zhou, et al., 2018; Kosson et al., 2020) shows the strength variations of cementitious composites with CNFs after 28 days. It can be concluded that strength variation is extremely content-dependent with CNFs. Gao et al. (2019) Table 1.15 The strength variations of ultra-high performance concrete (UHPC) and ordinary cementitious composites with carbon nano-fibers (CNFs) after 28 days. Matrix type

Ordinary cementitious composites

UHPC

CNF content

Increase/decrease (relative values) Compressive strength

Flexural strength

Split tensile strength

0.10%

21.80%

—

—

0.50% 1% 0.50%

21.10% 228.70% 5.10%

— — —

— — —

1% 2% 0.02%

21.50% 23.40% —

— — —

— — 24% (7 days)

0.20%

—

—

0.10%

—

18.10%

22% (7 days) —

0.10% 0.10% 10%

6% 10% 21.40%

20.10% — —

— —— —

15% 20% 25% 1%

12.40% 23.60% 22.60% 244% (7 days)

— — —

— — —

References

Jiang, Zhou, et al. (2018)

Galao et al. (2014)

Gay and Sanchez (2010)

Gao et al. (2019)

Gao et al. (2010)

Kosson et al. (2020)

Overview of tailoring cementitious composites with various nanomaterials

41

reported that CNFs are more efficient for improving the strength and elastic modulus of cement mortar and concrete than those of cement paste. They believed that the modification of CNFs to ITZ overweighs that to hydration products. The characterization of ITZ, where CNFs exist, shows that the Young’s modulus at nanoscale is increased by 36.7% for ITZ in cement mortar, and by 55%
63% in concrete. Gdoutos et al. (2016) reported that incorporating CNFs into cement mortar can effectively improve the fracture toughness, critical strain energy release rate, and critical crack tip opening displacement.

1.5.2.5 Durability CNFs turn out to be effective for improving the chemical attack resistance of cementitious composites. Brown and Sanchez (2018) observed that the cement paste with CNFs after being exposed to sulfate attack for 550 days exhibits less flexural strength loss than the control group. Galao et al. (2014) also confirmed that CNFs contribute to the resistance of cementitious composites against carbonation and chloride corrosion.

1.5.2.6 Functional properties Self-sensing properties Cementitious composites with CNFs can detect stress, strain, and damage from the electrical resistivity variation with static and dynamic load, loading duration, and environmental variation (Galao et al., 2014; Gao et al., 2010; Wang, Gao, et al., 2018). Wang, Gao, et al. (2018) reported that salt freeze-thaw cycles lead to the obvious degradation of sensing ability, demonstrating that the surrounding environment should by fully considered for application of cementitious composites with nanomaterials for structural monitoring. Cementitious composites with CNFs can also monitor fatigue cumulative damage and present a two-stage trend of electrical resistivity changes, including a small increase in early stage and a sharp increase when the cumulative damage reaches to 90% (Yang & Sun, 2019).

1.6

Tailoring cementitious composites with 2D nanomaterials

1.6.1 Graphene Graphene, a representative type of 2D nanocarbon materials, is formed by stacking monolayer graphene. The tensile strength, elastic modulus, and hardness of graphene reach 125 GPa, 1.1 TPa, and 110
121 GPa, respectively. At room temperature, the electronic mobility and resistivity of graphene are 15,000 cm2/(Vs) and 1026 Ωcm, providing the smallest electronic resistivity ever known for a material. Besides, the thermal conductivity of graphene can be as high as 5300 W/mK that is much higher than CNTs. These characteristics are beneficial for strength

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Recent Advances in Nano-Tailored Multi-Functional Cementitious Composites

improvement and abrasion performance of cementitious composites. Also, graphene exhibits excellent electrical and thermal conductivity, which can help the cementitious composites to realize the functional properties.

1.6.1.1 Hydration There still exists controversy toward the effect of graphene on hydration of cementitious composites. The first view believes graphene can accelerate the hydration process and improving hydration degree. Table 1.16 (Meng & Khayat, 2018) provides evidence for this observation, confirming that graphene diameter and content affect the hydration process (Meng & Khayat, 2018). On the other hand, Jing et al. (2017) concluded that graphene causes no effect on the hydration process and degree.

1.6.1.2 Rheology Rheology parameters of cement paste in Table 1.17 (Han, Zheng, et al., 2017) indicate that incorporation of graphene to cement paste increases the yield stress and plastic viscosity.

1.6.1.3 Workability Graphene demonstrates much less flowability deterioration for fresh cement paste among nano-carbon materials (GO, CNT, and graphene) (Alatawna et al., 2020). Nevertheless, flowability still shows a decreasing trend with graphene increasing. Chen, Yang, et al. (2019) indicated that the slump of concrete with graphene is almost linearly reduced with the increasing graphene content. It is also observed that the mixing process of fresh concrete becomes difficult when graphene at the content of 0.4% is added and the slump is only 57 mm, compared with 71 mm of the control specimen without graphene. Table 1.16 The effect of graphene on hydration of ultra-high performance concrete. Graphene diameter (nm)

Graphene content

End of induction period (mW/g)

Peak of hydration (mW/g)

Cumulative heat at 72 h (J/g)

Reference

25/30

0

0.258/0.258

2.001/2.001

126/126

Meng and Khayat. (2018)

0.05 0.1 0.15 0.2 0.3

0.238/0.256 0.222/0.260 0.211/0.266 0.256/0.265 0.301/0.265

2.056/2.109 2.111/2.235 2.218/2.313 2.337/2.476 2.496/2.561

128/130 130/165 132/141 142/160 164/182

Overview of tailoring cementitious composites with various nanomaterials

43

Table 1.17 The rheology parameters of cement paste without and with grapheme. Yield stress (Pa)

Plastic viscosity (Pa 3 s)

References

Control group

6.64

3.1

Han, Li, et al. (2017), Han, Wang, et al. (2017), Han, Zhang, et al. (2017), Han, Zheng, et al. (2017)

Graphene of 0.1%

8.18

3.27

Matrix type

Ordinary cementitious composites

Table 1.18 The mechanical performances of ultra-high performance concrete (UHPC) and ordinary cementitious composites with grapheme. Matrix type

Ordinary cementitious composites

UHPC

Graphene content

Increase (relative values) Compressive strength

Flexural strength

0.20%

3.87%

—

0.40% 0.60% 0.80% 1.00% 0.10% 0.05% 0.05% 0.10%

28.92% 63.93% 75.19% 65.76% 19.90% 28.30% — —

— — — — — — 82% 21%

0.20% 0.25% 0.50% 0.75%

154% 3.50% 21.90% 11.00%

— 3.70% 13.60% 20.70%

References

Rhee et al. (2016)

Fan (2014) Zohhadi et al. (2015) Huang (2012) Han, Li, et al. (2017), Han, Wang, et al. (2017), Han, Zhang, et al. (2017), Han, Zheng, et al. (2017) Li et al. (2020)

1.6.1.4 Mechanical properties Static mechanical properties Table 1.18 (Fan, 2014; Han, Zheng, et al., 2017; Huang, 2012; Li, Dong, et al., 2020; Rhee et al., 2016; Zohhadi et al., 2015) shows that the static strength

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Recent Advances in Nano-Tailored Multi-Functional Cementitious Composites

variation of cementitious composites with graphene as reported for few investigations in the literature. Incorporation of graphene can effectively increase compressive and flexural strengths of cementitious composites. Meng and Khayat (2018) investigated the effect of size of graphene on tensile strength of UHPC, showing that 25 nm-graphene can increase the tensile strength and energy absorption capacity by 40% and 190%, respectively. Furthermore, UHPC with 30 nm-graphene presents higher tensile strength about 45% and energy absorption about 150% than the ordinary UHPC.

Dynamic mechanical properties Researchers (Wang, Dong, Ashour, et al., 2020; Wang, Dong, Yu, et al., 2020; Wang, Zheng, et al., 2020) have investigated the dynamic mechanical behaviors of UHPC with graphene under different strain rates (200
800/s). Incorporation of graphene improves the dynamic compressive strength, peak strain, and ultimate strain and impact toughness of UHPC by 63.9%, 66.0%, 32.7%, and 117%, respectively.

1.6.1.5 Durability Chen, Yang, et al. (2019) reported that compared to the control group, graphene can reduce the strength loss after freeze-thaw circles, and the reduction is more obvious with the circle increasing. Fan (2014) investigated the durability of cementitious composites with graphene under 300 freeze-thaw cycles and five-month chloride corrosion, respectively, showing that the resistance of cementitious composites with graphene increased to weight loss, as well as spalling.

1.6.1.6 Functional properties Self-sensing properties Cementitious composites with graphene can act as sensors to detect strain, stress, damage, and environmental changes through electrical resistivity. In terms of sensing deformation, Sun et al. (2017) reported the electrical response is not time-dependent under the same loading rate. While the sensing ability is slightly dependent on dynamic loading rate because a trend toward reduced response is observed when the loading rate increases. Under a considerable compression strain range, cement paste still presents a linear pressure-sensitive behavior when the graphene exceeds 2.4% (Ahmadreza et al., 2014). Additionally, cementitious composites with graphene also can sense temperature changes (Ghosh et al., 2019; Sun et al., 2017).

Electromagnetic properties Incorporating graphene leads to a high electrical conductivity, dielectric loss angle tangent, and resonance absorption of cementitious composites, thereby improving the electromagnetic properties. Sun et al. (2017) reported that electromagnetic wave shielding effectiveness of cement paste with 10% of graphene is 1.6 times of specimen without functional fillers. Additionally, the highest electromagnetic wave

Overview of tailoring cementitious composites with various nanomaterials

45

absorption of cement paste with the presence of 10% graphene is nearly seven times of that of plain group.

Thermal properties Table 1.19 (D’alessandro et al., 2016; Ghosh et al., 2019; Han, Zheng, et al., 2017) shows that the thermal properties of cementitious composites with graphene. As shown in Table 1.19, the thermal conductivity of cement paste presents an increasing trend with the increasing content of graphene. The enhancement of thermal properties of cementitious composites is attributed to the unique plate morphology of graphene, providing larger area to bond with hydration products than other nanocarbon materials (CNTs, CNFs, and NCB) (D’alessandro et al., 2016). The improvement in thermal conductivity can reduce the thermal shrinkages and initial cracks inside matrix (Fan, 2014).

Thermoelectric properties Tailoring cementitious composites with graphene has the feasibility to achieve the energy saving purpose owing to the presence of thermoelectric properties. Ghosh et al. (2019) incorporated graphene into cement paste to fabricate a thermoelectric construction building material, converting the available ambient heat into electrical energy, thereby leading to a reduction of energy consumption. The experimental results demonstrated that cement paste with graphene exhibited p-type semiconductor behavior, where the charge carriers are contributed by the graphene. Although the thermoelectric efficiency up to 0.015% is small, there is still enormous available free heat energy absorbed by the building surface for conversion. Table 1.19 Thermal properties of ordinary cementitious composites with graphene. Matrix type

Graphene content (%)

Thermal conductivity (W/(m 3 K))

Specific heat (J/ (g 3 K))

References

Ordinary cementitious composites

0

0.679

—

Ghosh et al. (2019)

5 10 15 0

0.43 0.947 1.067 1.1

— — — 1.33/

1 2

1.22 0.3596

1.25 —

Han, Li, et al. (2017), Han, Wang, et al. (2017), Han, Zhang, et al. (2017), Han, Zheng, et al. (2017) D’alessandro et al. (2016)

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Recent Advances in Nano-Tailored Multi-Functional Cementitious Composites

Damping properties Researchers (Ruan, Han, Yu, Li, et al., 2018) have incorporated graphene into cement paste to enhance its damping properties. The results have shown that compared with the specimen without nano-modification, the damping ratio of cementitious composites with introduction of 1% and 5% of graphene increases by 16.22% and 45.73%, respectively.

1.6.2 Nano-BN Nano-BN, similar to graphene, is a type of 2D nanomaterial with plate structure. NanoBN possesses excellent performance in mechanical strength, thermal conductivity, heat resistance, lubricity, electrical insulation, and chemical stability. The tailoring cementitious composites with nano-BN is to develop cementitious composites with outstanding mechanical properties, durability, and functional properties.

1.6.2.1 Hydration Researchers (Zhang, Han, et al., 2018; Zhang, Ding, et al., 2018) have reported that the early hydration of cementitious composites can be obviously accelerated by incorporation of nano-BN at the content of 5% and 10% but deferred by 15% of nano-BN. While at late stage, nano-BN can only slightly increase the hydration degree.

1.6.2.2 Mechanical properties Experimental results listed in Table 1.20 (Rafiee et al., 2013; Zhang, Ding, et al., 2018; Zhang, Han, et al., 2018) indicate incorporating nano-BN with optimal content can increase mechanical performances of cementitious composites. It can be observed that UHPC containing a smaller size of nano-BN has higher strength values than the larger size (Zhang, Ding, et al., 2018; Zhang, Han, et al., 2018). Moreover, heat curing is more efficient for improving strength of UHPC containing 10% and 15% of nano-BN. While water curing is more preferable for UHPC with nano-BN at lower content. Additionally, Wang, Dong, et al. (2019) conducted an experiment exploring the effect of nano-BN on ITZ of cementitious composites by testing bond strength of the scaleup aggregate-cement mortar interface under the three-point flexural loading. The interfacial strength is improved by 42.8% with incorporation of nano-BN at the content of 0.3%. The modification of nano-BN to ITZ can also account for the strength improvement of cementitious composites with nano-BN.

1.6.2.3 Durability Researchers (Zhang, Han, et al., 2018; Zhang, Ding, et al., 2018) have studied the durability of UHPC with nano-BN, depending on different curing methods and nano-BN content. The experimental results are shown in Table 1.21 (Zhang, Han, et al., 2018; Zhang, Ding, et al., 2018)

Overview of tailoring cementitious composites with various nanomaterials

47

Table 1.20 The mechanical performances of ultra-high performance concrete (UHPC) and ordinary cementitious composites with nano-BN. Matrix type

Ordinary cementitious composites UHPC

Nano-BN content

Decrease/increase (relative values) Compressive strength

Flexural strength

1%

89%

—

1% (120 nm) 3% (120 nm) 5% (120 nm) 10% (120 nm) 15% (120 nm) 1% (500 nm) 3% (500 nm) 5% (500 nm)

12.70%

5.10%

12.10%

23.80%

25.10%

212.20%

2.10%

8.40%

26.50%

4.00%

217.2%

22.10%

212.30%

26.10%

218.80%

23.60%

References

Alireza et al. (2010) Han et al. (2009)

Table 1.21 Durability of UHPC with nano-BN. Matrix type

Nano-BN content

Abrasion loss (kg/m2)

Chloride transportation coefficient (10214 m2/s)

References

UHPC

0% (water/ heat curing) 5% (water/ heat curing) 10% (water/ heat curing) 15% (water/ heat curing)

0.63/0.49

1.5/1.9

Han et al. (2009)

0.28/0.32

1.2/0.8

0.4/0.42

0.8/0.02

0.43/0.52

0.02/0.8

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Recent Advances in Nano-Tailored Multi-Functional Cementitious Composites

1.6.2.4 Functional properties Oil
water separation properties Rafiee et al. (2013) reported that through experimental analysis, concrete with nano-BN at 5% content can effectively filter the crude oil from water, making the filtered water clear. They also conducted the molecular dynamics simulation to calculate the separation capacity of porous concrete composites with nano-BN, based on the oil absorption results of nano-BN, showing a consistent comparison between simulation and experimental results.

1.7

Applications of cementitious composites with nanomaterials

Based on the excellent mechanical performances, durability and smart/multifunctional properties, cementitious composites with nanomaterials have gained great achievements on field of structure health monitoring (beam, column deformation and damage, structural modal identification, and model-based damage detection), traffic flow detection (traffic flow, vehicle type), pollutant purification, and energy saving. Fig. 1.13 shows the typical application of cementitious composites with nanomaterials, and their specific introductions are stated as follows.

1.7.1 Structural health monitoring The applications of cementitious composites with nanomaterials have developed from structural component monitoring to the whole structural monitoring. Many studies have been conducted on monitoring structural beams and columns utilizing

Figure 1.13 The typical application of cementitious composites with nanomaterials. (A) Column strain monitoring (B) Traffic detection (C) structural monitoring (D) pollutant purifying.

Overview of tailoring cementitious composites with various nanomaterials

49

cementitious composites with nanomaterials, aiming to achieve self-sensing. The main studies are summarized in Table 1.22 (Baeza et al., 2013; D’Alessandro et al., 2017; Downey et al., 2017; Galao et al., 2017; Han, Ding, & Yu, 2015; Han, Sun, et al., 2015; Han, Zhang, et al., 2015; Howser et al., 2011; Saafi, 2009; Vossoughi, 2004). After verifying the feasibility of monitoring beams or columns utilizing cementitious composites with nanomaterials, the materials have been used to whole structure monitoring. Ding et al. (2020) developed a L-shaped sensor utilizing cementitious composites with CNT/NCB for structural modal identification and damage detection of a five-story building model through attached method. The

Table 1.22 Application of cementitious composites with nanomaterials on structural health monitoring. Methods

Nanomaterials

Parameter to monitor

References

Bulk method

CNFs

Damage of columns under reversed cyclic loading Damage of beams with using resistor mesh mode Strain of beam under fourpoint bending Strain of beam under fourpoint bending Strain and damage of cylinder column under compression Stress and strain of column under compression

Howser et al. (2011)

MWCNTs

Coating method Bonded method

CNFs CNFs CNFs

Embedded method

Self-assembled CNT/NCB

CNFs MWCNTs

MWCNTs

SWCNTs

Strain of beam under fourpoint bending Crack propagation and damage accumulation of beam under three-point bending Static and dynamic strain of full-scale reinforced concrete beam. Crack propagation and damage accumulation under monotonic and cyclic bending

Downey et al. (2017)

Baeza et al. (2013) Baeza et al. (2013) Galao et al. (2017)

Han, Ding, and Yu (2015), Han, Sun, et al. (2015), Han, Zhang, et al. (2015) Baeza et al. (2013) Vossoughi, 2004

D’Alessandro et al. (2017) Saafi (2009)

CNF, Carbon nanofiber; CNT, carbon nanotube; NCB, nano-carbon black; MWCNT, multiwalled CNTs; SWCNTs, single-walled CNTs.

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Recent Advances in Nano-Tailored Multi-Functional Cementitious Composites

sensor recorded satisfactory mechanical properties and pressure-sensitive reproducibility under dynamic loading, and its results of the identified modal frequency and their changes of damage are almost the same as the results obtained from strain gauges and accelerometer.

1.7.2 Traffic detection The cementitious composites with nanomaterials can help for traffic management on the basis of their sensing ability. Han et al. (2013) conducted a road test of pavement system for traffic detection utilizing sensor of cementitious composites with CNTs. The strip sensor is embedded into pavement to judge vehicle type with assistance of five-axle semi-trailer truck and a vane, and to monitor the traffic flow. The results showed that this method has a high detection precision to monitor traffic flow rates and traffic density, vehicular speed, and vehicle classification. And the method is of great simplicity of installation and convenience of maintenance. Monteiro et al. (2020) reported that cementitious composites integrating NCB can also provide accurate detection for traffic flow, vehicular speed, and weight in motion.

1.7.3 Pollutants purifying The utilization of cementitious composites with nano-TiO2 has been certified to be efficient for pollutant purifying. He et al. (2019) utilized the tailored composites to purify the vehicle exhaust. They reported iron coating nano-TiO2 particles is more efficient than the ordinary TiO2. Chen, Rad, et al. (2019) replaced cement by hybrid nano-TiO2 of rutile and anatase crystal (30%, 10%, and 3%) to design a novel fixed-bed reactor pond to purify the rainwater. The results showed, cementitious composites with nano-TiO2 is effective to the stormwater pollutant diminution, with efficiency enhancement of 16%, 25%, and 18% for NO32, H3PO4, and COD, respectively. Furthermore, the overall removal rates of tailored group after three weeks are averagely 33%, 84%, and 67%, while those of reference group are 6%, 26%, and 20%, respectively.

1.7.4 Other applications Ghosh et al. (2019) incorporated graphene into cement paste to fabricate a thermoelectric construction building material, which can convert the available ambient heat absorbed from surroundings into electrical energy, thereby leading to a reduction of energy consumption. Although the thermoelectric efficiency up to 0.015% is small, there is still enormous available free heat energy absorbed by the building surface for conversion. Shukla et al. (2012) tailored cementitious composites with MWCNTs to detect smoke by observing the electrical conductivity change with smoke concentration. The responsivity in the range of 26%
46% is obtained under smoke environment with the presence of CNTs.

Overview of tailoring cementitious composites with various nanomaterials

1.8

51

Prospects of cementitious composites with nanomaterials

Incorporating nanomaterials brings great changes in tailoring nano/micro/ macro scale structures of bulk cement paste phase and ITZ in the cementitious composites through the nano-core effect, thus making cementitious composites stronger, more durable, and smart/multi-functional (e.g., self-sensing, selfhealing, and pollutant purifying). Owing to the nano-modification to cement paste in combination with the supplement of nanoscale continuity for multiscale raw materials of cementitious composites, nanomaterials gradually show the potential to become the indispensable seventh component of cementitious composites besides cement, water, fine aggregates, coarse aggregates, chemical additives and mineral additives. Tailoring cementitious composites with nanomaterials provides a promising approach to develop the new generation of cementitious composites (e.g., ultra-high performance, smart/multi-functional and resilient) and sustainable infrastructures. For example, tailoring cementitious composites with nanomaterials can significantly improve dynamic mechanical properties. Up to now, cementitious composites with nanomaterials have gained great achievements on application of structural health monitoring (structural deformation and damage, structural modal identification and model-based damage detection), traffic detection (traffic flow, vehicle type), and pollutant purification. Tailoring cementitious composites with nanomaterials has the potential to improve the bond between new and old concrete, due to the nano-core effect in bulk cement paste phase and ITZ. Therefore cementitious composites with nanomaterials can become the restoring materials to repair or reinforce buildings after earthquake or impact damages. Additionally, the excellent strength performances, large deformation and toughness under dynamic impact demonstrate the prospective future for cementitious composites with nanomaterials applied in impact-resisting infrastructures, such as bridge pier and underwater foundation. Although tailoring cementitious composites with nanomaterials has gained great achievements, it still confronts plenty of challenges in the process to achieve the large-scale applications. Further work may focus on: (1) comprehensive understanding for the modified mechanisms and effects of nanomaterials to cementitious composites; (2) methods with simplicity, economy and efficiency for large-scale dispersion of nanomaterials, and fabrication of cementitious composites with nanomaterials; (3) development for theoretical models to predict behaviors of composites; (4) exploration of the optimal amount of nanomaterials added to cementitious composites to balance properties (mechanical properties, durability and functional properties), environment, and economy; (5) large scale-model research to achieve further engineering applications, such as smart/multi-functional constructions and resilient infrastructures. Tailoring cementitious composites with nanomaterials can enhance safety of structures, prolong the life span, reduce cement and energy consumption, and improve the living comfort. It is expected and believed that tailoring cementitious

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Recent Advances in Nano-Tailored Multi-Functional Cementitious Composites

composites with nanomaterials will bring a deep revolution to cementitious composites (e.g., ultra-high performance, smart/multi-functional, and resilient) and sustainable infrastructures.

Acknowledgments The authors would like to thank the National Science Foundation of China (51978127 and 51908103) and the China Postdoctoral Science Foundation (2019M651116) for providing funding to carry out this investigation.

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Cwirzen, A., Habermehl-Cwirzen, K., & Penttala, V. (2008). Surface decoration of carbon nanotubes and mechanical properties of cement/carbon nanotube composites. Advances in Cement Research, 20(2), 65
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Gao, D., Mo, Y. L., & Peng, L. M. (2010). Mechanical and electrical properties of carbonnanofiber self-consolidating concrete. In Proceedings of the 12th international conference on engineering, science, construction, and operations in challenging environments - Earth and space 2010 (pp. 2577
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Han, B., Sun, S., Ding, S., Zhang, L., Yu, X., & Ou, J. (2015). Review of nanocarbonengineered multifunctional cementitious composites. Composites Part A: Applied Science and Manufacturing, 70, 69
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Jin, M., Jiang, L., Lu, M., & Bai, S. (2017). Monitoring chloride ion penetration in concrete structure based on the conductivity of graphene/cement composite. Construction and Building Materials, 136, 394
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Li, W. W., Jia, Y. R., Shen, R. X., Xing, F., & Zheng, J. (2018). Compressive strength of cement mortar containing carbon nanotubes (CNTs) under sulfate attack and dry-wet cycling environment. Advances in Engineering Research, 146, 219
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Meng, W., & Khayat, K. H. (2018). Effect of graphite nanoplatelets and carbon nanofibers on rheology, hydration, shrinkage, mechanical properties, and microstructure of UHPC. Cement and Concrete Research, 105, 64
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Nazari, A., & Riahi, S. (2013). The effects of ZrO2 nanoparticles on strength assessments and water permeability of concrete in different curing media. Journal of Experimental Nanoscience, 8(4), 413
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Nano-tailored high-performance fiber-reinforced cementitious composites

2

Ismail Ozgur Yaman and Burhan Alam Department of Civil Engineering, Middle East Technical University, Ankara, Turkey

2.1

Introduction

The concept of adding fibers to construction materials is as old as adobe construction, in which straws were added to sun-dried mud bricks to increase their cracking resistance. However, it was in 1874 (50 years after Portland cement was patented), when the first patent on fiber reinforced concrete was obtained by A. Berard from the United States. After a few decades, in 1918 H. Alfsen from France patented the use of fibers of iron, wood, or other materials to improve the tensile strength of concrete (Naaman, 1985). The first use of fibers in a cementitious system, however, started with the invention of the so-called “Hatschek” process in 1989, where asbestos fibers were used to produce fiber cement sheets or boards. However, asbestos fibers were later abandoned because of their health risk potential, and alternative fiber types such as steel fibers (straight, crimped, twisted, etc.), synthetic fibers (polypropylene, nylon, polyester, etc.), glass fibers, and carbon fibers were all introduced to the market in the 1960s and 1970s (Daniel et al., 2009), and they are still being used together with new additions to that list. The formal use of the term “high-performance concrete” (HPC) started in the United States after the Strategic Highway Research Program (SHRP), which started in 1987. The aim of the SHRP program was to improve and extend the service life of bridges; therefore the focus was on the durability of concrete, and HPC was defined as any concrete that provides enhanced performance characteristics for a given application. Even though Volume 6 of the SHRP program incorporated fiber reinforced concrete, the focus was on very high early strength and durability rather than the postcracking performance of the concrete. At the same time frame, in the mid-1980s, the term “high-performance fiberreinforced cementitious composites” (HPFRCC) was started to be used by A.E. Naaman to distinguish the strain-hardening behavior of fiber-reinforced composites (FRC), which typically exhibit strain-softening behavior in tension. Naaman defined HPFRCC as “a class of FRC characterized by a strain-hardening behavior in tension after first cracking, accompanied by multiple cracking up to relatively high strain levels” (Naaman, 2007). In fact, the first workshop on HPFRCC took place in Mainz, Germany in 1991, which was organized under the auspices of RILEM and Recent Advances in Nano-Tailored Multi-Functional Cementitious Composites. DOI: https://doi.org/10.1016/B978-0-323-85229-6.00010-X © 2022 Elsevier Ltd. All rights reserved.

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ACI (Naaman & Reinhardt, 2003). This conference series was conducted on a regular basis, and the last one (HPFRCC-7) was held in Stuttgart, Germany in 2015. During the last two decades, the term “ultra high-performance fiber cementitious composites” (UHPFRCC) was also introduced, which refers to mixtures that have superior compressive strengths ranging from 150 to 800 MPa and fracture energy levels between 1200 and 40,000 J/m2 (Khan et al., 2017). In civil engineering applications, the properties of interest are usually mechanical properties such as compressive and tensile strengths, toughness, and energy absorption, in addition to durability properties such as freeze-thaw, sulfate attack, alkali
aggregate reactions, and corrosion resistance. At the beginning, the fiber usage in a cementitious system was to improve the cracking resistance or its general performance. However, eventually, it was realized that the use of fibers not only increased the cracking resistance but also improved the toughness and durability of the concrete. In addition, during the last decade, HPFRCC entered a new era by incorporating nanomaterials in the system to attain further improvements and provide some additional properties or functions that are not generally required in civil engineering applications. Such properties include electrical, thermal, self-sensing, and self-cleaning capabilities, which will all be explained in this chapter.

2.2

High-performance fiber-reinforced cementitious composites

When the term “fiber” appears along with the term “cement,” the concept of FRC immediately comes into mind. However, when the term “fiber” is used together with the term “high performance,” the concept of HPFRCC comes into mind. In the latter case, fiber is not only used to cover or improve a weakness of concrete but also used to obtain a different engineered products that brittle concrete could and should not be a part of. In such products, the amount of coarse aggregate is usually low, if at all utilized, and that is the reason why such products are more commonly referred to as “cementitious composites” rather than “concrete.” Different forms of HPFRCC, including slurry-infiltrated fiber concrete (SIFCON), reactive powder concrete, and glassreinforced concrete (GRC), carry the name concrete but do not include any coarse aggregate. The absence of coarse aggregates offers a better control on the fiber distribution and orientation, which in turn will highly improve the performance of those composites (Kuder & Shah, 2010). In addition, it is obvious that the main binding material in HPFRCC is cement, and the composite mixture demonstrates a higher performance in terms of mechanical and durability properties. There are different ways of classifying HPFRCC. In a general performancebased approach, one can consider that HPFRCC lies at the intersection of three groups of concrete, namely, FRC, where the performance parameter is the toughness or the ductility; HPC, where the performance parameter is durability; and selfcompacting concrete (SCC), where the performance parameter is the workability of the concrete. This general classification is depicted in Fig. 2.1.

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Figure 2.1 High-performance fiber-reinforced cementitious composites classification by performance criteria.

However, a more generally accepted classification is based on the tensile behavior of concrete. Being a quasibrittle material, concrete or cementitious materials in general show a strain-softening type of behavior once the fracture strength is exceeded upon being subjected to a tensile loading. With the addition of fibers to the plain cementitious composite, this behavior can be altered by a change in the postcracking response of concrete. The tensile stress
strain response for a conventional fiber reinforced cementitious composite (FRCC) will be a strain-softening type, whereas for a HPFRCC it will be a pseudo-strain-hardening, or rather called a strain-hardening type, which is clearly evidenced through multiple crack formation before failure (Kuder & Shah, 2010). The classification and a schematic response of FRC and HPFRCC together with a plain cementitious composite are depicted in in Fig. 2.2. Another classification was made by Reinhardt and Naaman after the discussions that took place at the end of the Fourth International Workshop on High Performance Fiber Reinforced Cement Composites (Naaman & Reinhardt, 2003). This classification is based on the type of testing utilized for FRC. Most of the time, in conventional FRCC, a low volume of fibers is used in slabs on grade to control the cracking of concrete, where tensile testing is not necessary to characterize. On the other hand, a moderate volume of fibers is used in beams in combination with conventional reinforcement, such as in seismic resistant structures, where bending tests are often performed instead of uniaxial tensile tests; in such a case, the material is termed as “deflection-hardening cementitious composites” (DHFRCC). In HPFRCC, a relatively higher volume of fibers is used, such as in stand-alone thin sheet applications, where uniaxial tensile testing is necessary. Finally, in UHPFRCC, a very high volume of fibers is utilized, which becomes

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Figure 2.2 Simple classification of cementitious composites based on their tensile response.

helpful when used in blast- and impact-resistant structures, which require high energy absorption capacity. It should be noted that—depending on the amount of fibers—a tension strain-softening composite can lead to structural elements with either deflection-hardening or deflection-softening behavior as described in Fig. 2.3.

2.2.1 Production and design parameters of high-performance fiber-reinforced cementitious composites HPFRCC can be produced using a wide array of processes, each requiring different construction and design parameters. Common production methods of HPFRCC have been summarized by Bentur and Mindess (Bentur & Mindess, 2007) as follows: 1. 2. 3. 4. 5. 6.

Premix process Sprayup process Shotcreting Pulp-type process Hand layup process Continuous production process

This list should most certainly be extended to include the recently developed additive manufacturing process such as 3D printing technology. It should also be noted that there are two different approaches for utilizing this technology. The most prevalent approach is to utilize 3D printing in the form of an extrusion process, and several publications utilizing HPFRCC (Arunothayan et al., 2020; Li et al., 2020) for this purpose exist in the literature. Another approach uses particle-bed 3D printing technology (Lowke et al., 2018). Besides these two approaches, another idea is about the 3D printing of a continuous network of fibers and filling that network with cement paste, similar to SIFCON (Lowke et al., 2018).

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Figure 2.3 Classification based on the type of testing.

The proportions of the HPFRCC ingredients depend on the process employed during manufacturing. Regardless of the manufacturing process, the following factors are known to most affect the properties of HPFRCC: 1. The amount and the composition of the cementitious system (cement, silica fume, slag, fly ash, etc.) 2. The water/binder ratio (the use of superplasticizers)

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3. The amount and characteristics of fibers (geometry and L/d ratio) 4. The compositions of sand, fine aggregate, and coarse aggregate

Considering the above-mentioned parameters involved, it can be clearly seen that the mix-design process must be engineered in order to obtain the desired performance using a suitable production process. Covering the details of all the production methods is beyond the purview of this work; however, the parameters involved in the most widely used one, that is, premixing, will be assessed below. While it may not be a substantial difference for ordinary concrete, mixing process is slightly different when it comes to HPFRCC. During the premix process, there are two key parameters in the production of HPFRCC: casting with the possible by the driest matrix and distributing the fibers in an even and randomly oriented way. By a dryer matrix, it is meant to use an amount of water low enough to provide the desired workability and high enough for complete hydration. Furthermore, the highest possible density refers to the best packing degree, where the voids are minimized and all the aggregates and fibers are surrounded by paste. To accomplish the same, after the mixture design is tailored, the first key parameter can be estimated by an optimization of the mixing time and effort along with the desired workability. The second key parameter can be estimated by adequately integrating time, speed, and torque information. In addition, the mixer type also plays a critical role in the production of HPFRCC. A planetary mixer can be used first to prepare the mortar, which can be added along with the fibers to a larger planetary mortar mixer or a concrete mixer. The first mixer will provide enough speed and shear force to obtain the flowable mortar, while the relatively slow rotation of the second mixer will provide enough time and space to mix and distribute the fibers evenly. In other words, it is commonly used to add the fibers at last, unless for SIFCON, when fibers are placed first and directly into the molds. When synthetic fibers are used, planetary mixers can be utilized in the whole mixing process, while a concrete mixer is preferred for steel fibers. In the literature, there is a vast amount of experimental research looking into different mixing schemes for various HPFRCCs (Felekoglu et al., 2014; Gao et al., 2017). Typically, binding materials are first mixed along with the filler aggregates. After that, the water premixed with the chemical admixtures are added and mixed till a complete dispersion of the solids is obtained and the required flowability is reached. Finally, the fibers are added, and the mixing continues until a homogenous composite is obtained. However, a different mixing sequence can always be applied and it might lead to a noticeable improvement in the performance of HPFRCC (Zhou et al., 2010). One of the main problems in the mixing procedure lies in the density difference between the mortar and the fibers. This difference might cause segregation problems (fiber float or sink) in the fresh mass. The viscosity of the mortar can play a positive role during the mixing if it allows a good adhesion on the fibers, while the geometry of the fibers can play a negative role if it leads to fiber balling. Moreover, most of those rules are predicated on laboratory experience; thus the outcome can be different for large-scale applications. In this case, the batching sequence can be modified to keep the mixture as fluid as possible through

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the whole mixing process, considering that the desired viscosity will be delivered at the end of the mixing job (Lepech & Li, 2008). The requirements from the matrix, fiber type, and volume of fibers that can be utilized in a particular HPFRCC may depend not only on the type of manufacturing process of HPFRCC but also on the approach utilized to obtain the tensile strainhardening response and the multicracking phenomenon. For the latter, various models have been developed. However, as summarized by Naaman and Reinhardt (1996), there are three models that utilize the following theories: 1. Mechanics of composite materials 2. Micromechanics of crack bridging and fracture 3. Fracture energy of debonding

Even though all the above-mentioned models use different characteristics, they simply consider (1) the properties of the matrix, (2) the properties of the fiber, and (3) the interaction between the two. This interaction is usually based on the assumptions that a good packing is obtained and a sufficient bonding between the two is achieved. Therefore the strain-hardening behavior of HPFRCC is known to be affected by the volume of fibers, fiber geometry, and bond strength between the fiber and the matrix. As for the fibers, an increase in the fiber volume typically increases the strainhardening response. On the other hand, this not only increases the cost of HPFRCC but also causes difficulties in mixing if the typical premix method is used in the manufacturing process. Besides the properties of the fiber material, the surface characteristics of the fiber are also known to impact the strain-hardening response of HPFRCC. For example, when a twisted steel fiber was developed and used to produce HPFRCC, an increase by 5.9 and 4.0 times occurred in pullout stress energy compared to smooth and hooked fibers, respectively. The same fiber displayed an increase in the number of multiple cracks by 4.0 times for 2% fiber volume, and in the bending strength by 2.6 times for 1% fiber volume and 1.3 times for 2% fiber volume, when compared to hooked fibers (Kim et al., 2009). In addition to steel fibers, synthetic fibers also have an important role in HPFRCC. A good example is the use of polyvinyl alcohol (PVA) fibers with a special coating on the surface, used in producing engineered cementitious composites (ECC), which continue to improve and find new applications (Qiu et al., 2018). Along with PVA, the availability of polypropylene (PP) fibers in different shapes and forms and the superior mechanical performance that polyethylene (PE) fibers can provide, make those two types of synthetic fibers widely used (Pakravan & Ozbakkaloglu, 2019). The use of PP fibers was reported to grant a compressive strength and flexural strength ratios of 69% and 74%, respectively, of the same UHFRCC mixture that uses steel fiber (Khan et al., 2017). Moreover, the use of basalt fibers in the production of HPFRCC is also a developing trend (Adesina, 2021). On the other hand, a hybrid use of steel and synthetic fibers was also reported to be highly effective in the production of HPFRCC (Khan et al., 2017). An important component of HPFRCC is the mineral admixture that can be used to improve some fresh and hardened properties of the mixtures. ECC mixtures typically include either fly ash or slag in order to control the cracking of the matrix.

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For instance, fly ash can enhance the workability, and granulated blast furnace slag can improve the fiber dispersion behavior (Keskinate¸s & Feleko˘glu, 2018). Besides ECC, other HPFRCC also utilized mineral admixtures. For example, zirconium silica fume was reported to reduce porosity in HPFRCC (Yoo et al., 2018), while a 50% replacement of Portland cement by fly ash and slag led to the production of greener composites (Aghdasi & Ostertag, 2018). Even though coarse aggregates are not typically utilized in HPFRCC, there are certain researchers investigating the effects of aggregate type on HPFRCC properties. For example, Wu et al. considered the used of granite, basalt, limestone, and steel slag aggregates with a maximum aggregate size of 16 mm (Wu et al., 2020); they demonstrated that the aggregate type and content have a remarkable effect on the compressive strength and elastic modulus of UHPFRC, whereas the effects on the flexural behavior were less than the ones on the compressive properties. Besides these, it was shown that heavy-weight HPFRCC can be produced with the unit weights up to 3005 and 3773 kg/m3 when barite sand and granulated ferrous wastes are used, respectively (Tufekci & Gokce, 2017). As for the chemical admixtures, the use of superplasticizers is a “panacea” solution for HPFRCC. However, some admixtures are also used to enhance durability through self-healing agents (Cuenca et al., 2021).

2.2.2 Evaluation of the mechanical properties of highperformance fiber-reinforced cementitious composites Although the nature of HPFRCC implies that all the properties—or at least the most of them—are improved, the focus can be made on a specific property to satisfy a specific requirement. This might be the reason behind an extensive classification for HPFRCC. In that scope, strength might be the most widely used property; yet in this case, it is not just the compressive strength, but the direct or indirect tensile strengths that are also important. Moreover, not only the ultimate strength is measured, but the whole stress
strain or load
deflection behavior is investigated. This is because of the huge improvement in the strain capacity of this type of mixtures, as depicted in Fig. 2.4. The properties of HPFRCC are usually determined on the basis of the efficiency of one or more design parameters. This investigation can start from the fiber
matrix interface, where the development of the microstructure in the transition zone is considered (Bentur & Mindess, 2007). Mainly, the stress transfer through the fibers, and the bond and pullout strengths are examined. Those factors can help characterize the stress
strain relation at both early and subsequent ages, where the focus can be made on enhancing the strain capacity or the strength limits. The yield strength, ultimate strength, and toughness are among the most studied parameters of HPFRCC (Rokugo et al., 2007). Unlike conventional concrete, the compressive strength is not typically used to characterize HPFRCC with the exception of UHPFRCC. The same tests that are used for the compressive strength of plain concrete are also applicable to HPFRCC. On the other hand, the Japan Concrete Institute (JCI) has developed a test method for determining the compressive toughness of such materials (Japan Society of

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Figure 2.4 Strain-hardening behavior of HPFRCC. HPFRCC, High-performance fiber reinforced cementitious composites.

Civil Engineers, 1984a). Typical compressive strength values of HPFRCC lies in the range of 50
100 MPa (Abbaszadeh et al., 2017; Dalvand & Ahmadi, 2021; Nguyen et al., 2018), whereas for UHPFRCC a range of 100
800 MPa has been reported (Richard & Cheyrezy, 1995; Wu et al., 2020; Yoo et al., 2013). It is worth noting that the compressive strength testing of such high values cannot be determined on conventional loading frames and conventional specimen sizes; therefore typically, smaller-size specimens are used in determining the compressive strength of UHPFRCC specimens. In order to obtain the strain-hardening response either under tension or bending, special servocontrolled hydraulic or electrical loading frames are needed to load the specimens in a strain- or deformation-controlled loading scheme. With the increasing availability of such loading frames, the number of researchers investigating the performance of HPFRCC is on the rise. These frames are nowadays equipped with digital image correlation systems to examine the evolution and the visualization of multiple cracking phenomenon (Talboys et al., 2012). There are numerous investigations that provide detailed information on the experimental test results obtained on the tensile or flexural response of HPFRCC. Fig. 2.5 presents the tensile stress
strain response of the most popular three HPFRCCs that have garnered massive attention in literature (Naaman, 2008). Furthermore, Fig. 2.6 presents the general range of experimental results of various HPFRCCs in literature. Even though direct tensile testing of HPFRCC is widely used since the 1960s, except for the recommendation made by RILEM Group TC 162-TDF

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Figure 2.5 Typical range of tensile stress
strain curves for HPFRCC showing the trade-off between strength and strain capacity. HPFRCC, High-performance fiber reinforced cementitious composites. Source: Adapted from Naaman (2008). High performance fiber reinforced cement composites. In High-performance construction materials, Volume 1 (pp. 91
153). World Scientific. https://doi.org/10.2174/1874149501711010650.

Figure 2.6 General description of the mechanical behavior of cementitious composites.

(Vandewalle et al., 2001), a standard test method is not yet devised to standardize the test conditions such as specimen geometry, loading rate, etc. The main difficulties associated with direct tensile tests are the boundary

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conditions on the testing machine and the effects of the inevitable eccentricity. On the other hand, both in Europe and the United States, there are few standard test methods that are used to characterize the response of HPFRCC specimens in bending (Japan Society of Civil Engineers, 1984b; ASTM International, 1997; ASTM International, 2015; BSI, 2005). Those often include notched beam specimens to characterize the fracture parameters of a specimen subjected to flexure. Other than the beam specimens, plate type of specimens are also standardized in Europe and the United States to calculate the energy absorption capacity from square and round panels, respectively (ASTM International, 2020; BSI, 2006). The shear performance of FRC is often studied on beam specimens of varying span-to-depth ratios, and the pure shear strength of FRC is seldom examined. One study from China utilized ECC specimens subjected to push-off loads (Zhao et al., 2017), and another one used various types of steel fibers to determine the shear strength of FRC plate specimens with fiber volumes up to 1.5% (Ishtewi & Toubia, 2015). Shear strength values up to 6.3 MPa were recorded. The superior performance of HPFRCC under static loading conditions is also expected to retain similar under different strain rates, such as the dynamic loading conditions (high cycle fatigue, seismic, impact, and blast loading), and the creep conditions. However, these effects are rarely studied, and there is still a variety of unknown parameters as depicted in Fig. 2.7. Among the abovementioned dynamic loading conditions, the fatigue properties of HPFRCC are seldom studied. For example, Alam and Yaman (2021) conducted high-cycle fatigue tests and looked at the stress-based fatigue performance of ECC. It was observed that ECC specimens underwent three main phases until failure. In the first one, the cracks initiated at the beginning of the test. In the second, the initiated cracks propagated and extended. Finally, in the third phase, the cracks became unstable, and failure occurred. The second phase was considered the dominant phase in which the crack propagation rate determined the fatigue life. Besides the fatigue properties, the dynamic response of HPFRCC subjected to impact and blast type of loading is also an area of interest. Alam et al. (2016)

Figure 2.7 Available test equipment and information on the behavior of HPFRCC under various strain rates and loading conditions. HPFRCC, High-performance fiber reinforced cementitious composites.

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conducted a series of experimental and numerical investigation on the ballistic performance of plates manufactured by various HPFRCC mixes. It was shown that a double layer steel FRC together with SIFCON performed the best against a projectile impact. Yoo and Banthia also investigated the mechanical behavior of UHPFRCC subjected to impact and blast, pointing out the fields for further research (Yoo & Banthia, 2017). When HPFRCCs are exposed to impact loads, a high performance is obtained under drop and blast types of impacts, while a completely different behavior can be demonstrated under projectile impacts. The reasons for that mainly lie in the difference in the speed and size of the contact surface in such test protocols. Drop weight tests are usually applied through large-size solid objects, and they have a relatively low impact speed, while projectile impact tests use a fine end solid object that travels at a high speed, which makes the impact very local. In this case, HPFRCC mixtures usually lack the heavy mass or the layers formation needed to absorb the applied energy, while the fibers are still unable to react quick enough to stop the projectile. On the other hand, blast type of impacts applies a quick, large pressure but on a large surface, which allows the large amounts of fibers to act all together at the same time. On the other side of the strain rate scale, creep behavior of HPFRCC is also seldom studied. Keskin et al. investigated the tensile creep properties of various ECC mixtures (Keskin et al., 2014). It was noted that ECC mixtures containing fly ash had a higher tensile creep compared to the mixtures with slag. Furthermore, as the amount of mineral admixture increased, tensile creep strains increased as well. Finally, they calculated the cracking potentials of ECC mixtures by tensile creep and shrinkage tests, which reflected the real cracking behavior of ECC specimens under restrained shrinkage conditions.

2.2.3 Evaluation of the other properties of high-performance fiber-reinforced cementitious composites Besides the mechanical properties, other properties of HPFRCC have also drawn attention. Among those, the permeability and durability properties, shrinkage and volume stability properties, and rheological properties can be listed. Investigations on the permeability and various durability properties of the standard ECC mixture started by Sahmaran and Li in the early 2000s (Sahmaran & Li, 2007; Sahmaran & Li, 2009; Sahmaran et al., 2008). Similarly, Gilani investigated the durability aspects of SIFCON (Gilani, 2007). Among the various durability aspects, freezing/thawing (He et al., 2020; Meng et al., 2020; Zhao et al., 2020) and corrosion (Tran et al., 2015; Yoo et al., 2020) are widely investigated not only in the original crack-free state but also in the cracked states. Of course, it is not surprising that HPFRCC are found to be durable against many environmental degradation processes owing to their lower water/binder contents, higher amounts of binding material, and tight crack-widths. However, another issue, especially in the case of glass fibers, is the alkaline susceptibility of such fibers. It is well known that the Si
O
Si bonds in the glass network are broken

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by the OH
ions, which are highly concentrated in the alkaline pore solution. For that, alkali-resistant glass fibers were developed by Majumdar with the incorporation of B16% ZrO2 in the glass composition (Majumdar, 1968). On the other hand, the utilized higher binder contents and the lack of coarse aggregates especially made most HPFRCC vulnerable to shrinkage. Therefore not only the drying shrinkage but also autogenous shrinkage properties of HPFRCC mixtures were widely investigated. For example, Keskin et al. used soaked expanded perlite aggregates to improve the dimensional stability of ECC (Keskin et al., 2013). The authors reported an improvement of 78% in the autogenous shrinkage strains to eliminate their adverse effects in early age cracking. In a more recent study, Fang et al. investigated the influence of steel fiber properties on the shrinkage of UHPFRCC (Fang et al., 2020). It was concluded that steel fiber volume and aspect ratio are the two main influencing factors to reduce the total and autogenous shrinkage, mainly due to the bond stress between the fibers and matrix resisting its self-desiccation. Another extreme loading condition that is commonly investigated is the resistance of HPFRCC to elevated temperatures and fire. In the case of the latter, it is well known that the water present in the concrete vaporizes rapidly, causing a spalling or a loss of cross-section. HPFRCCs are highly susceptible to fire-induced spalling because of their dense microstructure and lower permeability, and this can lead to lower fire resistance in HPFRCC members, as compared to conventional concrete members. It is also well known that plastic fibers are often used since they melt at temperatures about 150 C
200 C, creating open channels to evacuate the rapidly released water vapor. This phenomenon is also observed in HPFRCC beams incorporating polymeric fibers, as presented by Banerji et al. (2020).

2.2.4 Field applications of high-performance fiber-reinforced cementitious composites It is true that the addition of fibers minimized many disadvantages of concrete, but with the development of HPFRCC, new fields and uses for these cementitious materials have emerged. The idea that a type of concrete is now ductile allowed the use of HPFRCC in applications such as jointless concrete pavements (Zhang et al., 2013) or runway pavements (Pan et al., 2020). In a similar way, the ductility of HPFRCC helps to produce structures that are more resistant to seismic loads, and thus they were able to be used in members with shear-dominated response such as beam
column connections, low-rise walls, and coupling beams, as well as flexural members subjected to large displacement reversals (Parra-Montesinos, 2005; Rokugo et al., 2009). The use of HPFRCC in strengthening the infill walls of reinforced concrete frames is also extensively studied. Fig. 2.8 presents such a study utilizing the use of precast ECC plates to strengthen a R/C frame having an infill wall. That strengthening technique was found to be an effective, practical, and occupant-friendly technique for strengthening seismically vulnerable reinforced concrete structures (Ayatar et al., 2020).

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Figure 2.8 The use of HPFRCC for seismic strengthening of infill walls of R/C frames. HPFRCC, High-performance fiber reinforced cementitious composites. Source: Courtesy Prof. Dr. Erdem Canbay, Department of Civil Engineering, METU.

Figure 2.9 UHPFRCC storm gully tops produced according to EN 124:2015. UHPFRCC, Ultra high-performance fiber cementitious composites. Source: Courtesy Bursa Beton.

Besides the structural applications presented above, HPFRCC has also been utilized in precast members that are subjected to high dynamic loads. Nowadays, commercially available UHPFRCC precast units are frequently used to replace their counterparts made of different materials owing to their economical and reduced salvage values (Figs. 2.9 and 2.10). Moreover, the higher resistance of HPFRCC to impact loads is utilized in producing concrete nuclear waste containers (Othman et al., 2019), which can be also improved when heavy-weight HPFRCCs are produced (Tufekci & Gokce, 2017). Another common application of HPFRCC is in stand-alone applications such as the exterior fac¸ade of buildings. Both premix and sprayup production techniques

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Figure 2.10 UHPFRCC Fiber optic cable channels. UHPFRCC, Ultra high-performance fiber cementitious composites. Source: Courtesy ISTON.

Figure 2.11 UHPRCC manufactured by Ductal. Chinese Cultural Center, Belgrade. Source: Courtesy Fibrobeton.

can be utilized in producing those precast members such as the ones shown in Figs. 2.11 and 2.12, respectively. The hatschek process is also applied commonly to produce fibercement boards containing cellulose as the fibers. As shown in Fig. 2.13, 1250 3 3000 3 8 mm3sized fibercement boards show excellent deformability characteristics that can even be folded and unfolded.

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Figure 2.12 GRC exterior fac¸ade elements. Gori Justice House, Georgia. GRC, Glassreinforced concrete. Source: Courtesy Fibrobeton.

Figure 2.13 Fibercement boards produced by the hatschek process. Source: Courtesy Tepe Betopan.

In Japan, HPFRCC has also been used in the surface repair of concrete dams, water channels, and retaining walls, typically using shotcreting (Rokugo et al., 2009). HPFRCC are also suitable for 3D printing and additive manufacturing jobs ˇ (Hamidi & Aslani, 2019; Xu & Savija, 2019). Nowadays, with the introduction of

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nanomaterials, the strengthening part of HPFRCC can also be seen with its high ability to resist fire, which can be combined with self-healing property (Ming & Cao, 2020). This property is also an effective solution for infrastructure applications, where degradation is inevitable (Mircea et al., 2020). With the development of smart HPFRCC with self-sensing capabilities, continuous health monitoring of structures will be the next area of applications (Sasmal & Sindu, 2019), and when optical fibers are used, for example, cementitious composite matrix can be turned into a strain sensor (Saidi & Gabor, 2019).

2.3

Nanomaterials in high-performance fiber-reinforced cementitious composites

In the analysis of concrete structures, concrete is assumed to be a homogeneous and isotropic material on the macro level. On the other hand, while going from the macro- to microlevel analysis, concrete is generally regarded as a two-phase material composed of aggregate particles that are dispersed in a cement paste matrix. Nilsen and Monteiro were the first to consider the effects of interfacial transition zone (ITZ) to model the elastic behavior of concrete, and they defined concrete as a three-phase material (Ulrik Nilsen & Monteiro, 1993). Later, models that incorporate voids and the moisture state in those voids were also developed (Yaman et al., 2002). In multi-functional HPFRCC, with the incorporation of fibers and even the nano-fibers, a multiphase and multiscale heterogenous material is obtained. Even though the size of ordinary cement is around 1
60 μm, the hydration products, such as C
S
H gels, are primarily nanostructured materials. In other words, the first interaction between the ingredients of a HPFRCC occurs at a nanoscale. At the nano- and microscales, the structure of the cementitious system is composed of amorphous hydration products, nano-to-microsized crystals and multiscale voids/ pores (Bastos et al., 2016; Han et al., 2019). Perhaps silica fume, a byproduct of the silicon metal and alloy industries that contain 10
300 nm 2 sized spheres, can be regarded as one of the first nanomaterials widely used in the concrete production (Tokyay, 2016). On the other hand, there are also various publications comparing the effects of silica fume and the nano-silica as obtained from the sol
gel process, and on the properties of cementitious systems (Mondal et al., 2010). Therefore one of the first applications of nanomaterials in a cementitious system is said to date back to 1989, when nanozirconia was utilized to improve its toughness and strength (Han et al., 2017). Especially during the last decades, a wide variety of nanomaterials, ranging from nano-silica to nanotitanium oxide (nano-TiO2), and carbon nano-tubes (CNTs), have been utilized not only to improve the quite well-known properties of the “old concrete” but also add new functions to HPFRCCs. Although the utilization of nanomaterials could take place on each ingredient of HPFRCC (such as the coating of the aggregates and the fibers), the main focus in this section will be on the use of nanomaterials as a separate ingredient to change certain properties of the composite.

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2.3.1 Types of nanomaterials used in high-performance fiberreinforced cementitious composites In cementitious systems, nanomaterials are used to modify/enhance the fresh and hardened state properties, and the mechanical, thermal, and electrical performance of the system to add smart multi-functional properties to HPFRCC. Hence the type of the nanomaterial used highly depends on the targeted performance parameter. In HPFRCCs, nanomaterials can be categorized as reactive and inert based on their interference in the hydration reactions. The most widely used nanomaterials in HPFRCC and their briefly described production methods are enumerated below. 1. Nano-silica: Colloidal silica, which is produced by partially neutralizing alkaline silicate with a mineral acid, is usually used to synthesis nano-silica. In addition to that, the crystallization of nanosized quartz or porosils crystals can be used to produce nano-silica. The synthesis process determines the size and shape of nano-silica particles. Those particles tend to be in the size of 10
25 nm and have a spherical shape when amorphous colloidal silica is used. Larger particles are obtained by Sto¨ber synthesis when tetraethylorthosilicate is used. In addition to those processes, grinding and milling techniques (which lead to wider size range) are used to reduce the size of quartz, silica gel, and vitreous silica (Napierska et al., 2010). 2. Nano-CaCO3: Limestone ground to nanosizes have been also used in the production of blended cements. 3. Nano-clays and calcined nano-clays: Several forms of clays can be processed and used as nanomaterials to improve certain characteristics of cementitious systems. 4. Nanometals: There are many types that can fall under this topic, like nano-Al2O3, nanoTiO2, nano-ZnO, nano-Fe2O3, and nano-MgO. Wet chemistry is the most widely used process to produce this type of nanomaterials. 5. Cellulose nanomaterials: Two main types take place under this category, namely, cellulose nanofibrils and cellulose nanocrystals. The former is produced from mechanical refining of highly purified wood fiber and plant fiber pulps, while the latter is produced from the nanofibrils that stay after the cellulose fibers are hydrolyzed with acid. Cellulose nanofibrils have a width of 4
20 nm and a length of 0.5
2 μm, while cellulose nanocrystals are 50
500 nm rod-like materials with a width of 3
20 nm (Fu et al., 2017). 6. Graphene oxide: This material is produced from graphite in a form of a single layer that have been oxidized to intersperse the carbon layers with oxygen molecules. The thickness of the layer is around 1 nm. 7. Graphite nanoplatelets (GNP): Those are made with graphene layers with a thickness less than 100 nm, and a diameter of few micrometers. 8. CNT: These tubes are made by rolling a graphene sheet as single-walled (with an inner diameter between 0.4
2.5 nm) or multiwalled (with an inner diameter between 1 and 3 nm, and an outer diameter between 10 and 50 nm). The length of nano-tubes ranges between few micrometers to few millimeters (Yazdani & Mohanam, 2014). 9. Carbon nano-fibers (CNF): These fibers are made by stacking the wrapped graphene layers in the shape of cones, cups, or plates. The fibers have a diameter between 10 and 500 nm, and a length between 0.5 and 200 μm (Bastos et al., 2016; Han et al., 2019). 10. Carbon black (CB): This material has a form of amorphous carbon that is produced by an incomplete combustion. It is more like a crystalline array of condensed rings that

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have a high surface-area-to-volume ratio, with an average diameter between 10 and 400 nm, whereas the average diameter of CB aggregates ranges from 100 to 800 nm (Han et al., 2014).

2.3.2 Advantages of using nanomaterials in high-performance fiber-reinforced cementitious composites As mentioned earlier, adding nanomaterials to the cementitious systems has introduced various properties or functions that have opened the door to new applications for HPFRCC. In general, the ultrafine nature of nanomaterials allows even the nonreactive ones to enhance the hydration and hardening processes. For that, it is possible to have a combination of advantages when a nanomaterial is used in cementitious composites. However, it is obvious that each material plays a main role under a specific condition. The below list presents the main effects of the nanomaterials listed in the previous section when used in the production of HPFRCC. 1. Nano-silica: This material has a wide usability in cementitious materials. Its effect starts at the fresh state by increasing the amount of water needed to cover the high specific surface area of the ultra-fine particles. After that, nano-silica starts to play a role in the hydration reactions by forming H2 SiO22 4 . 2. Nanometals: In addition to some general enhancements, like improved compressive strength and modulus of elasticity (because of the increased compactness of ITZ and decreased porosity), nano-Al2O3 can show a pozzolanic reactivity related to its fineness. This is mainly because it contributes to the formation of calcium-aluminate-hydrate gel by reacting with calcium hydroxide (CH). Nano-TiO2 and nano-ZnO, on the other hand, are well known to have a self-cleaning effect, where they destroy the organic pollutants through photocatalysis process. In addition, the use of nano-TiO2 is also reported to accelerate the rate of hydration and increase the chemical shrinkage (Jayapalan et al., 2010). As for nano-Fe2O3, studies show that it can enhance the fire and gamma radiation resistance when added to concrete. Moreover, nano-Fe2O3 can also enhance the selfsensing ability of the mixture with the change of its electrical resistance under loads. On the other hand, by reacting with CH, nano-Fe3O4 generates a new phase called ironettringite, that help to reduce the porosity by precipitating in the pores of the matrix. Finally, the expansive nature of nano-MgO allows it to be used as an autogenous shrinkage reducing agent (Mahinroosta & Allahverdi, 2019). 3. Nano-CaCO3: The accelerating effect of CaCO3 on the hydration of cement is a wellstudied phenomenon. A greater accelerating effect is obtained on the hydration of tricalcium silicate (C3S) with the addition of a nano-CaCO3 in the presence of supplementary cementitious materials such as fly ash and slag (Sato & Beaudoin, 2011), which was later explained by the seeding effect due to the addition of nano-CaCO3 (Sato & Diallo, 2010). 4. Nano-clays and calcined nano-clays: As a kind of nanopozzolanic material, nano-clays are known to reduce the pore size and porosity of the cement matrix, thereby improving the hardened properties of cement paste and mortar (Farzadnia et al., 2013; Ramachandra Murthy et al., 2018). Moreover, calcined nano-clay is reported to decrease the porosity and water absorption, and increase the hardened properties (density, compressive strength, flexural strength, fracture toughness, impact strength, and Rockwell hardness),

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5.

6.

7. 8.

9. 10.

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as well as improve thermal stability when compared to the control cement paste (Hakamy et al., 2015). Cellulose nanomaterials: Cellulose nanofibrils can enhance the strength capacity of HPFRCC, but they do not contribute too much to strain capacity because of their inability of crack bridging. Moreover, cellulose nanocrystals reported to have increased the degree of hydration, especially at later ages. However, cellulose nanomaterials, in general, can lead to a delay in setting time due to their organic contents (Fu et al., 2017). Graphene oxide: Considering its availability and ease of production among the nanocarbon materials group, graphene oxide has a good cost-performance relation that is reflected on the general improvements of the matrix. In other words, it can show a combination of the enhancements that other carbon nanomaterials show, but at lower values for a lower cost. GNP: The performance of GNP is in between graphene oxide and the rest of carbon nanomaterials. CNT: If properly dispersed in the cementitious matrix, CNT can lead to a great improvement in the strength, fracture characteristics, durability, and electrical properties. CNT can also show a crack bridging behavior at the microscale. This can be related to the high mechanical properties of the CNT and their surface texture, which provide a good interfacial interaction along with the cementitious matrix. CNF: The nanoreinforcement effect of CNF is similar to the one of CNT. However, a better performance is obtained with CNF. CB: CB is mainly used for electrical conductivity purposes, like corrosion protection, deicing, and electrical grounding.

Table 2.1 presents a summary of the widely used nanomaterials and their common effects on the cementitious systems performance parameters.

2.3.3 Application challenges of using nanomaterials in highperformance fiber-reinforced cementitious composites When nanomaterials meet an aqueous compound such as the water used in HPFRCC, they tend to form agglomerates because of the attractive Van der Waals forces. Therefore a successful utilization of nanomaterials in the production of HPFRCC, starts with a good dispersion of it within the cementitious matrix, and later requires the adoption of that dispersion technique into an effective processing technology. Otherwise, the presence of aggregated nanoparticles can result in weak zones, in which nanomaterials act as an activator to promote pozzolanic reactions, which are then potential areas for stress concentration (Hanus & Harris, 2013). Dispersion techniques of nanomaterials in a cementitious system can be classified as chemical modification or functionalization, physical modification through the use of surfactants, and mechanical methods such as ultrasonication, ball milling, magnetically stirring and shear mixing. Of course, it should be mentioned that these techniques can be used alone or in combination with another, and most of them are still applicable at the lab-scale. Brief descriptions of those dispersion techniques are summarized below: 1. Mixing the nanomaterials first with the other solid particles is likely to prevent a good dispersion of other ingredients but will not allow the nanomaterials to agglomerate.

Table 2.1 The effect of nanomaterials on the properties of cementitious systems (Adesina, 2021; Bastos et al., 2016; Krystek & Go´rski, 2018).

Nano-silica Nano-alumina Nanotitanium dioxide Nanomagnesium oxide Nanozinc oxide Nanomagnetite Nanocalcium carbonate Nanometakaolin Nano-clay Graphene oxide Graphene nanoplatelets Carbon nano-tubes Carbon nano-fiber Carbon black

Mechanical properties

Porosity

Permeability

1 1 1

2 2 2

2 2 2

Water absorption 2

Hydration

Shrinkage

1 12

1

Elevated temperature 1 1

Frost resistance

Corrosion resistance

Selfcleaning

1 1

1

1

1

1

2

Antibacterial activity

Selfsensing

Electrical heating

1 1 1

1 1

2 2

1 1 1 1

2 2 2

1 1

1 1

2

2

2

2 2 2

1

1 1 2

1

1 1

1 1

1

1 1 1

1 1 1

In the above table, a “ 1 ” sign shows a positive effect and a “-” sign shows a negative effect of the corresponding nanomaterial on the property under investigation. Those left “(blank)” shows that the nanomaterial either does not have any effect or an effect is not yet reported in the literature

.

88

2.

3.

4.

5.

Recent Advances in Nano-Tailored Multi-Functional Cementitious Composites

However, if nanomaterials are mixed with cement through a ball milling process for a sufficient time, this will lead to a good dispersion. On the other hand, nanomaterials can always be dispersed in an aqueous solution, then added to the system. If applied for enough duration with a proper speed, magnetic or overhead stirring can give a satisfying result in terms of dispersion. The use of surfactants is also one of the most widely used applicable techniques for most nanomaterials. They help to increase the electrical repulsion between the particles by reducing the surface tension. When the working mechanism is based on preventing the formation of particles cluster in the suspension, these agents are called dispersants. Superplasticizers are a good example of surfactants. Moreover, nonionic polymers can be used as slurry stabilizer that prevent agglomeration. In the ultrasonication process, an ultrasonic excitation energy is used to break up the nanotube clusters at the expense of achieving decreased aspect ratios. It is applied on aqueous mediums through sonication by ultrasonic baths or probes. The latter method is known to be more efficient since it delivers the frequencies directly and can be locally applied. However, when bath sonication is used, multiple samples can be put in different tubes at the same time. Ultrasonication is usually used when stirring is not enough or cannot be applied. Among those techniques, chemical modification is the most common approach to achieve the satisfying dispersion of CNTs; more specifically by applying acid mixtures, which oxidize CNTs and add carboxylic (
COOH) or hydroxyl groups (
OH), thus increasing the solubility of CNTs in the aqueous matrix (Bastos et al., 2016). High-speed homogenizer (centrifuges) and high-pressure homogenizers are also highly effective dispersion methods. However, while the first method is suitable for laboratory use, the second is usually used at industrial scale.

In addition to the above-mentioned dispersion and the high cost of the nanomaterials, the need of preliminary research to obtain the desired performance and probably the assessment of that performance is another challenge. Finally, there has been an increasing awareness on the health and safety considerations related to the use of nanomaterials in the construction industry (Bastos et al., 2016). A common conclusion of the medical studies regarding health-related issues of nanomaterials is the need of further research to confirm these risks to human health. All of the above-mentioned challenges needed to be resolved before considering the use of nanomaterials at a large scale.

2.4

Influence of using nanomaterials in highperformance fiber-reinforced cementitious composites

Nanomaterials have a small effect zone, which implies that they should be used with systems composed of fine particles. In this context, the matrix of the cementitious composites forms a perfect medium to use nanomaterials. For that, the first concern here is to be able to find the optimum amount that will provide the desired enhancement, and then to be able to disperse that amount equally through the

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matrix. Once these concerns are eliminated, the care will be on what type of nanomaterials should be used to enhance a specific property. However, this topic is a kind of gray zone, because nanomaterials can have noticeable effects during different stages of hydration and on more than one feature of the cementitious composites.

2.4.1 Early and the hardening stages of nano-tailored highperformance fiber-reinforced cementitious composites The effect of using nanomaterials in cementitious composites starts with the rheological properties. Generally speaking, adding such a fine material will increase the total surface area of the solid particles, hence the water demand will increase. However, this is not true for all types of nanomaterials and for all addition amounts. In terms of flowability, the presence of nanoparticles increases the frictional force between the solid particles in the cement paste, which increases the resistance to initiate flow. The yield stress needed to initiate the flow highly depends on the surface characteristics of the nanomaterial. Nano-silica, for example, adsorbs more water when compared to other nanomaterials; hence the workability can be significantly reduced. Yet, when compared to CNTs and CNFs, the yield stress needed to initiate flow is more for the last two materials (CNT has higher yield stress because of its hollow cylindrical structure), because of the fiber twining. On the other hand, the plastic viscosity, which represents the flow resistance, is higher for nano-silica, because for CNT and CNF the particles are reoriented under high shear rate (Jiang et al., 2018). However, it was reported that when nano-silica effectively fills the voids between cement particles, it might improve the workability, since less water is needed to fill those voids (Mahinroosta & Allahverdi, 2019). The extreme fineness of the nanomaterials has a huge effect on the initial hydration process of the cement. The seeding effect of those materials facilitates the dissolution of cement compounds along with the precipitation of the hydration products (Nazar et al., 2020). Nano-silica for example, reacts with CH and form new bonds on its surface, which produces C
S
H gel, reduces CH, and refines CH crystals (Kuo et al., 2006). A pozzolanic activity is also observed with the use of nanometakaolin up to 10% (Abo-El-Enein et al., 2014). However, the hydration activation that is provided by nanomaterials leads to an increase in the autogenous shrinkage and the related drying shrinkage (Haruehansapong et al., 2017). Yet, when graphene nano-fibers were used, the size of shrinkage cracks was reduced by preventing crack propagation (Singh et al., 2013).

2.4.2 Mechanical and durability properties of nano-tailored highperformance fiber-reinforced cementitious composites The use of nanomaterials is expected to enhance the strength of the cementitious composite through the modification of the matrix porosity. While materials like nano-silica and nanometals can help to increase the compressive strength, carbon

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nano-fibers can also enhance the flexural toughness. However, when this use is incorporated with the use of the correct micro- or macrofiber, the results can be significantly enhanced. It was reported that the addition of nano-silica to a HPFRCC mixture incorporating basalt fiber pellets increased the compressive strength, flexural strength, and toughness by approximately 16%, 38%, and 46%, respectively. In the same mixture prepared with nano-silica, when basalt fibers were replaced by PVA, the compressive and flexural strengths were improved by about 25% and 58%, respectively, while the toughness decreased by about 51%. However, if the PVA fibers were added to that mixture, then the compressive strength, flexural strength, and toughness would show an improvement by about 19%, 50%, and 238%, respectively (Elhadary & Bassuoni, 2020). A remarkable improvement can also be seen in the modulus of elasticity value if an optimum tailoring was done using nanomaterials. An increase of about 87% in the modulus of elasticity was reported when a combination of CNT and GNP with a ratio of 4:1 was used in a cementitious composite mixture (Akbar et al., 2021). Under dynamic loadings, nanomaterials help to improve the performance of HPFRCC. When multiwall CNT is used, the impact toughness and dissipation energy can be increased up to 32.3% and 35.2%, respectively (Wang et al., 2020). The extraordinary ability of nanomaterials to fill the pores in the cementitious matrix leads to a significant improvement in the durability of HPFRCC. The use of CNT, for example, has been reported to decrease the porosity by 20% (Nochaiya & Chaipanich, 2011). Moreover, the addition of 4.5% nano-silica can reduce the chloride ion permeability at 56 days by about 65% for a HPFRCC mixture reinforced with 1% steel fiber and 0.25% polypropylene fiber. The same addition can reduce the weight loss due to immersion in MgSO4 by 53% at 56 days (Sujay et al., 2020). Nano-silica was also reported to reduce the carbonation depth at 28 days from around 11.0 mm to about 9.0 mm (Nochaiya & Chaipanich, 2011). Besides the above-mentioned improvements, nanomaterials can provide selfhealing and self-sensing capabilities to HPFRCC. For example, when nano-silica and CNT were added to ECC, the self-healing capability was increased by up to 39%, while the addition of CNT alone lead to an increment of 104% in the frac¨ ztu¨rk et al., 2020). Each of these materials tional changes in electrical resistivity (O will be investigated in the relevant chapters of this book.

2.4.3 Strain-hardening and crack propagation of nano-tailored high-performance fiber-reinforced cementitious composites As mentioned earlier, the main advantage of HPFRCC is probably its high ductility and the ability to show multiple cracking when subjected to tensile stresses. This helps to increase the crack width capacity before failure to a large extent. In this scope, adding nano-silica to ECC with an amount of 5% by binder weight can increase the total crack width by 275% (Li et al., 2016). Moreover, adding 0.10% of CNT to HPFRCC mixture incorporated 2% PVA fibers increases the stiffness by

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221%, the failure strain by 10%, the dissipated energy by 197%, and the fracture energy by 152% (Sindu & Sasmal, 2019). A similar addition of CNT to a HPFRCC mixture incorporated 2.4% PVA and 1.2% steel fibers leads to an increase of 195% in the crack opening distance difference between the distance before and after the maximum peak when the load is 95% of the ultimate load (Xu et al., 2019). Those values give a good example about how efficient can the nanomaterials be when it comes to improving the strain-hardening behavior of HPFRCC. The enhancement in the fracture and strain-hardening properties can be related to how the addition of nanomaterials affects the crack propagation. When CNT or CNF are dispersed homogeneously, the propagation of nanocracks to form microand macrocracks can be restricted (Parveen et al., 2013). Since the nanoparticles can easily fit between cement grains, they act ahead of the crack tip, which prevents the propagation of microcracks by elastic crack pinning (Ghazanlou et al., 2020). In Fig. 2.14, the effect of fibers on the crack propagation is presented (Lo¨fgren, 2005). Besides the growing research on the quasistatic performance of nanomaterials incorporated HPFRCC, there are also few articles investigating their performance on higher strain rates. For example, Chen et al. used Split Hopkinson pressure bar (SPHB) tests to characterize the behavior of nano-silica, nano-TiO2, and nanocalcium carbonate incorporated HPFRCC (Chen et al., 2019). They concluded that the dosage and type of nanomaterials did not have a measurable influence on the dynamic compressive strength and strain of HPFRCC at a constant strain rate level of 170/s: In another study, Wang et al. examined the effect of eight different types of multiwalled CNT (MWCNT) on the dynamic mechanical properties of HPFRCC. Again, SPHB testing was utilized, and the experimental results revealed that all types of MWCNT increased the dynamic compressive strength and ultimate strain of the composite, but the dynamic peak strain of the composite presented deviations with the MWCNT incorporation.

2.4.4 Structural applications of nano-tailored high-performance fiber-reinforced cementitious composites Although the incorporation of nanomaterials with HPFRCC mixtures is a popular topic, its large-scale structural applications are quite limited. Most of those applications are related to the self-cleaning ability of those concrete mixtures utilizing nano-TiO2. One of the first such applications took place in Rome, Italy on the interior and exterior walls of the Jubilee Church which was completed in 2003 (Han et al., 2017). Another application was on the use of photocatalytic pavement blocks (10,000 m2 in size) on the parking lanes of a main axe which was constructed in Antwerp in 2004
05 (Boonen & Beeldens, 2014). Besides Europe, similar applications also exist in United States, such as the photocatalytic concrete pavement section of a new highway which was constructed in 2011 in St. Louis, MO (Birgisson et al., 2012).

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Figure 2.14 Traction separation law for nano-tailored HPFRCC. HPFRCC, Highperformance fiber reinforced cementitious composites. Source: Adapted from Lo¨fgren, I. (2005). Fiber-reinforced concrete for industrial construction
A fracture mechanics approach to material testing and structural analysis. Doktorsavhandlingar Vid Chalmers Tekniska Hogskola, 2378, 1
162.

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As for the electrothermal properties of a HPFRCC, the carbon materials excellent electrical conductivity and a large heating capacity at low voltage offers its use as an excellent alternative as the heating element in electrical resistive deicing systems. There are some related pilot studies for its use in the bridge decks as well as airport pavements (Han et al., 2017). Finally, it is possible for cementitious composites to show sensitive piezoresistive effect when CNP are used with a volume ratio of 5%. Static and dynamic loads as well are able to be monitored through such types of application (Rao & Sasmal, 2020; Sun et al., 2017).

2.5

Challenges and future perspectives

Even though the existence of nanosized materials in nature and their presence in cementitious systems are quite well known, the recent developments in the visualization and measurement systems for characterizing and testing materials at the nanoscale level have led to a huge growth of the use of this technology in HPFRCC. Therefore as they did in many other industries, nanomaterials can become a game-changer in the construction industry, specifically in the subject matter of the book, that is, in the field of multi-functional HPFRCC construction. The research on the use of nanotechnology in cementitious systems, especially in the second decade of the new millennium was a huge milestone in the history of HPFRCC. Besides their contribution to the existing well-known properties of HPFRCC, such as strength, ductility, and durability, they also added new functions to those composites, thus it is obvious that nanomaterials now have a promising future, yet there are certain issues that need to be tackled. Among many challenges in the use of nanomaterials in cement composites, the most critical ones are the development of industrially viable dispersion and mixing techniques, and the cost issue that can be linked to the sectors price sensitivity. It should be noted that most of the parameters involved in HPFRCC production are well researched, fairly understood, and tied to firm standards. However, when it comes to the incorporation of nanomaterials, they still lack the necessary priorities in application, for example, the lack of mixing and dispersion methods at a commercial level, the required standards for production and processing, the marketing and supplying chains, and the economically feasible cost for large scale productions. In addition to the technical and economic issues, the potential health safety and environmental concerns for nanomaterials are not completely investigated and understood. It can be argued that the typically small addition rates of nanomaterials in HPFRCC may help to reduce the likelihood of adverse negative health and environmental effects. However, as their usage increases, human exposure to nanoparticles will be inevitable. Even though, there has been an increase in the number of articles that look at the health and safety considerations related to the use of nanomaterials in the construction industry, more is needed to better understand and detect their effects on the environment as well as the human health and safety (Bastos et al., 2016).

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Considering the fact that it took chemical admixtures about half a century to be an essential part of concrete, it can be stated that nanomaterials may probably need such a time period. On the other hand, with the rapid advances in technological development in this era and with the increasing amount of companies working on different cement additives, it is only a matter of time until nanomaterials are listed as an ingredient in the cement or concrete recipes either directly or indirectly. Of course, this is also associated with to what extent the advanced types of concrete like HPFRCC will be used in the future.

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Nano-tailored cementitious composites with self-sensing capability

3

2 Hocine Siad1, Mohamed Lachemi1, and Mustafa Sahmaran ¸ 1 Department of Civil Engineering, Ryerson University, Toronto, ON, Canada, 2 Department of Civil Engineering, Hacettepe University, Ankara, Turkey

3.1

Introduction

In recent decades, the incorporation of nanomaterials in cementitious composites has become an area of massive interest for academic and industrial specialists in construction materials. Their nano-sizes of less than 500 nm (Hou et al., 2015; Norhasri et al., 2017) and large surface areas of 10
1000 m2/g (Han et al., 2017), along with possible reactivity, have provided the potential for increased pore structure of concretes and related mechanical and durability performances (Mukhopadhyay, 2011; Reches, 2018). Various types of nano-admixtures, such as kaolin, slag, and clay, and oxide nanoparticles, such as silica, alumina, iron, titanium, and calcium carbonate have been included to enhance the in-service properties of concretes (Meng et al., 2017; Nazar et al., 2020; Pateriya et al., 2021; Sadawy & Nooman, 2020; Safiuddin et al., 2014). They were shown to improve the compressive and flexural strengths, elastic modulus, fracture toughness, compactness, and resistance against abrasion, fire and freeze-thaw, while also modifying the microstructural properties of the cement matrix by acting at the nanoscale level of C
S
H gel (Han et al., 2017; Paul et al., 2018). These outstanding effects have garnered the attention of specialists for the application of more nanomaterials in concrete mixtures and led to the appearance of a new concept of multi-functional cementitious composites, which can combine diverse structural and nonstructural features. Damage to reinforced concrete structures can occur as a result of the initiation of microcracks, which propagate and develop into macrocracks under continual and dilating strains. In addition to the brittle nature of most conventional concretes, the current changes in atmospheric conditions and the continual increase of mechanical loads have aggravated their vulnerability to cracking (Medeiros-Junior, 2018). Failure after excessive cracking can become inevitable if reliable and durable maintenance, renovation, and/or strengthening operations are not applied in a timely manner, which can also be accompanies by serious financial burdens and safety concerns. Thus the detection of cracking at its initial manifestation is highly desirable for increasing the serviceability and security of structures and lowering the Recent Advances in Nano-Tailored Multi-Functional Cementitious Composites. DOI: https://doi.org/10.1016/B978-0-323-85229-6.00014-7 © 2022 Elsevier Ltd. All rights reserved.

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cost of complicated repair and rehabilitation activities (Liew et al., 2019; Siad et al., 2018). The successful use of enabling nanotechnologies that offer autosensing behavior in concretes constitutes one of the most important recent advancements of researchers, helping to facilitate smarter control of damage in construction without the need for traditional sensors (Eddib & Chung, 2018; Han et al., 2020a). This was achieved principally through the introduction of electrically conductive carbon-based nanoparticles for self-sensing of microcracking and health monitoring of concrete structures (Sun et al., 2017; Tian et al., 2019). The self-sensing effectiveness was improved when nano-piezoresistive particles were present in the matrix of fiberreinforced cementitious composites, especially engineering cementitious composites (ECC), thanks to their tailored compositions with strain-hardening characteristics (Siad et al., 2018; Yıldırım et al., 2020). Although greater focus was given to damage/cracking detection under different mechanical loading and conditions, selfsensing cementitious composites were also applied to traffic, vibration, and temper¨ ztu¨rk et al., 2020; Wang & Aslani, 2019). Promising ature/humidity sensing (O work has been done by Siad et al. (2018), in which multiwalled carbon nano-tubes (MWCNT)-modified ECCs were presented to promote and sense self-healing after cracking, as can be seen in Fig. 3.1. The multi-functional performances were enhanced by a nano-combination approach of CNT and nano-silica in the recent ¨ ztu¨rk et al. (2020). investigation by O In general, the potential for nanomaterials in instigating the self-sensing ability of cementitious composites capitalizes on their increased conductivity when piezoresistive nanoparticles create continuous pathways, a tunneling effect and electric field emission (Wang & Aslani, 2019). However, research has demonstrated that

Figure 3.1 (A) Bridging CNT particles wrapped C
S
H, and (B) around polyvinyl alcohol (PVA) fibers for increased sensing and healing of microcracks (Siad et al., 2018). CNT, Carbon nanotube. Source: From Siad, H., Lachemi, M., Sahmaran, M., Mesbah, H. A., & Hossain, K. A. (2018). Advanced engineered cementitious composites with combined self-sensing and selfhealing functionalities. Construction and Building Materials, 176, 313
322. https://doi.org/ 10.1016/j.conbuildmat.2018.05.026. Bridging CNT particles to promote and sense selfhealing after cracking.

Nano-tailored cementitious composites with self-sensing capability

105

the self-sensing property has a strong correlation to other factors, thereby resulting in great challenges regarding the appropriate type and characteristics of nanoparticles and their optimized dispersity in the mixture, as well as the effect of moisture and microstructure of composites and testing conditions (Han et al., 2020a). This chapter begins with a review about the recent advancements in nano-piezoresistive materials, then investigates the important parameters influencing the sensing ability of nano-modified composites, such as intrinsic properties, concentration, and dispersion of nanoparticles, cement matrix characteristics and surrounding testing conditions, followed by discussion on the use of nanomaterials in cementitious composites for mechanical, cracking, vibration and special sensing applications, with special focus on recently developed concretes such as ultra-high-performance concrete (UHPC) and ECC. Furthermore, this chapter emphasizes challenges and proposed future research.

3.2

Nano-piezoresistive materials in cementitious composites: Recent advancements

Although the predominant objectives of incorporating nanomaterials in cementitious matrices are related to physico-mechanical and durability improvements, their use for electrical/sensing functionalities has greatly increased in the past few years, which has been motivated by various developments in nanotechnology. A plausible theory behind the efficacy of nanomaterials to guarantee the sensing ability in cementitious composites is increasing the volume of the electrical network, which depends greatly on the type of individual nanoparticles to be included (Wang & Aslani, 2019). Carbon-based nanomaterials (CBNMs) occupied the lead position of nano-conductive elements being or having been used in concretes since it was first utilized in 1993 as fibers to sense the cyclic compressive stress in mortars (Chen and Chung, 1993; Eddib & Chung, 2018; Han et al., 2020a; Liew et al., 2016). They are presented as the ideal nanomaterials for concretes because of their high intrinsic mechanical, electrical and thermal properties (Gao et al., 2019). The carbon family of CNTs, carbon nano-fibers (CNF), and nano carbon black (NCB) are defined with their 1D or 0D structures and lower carbon contents of less than 95 wt.%, while the graphite family of graphene nanoplatelets (GNPs) have a 2D structure and are more than 96% carbon (Huang, 2009). Unlike CNFs, which have flat graphene sheets as conical and tubular shapes, CNTs have assemblies of rolled graphene sheets like hollow cylindrical tubes (Fig. 3.2A and B) (Kang et al., 2006). Among all CBNMs, CNTs are currently the nanomaterials that are the most studied and included in cementitious composites for self-sensing purposes. They are manufactured as a single sheet of graphene called single-walled CNTs (SWCNTs) or multiple graphene sheets known as MWCNTs, which are generally produced with a larger diameter and length (1.4
50 nm and 0.1
100 μm, respectively) compared to SWCNT particles (0.4
3 nm and 1
50 μm, respectively) (Gao, 2018; Liew et al., 2016). In addition to their

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Figure 3.2 (A) SWCNTs; (B) MWCNTs (Liew et al., 2016). MWCNT, Multiwalled carbon nano-tubes; SWCNTs, single-walled carbon nano-tubes. Source: From Liew, K. M., Kai, M. F., & Zhang, L. W. (2016). Carbon nanotube reinforced cementitious composites: An overview. Composites Part A: Applied Science and Manufacturing, 91, 301
323. https://doi.org/10.1016/j.compositesa.2016.10.020. Rolled graphene sheets like hollow cylindrical tubes of single-walled and multiwalled carbon nanotubes.

enhanced elastic modulus of around 210
1000 GPa, tensile strength of 10
100 GPa and aspect ratio of $ 1000, they possess high electrical conductivity of 103
106 S/cm (volume resistivity of 1024
0.1 Ω cm) and current density of 106
109 A/cm2, depending on the type of CNTs (Liew et al., 2016; Shi et al., 2019). Their greater specific surface areas of .400 m2/g allowed them to be the most current effective nano-elements in decreasing the resistivity of concretes at a low content of 0.1 wt.% (Al-Dahawi et al., 2016; Wang & Aslani, 2019). Compared to CNTs, CNFs are larger, with an overall diameter of 50
150 nm and length of 50
200 μm, equivalent elastic modulus of 400
600 GPa and volume resistivity of 1023
0.1 Ω. cm, and lower tensile strength (2.7
7.0 GPa) and aspect ratio (100
500) (Blandine et al., 2016; Han et al., 2015b). Their large surface area of .200 m2/g helped in providing CNF-based cementitious materials with a promising piezoresistive response at concentrations of 0.1
0.5 wt.%. Furthermore, depending on their method of preparation, CNFs can be almost 100 times less expensive than CNTs (Sobolev, 2016). NCBs are another type of CBNMs that have been extensively used for sensing performance in concretes due principally to their high dispersion and lower cost, almost 20 times lower compared to CNFs (Wong, 2015). NCB particles are composed of disordered graphite sheets and have an acini-like amorphous structure of carbon (Fig. 3.3C), different in nature than the continuous graphite sheets of CNTs and CNFs (Al-Dahawi et al., 2016; Donaldson et al., 2006). They have a convenable particle size of around 20
100 nm and surface area of 30
100 m2/g and low structural properties (Han et al., 2017). Although NCB has a satisfactory intrinsic electrical conductivity of 1
100 S/cm (resistivity of 1021
2.3 Ω cm) and provides greater signal stability, its lower aspect ratio causes a minimum requirement of around 2 wt.% in order to clearly reduce the resistivity of cementitious composite

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Figure 3.3 Scanning electron microscopy micrograph images of (A) CNT, (B) carbon fiber (CF), (C) NCB, (D) GNP. Deferent microstructural shapes of CNT, CF, NCB, and GNP. CNT, Carbon nanotube; GNP, graphene nanoplatelet; NCB, nano carbon black.

(Al-Dahawi et al., 2016; Li et al., 2006). Unsuitably, such a large amount also generates a sharp reduction in the structural performance of concretes (Han et al., 2019). GNPs, either monolayer or multilayer graphene, are a more recent generation of nanomaterials, which have been utilized to enhance the sensing ability of concretes. They consist of 2D planar geometry of CBNMs exfoliated from graphite and composed of small stacks of graphene (Fig. 3.3D), with an average thickness of 0.35
100 nm, diameter of 5
10 μm and specific area between 13 and 500 m2/g (Cui et al., 2017; Wang & Aslani, 2019). Compared to CNTs, GNPs have a relatively larger elastic modulus of about 1000 GPa and electrical conductivity of 107
108 S/cm, comparable tensile strength of 10
120 GPa and much lower electrical resistivity of about 1026 Ω cm (Marinho et al., 2012). Although GNPs can greatly reduce the resistivity of cementitious materials at contents higher than 1.6 wt.% (Wang & Aslani, 2019), their use for self-sensing performance was reported to have lower capability than CNTs, CNFs, and NCB (Al-Dahawi et al., 2016; Yoo et al., 2017). In addition, GNPs have an almost two times inferior price compared to CNTs (Wong, 2015). Whereas the hybrid uses of nanomaterials look to be still at their initial stage, they are revealing outstanding results as a new tailored method for optimized piezoresistive and technological behaviors of cementitious composites. Combinations of CNT/CNF, CNT/NCB, CNT/GNP, and NCB/GNP were investigated and reported to have a better conductive effect compared to the single

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CBNMs (Abedi et al., 2020a; Zhang et al., 2018; Noiseux-Lauze and Akhras, 2013). For instance, it was possible to increase the electrical conductivity of mortars by more than 99.7% when CNTs/NCBs were incorporated at a combination of 40%/60% and a total dosage of 1.41 vol.% (Zhang et al., 2018). A combination of 50% MWCNTs/50% GNPs resulted in a sharp decrease of around 36% in the electrical resistivity of CNT-mortars (Abedi et al., 2020a). The addition of single and hybrid CBNMs in different fiber-reinforced concretes (FRCs) and ECCs showed a greater potential for developing nano-tailored multi-functional matrices, especially when combined with carbon fiber (Lee et al., 2017; Wang and Aslani, 2021) or nano-silica ¨ ztu¨rk et al., 2020). Although the 3D-printed nanomaterials are also gradually intro(O duced into piezoresistive cementitious materials and are revealed to have significant advantages over the common CBNMs, substantial research is yet to be conducted in this area (Abedi et al., 2020a,b; Noiseux-Lauze & Akhras, 2013; Zhang et al., 2020).

3.3

Parameters influencing the sensing ability of nano-tailored cementitious composites

3.3.1 Intrinsic properties Ordinary cementitious concretes have generally a non- or semiconductor medium with an important electrical resistivity of 106
109 Ω cm, even at their saturated state (Wang & Aslani, 2019). However, this condition can sharply change when appropriate types and portions of conductive CBNMs are incorporated (Sasmal et al., 2017). The intrinsic properties such as resistance, aspect ratio and morphology of nonconductive elements are the noteworthy parameters which can influence the resistivity and piezoresistivity of concretes. Regarding the intrinsic resistance, (Han et al., 2015a,b) categorized the piezoresistive response under an external force into three zones of poor (zone A), good (zone B) and low self-sensing (zone C), based on the percolation threshold, as shown in Fig. 3.4. The intrinsic resistance of the conductive nanoelements presented as one of the leading factors at the beginning, within and at the end of zone B, thereby contributing to the piezoresistive response of the composite. Also, the electron mobility inside the nanomaterials can influence one of the important sensing aspects related to the electrically conductive tunnels believed to manifest when the gaps between nanomaterials are around 1
10 nm (Han et al., 2020a), especially under deformation where the change in their length and diameter can alter the piezoresistive response of the matrix (Li and Chou, 2008). However, as CBNMs have extremely small size and high elastic modulus, their intrinsic conductivity under loading is expected to have negligible change; thus this parameter can be ignored when considering the piezoresistive response of concretes (Ackermann, 2018). The insignificant effect of the intrinsic resistance differences between CBNMs is also supported by several researchers (Yoo et al., 2017, 2018b,c), who investigated the effect of the same 1 wt.% amount of different CBNMs on the sensing ability of cement paste infused with

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Figure 3.4 Change in electrical resistivity as function of nanomaterials concentration (Han et al., 2015a). The piezoresistive response under an external force was categorized into three zones of poor (zone A), good (zone B) and low self-sensing (zone C). Source: From Han, B., Ding, S., & Yu, X. (2015). Intrinsic self-sensing concrete and structures: A review. Measurement, 59, 110
128. https://doi.org/10.1016/j. measurement.2014.09.048.

MWCNT90, MWCNT99, graphite nano-fibers (GNF), and graphene oxide (GO). The use of MWCNT with higher and lower carbon content (different conductivities) caused equivalent resistivity reductions and sensing ability under monotonic tensile and cyclic compression stresses compared to plain cement paste (Yoo et al., 2018c). Nevertheless, the influence of the type of CBNM was evident, as MWCNT presented much higher resistivity reduction (Fig. 3.5) and self-sensing ability than GNF and GO, though GNF has higher conductivity and electronic mobility than CNT. These results were explained by the morphology and random distribution of GNFs. Pisello et al. (2017) also studied the strain-sensing performance of cement pastes prepared with the same 2 wt.% contents of MWCNT, CNF, NCB, and GNP.

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Figure 3.5 Influence of 2 wt.% CNT, CNF, NCB, and GNP on the electrical conductivity of cement paste. The highest and smallest increases in conductivity were for GNP, CNF, and NCB than CNT pastes. CNF, Carbon nano-fibers; CNT, carbon nanotube; GNP, graphene nanoplatelet; NCB, nano carbon black. Source: From Pisello, A. L., D’Alessandro, A., Sambuco, S., Rallini, M., Ubertini, F., Asdrubali, F., Materazzi, A. L., & Cotana, F. (2017). Multipurpose experimental characterization of smart nanocomposite cement-based materials for thermal-energy efficiency and strain-sensing capability. Solar Energy Materials and Solar Cells, 161, 77
88. https://doi.org/10.1016/j.solmat.2016.11.030.

The highest and smallest increases in conductivity were registered for GNP and MWCNT composites, respectively (Fig. 3.5), the greater strain sensitivity was in MWCNT-incorporated samples, followed by NCB, GNP, and CNF. The authors describe the intrinsic conductivity of different CBNMs as similar when compared to that of plain cement paste, which makes the aspect ratio and morphology as the main differences between them. The numerical work done by Garcı´a-Macı´as et al. (2017) supported the important effect of the aspect ratio and size of nanomaterials, as the percolation threshold clearly reduced with the increased aspect ratio of CNTs. This is because for the same concentration of well-dispersed CBNMs there are a greater number of higher aspect ratio particles in a similar volume of cementitious composites. The previous experimental results of Yoo et al. (2017, 2018b,c) are in line with those of Sixuan (2012), who explored the resistivity and

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crack-sensing of pastes and mortars with the same dosage of three types of GNPs and found that GNPs with larger aspect ratios displayed greater conductivity and pizoresistivity. Li et al. (2007) demonstrated that acid-treated CNTs stimulated reduced conductivity and greater pressure-sensing than untreated CNTs because of the different shapes, distances, and number of contact points generated by each CNT. Dong et al. (2019) also mentioned that the sheets and fibers-based conductive nanomaterials have a higher probability for forming linkages and improving the conductivity than those of NCB powders. Although the aspect ratio and shape of nanomaterials were presented to affect the self-sensing of concretes, this seems more connected to their optimal concentration and dispersion levels, because the more the nanoelements are fibrous and have higher aspect ratios, the lowest contents are needed to reach greater electrical conductivity. For instance, Liew et al. (2016) concluded that the main parameter influencing the CNT
cement matrixsensing under an external effect is the change in network resistance between CNT particles, which is more related to their optimum contents and dispersion to reach the percolation threshold level rather than the intrinsic properties of the single CNT particle.

3.3.2 Concentration of nanomaterials Besides the clear discrepancy about the importance of intrinsic properties of CBNMs, there is a consensus on the key effect of nanomaterial’s concentration on the conductivity and piezoresistive response of cementitious composites due to its direct relation with the percolation threshold. Although this is also linked to the combined internal and external factors of the tested samples, the percolation threshold is generally connected to the optimized CBNMs concentrations, which can present a sharp reduction in the electrical resistivity of cementitious composites. However, the optimal piezoresistive response is not necessarily the result of an ideal threshold level at lower CBNMs content. When the amounts of CBNMs are higher than the percolation threshold, the tunneling (or field emission) effect of CBNMs can help to increase the self-sensing ability under tension. Similarly, lower contents than the percolation threshold are advantageous for the compression sensing, as tunneling distances will decrease under compression (Li et al., 2006; Liew et al., 2019). A concentration near the percolation threshold between the end of zone A and the percolation point in zone B of Fig. 3.4 is suggested when a lower dosage of nanomaterials and maximum strain sensitivity are targeted. Percolation threshold values of CNT-, CNF-, NCB-, and GNP-incorporated cementitious composites have been shown to be around 0.1
0.5, 0.1
1, 2
4, and 1.6
5 wt.%, respectively (Jiang et al., 2018; Konsta-Gdoutos & Aza, 2014; Sasmal et al., 2017). It is important to note that these values are not valid for all concretes and cement matrices and may vary under different compositions and materials. For example, Sasmal et al. (2017) indicated that no noticeable conductivity increments were registered in cement pastes with CNF or CNT beyond 0.1 wt.%, whereas their best piezoresistive responses for compressive cyclic loading were at 0.1 and 0.05 wt.%, respectively. Baeza et al. (2013) obtained the most sensitive dosage of CNF pastes

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at 2 wt.% for strain and traffic monitoring. The hybrid use of nanomaterials was more effective in reducing the threshold level compared to their single incorporation. The 0D/1D, 1D/2D and 0D/2D mixtures of 0D NCB, 1D CNT, and 2D GNP are shown to intensify the electron quantum tunneling and piezoresistive effects. Abedi et al., (2020b) were able to reduce GNP content to 0.25% when combined with 0.25% CNT in order to reach the percolation threshold in cement composite. NCB was also used with GNP by another group of researchers (Dong et al., 2021) and found to reduce the threshold volume from 2.4
3.6 to 0.67 vol.% when added with 3.35 vol.% (NCB 1 GNP 5 4.5 vol.%) (Fig. 3.6). The authors concluded that assembling 0D GNP and 2D NCB can endow the cement pastes with more stable piezoresistive property and maximum strain sensitivity, especially under compressive loading. It has been concluded by researchers (Han et al., 2015b) that the required concentrations of NCB and CNT reduced to 0.91 and 0.6 vol.% when used in a hybrid mortar composed of 60% NCB and 40% CNT, which were 8.79 and 1.5 vol.% for single NCB and CNT, respectively. The hybrid incorporation of carbon fiber (CF) and CNT was investigated by (Lee et al., 2017). The percolation thresholds of single CNTs and CFs were 1 and 0.5
1 vol.%, respectively, the hybrid CNT-CFs necessitated only 0.5 vol.% of CNT and 0.1 vol.% CFs to produce the same piezoresistive response as the single CNTs. Even under comparable compositions, different CBNMs concentrations were reported to attain the percolation threshold and optimum piezoresistivity in ECCs. In a current study by (Liu et al., 2019), the finest threshold and piezoresistive response of MWCNT-ECC1.2 (with fly ash/cement ratio of 1.2) were reached at 0.3 wt.% and the frictional change

Figure 3.6 Electrical resistivity of cement pastes with assembled NCB/GNP composite fillers. (A) direct current (DC), (B) alternating current (AC). Assembled GNP/NCB highly reduced the threshold volume to be 0.67/3.35 vol.% (NCB 1 GNP 5 4.5 vol.%). GNP, Graphene nanoplatelet; NCB, nano carbon black. Source: From Dong, S., Li, L., Ashour, A., Dong, X., & Han, B. (2021). Self-assembled 0D/2D nano carbon materials engineered smart and multifunctional cement-based composites. Construction and Building Materials, 272, 121632. https://doi.org/10.1016/j. conbuildmat.2020.121632.

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related to compressive loading was greatly reduced between 0.3 and 0.7 wt.% concentrations (Fig. 3.7). However, Al-Dahawi et al. (2016) reported the threshold level of MWCNT in ECC1.2 at 0.55 wt.%, and this percentage was shown effective in increasing the self-sensing under compressive loading. These authors also investigated the effect of the type of CBNMs on the threshold level of ECC and found it at 1, 0.55, 2, and 2 wt.% for CF, MWCNT, GNP, and NCB, respectively (Fig. 3.8). Although the sensing ability and related percolation threshold are related to the concentration of conductive nanoproducts, it is important to consider other mixing and mixture conditions when looking at an optimized piezoresistive capability of cementitious composites.

3.3.3 Dispersion The significant effect of CBNMs distribution on the threshold level and piezoresistivity of composite matrices is well documented in the literature and has been identified as the major challenge in developing self-sensing nano-tailored pastes, mortars, and concretes. Many authors have directed their research to finding the best way to disperse the nano-conductive particles more homogeneously, since adding them directly during mixing can generate agglomerations and lower the number of nano-cores and related conductive paths. Data indicate very high optimization in concentration and self-sensing properties when CBNMs are effectively dispersed in the matrix. (Coppola et al., 2011) specified clear differences in the minimum dosage of CNT needed for self-sensing capability; a better method of dispersion caused the cement composite with 0.1% CNT concentration to behave with the same sensing level as that with 1% dosage. The ability of CBNMs to be dispersed more easily

Figure 3.7 Electrical resistivity versus MWCNT concentrations in ECC1.2 (Liu et al., 2019). The threshold of MWCNT-ECC1.2 was reached at 0.3 wt.%. MWCNT, Multiwalled carbon nano-tubes. Source: Modified from Liu, C., Liu, G., Ge, Z., Guan, Y., Cui, Z., & Zhou, J. (2019). Mechanical and Self-Sensing Properties of Multiwalled Carbon Nanotube-Reinforced ECCs. Advances in Materials Science and Engineering, 2019. https://doi.org/10.1155/2019/2646012.

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Figure 3.8 Electrical resistivity versus MWCNT, GNP, and NCB contents in ECC1.2. The threshold level of MWCNT-ECC1.2 is reached at 0.55 wt.%. GNP, Graphene nanoplatelet; MWCNT, multiwalled carbon nano-tubes; NCB, nano carbon black.

varies according to their aspect ratio, surface structure, and entanglement density (Parveen et al., 2013). Generally, the morphology of 0D NCB and its lower surface area allows better and easier dispersion than 1D CNT and CNF, and 2D GNP presents even harder homogeneous dispersibility (Dong et al., 2021; Korayem et al., 2017). Also, MWCNTs and MWGNP have been suggested rather than SWCNT and SWGNP when considering their higher dispersibility aptitude (Li et al., 2021). Although different mechanical, physical, and chemical methods were investigated for CBNMs dispersion (Parveen et al., 2013; Wang & Aslani, 2019), the most common approach relies on their optimized dispersity in mixing water using a combination of chemical and physical techniques, or just one of them, and then pouring the prepared solution into the premixed solid materials. The use of an appropriate

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noncovalent surfactant such as superplasticizer (SP) (polycarboxylate or naphthalene types) has proven sufficient in the case of 0D CBNMs (Deng & Li, 2018; Korayem et al., 2017). For 1D and 2D CBNMs, ultrasonication is required with a noncovalent surfactant such as polycarboxylate-based (P-SP), sodium dodecyl sulfate (SDS), and Triton X100, or with a covalent surface modifier by sulfuric and nitric acids (Li et al., 2018). The lower tendency of GNPs to be dispersed in water is another issue for these 2D nanomaterials, since they continue to show a complicated dispersion character even with the combined actions of ultrasonication and surfactants (Korayem et al., 2017). Yu and Kwon (2009) compared the efficiency of combining ultrasonication with two different chemical methods, which involved acid treatment and noncovalent SDS solution. They stated that acid-treated CNTs were able to develop stronger self-sensing properties compared to noncovalent surfactants, which likely hindered the interactions between CNTs. However, the covalent-treated CNTs were also presented to show lower resistivity reduction and higher deterioration on the mechanical performance of cement matrices (Liu et al., 2019; Parveen et al., 2013). According to Adresi et al. (2016), using ultrasonication with a proper amount of P-SP or P-SP with SDS (1/9 ratio respectively) can solve the dispersion issues of CNTs. P-SP was also confirmed as one of the best surfactants with 0D, 1D, and 2D CBNMs since it can result in the double-dispersion effect of cementitious and conductive materials (Han et al., 2020a; Li et al., 2021; Wang & Aslani, 2019), while its incorporation as a water-reducing agent is typical in most concretes. Applying P-SP in CNT and GNP-based ECC-like mortars was shown by Al-Dahawi et al. (2016) to not even necessitate an ultrasonication process but only a high-speed shear mixing at 3000 rpm (method 8 in Fig. 3.9). CBNMs were also suggested to reaggregate when ultrasonication was applied. The same dis¨ ztu¨rk et al., persion method with P-SP and high shear speed was also utilized by (O 2020; Siad et al., 2018; Yıldırım et al., 2020) and revealed to generate a convenient sensing level at different curing ages of ECC. Nevertheless, the possible presence of 0D nano-and-micro silica particles and fly ash in ECC compositions was likely the reason for the good dispersity of CNT and GNP without the need for ultrasonication. This was also confirmed by some recent investigations where the combined use of 0D nanomaterials such as nano-silica, nano-clay, and silica fume with 1D CNT or CNF and 2D GNP were tested to increase the homogeneous dispersion of CBNMs and consequently the self-sensing of composites (Kim, Park, et al., 2014; ¨ ztu¨rk et al., 2020). The hybrid use of CBNMs looks more Morsy et al., 2011; O promising in terms of dispersibility and enhanced self-sensing capacity. Indeed, Zhang et al. (2018, 2017) obtained an increased conductivity cement-based multifunctional material by assembling 0D/1D NCB/CNT at 40%/60% ratio without involving any dispersion method other than using P-SP. Furthermore, 0D/2D NCB/ GNP were verified in different cement composites to require only a normal mixing speed when casted with a polycarboxylate SP in order to attain optimized dispersion efficiency and piezoresistive response (Abedi et al., 2020b; Da¸s et al., 2019; Qiu et al., 2019). However, a minimum ultrasonication time of 20 minutes was required with P-SP for an effective dispersity potential of hybrid 1D/2D CNT/GNP or CNF/ GNP nanomaterials (Gao et al., 2019; Noiseux-Lauze and Akhras, 2013).

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Figure 3.9 Influence of mixing methods on electrical resistivity (ER) of (A) CNT and (B) GNPmortars; Ref: without conductive materials; (1) mixing with water alone; (2) water 1 ultrasonication; (3) water 1 P-SP 1 ultrasonication; (4) water 1 nano-silica 1 ultrasonication; (5) nano-calcite 1 water; (6) water 1 P-SP 1 ultrasonication 1 methylcellulose agent 1 high mixing speed; (7) silica fume 1 water; (8) water 1 P-SP 1 high mixing speed. Applied P-SP in CNT and GNP-based ECC in method 8 showed higher ER results. CNT, Carbon nanotube; GNP, graphene nanoplatelet.

3.3.4 Cementitious matrix properties The conductivity and self-sensing response of nano-designed cementitious composites are also dependent on the network properties around CBNMs, including cement matrix and pore conditions. When diverse W/B ratios are involved, different

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CBNMs dispersions, percolation thresholds and self-sensing levels can be obtained. Kim et al., (2014) have considered the resistivity and piezoresistive response of cement mortars prepared by 0.3% MWCT and W/B ratios (binder composed of cement and silica fume) of 0.4, 0.5, and 0.6. They showed that CNTs dispersion and mortar conductivity and piezoresistivity improved by reducing the W/B ratio. Although the increased dispersion of CNTs can benefit the conductive network, this looks to be more associated with the presence of a suitable content of silica fume in their compositions, since this is known for its positive effect on the dispersity of CNTs (Kim et al., 2014). In addition, at the same SP dosage, the decreased W/B ratio may negatively influence the fluidity of cementitious materials; thus CNTs can re-agglomerate during the mixing process. This is supported by the results of Han et al. (2012), who revealed that the conductivity and self-sensing levels of 0.1% CNT-based cementitious pastes improved with increased water-to-cement ratios from 0.45 to 0.6, regardless of the same amount and type of surfactant. They described the increased dispersion level of CNT as one of the main outcomes of the ¨ ztu¨rk et al., 2020; Wang & enhanced W/B ratio. The results of other studies (O Aslani, 2019) also supported the relation between fluidity and dispersion of CNFs in cement pastes, while these authors suggested that only a proper increase of W/B or SP can positively influence the dispersion of CNFs, as an excessive W/B ratio or SP contents can cause floating CNFs and lower homogeneous dispersion and conductivity. Hence the average value of W/B ratio must be correlated to the optimized fluidity to reach better dispersion of CBNMs, which can also be controlled by an adjusted dosage of SP. Enhanced volume of aggregates yields lower conductivity for the same matrix materials and CBNMs concentration because of the nonconductive character of ordinary sands and gravels. Accordingly, their presence in mortars and concretes can undesirably affect the self-sensing of cementitious matrices (Tian et al., 2019). For example, the average dosage of MWCNT to reach percolation threshold was shown to increase from cement paste (around 0.1 wt.%) to mortar (0.4 wt.%) (Han et al., 2010, 2012). Also, the self-sensing level of the repeated compressive loading reduced when sand was introduced to the cement paste, regardless of MWCNT content. However, this was still proved highly dependent on the quality of nanomaterials dispersion as the percolation threshold of the same mixtures of cement paste, mortar and concrete was identified at 1 wt.% of MWCNTs by D’Alessandro et al. (2016), though the strain sensitivity was confirmed to reduce from concrete, to mortar to paste matrices with gauge factors of 130, 68, and 23 respectively. Fig. 3.10 shows reduced signal noise and increased sensing linearity from CNT-concrete to CNT mortar to CNT paste under cyclic compressive loading. The effect of aggregates can be different if they are conductive or semiconductive. Wang et al. (2018) found that replacing river sand with fine quartz sand in CNFs-based mortars leads the CNF content to reduce by a factor of three, from 1.5% to 0.5% CNFs, to achieve percolation threshold. In their study cement paste showed the same percolation as that of mortar prepared with river sand (1.5% CNFs). In addition, the gauge factors linked to the strain sensing capacity vastly increased from paste (38.1) to

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Figure 3.10 Piezoresistive response to cyclic compressive loading of CNT-based concrete, mortar, and paste. Reduced signal noise and increased sensing linearity from CNT concrete to CNT mortar to CNT paste. CNT, Carbon nanotube. Source: From D’Alessandro, A., Rallini, M., Ubertini, F., Materazzi, A. L., & Kenny, J. M. (2016). Investigations on scalable fabrication procedures for self-sensing carbon nanotube cement-matrix composites for SHM applications. Cement and Concrete Composites, 65, 200
213. https://doi.org/10.1016/j.cemconcomp.2015.11.001.

river sand mortar (183) to quartz sand mortar (476), though there were higher CNFs concentrations in paste and river sand mortar than in quartz sand mortar (Fig. 3.11). As discussed, the presence of some mineral admixtures was shown to help the dispersion of CBNMs and to further decrease the resistivity of cement composites. For instance, when silica fume was added to GNP mortar, the self-sensing properties were improved (Bai et al., 2018; Ozbulut et al., 2018). Also, in ECC compositions, (Hardy et al., 2016) stated that increasing fly ash to 2.8 by cement content achieved appropriate piezoresistivity even without adding CBNMs. The incorporation of a maximum portion of 0.333% CNFs increased the sensing ability of

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Figure 3.11 Strain sensing of mortars CNF-reactive powder concrete (RPC) with reactive quartz sand, carbon nanofiber mortar (CNFM) with river sand and carbon nanofiber paste (CNFP). FCR vastly increased from paste (CNFP) to river sand mortar (CNFM) to quartz sand mortar (CNFRPC). CNF, Carbon nano-fibers; FCR, fractional change of resistivity. Source: Modified from Wang, H., Gao, X., Liu, J., Ren, M., & Lu, A. (2018). Multifunctional properties of carbon nanofiber reinforced reactive powder concrete. Construction and Building Materials, 187, 699
707. https://doi.org/10.1016/j.conbuildmat.2018.07.229.

ECC2.8 by providing greater gauge factor correlation. Whereas the conductivity remarkably enhanced when incorporating slag in CNFs-composites due to its increased hydration effect, it displayed disadvantageous effects on the dispersion of CNFs and piezoresistive properties of cement pastes (Wang et al., 2017). Different than nonreinforced cementitious composites, FRCs are embedded with various types of conductive and nonconductive fibers such as steel, carbon, nickel, polyvinyl, polypropylene, nylon, etc. Using carbon and steel fibers was sufficient in increasing the self-sensing performance in FRCs, though the hybrid incorporation of CBNMs and CF can advance the piezoresistive capability at lower concentration levels of CBNMs. In addition, more stable sensing responses were obtained when combining conductive fibers and CBNMs, especially for UHPC concrete and cyclic mechanical loadings (Lee et al., 2017). The presence of PVA fibers in ECC and its appropriate chemical composition was favorable to self-sense the deformation and cracking even without the inclusion of conductive elements, though at reduced levels compared to the piezoresistive response of CBNMs-based ECCs (Al-Dahawi et al., 2017). However, this was not shown to decrease the required concentration to reach the threshold level (Al-Dahawi et al., 2016). The interconnectivity of the conductive pathways is also based on the development of hydration products with advanced curing ages, which are known to enhance

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the electrical resistivity of composites. Al-Dahawi et al. (2016), Yıldırım et al. (2020) considered the effect of curing ages of 7, 28, 90 and 180 days on the selfsensing behavior of CF-, CNT-, GNP- and NCB-ECCs. Although resistivity increased due to the developed hydration products, percolation threshold was independent of these increments because it relies mostly on the conductive constituents existed at different ages. The authors also reported that specimens tested at 7 days exhibited slightly higher self-sensing to compressive loading compared to those of 28 days, especially for NCB-ECC. However, when compared to greater ages, piezoresistive responses to compressive, splitting and flexural stresses were more accurate at 7, 28, and 90 days than 180 days and the superior behavior of MWCNTs compared to NCB was unchanged with curing age. A different finding regarding the clearer self-sensing behavior between 7 and 28 days was identified by (Galao et al., 2014) who studied the piezoresistivity of CNF-based cement pastes at 7, 14 and 28 days. They noticed that optimized sensing was 28 days compared to 7 and 14 days. This suggests a curing age of 28 days as the best option for self-sensing measurements of CBNMs-composites, particularly when considering the lower stability matrix structure and conductivity at 7 and 90 days, respectively.

3.3.5 Surrounding conditions Surrounding temperature and humidity conditions influence the conductivity and self-sensing ability of CBNMs cementitious composites due to their direct relation to the moisture content and ion movement across the matrix. However, this is also related to the concentration of CBNMs, as the effect is assumed to reduce with increased CBNMs content until being negligible when the percolation threshold is reached or exceeded (Gao et al., 2015). Below the percolation threshold, reduced humidity can cause the resistivity and sensitivity to increase to a certain level of moisture content. For instance, Song and Choi (2017) revealed that both resistivity and self-sensing increased with reduced humidity from 85% to 70%, 65% and 55% of MWCNT mortar specimens. Nevertheless, Han et al. (2010) registered a lower conductivity and piezoresistive response at a relative humidity of 30% and moisture content of 0.1% of MWCNT composites. Increased humidity is found to generate different impacts according to the type of CBNMs. As the electronic conduction from CBNMs dominates the total conductivity of matrix compared to the ionic conduction from moisture, the absorbent nature of NCB particles may result in decreased conductivity with increased humidity and moisture content, despite their positive effect on enhancing the ionic movement. Unlike NCB, the adsorbent texture of CNT, CNF, and GNP can preserve the existing electronic network. Thus the extra ionic pathways from enhanced humidity and moisture can increase the conductivity up to the saturation level of cementitious materials, to the point they become almost insensitive to loading (Han et al., 2010). The higher humidity and moisture content were also reported to have the ability to generate electrode polarization that negatively affects the repeatability of results. The synergetic effect of electronic and ion conduction on the piezoresistive response of CBNMs composites

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is expected to be at a relative humidity of 55%
65% when an ambient temperature of 23 C is applied (Lee et al., 2020; Suchorzewski et al., 2020; Yoo et al., 2017). The effect of temperature is also dependent on the threshold concentration of CBNMs. Around threshold levels, the electron mobility under thermal effect can be increased, leading to enhanced conductivity and sensing ability up to certain degrees of temperature. Commonly, an increased oven drying in the range of 20 C
60 C resulted in greater self-sensing responses of CBNMs-composites even if testing is conducted after air cooling, because of the reduced polarization caused by high temperature exposure. del Moral et al. (2021) recently investigated the effect of oven drying between 0 C and 60 C on the resistivity and self-sensing of cement pastes incorporated hybrid CNT and graphite powders. The high temperature caused only slight increments of electrical resistivity, while the sensing strain capacity significantly increased, particularly between 40 C and 60 C where the gauge factor was enhanced by 2.6 times. They highlighted the appropriate water evaporation as the reason for the enhanced balance between electron mobility and strain sensing ability. In another work (Hong et al., 2018), the pressure sensitivity of cement pastes incorporated mixed CF/CNT/NCB was best at 20 C and 40 C temperatures, becoming slightly lower at 60 C and 80 C. However, different results were reported for NCB composites, in which Dong et al. (2019) revealed independent sensitivity and repeatability responses from temperature changes between 220 C and 100 C. In addition to controlling temperature and humidity, the polarization effect can be reduced by using more suitable techniques for measuring the piezoresistive properties, such as utilizing alternating current (AC) rather than direct current (DC), a four-probe method rather than two probes, and an average amplitude voltage in the range of 20
30 V (Coppola et al., 2013; Dong et al., 2019; Konsta-Gdoutos and Aza, 2014), though these also associated with the type and contents of CBNMs and local testing conditions. It is worth noting that the above-mentioned parameters highly interact with each other and should be considered to control the self-sensing of nano-tailored composites, especially when enhanced accuracy is desired.

3.4

Use of nanomaterials in self-sensing cementitious composites

3.4.1 Sensing of deformation and cracking under mechanical loading One of the areas of significant interest for the incorporation of CBNMs in cementitious materials is related to the self-sensing of deformation under the effect of normal environment or mechanical loading. The majority of studies about the piezoresistive properties of nano-tailored composites have been published regarding the monotonic and cyclic loading under compression, flexion and tension. Although a strong relationship was established between ER measurements and stress changes,

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higher correlation was confirmed after using fractional change of resistivity (FCR), especially under recurrent loading. Gauge factor is typically utilized to describe the quantitative strain sensitivity that has a direct connection to FCR. Under monotonic actions, when in the elastic region, the internal deformation causes interruptions in the conductive network, which can be sensed by reductions or increments in resistivity due to compression and tension respectively; then ER sharply increased in the plastic region, and up to failure. Under cyclic loading, the varied stresses are usually shown as cyclic decrements and increments in the FCR. The distance between nanoparticles begins declining or increasing under compressive or tensile loading, resulting in decreased or increased FCR, respectively. The reoccurrence of the conductive pathways after unloading is presented to generate reversible sensing of the FCR changes up to 30% of the maximum strength under compressive loading, and up to reaching the plastic range under tensile loading (Sasmal et al., 2017; Tian et al., 2019). The sensing of FCR to the related damage turns gradually to irreversible with enhanced cyclic loading, to be completely reversible at around 75% of maximum strength under compression and starting from the plastic region under tension (Han et al., 2015b). CNT, in single and hybrid applications, was the most used CBNMs for stress/strain sensing because of the consensus about its outstanding self-sensing behavior in relation to compressive and tensile loadings compared to other single CBNMs (Jiang et al., 2018). Yoo et al. (2018c) reported positive self-sensing responses for all MWCNT-, CNF-, NCB-, and GNP-incorporated cement pastes at 2 wt.% and exposed to the effect of compressive loading cycles, with MWCNT-paste gauge factor 7, 12, and 16 times higher than that of CNF, NCB, and GNP, respectively (Fig. 3.12). However, the hybrid incorporation of CNT/GNP, CNT/NCB, CNT/GNP, and CNT/CF in various cementitious composites presented with much better piezoresistive properties of sensing, repeatability and

Figure 3.12 Self-sensing of strain under compressive loading of (A) MWCNT-sample (B) CNF- sample (C) NCB-sample and (D) GNP-sample. Higher self-sensing of strain under compressive loading of MWCNT sample. CNF, Carbon nano-fibers; GNP, graphene nanoplatelet; MWCNT, multiwalled carbon nano-tubes; NCB, nano carbon black. Source: From Pisello, A. L., D’Alessandro, A., Sambuco, S., Rallini, M., Ubertini, F., Asdrubali, F., Materazzi, A. L., & Cotana, F. (2017). Multipurpose experimental characterization of smart nanocomposite cement-based materials for thermal-energy efficiency and strain-sensing capability. Solar Energy Materials and Solar Cells, 161, 77
88. https://doi.org/10.1016/j.solmat.2016.11.030.

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FCR data stability to cyclic compressive, and tensile loading (Abedi et al., 2020a,b; Dong et al., 2021; Zhang et al., 2018). Abedi et al. (2020a,b) explained that 50% CNT 1 50% GNP at 0.7 wt.% mortars reached cyclic stress sensing with a gauge factor of 621, compared to 240 for single CNT at 0.15 vol.% content and 110 for mono GNP-mortars at 1.5 vol.%. Also, assembled NCB/GNP at 4.5 wt.% was presented by Dong et al. (2021) to increase the strain sensitivity of cementitious composites by more than 630% compared to that without nano-modification. Han et al. (2020b) have examined the sensing of monotonic and cyclic compressive loading of nano-tailored carbon fiber reinforced concrete (CFRC) incorporating hybrid CF (0.4%) and CNF (0.2
0.6 wt.%), NCB (0.4
0.8 wt.%) and steel slag powder (SS) (5
20 wt.%). They found that the addition of 0.4% and 0.6% CNF to CFRC at dry environment was more significant in increasing the sensing ability of CFRC under monotonic and repeated actions, mostly because of the larger number of conductive pathways generated by CNF/CF than NCB/CF and SS/CF. Also, the strain sensitive coefficient of CFRC increased by two times when adding 0.6% CNF, while higher concentrations than 0.8% were suggested for NCB to develop better sensing compared to normal CFRC. The use of CBNMs to increase the piezoresistive performance of ECCs have been subject to a number of investigations, particularly for their greater deformability and multi-functional characteristics. CNT, GNP, and NCB have been shown to significantly improve the self-sensing performance to monotonic compressive and tensile loadings of ECCs, even at long curing ages. Although FCR results were lower than CF-ECCs, reversibility in self-sensing was more effective in CNT- and NCB- than CF-ECCs (Al-Dahawi et al., 2016; Yıldırım et al., 2020). Huang et al. (2018) revealed that the addition of 1 wt.% carbon black with a surface area like nanomaterials was found to cause significant fluctuation in FCR under the uniaxial tension test. However, when NCB was introduced at 5 vol.% in ECC mixtures with high volume fly ash, it displayed significant improvements in the signal-to-noise ratios, with gauge factors increased from 52 6 19 and 84 6 12 for reference ECC to 247 6 24 and 344 6 31 for 5% NCB-ECC during the elastic stage of cyclic tension and compression (Li & Li, 2019). Yang and Qian (2020) investigated the direct addition and hydrophobic coating of 5% CNT by weight of polyethylene fiber in ECC and found that the gauge factor improved by more than 5 and 2.5 times, respectively, under tensile compared to control ECC. The effect of CBNMs on the self-sensing ability to compressive and tensile loading of UHPC and ultra-highperformance fiber-reinforced concrete (UHPFRC) was examined in a relatively limited number of papers. The piezoresistive response to compressive and tensile loadings of UHPC containing 0.5 vol.% CNTs and that of UHPFRC with combined 0.5 vol.% CNT and 2 vol.% steel fibers was considered by Yoo et al. (2018a) and Lee et al., (2018). CNT-UHPC and CNT-UHPFRC were not sensitive to the compressive stress and strain FCR measurement, despite their appropriate sensing of peak strength and concrete failure. This was explained by the high density of UHPC and UHPFRC that delayed the development of microcracks up to the near peak compressive strength. Insignificant fluctuations in FCR and better sensing of strain in pre- and postpeak tensile zones were registered in UHPFRC with CNT and

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steel fiber than in CNT-UHPC, which showed high signal noise in FCR. The feeble sensing to compression of UHPFRC was also supported by the results of Jung et al. (2020), who enclosed 2 vol.% steel fiber and 1.2 wt.% CNTs. Nevertheless, in other research (You et al., 2017), the self-sensing to monotonic compressive stress and strain of UHPFRC with 3% steel fiber imparted remarkable increments with enlarged CNT contents from 0.1% to 0.5%, as the maximum FCR increased by more than eight times between 0.1% and 0.5% CNTs. Thus, using balanced ratios of steel fiber and CNT is maybe the most important factor in reaching optimized self-sensing of FCR changes in CNT-UHPFRC. In addition, the value of FCR in CNT-UHPFRC changed dramatically after reaching the ultimate compressive, revealing outstanding sensing ability to cracking because of the collapse of the conductive network due to disconnected CNTs after failure, as shown in Fig. 3.13. The GF of cracking sensing increased from 113.3 in UHPFRC to 3610.7 for CNTUHPFRC (31.9 times). Sensing of deformation under flexural loading is based on the location of sensors if they are in the compression or tension areas or both of them if they are imbedded through the concrete specimens or on their sidewalls. Although the tension part was suggested by some authors to be with more effective piezoresistive response under flexion (Huang et al., 2018), other researchers showed that installing electrodes around the beam surfaces or inserting them inside the specimens would result in better sensing of the initial cracking (Jung et al., 2020; Naeem et al., 2017).

Figure 3.13 Stress, strain, and FCR changes with time of compressive loading for ultrahigh-performance fiber-reinforced concrete (UHPFRC) with CNT contents of (A) 0.1%, (B) 0.3% and (C) 0.5%. The self-sensing of UHPFRC with 3% steel fiber showed remarkable increments with increased CNT contents from 0.1% to 0.5%. CNT, Carbon nanotube; FCR, fractional change of resistivity. Source: Modified from You, I., Yoo, D. Y., Kim, S., Kim, M. J., & Zi, G. (2017). Electrical and self-sensing properties of ultra-high-performance fiber-reinforced concrete with carbon nanotubes. Sensors (Switzerland), 17(11). https://doi.org/10.3390/s17112481.

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The effect of 1% CF, 0.55% CNT, 2% GNP and 2% NCB on the self-sensing of repeated flexural loading-unloading of ECC compositions was considered by AlDahawi et al. (2017). They used imbedded electrodes inside the tensile region and presented its portion close to the support points as the best tensile region to better sense the flexural stress change. Unlike the low reversibility in FCR noticed for all ECCs in the elastic range (up to 30% of the maximum load), high self-sensing of flexural loading-unloading was recorded for CBNMs-ECCs in the plastic range up to 70% of the maximum load. CNT-ECC exhibited higher piezoresistive response under repeated flexural loading-unloading, with FCR in the first plastic cycle of 43% compared to 35% for GNP-ECC and 5% for NCB-ECC. The weak selfsensing of NCB-ECC to flexural stress and cracking was also identified by a number of researchers (Liew et al., 2016; Shi et al., 2017, 2019), which showed even lower piezoresistive response than micro steel fiber in ECC. However, NCB-ECC behaved better in sensing the tiny deformation in the elastic region under flexural loading (Naeem et al., 2017). Siad et al. (2018) confirmed an important increase in the self-sensing efficiency of cracking under flexural loading when including CNT at 0.25% and 0.5% in ECC and measuring the surface ER. Fig. 3.14, which presents the FCR change at different CF and CNT contents unveiled more than 4 and 3 times FCR at partially saturated and dry states for 0.5% CNT-ECC than 0.5% CFECC and control mixture. In contrast, the FCR changes to shear failure under static flexural loading were at least three times higher in CF- than in CNT-based reinforced mortar beams (Yıldırım et al., 2018). Therefore it is important to consider the probable failure modes of each type of concrete in order to choose the best carbon materials under flexural shear. The hybrid CNT and steel fibers in UHPC were studied under flexural loading with imbedded plates around support points in

Figure 3.14 FCR changes after cracking of ECC with carbon fiber (CF) and CNT. Increased FCR after cracking when using CNT at 025% and 0.5%. CNT, Carbon nanotube; ECC, engineering cementitious composites; FCR, fractional change of resistivity. Source: From Siad, H., Lachemi, M., Sahmaran, M., Mesbah, H. A., & Hossain, K. A. (2018). Advanced engineered cementitious composites with combined self-sensing and selfhealing functionalities. Construction and Building Materials, 176, 313
322. https://doi.org/ 10.1016/j.conbuildmat.2018.05.026.

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(Jung et al., 2020) and (You et al., 2017). According to You et al. (2017), the flexural behavior of UHPC was accurately simulated when including 2 vol.% steel fiber and 0.3 wt.% CNT, even at the elastic and hardening regions before peak point of cracking. The results of Jung et al. (2020) validated the increased sensing ability to flexural cracking of CNT-UHPFRC regardless of the method of curing, though this was possible only in the hardening and softening branches (Fig. 3.15). This highlights again the importance of the threshold amount of CNT, as the high CNT content of 1.2 wt.% has reduced the separation of conductive materials under flexural deformation, leading to insignificant sensing of cracking at the elastic range. The superior structural and durability properties of CBNMs-based ECCs and UHPCs and their accurate sensing of deformation and cracking can make these new

Figure 3.15 Flexural stress and FCR vs. time for (A) ultra-high-performance fiberreinforced concrete (UHPFRC), (B) CNT-UHPFRC cured at 90 C and (C) CNT-UHPFRC cured at low DC voltage. Increased sensing in the hardening and softening branches of CNTUHPFRC. CNT, Carbon nanotube; DC, direct current; FCR, fractional change of resistivity. Source: From Jung, M., Park, J., Hong, S. G., & Moon, J. (2020). Electrically cured ultrahigh-performance concrete (UHPC) embedded with carbon nanotubes for field casting and crack sensing. Materials & Design, 196, 109127.

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generations of concretes more suitable for structural health monitoring than other cementitious materials.

3.4.2 Sensing of dynamic actions for traffic monitoring Although some of the pressure-sensitive CBNMs tested for static stress/strain sensing are stated to be valid for traffic monitoring, timely sensing of varying loads under traffic flow requires more effective responses to dynamic and varied loading amplitudes and depends on the linearity and reversibility of sensing. Also, weight and speed of vehicular loads are important parameters to consider for an accurate cementitious composite for traffic detection and assessment in a timely manner. For this reason, self-sensing under traffic-like vehicular loading is gradually being involved in verifying the effectiveness of the piezoresistive responses. Unlike for structural health assessment where CFs presented with even better sensing ability than mono CBNMs composites in some investigations (Yıldırım et al., 2018), CFsconcretes were rarely used for traffic monitoring due to their irreversible response under large compressive strains. Thus, CBNMs were the main conductive ingredient introduced in cementitious materials for traffic monitoring purposes, with particular interest in CNT and CNB composites. Experiments in the lab included the piezoresistive sensing to repeated and impulsive compressive cyclic loadings under diverse amplitudes. Options for incorporating CBNMs in concrete pavement, in some locations of the concrete pavement or introducing them in the surface layer of concrete pavement were considered for real traffic sites. Han et al. (2013) showed a good relationship between the electrical resistance variation and the repeated and impulsive compressive loadings when testing 0.1% CNT cement paste samples in the laboratory. They also integrated CNT paste, mortar, and concrete specimens into a low volume traffic roadway to sense the real site vehicular loading and concluded that the CNT composites can effectively and repeatedly detect the vehicle movements and weight-in motion, especially for low weight vehicles like cars and vans (Han et al., 2013; Yu, 2012). However, the signal amplitude was more qualitative and was not reflecting the exact vehicle mass, since various amplitudes were obtained for a similar car (Fig. 3.16). Materazzi et al. (2013) studied the dynamic sensing of compressive strain of paste samples incorporated 2% CNT. They demonstrated a linear relationship between the dynamic loading at different frequencies and the sinusoidal change of the electrical resistance. When increasing amplitude loadings were applied on paste, mortar and concrete with the same amount of 1% CNT, the sensing ability was reduced from CNT-paste to mortar and concrete (Materazzi et al., 2013). D’Alessandro et al. (2016) explained that aggregates negatively influenced the interaction between CNT particles leading to reduced piezoresistive response during loading. The good dynamic sensing performance for static and random vibration was also confirmed by using a cement paste beam incorporated 1% CNT content, as presented in Fig. 3.17, and concrete beam integrated 2% CNT-based cement paste at its top surface (D’Alessandro et al., 2017). The linearity between loading and sinusoidal sensing increased with increased frequencies. Monteiro et al. (2020) tested the dynamic sensing properties of NCB mortars

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Figure 3.16 Sensing of passing cars in a real-site road test. The signal amplitudes changes with car passing without reflecting the vehicle weight. Source: From Yu, X. (2012). Intelligent Pavement for Traffic Flow Detection
Phase II.

Figure 3.17 Piezoresistive response to dynamic loads by vibration of CNT-paste beam. Good dynamic sensing for static and random vibration when using a 1% CNT-paste beam. CNT, Carbon nanotube. Source: From D’Alessandro, A., Ubertini, F., Garcı´a-Macı´as, E., Castro-Triguero, R., Downey, A., Laflamme, S., Meoni, A., & Materazzi, A. L. (2017). Static and dynamic strain monitoring of reinforced concrete components through embedded carbon nanotube cementbased sensors. Shock and Vibration, 2017. https://doi.org/10.1155/2017/3648403.

(6.5% NCB concentration) under various amplitude levels and orders to simulate the traffic-like dynamic action. The natural condition factor was also considered by investigating the effect of temperatures between 25 C and 45 C. NCB mortar was shown with high sensing linearity and reversibility to all applied constant, including varied order load cycles. In addition, a reduced gauge factor of sensing was noticed with increased temperature conditions, though linearity was not disrupted. The utilization of CBNMs-ECC and CBNMs-UHPC for dynamic, vibration sensing or real traffic monitoring was not reported in the literature, though the great technological and economical advantages that can be gained from combining

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multiple structural and multi-functional properties in ECC and UHPC increasingly employed in bridges and tunnels.

3.4.3 Special self-sensing applications The significant effect of CBNMs on improving the piezoresistive sensing to different types of external loadings and internal damages led the researchers to explore more advanced applications for multi-function cementitious composites. Other important applications were the monitoring of environmental conditions, such as temperature and humidity variations, and durability properties after exposure to cyclic freezing/thawing or chemical chloride attack (Ding et al., 2015; Du et al., 2020; Kim, 2015). For instance, the inclusion of hybrid CNT/CNF at 1.5%/0.4% in cement pastes caused their temperature sensitivity coefficients to increase by four times at the range of 20 C
80 C (Du et al., 2020). Also, when tested for temperature changes between 220 C and 50 C, concretes prepared with hybrid CF and NCB or CNF exhibited around two times higher coefficient of sensitivity compared to that without nanomaterials (Fig. 3.18). Ding et al. (2015) found that the hybrid incorporation of various concentrations of CF, NCB, and steel fiber in concrete beams generated correlation coefficients of 0.977
0.994 between the measured FCR and mass loss resulted from freeze
thaw sequences. A pioneering study (Siad et al., 2018) was conducted to synthesize a new class of ECC with tensile ductility, self-healing and self-sensing functionalities. Through adjusting the composition of ECC at nanoscale level by adding CNT at 0.25 and 0.5 wt.% contents, it was possible to achieve CTN-ECCs with greater cracking performance and stronger self-healing and self-sensing abilities of cracking. A more attractive capacity of the developed CNT-ECCs was the self-sensing of self-healing progress, which also proved to be highly accurate when correlated to the mechanical recovery. Fig. 3.19 shows very small errors in the correlation coefficients between the recovery of flexural strengths and ERs, with an average R-square coefficient of around 99% for both CNT-ECCs. The multi-functional characteristics of CNT-ECCs were further tailored by the hybrid incorporation of nano-silica with ¨ ztu¨rk et al., 2020), in which nano-silica caused the mechanical and selfCNT (O healing properties of CNT-ECC to improve while reaching appropriate sensing to cracking.

3.5

Perspectives and conclusions

It is clear that the inclusion of CBNMs such as CNT, CNF, NCB, and GNP in cementitious composites has tremendous advantages in enhancing their self-sensing performance regarding different types of deformation/damage from internal and external sources. Although their optimized piezoresistive ability requires more interacted design elements than other conductive powders and fibers with enhanced surface areas, reaching a threshold level based on a specific concentration of each

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Figure 3.18 Coefficient of resistivity variations based on temperature changes of carbon fiber (CF) concretes with CNF, NCB, and steel slag powder. Hybrid CF and NCB or CNF showed higher coefficient of sensitivity to temperature changes. CNF, Carbon nano-fibers, NCB, nano carbon black.

CBNMs can certainly ease the tailoring of the other parameters. This is also dependent on the efficient dispersion of the conductive nanoparticles during mixing, which still looks complicated in most cases since it necessitates sonication with the existing SPs. It is therefore essential to spend more effort to develop other, easier techniques of dispersion, especially for mono or binary 1D and 2D CBNMs, so that they can be incorporated in a more practical way in laboratories and construction sites. Such new processes would use the ordinary mixing method by involving new multieffect SPs, while avoiding the intricacy of surface treatment and sonication. The hybrid uses of 0D/1D and 0D/2D CBNMs is also promising in terms of increased dispersibility, with the possibility to eliminate the extra mixing process from the normal method. CNTs were the best mono nanomaterials used up to now for piezoresistive cementitious composites, 0D/1D, 0D/2D, and 1D/2D CBNMs

Recovery in Flexural Strength (%)

Nano-tailored cementitious composites with self-sensing capability

R² = 0.9941

40

131

R²² = 0.986

30

R² = 0.374

20

ECC--Control 0.25% %CNT 0.5% %CNT 0.5% %CF 1%CF F

R² = 0.798 87

10

R² = 0.8892 0

0 0

30

60

90

120

1500 180 210 Recoveery of ER (%))

240

270

300

Figure 3.19 Recovery in flexural strengths vs. recovery in elecrical resistivity (ER) of ECC incorporated CF and CNT. High correlation between mechanical self-healing and ER changes. CNT, Carbon nanotube; ECC, engineering cementitious composites.

were verified to have lower threshold concentrations and higher self-sensing ability, repeatability and response accuracy to static and dynamic loadings. In addition, including CBNMs, particularly at hybrid form, in UHPC and ECC compositions can provide extra multi-functional properties in addition to self-sensing capability, such as ultra-high mechanical strengths, ductility and self-healing. This further advances the technological benefits from the nano-tailored cementitious composites and renders their multiple structural, durability and safety outcomes outstanding compared to normal conductive concretes. However, this also highlights the necessity for more research about the CBNMs-based UHPC and ECC, specifically for large-scale and real-site applications. Furthermore, new standards or more compatible measurable indices should be defined to determine the quality of dispersion of CBNMs and self-sensing performance, as there are no common qualitative or quantitative references for the good or poor indices.

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Nanomaterials in self-healing cementitious composites

4

Gerlinde Lefever1,2, Dimitrios G. Aggelis2, Nele De Belie1, Danny Van Hemelrijck2, and Didier Snoeck1,2,3 1 Magnel-Vandepitte Laboratory for Structural Engineering and Building Materials, Department of Structural Engineering and Building Materials, Faculty of Engineering and Architecture, Ghent University, Tech Lane Ghent Science Park, Ghent, Belgium, 2 Department of Mechanics of Materials and Constructions, Vrije Universiteit Brussel (VUB), Brussels, Belgium, 3BATir Department, Universite´ Libre de Bruxelles (ULB), 50 F.D. Roosevelt Ave., Brussels, Belgium

4.1

Introduction

Concrete, being the most-used construction material worldwide, contributes largely to the emission of greenhouse gases (Adesina, 2020). The cause of the excessive amount of CO2 generated lies in the production of Portland cement. During the calcination process, the raw ingredients are heated to a temperature of approximately 1450 C, leading to a chemical reaction that transforms limestone into CaO and CO2. Both the fuel combustion necessary to reach such high temperatures and the calcination reaction emit large amounts of carbon dioxide, which are estimated to be equal to the mass of Portland cement produced (Andrew, 2018). Additionally, concrete is known to have an excellent resistance against compressive loads, but a large cross-section, and thus large quantity, of concrete is necessary. On the other hand, concrete is highly sensitive to cracking, caused by its relatively low tensile strength. The occurrence of cracks increases the risk of deleterious substances penetrating into the material’s inner layers, thereby causing damage to the cementitious matrix and the possibly present reinforcement. The maintenance and repair of concrete structures are thus crucial to extend the construction’s service life but are costly and labor-intensive (Cailleux & Pollet, 2009). Moreover, most repairs only have a limited lifespan and require repeated restorations. It is clear that a solution should be found in order to obviate these complications, both from the ecological and the economical perspectives. Therefore the concept of self-healing of cementitious materials was introduced. The mechanisms of self-healing are divided into two main categories, namely, autogenous healing and autonomous healing. In case of autonomous healing, chemicals and/or other materials are introduced inside the cementitious matrix to heal cracks, whereas autogenous healing comprises the processes that utilize the inherent self-healing capability of the present cementitious matrix and its building blocks. Subdivisions of the latter category separates the Recent Advances in Nano-Tailored Multi-Functional Cementitious Composites. DOI: https://doi.org/10.1016/B978-0-323-85229-6.00013-5 © 2022 Elsevier Ltd. All rights reserved.

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mechanisms that are fully autogenous from the ones that are improved and/or promoted by the use of additives (De Belie et al., 2018). In this chapter, the focus is laid on the inclusion of nanomaterials to enhance the self-healing ability of cementitious composites. First, the mechanisms of autogenous and autonomous healing are briefly explained. Afterward, various nanoinclusions such as nano-clays, nano-silica, carbon nano-tubes (CNTs), and many other nano-sized additives and their contribution to both healing approaches are detailed.

4.2

Toward self-healing concrete

4.2.1 Autogenous healing Autogenous healing can be attributed to multiple processes (de Rooij et al., 2013; Snoeck & De Belie, 2015a, 2015b), as shown in Fig. 4.1. The most essential mechanisms are the hydration of unhydrated cement particles and/or pozzolanic activity by included supplementary cementitious materials, and the recrystallization and carbonation of Ca(OH)2 (Huang et al., 2016; Van Tittelboom & De Belie, 2013). Whichever of these mechanisms prevails, is dependent on the presence of CO2 and on the age of the cementitious material. At early age, a high amount of unhydrated cement particles is still present in the mixture, meaning that continued hydration is the most critical contributing factor, while later on the recrystallization and carbonation of Ca(OH)2 (CH) become more important (Snoeck & De Belie, 2019). In both cases, however, water is essential to be present, which limits the healing efficiency. The inferior autogenous healing mechanisms are the swelling of calcium
silicate
hydrates (C
S
H) next to the physical blockage by impurities originating from either broken-off particles from the cementitious matrix or

Calcium carbonate precipitaon

Connued hydraon

Swelling

Parcles broken of from surface

Figure 4.1 Mechanisms of autogenous healing in cementitious materials.

Parcles in water

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included particles from an intruding fluid. Even though these mechanisms are inferior, they may contribute to the overall autogenous healing effect. To enhance the availability of water inside the cracks, internal water reservoirs could be added. A well-established example of these are superabsorbent polymers (SAPs) (Snoeck & De Belie, 2015a). These materials are able to absorb and retain large amounts of water. The swelling of the SAPs causes physical blocking of the cracks, leading to self-sealing (Lefever et al., 2020; Snoeck et al., 2012), while after the release of the water, further hydration and CaCO3 precipitation inside the cracks are promoted. In this way, the water supply could be prolonged and self-healing is encouraged. Additionally, whereas the technique of autogenous healing seems to be an easily accessible and valuable solution for the curing of cracks, healing is most effective in case of small cracks (Reinhardt & Jooss, 2003), having a maximum width of 200
300 μm (Edvardsen, 1999). For this reason, fibers are often included to obtain a restriction of the crack width (Homma et al., 2009; Snoeck et al., 2014). Moreover, the fibers act as precipitation surfaces, inducing a higher extent of crack healing (Homma et al., 2009; Snoeck et al., 2014). The use of fibers to limit the crack width is a commonly accepted method, using either synthetic microfibers (Snoeck & De Belie, 2015a), natural fibers (Snoeck et al., 2015; Snoeck & De Belie, 2012), or glass fibers (Snoeck et al., 2020a, 2020b; Snoeck et al., 2014; Snoeck & De Belie, 2015b). With an adapted mix design, a strain-hardening mixture can be obtained that also has enough building blocks available to sustain autogenous healing. Closely related to the restraining of the crack openings by the addition of fibers is the adoption of engineered cementitious composites (ECC) or ultra-highperformance fiber-reinforced composites (Huseien et al., 2019), which are extremely ductile (3%
7%) and contain a relatively low amount of fibers, being below 2%, leading to narrow crack widths up to 60 μm (Zhou et al., 2010). ECC shows a tensile strain-hardening behavior, similar to metals. Thanks to its high fraction of cementitious material and the controlled crack formation, ECC allows for a continued hydration of cement particles to take place and improves both the autogenous and autonomous self-healing capacities by its tight crack widths (Li et al., 1998). Third, the inclusion of mineral additions can be used to stimulate autogenous healing (De Belie et al., 2018). Mineral additions, such as fly ash, blast-furnace slag, silica fume, etc., benefit from their siliceous and/or aluminous content, inducing a pozzolanic reaction. This pozzolanic reaction promotes a continued hydration, increasing the extent of autogenous crack healing (Van Tittelboom et al., 2012) (Sahmaran et al., 2013). The fourth and last example is the use of expandable materials, such as crystalline admixtures (Roig-Flores et al., 2015).

4.2.2 Autonomous healing Besides the intrinsic healing capacity of cementitious materials, chemical agents and/or other materials can be introduced. One way of incorporating these healing

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products is by means of capsules (Dry, 1994) (Tsangouri et al., 2013; Yang et al., 2011), which is depicted in Fig. 4.2. Upon the occurrence of cracks, the capsules break and release their content to the surrounding matrix. Some of these agents react upon contact with the cementitious matrix or moisture inside the crack, while others may react with a second component, which can be introduced by means of other capsules or those present in the matrix. Furthermore, healing by use of a vascular system can be adopted, which is a bio-inspired technique where a network of hollow tubes, linked to a reservoir with healing agent, is embedded in the matrix (Minnebo et al., 2017; Selvarajoo et al., 2020). A graphical representation of this mechanism is illustrated in Fig. 4.2. The advantage of vascular systems over capsules is the repeatability of crack healing, as the reservoir is refillable. However, when the tubes are exposed to multiple cracking, the healing agent will eventually not be able to reach the regions within the cementitious specimen that are further away from the reservoir. Also, some issues concerning both methods are addressed, being the proper material choice for the vascular tubes or capsules, which should be brittle but able to survive mixing and casting, as well as the ideal healing agent, having a viscosity that allows to flow easily through the crack without leaking out or being absorbed by the matrix. A third method is the use of shape-memory alloys, which return to their initial size upon heating (Jefferson et al., 2010). When cracks are formed and the specimen is heated, the shape-memory alloy shrinks and pulls the crack faces toward each other, thereby closing the crack. The fourth option is mimicking the bone healing using a porous concrete through which a healing product can be injected (Sangadji & Schlangen, 2012).

Capsule with polymeric healing agent

Crack

Reservoir with healing agent

Crack

Capsule-based healing

Tube connected to reservoir

Vascular healing

Figure 4.2 Autonomous healing mechanisms: capsule-based healing (top) and vascular healing (bottom).

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Finally, a fifth approach of improving the autogenous healing capacity is the use of mineral producing bacteria. Formerly, this technique was used for surface consolidation and as an external method for crack repair (De Muynck et al., 2008) (De Muynck et al., 2010) (Van Tittelboom et al., 2010). Protected ureolytic, denitrifying, or aerobic heterotrophic bacteria are introduced into the concrete (De Belie et al., 2018). When considering the ureolytic pathway, urea is hydrolyzed into carbonate and ammonium ions. These ions react with calcium ions, which are either present in the cementitious material or added as a mineral precursor, and form calcium carbonate. The different types of bacteria are incorporated during mixing and should be able to withstand the mechanical stresses and remain viable in a highly alkaline and oxygen-poor environment. Jonkers et al. found a suitable candidate within the genus Bacillus (Jonkers et al., 2010; Jonkers & Schlangen, 2008). However, the viability of unprotected spores was found to be limited to approximately 2 months. For this reason, a protective barrier should be applied. Many possible solutions are available, such as the incorporation of the bacteria into porous expanded clay particles (Jonkers, 2011), SAPs (Wang et al., 2014), polyurethane immobilization (Wang et al., 2012; Wang et al., 2014), microencapsulation (Wang et al., 2014), and the use of graphite nano-platelets (GNPs) (Khaliq & Basit Ehsan, 2016). In the latter study, GNP particles were soaked with a bacterial solution before mixing. Results showed that the self-healing ability was largely improved by the use of GNP particles in comparison to direct incorporation of bacteria in case of precracking at early age, owing to their relatively small particle size, which provides a uniform distribution of the bacteria and acts as a filler material. In a later stage, however, the viability of the bacteria is compromised, because of the pressure exerted on the GNP particles as hydration continues.

4.3

Nanomaterials for self-healing purposes

The use of nanomaterials in cementitious composites has garnered massive attention in the last decades. These nano-sized inclusions have already proven their effectiveness in enhancing the durability and mechanical performance of cementitious mixtures (Qing et al., 2007; Li et al., 1998), leading to the so-called ultra-high-performance concrete. The application of nanomaterials to cementitious mixtures exhibits three advantages over larger-sized additives of the same composition. Firstly, these particles act as a filler material and increase the density of the matrix, due to their relatively small volume. Additionally, as the particles have a large surface area, they act as nucleation sites for the formation of C
S
H, thereby accelerating the hydration of cement. Lastly, when reactive materials, such as pozzolans, are used, the high specific surface area of the particles leads to an increased reactivity. These characteristics already give the impression that nanomaterials could be beneficial for the promotion of the self-healing ability. The combination of these technologies, meaning that both an increased performance and an improvement in the self-healing ability are induced by nanomaterials, contributes to the development of sustainable cementitious materials.

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4.3.1 Nano-sized superabsorbent polymers In recent studies, submicron spherical SAPs based on acrylic acid and ethylene glycol dimethacrylate were developed with a diameter of around 200 nm (Karatzas et al., 2012) and encapsulated in organic shells (Kanellopoulou et al., 2019). In the latter study, upon an inclusion of 2% of these encapsulated SAPs, a reduction of only 12% of the compressive strength was obtained, whereas much larger decreases up to 50% were reported in literature in case of larger SAP particles, even for amounts of SAP addition between 0.3% and 1% only (Justs et al., 2015) (Pelto et al., 2017) (Sun et al., 2019). This effect was caused by the fact that the workability of the cementitious mixtures was maintained upon SAP inclusion, so that no additional water was needed. As the SAPs absorb water during mixing, the waterto-cement ratio was initially lowered, leading to an increased mechanical performance, counteracted by the creation of voids upon water release by the SAPs. Thanks to their small size, a homogeneous distribution of the porosity was obtained. Results showed that their high absorption capacity (up to 600%) promoted the sealing and healing of cracks with an opening of about 100 μm, demonstrated by a lowered permeability and visible precipitation of CaCO3.

4.3.2 Nano-clays Another means of providing additional water, similar to the inclusion of SAPs, is the addition of nano-clays. In a study by Qian et al. (2010), nano-clays were utilized as internal water reservoirs, as these clays are able to hold large amounts of water between their platelets. By releasing their absorbed water inside the cracks, a promotion of the autogenous healing mechanisms takes place. Results showed that nano-clays were able to promote healing of precracked samples, but the effect was considerable within the first 2 months only. This limited healing efficiency at later age is likely caused by the consummation of unhydrated cement and CH over time. Research of Shaheen et al. (2019) examined the use of nano and micro-bentonite to immobilize bacteria Bacillus subtilis. It was observed that bentonite was ineffective as a carrier for bacteria, caused by its pozzolanic nature, forming C
S
H from CH. A limited regain in compressive strength and partial crack closure could be reached, although being reduced in comparison to the direct addition of bacteria through the mixing water. Lastly, a nano-clay that is often used as an additive for cementitious composites is kaolinite or kaolin clay. Kaolin is a layered silicate mineral with chemical formula Al2Si2O5(OH)4, consisting of a tetrahedral silica sheet connected to an octahedral alumina layer. Compared to other clay types, kaolin is known to have a more limited swelling capacity, caused by its two-layered morphology. In its crystalline structure, kaolin is fairly nonreactive during cement hydration. However, a transformation towards metakaolin, having an amorphous structure, makes this mineral a highly reactive pozzolan, depending on the origin of the kaolin clay (Fernandez et al., 2011). The transformation is induced by a thermal activation between 650 C and 800 C (Sabir et al., 2001), also called the calcination process. This procedure is

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similar to the calcination stage during Portland cement production, but takes place at a much lower temperature compared to the cement case (typically around 1450 C), leading to a reduced environmental impact. The partial replacement of cement by metakaolin and nano-metakaolin resulted in an increased tensile and compressive strength, as observed by various researchers (El-Gamal et al., 2017; Morsy et al., 2010, 2011; Shafiq et al., 2015). Because of its pozzolanic nature, nano-metakaolin reacts with CH to C
S
H in aqueous media. Upon water ingress into a crack, a humid environment is created that encourages the initiation of the pozzolanic reaction with CH present inside the crack. Together with its fine particle size, providing nucleation sites for cement hydration, nano-metakaolin could improve the self-healing ability of cement-based materials.

4.3.3 Nano-silica Nano-silica is one of the most-used nanomaterials for incorporation in cementitious mixtures and can be seen as the engineered form of silica fume. Also, it is frequently used within ultra-high-performance concrete. Similar to other nanomaterials, nano-silica acts as nucleation site for the production of C
S
H and increases the density of the hardened matrix by means of the filler action. Additionally, their large surface area makes them highly reactive, thereby leading to an early pozzolanic reaction. These features contribute to a higher mechanical performance and increased durability of cementitious composites (Aggarwal et al., 2015; Li et al., ¨ ztu¨rk et al. (2020) investigated the 2015; Shih et al., 2006; Singh et al., 2013). O effect of nano-silica on the closure of cracks. On the basis of electrical resistivity and microscopic analysis, the addition of nano-silica was demonstrated to increase the self-healing efficiency compared to control specimens, by the precipitation of CaCO3 and some continued production of C
S
H. Sirajuddin et al. examined the effect of nano-silica together with a crystalline admixture (Sirajuddin et al., 2019). The maximum regain in compressive strength was obtained for concrete holding 2% nano-silica and 1.1% crystalline admixture and this after 42 days of immersion in water. Apart from the direct inclusion of nano-silica particles to cementitious mixtures, thin-walled soda glass capsules containing colloidal nano-silica were utilized (Kanellopoulos et al., 2015). This technique is considered to be autonomous healing but still makes use of the cementitious composite’s building blocks to obtain crack healing. The release of the colloidal silica triggered the further hydration of unhydrated cement particles and stimulated the pozzolanic reaction. Results showed a regain in mechanical performance and improvement in the gas permeability and water uptake by the closure of cracks in presence of nano-silica. The presence of water is however crucial. The positive effect of nano-silica on the concrete strength, was used in a mixture containing SAPs. As these SAPs decrease the mechanical properties (Mechtcherine & Reinhardt, 2012), the combination of both materials resulted in a cementitious composite without a strength reduction and with a superior healing mechanism (Lefever et al., 2020a, 2020b, 2020c; Snoeck et al., 2012). The self-sealing capacity was

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SAP-nano-silica

Reference

assessed by means of microscopic analysis and a water permeability test (Lefever et al., 2020). The evolution of a specific but representative crack of a reference and a SAP-nano-silica mixture are shown in Fig. 4.3. It was seen that the addition of SAPs and nano-silica significantly improved the visible closure of cracks after 3 days of wet
dry curing. A confirmation of crack filling was found by a reduced water flow through cracked samples. The improved healing ability was obtained through calcium carbonate precipitation and a pozzolanic reaction with nano-silica, induced by the retained water inside the SAPs. The combination of SAPs and nano-silica benefitted from both the promoted self-healing ability and the improved mechanical performance with respect to the use of SAPs solely. Nano-silica was not only used for healing purposes because of its pozzolanic reaction or availability of nucleation sites. In a study by Perez et al., amine functionalized nano-silica was added next to silica microcapsules, filled with an epoxy. The inclusion of the nano-particles, reacting with clinker, led to the creation of an amine functionalized cementitious matrix. When the microcapsules opened, caused by fracture of the specimen, the epoxy comes into contact with the amine groups and their reaction induces crack healing. Cementitious specimens with 5% of selfhealing dosages were shown to reduce the water absorption compared to a reference mixture without additions and this in the cracked as well as the uncracked state (Garcia Calvo et al., 2017). A final application of silica nano-particles for self-healing cementitious composites is their use as reinforcing material for microcapsules. As already mentioned, the choice of a capsule material is a critical parameter to obtain effective healing of cracks. The material should be brittle, so that it breaks easily when the matrix

Ini al crack

3 days

7 days

14 days

Figure 4.3 Evolution in crack width during wet-dry curing of reference (top) and SAP-nanosilica samples (bottom). The red scale bar has a width of 500 μm. SAP, Superabsorbent polymer.

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fractures, but should also be able to resist the mixing and/or casting process. The use of nano-silica together with paraffin and polyethylene wax showed an increased encapsulation ability and mechanical performance compared to microcapsules without nano-particles and demonstrated a higher recovery rate in compressive strength (Du et al., 2020).

4.3.4 Carbon nano-tubes CNTs are large cylindrical atomic structures that are formed by the rolling up of graphene sheets. These sheets are characterized by a hexagonal arrangement of carbon atoms. The rolling up of the sheets is not arbitrarily executed, but follows a certain revolving angle, which characterizes the final nano-tube. CNTs have a diameter in the nanometer-range, while their length can go up to several centimeters. They can be divided into single-walled or multiwalled nano-tubes. Nowadays, these nanomaterials are often used to provide electrical conductivity and piezoresistivity to a cementitious blend, while only including small amounts (,1% by weight of cement) (Yu & Kwon, 2009; Han et al., 2011; Kim et al., 2014). Thanks to the introduction of piezoresistivity, a self-sensing composite can be created, meaning that the material is able to monitor itself and detect present damage (Han et al., 2009). Additionally, an increase in compressive and tensilesplitting strength is noticed (Kumar et al., 2012). Nonetheless, care should be taken on the amount of CNT included, as the particles tend to cluster. Regarding the selfhealing capacity of cementitious mixtures with CNTs, an improvement in visual crack closure was observed, as well as an increased restoration of electrical resistiv¨ ztu¨rk et al., 2020). Being an inert material, the ity compared to virgin mixtures (O CNTs acted as nucleation sites for further hydration and deposition of CaCO3. The effect was even more pronounced in combination with nano-silica. The combination of both is interesting as in combination with the improved self-healing efficiency, damage may be detected at an early stage and the possible healing mechanisms may be monitored over time. In a study of by Lanzara et al. (2009), the feasibility of using CNTs as healing reservoirs was investigated by a simulation of the liquid flow from the ruptured CNT. It was observed that organic molecules, hosted inside the CNTs, were able to escape to the outer environment. Nonetheless, the process showed saturation before all molecules were released. The CNTs should therefore be tailored in order to increase the amount of healing agent that is transferred to the matrix material. By including them into cementitious materials, a combined strengthening and selfhealing effect could be obtained.

4.3.5 Nano-iron Because of its large surface area, nano-sized iron oxide Fe2O3 was employed to increase the amount of hydrates formed, leading to a denser microstructure (Li et al., 2004). This feature of nano-iron makes it a potential candidate for selfhealing purpose, together with the reaction that takes place upon contact of calcium

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hydroxide and Fe2O3, forming iron(III) hydroxide and calcium oxide, which could fill the cracks. However, the ability of nano-Fe2O3 to promote the closure of cracks was only studied indirectly, as being a carrier for bacteria (Shaheen et al., 2019). Several carriers to improve the survivability of bacteria were already studied, but the majority of them affect the mechanical performance of cementitious materials (De Belie, 2016). Results with Fe2O3 as carrier material revealed an improvement in the sealing of cracks, up to 1.2 mm wide, and a regain in compressive strength up to 85%. Also, the initial mechanical performance was increased compared to control mixtures, due to the filler effect. Another oxide based on iron that has been investigated in terms of bacteriabased crack healing is Fe3O4 (Seifan et al., 2018). The immobilization of bacteria by means of magnetic iron oxide showed to decrease the capillary water absorption compared to reference mixtures, caused by the precipitation of CaCO3 by the bacteria and the filler effect of the nano-particles (Seifan et al., 2018). Moreover, a higher compressive strength with respect to control samples was obtained (Seifan et al., 2018). Lastly, nano-Fe3O4 could also be used for the design of microcapsules, similar to the application of nano-silica. Li et al. (2020) created electromagnetic controlled capsules by use of Fe3O4 and paraffin, containing toluene-di-isocyanate as healing agent. In this way, the self-healing ability could be triggered by an electromagnetic field. An improvement in compressive strength regain was observed upon the addition of microcapsules, as well as an enhancement of the initial mechanical properties.

4.3.6 Nano-alumina Next to silica, alumina is the most important reactive component during the hydration of cement. Whereas silica is known to mainly influence the strength of the cementitious mixture, alumina controls the setting time, thereby affecting various properties of the hardened material (Muhd Norhasri et al., 2017). Nano-Al2O3 has been utilized by several researchers to modify the properties of cementitious composites. By its inclusion, the formation of calcium aluminate hydrates (C
A
H) is promoted, which improved the compressive strength of concrete (Nazari et al., 2010). Similarly, Li et al. (2006) included nano-alumina to modify the mechanical properties of cementitious composites. Because of its high surface energy, nanoalumina was absorbed on the surface of sand particles. A reaction between CH and Al2O3, on top of the filler effect, led to a densification of the interfacial transition zone, increasing the elastic modulus and the compressive strength, also confirmed by Hase and Rathi (2015). Moreover, the use of nano-alumina in cementitious mixtures showed to reduce the corrosion rate of embedded steel bars (Ann et al., 2010) and to enhance the frost resistance (Behfarnia & Salemi, 2013). The self-healing ability of nano-alumina has not been studied in cementitious materials yet, but showed to increase the fatigue life of bitumen (Akbari & Modarres, 2018). The latter might be a first step towards the use of nano-Al2O3 for self-healing objectives.

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4.3.7 Nano-titania Titania or titanium oxide is known as one of the most used photocatalysts in construction industry. Photocatalysts are materials that easily absorb UV radiation to activate electrons in their valence band. In the presence of water or air, radicals are formed which on their turn react with substances adsorbed to the surface of TiO2 (Chen et al., 2012; Shafaei et al., 2020). Their use in cementitious composites promotes the self-cleaning effect of the outer surfaces (Maury Ramirez & De Belie, 2009), so that the esthetics of a construction are maintained. Additionally, the use of titanium oxide reduces air pollution by the decomposition of NOx and volatile organic compounds (Maury Ramirez et al., 2010), which makes it an ideal additive for cementitious materials in urban areas. Recent studies concerning the use of nano-TiO2 in cementitious mixtures show that the nano-particles are nonreactive regarding the formation of hydration products but provide nucleation sites for the hydration of cement and act as a filler, similar to other nanomaterials studied (Jayapalan et al., 2009; Yousefi et al., 2020). For this reason, nano-TiO2 seems to be an interesting candidate to promote the healing of cracks, but further research on this topic needs to be performed. Similar to the encapsulation of nano-silica, nanoTiO2 was included in polymeric capsules (Taheri et al., 2020). In this way, the titania particles become available after fracture of the cementitious matrix, so that nucleation sites are created for continued hydration of unhydrated cement particles.

4.3.8 Nano-fibers Whereas fibers are often used in self-healing cementitious composites to limit the crack width opening and provide additional surfaces for the deposition of hydration products and CaCO3 (Snoeck & De Belie, 2015b) (Homma et al., 2009), the use of hollow fibers is closely related to the use of a vascular healing system. In this case, the healing agent is injected into the fibers, which can be simply included in the cementitious matrix as a type of capsules or connected to a reservoir. Similar to autonomous self-healing, this technique showed to effectively repair cracks (Dry, 1994; Kuang & Ou, 2008), but the efficiency strongly depends on the healing agent used. The adoption of hollow nano-fibers was already investigated for polymeric matrices (Khan et al., 2019; Lee et al., 2016; Pulikkalparambil et al., 2018). Because of their nano-sized diameter, these fibers act as independent healing reservoirs and are not connected to a container with the healing liquid. The use of hollow nano-fibers has exhibited potential to be applied in cementitious materials as well.

4.4

Conclusions and future perspectives

As cracking of concrete poses severe problems for the durability and lifespan of structures, the adoption of self-healing cementitious composites is an essential step toward sustainable constructions. To improve the self-healing capacity of such

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materials, various additives can be used, ranging from polymeric healing agents, which harden by means of a chemical reaction with other constituents or upon contact with air, to materials that promote the hydration of cement inside the crack. Nano-sized additives have shown to improve various properties of cementitious composites. More specifically, they contribute to an increase in mechanical performance, a reduction in porosity and an enhanced self-healing ability. Whereas the topic of nanotechnology has been studied more intensively over the last years, further research on the use of nanomaterials should be performed in order to fully understand their working principle for self-healing purposes. An issue that was raised is the potential toxicity of nano-particles for the environment and the human body. Increasing their use should therefore take into consideration the possible risks during exposure, both in the processing stage and later on, being so-called nano-waste (Bystrzejewska-Piotrowska et al., 2009). In case of release to the atmosphere, only little time will pass before the nano-particles are dispersed in air, water, and soil, where they can be picked up by living organisms. For the human body specifically, these particles come into contact with organs through inhalation, ingestion, or skin contact. Research has shown that this exposure leads to the production of reactive oxygen species, which causes inflammation and in a later stage even damage of DNA (Sengul & Asmatulu, 2020). An example of such study investigated the toxicity of TiO2, present in sunscreen (Dunford et al., 1997). While TiO2-containing sunscreen was considered to be safe and effective, it was shown that the absorption of UV by TiO2 catalyzes the formation of hydroxyl radicals, leading to oxidative reactions that cause DNA damage. Besides this chemical toxicity, the particle size, surface area and shape is of high importance, as these influence the ease of penetration through cell membranes (Buzea et al., 2007). It is clear that these aspects of nanomaterials should be taken into account during future research. Before taking the step towards the extensive use of nanomaterials, its impact on the environment and the health of human beings should be assessed, so that their application in cementitious composites (and other products) occurs without any risk. Besides the sustainability question, the cost of these nanomaterials should be taken into account. Nano-particles fall into the category of engineered materials and exhibit unique characteristics thanks to their specific manufacturing process. This feature increases their cost compared to the bulk material (Gkika et al., 2017). As one of the objectives of using nanomaterials for self-healing purpose is to limit the repair and/or rebuilding expenses, an analysis considering all costs should be performed. However, this issue will be solved partially by the upscaling of nanomodified cementitious composites (Huseien et al., 2019). At the present time, nanotechnology is still in a developing stage, meaning that the production of nano-sized particles is limited. The machinery and processing costs are therefore relatively high compared to the amount of nano-particles that is generated. By increasing the manufacturing volume, the cost per gram of produced nanomaterial will be reduced, which makes their use as self-healing additives more accessible. It can be concluded that nanomaterials are a promising additive for concrete and mortar mixtures. Research has shown that the inclusion of these particles not only

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increases the mechanical strength and durability, leading to (ultra-)high-performance cementitious composites but also improves the self-sealing and self-healing ability of these blends. The latter aspect is highly important in structures with limited accessibility, not allowing for manual repairs to take place, and reduces the costs as well as the carbon dioxide emissions related to repairing and rebuilding of damaged constructions. As the topic of nanotechnology is only recently studied, further research will likely uncover new nanomaterials and their applicability for cementitious mixtures. Additionally, more in-depth investigations of the presented nano-additives are anticipated to lead to a better understanding of their working principle and the environmental impact that originates from their use. In this way, nano-modified self-healing cementitious composites will become an approved building solution in the near future.

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Nano-tailored TiO2-based photocatalytic cementitious systems for NOx reduction and air cleaning

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1,2 ˘ Oguzhan Sahin ¸ , Emrah Bah¸si1, Gu¨rkan Yıldırım3, and 3 Mustafa Sahmaran ¸ 1 Hacettepe University, Institute of Science, Beytepe, Ankara, Turkey, 2Department of Civil Engineering, Kır¸sehir Ahi Evran University, Kır¸sehir, Turkey, 3Department of Civil Engineering, Hacettepe University, Ankara, Turkey

5.1

Introduction

As a consequence of the rapidly increasing urbanization, population, number of vehicles, advancing industry among many other reasons, air pollution has reached serious levels and become one of the major environmental problems of the countries to be urgently overcome. The World Health Organization (WHO) reported that air pollution is responsible for the death of 8 million people around the world annually and 9 out of 10 people living in low- and middle-income countries are exposed to environmental air pollution (WHO, 2020), pointing out the disturbing effect of air pollution on the quality of life of our society, which is increasingly urbanizing every day. Air pollution is caused by different types of solid, liquid, and gaseous materials suspended in the air; the most important of which are the harmful gases of carbon monoxide (CO), sulfur dioxide (SO2), and nitrogen oxides (NOx). These gases are particularly important, because if inhaled without any protection up to certain level, they can be very dangerous health-wise, especially for the upper respiratory tract given their toxic nature. In order to reduce/eliminate the amount of these gases, several solutions focusing on their reduction/elimination on the source level and their existence after their production have been attempted. Given the situation of these gases, however, it is evident that current measures are not adequate and needed to require advancements to be made. Among air pollutant gases that are harmful to human health, NOx requires special attention since they are highly reactive, colorless, odorless, and waterinsoluble. They are generated in the compound forms of NO and NO2. The exhaust gases of vehicles are one of the main contributors to the world’s NOx emission alongside with industrial activities (Cunha-Lopes et al., 2019). It has been stated by a report related to the worsening in air quality of the Sa˜o Paulo State of Brazil that Recent Advances in Nano-Tailored Multi-Functional Cementitious Composites. DOI: https://doi.org/10.1016/B978-0-323-85229-6.00015-9 © 2022 Elsevier Ltd. All rights reserved.

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major factors found responsible for the air pollution are around 2000 industrial establishments located in the city and around 9.7 million registered vehicles. It was also reported that the NOx release amount from these sources was around 376k tons in 2009, and the vehicles were responsible for approximately 96% of the NOxrelated air pollution (Prodesp, 2009). According to a report published by the South Korea National Air Pollutants Emission Service (National Air Pollutants Emission Service of Korea. Emission Rate Inquiry, 2020), NOx emitted from different sources, which is the substance with the highest emission rate, is responsible for about 26% of the total pollution. Total NOx emission in South Korea measured in 2017 was about 1189k tons, and the road transport had the biggest share with 434k tons/year. Considering that the cases in regard to NOx emission in other countries are similar to the above examples, many developed countries have taken measures to limit/reduce their NOx emission over the years. In order to control/limit the rate of NOx emission to the atmosphere, United States Environmental Protection Agency (EPA,2010, 2018) has set a one-hour ambient air quality standard for NO2 at a level of 100 parts per billion (ppb) and an annual NO2 standard at a level of 53 ppb based on annual average NO2 concentrations, and strict control measures were applied to interested parties to properly comply with this matter. NOx release is a major concern in European countries (the United Kingdom and EU-27) as well. They have made a number of adjustments to limit NOx emission rates and minimize its detrimental effects. For the European countries, the determined critical/limit value for NOx concentration in air is 40 μg/m3 as an annual average. The vegetation critical level set for the protection of vegetation (beyond/above this level, direct adverse impacts on sensitive vegetation may happen) is 30 μg/m3 as an annual average and the official 1-hour NO2 standard is for a level at or below 200 μg/m3 (EEA, 2018; EU, 2008; WHO, 2000). Owing to such adjustments related with NOx, the rate of decrement in the amount of NOx emission from 2017 to 2018 was 4.1% and this decrement rate was 60% for 1990
2018 (EEA, 2020). Despite the availability of measures taken to combat the high levels of NOx, additional solutions must be sought since the problem of air pollution is far from being reduced currently. One innovative and new-generation way to eliminate NOx in the air is using it as photocatalyst in photocatalytic degradation reactions via photoactivation. Photocatalysts induce the formation of photoinduced negative electrons and positive holes under the irradiation of visible or UV light. A series of reduction and oxidation reactions (redox) are initiated by these surface charge carriers resulting in the generation of reactive radical species (mainly hydroxyl and/or superoxide radicals) and these radicals exhibit capability to oxidize/degrade various pollutants such as NOx (Martyanov & Klabunde, 2003; Nishikawa & Takahara, 2001; Ohama & Gemert, 2011). There are various photocatalyst materials used in the photocatalytic applications such as titanium dioxide (TiO2), zinc oxide (ZnO), and cadmium oxide (CdO). Among them, TiO2 is one of the most popular photocatalysts, most probably because of its enhanced photocatalytic activity (Hager et al., 2000), chemical inertness in the absence of irradiation, safety, cost-efficiency, and nontoxicity (Husken & Brouwers, 2008; Wang et al., 2007; Zhao & Yang, 2003). Moreover, it is a widely used material in different applications such as different self-cleaning purposes including water and air purification, cancer treatment,

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development of dye-sensitized solar cells, sterilization, development of antifogging surfaces, water splitting, and CO2 reduction (Anpo et al., 1995; Blake et al., 1999; Cai et al., 1991, 1992; Fujishima et al., 1986, 2000; Mills et al., 2003; Mor et al., 2006; Supeno & Siburian, 2018). It also has very wide and rapidly expanding area of use in many subbranches of different industries through its superior photocatalytic degradation capability. The photocatalytic degradation capability of TiO2 has become of interest particularly to those working on the construction-related sectors utilizing construction materials heavily. This is mainly because of the structural members having large surface areas that can be easily subjected to irradiation from sun and/or any other artificial source to trigger photocatalytic activity and can be useful instruments for air purification purposes. In line with this idea, incorporation of photocatalysts (e.g., nano-TiO2) into cementitious systems has been an option to provide air purification property for the construction materials in favor of achieving multifunctionality alongside with the improvement of other performance indicators (e.g., strength, permeability, durability, material greenness). In this chapter, literature studies focusing on the photocatalytic cementitious systems are reviewed extensively outlining the current understanding, situation, and research directions to equip cementitious systems with the air purification property.

5.2

TiO2 as a photocatalyst

Photocatalytic reactions occur as a result of the activation of heterogeneous semiconductive photocatalysts through irradiation. Owing to their considerably high activity in NOx degradation through photocatalytic reactions, TiO2, ZnO, and CdO are often preferred as photocatalysts (Chen et al., 2008; Zhao & Yang, 2003). Among these materials, TiO2 is the most commonly used semiconductive photocatalyst, given its superior characteristics as above-noted. Under this title, detailed information in regard to the use of TiO2 as a photocatalytic material are shared.

5.2.1 Structure of TiO2 There are three crystalline phases of TiO2 in nature, under atmospheric pressure; rutile (tetragonal), anatase (tetragonal), and brookite (orthorhombic) (Ding et al., 1996; Gamboa & Pasquevich, 1992; Ghosh et al., 2003; Mo & Ching, 1995; Smith et al., 2009) crystallographic structures of which are shown in Fig. 5.1. Rutile is the most thermodynamically stable and common form (Fazli et al., 2017; Gouma & Mills, 2001; Lin et al., 2013; Mayabadi et al., 2014). Anatase can transform into rutile phase at elevated temperatures and this transformation is not reversible. There are also other phases of TiO2 formed under high pressure, which are TiO2 II or srilankite, cubic fluorite, pyrite-type, monoclinic baddeleyite-type and cotunnite-type phases (Hanaor & Sorrell, 2011). The generation of brookite phase is more difficult; therefore its presence is less common compared to other natural phases of TiO2,

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Figure 5.1 Illustration of crystallographic structure of (A) anatase, (B) rutile, (C) brookite.

which led researchers to focus on more common phases of metastable anatase and stable rutile. Anatase and rutile phases have a tetragonal ditetragonal dipyramidal crystal system, where Ti41 is bonded with six O22 resulting in the formation of TiO6 octahedra chain (oxygen atoms around a titanium atom), with different lattice parameters of a 5 0.378 nm; b 5 0.951 nm and a 5 0.459 nm; b 5 0.296 nm, respectively (Austin & Lim, 2008; Burdett et al., 1987; Cromer & Herrington, 1955; Fisher & Egerton, 2001; Peters & Vill, 1989). They are both inert and insoluble in water (Fisher & Egerton, 2001). The number of atoms per unit cell (Z) is 4 for anatase and 2 for rutile (Burdett et al., 1987; Cromer & Herrington, 1955; Mo & Ching, 1995; Peters & Vill, 1989). It can be seen from lattice parameters of these two forms of TiO2 that rutile has a relatively shorter and wider lattice structure compared to anatase making it different in terms of physical and chemical properties. The unit cell volumes (volumes of the lattices) of rutile and anatase phases are approximately 0.0624 and 0.1363 nm3 (Burdett et al., 1987; Hanaor & Sorrell, 2011; Hanson, 2014; Peters & Vill, 1989), while their hardness values are 5.5
6 and 6
6.5 Mohs, respectively (Hanaor & Sorrell, 2011; Oi et al., 2016). While rutile can be excited by both visible and UV light, anatase is only excited by the UV light (Hanson, 2014). The band gap energy (Eg) values for anatase and rutile are 3.2 and 3.0 eV, which correspond to wavelengths of 388 and 413 nm, respectively (Beltra´n et al., 2006; Daude et al., 1977; Hanaor & Sorrell, 2011; Kavan et al., 1996; Reddy et al., 2003; Serpone, 2006; Wang & Lewis, 2006).

5.2.2 Utilization of TiO2 for air purification There are several photocatalyst used for air purification purposes including TiO2, ZnO, iron oxide or hematite (Fe2O3), tungsten trioxide (WO3), CdO, CdS, and

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Table 5.1 Band energies and beam wavelengths of some semiconductive photocatalysts. Semiconductive photocatalysts

Wavelength (nm)

Band energy (eV)

Titanium dioxide (TiO2)—rutile Titanium dioxide (TiO2)—anatase Zinc oxide (ZnO) Zinc sulfide (ZnS) Cadmium sulfide (CdS) Hematite (Fe2O3) Tungsten trioxide (WO3)

413 388 388 335 516 539 443

3.0 3.2 3.2 3.6 2.4 2.3 2.8

bismuth (Bi) and others (Chen et al., 2008; Ibhadon & Fitzpatrick, 2013; Zhao & Yang, 2003). Band energies and beam wavelengths of some of these semiconductive materials are presented in Table 5.1. Photocatalytic reaction is a photochemical process, where semiconductive photocatalyst absorbs energy of the incident light, which is stimulated to be used especially in removing air/water pollution. Photocatalytic activity of semiconductive materials, used as a catalyst, under light irradiation depends on the amount of energy difference between the valence band and conduction band. Although the band energy required for the stimulation of TiO2 (Table 5.1) is somewhat higher than that for the CdS, Fe2O3, and WO3; TiO2 is more preferred than other semiconductors. This is partly because of the fact that the wavelength required for the activation of TiO2 falls in the range of 315
400 nm, which is the range of UV-A rays emitting from the sun and reaching to the atmosphere and TiO2 is photochemically stable, chemically inert in the absence of irradiation, safe, cost-efficient, nontoxic and with enhanced photocatalytic activity (Hager et al., 2000; Husken & Brouwers, 2008; Wang et al., 2007; Zhao & Yang, 2003). Enhanced photocatalytic activity of TiO2 which brings about effective NOx degradation capability (Husken & Brouwers, 2008), is largely dependent on the size, shape, type, phase and particle properties of TiO2 (Znaidi et al., 2001). Given both anatase and rutile phases of TiO2 are thermally/chemically stable, have high oxidation capability, permeability against visible rays and high hydrophilic properties, their use as photocatalytic materials in numerous applications has been more comment compared to other semiconductive photocatalysts, (Banerjee et al., 2006; Sakai et al., 2004), which therefore have been of focus to current review.

5.2.3 Photocatalytic property of TiO2 5.2.3.1 NOx degradation mechanism of TiO2 A photocatalyst is a material capable of improving the quality of atmospheric air, which degrades or removes air pollutants such NOx by absorbing the energy of light (i.e., UV irradiation) to stimulate a chemical reaction called photocatalytic degradation. It is necessary to trigger this photochemical degradation process via UV and/or visible light irradiation having a photon energy equivalent to or higher than

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the band-gap energy of the semiconductive photocatalysts. Mechanism of photocatalytic degradation process of TiO2 is explained step-by-step below and schematized in Fig. 5.2. First, photons emitted from an irradiation source reach the surface of photocatalysts to start the photochemical degradation process. When the energy (hν ) of photons reaching to the surface of photocatalysts is equivalent to or greater than the electronic band gap value (Eg) (hν $ Eg) of the photocatalysts, the photocatalysts absorb the light energy of irradiation (visible light or UV rays). In this first step of the photocatalytic reaction, with the help of photon from an irradiation source, a gap is formed in the valence band via electron movement from the valence band of the photocatalyst to the conduction band. In this way, photogenerated electron hole (e2/h1) pairs are formed (Zhu & Zhou, 2019) following the reaction below: TiO2 1 hv 2h1 1 e2 The photogenerated holes (h1) are one of the most critical reactive oxidation species stimulating oxidation of electron donors in photocatalytic degradation

Figure 5.2 Mechanism of photocatalytic degradation process of TiO2. Source: Redrawn after Poon, C.S., & Cheung, E. (2007). NO removal efficiency of photocatalytic paving blocks prepared with recycled materials. Construction and Building Materials, 21(8), 1746
1753. https://doi.org/10.1016/j.conbuildmat.2006.05.018; (Boonen & Beeldens, 2013; Dylla et al., 2011; Janus & Zajac, 2016).

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process. In the second step, the holes oxidize the hydroxyl groups to form highly active hydroxyl radicals (OH ) (Yang et al., 2007; Zhao et al., 2007) Another critical highly reactive oxidation specie for the photodegradation process is superoxide radical (O22) obtained from the reduction of oxygen (O2) by using the free electron available in the conduction band as an electron acceptor (Banerjee et al., 2006; Husken & Brouwers, 2008; Zhu & Zhou, 2019). This oxidation
reduction process which is schematized in Fig. 5.2 takes place following the reactions given below. It is of significance to stress that generally, in the absence of O2, photocatalytic degradation process stops almost completely. Since the resulting superoxide ion (O22) obtained from the reduction of O2 is highly active, it either directly attacks the pollutant molecules oxidizing and decomposing them, or causes the formation of hydroxyl radicals. Therefore oxygen is very important for the photocatalytic degradation process. Hole trapping; OH2 1 h1 ! OH Electron trapping; O2 1 e2 ! O2 2 Finally, highly reactive hydroxyl radicals oxidize the pollutants such as NOx. Airborne pollutant molecules can be adsorbed on the surface of the photocatalytic particles via reactions, steps of which are as follows: O2 2 1 H1 ! HO2  NO 1 HO2  ! NO2 1 OH NO2 1 OH ! HNO3

5.2.3.2 Factors affecting photocatalytic activity There are numerous factors affecting the photocatalytic activity of TiO2 particles including photocatalyst-related parameters such as specific surface area/particle size, crystal structure, crystallite size, type of phase, and operational parameters such as amount of TiO2, concentration of the pollution, type/intensity/wavelength/ duration of the irradiation, temperature/humidity/pH of the ambient, amount/type of substituted metal/non-metal materials. All these factors play incontrovertible and essential role on the formation of effective photocatalytic degradation and are mainly influential on the energy released through the combination/recombination of electrons and holes, photon (light) absorption rate of TiO2, concentration/number of active surface sites and diffusion/capture rates of pollutant compounds. The influence of some of these factors on the photocatalytic degradation capability is detailed in the following sections.

Specific surface area and particle size of TiO2 Photocatalytic activity of TiO2 is in direct relationship with its surface morphology, specific surface area and particle size, which are influential on the amount of available active sites for the photocatalytic reactions to take place, light absorption rate,

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photon conversion efficiency, photoactivation of electron-hole pairs on its surface, carriage of the photoinduced charges (electron [e2] and hole [h1]) on its surface, recombination of e2 and h1 and mass transfer of reactants to its surface reaction sites (Koˇc´ı et al., 2012, 2009; Lin et al., 2006; Serpone et al., 1995). However, there are several other material-, production-, target substance
, and operation-related parameters having clear effect on the photocatalytic activity of TiO2 apart from the abovementioned parameters TiO2 itself. Therefore different photocatalytic activity results are generally observed in the available literature studies. In accordance with Vorontsov et al. (2018), the particle size and specific surface area of TiO2 particles are only secondary factors affecting the photocatalytic activity while the productionrelated parameters such as calcination temperature and pH are more influential on photocatalytic activity (Vorontsov et al., 2018). Several studies reported that the reduction in particle size of TiO2 particles up to a certain level has a positive effect on the photocatalytic activity, although different particle size levels have been reported to be optimum in these studies. For example, Zhang et al. (1998) stated that the optimum particle size of TiO2 for photocatalytic decomposition activity for chloroform (CHCl3) (oxidation of CHCl3 in liquid phase) is about 10 nm (Zhang et al., 1998). Maira et al. (2000) found that optimum particle size of TiO2 for the photocatalytic degradation of trichloroethylene (TCE) in gas phase is 7 nm (Maira et al., 2000). There are also other TiO2 particle size and/or size ranges that are reported to be optimum from different studies for the photocatalytic degradation of different substances. For example, Almquist and Biswas (2002) stated that the optimum particle size range of TiO2 for the oxidation of phenol is 25
40 nm (Almquist & Biswas, 2002), while Wang et al. (1997) stated that 11 nm is the optimum particle size for the liquid-phase decomposition of chloroform (Wang et al., 1997). Studies from the literature therefore clearly show that there is no unified particle size and/or range of TiO2, as there are many other parameters being influential on the achievement of effective photocatalytic activity. Decrement in the particle size of TiO2 causes increment in the specific surface area and the large specific surface area of smaller particles contributes to the enhanced photocatalytic activity, most probably due to modifications in the physical and electrochemical properties of TiO2 resulting in higher photon energy absorption, stimulation of the radiation-free transfer of the absorbed photon energy, increase in the number of active surface sites to absorb pollutant molecules and thereby higher electron
hole generation and higher rate of surface reaction of e2 and h1 (Carneiro et al., 2010; Cheng et al., 2014; Chou et al., 2007; Dodd et al., 2006; Jang et al., 2001; Kim et al., 2007; Kominami et al., 1998; Lin et al., 2006; Masakazu & Takahit, 1987; Retamoso et al., 2019; Xu et al., 1999; Yamashita et al., 1996; Yu et al., 2002). Although the increase in particle size improves the optical properties of TiO2, which is beneficial for its photocatalytic activity, the gain in the photocatalytic activity due to improved optical properties is compromised because of decrements in the specific surface area of TiO2 when the particle size increases (Lin et al., 2006). Moreover, the conduction band energy and the band gap energy of photocatalysts increase with the decreasing particle size (Brus, 1986; Lippens & Lannoo, 1989; Retamoso et al., 2019). Increments in the

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conduction band energy level lead to increments in the redox potential stimulating the reduction of oxygen on the catalyst surface and contribute to efficient photocatalytic degradation (Abreu et al., 2012; Wang et al., 2000). This increment in the band gap energy can lead to a decrease in the photocatalytic activity (Koˇc´ı et al., 2012, 2009). However, some studies have stated that the observed increments in the band energy are not possible when the particle size of TiO2 gets smaller than a certain limit and under such cases, band energy can decrease (Lin et al., 2006). It is also known that when the particle size of the photocatalyst is smaller, the charges of e2 and h1 are closer to each other, and this closeness can increase the interaction between them (Anpo et al., 1984). Photogenerated electron
hole (e2/h1) pairs are also formed sufficiently close to the surface, when the particle size of the photocatalyst is smaller, leading the pairs to reach the surface more quickly (Zhang et al., 1998). The closeness in the charges of e2 and h1 can increase the rate of surface recombination (Almquist & Biswas, 2002; Cheng et al., 2014; Harada & Ueda, 1984; Pellegrino et al., 2017; Wang et al., 1997; Yeung et al., 2003; Yuangpho et al., 2015; Zhang et al., 1998) and accumulation of charge carriers (Gao & Zhang, 2001; Yeung et al., 2003; Zhu et al., 2017). As the particle size decreases, the density of recombination centers increases and thus the recombination of photoinduced charges (e2 and h1) is also enhanced (Grela & Colussi, 1996; Lin et al., 2006). Below a certain particle size, the surface recombination process is more dominant than the carriage of photoinduced charges, which affects the photocatalytic activity negatively. This is most probably the reason why an optimum particle size is reported in numerous studies for the achievement of the best photocatalytic activity (Dodd et al., 2006; Zhang et al., 1998) and is in line with the literature, which states that the surface reactions and recombination of charge carriers (e2 and h1) should be balanced to guarantee the achievement of highly effective photocatalytic activity of semiconductors Asilturk et al., 2006; Kominami & Ohtani, 2010; Zhang et al., 1998). In addition, decrement in particle size of TiO2 beyond a certain limit can ease the formation of rapid flocculation and agglomeration, reducing the availability of active surface areas and interaction of pollutants on the surface of the photocatalyst, thereby lowering the photocatalytic activity (Lin et al., 2006; Maira et al., 2000; Pellegrino et al., 2017; Yuangpho et al., 2015).

Crystal structure, crystallite size and crystalline phase of TiO2 TiO2 is a widely preferred photocatalytic material and has two polymorphs (phases), namely, anatase and rutile. Although there are disputed results about the photocatalytic activity of these two phases in the literature, it is generally reconciled that anatase has a more effective photocatalytic degradation capability (Arunmetha et al., 2013; Augustynski, 1993; Bilmes et al., 2000; Brown et al., 1985; Etacheri et al., 2015; Fox & Dulay, 1993; Garcia-Segura & Brillas, 2017; Hassan et al., 2010a,b; Ilie et al., 2017; Jung & Park, 1999; Linsebigler et al., 1995; SafarzadehAmiri et al., 1996; Sclafani et al., 1990; Tanaka et al., 1991; Wang et al., 2015). Despite this general agreement in the literature, it has also been reported by several other researchers that rutile has a better photocatalytic activity than anatase (Jia et al., 2018; Mills et al., 2003; Vijayarangamuthu et al., 2020; Watson et al., 2003).

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Furthermore, a number of studies have reported that the combined use of anatase and rutile phases leads to a better photoelectrochemical activity and degradation of pollutant compounds than the single use of anatase phase (Bacsa & Kiwi, 1998; Kim & Ehrman, 2009; Muggli & Ding, 2001; Ohno et al., 2001; Poon & Cheung, 2007). The differences between the photocatalytic activities of anatase and rutile phases of TiO2 are directly related to their crystal structure, which affects the kinetic behavior of the charge carriers, including their optical and transport properties. It was reported that the band gaps of rutile and anatase TiO2 are 3.00 and 3.20 eV, respectively, indicating that the conduction band of anatase phase is 0.2 eV more than that of rutile phase. Thanks to the excess electrons in the conduction band of anatase resulting in lower oxygen absorption capacity and higher degree of hydroxylation (Gerischer & Heller, 1992; Maruska & Ghosh, 1978; Tanaka et al., 1991), the reduction of reactants is more stimulated in the anatase phase than the rutile phase (Banerjee et al., 2006; McLendon, 1983; Reza et al., 2017). Anatase also exhibits longer lifetimes of photogenerated excess charge carriers because of its lower recombination rate and higher specific surface area owing to the presence of differently oriented facets on its surface, resulting in better photocatalytic degradation capability than rutile (Schindler & Kunst, 1990; Schneider et al., 2014; Xu et al., 2011). Another possible reason for the better photocatalytic activity of anatase is related to the wider optical absorption gap and smaller electron effective mass of anatase, which provide higher mobility of photogenerated e
and h1 compared to rutile (Banerjee et al., 2006; Mo & Ching, 1995). Although it has been reported in many studies that anatase has a more effective photocatalytic activity in general, it should be kept in mind that there are many other factors other than phase type largely affecting the photocatalytic activity such as crystal size, surface area, defects, porosity, and pore size distribution of photocatalyst and others.

Amount of TiO2 In photocatalytic degradation, the amount of TiO2 is one of the key factors for the photocatalytic activity of the process. The utilization of TiO2 at optimum levels is necessary in order to ensure complete/efficient absorption of the photons on the surface of TiO2 particles and lower the excess use of TiO2 in favor of achieving cost-efficiency. The optimum amount of TiO2 to be used for the achievement of a more effective photocatalytic activity varies in accordance with the material-, production-, target substance
, and operation-related factors such as the process of synthesis, particle size/specific surface area of the photocatalyst, concentration, and structure of degraded substances together with the concentration of the pollution (Kry´sa et al., 2004). Many studies in literature have reported that an increase in the amount of photocatalyst leads to an increase in the rate/amount of photocatalytic activity (Azad & Pandey, 2017; Hung & Yuan, 2000; Kumar & Pandey, 2017c; Qamar et al., 2005; Saggioro et al., 2011; Saquib & Muneer, 2003). On the other hand, some other studies claimed that the increase in the photocatalytic activity is valid up to a certain TiO2 amount, after which, there was either decrement or no change in the

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photocatalytic activity (Chen & Liu, 2007; Coleman et al., 2007; Daneshvar et al., 2003; Huang & Wen, 2019; Neppolian et al., 2002; Sohrabi & Ghavami, 2008; Wei & Wan, 1991; Zhang et al., 2002). As the amount of photocatalyst increases above the optimum level, although some of the photocatalyst particles can be activated as a result of the stimulation through the irradiation reaching their surface, there is a likelihood that some particles cannot be activated due to inadequate interaction with the irradiation, resulting in lower overall photocatalytic activity (Chun et al., 2000). Moreover, TiO2 particles are prone to agglomeration and sedimentation, which limits the interaction of the particles with photons and thereby the formation of further photocatalytic reactions especially in the presence of high dosages of TiO2 (Garcia & Takashima, 2003; Kaneco et al., 2004; Li et al., 2010; Pellegrino et al., 2017; So et al., 2002). It was also reported that in the presence of high concentrations of photocatalyst, photoinduced and nonphotoinduced particles can collide with each other before they allow photocatalytic reactions to occur, resulting in the inactivation of induced particles (Asilturk et al., 2006; Neppolian et al., 2002). All these findings from the literature therefore clearly show that there is an optimum amount of TiO2 for the achievement of effective photocatalytic activity, and this situation is also closely related to other influential parameters.

Pollutant concentration Photocatalytic activity of TiO2 depends on the pollutant type and concentration as well. It was stated that the increase in the pollutant (substrate) concentration up to a certain level increases the pollutant degradation rate of the photocatalyst although there are studies found that increment in pollutant concentration causes continuous increments (Mazierski et al., 2017) or decrements (Behnajady et al., 2011; Lucas, 2017; Martinez et al., 2011; Tayeb & Hussein, 2015; Thennarasu & Sivasamy, 2016) in degradation rate. However, beyond a certain level, further increase in the pollutant concentration causes a decrease in the photocatalytic degradation rate (Augugliaro et al., 2002; Kabra et al., 2004; Kiriakidou et al., 1999; Sakthivel et al., 2003; Saquib & Muneer, 2003; Sivalingam et al., 2003; Sohrabi et al., 2009). One of the possible reasons for the reported decrement in photocatalytic activity is that higher pollutant concentrations prevent the irradiation-based induction of the active surface areas on the photocatalysts’ surface, hindering further generation of OH and O22 radicals required for the photocatalytic degradation process (Augugliaro et al., 2002; Chong et al., 2010; Kiriakidou et al., 1999; Kumar & Pandey, 2017a, 2017b; Mazierski et al., 2017; Saquib & Muneer, 2003; Tayeb & Hussein, 2015). One of another possible reason reported for the decrement in photocatalytic degradation rate with increased pollutant concentration is unavailability of sufficient active sites and thereby intermediate products to meet available pollutant substrates for an efficient degradation process (Lucas, 2017). Another possible reason can be that in the case of higher pollutant concentrations, a significant portion of the irradiation triggering the formation of photocatalytic degradation reactions are absorbed more by the pollutant molecules themselves rather than TiO2 molecules, and the formation of OH and O2- radicals disposing the pollutant molecules G

G

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on the photocatalyst surface is prevented, resulting in decrement in the rate of photocatalytic reactions (Jun et al., 2006; Tayeb & Hussein, 2015).

Temperature Reaction temperature has significant role in the kinetic behavior of charge carriers, which are influential on photocatalytic degradation of the pollutants. For a more effective photocatalytic degradation process, the balance between the adsorption and desorption processes needs to be ensured. Considering the effect of temperature on these processes, there is an optimum temperature range and/or value for the photocatalytic degradation process, as reported by the literature (Azad & Pandey, 2017; Gogate & Pandit, 2004; Hermann, 1999; Kim & Hong, 2002; Ollis et al., 1991; Pichat & Hermann, 1989; Soares et al., 2007). Generally, as the temperature increases, the speed of recombination of electron-hole pairs on the photocatalyst surface increases (Kumar & Pandey, 2017a, 2017b; Moser et al., 1987; Zeltner & MA, 1993). Since the combination and recombination of charge carriers are competitive reactions, excessively high temperatures reduce the photocatalytic activity by preventing the formation of redox reactions on the surface of photocatalyst. Lowering the temperature promotes the adsorption of reactants (e.g., NO, NO2, dye) and final reaction product. On the other hand, increase in the temperature increases the desorption of the final reaction product of the redox reactions from the surface of the photocatalyst inhibiting the reactions, thereby reducing the number of radicals formed on the photocatalyst surface. Therefore the desorption of the final reaction products determines the level of reaction in the case of low reaction temperatures while adsorption of reactants determines the level of reaction in the case of high reaction temperatures (Fu et al., 1996; Hashimoto et al., 2005; Hussein, 2016; Peral et al., 1997; Soares et al., 2007).

Irradiation The light absorption intensity of a photocatalyst is significantly effective on its photocatalytic performance and the photocatalytic activity of a photocatalyst improves with the increased light absorption intensity (Zhang et al., 2020). One of the main requirements for the initiation of the photocatalytic activity is to have the energy of photons reached to the surface of photocatalyst to be equivalent to or greater than its electronic band gap. Therefore in addition to the light absorption performance of the photocatalysts, irradiation-related factors affect photon absorption rate of a photocatalyst and thereby the photocatalytic reaction process. For one of these factors, namely, the intensity of light irradiation, it was reported that the reaction rates of photocatalyst increases with the increased light intensity, which results in higher amount of light absorption and further electron
hole formation (Aguado et al., 1994; Bahnemann, 1999; Curco´ et al., 2002; Hung & Yuan, 2000; Xiao et al., 2007). However, despite the availability of increased number of photons with the increased light intensity resulting in further potential for photon activation, owing to limited number of activation areas, the increase in the reaction rates becomes limited and after a certain level; therefore the increments in the light intensity become ineffective on the photocatalytic degradation rate (Ollis & Al-Ekabi,

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1992; Reza et al., 2017; Xiao et al., 2007). In addition, exposure to higher light intensity can lead to increments in the rate of recombination of electron
hole pairs (Aguado et al., 1994).

Humidity Water molecules have a very important role to play in the photocatalytic degradation process of pollutants, namely, transforming the pollutant molecules into harmless products (Zhang et al., 2020). The presence of water molecules in the photocatalytic reaction medium is a necessity for the photocatalytic degradation. Briefly, upon irradiation of the photocatalyst with light, the interaction of the photocatalyst with water molecules is a critical step for the photocatalytic oxidation to take place, which leads to the formation of reactive radical species (mainly hydroxyl and/or superoxide radicals) (Kim et al., 2002). On the other hand, the water molecules also increase the rate of recombination of photoinduced charges (e2 and h1) (Fox & Dulay, 1993; Mark et al., 1993). There is also a tough competition between the water molecules and pollutant molecules for adsorption on the catalyst surface sites decreasing the generation of the number of radicals (Assadi et al., 2012; D’Hennezel et al., 1998; He´quet et al., 2018; Kim & Hong, 2002; Luo & Ollis, 1996; Wang et al., 1998; Zhang et al., 2020). The concentration of water molecules also affects the generation of carbon deposits on the surface of photocatalyst, which decrease the activity of photocatalyst (Einaga et al., 1999, 2002). At higher humidity levels, formation of a film on the surface of the photocatalyst is also possible, which decreases the adsorption of pollutant molecules and thereby blocks the photodegradation reactions (Akbari et al., 2014; Zhang, et al., 2007, 2020). It can be stated in this regard that humidity can impact the photodegradation rate of a pollutant both positively and negatively, depending on its level, hydrophilic/hydrophobic characteristics of the pollutant compounds, and other factors as concluded by several studies available in the literature (Ao et al., 2003; Einaga et al., 1999, 2002; Hay et al., 2015; Kim et al., 2002; Maggos et al., 2007; Zhang et al., 2007, 2020). Therefore in order to ensure an ideal and stable photocatalytic degradation process, the humidity level of the reaction environment needs to be balanced.

5.3

Utilization of TiO2 in cementitious systems for air purification purposes

Researchers with innovative opinions who consider themselves responsible for contributing to sustainability have achieved remarkable progress in developing new generation advanced construction materials in recent years. Environment-friendly multi-functional construction materials with air purification capability act as valuable options in contributing to the sustainability perspectives, and there are increasing efforts in the route of solving/combating the problems related to low air quality, which affects our lives as a whole. The incorporation of different types of

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photocatalysts into the cement-based mixtures, with the aim of equipping the resultant construction material with NOx degradation capability, is one of the initiatives to tackle air pollution. Large surface areas of structural/nonstructural members manufactured by the use of cement-based mixtures make them unique and ideal options that can be used for the stimulation of photocatalytic reactions. One of the most important challenges to achieve effective photocatalytic activity from TiO2-based cementitious composites is to allow the irradiation to reach all surfaces together with the in-depth regions of cement-based specimens/elements. Therefore for an effective photocatalytic activity in the case of cementitious systems, optimized design, development, and production processes are required by taking into account all related factors. There are many studies available in the current literature focusing on the utilization of TiO2 particles within cementitious systems to develop sustainable photocatalytic construction materials with NOx degradation capability. In the following sections, these studies are reviewed, especially from the perspectives of material-, production-, and operation-related parameters.

5.3.1 Relationship between the quality of distribution of TiO2 particles in cement-based systems and NOx degradation capability Agglomeration of nanomaterials including nano-TiO2 is a general problem, which arises because of high surface area/energy of the nanomaterials triggering very strong attractive forces and intense surface interactions among nanosized particles (Kawashima et al., 2014). Furthermore, highly alkaline and calcium-rich environment of cement-based systems further stimulate the agglomeration of nano-TiO2 particles. Agglomeration leads to the formation of nonhomogeneous microstructure, thereby reducing the UV light reachable photocatalyst surface area in the photocatalyst-incorporated matrix, lowering the number of active sites and leading to the occurrence of weak zones in the matrix. Therefore one of the most important requirements to obtain the optimum photocatalytic performance and make sure reliable reproducibility of TiO2-based photocatalytic systems identically is the achievement of homogeneous distribution of nano-TiO2 particles throughout the cementitious matrix (Fig. 5.3). Other characteristic properties of TiO2-incorporated photocatalytic cementitious systems such as microstructure, mechanical properties, etc., are also affected by the homogeneous distribution of TiO2. As in the case of other nanomaterials, selection of proper treatment method to ensure homogeneous distribution of nano-TiO2 particles throughout the cement-based systems requires specific attention, especially in order not to risk the chemical state of the cement-based systems and hydration process. Several studies available in the literature confirm the importance of surface charge on the distribution of nano-TiO2 in cementitious systems and show that pH of the environment can markedly alter the magnitude of the surface charge and hence affect the degree of distribution of photocatalyst (Ca´rdenas et al., 2012; Folli et al., 2010; MacPhee & Folli, 2016). The attraction or repulsion of nano-TiO2

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Figure 5.3 Nano-TiO2-incorporated cementitious systems with (A) nonhomogeneous and (B) homogeneous distribution of nano-TiO2 particles.

particles determines the stability and distribution quality/state of the system depending on their electrical charge (Folli et al., 2010; Oliveira, 2000). Therefore an environment with specific/proper pH is usually required to obtain negatively or positively charged surface properties, which leads to the effective distribution of nano-TiO2 particles. When the pH of the environment increases, a negatively charged surface is obtained, on the contrary, when the pH decreases, a positively charged surface is obtained. Thus via surface charging, particles repellent to each other are obtained so as to avoid agglomeration; and the larger the magnitude of charge, the stronger the repulsion and nonagglomeration property of nano-TiO2 particles is (Fig. 5.4) (MacPhee & Folli, 2016). Zeta potential is an indicator of the surface charge status of the particles and measurement of the magnitude of zeta potential of a system gives valuable information about the stability and surface properties. At pH levels, where the zeta potential is or close to zero, an environment in which the particles can agglomerate, resulting in instability and insufficient distribution can be obtained (Suttiponparnit et al., 2011). Positive and/or negative threshold values for the zeta potential, that can eliminate the agglomeration of particles, have been reported in the available literature such as |25
30| mV (Mandzy et al., 2005), , 2 30 mV (Ca´rdenas et al., 2012), 6 30 mV (Ca´rdenas et al., 2012; Jiang et al., 2009; Pe´rez-Nicola´s et al., 2017), although these values have the potential to be seriously affected by different conditions. It is therefore essential to obtain an environment with an appropriate distribution-oriented pH value (i.e., zeta potential value), considering the characteristics of the developed systems, especially alkaline medium of cementitious paste together with the presence of complex ions in this environment. Accordingly, various studies have been carried out to investigate the distribution of nano-TiO2 particles throughout the cementitious systems and its effects on the properties of such systems specifically considering the photocatalytic activity (Dantas et al., 2019; MacPhee & Folli, 2016; Sahin ¸ et al., 2021; Yousefi et al., 2013). Different procedures have been utilized in the literature in order to uniformly distribute nano-TiO2 particles throughout the cementitious systems with the aim of acquiring photocatalytic capability through manipulating the status of surface charge of the particles. Such procedures include optimization by curing process (Strini et al., 2016), proper

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Figure 5.4 Nano-TiO2 distribution through surface modification (surface charging) Source: Redrawn after Li, Z., Ding, S., Yu, X., Han, B. & Ou, J. (2018). Multifunctional cementitious composites modified with nano titanium dioxide: A review. Composites Part A: Applied Science and Manufacturing, 111, 115
137. https://doi.org/10.1016/j. compositesa.2018.05.019.

selection of the constituents (Giosue` et al., 2018; Strini et al., 2016), mixing of the mixture ingredients with high-shear energy (Dantas et al., 2019) or handheld electric mixer (Jin et al., 2019), incorporation of TiO2 particles as suspension instead of powder form (Hunger et al., 2008), particle surface modification/treatment through chemical and physical processes including the implementation of different mixing procedures via sonication (Karapati et al., 2014; Rhee et al., 2018; Sahin ¸ et al., 2021; Xu, Clack, et al., 2020; Xu, Jin, et al., 2020; Yousefi et al., 2013), utilizing different surfactant materials such as modified polycarboxylate polymer (Ca´rdenas et al., 2012; Pe´rez-Nicola´s et al., 2018), polyacrylic acid
and polycarboxylic ether
based superplasticizer (Sahin ¸ et al., 2021), incorporating saturated Ca(OH)2 solution (Yousefi et al., 2013), doping different metal/non-metal materials such as P
OH surface modification (Folli et al., 2010), Fe doping (Pe´rez-Nicola´s et al., 2017), etc. In more details about some of these studies which are directly focused on ensuring of effective dispersion of TiO2 powder in cementitious systems, Dantas et al. (2019) evaluated the TiO2 dispersion capability of two different dispersion procedures based on two different mixing equipment including “standard energy mix” (standard shear energy) and “high energy dispersion” (high shear energy) and observed that mixing with “high energy dispersion” is more efficient more deagglomeration providing better dispersion quality than that with “standard energy mix.” Sahin ¸ et al. (2021) investigated five different mixing procedures created with combination of different surfactant material content (single or binary utilization of polyacrylic acid
and polycarboxylic ether
based superplasticizer) and different type of mixing equipment (ultrasonic mixer, high-speed kitchen-type hand blender, conventional cement mixer, mixer combinations) to evaluate their TiO2 dispersion capability in cement pastes. According to their results, the mixing procedure based on ultrasonication process and binary utilization of surfactant (mentioned above) provides more homogeneous cementitious system with higher photocatalytic activity compared to those prepared with other mixing methods. They stated that greater

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fluidity can improve the homogeneous dispersion of ingredients in cementitious systems. Yousefi et al. (2013) also made some efforts to obtain a method for efficient dispersion of nano-TiO2 powder in cement media. They prepared nano-TiO2 dispersions by using two different dispersants (deionized water and saturated Ca (OH)2 solution) and two different mixing methods (mechanical stirring or ultrasonication) and obtained better nano-TiO2 dispersion capability from the suspension prepared with saturated Ca(OH)2 solution and ultrasonication. Pe´rezNicola´s et al. (2018) experimented with different superplasticizer molecules to evaluate their compatibility with cement-based systems in terms of dispersion of nanosized photocatalytic particles. According to their results, the use of polycarboxylate-based superplasticizers was an effective method in terms of preventing TiO2 particles from agglomeration although the use of a naphthalene sulfonate formaldehyde polycondensate caused formation of large agglomerations of nanosize TiO2 particles.

5.3.2 Relationship between the particle size of TiO2 in the cement-based systems and NOx degradation capability Considering that the particle size of TiO2 is a significantly influential factor on the activity of photocatalytic systems, studies addressing the relationship between the particle size of TiO2 incorporated into the cement-based systems and photocatalytic activity of such systems have been carried out. Seo and Yun (2017) used two different-size (35 and 100 nm) TiO2 particles and obtained higher NO absorption/ removal rate from mortar mixtures produced with smaller TiO2 particles (35 nm) compared to those with larger TiO2 particles (100 nm) (Seo & Yun, 2017). Their results were found attributable to two factors: (1) smaller-size particles mean the presence of larger number of particles and thus large surface area and high porosity beneficial for NO absorption capability, and (2) smaller-size particles provide higher amount of active surface areas. Similarly, Chen and Poon (2009a), (Husken and Brouwers, 2008); Hunger et al. (2008), Meng et al. (2020), and Folli et al. (2010) found results indicating that TiO2 particles with higher surface area exhibits better photocatalytic degradation capability compared to TiO2 with lower surface area (Chen & Poon, 2009c; Folli et al., 2010; Hunger et al., 2008; Husken & Brouwers, 2008; Meng et al., 2020). Considering the results obtained from abovementioned studies, other factors affecting the photocatalytic degradation capability of the specimens were reported such as 3D structure of the TiO2, porosity of the, kinetic of the reactions in cement-based system, dispersion of the TiO2 particles etc. Contrary to results from above-mentioned studies, Poon and Cheung (2007) obtained better results with the use of TiO2 particles having larger particle size, although they have made their comparison by taking three different types of commercially available TiO2 powders and stated that their findings require further research in order to take other factors influential on the photocatalytic activity (Poon & Cheung, 2007). It was also reported in the works of Pe´rez-Nicola´s et al. (2017), (Husken and Brouwers, 2008), and Folli et al. (2010) that although smaller-size TiO2 particles

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are better, they tend to exhibit more agglomeration, which may lead to the realization of certain decrements in the photocatalytic degradation capability.

5.3.3 Relationship between the amount of TiO2 in the cementbased systems and NOx degradation capability The amount of irradiation-reachable photocatalyst material is an important parameter to be considered in order to obtain an optimum cementitious product that is affordable and characterized with the enhanced photocatalytic degradation capability. Many studies have addressed this issue, yet there is no consensus on the best/ optimum utilization rate of TiO2 in cement-based systems. Considering that the photocatalytic reactions can occur on the surfaces accessible to irradiation and pollutant NOx gas, establishing a direct relationship between the amount of TiO2 substituted and photocatalytic activity is a complex process. Considering that there are many material-, production-, target substance- and operation-related parameters that can affect the performance of cement-based photocatalytic systems, it can be anticipated that the optimum utilization rate to obtain the maximum performance may vary based on the application. Furthermore, the presence of large amounts of nanomaterials can increase the tendency of agglomeration and make their distribution difficult, which can lead to a decrease in photocatalytic degradation activity (Senff et al., 2013; Xu, Clack, et al., 2020; Xu, Jin, et al., 2020). As also reported by, Chen and Poon (2009a), a marked increase in the utilization rate of TiO2 cannot exhibit a proportional increase in photocatalytic degradation capability. Therefore it can be stated that there is a certain threshold for TiO2 utilization rate/range to ensure efficient photocatalytic degradation capability (Karapati et al., 2014; Rhee et al., 2018; Sahin ¸ et al., 2021; Xu, Clack, et al., 2020; Xu, Jin, et al., 2020; Yousefi et al., 2013). Lucas et al. (2013) found different TiO2 incorporation rates (0.5, 1.0, 2.5, 5.0wt.% of cement) as optimum for NOx degradation, which are changeable according to the utilization of different ingredients and compositions. They have found a decreasing trend in the NOx degradation capability of cementitious mortars with the increased utilization rate of NOx and attributed this general outcome to the electron
hole pair deactivation in the case of excess amounts of photocatalyst. It was also stated in this work that the utilization rate of TiO2 and the composition of the mixtures can affect the microstructural properties of the specimens and the photocatalytic degradation capability. Xu, Clack, et al. (2020), Xu, Jin, et al. (2020) observed that utilizing 5% TiO2, by the weight of cement, is optimum for the best photocatalytic capability and the reaction rate of NOx degradation decreased as the TiO2 content was increased from 5% to 10%. Similarly, according to Seo and Yun (2017), TiO2 utilization rate of 5%, by the weight of cement is optimum for the photocatalytic activity and there is no significant increase in the NO removal rate with the increase in the TiO2 utilization rate from 5% to 10%. Senff et al. (2013) also pointed out the optimum TiO2 utilization rate to be 5%, by the weight of cement, considering the NO reduction levels of mixtures having TiO2 with utilization rates ranging from 5% to 20%. Rhee et al. (2018) obtained the best

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photocatalytic activity from cement paste specimen prepared by using utilization rate of 10wt.% of cement among cement paste specimens prepared with utilization rates of 0, 5, 10, 20wt.% of cement and attributed this result to the increase in electron
hole recombination due to relatively large amounts of TiO2 beyond this rate (10%) (Rhee et al., 2018). However, for mortar mixtures, the continuous increase was observed in photocatalytic degradation with increasing TiO2 utilization rate in mortar specimens prepared at the same utilization rates with cement paste specimens (0, 5, 10, 20wt.% of cement), most probably due to relatively less TiO2 by volume compared to paste specimens because of presence of sand providing balance in the redox reactions (preventing excess electron-hole recombination). In some other studies, different utilization ranges of TiO2 were also given such as 0%
10% (Pe´rez-Nicola´s et al., 2015), 3%
10% (De Melo et al., 2012; Hunger et al., 2008), 0%
5% (Ca´rdenas et al., 2012; Guo & Poon, 2018; Hassan et al., 2010a,b; Janus et al., 2019) 0%
3% (Janus et al., 2020), 5%
10% (Chen et al., 2011; Chen & Poon, 2009c), by the weight of cement; replacing 5%
10% weight of cement (Herna´ndez-Rodrı´guez et al., 2019); 0.5%
2.5%, by the weight of binder (Pe´rez-Nicola´s et al., 2017); 0%
5%, by the weight of cementitious materials (Guo et al., 2012); 0.1%
2% (Matˇejka et al., 2012), 0%
10% (Poon & Cheung, 2007), by the weight of the mixture without water and 2%
4%, by the volume of binder (Giosue` et al., 2018). It has been reported in these studies that increasing the utilization rate of TiO2 within the given range increases photocatalytic degradation capability continuously.

5.3.4 Relationship between the type of TiO2 in the cementbased systems and NOx degradation capability The effect of different types (phases) of TiO2 incorporated into the photocatalytic cement-based systems is another point that drove the researchers’ attention with the aim of ensuring optimum photocatalytic performance in terms of cost, producibility, and NOx degradation activity. Numerous papers dealing with the effect of different types of TiO2 on the photocatalytic performance of cement-based systems indicated that cement-based systems containing the combination of anatase and rutile phases of TiO2 in the form of a commercial product named “Degussa P25” outperform those containing the different forms of TiO2 singly in terms of photocatalytic performance (Chen & Poon, 2009b; Poon & Cheung, 2007; Rhee et al., 2018), although there are other works which state the opposite most probably due to production- and operation-related parameters effecting overall performance (Park et al., 2020). Herna´ndez-Rodrı´guez et al. (2019) compared two different commercial TiO2 products including “Kronos” [100% Anatase (7 nm)] and “Degussa P25” [82% Anatase (23 nm) 118% Rutile (44 nm)] at different utilization rates and generally obtained better photocatalytic degradation performance from Kronoscontaining mortars with some exceptions due to operational and content-related factors (Herna´ndez-Rodrı´guez et al., 2019). On the other hand, although it is accepted that anatase is more effective in degrading not only NOx gas but also other

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pollutants (De Melo et al., 2012; Yu & Brouwers, 2009) an opposite behavior was observed in the work of Poon and Cheung (2007), who tested cement-based mortar mixtures with different types of TiO2 for NOx degradation capability and found better results for mixtures with rutile than mixtures with anatase. Ca´rdenas et al. (2013) produced cement paste mixtures containing the combination of anatase and rutile particles with different utilization rates (anatase:rutile—“0:0,” “100:0,” “85:15,” “50:50”) and achieved better NOx degradation performance from mixtures containing anatase-rutile combination of 85:15 at early age of 65 hours. This result was found attributable to the reduction in the charge recombination due to the difference in Fermi levels (the highest energy level of an electron at absolute zero temperature) of anatase and rutile, and ease of transfer of holes/electrons due to closer contact of the anatase and rutile particles. It was also noted that as the anatase-rutile combination was changed to be 50:50, early age NOx degradation performance similar to the case of anatase-rutile combination of 85:15 could not be obtained because of decrements in the surface active sites as a result of the increments in the number of larger-size particles. On the other hand, at 28 days, mixtures containing only anatase (100:0) showed better photocatalytic degradation performance, possibly because of the fact that the microstructure got densified thanks to ongoing hydration reactions further blocking the mixtures containing the combination of anatase-rutile in terms of photocatalytic activity compared to those containing anatase solely.

5.3.5 Relationship between the combined presence of metal/ non-metals and TiO2 in the cement-based systems and NOx degradation capability Doping TiO2-based cementitious systems with metallic/nonmetallic materials is a method to improve the photocatalytic degradation capability. Lucas et al. (2013) developed mortar mixtures containing Fe-doped nano-TiO2, which outperformed the mortar mixture containing nonFe-doped nano-TiO2 in terms of NOx degradation. This result was associated to the narrowing of the band gap (Fig. 5.5) and inhibition of the electron
hole recombination as a result of Fe doping (Moser et al., 1987; Yamashita et al., 1996). Similarly, Hunger et al. (2008) obtained better degradation results from concretes containing carbon-doped TiO2 than those containing noncarbon-doped TiO2 (Hunger et al., 2008). They stated that this finding was obtained as concrete products containing carbon-doped TiO2 use UV-A radiation more efficiently. MacPhee and Folli (2016) stated that although doping of TiO2s (such as with tungsten and niobium) can affect the photonic efficiency negatively, doped TiO2s have better visible light response and nitrate selectivity providing removal of NO and highly toxic NO2 from the atmosphere, thanks to high nitrate conversion rates, which is very important to improve the air quality in urban areas (MacPhee & Folli, 2016). Although the effect of different parameters was noted, results similar to the work of MacPhee and Folli (2016) were also observed in the work of

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Figure 5.5 The influential mechanism of metal ion doping TiO2/Bad gap (Eg) narrowing. Source: Redrawn after (Li et al., 2020).

Pe´rez-Nicola´s et al. (2017), where visible light-sensitive photocatalytic performance was obtained from cement mortars through the incorporation of iron- and vanadiumdoped TiO2. Pe´rez-Nicola´s et al. (2015) obtained the worst photocatalytic degradation results from mortars containing iron-rich calcium aluminate cement compared to those containing iron-lean cement. The reason for this finding was stated to be the generation of iron titanates (with lower band gaps and photocatalytic activity) as a result of the interaction between the TiO2 and ferrite phase available in the iron-rich calcium aluminate cement. Laplaza et al. (2017) observed that incorporation of iron oxide
based pigments into cement-based systems containing TiO2 can enhance the visible light performance of TiO2 and is influential on the separation/occurrence of photoinduced electron
hole pairs, recombination of the photogenerated electron
hole pairs, and formation of OH radicals, which can sabotage or enhance the photocatalytic activity (Laplaza et al., 2017). They also stated that Fe/Ti ratio and the band edge position of pigment
cement heterostructure are important parameters affecting the electron transfer between the conduction band of the pigment and the photocatalytic colored cement mortar, which are determining factors on the abovementioned behavior. The effect of the presence of transition metals or mixed oxides on the photocatalytic activity is a complex issue, as there are many factors influencing the phenomenon and there is possible variability potential depending on the nature/properties/ proportion of the material used, the method selected and so on. Herna´ndezRodrı´guez et al. (2019) obtained better photocatalytic degradation results from mortar mixtures manufactured with gray cement compared to white cement, and they attributed this result to the higher Fe2O3 content of gray cement, which is regarded as photocatalytic activity enhancer up to a certain level. They also obtained better

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NOx removal rates from mixtures containing sand compared to mixtures without sand most probably because of higher aggregation behavior of the nano-TiO2 particles and possibility of chemical interaction between the TiO2 and smaller-size cement particles. However, Chen and Poon (2009a) arrived at a contrasting result and stated that the utilization of white cement is better for the increased photocatalytic degradation capability due to light absorption characteristics of white cement. The presence of transition metals (manganese and iron) in gray cement, which results in absorption or blocking a portion of incident photons usable for the photocatalytic reactions and electron
hole recombination was also stated to be responsible for the lower degradation capability of gray cement. Similarly, in the work of Guo and Poon (2018), NOx degradation activity of white cement paste was found to be better compared to that of gray ordinary Portland cement paste. They attributed this result to the presence of higher quantities of Fe2O3 in the ordinary Portland cement, which led to higher light absorption ability and stronger charge transfer resistance in ordinary Portland cement resulting in higher rate of electron
hole pair recombination compared to that of electron
hole pair generation. They also incorporated different dosages of Fe2O3 (2, 5, and 10 wt.% of white cement) into the pastes as a replacement of white cement and observed that increase in the Fe2O3 dosage caused decrease in the NOx removal rate. Park et al. (2020) also obtained better NO removal capacity from mortars with white cement compared to those with gray cement due to higher light absorption ability of the gray cement leading to fewer electron
hole pairs (Park et al., 2020).

5.3.6 Relationship between the mixture composition of cementbased systems and NOx degradation capability Although the properties of TiO2 used as photocatalyst in cement-based systems have noticeable effects on the NOx degradation capability, the mixture composition of such systems undeniably affects the activity as well, especially considering the content-specific microstructural changes. Pe´rez-Nicola´s et al. (2017) manufactured mortars with different types of binders, namely, Portland cement, high-alumina cement, low-alumina cement, and dry slaked lime and obtained relatively higher photocatalytic degradation activity from mortars prepared by using high-alumina cement and lime separately, which contained higher amounts of pores with diameters above 0.05 μm allowing easier reach of NO molecules into more in-depth parts of the mortars in favor of further contact between the surface of photocatalytic materials and pollutants. In addition, they attributed the relatively higher photocatalytic activity of calcium aluminate cement
containing mixtures to the sensitivity of calcium aluminates to illumination and concluded that the availability of significant amounts of calcium carbonate in these mortars (both high-alumina cement and lime-based mortars) is also influential on the better photocatalytic degradation capability of these mortars. Pe´rez-Nicola´s et al. (2015) stated that the presence of calcium aluminates in the cement-based systems is beneficial for NOx abatement rate. Lucas et al. (2013) prepared several mortars by using different types of binders

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(e.g., lime, cement and gypsum) and established links between the microstructure and photocatalytic performance of these mortars. They emphasized that high porosity promotes the photocatalytic activity by facilitating gas diffusion in an effort to assist the contact between the photocatalyst and NOx gas at the irradiated surface, and the diameter of pores should not be less than 1 μm (nanopores) to restrict the gas diffusion, as also reported by Giosue` et al. (2018). On the other hand, Lucas et al. (2013) also concluded that exceedance of the optimum TiO2 content, the presence of excessive porosity and overabundant light exposure of TiO2 on the surface may trigger higher electron
hole recombination and thus decrease the photocatalytic degradation capability. Similarly, Sugran˜ez et al. (2013) stated that the formation of higher amount of hydration products provides a denser microstructure through the filling of the pores, while lower amount of hydration products and the amount of adsorbed water filling the space between the cement grains provide a more porous structure, which is beneficial for higher photocatalytic activity (Sugran˜ez et al., 2013). They also concluded that photocatalytic activity is enhanced as the total porosity increases because of higher possibility of having larger number of active sites accessible to the molecules needed for the photocatalytic reactions and increasing in the generation/amount of hydroxyl radicals to react with NOx, and they proposed that poor packaging of aggregate particles (through modification of particle-size distribution) is a method that can be used to create a macroporosity structure. In order to achieve improved photocatalytic activity through altering the porosity of cementitious materials, Chen and Poon (2009a,b) and Poon and Cheung (2007) recommended changing the particle-size distribution of aggregates or decreasing the cement content of the mixtures. Xu, Clack et al. (2020), Xu, Jin et al. (2020) obtained a more porous structure by using fly ash. Meng et al. (2020), Matˇejka et al. (2012), (Lee et al., 2014), (Guo et al., 2015b), and (Boonen & Beeldens, 2014) observed increments in the NO removal rate with increased porosity as well. Chen et al. (2020) also noted the positive contribution of porosity to the photocatalytic activity, as it provides a channel for pollutants to easily flow through, more surface area for the deposition of photocatalysts, and more space for holding/ accumulation of final products, which prevents the covering of the surface of the photocatalyst (Fig. 5.6). In contrast to above-mentioned works, Giosue` et al. (2018) stated that macroporosity is not beneficial for increasing the photocatalytic capability of mortar specimens. Jin et al. (2019) stated that the presence of larger amount of micropores (,5 nm) in cement-based mortars is beneficial for NOx uptake (Giosue` et al., 2018; Jin et al., 2019). In addition, Jimenez-Relinque et al. (2015) achieved a more effective NOx reduction process in a given porosity range (which was below a specified limit value of 1 μm as reported by Lucas et al. (2013) for the enhanced photocatalytic performance), and stated that excessive porosity reduced the photocatalytic activity by letting the pollutant gas into the nonirradiated regions of the cementbased matrix (Jimenez-Relinque et al., 2015). In addition to the effect of the cement-based system’s microstructure formed depending on the mixture composition on the photocatalytic activity, Jimenez-Relinque et al. (2015) stated that difference in mixture composition (e.g., utilization of different binder materials) can also

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Figure 5.6 Penetration of rays and NOx gases into deeper layers of cement-based systems with pores and crushed glass. Source: Redrawn after Chen, J. & Poon, C. S. (2009b). Photocatalytic activity of titanium dioxide modified concrete materials
Influence of utilizing recycled glass cullets as aggregates. Journal of Environmental Management, 90(11), 3436
3442. https://doi.org/ 10.1016/j.jenvman.2009.05.029.

affect the photocatalytic activity by causing chemically mixture-specific/different photocatalytic reaction mediums (e.g., chemically different aqueous phases in the pores and different photoabsorption energies resulting different oxidation-reduction potentials). Poon and Cheung (2007) and Chen et al. (2020) observed that utilization of aggregates with higher porosity and low specific density provides an increase in the NO removal capability of the final cementitious products. They also revealed that owing to the high light transmitting characteristic of the recycled glass particles, incorporating recycled crushed glass cullet into the cement-based systems as aggregates let photocatalytic reaction-provocative light reach to deeper TiO2 particles, activate them and lead to an enhanced photocatalytic degradation process (Fig. 5.6). Chen and Poon (2009b), Guo et al. (2012), and (Guo & Poon, 2013) observed similar results that utilization of glass cullet as aggregates promises better NO removal performance for the cement-based systems. They also indicated that light-tinted glasses are more effective in increasing the photocatalytic activity thanks to their low UV absorbance and the light-transmitting property of glass aggregates has significant impact on the photocatalytic performance of cementbased systems. In their work, (Guo & Poon, 2013) also used pigments with different colors and observed that the utilization of different-colored pigments in cementbased systems reduces the NO removal rate of photocatalytic products most probably due to increase in the light absorption capability, covering of TiO2 particles and filling of the pores with the pigments. They further concluded that pigments with lighter colors lower the photocatalytic performance less than those with darker colors having higher light absorption capability. Giosue` et al. (2020) used aggregates with different colors and obtained the lowest NO removal activity from the mortar mixtures containing activated carbon (Giosue` et al., 2020). This was found attributable to the dark color of the activated carbon, which results in lower

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reflectance of radiation. Chen et al. (2018) prepared separate mortar mixtures by using two different types of aggregates and found that the mortar mixture containing aggregates with the rough surface, high water absorption, many cracks and high porosity had higher photocatalytic activity, most probably because of the higher potential of this type of aggregate to accommodate more TiO2 particles in their pores and cracks. Similarly, Giosue` et al. (2020) obtained better NO removal rate and selectivity (low) from mortars containing aggregates with higher water absorption due to increased availability of absorbed water providing enhancement in the production of OH radicals.

5.3.7 Relationship between the abrasion/wearing/weathering of the surface of cement-based systems and NOx degradation capability Abrasion/wearing of the photocatalytic surface is an important issue considering the real-time exposure conditions of the photocatalytic cement-based systems. In this route, some researchers performed studies positioned on the assessment of abrasion/ wearing/weathering effects on the photocatalytic activity of the cement-based systems. Hassan et al. (2010b) performed a study to evaluate wear and abrasion resistance of the TiO2 incorporated cement-based systems by using an accelerated loading test and rotary abrasion (Hassan et al., 2010a,b). Although they observed a small decrease in the NO removal performance for the cement-based mixtures with high amount of TiO2 substitution (5%) after abrasion and wearing, they obtained better NO removal performance from mixtures with low amount of substitution (3%) after the same processes. They also observed from the energy-dispersive X-ray spectroscopy analysis that the relative concentration of Ti on the worn specimens, which underwent the abrasion and wearing process, did not essentially change in comparison to the original samples. It was observed in the work of Chen et al. (2020) that the photocatalytic cement-based specimens can show some loss of photocatalytic activity after abrasion. They stated that increase in the rough and porous material content can reduce the loss potential of photocatalytic activity of the specimens after abrasion, namely a more preferable antiabrasion performance since the rough surfaces can enable photocatalysts to clamp and photocatalysts accommodating in deeper pores/spaces can become available to receive UV rays after the abrasion. (Guo & Poon, 2013) observed not-so-evident fluctuations in the NO removal rates at the end of abrasion applied to concrete specimens for different periods of time most probably due to the scattered distribution of TiO2 particles throughout the photocatalytic specimens and stated that the incorporation of TiO2 into cement-based systems is a robust and good option to prevent loss of photocatalytic performance caused by abrasion when compared to coating/spraying applications (Fig. 5.7). (Boonen & Beeldens, 2014) also stated that incorporation of TiO2 into cement-based systems provides more stable photocatalytic performance under wearing action although only the TiO2 at the irradiation-reached surface can be stimulated for the degradation process. (Guo et al., 2015a) tried to simulate

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Figure 5.7 TiO2-incorporated cement-based systems (A) new material, (B) after abrasion.

weathering actions on the air cleaning properties of cement-based systems by using “Rain/dry” and “day/night” cycles. They observed reduction in the photocatalytic activity of the TiO2-incorporated specimens after weathering which was attributed to the reduction in the blocked pores by the deposition of impurities moved due to repeated raining process reducing NOx removal ability by preventing penetration of light and NO gas to deeper to contact with the TiO2 particles.

5.3.8 Relationship between the curing age/condition of cementbased systems and NOx degradation capability Curing age/condition applied to photocatalytic cement-based specimens/products have direct influence on the development of microstructure, therefore can be considered as critical factors affecting the NOx degradation capability. In general, NOx degradation capability of cement-based systems shows a decreasing trend with the prolonged curing due to time-dependent changes in the microstructure of the cement-based systems (Chen & Poon, 2009a,b; Chen et al., 2020; Garcı´a et al., 2018; Guo & Poon, 2018; Poon & Cheung, 2007; Sahin ¸ et al., 2021; Sugran˜ez et al., 2013). One of the main reasons for the observed decrease in the photocatalytic activity with prolonged curing is ongoing hydration reactions affecting pore structure and ion concentration by reducing the size/volume/connectivity of the pores together with the ion concentration of the pore solution. Ongoing hydration also changes the microstructure making the cementitious matrix denser, coating TiO2 particles with the ongoing hydration products and lowering the adsorption capacity of the photons and target pollutants (Fig. 5.8). Another possible reason for the lower photocatalytic performance with the extended curing periods is the formation of carbonation products on the surface of cement-based specimens causing the surface to be coated. Carbonation is an influential mechanism on the pore characteristics of cement-based systems; therefore it can manipulate/affect the overall photocatalytic degradation capability of the cement-based systems. In the work of Chen and Poon (2009a), cement-based specimens were subjected to accelerated carbonation, which caused decrements in the NOx removal performance. This finding was

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Figure 5.8 Representative scheme showing the blockage of photocatalytic reactions due to ageing (A) new material, (B) aged material. Source: Redrawn after (Vittoriadiamanti & PedeferriConcrete, 2013).

attributed to the increased volume of carbonation products as a result of the conversion of Ca(OH)2 into CaCO3, filling the pores, reduction the total porosity, thereby blocking the diffusivity of NOx into the cement-based matrix.

5.3.9 Relationship between the final surface texture of cementbased systems and NOx degradation capability It has been verified by a number of studies that the design of cement-based materials/products with a rougher surface (more open roughness) provides higher photocatalytic activity because of the availability of higher active surface areas for the photocatalytic reactions and exposure of larger amounts of TiO2 particles to lights and pollutants (De Melo et al., 2012; Jimenez-Relinque et al., 2015; Jin et al., 2019; Laplaza et al., 2017; Rhee et al., 2018; Sugran˜ez et al., 2013). However, photocatalytic materials with high surface roughness are likely to accumulate dirt, dust, or any blocking materials on their surface that will prevent photocatalytic reactions (Fig. 5.9) (Rhee et al., 2018). (Poon & Cheung, 2006) also reported that because of low possibility of accumulation of dust, dirt, oil, and grease on the surface, a product with dense surface texture provides much more stable photocatalytic degradation activity over time, (i.e., lower decrease in degradation capability with prolonged time).

5.3.10 Relationship between operation-related parameters and NOx degradation capability In addition to the material- and production-related parameters detailed above, environment- and operation-related parameters including humidity, pollutant concentration, and irradiation to be exposed may affect the photocatalytic activity of cement-based materials. In terms of pollutant concentration (amount), although there are contradicting results in the literature, there is a general consensus that increased pollutant concentration decreases the rate of photocatalytic degradation. In the work of Lucas et al. (2013), it was observed that the decrease in the pollutant (NOx) concentration (from 0.7 to 0.5 ppmv and from 0.5 to 0.3 ppmv) increased the photocatalytic

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Figure 5.9 Blocking the interaction between TiO2 particles and irradiation/pollutants because of the accumulation of blocking materials on rough concrete surface, (A) new material (B) old material (under service).

activity of specimens. They also reported that for pollutant concentration beyond a certain level, in which all active sites can be used in the NOx degradation process, an excessive amount of intermediate reaction products is formed, which sabotage the degradation process. It was observed in different studies that the increase in the inlet NO concentration (Ballari et al., 2010, 2011; Bolte, 2009; Hunger et al., 2008) and flow rate (Ballari et al., 2010; Bolte, 2009; Folli et al., 2010; Hunger et al., 2008) decrease the photocatalytic degradation rate. Contrary to these results, (Yang et al., 2019) found results indicating that the increase in NO flow rate (L/min) and initial NO concentration (ppb) cause increase in the photocatalytic activity of mortars. It was also noted that the increased blockage of the photocatalyst surface with NO molecules can increase the rate of photocatalytic reaction between photocatalyst particles and NO molecules. There are studies that have been performed to appraise the effect of irradiation to be exposed on the photocatalytic activity of cement-based systems. (Yang et al., 2019) indicated that the utilization efficiency of incident photons from light source is reduced with the increase in incident UV-A light intensity, causing decrement in the photocatalytic activity of photocatalytic products. In contrast to the results found in (Yang et al., 2019), Ballari et al. (2011) stated that better NOx conversion rates are obtained when the irradiance is increased, and they attributed this result to the increment in the production of electrons/holes leading to the generation of higher amount of hydroxyl radicals. Similarly, Bolte (2009) and Hunger et al. (2008) observed increments in the NOx degradation rates with the increments in UV-A intensity. In addition to light intensity, type of irradiation and light source are influential on the photocatalytic degradation capability of cement-based systems as well. In this regard, (Guo et al., 2015b) obtained lower NO conversion results under sunlight irradiation compared to those under UV-A irradiation. (Guo et al., 2015a) and

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Hunger et al. (2008) obtained better photocatalytic degradation results under UV-A irradiation compared to visible light. Pe´rez-Nicola´s et al. (2017) used three different light sources including UV light, solar light and visible light. They obtained the highest NO removal results from the specimens irradiated by UV light regardless of other factors. The NO removal capability of identical specimens under two other light sources (solar and visible) varied depending on the mixture content and curing condition. Relative humidity is another influential parameter on the photocatalytic degradation capability of the cement-based systems. Hunger et al. (2008), Ballari et al. (2011), Herna´ndez-Rodrı´guez et al. (2019), (Boonen & Beeldens, 2014), (Husken and Brouwers, 2008), and Seo and Yun (2017) obtained higher NO degradation rates under lower relative humidity conditions, which was associated to the competition between excess water molecules at high humidity and pollutant molecules to occupy the active TiO2 sites. Presence of excessive water can prevent further interaction between the pollutant and TiO2 particles because of the blocking effect of absorbed relative humidity on the penetration of pollutant gases to places where TiO2 particles are.

5.4

Conclusions

In this chapter, an overview of the photocatalytic cement-based materials incorporated with TiO2 is given. Utilization of TiO2 within the cement-based systems is a very attractive approach for the development of multi-functional construction materials characterized with air purification (NOx degradation) capability, and recent studies have clearly demonstrated that functional photocatalytic cement-based materials can be developed with the use of TiO2. Research studies available in the literature also confirm the existence of many different parameters such as mixture composition (e.g., presence of glass as aggregate), color of the ingredients, presence of transition metals/mixed oxides, microstructure, type/amount/particle size of the TiO2 utilized, dispersion quality of the TiO2 powder throughout the matrix, relative humidity, irradiation (type, intensity), pollutant concentration, curing age, weathering/abrasion conditions, pore surface texture etc. being influential on the NOx reduction capability of TiO2-incorporated cement-based materials. Generally speaking, establishing a state to stabilize the redox reactions occurring during photocatalytic reactions is essential to achieve optimum degradation activity. In addition, it is necessary to eliminate the conditions that would sabotage/block the sufficient interaction of TiO2, pollutant (NOx) and irradiation as the main reactants of photocatalytic degradation reactions. Therefore as a conclusion, it is necessary to provide a standardized and balanced production process, taking into account the effects of many different parameters for high NOx degradation activity. In addition, an optimized production process ideally leads to obtain a commercializable and widely available/applicable product or process for air purifying applications in construction industry with a compelling value vision of the future. Considering that

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transformation from conventional construction to sustainable construction is advancing at a rapid pace, the merging photocatalytic degradation phenomenon with cementitious systems/construction industry is a promising approach with a great potential to become widespread.

Acknowledgment The authors gratefully acknowledge the financial assistance of the Scientific and Technical Research Council (TUBITAK) of Turkey provided under Project: 118M197.

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Nano-modification of the rheological properties of cementitious composites

6

Ayoub Dehghani and Farhad Aslani Materials and Structures Innovation Group, School of Engineering, The University of Western Australia, Crawley, WA, Australia

6.1

Introduction

The term “rheology” was coined by Bingham in 1920 as the science of the deformation and flow of materials, especially fluids. Rheology considers those properties of materials that provide an overview of the viscoelastic flow behavior of materials. Fluids can be classified on the basis of different properties, including viscosity, conductivity, density, and compressibility. Fluids are separated into five primary groups according to viscosity: ideal, real, Newtonian, non-Newtonian fluid, and ideal plastic. The flow properties of Newtonian fluids are defined using a single coefficient of viscosity for a specific temperature, which does not change with strain rates. However, the behavior of non-Newtonian fluids is studied by rheology, which relates applied stresses with strain rates. Cementitious materials are classified as non-Newtonian fluids. Therefore rheological properties can be used to control the fresh properties of these materials. Flowability, compatibility, and stability are the essential fresh properties of cementitious materials correlated with rheology (Banfill, 1990). However, the rheological characteristics of cementitious materials are not straightforward to measure owing to the presence of a wide range of grain sizes
from 1 μm cement particles to 20 mm aggregates. Hence the flow properties of cementitious materials are typically determined using approaches defined in codes, provided that most of these methods partially measure the flow characteristics of cementitious materials. This book chapter first provides a theoretical background regarding fluid and cementitious composites rheology. It is followed by available test methods to measure the rheological properties of cementitious composites. Finally, the effects of nanomaterials on the rheological behavior of such composites are discussed.

6.2

Theoretical background

6.2.1 Suspension rheology and rheological models for cementbased systems Cementitious composites in the fresh state can be regarded as the suspension of solid particles in cement paste acting as a viscous and nonhomogeneous fluid. Recent Advances in Nano-Tailored Multi-Functional Cementitious Composites. DOI: https://doi.org/10.1016/B978-0-323-85229-6.00002-0 © 2022 Elsevier Ltd. All rights reserved.

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The deformation of such suspension in the fresh state is characterized by shear flow as one of the two basic types of flow, shear, or extensional flow. Fluid elements move over or pass each other in a shear flow, whereas they move toward or away from each other in extensional flow. The latter is rarely observed in cementitious composites (Banfill, 2003) and will not be discussed further in this chapter. In a shear flow, hypothetical fluid layers move because of external shear force (F),
dv and a velocity gradient dh is induced in the fluid (see Fig. 6.1). In Newtonian liquids such as water and thin motor oils, the velocity gradient, which is equal to the shear rate _ is proportional to shear force through viscosity ðηÞ, as given by Eq. (6.1). γ, τ5

F dv 5 ηU 5 ηU γ_ A dh

(6.1)

where τ is shear stress and represents the force per unit area required to produce the shearing action, and A is the area of hypothetical layers. In this formula, viscosity represents the internal friction of the fluid. When the viscosity of a fluid increases, a higher shear force is required to move it physically for pouring, casting, spraying, or mixing purposes. In contrast to Eq. (6.1), a non-Newtonian fluid is generally defined when the relationship between shear stress and strain is not linear. Hence the viscosity of such fluid changes when the shear rate is changed. Liquids or suspensions containing molecules and particles with different shapes and sizes mostly exhibit nonNewtonian flow. This is because the size, shape, and cohesiveness of molecules and particles control the level of force required to pass by each other during the flow. Therefore by changing the applied shear rate, particle alignment is likely to change, leading to a distinct demand force to maintain fluid motion. Fig. 6.2 compares the idealized rheological performance of the most common types of non-Newtonian flows with the Newtonian flow. The change in the viscosity of fluids over time at a constant shear rate is typically defined by thixotropy and rheopexy. At a constant shear rate, the viscosity decreases over time in a thixotropic fluid, whereas it increases in the case of rheopexy behavior. Thixotropic and rheopectic behaviors may be observed during any of the flow behaviors shown in Fig. 6.2. Cementitious composites, such as conventional concrete, are known to exhibit thixotropic behavior (Roussel, 2006). Fig. 6.3 shows how a thixotropic flow reacts at a viscometric test cycle. Researchers have typically followed two approaches to formulate the rheological behavior of concentrated suspensions, such as cementitious composites including

Figure 6.1 Viscous flow in a Newtonian fluid.

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Figure 6.2 Non-Newtonian flows in comparison to the Newtonian flow

Figure 6.3 Schematic response of a thixotropic fluid to varying rates of shear (A) shear stress
shear rate and (B) viscosity
shear rate.

conventional concrete. In the first approach, a relationship will be established between the viscosity and concentration of a suspension. In contrast, the second approach shows a relationship between shear stress and shear rate. The most famous formulas developed on the basis of these two approaches are listed in Tables 6.1 and 6.2. The models listed in the former cannot be used to describe the rheology of the cementitious composite containing aggregates such as concrete since such suspensions exhibit complex flow behavior. However, they may use to represent the flow of cement paste (Ferraris, 1999). Table 6.2 includes most of the rheology modelsused to describe cement concrete and paste flow behavior. As can be seen, most of these models incorporate the yield stress, representing the minimum shear stress required to initiate flow. When the applied shear stress is less than the yield stress, no permanent deformation occurs, and the material behave elastically.

Table 6.1 Formulas proposed to relate viscosity to filler concentration. Reference

Mathematical formulationa

Comment

Einstein model (Mendoza et al., 2009) Roscoe model (Roscoe, 1952)

η=η0 5 1 1 ½η[

(6.2)

η=η0 5 ð12[=[c Þ22:5

(6.3)

Krieger
Dougherty model (Krieger & Dougherty, 1959)

η=η0 5 ð12[=[m Þð2½η[m Þ

(6.4)

A semiempirical formula considering the critical fraction ([m ) typically falling between 0.6 and 0.7

Mooney model (Mooney, 1951)

η=η0 5 exp½ð½η[Þ=ð1 2 [=[m Þ

(6.5)

A suspension of finite concentration

An infinitely dilute suspension of rigid and noninteracting spherical particles ([η] 5 2.5) For spherical particles in different sizes with interaction

a Note: η0 is the viscosity of the media without solid filler (i.e., pure liquid), Φ is the volume fraction of filler, Φm is the maximum solid concentration at which solid filler would prevent any flow (i.e., critical fraction) and depends on the particle shape and size distribution, η is the intrinsic viscosity, which is a measure of the influence of particles on viscosity.

Table 6.2 Formulas proposed to relate shear stress to shear rate. Reference

Mathematical formulation

Comments

Newtonian model (Chhabra, 2010) Bingham model (Tattersall & Banfill, 1983)

τ 5 ηγ_

(6.6)

τ 5 τ 0 1 ηγ_

(6.7)

Power-law model (Atzeni et al., 1985) Herschel
Bulkley model (Atzeni et al., 1985)

τ 5 Aγ_ n

(6.8)

τ 5 τ 0 1 Aγ_ n

(6.9)

Eyring model (Atzeni et al., 1985)

τ 5 Asinh21 ðBγ_ Þ

(6.10)


 _ τ 5 τ 0 1 Bsinh21 γ=C

(6.11)

_ τ 5 Aðγ1B Þb

(6.12)

γ_ 5 ατ 2 1 βτ 1 δ

(6.13)

vom Berg model (vom Berg, 1979) Robertson
Stiff model (Barness et al., 1989)

Limited to linear rheological behavior and no estimation of yield stress. Simple model facilitating analytical solutions; limited to linear flow curves and is not recommended for highly pseudoplastic cement systems such as those with low w/b, and those containing viscosity modifying admixtures (VMAs) and supplementary cementitious materials (SCMs); inefficient to model the nonlinear portion of the flow curve at low shear rates. n 5 1: Newtonian flow; n . 1: shear-thickening; n , 1: shearthinning; no estimation of yield stress. Embraces all Newtonian, Bingham, and Power models; applicable for model describes shear-thickening and shearthinning flows; inaccurate prediction of yield stress at a low shear rates. No estimation of yield stress; needs complex calculation; weak correlation with experimental flow data for pseudoplastic cement systems even with limited shear-thinning behavior. Only suitable for cement pastes at the low shear rates; complex calculation. Less accurate in terms of correlation; complex calculation.

(Continued)

Table 6.2 (Continued) Reference

Mathematical formulation 21

Atzeni et al. model (Atzeni et al., 1983) Modified Bingham model (Yahia & Khayat, 2001) Casson model (Nehdi & Rahman, 2004)

τ 5 τ 0 1 aγ_ 1 bsinh




Comments

_ γ=c



(6.14)

τ 5 τ 0 1 ηγ_ 1 cγ_ 2

(6.15)

τ 1=2 5 τ 0 1=2 1 ðηγ_ Þ1=2

(6.16)

De Kee model (Vikan et al., 2007)

τ 5 τ 0 1 ηe2αγ_

(6.17)

Sisko model (Papo, 1988) Williamson model (Papo, 1988)

τ 5 ηN γ_ 1 bγ_ c

(6.18)

Yahia
Khayat model (Yahia & Khayat, 2003)

γ_ γ_ 1 Γ

(6.19)

pffiffiffiffiffiffiffiffiffiffiffipffiffiffi τ 5 τ 0 1 ηN γ_ 1 2 τ 0 ηN γ_ e2αγ_

(6.20)

τ 5 ηN γ_ 1 τ f

Eq. (6.14) needs four parameters and complex calculation; the parabolic model (Eq. 6.13) was recommended due to its simplicity and accuracy in data elaboration. More accurate for cementitious materials with nonlinear flow curve; weak correlation with experimental flow data for highly shear-thickening flow. Not suitable for highly concentrated suspensions such as cement pastes with low water-to-cement ratio; weak correlation in the case of graphene-based nanomaterials. Adequate to model the rheology of cement systems with shearthinning or shear-thickening behavior; needs relatively complex calculation. No estimation of yield stress; shows adequate fitting with flow data of cement pastes. No estimation of yield stress; it has been rarely used for cement systems. Needs complex calculation; has a significant limitation on predicting the yield stress of pseudoplastic mixtures.

Note: η0 is the viscosity of the media without solid filler (i.e., pure liquid), Φ is the volume fraction of filler,Φm is the maximum solid concentration at which solid filler would prevent any flow (i.e., critical fraction) and depends on the particle shape and size distribution, ½η is the intrinsic viscosity, which is a measure of the influence of particles on viscosity. τ 0 5 yield stress; A, B, C, a, b, c, α, β, δ 5 constant;ηN 5 viscosity at infinite shear rate; Γ 5 deviation from the Bingham behavior, and τ f 5 intercept of the asymptote of the flow curve with the shear stress axis.

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It is known that a single shear stress
shear rate model cannot fully describe the complex rheology of cementitious composites (Nehdi & Rahman, 2004). Several physical and chemical factors affect the rheological behavior of cementitious composites, such as water to binder ratio, the type of binder, particle-size distribution, particle shape and interactions (Nguyen et al., 2011; Rahman et al., 2015; Svermova et al., 2003; Yahia & Khayat, 2003; Yahia, 2011). Models introduced in Table 6.2 can achieve various levels of success in describing the flow of cementitious composites (Atzeni et al., 1985; Papo, 1988; Yahia & Khayat, 2003). Their accuracy mainly depends on their abilities to appropriately fit the nonlinear part of the shear stress-shear strain curve at low shear rates. It has been established that cement-based systems mostly exhibit non-Newtonian flow properties, indicating a nonlinear relationship between shear stress and shear rate. Therefore models such as Newtonian and Bingham characterized by a linear flow curve (i.e., shear stress and shear rate curve) cannot precisely describe the rheological behavior of cement-based systems (Atzeni et al., 1985). However, the Bingham model has been used by several researchers to define the rheological behavior of cementitious materials (Ferraris et al., 2001; Papo, 1988; Rehman et al., 2018; Vikan et al., 2007; Yahia & Khayat, 2001). Such a wide usage of the Bingham model roots in its simplicity that facilitates analytical solutions and its capability to fit a wide range of data over a limited range of shear rates (Yahia & Khayat, 2003). However, this model is not recommended for highly pseudoplastic cement systems, such as those prepared with low water to binder ratio (w/b) or those containing viscosity modifying admixture (VMA) and supplementary cementitious materials (SCMs). For such mixtures, it is reported that the Bingham model exhibited the least fit for shear stress
shear rate data compared to other analytical models (Yahia & Khayat, 2001, 2003). This can be attributed to its inefficiency in predicting the nonlinear portion of the shear stress-shear strain curve observed at low shear rates (Nguyen et al., 2011; Yahia & Khayat, 2001). Such a conclusion agrees well with Papo’s findings (Papo, 1988). According to Papo (1988), the flow data of cement pastes correlated well with the Bingham model when the regression was limited to high-shear data. It is also reported that the Bingham model fails to predict the flow curve in self-compacting concrete (SCC) with very low yield stress and shear-thickening effect (Feys et al., 2008). The Bingham model can result in a negative value for apparent yield stress for the mixtures with a very low yield stress, where the model is not valid anymore (De Larrard et al., 1996; Feys et al., 2008). The Herschel
Bulkley model has been suggested in such cases to avoid the problem of encountering negative apparent yield stress (De Larrard et al., 1996). The Herschel
Bulkley model developed in 1926 to overcome the Bingham model’s shortage in describing the nonlinearity of the flow curve through defining a power index (i.e., n) for shear rate. This model shows the nonlinearity of pseudoplastic fluid. The model describes shear-thickening behavior when n is larger than one, whereas n less than one shows shear-thinning behavior (Nehdi & Rahman, 2004; Rehman et al., 2018; Yahia & Khayat, 2003). The Herschel
Bulkley model adequately fitted to flow curve data for neat cement grout prepared with low w/c

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ranging between 0.3 and 0.45 (Atzeni et al., 1985; vom Berg, 1979), cement pastes (Jones & Taylor, 1977; Papo, 1988), and SCC (De Larrard et al., 1996; Feys et al., 2008). Although this model described the rheological behavior of SCC as a thixotropic fluid better than the Bingham model, it overestimated the yield stress of SCC due to its mathematical restriction at low shear rates (Feys et al., 2007). It was also found that this model gave the lowest estimate of yield stress for cement grouts prepared with w/c 5 0.4, VMA, and SCMs compared to other rheological models, including Bingham (Yahia & Khayat, 2001). Such grouts tended to show high pseudoplastic shear-thinning behavior and could be described by a new rheological model (Yahia & Khayat, 2001). The Eyring model was proposed to describe the rheological behavior of simple fluids through two adjustable parameters. The model does not explicitly offer a value for yield stress. The efficacy of this model to fit flow curve data of cement pastes has been presented by Atzeni et al. (1985) for cement pastes with w/c ranging from 0.3 to 0.45. They found optimum results for the Eyring model, Herschel
Bulkley model, and a newly proposed parabolic model (see Table 6.2). However, they recommended using the proposed parabolic model owing to its simplicity and accuracy in data elaboration (Atzeni et al., 1985). Atzeni et al. (1985) also combined the Eyring and Vom Berg model to consider both the linear term and the yield stress of cement pastes. However, the combined model requires a more complicated computation since it includes four different constants. Hence they concluded that the proposed parabolic model is more practical (Atzeni et al., 1985). In contrast to the finding of Atzeni et al. (1985) for the Eyring model, Papo obtained the least fit using the Eyring model for cement pastes prepared with low w/c (ranging between 0.34 and 0.42) (Papo, 1988). Likewise, this model could not fit the flow curve data of grout mixes prepared with w/c 5 0.4, VMA, and SCMs, even those mixes with limited shear-thinning behavior (Yahia & Khayat, 2003). Vom Berg developed his model in accordance with the Eyring model and included the yield stress term in the equation. The model was initially formulated to relate the yield stress and plastic viscosity to the specific surface and concentration of solids in the cement pastes prepared with a w/c of 0.4
0.8 corresponding to solid concentrations of 0.44
0.29 (vom Berg, 1979). This model was only suitable for elaborating experimental shear stress
shear rate data of cement pastes at the low shear range (Atzeni et al., 1985; vom Berg, 1979). Similar to the Eyring model, this model failed to fit the flow curve of grouts containing VMA and SCMs with low w/c, even for grouts with limited shear-thinning response (Yahia & Khayat, 2003). The model proposed by Robertson and Stiff (1976) proposed a model to approximate the rheology of yield
pseudoplastic fluids, such as drilling fluids and cement slurries. These fluids exhibit yield stress as well as a nonlinear flow curve. Published findings show the model’s adequacy in describing the flow curve of drilling fluids and cement slurries prepared with a w/c ranging between 0.3 and 0.5 (Fordham et al., 1991; Jones & Taylor, 1977; Kelessidis & Maglione, 2006; Robertson & Stiff, 1976). The modified Bingham model for cement pastes suggested by Yahia and Khayat (2001) includes a second-order term of shear rate. Such a term can describe the

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shear-thickening behavior of pseudoplastic cementitious grout. This model can also be considered as Taylor development of the Herschel
Bulkley model since no experimental data show a power index n bigger than 2 for cement-based systems (Feys et al., 2009, 2007). Hence this model avoids the need for computing variable n while describing the shear-thickening behavior of cement systems as long as they are not highly shear-thickening (Feys et al., 2013). The modified Bingham model was also resulted in the most stable estimate of the yield stress of SCC compared to Bingham and Herschel
Bulkley models (Feys et al., 2013). This finding was due to the independence observed between the estimated values for yield stress and the nonlinearity of the rheological curve (Feys et al., 2013). The Casson model was derived by assuming certain interaction forces between dispersed particles in a suspension, leading to the formation of hypothetical chains. However, it seems that the model is not applicable for highly concentrated suspensions since it cannot consider the interaction between chain structures (Papo, 1988). The correlation between this model and the experimental flow data of cement paste with w/c ranging between 0.30 and 0.45 was unsatisfactory (Atzeni et al., 1985). This observation was attributed to the higher solid concentration of cement pastes than that considered in developing the Casson formula (Atzeni et al., 1985). Similarly, the Casson model exhibited the least fit and highest average standard error for graphene-based cement composites compared to Bingham, modified Bingham, and Herschel
Bulkley models (Rehman et al., 2018). However, Papo’s conclusion was different. He found the best fit between the Casson model and the flow data of cement pastes with w/c ranging from 0.34 to 0.42, compared to other rheological models with two adjustable parameters (Papo, 1988). Similarly, Gu¨llu¨ (2016) found the Casson model unacceptable to predict the shear stress in cement systems containing stabilizers such as clay, sand, lime, and bottom ash. The De Kee model was initially developed to model the rheology of fluids exhibiting shear-thinning, shear-thickening, thixotropy, and antithixotropy behaviors at various shear rates. A hydrolyzed polyacrylamide solution was used to evaluate the model. A good agreement between the predicted and experimentally obtained viscosity and primary normal stress coefficient was reported for such a solution (De Kee & Chan Man Fong, 1994). The model has been rarely used to describe the flow curve of cement systems. However, Yahia and Khayat (2003) used this model to describe the flow curve of high-performance cementitious grouts. They found the De Kee model can accurately predict the yield stress of such grouts (Yahia & Khayat, 2001, 2003). The De Kee model obtained moderate ranking in a ranking scheme presented by Gu¨llu¨ (2016) to compare available rheological models in estimating the rheology of stabilized cement grouts for jet grouting. In this study, the Robertson
Stiff model achieved the highest ranking, whereas the Herschel
Bulkley and modified Bingham models obtained low to moderate ranking. According to Gu¨llu¨ (2016), all considered models except for the Casson were adequately correlated to experimental data with low errors and high correlation coefficient (R . 1). The Sisko model with three adjustable parameters does not offer any explicit value for the yield stress. However, it was used by Papo (1988) for cement pastes

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where the model was adequately fitted to the flow data of cement paste, similar to the Herschel
Bulkley Robertson
Stiff models. Nevertheless, the Sisko model was not recommended by Papo (1988), since this equation does not estimate any value for the yield stress, an essential parameter in describing the rheology of cementitious composites. The Williamson model with three constants was developed to model the flow of dispersions, which show flowing properties similar to the ideal plastic flow in specific respects but with no real yield value (Williamson, 1929). Hence the model does not involve a yield stress term. However, it considers the intercept of the asymptote of the flow curve with the shear stress axis as the stress of plastic resistance at an infinite shear rate. Nehdi and Rahman (2004) used the Williamson and Sisko models to predict the theoretical viscosity of cement pastes containing various additives at zero and infinite rate of shear. In general, this model has been rarely used for cement systems. The model proposed by Yahia and Khayat (2003) for high-performance cement grouts was a combination of the Casson and De Kee models. The Casson model estimated higher shear stress at low shear rates for such mixtures, whereas it underestimated shear stresses at large shear rates. In contrast, the calculated shear stresses by The De Kee model were lower than the Casson model but slightly higher than the experimental data. Also, the De Kee model showed a better correlation with experimental flow data at high-shear rates. Hence the combination of these models was proposed to predict the flow behavior of the tested cement systems more accurately. Yahia and Khayat (2003) observed a stronger correlation between the combined model and the flow curve of the grouts. This correlation was more pronounced for the mixes containing VMA and SCMs with a relatively high degree of pseudoplasticity (Yahia & Khayat, 2003). The limitation of the model on predicting the yield stress of pseudoplastic cement mixtures has been significant, similar to the Bingham, Casson, Herschel
Bulkley, and De Kee models (Yahia & Khayat, 2001).

6.3

Test methods

The rheological behavior of cement-based systems can be tested directly and indirectly. In the direct method, a suitable rheometer is used to measure rheological parameters. In the indirect tests, however, the rheology of cement-based systems is evaluated on the basis of conventional flowability tests such as the slump test.

6.3.1 Rheometer test A rheometer test provides more information and scientific description about the rheology of cement systems compared to conventional flowability tests. It enables to determine yield stress, plastic viscosity and their variations with time, as well as the evaluation of shear-thinning or shear-thickening behavior. Such comprehensive

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rheological information is beneficial, for instance, in developing chemical admixtures to adjust the flow performance of cement-based systems (Khayat, 1998). However, obtaining the right flow properties of cementitious composites is not straightforward due to the inherent rheological properties of these materials such as shear-thickening and segregation and complexity of rheometers (Feys et al., 2013). Furthermore, the rheological properties measured by rheometers are significantly dependent on the testing method and data interpretation (Geiker et al., 2002; Macosko, 1994; Tattersall & Banfill, 1983). Rheometers can be used to describe the flow behavior of different suspensions, including cement-based systems. However, existing rheometers cannot measure shear stress and shear rate directly. During a rheology test, torque or force is measured and converted to shear stress. Shear rates are also obtained from linear or rotational velocity recorded by the rheometer. A simple rheometer would consist of two parallel steel plates that can pass each other, as shown in Fig. 6.1. In such a hypothetical rheometer, the shear stress and shear rate are determined by measuring dv parameters given in Eq. (6.1), that is, A, F, and dh (Macosko, 1994). However, measuring the rheological properties of cement-based systems are not feasible with this hypothetical rheometer (Wallevik et al., 2015). The most suitable rheometers for cement-based systems are coaxial cylinder and parallel rotating plate geometries since they offer analytical converting equations to calculate shear stress and rate. The use of other rheometers to obtain real rheological characteristics is more challenging since there are no analytical converting equations for other rheometers (Wallevik et al., 2015). However, rheological parameters of cement-based systems may be obtained by comparing the results from different devices (Feys et al., 2007), numerical modeling (Wallevik et al., 2015), or using a certain rheometer for comparative studies.

6.3.1.1 Coaxial cylinder rheometer The coaxial cylinder rheometer, also called concentric cylinder rheometer, consists of an outer cylinder with a diameter of Ro as a container for the material and an inner rotating cylinder with a diameter of Ri , that is placed coaxially to the other cylinder. The rheometer cannot measure rheological properties (i.e., shear stress and shear rate) directly. Instead, it measures relative rheological parameters, such as torque (T in N  m) and angular velocity (Ω in rad=s). Fig. 6.4 shows schematic flow curves measured by a coaxial cylinder rheometer. Fig. 6.4A illustrates a complete torque
velocity cycle along the Bingham model fitted to the descending portion of the torque
velocity cycle. The hysteresis loop presented in this figure is formed because of the thixotropic behavior of the material (Jiao et al., 2017, 2019). This behavior in cement-based systems is related to coagulation, dispersion and recoagulation of cement particles (Wallevik, 2009). Cement particles coagulate as a result of the interaction that exists among them owing to combined interparticle forces (i.e., van der Waals, Brownian, electrostatic repulsion, and steric hindrance forces) and gravitational forces (Geiker et al., 2002; Macosko, 1994; Tattersall & Banfill, 1983). As shown in Fig. 6.4A,

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Figure 6.4 Schematic flow curves measured by a coaxial cylinder rheometer; (A) one hysteresis torque
angular velocity cycle and (B) torque versus time curve.

the measured torque increases gradually by applying low shear rates until it reaches a peak point, showing the static yield stress of the material. The peak point can be seen in the torque
time curve at the early test stages (see Fig. 6.4B). The static yield stress (τ s ) represents the minimum shear stress required to overcome flocculated structures inside the cement-based mixture and initiate flow. After the peak point, the torque
time curve reaches a plateau showing the dynamic yield stress (τ 0 ). There are almost no fluctuated structures at this stage since they were broken at the beginning of the test. The dynamic yield stress refers to the lowest shear stress applied before the material stops flowing. Note that these two stresses are the same for a simple yield stress flow, whereas they are different for a system with thixotropic behavior (Balmforth et al., 2014; Coussot, 2014). In general, the descending portion of the hysteresis loop is used for fitting purposes to mitigate the fitting error. In Fig. 6.4A, the Bingham model fitted to the descending portion of the torque
velocity curve specifies a linear relationship between these two parameters, as given in Eq. (6.21). T 5 G 1 HΩ

(6.21)

where Ω is the angular velocity, G is the yield torque obtained by T-line, and H is the slope of T-line. G and H are referred to as relative rheological parameters that can be transferred to τ 0 and η. The transformation equations developed for the Bingham model are discussed in the following paragraphs. For such a coaxial cylinder rheometer, the shear stress can be obtained by the following equation (Schramm, 1994): τ5

T 2πr 2 L

(6.22)

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where τ is the shear stress (Pa) at the radial coordinate and is the effective length of the inner cylinder (m). For the shear rate, two different conditions have been considered in the literature to convert angular velocity to shear rate. First, when the annular gap between the two cylinders is small (Ri =R0 $ 0:99) and second, when it is large. Eq. (6.23) converts angular velocity (Ω, rad=s) to the shear rate at the inner cylinder in the case of a small annular gap (Schramm, 1994). For the second case, the conversion equation is more complex and can be given as Eq. (6.24) (Macosko, 1994). γ_

5 2Ω

γ_

5 2Ω

R2o

R2o 2 R2i






(6.23) 1

n 1 2 Ri =Ro

2=n 

(6.24)

In this formula, n stands for the slope of torque
angular velocity plotted on a logarithmic scale. The small annular gap geometry is not applicable for cementitious composites, especially for those containing relatively large aggregate. The annular gap is a crucial parameter restricting the aggregate size. An annular gap four times larger than the maximum aggregate size has been suggested to provide an adequate homogeneity level in cement-based systems. Such a geometry also prevent the intensification of interaction between aggregate (Lu et al., 2015). The annular gap size was found to affect the rheological characteristics obtained for cement-based systems containing large aggregate such as conventional concrete (Feys & Khayat, 2017). In contrast, it did not show any notable effect for the systems containing small aggregate such as cement mortars. Also, increasing aggregate content intensified the effect of gap size on the rheological characteristics of cement-based systems. This effect was found to be minimal for flowable concrete, whereas it could make rheological data even unreliable for conventional concrete (Feys & Khayat, 2017). Even Eq. (6.24) developed for the large annular gap is not straightforward for cement-based systems since n depends on the angular velocity. An alternative approach for cement-based systems is to convert the applied torque and angular velocity into fundamental rheological units using the Reiner
Riwlin equation (Eq. 6.25). T5



4πLτ o

1 R2i

2

1 R2o



ln

  Ro Ri

1



8ηπ2 L

1 R2i

2

1 R2o



Ω

(6.25)

As can be seen, this equation follows the Bingham model, that is, T 5 G 1 HΩ. Hence τ 0 and η can be calculated when G and H are obtained using a given coaxial cylinder rheometer characterized with Ro, Ri, and L. To this end, a set of angular velocity and corresponding torque values are recorded by the rheometer. Then, G and H are obtained through the least-squares linear fitting method. As can be seen, this approach can be used to obtain the yield stress (τ 0 ) and plastic viscosity (η) for

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cement-based systems that flow like Bingham fluid. Note that any nonlinearity in the flow curve makes this method inaccurate. The Reiner
Riwlin equation has also been extended by Heirman et al. (2007) for nonlinear flow curves based on the Herschel
Bulkley model and by Feys et al. (2013) for the modified Bingham model. The use of the coaxial cylinder rheometer and its data interpretation is not a straightforward task. Hence special care should be taken to obtain the rheological properties of cement-based systems correctly (O. H. Wallevik et al., 2015).

6.3.1.2 Parallel rotating plates rheometers In this rheometer, a relative rotation is applied to the two parallel plates (disks). It should not be confused with the hypothetical rheometer consisted of two parallel plates sliding over each other (see Section 6.3.1). The transformation equation for angular velocity to shear rate for this type of rheometer is more straightforward than coaxial rheometers. However, the transformation equation for shear stress is more complex since the shear rate varies from 0 at the disk center to a maximum value at the outer disk edge. The following equations transfer the torque and angular velocity to fundamental rheological units for this type of rheometer (Macosko, 1994). γ_ R τ5

5

RΩ h

T 2πR3

(6.26)   dlnT 31 dlnγ_ R

(6.27)

where h is the gap between two disks (in m), R is the disk radius (in m). The edges of the sample are of particular importance and should be selected carefully (Broyer & Macosko, 1975). For instance, when the disks have different diameters, the shearing sample might be too big. In such a case, the material located at a distance larger than the outer radius of the smallest disk should be removed. Also, a lower estimation of real rheological properties may be obtained if the material between the disks moves toward the outside. This inaccuracy can be mitigated by reducing the gap between the disks, although it is not guaranteed.

6.3.1.3 Other rheometers Several other rheometers have been developed and used to measure the rheological properties of cement-based systems, including those with coarse aggregate. Since all cement-based systems contain solid particles, the flow should occur in a sufficiently large space (i.e., the gap between the shearing surfaces) to ensure flow homogeneity. A gap size at least 10 times larger than the maximum size of particles have been suggested by rheologists. However, a gap larger than 4
5 times of aggregate size has been reported to be adequate for cement-based systems (Hackley & Ferraris, 2001; Lu et al., 2015). The use of a large gap is not possible in some geometries, such as cone and plate geometries. Hence efforts have been made to

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develop suitable rheometers for measuring rheological properties of cementitious composites containing fine and coarse aggregate. Some of these rheometers are as follows: ConTec
BML (Wallevik & Wallevik, 1998), BTRHEOM (Hu et al., 1996; Hu & de Larrard, 1996), Tattersall Mk-II and -III (Geiker et al., 2002; Macosko, 1994; Tattersall & Banfill, 1983), Cemagref-IMG (Banfill, 1990), IBB (Yun et al., 2015), the vane rheometer, which is suitable for measuring the yield stress but not the viscosity (Mahaut et al., 2008), and eBT2 (Chen et al., 2019).

6.3.2 One-factor tests The rheological performance of cementitious composites is characterized on the basis of at least two parameters (e.g., yield stress and plastic viscosity). However, as mentioned before, a large gap in the rheometer is typically needed to ensure a homogeneous flow without aggregate interlocking. Such requirements limit the ease of rheometry test in the filed and resulted in the development of various empirical standard and nonstandard tests which do not measure the rheological properties in fundamental units. These tests designed to replicate concrete casting in the field measure only one parameter related to the flowability and workability of cementbased systems. Since parameters measured by these tests are not necessarily related to the fundamental rheological parameters, it is difficult or even not feasible to connect results obtained by these tests. However, such tests are useful when comparing results from the same testing device is of interest. A detailed discussion about these empirical tests is out of the scope and can be found in the literature (Ferraris, 1999; Rasekh et al., 2020). A list of these tests, along with their possible relation to rheological parameters, is presented in Table 6.3. An external source of stress is needed to overcome the yield stress and initiate the flow. Among various approaches that can be used, material weight and vibration are the two most common methods. The flow behavior of cementbased systems observed for these two methods is entirely different. The results of these tests are indirectly related to the yield stress and plastic viscosity.

6.4

Rheology of nano-modified cementitious composites

6.4.1 Nanoscale particles 6.4.1.1 Nano-silica Nano-silica (nano-SiO2) is a high purity amorphous silica powder with a particle size about 1000 times smaller than cement particles (Lavergne et al., 2019). It typically produced from micron-sized silica and is the most common type of nanomaterials used in cementitious composites. In terms of hydrophilicity, nano-SiO2 can be categorized as hydrophilic and hydrophobic nano-SiO2. Mainly, hydrophilic nano-SiO2 is used in cementitious composites because of its good dispersion ability in the water. This dispersion has considerable effects on the rheological and mechanical properties of cementitious composites. Nano-somia has a high specific

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Table 6.3 One-factor tests related to the flowability of cement-based systems. Test or measurement

Stress source

Relation to rheological parameters

Slump

Material weight

T500 Penetrating rod: Kelly Ball, Vicat, and Wigmore tests K-slump

Material weight Weight of the device or the force applied to the device Material weight

Material flows when stress is larger than the yield stress, and the flow stops when it is lower Related to plastic viscosity. The penetration depth depends on the yield stress of the material

Ve-be time and remolding tests

Vibration

LCL apparatus

Vibration and Material weight

Filling ability tests: L-box, U-box, Vfunnel Fritsch test Flow cone Orimet test

Material weight

The measurement value is related to the yield stress and slump test results. It is suitable for systems with low yield stress. For mixtures with large aggregate ( . 9.4 mm), it can only evaluate segregation. Related to plastic viscosity but not directly. Suitable for mixes with high yield stress Related to plastic viscosity. For low amplitude of vibration, the measurement can be connected to yield stress as well. Related to the plastic viscosity.

Vibration Material weight Material weight

Related to the plastic viscosity. Related to the plastic viscosity. Related to the plastic viscosity.

surface area allowing it to react with cement particles effectively and enhance the durability and mechanical properties of cementitious composites (Amiri et al., 2012; Bahari et al., 2011; Barkoula et al., 2016; Du et al., 2014; Reches et al., 2018; Sadeghi-Nik et al., 2019). It is established that the incorporation of nano-SiO2 in cementitious composites increases the water demand to retain mixture workability (Beigi et al., 2013; Quercia et al., 2012; Senff et al., 2010; Sonebi et al., 2015). The increase in water demand was attributed to the high specific surface area and unsaturated bonds in nano-SiO2, causing nano-SiO2 to adsorb surrounding water molecules for forming chemical bonds. Hence bleeding and segregation are not typically observed for the mixtures containing nano-SiO2 (Rezania et al., 2019; Singh et al., 2015). The attraction of water by nanoparticles decreases free water required to lubricate mixture particles, increasing internal friction. Therefore nano-silica addition leads to increased flow time, yield stress, and plastic viscosity of cementitious composites. Many researchers have reported this phenomenon upon adding nano-SiO2 to various

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cementitious mixtures such as grout, concrete, etc. (Bjo¨rnstro¨m et al., 2004; Jalal et al., 2013; Khaloo et al., 2016; Peng et al., 2019; Qing et al., 2007; Senff et al., 2009; Sonebi et al., 2015). It was also found that nano-silica reduces both the initial and final setting time due to accelerating the hydration process and CSH formation (Jalal et al., 2013; Senff et al., 2009; Zhang et al., 2012; Zhang & Islam, 2012). According to Collepardi et al. (2005), nano-SiO2 was more effective than silica fume to increase cohesiveness in fresh cement mortar and reduce bleeding and segregation. Fig. 6.5 shows the effects of nano-SiO2 addition on rheological properties of cement mortars reported by Senff et al. (2009). In this figure, T is the torque applied by rheometer in N  mm and is defined as a function of rotation speed (N, mm21) by T 5 g 1 hN. The yield stress and plastic viscosity of mixture are also proportional to g and h, respectively. The recorded torque is higher for the mixtures containing nano-SiO2 because of increased plastic viscosity and yield stress during the measuring period (150 min). Fig. 6.5B indicates an evident increase in plastic viscosity due to nano-SiO2 addition. However, it does not change significantly by increasing the nano-SiO2 content of the mixture. In contrast, yield stress growth is evident after adding more nano-SiO2, as shown in Fig. 6.5C and concluded by others (Peng et al., 2019; Senff et al., 2010). For instance, the mixture containing 2.5% nano-SiO2 exhibits a maximum yield stress value after 75 min, corresponding to the torque capacity of the rheometer used. A similar trend is observed in Fig. 6.6 for the effect of nano-SiO2 content on plastic viscosity and yield stress of cement
fly ash past tested by Peng et al. (2019). The increase in nano-SiO2 content improves the packing density of cementitious composite, reducing the volume between particles and free water. Hence the internal friction between particles at the fresh state increases, which affects yield stress significantly. As mentioned before, the incorporation of nano-SiO2 decreases the flowability of cementitious composites, which can be readily seen by the flow table test. The spread on the flow table is closely related to yield stress for cementitious composites containing nano-SiO2 (Senff et al., 2010, 2009). The reduction in flowability is attributed to the increase of cohesion in the matrix. The reported spread diameter on the flow table for the mixture with 2.5% nano-SiO2 was 19.6% and 32.8% lower than that of the 1% nano-SiO2 containing mixture and control mixture, respectively (Senff et al., 2009). However, the adverse effect of nano-SiO2 on flowability can be mitigated using a proper dosage of superabsorbent polymers (Pourjavadi et al., 2012; Safi et al., 2018). It is reported that improved rheological behavior can be obtained if nano-SiO2 is mixed with superplasticizer comparing to the use of nanosilica in powdered form (Safi et al., 2018).

6.4.1.2 Nano-titania Nano-titania (nano-TiO2) is well known for its wide application in the industry as a photocatalyst coating agent. In the construction industry, nano-TiO2 has been used to manufacture cementitious composites with self-cleaning and self-disinfection properties (L. Chen et al., 2019). Like nano-SiO2, the addition of nano-TiO2 into

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Figure 6.5 Effects of nano-SiO2 content on the time evolution of the rheological performance of cement mortar; (A) applied torque, (B) plastic viscosity, and (C) yield stress. Source: With permission from Senff, L., Labrincha, J. A., Ferreira, V. M., Hotza, D. & Repette, W. L. (2009). Effect of nano-silica on rheology and fresh properties of cement pastes and mortars. Construction and Building Materials, 23(7), 2487
2491. https://doi.org/ 10.1016/j.conbuildmat.2009.02.005.

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Figure 6.6 Influence of nano-SiO2 content on (A) yield stress and (B) plastic viscosity of cement
fly ash paste. Source: Modified from Peng, Y., Ma, K., Long, G. & Xie, Y. (2019). Influence of nanoSiO2, nano-CaCO3 and nano-Al2O3 on rheological properties of cement-fly ash paste. Materials, 12(16). https://doi.org/10.3390/ma12162598.

cementitious matrices accelerates CSH gel formation (Nik & Bahari, 2011; Sadeghi-Nik et al., 2011), especially at the early age of hydration. The acceleration occurs because of the seeding effect, whereby nano-TiO2 provides new nucleation sites where more hydration products can precipitate (Hubler et al., 2011; Li et al., 2007; Sadeghi-Nik et al., 2017). Hence, the addition of nano-TiO2 decreases both the initial and final setting time (Jalal et al., 2013; Senff et al., 2009; M.-H. Zhang et al., 2012; M.-H. Zhang & Islam, 2012). It has been reported that nano-TiO2 has an insignificant effect on cement mortar and concrete fluidity (Jalal et al., 2013; Liu et al., 2015). Fig. 6.7 shows the spread diameter on the flow table for mortars containing nano-TiO2 compared to other nanoparticles. As can be seen, the addition of nano-TiO2 up to 3.0 wt.% has almost no adverse impact on mortar workability. According to Liu et al. (2015), the spread diameter first slightly increased by adding nano-TiO2 up to 1.0 wt.%, and then, it slightly decreased upon adding further nano-TiO2 up to 3.0 wt.%. This observation was attributed to the less specific surface area of nano-TiO2 compared to nano-SiO2 (Liu et al., 2015). Others have also reported a similar conclusion except for the slight increase in workability by adding nano-TiO2 up to 1.0 wt.% (Jalal et al., 2013; Nazar et al., 2020). For instance, Senff et al. (2010; Beigi et al., 2013; Quercia et al., 2012; Sonebi et al., 2015) reported a reduction of only 7% in the spread diameter after adding 2.6 wt.% nano-TiO2 to cement pastes. Senff et al. (2012) also reported no further decrease in flowability by increasing nano-TiO2 content to 5.2 wt.%. In contrast, R. Zhang et al. (2015) reported a more significant reduction in cement mortar flowability upon adding nano-TiO2 at the dosage of 3.0 and 5.0 wt.%. They observed a decrease by 19.8% and 20.8% at the spread diameter for these two dosages, respectively. However, a negligible decrease in flowability upon increasing nano-TiO2 content from 3.0 to 5.0 wt.% observed by Zhang et al. (2015) is in agreement with the conclusion made by Senff et al. (2012).

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Figure 6.7 The effects of different nanoparticles on cement mortar fluidity based on flow table test. NS, nano-SiO2; NT, nano-TiO2; NA: nano-aluminum oxide, that is, nano-Al2O3; NZ, nano-zinc oxide, that is, nano-ZnO. Source: With permission from Liu, J., Li, Q. & Xu, S. (2015). Influence of nanoparticles on fluidity and mechanical properties of cement mortar. Construction and Building Materials, 101, 892
901. https://doi.org/10.1016/j.conbuildmat.2015.10.149.

In contrast to nano-TiO2, nano-SiO2 at 3.0 wt.% decreases mixture flowability by 50% (Liu et al., 2015), see Fig. 6.7. This figure also indicates that the adverse effect of nano-Al2O3 on cement mortar fluidity is much more intense than nanoSiO2 so that a 45% reduction in the spread diameter is seen after adding only 1.0 wt.% nano-Al2O3. The effects of adding nano-TiO2 on the rheological properties of cement mortars can be seen in Fig. 6.8 reported by Senff et al. (2012). This figure also provides a comparison between the rheological performance of mixtures containing nano-TiO2 and nano-SiO2. Fig. 6.8 indicates that the use of nano-TiO2 at low dosage (less than 3.0 wt.%) has negligible effects on the applied torque and yield stress during the measuring period up to 75 min. After this period, torque and yield stress increase slightly due to the addition of nano-TiO2 at a low dosage. This observation is consistent with the results of flow table tests discussed in the previous paragraph. Senff et al. (2012) concluded that the spread diameter on the flow table is closely related to yield stress for cementitious composites containing nano-TiO2. In contrast, they reported no correlation between plastic viscosity and the spread on the flow table. Fig. 6.8 also indicates that nano-TiO2 at low dosage does not affect plastic viscosity (see Fig. 6.8C).

6.4.1.3 Nano-zinc oxide The study on the rheological properties of cementitious composites containing nano-ZnO is minimal. Research conducted by Liu et al. (2015) showed that mortar flowability did not decrease considerably for nano-ZnO contents less than 1.5 wt.%.

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Figure 6.8 Effects of nano-TiO2 content on the time evolution of the rheological performance of cement mortar in comparison to nano-SiO2; (A) applied torque, (B) plastic viscosity, and (C) yield stress. Source: With permission from Senff, L., Hotza, D., Lucas, S., Ferreira, V. M. & Labrincha, J. A. (2012). Effect of nano-SiO2 and nano-TiO2 addition on the rheological behavior and the hardened properties of cement mortars. Materials Science and Engineering: A, 532, 354
361. https://doi.org/10.1016/j.msea.2011.10.102.

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The decrease in the spread diameter on flow table was reported as less than 10%. However, they reported a 23% reduction in the spread diameter after increasing the nanoparticles content to 2.0 wt.% (see Fig. 6.7). In comparison, Gowda et al. (2019) reported only 7% reduction in the spread diameter on flow table test due to the addition of 5.0 wt.% nano-ZnO to cement mortars. The flowability of silica fume
based cement composites containing ZnO nanoparticles presented recently by Garg and Garg (2020) is not consistent with findings by Liu et al. (2015). According to Garg and Garg (2020), the addition of only 0.35 wt.% nano-ZnO decreased the spread diameter of silica fume
based cement composites by 18%. The flowability reduction increased by adding further nanoZnO and reached 31% for 1.4 wt.% nano-ZnO content. The observed decrease in composite fluidity was attributed to the higher pozzolanic activity of silica fume in the presence of nano-ZnO (Garg & Garg, 2020; Saleh et al., 2018). There are no published findings regarding the effects of nano-ZnO on yield stress and plastic viscosity as the main rheological properties of cementitious composites. More research studies should be performed to fill this research gap.

6.4.1.4 Nano-aluminum oxide Experimental studies have shown that nano-Al2O3 decreases the flowability of cementitious composites significantly (Liu et al., 2015; Rashad, 2013). According to Liu et al. (2015), the adverse effect of nano-Al2O3 on cement mortar workability is more intense than nano-SiO2 (see Fig. 6.7). As shown in Fig. 6.7, the addition of 1.0 wt.% nano-Al2O3 decreases the spread diameter by 45%, whereas this value is about 30% for nano-SiO2. Such an intense effect of nano-Al2O3 on workability has also been reported for concrete mixtures containing 2.0 wt.% nano-Al2O3 by Agarkar and Joshi (2012). The intense effect of nano-Al2O3 on concrete workability is attributed to the ultrahigh specific surface area of nano-Al2O3, causing stronger water attraction than cement particles. Consequently, the available water to lubricate particles in fresh concrete decreases significantly, resulting in a reduced slump value. Research studies on the effects of nano-Al2O3 on yield stress and plastic viscosity of cementitious composites are minimal. Recently, Peng et al. (2019) published experimental data on yield stress and plastic viscosity of cement
fly ash paste containing nano-Al2O3. The obtained shear stress versus shear rate curves obtained at three different resting time is shown in Fig. 6.9. This figure indicates that the addition of nanoAl2O3 changes the rheological curves of cement
fly ash pastes, especially at high dosages of nano-Al2O3. When nano-Al2O3 content was increased to 3.0 wt.%, first, shear stress experienced a considerable decrease and then increased upon increasing shear strain. Fig. 6.10 shows the influence of nano-Al2O3 on yield stress and plastic viscosity. This figure indicates a significant increase in both yield stress and plastic viscosity of cement
fly ash pastes after the addition of nano-Al2O3 at high dosages (i.e., 2 and 3.0 wt.%). According to Peng et al. (2019), however, cement
fly ash pastes containing 1.0 wt.% nano-Al2O3 did not show any notable change in yield stress and plastic viscosity. More experimental studies should be carried out to

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Figure 6.9 Rheological behavior of cement
fly ash paste with nano-Al2O3 at (A) 5 min, (B) 60 min, and (C) 120 min resting time. Source: With permission from Peng, Y., Ma, K., Long, G. & Xie, Y. (2019). Influence of nano-SiO2, nano-CaCO3 and nano-Al2O3 on rheological properties of cement-fly ash paste. Materials, 12(16). https://doi.org/10.3390/ma12162598.

Figure 6.10 Influence of nano-Al2O3 (NA) content on (A) yield stress and (B) plastic viscosity of cement
fly ash paste. Source: Modified from Peng, Y., Ma, K., Long, G. & Xie, Y. (2019). Influence of nanoSiO2, nano-CaCO3 and nano-Al2O3 on rheological properties of cement-fly ash paste. Materials, 12(16). https://doi.org/10.3390/ma12162598.

evaluate the influence of nano-Al2O3 on yield stress and plastic viscosity of cementitious composites.

6.4.1.5 Nano-zirconium oxide Zirconium oxide nanoparticles or nano-zirconium oxide (nano-ZrO2) are inorganic nonmetallic particles characterized with corrosion and wear resistance (Lu et al., 2015). Several studies using nuclear magnetic resonance and X-ray diffraction showed that the contribution of nano-ZrO2 in the cement hydration process is minimal (Gogtas, 2012; Silva et al., 2014). Only a handful of studies have addressed the effect of nano-ZrO2 on rheological properties of cementitious composites. It seems that these nanoparticles have an insignificant impact on the flowability of cementitious composites. Lu et al. (2019)

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showed that the flow index and consistency coefficient of cement slurry with 1 wt. % nano-ZrO2 is almost similar to that of the control mix. The control mix exhibited a flow index and consistency coefficient of 0.43 and 2.70 (Pa.s), respectively. The addition of 1 wt.% nano-ZrO2 changed these rheological properties to 0.41 and 2.44 (Pa.s). However, this finding did not agree well with the conclusion made by Umarajyadav and Vahini (2017), who found that concrete workability decreased by increasing nano-ZrO2 dosage from 0.2 to 1.2 wt.%.

6.4.1.6 Nano-calcium carbonate Published findings regarding the effect of nano-calcium carbonate (nano-CaCO3) on the fluidity of cementitious composites are not similar. Some of the reported data indicate the decrease in flowability of cement pastes and mortars after the addition of nano-CaCO3 (Chen et al., 2012; Chen & Poon, 2009; Liu et al., 2015). X. Liu et al. (2012) observed a 16% decrease in the flowability of cement paste after adding 1.0 wt.% nano-CaCO3. The reduction in flowability increased to 26% when nano-CaCO3 was increased to 3.0 wt.%. They related the observed reduction to the increased surface area of particles, requiring more water for lubrication. Similarly, Supit and Shaikh (2014) observed a smaller spread diameter on the flow table after adding nano-CaCO3. However, Supit and Shaikh (2014) reported only 11% reduction in workability after the addition of 4.0 wt.% nano-CaCO3, which is much less than what was observed by X. Liu et al. (2012). On the contrary, some studies are contradicting the finding mentioned above. In an experimental study carried out by Camiletti et al. (2013), larger flowability was observed for mixtures containing micro- and/or nano-CaCO3 as cement replacement. For example, the workability of the mixture containing 2.5 wt.% micro-CaCO3 increased 14.5% and 40% after the addition of 2.5 and 5.0 wt.% nano-CaCO3. They explained this observation based on the lubrication effect caused by fine CaCO3 particles (Camiletti et al., 2013). Similarly, Xu et al. (2012) reported an 8.5% and 18% increase in slump height and slump flow diameter by adding 2.0 wt.% nano-CaCO3. As can be seen, the published findings are not congruent with each other. Hence further experimental research should be performed to make a robust conclusion about the impact of nano-CaCO3 on the flowability and rheological properties of cementitious composites. Recently, Peng et al. (2019) tested the rheological performance of cement
fly ash paste containing nano-CaCO3, as illustrated in Fig. 6.11. The figure indicates that yield stress after 5 min resting time decreases slightly by increasing the content of nano-CaCO3. For a higher resting time, 60 and 120 min, the yield stress increases slightly by increasing the dosage of nano-CaCO3. In the case of plastic viscosity, a minor increase is observed by increasing nano-CaCO3 content. Peng et al. (2019). concluded that nano-CaCO3 did not cause a notable change in the rheological performance of cement
fly ash paste. Such rheological properties seem to be matched more with the observation made by Supit and Shaikh (2014).

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Figure 6.11 Influence of nano-CaCO3 (NC) content on (A) yield stress and (B) plastic viscosity of cement
fly ash paste. Source: Modified from Peng, Y., Ma, K., Long, G. & Xie, Y. (2019). Influence of nanoSiO2, nano-CaCO3 and nano-Al2O3 on rheological properties of cement-fly ash paste. Materials, 12(16). https://doi.org/10.3390/ma12162598.

6.4.2 Nano-tubes and fibers 6.4.2.1 Carbon nano-tubes and nano-carbon fibers The incorporation of carbon nano-tubes (CNTs) and nano-carbon fibers (CNFs) has been found to increase the yield stress of cementitious composites significantly (Farooq et al., 2020; Jiang et al., 2018; Mendoza Reales et al., 2018). This increase becomes more pronounced by increasing the content of CNTs and CNFs. Rheological data reported for CNT- and CNF-modified cementitious composites by Jiang et al. (2018) based on the modified Bingham model shows a growth of 169% and 205% in the yield stress after the addition of 0.1 wt.% CNT and CNF, respectively. The yield stress increased 48 times more than the control mix by increasing the content of CNTs and CNFs to 0.5 wt.%. The yield stress of these mixtures was also 3
4 times than cement pastes containing 0.5 wt.% nano-SiO2 even though CNFs had a similar specific surface area to nano-SiO2. Apart from increasing internal friction because of the decrease in free water available for lubricating particles, nano-fibers are prone to fiber entanglement because of their higher aspect ratio comparing to nanoparticles such as nano-SiO2. The fiber entanglement forms an extensive network structure, arresting mixture particles and resisting the mixture to flow. This results in a further increase in the yield stress of cementitious composites. Furthermore, in CNTs, the hollow structure leads to a larger specific surface area compared to nano-SiO2. Therefore the presence of CNTs increases the amount of water absorbed, leading to lower water for lubricating particles and increased yield stress. This is why composites prepared with 0.5 wt.% CNTs exhibited the highest yield stress comparing to those containing nano-SiO2 and CNFs. The interpretation mentioned above is based on the fact that nanoparticles having a specific high surface area decrease free water available for particle lubricating.

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However, it does not apply to CNTs and CNFs directly due to their intrinsic hydrophobic nature. Nonetheless, when surfactants are used, as they are essential for adequate dispersion of CNTs and CNFs, the hydrophobic tail of the surfactant molecule absorbs on the surface of CNTs and CNFs, pointing inwards due to hydrophobic interaction. Simultaneously, the hydrophilic head points outward and provides the interaction between CNTs and CNFs with water molecules (Sobolkina et al., 2014; Wang, 2009) and cement particles based on the type of surfactant used. Mendoza Reales et al. (2018) reported a 37% increase in the surface area of solid in cement-based systems after incorporating 0.15 wt.% of multiwall CNTs. They concluded that the additional surface due to CNTs with surfactant molecules could interact with water molecules and cement particles. Mendoza Reales et al. (2018) observed a significant increase in the yield stress but no notable change in the plastic viscosity after the addition of 0.15 wt.% multiwall CNTs to cement pastes. Their study suggested modifying the interaction among particles due to the addition of multiwall CNTs rather than interparticle locking or water demand (Mendoza Reales et al., 2018). The greater interlocking among particles would increase the plastic viscosity, whereas water demand growth could lead to a change in both plastic viscosity and yield stress (Banfill, 1990). In contrast, some studies reported higher plastic viscosity for cement-based systems after incorporating CNTs and CNFs (Farooq et al., 2020; Jiang et al., 2018). The addition of CNTs exhibited a more significant influence on plastic viscosity than CNFs (Jiang et al., 2018). According to Jiang et al. (2018), CNTs and CNFs could reduce available water for particle lubricating and fill voids among cement particles. As a result, the material tended to flow with a lower velocity when the yield stress exceeds (Jiang et al., 2018). It is worth mentioning that the rheology of CNTs and CNF-modified cement-based systems is a complex subject, depending on various parameters such as the type of CNTs and CNFs, aspect ratio, the amount and type of surfactant used, and dispersion approach of these nanomaterials.

6.4.3 Nano-plates 6.4.3.1 Nano-clay The addition of nano-clay has proven to decrease the flowability of cementitious composites (Kafi et al., 2016). Hosseini et al. (2017) appraised the flowability of self-compacting concrete containing nano-clay and observed a notable reduction in slump flow diameter by increasing nano-clay content up to 1.0 wt.%. The mixture showed a slump flow diameter of 640 mm at this dosage compared to 740 mm slump flow diameter for the control mix. Results also revealed that slump flow time (i.e., T500) and V-funnel flow time increased by 51% and 112% after the addition of 1.0 wt.% nano-clay, respectively. Slump and V-funnel flow time are considered indicators for the viscosity of fresh concrete. A similar observation has been reported by Langaroudi and Mohammadi (2018). The effect of nano-clay on the flowability of cement mortar was found to be comparable to self-compacting concrete. For instance, Dejaeghere et al. (2019) increased nano-clay content in cement

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mortar between 0.5 and 2.5 wt.% and observed lower minislump flow for higher nano content. It has also shown that the adverse effect of nano-clay on workability is less than nano-SiO2 and nano-Al2O3 (Shahrajabian & Behfarnia, 2018). The inclusion of nano-clay has proven to increase the yield stress, viscosity, and thixotropy of cementitious composites. Fig. 6.12 shows the influence of nano-clay and time on these rheological parameters tested by Quanji et al. (2014). It can be seen that all these three parameters increase with increasing nano-clay content and resting time. However, yield stress shows a larger rate of increase with time for the mixture containing 1.0 wt.% nano-clay. Also, the increase in viscosity is more significant for nano-clay dosages of 0.5 and 1.0 wt.% compared to other dosages tested in this study. Quanji et al. (2014) also concluded the rate of thixotropy change in cement mortars increases when a low dosage of nano-clay (0.5
1.0 wt.%) is used. In contrast, the use of a significant amount of nano-clay (2.0
3.0 wt.%) reduces the rate of thixotropy change. Others also reported similar changes in the rheological behavior of cement mortars after replacing cement with nano-clay in the presence or absence of superplasticiser (Liu et al., 2018; Panda et al., 2019; Qian & De Schutter, 2018; Zhang et al., 2019).

Figure 6.12 Influence of nano-clay content and time on the rheology of cement pastes; (A) yield stress (B) plastic viscosity, and (C) thixotropy. Source: Modified from Quanji, Z., Lomboy, G. R. & Wang, K. (2014). Influence of nanosized highly purified magnesium alumino silicate clay on thixotropic behavior of fresh cement pastes. Construction and Building Materials, 69, 295
300. https://doi.org/10.1016/j. conbuildmat.2014.07.050.

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For instance, Y. Liu et al. (2018) reported a substantial increase in rheological properties (i.e., thixotropy, yield stress and viscosity) of cement mortars due to replacing cement with nano-clay up to 2.0 wt.%. In their experiment, mortars with a water to binder ratio (w/b) of 0.25 showed a more pronounced change in the rheological properties than w/b 5 0.4. This difference can be seen in Fig. 6.13, as reported by Y. Liu et al. (2018).

6.4.3.2 Nano-graphene oxide Various studies have shown that nano-graphene oxide (nano-GO) significantly decreases the fluidity of cementitious composites. The slump flow of cement pastes containing 0.05 wt.% nano-GO was reported to be 180 mm compared to the 221 mm slump flow of the control mix (Sobolkina et al., 2014; H. Wang, 2009). This shows an 18% reduction in cement paste fluidity due to the addition of a low dosage of nano-GO. A lower reduction rate in the fluidity of cement composites due to increasing nano-GO content was reported by (Rehman et al. (2015, 2017,

Figure 6.13 Influence of nano-clay content on (A) yield stress (B) plastic viscosity, and (C) thixotropy of cement mortars with w/b of 0.25 and 0.4. NP stands for nano-clay as a nanoscale viscosity modifier. Source: Modified from Liu, Y., Han, J., Li, M. & Yan, P. (2018). Effect of a nanoscale viscosity modifier on rheological properties of cement pastes and mechanical properties of mortars. Construction and Building Materials, 190, 255
264. https://doi.org/10.1016/j. conbuildmat.2018.09.110.

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2018); Ferraris et al., 2001; Papo, 1988; Vikan et al., 2007; A. Yahia & Khayat, 2001). Wang, Yao, et al. (2017) observed a more significant decrease in slump flow due to replacing cement with nano-GO. Their experiments showed a 13.2% and 32.1% reduction in cement paste fluidity upon using 0.01 and 0.03 wt.% nano-GO, respectively. However, the adverse effect of nano-GO on fluidity was effectively mitigated by partially replacing cement with fly ash, as can be seen in Fig. 6.14. The positive impact of fly ash on fluidity was attributed to less flocculation structure formed in cement pastes (Wang et al., 2017). This could be due to smooth spherical fly ash particles, reducing water demand for lubrication. Furthermore, fly ash particles with less fineness than cement particles can fill voids between cement particles and improve cement workability (Wang et al., 2017). The experiment carried out by Wang et al. (2016) showed a severe adverse effect of nano-GO on cement paste fluidity. In their experiments, the fluidity of nano-GOblended paste containing 0.01, 0.03, and 0.05 wt.% nano-GO showed a reduction of 20%, 61% and 70%, respectively, compared with the control mix. The significant reduction in cementitious composite fluidity by increasing nanoGO content is related to the large surface area of nano-GO compared to cement particles. The mixture containing a high amount of GO need additional water to lubricate graphene sheets. Increasing nano-GO content of cement composites at a fixed w/b reduces the amount of free water available for lubricating mixture particles. Therefore the increase in GO content adversely affects the flow diameter and workability of cementitious composites. The incorporation of nano-GO in cement composites has proven to increase the yield stress, plastic viscosity and thixotropy as three main rheological parameters of cement composites. As reported by Wang et al. (2016), Fig. 6.15 illustrates how the incorporation of nano-GO in cement pastes changes shear stress-shear

Figure 6.14 The effect of nano-GO on the fluidity of cement paste at different resting times in the presence and absence of fly ash. Source: With Permission From Wang, Q., Cui, X., Wang, J., Li, S., Lv, C. & Dong, Y. (2017). Effect of fly ash on rheological properties of graphene oxide cement paste. Construction and Building Materials, 138, 35
44. https://doi.org/10.1016/j.conbuildmat.2017.01.126.

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Figure 6.15 The effect of nano-GO on (A) shear behavior and (B) yield stress and plastic viscosity of cement pastes. Source: With permission from Wang, Q., Wang, J., Lu, C. X., Cui, X. Y., Li, S. Y. & Wang, X. (2016). Rheological behavior of fresh cement pastes with a graphene oxide additive. Xinxing Tan Cailiao/New Carbon Materials, 31(6), 574
584. https://doi.org/10.1016/S18725805(16)60033-1.

rate curves, yield stress and plastic viscosity. Fig. 6.15A shows the increase in shear stress by increasing the shear rate at all dosages of nano-GO. It also shows that the growth rate in shear stress is more significant for a higher dosage of nano-GO. In the case of yield stress and plastic viscosity, Fig. 6.15B reveals a sharp increase in both of these parameters by adding nano-GO to cement pastes, mainly when nano-GO dosage is higher than 0.01 wt.%. It reflects that the mixtures containing a higher nano-GO content are more difficult to be deformed in the fresh state. Wang et al. (2016) also concluded that incorporating nano-GO decreased the level of shear-thickening and improved the stability of fresh cement pastes. The increase in yield stress and plastic viscosity by increasing nano-GO content of cement composites have also been reported by Rehman et al. (2018) and Shang et al. (2015). Both studies reported a dramatic increase in plastic viscosity (between 70% and 80%) after the addition of 0.04 and 0.1 wt.% nano-GO. The increase in yield stress can be attributed to the high surface area of nanoGO which significantly increases the free water needed to lubricate mixture particles (Chuah et al., 2014). Furthermore, significant Van der Waals forces between nano-GO particles result in bundles forming in an aqueous solution. When such an aqueous solution is mixed with cement, the electrostatic forces increase remarkably, resulting in entrapping water molecules and reducing free water. Agglomerated nano-GO and the formation of nano-GO bundles in cement mixtures have been observed in several studies (Rehman et al., 2018; Shang et al., 2015; Wang et al., 2016). Therefore incorporating more nano-GO in cementitious composites increases the degree of particles flocculation, leading to a rise in the yield stress. The high surface area of nano-GO and the increase in free water for lubrication can also explain the rise in plastic viscosity by adding nano-GO to cement composites. When free water is reduced, the internal friction between nano-GO and cement particles increases, leading to a higher value of plastic viscosity.

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Conclusions

The theoretical background of fluid and cementitious composites rheology and test methods for measuring the rheological properties of cementitious composites were reviewed. More specifically, the effects of different nanomaterials on the rheological behavior of cementitious composites were discussed in detail. In general, nanoparticles mostly increases the water demand of cementitious composites to retain mixture workability. This is mainly due to the high specific surface area of the nanoparticles. The attraction of water by nanoparticles decreases the free water required to lubricate mixture particles and increases internal friction. Therefore, in most cases, the incorporation of nanoparticles decreases the flowability of cementitious composites and increases flow time, yield stress, and plastic viscosity of cementitious composites. Examples of such nanoparticles are nano-SiO2, nano-Al2O3, nano-ZnO, nano-clay, and nano-GO. A similar conclusion can be drawn for CNTs and CNFs when mixed with cement-based composites using surfactants. However, some nanoparticles have an insignificant effect on the flowability and rheology of cement-based composites, especially at low dosage (less than 3.0 wt.%), such as nano-TiO2, nano-ZrO2. Published findings do not agree well regarding the effect of some nanoparticles, such as nano-CaCO3, on the rheology of cementitious composites. Hence further experimental studies should be performed to make a robust conclusion for those nanoparticles. The rheological performance of cement-based systems containing nanoparticles can be tested directly using a suitable rheometer and indirectly by conventional flowability tests such as the slump test. The former provides more information and scientific description about the rheology of cement systems, including yield stress, plastic viscosity and their variations with time. However, this test is not straightforward owing to the complex rheological behavior of cement-based systems such as shear-thickening and segregation and the complexity of rheometers. Furthermore, the rheological properties measured by rheometers are significantly dependent on the testing method and data interpretation. The most suitable rheometers for cement-based systems are coaxial cylinder and parallel rotating plate geometries since they offer analytical converting equations to calculate shear stress and rate. Several rheological models have also been used to describe the rheology of cement-based systems with and without nanoparticles, such as the Bingham model, modified Bingham model, Herschel
Bulkley model, De Kee model, Yahia
Khayat model etc. Some of these models are simple facilitating analytical solutions, such as the Bingham model. In contrast, some others, like the Yahia
Khayat model, need complex calculation. Rheological models introduced in this chapter can achieve various levels of success in describing the flow of cementitious composites, as discussed in detail in Section 6.2.1. Their accuracy mainly depends on their abilities to appropriately fit the nonlinear part of the shear stress
shear strain curve at low shear rates.

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891. Available from https://doi.org/10.1016/j.conbuildmat.2017.08.107. Safi, B., Aknouche, H., Mechakra, H., Aboutaleb, D., & Bouali, K. (2018). Incorporation mode effect of nano-silica on the rheological and mechanical properties of cementitious pastes and cement mortars. IOP Conference Series: Earth and Environmental Science, 143 (1), 012015. Available from https://doi.org/10.1088/1755-1315/143/1/012015,
012015. Saleh, H. M., El-Saied, F. A., Salaheldin, T. A., & Hezo, A. A. (2018). Macro- and nanomaterials for improvement of mechanical and physical properties of cement kiln dust-based composite materials. Journal of Cleaner Production, 204, 532
541. Available from https://doi.org/10.1016/j.jclepro.2018.08.303. Schramm, G. (1994). A practical approach to rheology and rheometry. Rheology (p. 291) Gebrueder HAAKE GmbH. Senff, L., Hotza, D., Lucas, S., Ferreira, V. M., & Labrincha, J. A. (2012). Effect of nanoSiO2 and nano-TiO2 addition on the rheological behavior and the hardened properties of cement mortars. Materials Science and Engineering: A, 532, 354
361. Available from https://doi.org/10.1016/j.msea.2011.10.102. Senff, Luciano, Hotza, D., Repette, W. L., Ferreira, V. M., & Labrincha, J. A. (2010). Mortars with nano-SiO2 and micro-SiO2 investigated by experimental design. Construction and Building Materials, 24(8), 1432
1437. Available from https://doi.org/ 10.1016/j.conbuildmat.2010.01.012. Senff, Luciano, Labrincha, J. A., Ferreira, V. M., Hotza, D., & Repette, W. L. (2009). Effect of nano-silica on rheology and fresh properties of cement pastes and mortars. Construction and Building Materials, 23(7), 2487
2491. Available from https://doi.org/ 10.1016/j.conbuildmat.2009.02.005.

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Shahrajabian, F., & Behfarnia, K. (2018). The effects of nano particles on freeze and thaw resistance of alkali-activated slag concrete. Construction and Building Materials, 176, 172
178. Available from https://doi.org/10.1016/j.conbuildmat.2018.05.033. Shang, Y., Zhang, D., Yang, C., Liu, Y., & Liu, Y. (2015). Effect of graphene oxide on the rheological properties of cement pastes. Construction and Building Materials, 96, 20
28. Available from https://doi.org/10.1016/j.conbuildmat.2015.07.181. Silva, G. F., Bosso, R., Ferino, R. V., Tanomaru-Filho, M., Bernardi, M. I. B., GuerreiroTanomaru, J. M., & Cerri, P. S. (2014). Microparticulated and nanoparticulated zirconium oxide added to calcium silicate cement: Evaluation of physicochemical and biological properties. Journal of Biomedical Materials Research Part A, 102(12). Available from https://doi.org/10.1002/jbm.a.35099, n/a-n/a. Singh, L. P., Goel, A., Bhattachharyya, S. K., Ahalawat, S., Sharma, U., & Mishra, G. (2015). Effect of morphology and dispersibility of silica nanoparticles on the mechanical behaviour of cement mortar. International Journal of Concrete Structures and Materials, 9(2), 207
217. Available from https://doi.org/10.1007/s40069-015-0099-2. Sobolkina, A., Mechtcherine, V., Bellmann, C., Khavrus, V., Oswald, S., Hampel, S., & Leonhardt, A. (2014). Surface properties of CNTs and their interaction with silica. Journal of Colloid and Interface Science, 413, 43
53. Available from https://doi.org/ 10.1016/j.jcis.2013.09.033. Sonebi, M., Bassuoni, M. T., Kwasny, J., & Amanuddin, A. K. (2015). Effect of nanosilica on rheology, fresh properties, and strength of cement-based grouts. Journal of Materials in Civil Engineering, 27(4), 04014145. Available from https://doi.org/10.1061/(asce) mt.1943-5533.0001080,
04014145. Supit, S. W. M., & Shaikh, F. U. A. (2014). Effect of Nano-CaCO3 on compressive strength development of high volume fly ash mortars and concretes. Journal of Advanced Concrete Technology, 12(6), 178
186. Available from https://doi.org/10.3151/ jact.12.178. Svermova, L., Sonebi, M., & Bartos, P. J. M. (2003). Influence of mix proportions on rheology of cement grouts containing limestone powder. Cement and Concrete Composites, 25(7), 737
749. Available from https://doi.org/10.1016/S0958-9465(02)00115-4. Tattersall, G. H. (1973). The rationale of a two-point workability test. Magazine of Concrete Research, 25, 169
172. Available from https://doi.org/10.1680/macr.1973.25.84.169. Tattersall, G. H., & Banfill, P. F. G. (1983). The rheology of fresh concrete. Great Britain: Pitman Books Limited. Tattersall, G. H., & Bloomer, S. J. (1979). Further development of the two-point test for workability and extension of its range. Magazine of Concrete Research, 31, 202
210. Available from https://doi.org/10.1680/macr.1979.31.109.202. Umarajyadav., & Vahini, M. (2017). Study of mechanical properties of concrete with nano zirconia. International Research Journal of Engineering and Technology, 4(8), 90
94. Vikan, H., Justnes, H., Winnefeld, F., & Figi, R. (2007). Correlating cement characteristics with rheology of paste. Cement and Concrete Research, 37(11), 1502
1511. Available from https://doi.org/10.1016/j.cemconres.2007.08.011. vom Berg, W. (1979). Influence of specific surface and concentration of solids upon the flow behaviour of cement pastes. Magazine of Concrete Research, 31(109), 211
216. Available from https://doi.org/10.1680/macr.1979.31.109.211. Wallevik, J.E. (2009). Development of parallel plate-based measuring system for the contec viscometer. In 3rd International RILEM Symposium on Rheology of Cement Suspensions such as Fresh Concrete (pp. 18
24). RILEM.

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Wallevik, J. E., & Wallevik, O. H. (1998). Effect of eccentricity and tilting in coaxial cylinder viscometers when testing cement paste. Nordic Concrete Federation, 21, 144
152. Wallevik, O. H., Feys, D., Wallevik, J. E., & Khayat, K. H. (2015). Avoiding inaccurate interpretations of rheological measurements for cement-based materials. Cement and Concrete Research, 78, 100
109. Available from https://doi.org/10.1016/j. cemconres.2015.05.003. Wang, H. (2009). Dispersing carbon nanotubes using surfactants. Current Opinion in Colloid & Interface Science, 14(5), 364
371. Available from https://doi.org/10.1016/j. cocis.2009.06.004. Wang, M., Yao, H., Wang, R., & Zheng, S. (2017). Chemically functionalized graphene oxide as the additive for cement
matrix composite with enhanced fluidity and toughness. Construction and Building Materials, 150, 150
156. Available from https://doi. org/10.1016/j.conbuildmat.2017.05.217. Wang, Q., Cui, X., Wang, J., Li, S., Lv, C., & Dong, Y. (2017). Effect of fly ash on rheological properties of graphene oxide cement paste. Construction and Building Materials, 138, 35
44. Available from https://doi.org/10.1016/j.conbuildmat.2017.01.126. Wang, Q., Wang, J., Lu, C. X., Cui, X. Y., Li, S. Y., & Wang, X. (2016). Rheological behavior of fresh cement pastes with a graphene oxide additive. Xinxing Tan Cailiao/New Carbon Materials, 31(6), 574
584. Available from https://doi.org/10.1016/S1872-5805 (16)60033-1. Williamson, R. V. (1929). The flow of pseudoplastic materials. Industrial and Engineering Chemistry, 21(11), 1108
1111. Available from https://doi.org/10.1021/ie50239a035. Xu, Q., Meng, T., & Huang, M. (2012). Effects of Nano-CaCO3 on the compressive strength and microstructure of high strength concrete in different curing temperature. Applied Mechanics and Materials, 121
126, 126
131. Available from https://doi.org/10.4028/ http://www.scientific.net/AMM.121-126.126. Yahia, A. (2011). Shear-thickening behavior of high-performance cement grouts — Influencing mix-design parameters. Cement and Concrete Research, 41(3), 230
235. Available from https://doi.org/10.1016/j.cemconres.2010.11.004. Yahia, A., & Khayat, K. H. (2001). Analytical models for estimating yield stress of highperformance pseudoplastic grout. Cement and Concrete Research, 31(5), 731
738. Available from https://doi.org/10.1016/S0008-8846(01)00476-8. Yahia, A., & Khayat, K. H. (2003). Applicability of rheological models to high-performance grouts containing supplementary cementitious materials and viscosity enhancing admixture. Materials and Structures, 36(6), 402
412. Available from https://doi.org/10.1007/ BF02481066. Yun, K.-K., Choi, S.-Y., & Yeon, J. H. (2015). Effects of admixtures on the rheological properties of high-performance wet-mix shotcrete mixtures. Construction and Building Materials, 78, 194
202. Available from https://doi.org/10.1016/j.conbuildmat.2014.12.117. Zhang, M.-H., & Islam, J. (2012). Use of nano-silica to reduce setting time and increase early strength of concretes with high volumes of fly ash or slag. Construction and Building Materials, 29, 573
580. Available from https://doi.org/10.1016/j.conbuildmat.2011.11.013. Zhang, M.-H., Islam, J., & Peethamparan, S. (2012). Use of nano-silica to increase early strength and reduce setting time of concretes with high volumes of slag. Cement and Concrete Composites, 34(5), 650
662. Available from https://doi.org/10.1016/j. cemconcomp.2012.02.005. Zhang, R., Cheng, X., Hou, P., & Ye, Z. (2015). Influences of nano-TiO2 on the properties of cement-based materials: Hydration and drying shrinkage. Construction and Building Materials, 81, 35
41. Available from https://doi.org/10.1016/j.conbuildmat.2015.02.003.

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Zhang, Y., Zhang, Y., She, W., Yang, L., Liu, G., & Yang, Y. (2019). Rheological and harden properties of the high-thixotropy 3D printing concrete. Construction and Building Materials, 201, 278
285. Available from https://doi.org/10.1016/j. conbuildmat.2018.12.061.

Further reading Banfill, P. F. G., Brower, L. E., Banfill, P., Beaupre´, D., Chapdelaine, F., Larrard, F., de Domone, P., Nachbaur, L., Sedran, T., Wallevik, O., & Wallevik, J. E. (2001). Comparison of concrete rheometers: International tests at LCPC (Nantes, France) in October, 2000. National Institute of Standards and Technology. Domone, P. L. J., Yongmo, X., & Banfill, P. F. G. (1999). Developments of the two-point workability test for high-performance concrete. Magazine of Concrete Research, 51(3), 171
179. Available from https://doi.org/10.1680/macr.1999.51.3.171. Ferraris, C. F., Brower, L. E., Beaupre´, D., Chapdelaine, F., Domone, P., Koehler, E., Shen, L., Sonebi, M., Struble, L., Tepke, D., Wallevik, O., & Wallevik, J. E. (2004). Comparison of concrete rheometers: International tests at MB (Cleveland OH, USA) in May, 2003. National Institute of Standards and Technology. Rehman, S. K. U., Ibrahim, Z., Memon, S., Javed, M., & Khushnood, R. (2017). A sustainable graphene based cement composite. Sustainability, 9(7), 1229. Available from https://doi.org/10.3390/su9071229,
1229. Rehman, S. K. U., Ibrahim, Z., Memon, S. A., Aunkor, M. T. H., Javed, M. F., Mehmood, K., & Shah, S. M. A. (2018). Influence of graphene nanosheets on rheology, microstructure, strength development and self-sensing properties of cement based composites. Sustainability (Switzerland), 10(3). Available from https://doi.org/10.3390/su10030822. Tattersall, G. H. (1973). The rationale of a two-point workability test. Magazine of Concrete Research, 25(84), 169
172. Available from https://doi.org/10.1680/macr.1973.25.84.169. Tattersall, G. H., & Bloomer, S. J. (1979). Further development of the two-point test for workability and extension of its range. Magazine of Concrete Research, 31(109), 202
210. Available from https://doi.org/10.1680/macr.1979.31.109.202.

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Nano-modification in digital manufacturing of cementitious composites

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˘ ˘ Fernando Franc¸a de Mendonc¸a Filho, Yu Chen, and Oguzhan C¸opuroglu Faculty of Civil Engineering and Geosciences, Section Materials and Environment, Delft University of Technology, Delft, The Netherlands

7.1

Introduction

As new construction technologies emerge, their development requires nonconventional approaches, which include using innovative functional components. The history of concrete technology has seen several significant leaps forward by adopting this strategy. A well-known example is the development and introduction of superplasticizers in the 1970s which opened the doors for high-performance concrete (Mehta & Aietcin, 1990). Later, during the 1990s and 2000s, mineral additions and supplementary cementitious materials became increasingly essential for the development of ultra-high performance concrete (Acker & Ulm, 2013). However, in recent years it has become clear that the highest demand for innovation in concrete is not in its mechanical properties, but in its digital manufacturing (Attaran, 2017). This new approach is quickly becoming a strong candidate for future construction technologies, and even for extra-terrestrial endeavors (Matsumoto et al., 1992; Reches, 2019). However, conventional materials appear to have limited resources to offer for further enhancing the digital manufacturing capabilities. Therefore there is a dire need for adopting nonconventional material
based solutions, for which nanomaterials stand out for the development of this additive manufacturing technology (Khan et al., 2020). As defined by ASTM (ASTM & F2792
10, 2010), additive manufacturing is “a process of joining materials to make objects from 3D model data, usually layer upon layer, as opposed to subtractive manufacturing methodologies.” Until now, main techniques in the context of digital concrete manufacturing include layer extrusion (contour crafting, 3D concrete printing), particle bed printing (D-shape, binder jetting, or sand jetting), formwork printing (mesh mold), and temporary supports (flexible formwork, knitted textiles) (Wangler et al., 2019), as shown in Fig. 7.1. Detailed classifications of digital fabrication with concrete and descriptions of each specific technique are given in (Buswell et al., 2020; Reiter et al., 2020; Wangler et al., 2019). Extrusion-based method is the most popular and investigated 3D printing technology with concrete so far (Buswell et al., 2018; Marchon et al., 2018; Recent Advances in Nano-Tailored Multi-Functional Cementitious Composites. DOI: https://doi.org/10.1016/B978-0-323-85229-6.00009-3 © 2022 Elsevier Ltd. All rights reserved.

Figure 7.1 Main techniques for digital fabrication with concrete. Source: Adapted from Buswell, R. A., da Silva, W. R. L., Bos, F. P., Schipper, H. R., Lowke, D., Hack, N., Kloft, H., Mechtcherine, V., Wangler, T., & Roussel, N. (2020). A process classification framework for defining and describing Digital Fabrication with Concrete. Cement and Concrete Research, 134. https://doi.org/10.1016/j. cemconres.2020.106068.

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Wangler et al., 2016). Remarkable attention from both academia and industry has been given to extrusion-based 3D concrete printing (3DCP) during the last decade. Many companies in the Netherlands, for example, Royal BAM Group, CyBe, Twente Additive Manufacturing, and Bruil, are attempting to implement this technology in practice. 3DCP is the digital concrete manufacturing technique under focus in this study. The development of printable cementitious composites is possibly the most critical aspect in 3DCP. Compared to mold-cast concrete process, several essential material parameters need to be controlled in 3DCP process, that is, pumpability, extrudability, buildability, and others (Le et al., 2012; Lim et al., 2012). Conventional material
based technology appears to have limited resources to offer for further enhancing the capabilities of 3D printing. Therefore there is a dire need for adopting nonconventional material
based solutions for which nanomaterials can play a vital role. Controlling the rheology is the key to successful 3DCP, as achieving dimensional stability and the minimum required mechanical properties in green state are the main challenges. Furthermore, achieving the required strength development rate and enabling smart monitoring of the 3DCP are the other goals that are desired in designing such materials. Recent research shows that successful modification of cementitious materials can be achieved by incorporating nanomaterials in the material design for the enhanced fresh and hardened state properties. In this chapter, a summary of these developments is compiled in the light of potential applications, safety issues, and technological challenges.

7.2

Implementation of nanomaterials in extrusion-based 3D concrete printing

7.2.1 Printing processes and required material behaviors A typical 3DCP setup, which consists of three primary components, namely, a deposition setup (three-/four-axis gantry-based, or six-axis robotic system), a control unit, and a material extrusion system (ram extrusion
based or rotor and statorbased printing setup and printhead), is illustrated in Fig. 7.2. The working mechanism of 3DCP is explained as a layer-wise construction process without a formwork (Lim et al., 2012; Wolfs et al., 2019). Because of the absence of formwork, the cementitious materials for 3DCP are expected to show a unique material behavior compared to the conventional mold-cast concrete. Specifically, the fresh cementitious materials should meet contradicting rheological requirements, that is, sufficiently flowable (low initial yield stress) under the shear during pumping and extrusion, and high shape retention and structuration rate at rest after deposition to build the layered structure (Chen et al., 2020; Marchon et al., 2018). To date, two main printing strategies have been employed by most of the studies in the context of 3DCP. The first one is defined as the extrusion of highly stiff or sufficiently stiff material without using additional energy or adding reactive agent in the printhead (Mechtcherine et al., 2020), which is the most common printing

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Figure 7.2 Extrusion-based 3D concrete printing setup at TU Eindhoven. Source: Adapted from Chen, Y., Chaves Figueiredo, S., Li, Z., Chang, Z., Jansen, K., C ¸ opuro˘glu, O., & Schlangen, E. (2020). Improving printability of limestone-calcined claybased cementitious materials by using viscosity-modifying admixture. Cement and Concrete Research, 132, 106040. Available from https://doi.org/10.1016/j.cemconres.2020.106040.

approach (see Kazemian et al., 2017; Le et al., 2012; Nerella et al., 2019; Panda, Lim, et al., 2019; Rahul et al., 2019). In this case, the printability of fresh mixture is dominated by its thixotropy. For a fresh cementitious material, the yield stress is not the same at dynamic (flow under shearing) and static (after deposition) states. The dynamic and static yield stresses are defined as the minimum stress to stop flow under shear and the stress necessary to initiate flow from rest, respectively (Marchon et al., 2018). The magnitude of the difference between dynamic and static yield stresses is regarded as thixotropy (Marchon et al., 2018; Perrot et al., 2016). A low dynamic yield stress is required to guarantee the ease of pumping and extrusion, whereas a high static yield stress is essential for constructing a layered structure. From the microstructure perspective, the static yield stress development of fresh cementitious materials is dominated by the flocculation of particles and ongoing hydration. During the pumping and extrusion processes, the links between particles are broken under constant shearing, which reduces colloidal suspension viscosity. After deposition, the viscosity of material at rest is recovered since the interparticle links are rebuilt (Panda, Ruan, et al., 2019; Roussel, 2018; Roussel et al., 2012). The addition of nanomaterials in printable mixtures (nano-clay and nano-silica) can promote this re-flocculation process (see Fig. 7.3), which has been reported by a few recent studies (Kruger et al., 2019; Panda, Ruan, et al., 2019). In contrast, the second printing strategy is known as set-on-demand printing, which was proposed and developed by (Gosselin et al., 2016; Marchon et al., 2018; Reiter et al., 2018, 2020). The flowable mixture with a low initial yield stress close

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Figure 7.3 Buildability assessment of (A) Control mixture (without nano-clay) and (B) 5NC mixture (containing 0.5 wt.% (of binder) nano-clay). (C) Flocculation characterization—floc size under low and high shear rates. Source: Adapted from Panda, B., Ruan, S., Unluer, C., & Tan, M. J. (2019). Improving the 3D printability of high volume fly ash mixtures via the use of nano attapulgite clay. Composites Part B: Engineering, 165, 75
83. https://doi.org/10.1016/j. compositesb.2018.11.109.

to that of the self-compacting concrete can be easily transported through the hose to the nozzle. The reactive agent, that is, accelerator, or other fast-setting slurries, is injected and mixed with the fresh cementitious materials in the printhead (Mechtcherine et al., 2020). Consequently, the deposited fresh mixture can reach a very high stiffness because of the addition of reactive agents. Unlike the first printing strategy, the change of yield stress does not rely on the material thixotropy. Nanomaterial is not the key ingredient in this process; therefore this strategy is not discussed further.

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7.2.2 Nanomaterials as thixotropic agents Nanoparticles, typically with a particle size smaller than 100 nm, exhibit significant influences on the rheology of concrete, for example, increasing the stiffness and viscosity of fresh concrete even at a low dosage (Kruger et al., 2019; Senff et al., 2009). In recent years, it has been found that nanoparticles can be employed as thixotropic/thickening agents for 3D-printable cementitious materials (Kruger et al., 2019; Panda, Ruan, et al., 2019; Panda, Mohamed, et al., 2019). Until now, nanomaterials such as nano-clay, nano-silica, and nanocalcium carbonate have been attempted to be used in 3DCP. A summary of using these nanomaterials as thixotropic agents in 3D printable cementitious materials is shown in Table 7.1. As shown Table 7.1 A summary of using nanomaterials as thixotropic agents in 3D printed cementitious materials.

Chemistry (phase compositions) Particle size Morphology Effect on 3D printing

References

Nano-clay (highly purified attapulgite clay)

Nano-silica

Nanocalcium carbonate

Si
(Mg, Al)
O

Si
O

CaCO3

30 nm diameter, 1.5
2 μm length Rod-like/fibrous shape

15
20 nm

B150 nm

Spherical and porous particles (1) Thickening effect. (2) Increase yield stress, particle (re-) flocculation rate, and structuration rate (structural build-up). (3) Enhance shape stability and buildability.

Granular shape

(1) Increase yield stress and plastic viscosity. (2) Enhance shape stability and buildability. (3) Improve thixotropy (accelerating green strength development without compromising flowability). Kazemian et al. (2017), Marchon et al. (2018), Moeini et al. (2020), Panda, Lim et al. (2019), Panda, Ruan et al. (2019), Panda et al. (2020), Rahul et al. (2019), Yuan et al. (2018), Zhang et al. (2018)

Kruger et al. (2019), Mendoza Reales et al. (2019), Yuan et al. (2018)

Enhance shape stability, buildability, and structural build-up.

Chu et al. (2021), Yuan et al. (2018)

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in Fig. 7.3, the addition of such nanomaterials could primarily improve shape stability, buildability, thixotropy, and structural buildup of fresh cementitious materials (Mendoza Reales et al., 2019; Zhang et al., 2018), which may be attributed to the agglomeration and high water absorption of nanoparticles (Khayat et al., 2019; Nazar et al., 2020). On the other hand, nanoparticles could fill the gap between cementitious particles, increasing the packing density, which consequently reduces the dynamic yield stress and plastic viscosity of fresh cementitious materials (Khayat et al., 2019). However, nanomaterials generally display a very high specific surface area (SSA)-to-volume ratio, which significantly decreases the free water content, adversely affecting the fluidity of fresh mixtures (Nazar et al., 2020). Thus it is difficult to predict the influence of nanomaterials on flowability, which may be varied using different dosages or types of nanomaterials. Among the nanomaterials listed in Table 7.1, nano-clays appear to be the most generic ones employed to improve the 3D printability of fresh cementitious materials. Generally, nano-clay is a common class name, which can be further specified as attapulgite, bentonite (montmorillonite-based), kaolinite, halloysite, sepiolite, and contaminated clays depending on their chemical composition and particle morphology (Marchon et al., 2018; Nazar et al., 2020). The initial attempt (Kazemian et al., 2017) of using nano-clay (highly purified attapulgite clay) in printable concrete was made by researchers from Contour Crafting group at the University of Southern California. Recent studies on employing attapulgite nano-clay as the thixotropic agent in 3D-printable cementitious materials are given by (Panda, Lim, et al., 2019; Panda, Ruan, et al., 2019). Unlike the nano-silica or calcium carbonate with a round/granular particle shape, attapulgite nano-clay exhibits rod-like particle morphology.

7.2.3 Comparison between polymeric viscosity modifying admixtures and nanomaterials In 3DCP, polymeric viscosity modifying admixtures (VMAs), including celluloseether derivatives and anionic polyacrylamides (Marchon et al., 2018), can be used as thixotropic agents, which exhibit the high capacity to catch water molecules and increase the dynamic viscosity of the cement paste (Palacios & Flatt, 2016). In many available 3DCP recipes (see Chaves Figueiredo et al., 2019; Chen et al., 2018; Rahul et al., 2019), both superplasticizers (mainly polycarboxylate ether superplasticizers) and cellulose-ether-derived VMAs (such as hydroxypropyl methylcellulose) were utilized for adjusting the rheology and 3D printability of fresh mixtures. The issues of incompatibility between two polymers must be highlighted. The addition of cellulose-ether-derived VMAs could compete with superplasticizers to adsorb onto the surface of particles, which affects the dispersion of superplasticizers (Khayat & Mikanovic, 2011; Palacios & Flatt, 2016). Besides, the nonadsorbed polymers could increase the viscosity of the pore solution, resulting in depletion flocculation. The competitive effects between superplasticizer and VMA may increase yield stress and plastic viscosity (Palacios & Flatt, 2016), which can significantly affect pumpability and extrudability. Using nanomaterials (nano-silica) as

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thixotropic agents may also affect the compatibility between the cement and superplasticizers. The yield stress of fresh cementitious materials and the demand for superplasticizers are significantly increased by increasing the dose of nanomaterials (Sonebi et al., 2015). However, the presence of superplasticizers could contribute to the uniform distribution of nanoparticles, which was discussed in Section 7.4.1. Furthermore, the addition of polymetric VMAs may perturb the hydration of cement, for example, prolonging the induction period, reducing the reaction rate in the acceleration period, and decreasing the maximum heat release. The negative effect of cellulose-ether-derived VMAs on cement hydration has been reported by many previous studies (Chen et al., 2020; Palacios & Flatt, 2016; Pourchez et al., 2006, 2010). As explained by Pourchez et al. (2010), cellulose-ether-derived VMAs could strongly be adsorbed on hydrated phases, that is, C
S
H and portlandite, which inhibits the growth of these hydrates. However, using nanomaterials may not adversely affect cement hydration. The presence of nanomaterials could introduce an additional amount of nucleation and/or hydrate growth surface for consuming the ions from dissolved anhydrites, which can accelerate the structural buildup and hydration (Palacios & Flatt, 2016; Roussel et al., 2019; Sanchez & Sobolev, 2010). Therefore in comparison with polymetric VMAs (especially cellulose-ether-derived VMAs), nanomaterials may be a better thixotropic agent for developing 3Dprintable cementitious materials when retardation is an issue.

7.3

Effects of nano-additions on fresh and hardened state of concrete

The addition of nanoscale materials to concrete can be broadly divided into two categories, carbon-based and noncarbon-based. As cement relies on the reaction and hydration of Ca
Si
Al systems, it is natural that many efforts focus on the addition of nano-sized oxides based on calcium, silicon, or aluminum (and the combinations thereof). On the other hand, novel materials such as carbon nano-tubes (CNTs), graphene oxide (GO), and carbon nano-fibers (CNF) display advanced mechanical and electrical properties, which are sought after additions to concrete. As carbon-based additives have much higher aspect ratios, these are usually added in much smaller quantities, for example, 0.1% of binder mass, while noncarbonbased additions are typically added in values of 1%
5% of binder mass.

7.3.1 Nano-silica Nano-silica is the cheapest and the most consumed nano-addition by the construction industry (Chithra et al., 2016). It can be found as a powder or as a suspension. The average particle size varies between 8.5 and 15 nm (Pavan Kumar et al., 2021), its chemical composition is SiO2 However, any nanoparticle presents a much higher SSA than the other components of concrete, as stated in the previous section. Thus workability of the mix is

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considerably affected. Furthermore, the properties of the material can only be tailored if uniform dispersion is achieved (Li et al., 2004). Although nano-silica is considered a zero-dimensional addition, it also suffers from this disadvantage because the grains tend to flocculate. As with other additions, researchers have successfully used superplasticizers (Senff et al., 2012), ultrasonication (Wang et al., 2015), superabsorbent polymers (Pourjavadi et al., 2012), and the application of the addition in suspension (Safi et al., 2018) as means to mitigate the effects on workability. Additionally, the high SSA improves reactivity, which has the adverse effect of reducing the setting time of the mix (Abhilash et al., 2021). The improvement in compression strength of concrete boosted by nano-silica is because of two reasons: the pozzolanic reaction of the material, which takes place in the same manner than silica fume; and the filler effect, which aids the diminishment of voids (Abhilash et al., 2021). Research has also shown that flexural strength (Zhang et al., 2016), permeability (Pavan Kumar et al., 2021), freeze-andthaw resistance (Gonzalez et al., 2016), and steel
matrix bond (Ismael et al., 2016) are improved by the addition of nano-silica.

7.3.2 Nano-titania Nano-titania is the second most common nanoparticle additive for concrete (Mendes et al., 2015). These are relatively easy to obtain and are available in varying sizes, with average particle diameter ranging from 25 to 600 nm. Its typical chemical composition is TiO2 The effects in the fresh state of concrete are similar to nano-silica, with the additional effect that nano-titania was found to increase the heat of hydration considerably (Nazar et al., 2020). Although this addition also provides the standard improvement of mechanical properties, its main application is the development of “self-cleaning” concrete. Chen and Poon (2009) described in details the photocatalytic effect in such concretes, which can convert organic pollutants and oxides (such as NO, NO2, and SO2) into harmless components using only UV radiation from daylight. This, however, should not be taken as literally self-cleaning, as the material still requires common cleaning, just less frequently. As the main advantage is only present on the surface, there are also commercial coatings available for concrete.

7.3.3 Nano-clay Nano-clay is a white powder with dimensions depending on the parent mineral silicate. Many studies focus on nano-montmorillonite because of its different structure. In fact, this is a 2D addition with platelet shape, about 1 nm thickness and 70
150 nm width (Norhasri et al., 2017). In the fresh state, this addition also increases viscosity and flocculation. However, the increase in shape stability of the fresh mix combined with the overall improvement in buildability makes this a very promising material for 3D printing of concrete (Nazar et al., 2020; Panda, Ruan, et al., 2019).

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Nano-clay has certain pozzolanicity and acts well as filler, but it was shown to work better in combination with other additions. Al-Rifaie and Ahmed demonstrated that the combination of nano-silica and nano-clay is far more effective on the improvement of mechanical properties, having doubled the compression strength and quadrupled the tensile strength with respect to reference mortar samples (Al-Rifaie & Ahmed, 2016).

7.3.4 Nano-alumina Nano-alumina is another white powder material, with a chemical composition of Al2 O3 . Its influence on the fresh state of concrete is similar to all other nano-additions given its particularly high SSA. One notable exception is that it can be used as a hydration retarder (Bastos et al., 2016), as alumina has preferential reaction during the very early ages (Krishnaveni & Senthil Selvan, 2021). Besides the improvement in compressive and flexural strength, it strengthens the bond between concrete and steel reinforcement by 25% (Ismael et al., 2016). In addition, it was found that at a 3% dosage, it can be used as replacement to binder, still improving the overall mechanical behavior and durability properties (Nazari & Riahi, 2011).

7.3.5 Other mineral additions Many other nanomaterials have been added to concrete in order to change its properties. However, most research focuses on the four nanomaterials mentioned earlier, because of their availability and cost. Yet, some new additions are worth mentioning because of their unique properties. Nanocalcium carbonate has been found to combine a number of advantages of other additions. It provides a much more consistent control of concrete flowability to be used in digital manufacturing (Chu et al., 2021). It also provides enhanced strength, while reducing shrinkage during hydration (Cai et al., 2016). Finally, it drastically reduces the loss of water at temperatures as high as 800 C (Salih et al., 2020). It is expected that more research will be devoted to this material once its manufacturing and distribution becomes more widespread. In a similar manner that the use of silica fume grew interest in nano-silica for the construction industry, the use of metakaolin generated interest in nanometakaolin. This has, nonetheless, been studied at a much slower pace because of the low availability of kaolin in certain countries (Norhasri et al., 2017). Similar to nano-clay, this material is composed of amorphous alumina and silica; however, its structure comprises long-order hexagonal layers (Zhang et al., 2010). This addition increases the compressive and flexural strength of concrete considerably and has an optimal dosage ranging from 4% to 12% (Abdel Gawwad et al., 2017; Nitish et al., 2020). However, further research must be conducted for acquiring insights into the exact reaction mechanism and its effects on shrinkage before this material is popularized in construction (Zhan et al., 2020).

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Nanomagnesia or nano-MgO is another relatively new additive for concrete. This has been used as a shrinkage reducer, decreasing as much as 80% in a dosage of 7.5% of cement mass (Polat et al., 2017). This is particularly useful in mass concrete, as this is an expansion agent that requires less water than usual alternatives (Shah et al., 2015). Still, the contributions to mechanical properties are not well understood as some researchers have found contributions to compressive strength as low as 1%
8% (Polat et al., 2017), while other researchers claim an increase of 80% in strength with only 1% addition of the material (Moradpour et al., 2013). Another new addition gaining popularity is nano-zinc oxide. This addition has a distinct capacity of inhibiting the hydration of C3 S and C2S, thus considerably increasing the induction period previous to initial setting of the cement. Liu et al. (2019) reported quantities as low as 0.2% in mass replacement of cement quadrupling the period before setting. As these particles do not react with cement or increase reactivity, the gains in compression and tensile strength are small, only caused by the void filling effect (Garg & Garg, 2021). However, concretes with ZnO addition have also been reported to gain photocatalytic properties. While TiO2 is more efficient at degrading NOx components, ZnO shows a much better performance against bacterial activity, even at night when no UV source is available (Kumar et al., 2021). As biodeterioration of constructions remains an open problem, it is expected this addition to find niche applications in the coming years.

7.3.6 Carbon nano-tubes CNTs can be single- (diameter 5 1
3 nm) or multiple-walled (diameter 5 10
40 nm), both with a typical length of 1 μm. The elastic modulus of this material is close to 1 TPa, while the tensile strength varies between 11 and 63 GPa and the elongation at break is about 12% (Yu et al., 2000). However, the bond of CNTs with cement products was found to be very weak, which hinders the reinforcement capacity of the addition (Fattah et al., 2015). Furthermore, acquisition remains costly, and dispersion requires special surfactants or sonication (Nazar et al., 2020). Therefore optimal dosage varies between 0.1% and 0.2%, which leads to a modest improvement of mechanical properties. Similarly, researchers also found modest improvement in residual mechanical properties after high-temperature exposure and overall higher maximum temperature before failure (Afzal & Khushnood, 2021). For the improvement of bond with reinforcement, CNTs are found to be more effective than nano-silica, with an increase up to 50% (Song et al., 2020). Nonetheless, the main application for CNTs in concrete is not on the enhancement of mechanical properties but on previously neglected properties. Additions as low as 0.05 wt.% in binder can drastically increase the capacity of self-sensing (Suchorzewski et al., 2020) and thermal sensing (Zuo et al., 2021), which can further be used for damage monitoring. In addition, this material also allows for electrical heating in concrete, which can be used to protect pavements from freeze-thaw damage (Choi, 2021). Finally, a much higher dosage (1%
3%) can create a

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material with good service life under extreme polarization, which would be immune to chloride attack (Qiao et al., 2015). As research progresses, it is expected that the use of CNTs in concrete will increase proportionally to the decrease in production cost or with the advent of better dispersion methods.

7.3.7 Carbon nano-fibers, graphene oxide, and carbon black While CNTs’ production price does not decrease, a few alternative solutions arise as additives for cementitious materials. CNFs are cylindrical graphene structures with a diameter of 70
200 nm and a length of 50
200 nm (Yazdani & Mohanam, 2014), displaying a far smaller aspect ratio than CNTs. This material is much weaker than CNTs (σt D8GPa) and much easier to disperse. At the same addition level with CNTs, it shows superior improvements in compressive and flexural strength (Danoglidis et al., 2016; Yazdani & Mohanam, 2014) and improves electrical and thermal properties at the same rate (Gomis et al., 2015). The main drive for the research and use of this material is the production cost, which is 50 times cheaper than for CNTs (Yazdani & Brown, 2016). As such, this is growing as a more viable alternative for addition. GO is competitive with CNTs because of its cheaper production cost and higher dispersibility in water (Bastos et al., 2016). The former a 2D surface, which works as a nucleation site for C
S
H, improving the overall mix reactivity (Babak et al., 2014). An addition as low as 0.05% can increase the compressive strength by 30% and flexural strength by 60%, while also producing a ductile material (Pan et al., 2013). Furthermore, at higher quantities (0.1%), it has been shown that GO makes possible the design and use of 100% recycled aggregates concrete (Devi & Khan, 2020). It is also possible to design GOs with hydrophobic properties, then it is applied as a coating material, decreasing water absorption, capillary absorption and chloride ingress considerably (Habibnejad Korayem et al., 2020). However, some researchers (Dalal & Dalal, 2021) have reported problems with excessive shrinkage, which requires further study. Finally, carbon black (CB) is formed from the incomplete combustion of carbonaceous materials, creating amorphous carbon molecules with open carbon bonds (Bastos et al., 2016). Because of this method of production, this material is much cheaper than the other options cited in this section. While CB does not improve the mechanical properties of concrete considerably, it can improve the electrical, thermal and self-sensing properties (Masadeh, 2015). As such, researchers (Wen & Chung, 2007) looked into replacing 50% of carbon fibers by CB and found that the conductivity could be maintained at the same level while the material price was lowered. In a similar fashion, it can also be used for electrical heating of concrete in cold climates, which allows for casting in negative temperatures (C ¸ ınar et al., 2020). Fig. 7.4 is presented as a short summary of the main nano-additions and their contribution to the hardened state properties of concrete.

Nano-modification in digital manufacturing of cementitious composites

Nanoparticles Nanomaterial

Nano-SiO₂ Nano-Al₂O₃

NanoFe₂O₃

NanoCaCO₃

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Carbon based Carbon nano fibres

NanoCarbon Graphene Nano-clay Nano-MgO Nano-TiO₂ Nano-ZnO metakaolin nano tubes oxide

Carbon black

Compressive strength and pore-filling Flexural strength Freeze and thaw resistance Steel-matrix bond Hydration accelerator Hydration retarder Shrinkage reducer Photocatalytic Hydrophilic coatings Hydrophobic coatings Strain-sensing Thermal sensing Electrical heating Cathodic protection enhancer

Figure 7.4 Main contributions for concrete by nano-additions. Source: Adapted from Bastos, G., Patin˜o-Barbeito, F., Patin˜o-Cambeiro, F., & Armesto, J. (2016). Nano-inclusions applied in cement-matrix composites: A review. Materials, 9(12). https://doi.org/10.3390/ma9121015.

7.4

Challenges with using nanomaterials as additives

7.4.1 Dispersion of nanomaterials Uniform dispersion is the main challenge when using nanomaterials in cementitious materials for 3DCP. The rheology of fresh cementitious material can be strongly influenced by the dispersion of nanomaterials. Insufficient dispersion can lead to a severe reduction in workability and weak flow consistency of fresh mixtures, which essentially limits their application (Nazar et al., 2020). Different nanomaterials show various particle morphologies, categorized as zero-, one-, and twodimensional (0D, 1D, and 2D) nanoparticles. The dispersion of different nanomaterials with different geometries is dissimilar. As Habibnejad Korayem et al. (2020) reported, because of the lower complexity of particle geometry, 0D nanoparticles (e.g., nano-CaCO3, TiO2, ZnO, and SiO2) are much easier to be dispersed than 1D and 2D nanoparticles. It is quite complex to properly disperse 1D nanoparticles (e.g., highly purified attapulgite clay and nano-CNTs) because of their bundling and entanglement. In contrast, uniform dispersion of 2D nano-sheets (e.g., nanographene) is extremely difficult due to their high surface energy.

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For improving the dispersion of nanomaterials, sufficient energy is required to break down agglomerated particles initially. After that, the particles need to be stabilized via chemical modifications, for example, steric and electrostatic repulsion, to avoid their re-agglomeration (Korayem et al., 2017). The methods applied to enhance the dispersion of nanoparticles are discussed by (Korayem et al., 2017; Parveen et al., 2013). Both mechanical and chemical approaches can be employed. In many cases, the combination of two approaches was conducted. As summarized by Korayem et al. (2017), high-shear mixing, mechanical stirring, ultrasonication, and ball milling are the common mechanical approaches to disperse nanoparticles in aqueous solutions and cement composites. Among them, the ultrasonication method appears very frequently in literature. Applying a proper ultrasonication energy can effectively reduce the agglomeration of nanoparticles. This method is already employed to disperse CNTs (Nochaiya & Chaipanich, 2011; Zou et al., 2015), nano-CaCO3 (Kawashima et al., 2014), nano-titania (Yousefi et al., 2013), and nano-silica (Sobolev et al., 2009) for use in cementitious pastes. The chemical approach focused on the physical and chemical surface modifications of nanoparticles, which is generally performed after breaking down agglomerates via mechanical methods. Surface active agents, also known as surfactants, are usually employed in the physical surface modification of nanomaterials (Korayem et al., 2017). Surfactants used for physical surface treatment of nanoparticles can benefit solution stabilization through the mechanism of electrostatic repulsion and/ or steric hindrance for reducing attractive Van der Waals forces between particles (Korayem et al., 2017; Lewis, 2000). As the most common surfactant in the industry, superplasticizer is used for this purpose (see Fakhim et al., 2015; Sobolev et al., 2009). However, it is worth noting that the addition of some surfactants may adversely affect cement hydration (Marchon et al., 2016). Besides, chemical surface modification is also utilized to disperse nanoparticles. As the most typical chemical treatment, functionalization is executed for attaching functional groups to the surface of hydrophobic nanoparticles to improve their hydrophilic properties (Korayem et al., 2017). This method is occupied for the dispersion of CNTs (Li et al., 2005, 2007; Zhu et al., 2003) and graphene/GO (Singh et al., 2011; Zhao et al., 2014).

7.4.2 Safety issues Except for the high cost and uncertainties in long-term material performance, a critical concern impeding the use of nanomaterials in construction industry is their adverse biological and toxicological effects (Van Broekhuizen et al., 2011). According to earlier works (Buzea et al., 2007; Karlsson et al., 2008; Lam et al., 2006; Spitzmiller et al., 2013; Xia et al., 2009), most of engineered (carbon-based, metal-containing, and nonmetallic) nanomaterials are confirmed to have toxic effects. Spitzmiller et al. (2013) pointed out that it is quite challenging to prevent the release of nanomaterials into the environment, which may be from the structure construction, throughout the use of structure, to the end of its service life (deconstruction and disposal). Thus, the consideration and approaches are essential to

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respond to the effect of nanomaterials on the health of construction workers and users, and the environment at all stages of the life cycle (Lee et al., 2010). For developing and using nanomaterials in a green and less toxic way, Spitzmiller et al. (2013) proposed a number of suggestions. First, tuning down the toxicity of nanomaterials during the manufacturing process. For example, blending 1%
10% of iron doping in nano-ZnO can reduces its toxicity. Similar approaches are expected for other nanomaterials. Second, yielding the universal design rules for green nanomaterials. Third, employing high throughput screening to test various nanomaterials in different doses, time points, and assay systems. Fourth, the research regarding the fewer environmental and health impacts should be prioritized. It suggests the tight cooperation between all related researchers at the beginning of new nanomaterials development and application.

7.5

Conclusions and future prospects

In extrusion-based 3DCP, nanomaterials are employed as thixotropic agents to rapidly increase viscosity of fresh mixture at rest (after deposition). Nano-clay (highly purified attapulgite clay) is the most frequently reported nanomaterial for improving the 3D printability of fresh cementitious materials. Compared to polymetric VMAs, nanomaterials can accelerate the structural buildup and hydration of cementitious materials. While nano-clay improves printability, a number of other nano-additions have been successfully incorporated in concrete mixes to vastly enhance its hardened properties. Most materials have the common effect of increasing compressive and tensile strength. However, specific additions contribute to novel applications such as hydrophobic coatings, photocatalytic effect, electrical-heating, cathodic protection enhancement, which presents great prospects for more durable concrete. The main challenge with using nanomaterials in 3D printable cementitious materials is the uniform dispersion of nanoparticles. Mechanical, chemical, or combined dispersion approaches can be applied to nanoparticles with different geometries (0D, 1D, or 2D). For 0D nanoparticles, mechanical methods, including high-shear mixing, stirring, ultrasonication, and ball milling, are commonly used. For 1D and 2D nanoparticles with high-complexity particle geometries, it seems that chemical approaches, that is, physical and chemical surface treatments, are required after the particle agglomeration is broken down by mechanical methods. The three main obstacles for the implementation nano-modification in the digital manufacturing of a cement composites remains the price, availability, and ease of use. The price is usually controlled by the production processes. As mentioned in Section 7.3, several advances in the generation of nanoparticles have been reported recently (Bastos et al., 2016; Chithra et al., 2016; Nazar et al., 2020; Yazdani & Brown, 2016), which contributes for cheaper materials. Availability is often changed by market demand, which means that ease of use is key for the adoption of this technology, which was discussed in Section 7.4.1.

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Nevertheless, the incentive to use nanomaterials in concrete is considerably strong. Governments and companies are pushing for better, more durable and more sustainable constructions (Le´le´, 1991; Palacios-Munoz et al., 2019), which require concrete with enhanced properties and diminished cement consumption (Peris Mora, 2007). To achieve this, mix optimization and the replacement of normal supplementary cementitious materials by nanomaterials is seen as the most successful strategy (Ghafari et al., 2015). Thus it is likely that research and applications for nano-additions, especially in digital manufacturing, will keep expanding until natural adoption by the construction market.

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Thermal insulation of buildings through classical materials and nanomaterials

8

Anwar Khitab, Zain Ul Abdin, Imtiaz Ahmed, and Taimur Karim Department of Civil Engineering, Mirpur University of Science and Technology (MUST), Mirpur, Pakistan

8.1

Introduction

Buildings are thermally insulated for reducing energy requirements and for providing comfort to the occupants. Insulation can be provided through various materials and methods or techniques. Building construction is old technology and was customized in different regions primarily looking into the locally available materials and the surrounding environmental conditions. It is believed that humans initially built shelters for protecting themselves from deadly creatures; timber, plants, and stones are considered to be the oldest building materials (Khitab & Anwar, 2016). With the passage of time, new materials and new technologies emerged. Different clayey and ceramic materials were devised and were joined together with binding materials mainly based on hydraulic lime. With the invention of Portland cement in the 19th century, new series of cementitious composites emerged. The cementitious materials gained high popularity thanks to their rapid gaining strength. Steel is another giant material that added immense strength to the buildings and structures. In the contemporary world, almost all the important buildings are the outcome of cementitious materials, steel, or a combination of the two. While these giant materials provide strength and durability, they have also increased the energy demand of the buildings and structures. The contemporary world is seeking methodologies, which not only address the strength and durability of the structures but also sustainability. Hence thermal insulation of buildings is of utmost importance. This chapter addresses the use of nanotechnology for constructing thermally efficient buildings. Nanotechnology is a relatively new branch of materials science, and like many other fields, the construction industry has also duly benefited by opting for nanotechnology-based techniques and materials (Khitab & Anwar, 2016).

8.2

Fundamentals of building physics

8.2.1 Heat transmission Heat transmission in buildings refers to the flow of heat from a higher temperature atmosphere to a lower temperature one. The deciding factor for this heat transfer is Recent Advances in Nano-Tailored Multi-Functional Cementitious Composites. DOI: https://doi.org/10.1016/B978-0-323-85229-6.00011-1 © 2022 Elsevier Ltd. All rights reserved.

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the temperature difference. A larger temperature difference yields a higher heat transfer rate. Nevertheless, heat transfer is irreversible. Heat transmission varies with temperature gradient and is shown in Fig. 8.1. A higher temperature gradient results in rapid heat transfer and vice versa. Heat transmission can take place in three different ways (Lienhard & Lienhard, 2019). 1. Thermal Conduction 2. Thermal Convection 3. Radiation

The three fundamental modes of heat transfer are summarized in Table 8.1. For evaluating the thermal insulation level of the buildings and the energy demand, thermal characteristics of the materials, and building components are required. The important characteristics include but are not limited to thermal resistance (R-value), conductivity (λ), transmittance (U-value), and capacitance (c) (Aldawi & Alam, 2016; Lobontiu, 2010; Willoughby, 2002).

8.2.2 Resistance (R-value) Heat transmission paths can be illustrated by resistances, which are indicated by symbol R (m2 K/W), as shown in Fig. 8.2. Every material possesses an R value, which will make it more or less suitable to be used as an insulation material. The heat transfer, using the concept of R-value can be calculated by using Eq. (8.1) (Kung et al., 2015): Q5

A:ðT1 2 T2 Þ R

(8.1)

Heat transfer rate depends on the temperature gradient

Temperature

T1

T2

Time Figure 8.1 Heat transfer rate as a function of temperature.

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Table 8.1 Three modes of heat transfer. Conduction (via physical contact)

Convection (via fluids)

Radiation (via electromagnetic waves)

Transfer of heat within or between materials in physical contact

Transfer of heat by the movement of fluids from one place to another and between a solid surface and adjacent fluid

Transfer of heat by electromagnetic waves between the matter not in direct contact

Figure 8.2 Heat transfer through a material with value R.

where Q presents the heat, either lost or gained, through the material (Watts), A is the Surface area (m2) (wall, window), Temperature T1 is the larger temperature (K), Temperature T2 is the smaller of the two temperatures (K), and R is the resistance (m2 K/W). The resistance (R) can be calculated using Eq. (8.2) (Levy, 2012). R5

l λ

(8.2)

In Eq. (8.2), l is the thickness of the material and λ is the thermal conductivity in W/K. Thermal conductivities of various building materials are shown in Table 8.3. For a 150 mm thick concrete wall (thermal conductivity 0.18 W/m K) (Solla, 2010), the R value in accordance with Eq. (8.2) will be 0.83 m2 K/W. The calculation of heat transfer for such a wall with given environmental conditions is shown in Fig. 8.3. Nevertheless, higher R-values are better for insulation. The higher thicknesses do not typically indicate higher R-values. For example, 50-mm polystyrene has the same R-value as 80-mm glass wool (Aldawi & Alam, 2016). In accordance

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R-value (0.83 m2K/W)

T1 20ºC (294K)

Surface-area = height × width (2.5m ×3m) = 7.5 (m2)

Height

Indoors

Outdoors

T2 -1ºC (272K)

Width

heat transfer = (7.5 × (294 – 272)) /(0.83) = 198.8 Watts

Figure 8.3 Example of heat lost through a material with a specific R-value.

with Eq. (8.2), we need to know the thermal conductivity and thickness of the building materials for assessing the R-values. The R-values of different building materials as a function of thermal conductivity and materials thickness are shown in Table 8.2. If a wall is constituted by the materials as described in Table 8.2, the R-value of the wall will be the sum of the R-values of its constituent materials, that is, 4.56 K m2/W: It is calculated in the same fashion as resistances in series in an electrical circuit.

8.2.3 Thermal conductivity, λ Thermal conductivity is defined as the amount of heat transfer in unit thickness per unit difference of temperature per unit surface area: Hence its SI unit is W/m K. It can be calculated using Eq. (8.3) (Fourier’s law) (Chebbi, 2019). λ5

QL AðT2 2 T1 Þ

(8.3)

where Q is the heat transfer (W), L is the thickness of the material (m), A is the surface area (m2) and T2 2T1 is the temperature gradient (K). Thermal conductivity is an intrinsic property of the material and its values for different materials are described in Table 8.3 (Incropera, 1990). Thermal conductivity is a significant parameter in the design of energy-efficient houses. Materials with low thermal conductivities enhance the thermal insulation of the house and permit low energy demand for maintaining the internal temperatures.

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Table 8.2 R-Values. Material

Thickness (m)

Thermal conductivity (W/m K)

R-Value (K m2/W)

Clay bricks Glass wool Concrete Plaster

0.125 0.125 0.125 0.0125

0.710 0.035 0.18 0.16 Total

0.18 3.6 0.7 0.08 4.56

Table 8.3 Thermal conductivity of common metals and building materials. Material

Thermal conductivity (W/m K)

Diamond silver Copper Gold Zinc Aluminum Iron Steel Brick Dense concrete lightweight concrete Glass Wood

2000
2200 429 398 315 116 247 79.5 50.2 0.7 1.4 0.4 0.8 0.12
0.16

8.2.4 Thermal transmittance (U-value) Thermal transmittance is the capacity of a material to transmit heat under steady state circumstances. It is a measure of the amount of heat that flows through unit area per unit time unit temperature gradient: Hence its units are W/m2 K. It is calculated as the reciprocal of the sum of R-values of each part of the structural member including cavities. The smaller the U-value the better thermally insulated is the building. The U-value is calculated in accordance with Eq. (8.4) (BienvenidoHuertas et al., 2018). U5

1 R1 1 R2 1 R3 1 . . . 1 Rn

(8.4)

Hence, the U-value of the wall system, presented in Table 8.2 will be 1/4.56 or 0.22 W/m2 K.

8.2.5 Thermal capacity (C-value) Thermal capacity of a material is a measure of how much heat it can store. This property enables the material to absorb heat when heated and to release it while

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cooled. The thermal capacity of the system can be defined as given by Eq. (8.5) (The Basic Properties of Building Materials, 2011). C5

Q mðT2 2 T1 Þ

(8.5)

Where C is the specific heat of the material (J/g K), Q is the quantity of heat absorbed or released (J), m is the mass of the material (g), and (T2 2 T1) is the change of temperature before heating or cooling with time (K). The thermal capacities of some of the materials are given in Table 8.4. The higher values of thermal capacities ensure more stable indoor temperatures and vice versa.

8.3

Energy-efficient buildings

The energy-efficient design involves modulation of the conditions as near as possible to the comfort zone. Modulations including the landscape, architecture, envelope, materials, and other regulating measures carry the conditions within the limits round the clock. The concept of “energy efficiency” is to use energy and produce the same or more amount of energy to maintain the requisite balance. Following are the ways in which a building can be made energy efficient. 1. 2. 3. 4. 5. 6.

Insulating roof; Installing solar heating panels; Energy-efficient glazing; Insulating external walls; Using modern heating systems; Insulating cellar ceiling.

Energy efficiency aims to: 1. 2. 3. 4.

Reduce energy for services; Combat climate change; Have positive macroeconomic impacts; Reduce energy costs.

Table 8.4 Thermal capacities of building materials (The Basic Properties of Building Materials, 2011). Materials

Thermal capacity (J/g K)

Steel Granite Ordinary concrete Ordinary clayey bricks Copper

0.9 1.3 1.0 4.19 1.46

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Benefits of energy-efficient buildings: 1. 2. 3. 4. 5. 6. 7.

Minimize maintenance costs; Reduce utility expenses; Enhance comfort; Create healthy environment; Improve durability; Increase safety; Reduce noise.

8.4

Conventional insulation materials and methods

Traditionally, a wide range of insulation materials are available in the market. These materials are subdivided into different categories on the basis of their chemical composition or physical structure (Kylili & Fokaides, 2017). These materials include organic materials (polyurethane foam, extruded polystyrene, expanded polystyrene (EPS), phenol foam, melamine foam, sheep wool, cellulose, cotton wool, and coconut fiber), inorganic materials (stone wool, foam glass, and glass
wool), combined materials (siliconated calcium, wood wool, and gypsum foam), and nanotechnology-based materials (transparent and dynamic material such as aerogel). The major requirements for thermal insulation materials include strength, better thermal insulation characteristics, good fire resistant, vapor permeability, dimensional stability, hydrophobicity, chemical inertness, capable to resist aging, vibration and deterioration, and environmentally acceptable. As described above, the important parameters that must be under consideration while dealing with thermal insulation materials include thermal conductivity λ (W/m K), thermal transmittance U-value, and thermal capacity C-value. Nevertheless, thermal conductivity depends on insulating thickness of the material layer and low thermal conductivities allow the applications of relatively thin buildings envelopes. Different conventional insulation materials include but are not limited to the materials described as follows.

8.4.1 Mineral wool Mineral wool is an inorganic material obtained from stones or silica. The raw materials are heated to melting. The molten mix is spun and transformed into a fibrous material. In framed houses and some other structures, soft mineral wool and light products are introduced with cavities (Liu et al., 2020). In case for carrying intended load, harder and heavier mineral wool boards are also used on floors or roofs. This material may also be utilized to fill the hollow openings and cavities. The thermal conductivities of mineral wool depend on mass density, moisture content, and temperature (Siwi´nska & Garbali´nska, 2011; Zhu et al., 2020). Thermal conductivities of mineral wool usually vary in the range of 0.03
0.04 W/m K (Youngquist et al., 1994), whereas its thermal capacity and transmittance value are B1 (J/g K) (Nagy, 2020) and 0.16
0.38 W/m2 K (Salem et al., 2018).

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8.4.2 Expanded polystyrene EPSs are prepared from small spheres of polystyrenes incorporating the expansion agent that can expand by heating with water vapor. Pentane (C5H12) is the most frequently used expansion agent (Hanif et al., 2016; Liu et al., 2020). Owing to the presence of small spheres, EPSs have partially open pore structures. Thermal conductivities for EPS vary in the range of 0.030
0.040 W/m K. Likewise, the thermal capacity and transmittance values are B1.5 J/g K (Yucel et al., 2003) and 0.30 to 0.45 W/m2 K (Center, n.d.). Same as mineral wool, thermal conductivity of EPS depends upon mass density, moisture content, and temperature. EPS
incorporated products are perforated in nature and can be used by cutting in different shapes without any loss of thermal resistance (Kylili & Fokaides, 2017).

8.4.3 Extruded polystyrene Extruded polystyrenes (XPS) are made from melted polystyrene by adding the expansion gases, where the melted polystyrenes are obtained from crude oil. Hydrofluorocarbon (HFC), CO2, or pentane can be used as expansion agent (Leng et al., 2019). XPS have closed pore structures. Usually, the thermal conductivity values for XPS fall in the range of 0.030
0.040 W/m K. Thermal conductivities of XPS vary with mass density, moisture content, and temperature. XPS products may be perforated. They are also adjusted or cut at the building sites that do not have any thermal resistance loss (Zeitler, 2010).

8.4.4 Cellulose Cellulose comprises thermal insulations made from wood fiber mass or recycled paper (Ali, 2011; Elanchezhian et al., 2018; Rowel & Keany, 1991; Youngquist et al., 1994; Zhu et al., 2020). For improving the products properties, borax (Na2[B4O5(OH)4]  8H2O, sodium borate, or Na2B4O7  10H2O), and boric acid are added during the production process. Cellulose insulations are employed as filler materials for filling voids and cavities. Additionally, cellulose insulation mat and boards are also manufactured. Usually, thermal conductivity values for cellulose insulations range from 0.040 to 0.050 W/mK. Thermal conductivities of cellulose insulations vary with mass density, moisture content, and temperature. Cellulose insulation products can also be manufactured in perforated form. They can be adjusted or cut at the building sites with no effect on its thermal resistance.

8.4.5 Cork Cork thermal insulations are basically made from cork oak, and these can be produced as both—boards or as filler materials (Cetiner & Shea, 2018; Gupta & Maji, 2020). Usually, thermal conductivity values for corks fall in the range of 0.040
0.050 W/m K. Cork insulations products may be perforated. They may also be adjusted or cut at the building sites without any loss of thermal resistance.

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8.4.6 Polyurethane Polyurethanes are formed by the reactions between polyols (alcohol containing hydroxyl groups) and isocyanates (Fallah & Medghalchi, 2020; Petter Jelle, 2016; Sapuan et al., 2016; Yildiz et al., 2019). Closed pores are filled during the expansion process with an expansion gas such as cyclohexane (C6H12), CO2, or HFC. Insulation materials are produced as boards or continuously on the production lines. Polyurethanes can also be used as expanding foams at building sites, for example, to seal doors, windows and to fill numerous cavities. Usually, thermal conductivity values for polyurethanes fall in the range of 0.020
0.030 W/m K, and they are lower than the cellulose products, polystyrene, and mineral wool. Loss of gases from pores and consequent air infusion due to degradation or diffusion with age may enhance thermal conductivities. Thermal conductivity values of polyurethanes vary with mass density, moisture content, and temperature (Abu-Jdayil et al., 2019; Fallah & Medghalchi, 2020; Petter Jelle, 2016; Sapuan et al., 2016; Ustaoglu et al., 2020; Yildiz et al., 2019).

8.4.7 Non-zero energy buildings (nZEB) The building industry standards impose many practical features for continuously improving the construction specifications. Among many advancements, European Union has been actively advocating for optimizing the building components for thermal insulation. In order to satisfy the specifications, set for nonzero energy building (nZEB), the conventional insulations get thicker and thicker representing many hindrances in practical application as well as economy. The increased energy-efficiency restrictions through optimization of the thermal behavior of the building components is one of the modern factors to be considered from the early phase of building construction. Although the conventional and older thermal insulations represent many advantages, they require higher insulation thickness in order to meet the modern specification standards, which is undesirable both in terms of practical application and economy. It sometimes also compromises the building’s architectural esthetics and may not comply with the departmental guidelines. It may also need withdrawal of windows in the wall section minimizing the solar light and energy input. This problem may well be addressed by the use of the larger window sections, which may come at a price of increased thermal bridges near the glazing area. This, in turn, represents another problem of greater materials and energy usage at the manufacturing stage resulting in higher ecological impact and overall project cost. The conventional insulations represent many limits irrespective of the thermal insulation of the building. Nano insulation materials have thermal conductivities three to five times lower as compared to the conventional ones. Nevertheless, the smaller material thickness is devised to have the same thermal performance. This comes with many advantages over conventional materials. The most important advantage is the reduced materials thickness requirement for achieving the equivalent to better thermal insulation for new and existing buildings. In the case of new buildings, the thickness of the reduced material results in a more

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built area without compromising the urbanistic limitations imposed by the authorities. For the older and existing buildings, the use of nano insulation materials does not alter the fac¸ade esthetics. Over the years, many nano insulation materials have been developed with each having its merits and shortcomings (Baetens et al., 2011; Boˇzi´c, 2015; Bozsaky, 2016; Norris & Shrinivasan, 2005).

8.5

Role of nanotechnology for building insulation

8.5.1 Nanotechnology and the construction industry In the recent times, an unprecedented rise in application of nanotechnology in almost all the fields of science and research including space research, pharmaceuticals, electronics, and chemical industries has been observed (Anwar & Khitab, 2019). The same has also led toward the construction industry and resulted in development of the nanotechnology-based materials for thermal insulation of the buildings (Pacheco-Torgal and Jalali, 2011). The practical application of currently underused materials is expected to boost in the coming days as it presents a multitude of potential applications (Khitab et al., 2015). The use of nanotechnology in construction industry goes back in time where researchers investigated the potential application of the latest scientific development in civil engineering. The role of nanotechnology for the betterment of the building industry has been investigated by a number of researchers (Siwi´nska & Garbali´nska, 2011; Zhu et al., 2020). Nanotechnology has influenced the construction industry in numerous ways as it represents many influential characteristics including resistance to moisture, heat and chemical and/or bacterial attacks, as well as air quality improvement, selfcleaning abilities, and energy efficiency (Khitab et al., 2019). Numerous studies are available in which an increase in the durability and compressive strength of the cement-based materials was reported by adding nanomaterials such as silica, alumina, and different clays (Khitab & Anawar, 2016). Several researchers have also reported an increase in fluidity or water permeability of concrete. In some other studies, it was found that the tensile and bending strength of concrete could be enhanced by adding nano-tubes/nano-fibers (Ahmed et al., 2018; Gillani et al., 2017). A nano-tube-/nano-fiber-modified wood is reported to gain as much as double the strength of steel (McIntyre, 2012). Nano-titanium dioxide (TiO2) is used for coating, owing to its disinfecting and antifouling characteristics (Mohamed, 2010).

8.5.2 Nanotechnology applied to thermal insulation The use of nanotechnology for manufacturing high-performance thermal insulation materials has taken giant leaps in the recent times (Khitab & Arshad, 2014; Bittnar et al., 2009). Generally, the nanotechnology is concerned with controlling matter (particles) having dimensions in the range of 0.1
100 nm (Zhu et al., 2004). However, for manufacturing thermal insulation materials based on nanotechnology,

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the focus shifts from particles to pores in the same nanorange. This phenomenon has been well described by Jelle and shown in Fig. 8.4 (Jelle, 2011). The application of nanomaterials and nanotechnology in various civil engineering fields has gained immense popularity during the last few decades (Khitab & Anawar, 2016). Likewise, it can also be workable in improving the thermal insulation of buildings, if applied with proper methodology with respect to the needs of the end-users, the nature of the structure, and the environmental, economic, and cultural perspectives of the region. It has been reported that nanotechnology may improve the energy efficiency of buildings by employing advanced materials and methods with enhanced static or dynamic performance (Casini, 2016; Moga & Bucur, 2018). Nano insulating materials are based on the principle of energy transfer by the collision of gas molecules (Casini, 2016; Jelle et al., 2019; Luka, 2020; Rostam et al., 2015; Sa´ez de Guinoa et al., 2017). When pore size in the materials is reduced, for example, of the order of 200 nm in diameter, the principle of Knudsen prevails, that is, the molecules mostly collide with the walls of pores and not with the other gas molecules (Ruthven, 2003). The elimination of intermolecular collisions results in the reduction of thermal conductivity and the enhancement of the efficiency of nano insulation material. Nanothermal insulation materials are developed with large porous volume having pores in nanorange. Various nano-modified thermal insulation materials are actively available in the market including but not limited to the following: EPS Aerogels Vacuum and gas-filled insulation panels Nano-coatings

Figure 8.4 Nanotechnology and its application for high-performance thermal insulation materials. Source: From Jelle, B. P. (2011). Traditional, state-of-the-art and future thermal building insulation materials and solutions e properties, requirements and possibilities. Energy and Buildings, 43, 2549
2563. https://doi.org/10.1016/j.enbuild.2011.05.015.

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8.5.3 Aerogels It is an ultra-light material of superior thermal properties and is shown in Fig. 8.5 (Dowson et al., 2012). It possesses equally strong acoustic insulation properties. Aerogel represents a state-of-the-art thermal insulation solution (Ganga˚ssæter et al., 2017; Gupta & Maji, 2020; Kylili & Fokaides, 2017). Commercially available aerogels are reported to have thermal conductivities in the range 0.013
0.014 W/m K at ambient pressure. Still, its production cost is very high. Aerogel has relatively high compression strength. However, it is a fragile material because of its low tensile strength. Tensile strength can be increased by providing carbon fiber matrix. It is an interesting aspect about aerogel that it can be created in transparent, translucent, or opaque phases. The cost should be substantially lowered for aerogel to make it a widespread material, which is based on thermal insulation for opaque application (Rostam et al., 2015).

8.5.4 Vacuum insulation panels Vacuum insulation panel (VIP) is a high-tech insulation material with strong insulation properties (Liu et al., 2020; Zhu et al., 2020). VIPs are nonhomogeneous insulating panels, which are made of distinctive multilayers films

Figure 8.5 Samples of transparent aerogel, made in the high temperature; on the top supercritical drying and in the bottom low-temperature supercritical drying. Source: From Dowson, M., Grogan, M., Birks, T., Harrison, D., & Craig, S. (2012). Streamlined life cycle assessment of transparent silica aerogel made by supercritical drying. Applied Energy, 97, 396
404. https://doi.org/10.1016/j.apenergy.2011.11.047.

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impermeable to moisture, air, and with an insulating core. The insulation core is made of an open porous structure of silica fumes enveloped by the numerous metallized polymer laminate layers and is shown in Fig. 8.6 (Simmler et al., 2005). Nowadays VIPs are manufactured with thermal conductivity of the central panel as low as 0.004 W/m K (Peng & Yang, 2016). The main disadvantage with VIPs is that they cannot be cut for adjustment at building sites or perforated without losing a considerable proportion of their thermal insulation performance. Thermal conductivity increases from 5 to 10 times depending on the aging time; however, it is still lower than those of the traditional thermal insulation materials such as mineral wools and polystyrene product (Kylili & Fokaides, 2017; Zeitler, 2010).

8.5.5 Gas-filled panel Gas-filled panels (GFPs) are manufactured using the same technique as that for VIPs. GFP applies a gasless thermal conductivity rather than air (e.g., Xenon (Xe), Krypton (Kr) and Argon (Ar)) in lieu of vacuum as in VIP (Kylili & Fokaides, 2017; Zhu et al., 2020). The cellular structure (baffle) and barrier foil in GFP (LBNL 2015, 2015). Maintaining low conductivity inside GFP and avoiding moisture and air penetration are crucial to thermal performance of the panels. Vacuums are better thermal insulators than various gases employed in GFP. GFPs grid structures cannot withstand the inner vacuum as VIPs. Low-emissivity surface inside GFP also decreases radio-active heat transfer. Thermal conductivity for GFP is relatively higher than that of the VIPs, that is, 0.011
0.02 W/m K, depending upon the gas filled (Kralj et al., 2011).

Figure 8.6 Typical VIP (Left), Comparison of equivalent thermal resistance thickness of traditional thermal insulation and VIP (Right). VIP, Vacuum insulation panel. Source: From Simmler, H., Heinemann, U., Schwab, H., Que´nard, D., Ku¨cu¨kpinar-niarchos, E., & Stramm, C. (2005). Vacuum Insulation Panels. HiPTI-Hi(September 2005), 159.

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8.5.6 Phase change materials Phase change materials (PCMs) are not like typical thermal insulation materials. However, being the part of the thermal building envelopes, they may be adapted for thermal insulation applications (Luka, 2020). PCMs change phase from solid to liquid when heated. Consequently, they absorb energy in the endothermic process. When ambient temperature drops again, the liquid PCM turns into solid state material while giving off earlier absorbed heat in the exothermic process. This phase changing cycle stabilizes the indoor buildings’ temperature and decreases cooling and heating requirements. Paraffin is an example of PCM (Pezeshki et al., 2018). An appropriate phase can change temperature range, which depends on climate conditions and desired comfortable temperature. They are capable of releasing and absorbing considerable amount of heat, which makes them suitable for thermal insulation for buildings (Casini, 2016).

8.5.7 Nano-coatings for buildings Historically, many different materials have been used as coatings for thermal insulation of the buildings. The trend is slowly shifting toward using nano-coating for the purpose due to its durability, enhanced engineering properties, longer life, and economic benefits. Nano-coating is utilized in the building industry with the ability of application on all types of surfaces including walls, doors, and windows. The easy application, durability, and excellent insulation performance of nano-coating contribute to sustainable and environment-friendly buildings. They form a layer of the anticipated shielding and functional properties, when applied to the base materials and provide better adhesion, transparency, self-cleaning, corrosion, and fire protection (Mohamed, 2010). The basic mechanism concept of nano-coating comes from their self-healing properties attained through means of self-assembly (Abhiyan et al., 2013). The self-assembly concept involves the spontaneous self-fabrication of a system via an interaction resulting in the creation of a larger functional unit as shown in Fig. 8.7. This spontaneous system builds because of primarily direct contact or secondarily from its environment (Whitesides, 2002). These special arrangements of the self-fabricated nanoparticles present the basic idea of nano-coating employment.

8.5.8 Types of nano-coatings Many different types of nano-coatings for thermal insulation of the buildings have been developed by the researchers, and are described below.

8.5.8.1 Hydrophilic and hydrophobic coatings The hydrophilic and hydrophobic nano-coatings are shown in Figs. 8.8 and 8.9, respectively. A self-cleaning glass system based on a thin film of TiO2 coating is shown in Fig. 8.8 (Boostani & Modirrousta, 2016), which cleans itself in two steps. The different stages of its working mechanism are shown in different parts of Fig. 8.8, where part A shows the applied nano-coating on a surface. The second stage (B) of the cleaning

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Figure 8.7 Self-assembly in nanoparticles. Source: From Boostani, H., & Modirrousta, S. (2016). Review of nanocoatings for building application. Procedia Engineering, 145, 1541
1548. https://doi.org/10.1016/j. proeng.2016.04.194.

Figure 8.8 Self-cleansing glass system using TiO2 film coating. Source: From Boostani, H., & Modirrousta, S. (2016). Review of nanocoatings for building application. Procedia Engineering, 145, 1541
1548. https://doi.org/10.1016/j. proeng.2016.04.194.

process is called photocatalytic and involves crashing down the dust on the surface by using ultraviolet light beams making the glass hydrophilic (Khitab et al., 2018). After this hydrophilic stage, the dust on the glass surface is washed away by the rainwater as it spreads uniformly on hydrophilic surfaces (Drelich et al., 2011). The thin films are generally equipped with good photo-induced antireflective and antibacterial characteristics. These antibacterial properties of the TiO2 porous films can be augmented by the addition of a small amount of silver without ultraviolet light radiation. In contrast to hydrophilic coatings, hydrophobic coatings (Caldarelli et al., 2015; Kumar et al., 2015; Nakajima et al., 2014; Wang et al., 2015) render the surfaces resistant to moisture and corrosion. Hydrophobic systems are composed of silicon dioxide (SiO2) coating is

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Figure 8.9 SiO2-coated hydrophobic surface. Source: From Boostani, H., & Modirrousta, S. (2016). Review of nanocoatings for building application. Procedia Engineering, 145, 1541
1548. https://doi.org/10.1016/j. proeng.2016.04.194.

shown in Fig. 8.9. The researchers showed that the contact angle of a water droplet goes beyond 150 and the roll-off angle is smaller than 10 (Lafuma & Que´re´, 2003). These hydrophilic and hydrophobic coatings can be applied to the plane surfaces of the building as well as the base materials including stones, bricks, wood, and tiles, etc.

8.5.8.2 Flame-retardant coatings Flame-retardant coatings are another important type of nano-coatings that can be prepared by the addition of nano-sized Mg
Al layered double hydroxides (LDHs), TiO2 and SiO2 as proposed by Wang et al. (Caldarelli et al., 2015; Kumar et al., 2015; Nakajima et al., 2014; Wang et al., 2015). Another study (Mizutani et al., 2006) represented using nano-sized silica and poly-acrylate particles containing nanocomposite emulsion in wall paints. The products formed by the addition of nanomaterials have demonstrated excellent resistance to the high flames and pollution (Wang et al., 2005, 2006).

8.5.8.3 Wear-resistant coatings Wear-resistant coatings, as the name suggests, are used as a protection against wear. These coatings also help in maintaining the performance level, which results in extending the life of the structural component. An increased friction between two surfaces causes the wear in the structure and the coatings are used to protect those surfaces from getting damaged. The mechanical properties and hardness of the building coatings can easily be increased by incorporating nanoparticles such as SiO2, TiO2, alumina (Al2O3), and zirconium dioxide (ZrO2) as reported by Barna et al. (2005). The improved mechanical properties result in improved resistant to wear and scratch. Another advantage due to nanoparticle’s small size is their tendency of not affecting the transparent look or gloss of the building coating. Therefore these nano-modified coatings could easily be used for two purposes including the durability enhancement and maintaining the physical appearance of the treated surfaces like floors, windows panes etc.

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8.5.8.4 Antigraffiti coatings TiO2-based nano-coating was found out to be very beneficial to protect the old historical stone surfaces as revealed by various researchers (Munafo` et al., 2014; Quagliarini et al., 2012). It has been reported that Nano-structured Titanium Oxide based coatings exhibit favorable results on archeological and ancient stone exteriors. Rabea et al. (2012) employed nano-SiO2 particles on a stable antigraffiti polyurethane coating. They reported an improvement in the antigraffiti performance against ageing cycles.

8.5.8.5 Corrosion-resistant coatings The aluminum alloys were successfully protected from the corrosion by applying the chromate conversion nano-coating as reported by Hamdy et al. (Hamdy, 2006; Hamdy & Butt, 2007). The oxidation and corrosion resistance of the composite coating could be enhanced by addition of the nano-Al2O3 particles (Wang et al., 2010). The latest alternatives to the self-healing and insulation coating to improve the protection against corrosion were documented in a review paper (Montemor, 2014). Corrosion-resistant coatings protect the metal components of the building from rusting and incur protection against moisture, oxidation, fog and different environmental and industrial chemicals. The barrier against all these influencing factors helps increasing the life period of the coatings by providing protection. Various metal coatings could be used for providing shield against corrosion. These include fluoropolymer coatings, polytetrafluoroethylene coatings, nano-Al2O3 coatings, and ceramic epoxy coating for excellent corrosion protection, to withstand high temperatures, corrosion, and oxidation resistance and protect ceramic particles by binding them to a resin system, respectively. Many other metal coatings such as tungsten oxide (WO3), TiO2, nickel oxide (NiO2), and vanadium oxide (V2O5) can also be applied as energy-efficient coatings as a thin film layer on the window glasses.

8.6

Energy-efficient coatings

8.6.1 Phase change materials The performance of PCMs as a latent heat storage (LHS) system incorporated in the building coating has been probed by the researchers (Karlessi et al., 2011). Generally, these are applied to the external building surfaces, which include walls, windows (Entrop et al., 2011; Ismail et al., 2008), and get exposed to the air and thus a range of temperature variations. They have reported a relatively lower temperature of the PCM coatings applied surfaces as compared to the traditional coatings. Nano-paraffin is a good composite for coatings, which possesses high thermal conductivity (Li, 2013). These composites can store and release greater amounts of energy by heat absorption or release on changing the state of the material from solid to liquid and vice versa (Agyenim et al., 2010; Khudhair & Farid, 2004; Kuznik et al., 2011; Zalba et al., 2003; Zhou et al., 2012). Fig. 8.10 represents the

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Figure 8.10 Process of PCMs functional mechanism. PCMs, Phase change materials. Source: From Boostani, H., & Modirrousta, S. (2016). Review of Nanocoatings for building application. Procedia Engineering, 145, 1541
1548. https://doi.org/10.1016/j. proeng.2016.04.194.

functional mechanism of PCMs. In principle, they are applied to the external surfaces, for example, walls, windows, floors, which are exposed to the air for a certain temperature range (Boostani & Modirrousta, 2016; Entrop et al., 2011).

8.6.2 Electrochromic materials Window glasses in the buildings can be protected by energy-efficient insulation coatings applied in thin layers and prepared by nano-chromatic materials including WO3 (Deb, 2008), NiO2, TiO2, and V2O5 (Baetens et al., 2010; Granqvist et al., 2009). Baetens et al. (2010) reported that electrochromic windows are one of the most effective solution for reducing heating, cooling and lightning loads in buildings: They are not only reliable but also capable of modulating the transmittance as much as 68% of the total solar spectrum.

8.6.3 Photovoltaic coatings Fig. 8.11 presents a photovoltaic (PV) arrangement, which is capable of converting solar energy into electric energy (Bagnall & Boreland, 2008). In another study, it

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Figure 8.11 A PV system. PV, Photovoltaic. Source: From Bagnall, D. M., & Boreland, M. (2008). Photovoltaic technologies. Energy Policy, 36(12), 4390
4396. https://doi.org/10.1016/j.enpol.2008.09.070. Table 8.5 Nano-coating categorization. Type of coating

Application

Synthesis

Energy efficiency

Hydrophilic

Window frames and window panes, tiles, bricks, stones, paints Tiles, bricks, stones, timber, paints Aluminum

Thin films comprising of TiO2 and Silver (Ag) SiO2

Nonenergy efficient

Hydrophilic Flame retardant Wear/Scratch resistance

Antigraffiti

magnesium, aluminum hydroxides (LDHs), TiO2, and SiO2 SiO2, TiO2, Al2O3, and ZrO2

transparent surfaces, parquet floorings, glasses, and window panes Stone, fac¸ade plasters

TiO2

Corrosion resistant Phase change Materials Electrochromic

Aluminum alloys

Al2O3

external walls, window panes, flooring Window panes

Mesoporous silicon dioxide (MPSiO2) TiO2

Photovoltaic

Solar cells

TiO2, SnO2

Nonenergy efficient Nonenergy efficient Nonenergy efficient

Nonenergy efficient Nonenergy efficient Energy efficient Energy efficient Energy efficient

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was found that even more electricity could be generated if a nano-porous TiO2 film is used onto a thin film of stannic Oxide (SnO2) (Jayaweera et al., 1999).

8.6.4 Nano-coating categorization Different types of nano-coatings categorized on the basis of type, their application and synthetics are summarized in Table 8.5.

8.7

Conclusions

Energy efficiency of the buildings has received enhanced attention over the last few decades. One of the solutions to reduce the energy demands of the buildings is employing better insulating materials and components. Various traditional thermal insulation materials are available. The list includes but is not limited to: mineral wool, EPS, extruded polystyrene, cellulose, cork, and polyurethane. Nanothermal insulation materials involve the reduction of their pores to nano-sizes: This in turn reduces the thermal conductivity to a great extent. The conventional materials may enhance the size of the building components, nanomaterials can be used without compromising the size or building esthetics. The important nanothermal insulation materials include aerogels, VIPs, GFPs, and PCMs. Lastly, the building components can be coated with nano electrochromic and PV materials.

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Further reading Anwar, W., & Khitab, A. (2019). Nanotechnology From Engineers to Toxicologists. International Journal of Applied Nanotechnology Research, 4(2), 1
25. Available from https://doi.org/10.4018/IJANR.2019070101. ˇ Bittnar, Z., Bartos, P. J. M., NemeCek, J., Smilauer, V., Zeman, J., Bittnar, Z., Bartos, P. J. M., ˇ Nˇemeˇcek, J., Smilauer, V., & Zeman, J. (2009). Nanotechnology in Construction 3. https://doi.org/10.1007/978-3-642-00980-8. Khitab, A., & Arshad, M. T. (2014). Nano construction materials: Review. Reviews on Advanced Materials Science, 38, 2. Pacheco-Torgal, F., & Jalali, S. (2011). Nanotechnology: Advantages and drawbacks in the field of construction and building materials. Construction and Building Materials, 25(2), 582
590. Available from https://doi.org/10.1016/j.conbuildmat.2010.07.009. Wang, Z., Han, E., & Ke, W. (2006). An investigation into fire protection and water resistance of intumescent nano-coatings. Surface and Coatings Technology, 201(3
4), 1528
1535. Available from https://doi.org/10.1016/j.surfcoat.2006.02.021.

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Wang, Z., Han, E., & Ke, W. (2005). Influence of nano-LDHs on char formation and fireresistant properties of flame-retardant coating. Progress in Organic Coatings, 53(1), 29
37. Available from https://doi.org/10.1016/j.porgcoat.2005.01.004. Zhu, W., Bartos, P. J. M., & Porro, A. (2004). Application of nanotechnology in construction. Materials and Structures, 37(9), 649
658. Available from https://doi.org/10.1007/ BF02483294.

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Nano-modified green cementitious composites

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Salmabanu Luhar1, Ismail Luhar 2, and Faiz Shaikh3 1 Frederick Research Center, Nicosia, Cyprus, 2Shri Jagdishprasad Jhabarmal Tibrewala University, Jhunjhunu, India, 3School of Civil and Mechanical Engineering, Curtin University, Perth, WA, Australia

9.1

Introduction

Since the innovative technology for nanomaterials was first introduced by Nobel laureate physicist Richard Feynman in 1959 (Feynman, 1959), the investigations on these groundbreaking materials were hot subject matter in the field of research and development. Richard Feynman shed light on the possibility of employing atoms as building particles in order to create nanosized yields. However, the term “nanotechnology” was coined in the year 1974 by Norio Taniguchi (Taniguchi et al., 1974). Nanomaterials can be defined as the physical substances having at least one attribute of dimension ranging between 1 and 150 nm. Amazingly, their characteristics may be different from those of the parent materials, and they have merely nano- or micron-scale dimensions. They can be regarded as building blocks of practical nanotechnology and can also be chemically and physically manipulated for some definite value-added uses. Since mid-2000, the insurgency of these cutting-edge and superior 21st-century complex materials is on the headway in the form of the nanomaterials achieved through this new-fangled nanotechnology. The recent advancements in the context of nano-modification of green cementitious composites generally by using nanomaterials as performance enhancer such as nano-silica (nano-SiO2), nanokaolin, nano-clay, nano-alumina (nano-Al2O3), nano-titania (nano-TiO2), and carbon nanomaterials such as carbon nano-tubes (CNTs), and carbon nano-fibers (CNFs), graphene oxide (GO), graphene (C140H42O20), polycarboxylates, nano-CaCO3; nanoiron oxide (nano-Fe2O3), nanocarbonate, etc. (Li et al., 2004a,b) have turned out to be the center of interest for the construction community. On the other hand, with the worldwide infrastructure development, the acrossthe-board applications of cementitious composites such as paste, mortar, concrete, and composite concrete have increased the international exigency and production in the construction industry (Anwar et al., 2020). This is because of their advantages, viz., elevated compressive strength, straightforward course for the manufacturing, low production cost, and expediency to use (Han, Ding, et al., 2015). However, undesirably, the cementitious composites are well known for their high cracking propensity, poor performance on deformation, and brittleness because of substandard tensile strength. Thus inevitable crack development in cementitious composites Recent Advances in Nano-Tailored Multi-Functional Cementitious Composites. DOI: https://doi.org/10.1016/B978-0-323-85229-6.00003-2 © 2022 Elsevier Ltd. All rights reserved.

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is accountable not only to deteriorate the integrity but also the bearing capacity of the structures severely impacting their safety, service life, and durability, ultimately piloting to their failure. This means that the serviceability of concrete structures, namely, pavements, runways, bridge decks, etc., relies upon various aspects such as the rheology, durability, and mechanical attributes of the materials employed, besides loading configurations and exposure conditions. Such interconnected factors may directly influence the performance and longevity of concrete structures on the whole, particularly in locations vulnerable to deteriorations such as joints in bridges and pavements (Jones et al., 2013). Also, the findings of research by Colston et al. (2000) on the green cementitious composites modified by nanoparticles have strongly supported that the supplement of nanoparticles into cementitious composites can bring about a noteworthy improvement with respect to their microstructures. Subsequently, some researchers (Rahim & Nair, 2016; Shekari & Razzaghi, 2011) revealed that a few nanomaterials not only enhance the durability and brittleness of cementitious composites but also extend the multi-functionality to them. It has been put forward that a variety of stronger, more durable, and greener cementitious composites can be achieved in the course of nano-modifications. Quite recently, the application of nanotechnology in constructions and infrastructure industries has become the center of massive attention. Also, the interest and concentration of many global researchers are focusing on these novel innovative nanomaterials in order to develop numerous pristine nano-modified green cementitious composites as innovative building materials. The outcomes of nano-modification of green cementitious composites have piloted to remarkable improvement with regard to not merely their mechanical properties but also with their durability and compactness in microstructure.

9.2

Types of nanomaterials used for modification of green cementitious composites

One of the most sustainable approaches to mitigate the carbon footprint and operational energy to improve the concrete performances is to develop novel innovative green cementitious construction composites by using binder systems comprising supplementary cementitious materials (SCMs) such as fly ash, ground granulated blast furnace slag (GGBFS), metakaolin, silica fumes, and volcanic ash, either alone or in combinations of two or more SCMs in diverse volume fractions as a partial substitution of cement (Cheah & Ramli, 2014). On account of the demonstration of extraordinary properties by nanomaterials, they are found competent enough to improve not only the fresh and hardened properties but also the microstructural properties of cement composites; therefore in this day and age, they are broadly employed in manufacturing green cementitious composites. The green cementitious nanocomposites have gained the attention of world researchers intending to promote them as nanocomposite building materials because of the ultrafine particle size of nanomaterials leading to display unique physical and chemical properties. It is

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worth noting that nanoparticles improve the microstructure and strength properties of the green mortar owing to their capability of getting dispersed homogeneously in the binder paste. This improvement is assigned to the outstanding act of nanoparticles as a filler, which manipulates the cement matrix structure at nanolevel, boosts packing model structure, and refines the intersectional zone in cement resulting in a more densely packed microstructure (Adak et al., 2014). Nano-SiO2, nano-Al2O3, nano-TiO2, CNTs, nanokaolin, nano-clay, CNFs, carbon black (CB), etc., could find their scope as nanomaterials for use in manufacturing green nanocomposite as performance enhancers (Assaedi et al., 2019) have been taken into account in the present chapter.

9.2.1 Nano-silica Silicon dioxide or silica nanoparticles are also known as “nano-silica” (nano-SiO2), which can be further classified as P-type containing numerous nanopores and Stype possessing a comparatively smaller area. This nanomaterial compound belongs to the pozzolanic group. It is the most studied nanoparticle, which is used in most of the concretes internationally (Said et al., 2012). The referred features including the densification and chemical bonding of pozzolans are proportionally fostered since the scale is decreased. Contrary to carbon nanoallotropes, the competence in the nanoparticles production has enhanced to a great extent during the last few decades, piloting to a significant cutback in the cost while applying volumetric admixtures (Xie, 2016). The said cost-effectiveness has smoothed the progress for the application of nanoparticles in building materials. Enhancement in the compressive strength and water tightness of concrete containing nano-SiO2 has been examined by Gonzalez et al. (2016), who reported an elevated resistance of concrete pavement, especially in colder climatic conditions. They conducted an assessment of freeze/thaw cycles and the consequent scaling response. The scaling took place when the impact of freezing and thawing cycles created local failures or mortar degradation on the surface. Nano-SiO2 facilitates the utilization of recycled materials in the cement matrix. For instance, researchers (Mohammed, Awang, et al., 2016; Mohammed, Sanjayan, et al., 2016) have revealed that enhancement in the compressive strength of concrete was reported when the use of rubber from waste tires and nano-SiO2 was made, permitting a structural application of concrete enclosing a higher rubber content. When a decline in toughness is not needed, nano-SiO2 assists to maintain this attribute within the same values as conventional concrete. Li et al. (2004a,b) employed nano-SiO2 and nano-CaCO3 to maximize the utilization of the recycled aggregate concrete, which is highly used recycled material in construction. On account of possessing higher content of silica, Harbec et al. (2015) investigated recycled glass nanoparticles with cement paste to obtain a mortar with compressive strength and permeability equivalent to high-performance concrete (HPC) with silica fume (SF). Mortar incorporated with nano-SiO2 and glass microparticles was studied by Aly et al. (2012), and they concluded that nano-SiO2 seemed to encourage the utilization of glass particles as a high-volume cement substitution. It was shown that 20% of glass powder and 3% of nano-SiO2 enclosing

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sample achieved an augmentation by 31% in compressive and 55% in flexural strengths. A method that can be potentially applied in the upholding of concrete structures is based on the electromechanical migration of the species. The extraction of chloride from corroded reinforced concrete has been an eminent practice for the years: chloride ions (Cl21) are alienated from rebar by applying an electric field to the concrete element. Several experiments have obtained a compaction effect on concrete by electromechanical injection of nanoparticles in the concrete surface. Dı´az-Pen˜a et al. (2015) documented a 1.5
2 mm deep protective film by introducing nano-SiO2. An identical pore-filling effect and protection against carbonation utilizing Si14 ions were obtained by Fajardo et al. (2015). A solution of SiO322 ions was used by Shan et al. (2016), since it is easier to get ready as they are more stable than nano-SiO2 particles. The application of such substances and subjecting them to an electric field on-site structures is a complex job. That is why researchers ´ ngel et al., 2016) have augmented the competence (Climent et al., 2016; Miguel-A of the progression through a graphite
cement paste coating employed as an anode. Although it performs at an efficiency of 80% in comparison with conventional anodes, it is considered to be a low-cost and durable anode, which is adaptable to any surface and less sensitive to the anisotropic electric properties of concrete created by the spatial distribution of rebars.

9.2.2 Nano-titania Titanium dioxide (TiO2) provides a different and more advanced attribute called the “photocatalytic effect.” Nano-TiO2 is the most extensively utilized photocatalyst in the production of building materials and is the second most used nano-oxide particle (Mendes et al., 2015). It is used in the production of self-cleaning concrete contributing to obliterate organic contaminants. The elimination of NOx occurs because of the photocatalytic reaction (Fig. 9.1). A few most recent investigations on this subject by Cerro-Prada et al. (2016) and Ganji et al. (2016) have utilized methylene blue as the organic dye and malachite green, respectively. Of the two, the latter is severely toxic and difficult to eliminate from aqueous solutions. Both research teams applied UV radiation. Ganji et al.

Figure 9.1 Photocatalytic of concrete (Janus & Zaja˛c, 2016).

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(2016) monitored that the samples of cement enclosing nano-TiO2 exhibited stronger photocatalytic attributes in comparison with those encompassing the similar quantity of pure TiO2. Cohen et al. (2015) noted a more prominent cleaning effect utilizing nano-TiO22xNy than the one achieved with nanoparticles of TiO2, being activated by both UV and visible radiations. The self-cleaning characteristic of the TiO2 embedded in the cement matrix was used in the construction of the unique Jubilee Church and Ara Pacis archeological museum, both in Rome. Nano-TiO2 particles are also applied on concrete surfaces as coatings, which is the technique studied by Jafari and Afshar (2016), who coated the concrete blocks by immersing them in a solution with nano-SiO2 and nanoTiO2. Nano-TiO2 possesses a transparency feature and is also a durability enhancer. Consequently, when this admixture is applied as a coating, it can act for maintenance, particularly for higher-cost maintenance structures or valuable cultural heritage structures. This characteristic was studied by Quagliarini et al. (2012) through an experiment on a limestone rock known as Travertine, which is a form of limestone deposited by mineral springs with a fibrous or concentric appearance and was commonly employed in the construction of historical monuments. Faraldos et al. (2016) manufactured a coating, which brought together the photocatalytic and hydrophobic effects by combining nano-TiO2 particles with a siloxane sealant. A coating to cement samples to provide them with hydro- and ice-phobic capabilities was applied by Ramachandran et al. (2016). This coating comprised a water-based siloxane emulsion for hydrophobic alteration, poly-methyl-hydroxy-silane for the making of the hydrophobic agent, and polyvinyl alcohol (PVA) to be employed as a surfactant. Consequently, the coated samples could repel falling water droplets at 25 C. In the last couple of years, other superhydrophobic coatings like nano-SiO2 coatings have been developed and investigated (Boostani & Modirrousta, 2016), which may be applied as an anticorrosion protection because water is the core cause of most of the pathologies affecting the foundations of the buildings.

9.2.3 Carbon nano-tubes CNTs are carbon-based nanoparticles. They are graphene sheets rolled into a tube and employed for the structural reinforcement because they are 100-fold stronger than steel. They are distinctive with respect to being thermally conductive along the length but nonconductive across the tube. Iijima (2002) reported regarding the CNTs for the first time. Generally, they are classified as single-walled CNTs (SWCNTs) and multiwalled CNTs (MWCNTs), of which, the former is made of a graphene sheet rolled up into a cylinder and closed at both ends by two semispherical caps. The internal diameter of SWCNTs ranges between 0.4 and 2.5 nm, while their length varies from few microns to several millimeters (Serp et al., 2003). On the other hand, MWCNTs are composed of more than one graphene cylinder nested into one another. Typical MWCNTs possess an inner diameter of approximately 1
3 nm and an outer diameter of about 10 nm. It is worth noting that their length can be millions of times larger than these dimensions. Even though CNTs possess simple chemistry, they put on display one of the most tremendous diversity amongst nanomaterials concerning their structure
property relations (Dai, 2002).

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Regarding mechanical behavior, CNTs are highly resilient with Young’s modulus approximately 1.2 TPa and exhibit a tensile strength of about 100 times greater than steel (Kumar & Sharma, n.d.). Conversely, a few downsides have also been reported since they are inclined to cluster in bundles and accordingly make the interaction with the cement matrix incompetent, and the bonding among CNTs and the matrix is feeble (del Carmen Camacho et al., 2014). With respect to mechanical enhancements, CNTs are aimed to move the reinforcing behavior of carbon fibers from the macroscopic to the nanoscopic level (Fattah et al., 2015). The said nanofilaments restrain the crack formation and hold the growth at the nanoscale. Furthermore, they are useful as fillers, which make the C
S
H gel denser and improve the quality of the interface between cement paste and aggregate (Vera-Agullo et al., 2009) (Fig. 9.2). The momentous endeavors to achieve CNT composites have already contributed to encouraging outcomes with dissimilar compounds. For example, CNTs are dispersible in a sufficient solvent in the polymeric matrix, with or without functionalization. The mechanical methods can be applied acceptably in the case of ceramic or metallic matrices. However, the cement matrix is unsuitable for the referred approaches. The functionalization of CNTs can cause alterations in their chemical structure and influence the electrical behavior of the nanoelement. The cause resides in the cleavage of the bond that leads to a decline in the electrical conductivity (Bekyarova et al., 2013). Even so, Fattah et al. (2015) studied the functionalization with the
COOH carbonic group and reported the increase in the solubility and the bonding strength of CNTs in the cement matrix. Isfahani et al. (2016) did not find any enhancement in the dispersion course when applying sonication to the CNT suspension in H2O and then adding it to the cement mortar. With the possibility of adding CNTs to cement matrices, Stynoski et al. (1996) improved the solubility of CNTs in H2O by utilizing their functionalization with particles of nano-SiO2. In the context of CNTs, the researchers have concentrated

Figure 9.2 Carbon nano-tubes type (Siahkouhi et al., 2021).

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on the dispersion techniques, which are well matched with the cement paste chemistry. A well-known line of action lies with the application of superplasticizers as dispersing agents. Alrekabi et al. (2016) monitored an elevated augment in compressive strength while applying physical alteration (superplasticizer/surfactant) in comparison with the influence of chemical functionalization. Chuah et al. (2014) made public their relevant reviews for the investigations on the enhancement in the fundamental mechanical characteristics of cement matrices, applying the synthesis technique, the dispersion method, and the kind of superplasticizer and surfactant employed (Fig. 9.3). An analysis of the relation between the duration of the sonication applied to CNTs and the flexural strength attained was carried out by Mohsen et al. (2016). They employed a poly-carboxylate-based superplasticizer. The outcome achieved with 0.15wt.% of CNTs showed that the flexural strength practically stood at a standstill from 15 to 60 minutes of the sonication procedure, whereas in the case of 0.25wt.% of CNTs, the flexural strength boosted linearly with sonication time. Collectively with the utilization of ultrasonication, the investigations on MWCNTs by Konsta-Gdoutos et al. (2010a,b) revealed that a weight ratio of surfactant to MWCNTs in the vicinity of 0.4 was necessitated to have an optimum dispersion. More to add, they observed that long MWCNTs were more efficient at reinforcing the cement matrix than the shorter ones. The carbon allotropes are found fitting owing to their nanosize for combining them with elevated scaled inclusions, as it can be deduced from the presence of PVA fibers, silica fume, CNFs, and fly ash (Fig. 9.4). Both silica fume and fly ash are microsized pozzolanic materials utilized in HPC manufacturing. In the pore-filling function, a gradation in the size and cost of the materials appear to be more competent than the inclusions of dissimilar sizes utilized separately. CNTs can help observe the structural health of a cement
matrix element, which is assigned to their piezo-resistive strain-sensing abilities. The electric resistivity of the building material can be utilized as a parameter of the stresses applied over the diverse structures. The strain-sensing aptitude can either be reversible or irreversible. The irreversible strain detection is an indication of health problems in a structure provided, while reversible strains can be observed in the interest to measure dynamic loads (Mo & Roberts, 2013). Characteristically, the sensing of reversible strains is more complex because of their usually smaller size than those irreversible strains, and it is a progression that necessitates real-time monitoring (Chen & Chung, 1996). D’Alessandro et al. (2016) studied the influence of this electrical feature on the cement matrix, concentrating on the use of chemical dispersants and diverse mixing strategies that did not lower down the electric conductivity of CNTs. Based on upshots of their study, they concluded that regardless of the lower cost of the electrical equipment employed, permanent monitoring of the structural stress may not be an attractive tool for all structures technically and financially. However, it may find utilization in civil structures exposed to complex dynamic loads. The electrical resistivity of such nano-modified concrete is also found very much dependable on temperature (Han, Sun, et al., 2015). Furthermore, Zuo et al. (2012) put forward that this attribute could be applied to structures and

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Figure 9.3 Crack bridging in geopolymeric nanocomposites with MWCNTs (A) 0.1%, (B) 0.5% and (C) 1%.

traffic pavements for monitoring the traffic: weigh-in-motion measurement, number of vehicles, temperature sensing, and vehicle speed. Another use of CNTs is associated with the electromagnetic shielding, which guards not merely the devices but also human health. Even so, CNTs are not presently being employed for the said purpose on a large scale, since a variety of low-cost optional materials are available in this regard. On the other hand, the application of carbonaceous nano/microinert derived from the carbonization of wastes from agriculture was advocated by

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Figure 9.4 Heterogeneous photocatalysis of titanium dioxide for cementitious material (Cerro-Prada, 2018).

Khushnood et al. (2015), since they are pretty effectual at improving the electromagnetic interference. More to the point, they are found extremely cost-effective and very competent in comparison with CNTs so far the dispersion is concerned.

9.2.4 Carbon nano-fibers CNFs are cylindrical nanostructures with graphene layers, which can be stacked according to three dissimilar patterns, that is, in the shape of cones, cups, or plates. The average length and diameter of them range from 50 to 200 μm and 70 to 200 nm, respectively (Yazdani & Mohanam, 2014). They exhibit a tensile strength of circa 8 GPa, which is somewhat poorer than CNTs, and their cost is 50 times inferior to the value of these nanofilaments (Yazdani & Mohanam, 2014). The vapor deposition fabrication process makes possible the CNFs being produced at such marketable feasible costs (Kim et al., 2013). The reinforcement obtained in a mortar with CNTs and with CNFs has been compared by Yazdani and Mohanam (2014). They reported by using 0.1wt.% of CNTs that the flexural and compressive strengths obtained an improvement of 14% and 54% in that order. The constant weight-to-cement ratio of CNFs has attained an enhancement of 68% and 8% in the context of the same parameters. Also, this supremacy of CNFs to enhance the flexural strength was accounted by Danoglidis et al. (2016), which is an enhancement of 87% and 106% with CNTs and CNFs correspondingly. The efficiency of CNFs in the cement paste, both on their own and collectively with PVA fibers was studies by Metaxa et al. (2010). Subsequent to the curing period of 28 days, a 0.048wt. %

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dose of CNFs augmented the toughness and the flexural strength by 21.7% and 36.4%, respectively. On the other hand, a 0.54wt.% dose of PVA fibers yielded an increase of 5.4% in the flexural strength and of 28 times in the toughness. On employing both fibers concurrently, an enhancement of 32.7% in the flexural strength was obtained while the toughness improved by 30 times. The electrical conductivity of CNFs, at 20 C, is 105 S/m, whereas the value for CNTs varies from 105 to 107 S/m; and the value for copper is 6 3 107 S/m. Given such higher conductivity, a report by Galao et al. (2016) revealed that a CNF quantity of 2wt.% of cement and a fixed voltage of 20 V were found competent enough to prevent the freezing of the concrete sample having dimension 30 3 30 3 2 cm3. The referred heating function on the cement paste with dissimilar carbonaceous materials was investigated by Gomis et al. (2015) using graphite powder with 13 μm diameter and 130 μm length, carbon fibers with 13 μm diameter and 3 mm length, CNFs with 20
80 nm diameter and more than 30 μm length, as well as MWCNTs with aspect ratio more than 150. Gao et al. (2009) confirmed the optimal content ratio of CNFs to maximize the piezo-resistive performance of concrete samples. They employed three diverse kinds of CNFs yielded through numerous synthesis techniques, which produced varied values of electrical conductivity. The significance of a sufficient dispersion of CNFs and the presence of a threshold of fiber concentration beyond which the electrical resistance stayed constant despite the disparity of the strain according to the study of Gao et al. (2009). Sanchez (2009) attained an enhancement concerning the dispersion in the cement paste by employing nitric acid as a surfactant. Mo and Roberts (2013) concentrated on the strain sensing of selfcompacting concrete by adding CNFs and concluded that nano-modified concrete is about to work as a permanent strain sensor that would lend a hand to detect the smash up in the structure. Nevertheless, the correspondence among the loads and deformations was not precise to presume safely that the referred concrete could be utilized as a reversible strain sensor.

9.2.5 Carbon black nanoparticles CB nanoparticles can be produced through partial combustion of fundamentally made up of carbon atoms in the form of an amorphous molecular structure. This means that it is a structure of crystalline arrays of condensed rings. Because of the random orientation of arrays, they possess a few open edges with unsaturated carbon bonds involving chemical reactivity. In other words, CB nanoparticles or nanopowder is a conductive, noncontaminating powder produced using elevated temperature carbonization at about 1300 C in a cautiously controlled course of incomplete combustion. It is worth noting that the CB nano-powder is useful in electronics, coating, plastics, and inks, as well as green technology to produce green building composites. Wen and Chung (2007) made a comparison of the electrical attributes extended by fibers of carbon and CB to the cement paste by keeping a constant content ratio and found that carbon fibers were more effectual than CB at escalating electrical conductivity of the cement matrix and shielding the electromagnetic interferences. The partial substitution of up to 50% of the carbon fibers for CB kept the

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conductivity at a lesser cost and thus made it easy to apply the said composite for cathodic protection, deicing, and electrical grounding. The electrical conductivity of CB and its condensed size made it a cost-effective technique for guarding the steel rebar against the corrosion. The study by Masadeh (2015) on this feature in the context of concrete enclosing CB was carried out wherein he left the concrete samples in a tank with 3.5% NaCl solution for 6 months. The succeeding samples analyses helped conclude that corrosion declined as the CB quantity augmented. The use of a weight-to-cement ratio equal to or greater than 0.4% led to the very low chloride permeability of the samples. On the other hand, Qiao et al. (2015) concentrated on the study of triple-scaled inclusions of carbon into the concrete to employ the amalgamation of CNTs, carbon fibers, and CB as an anode for the large current cathodic protection in reinforced concrete structures. The best possible doses in the weightto-cement ratio resulted to be 1.5% for CNTs, 3% in the case of carbon fibers, and 2% for CB. The referred mixture obtained a great service life under the tremendous polarization potential and was resistant to the chloride attack. Regardless of being less effectual than the carbon fibers, Xiao et al. (2003) revealed a good self-sensing behavior of the cement matrices enclosing the CB, that is, a boost in compressive stress leading to a linear decline in the fractional changes of the electrical resistance was observed. Both the mechanical and the piezoresistive performances of mortar samples were measured by Monteiro et al. (2016), who claimed that the supplements of the CB with a content of approximately 4% weight-to-binder (w/b) ratio were favorable in enhancing the tensile as well as compressive strengths, while the optimal piezo-resistive performance fell within the range of 7%
10% of w/b ratio. Also, Chung (2012) explained the differences in result from the application of CB, CNFs, and graphite nanoplatelets (GNPs) in the context of the electromagnetic performance of the cement matrix.

9.2.6 Most relevant lines of study using other nanoparticles Nowadays, the comparison or amalgamation of diverse nanoparticles with other supplements is one of the most active research subject matters in nanoengineering. A research study by Mutuk et al. (2016) is an excellent illustration in this context whereby they significantly made the comparison of the hardening effect obtained during application of different kinds of nanoparticles. In the case of an ordinary Portland cement (OPC) mortar, they monitored an enhancement concerning its compressive strength for 16.4%, 15.4%, and 10.5% by adding 1% of nano-SiO2, nano-Al2O3, and nano-Fe2O3, respectively. Furthermore, the pieces of research study by Zhang, Ma, et al. (2016) have been made public between 2004 and 2013, which summed up the outcome of enhancements attained in the strengthening effect of the cement
matrix enclosing nano-SiO2, nano-Al2O3, and nano-Fe2O3, as well as nano-TiO2. Remarkably, Liu et al. (2016) quantified how the synergy among the pozzolanic GGBFS and nano-SiO2 could augment the compressive strength. They also monitored that low-quantity use of nano-SiO2 trimmed down the GGBFS hydration; thus the matrix porosity also reduced. OPC was employed to attain a compressive strength of 59.42 MPa subsequent to curing for 28 days while adding

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30wt.% of GGBFS and 3wt.% of nano-SiO2. The different mixtures of micro- and nano-SiO2 have been brought under consideration by Garg et al. (2016) for investigations, and they concluded that using a proportion of 1wt.% of nano-SiO2 and 10% of microSiO2 exhibited the highest enhancement about the split tensile strength, apart from offering resistance to the chloride infiltration. During the investigations on the impact of nano-SiO2 and nano-Al2O3 on the steel
concrete bonding of steel-reinforced concrete, Ismael et al. (2016) monitored that the bond stress was found enhanced in the region of 25% in the case of both of the referred nanoparticles when a higher dose of cement was there. When plain rebars were employed, nano-Al2O3 was found efficient in slimming down the cracks width as well as the spacing present among them. However, no effect was observed by them in the case of fiber-reinforced concrete when studied in a similar way using nano-SiO2 and nano-Al2O3 with the same objective. Other than the fundamental characteristics essential for structural elements, researchers (Land & Stephan, 2015) have analyzed how the hydration kinetics of cement can be controlled by nanoparticles. Nano-Al2O3 was identified as a retarder, whereas the nano-SiO2 resulted to be an accelerator for cement hardening. Following Cai et al. (2016), nano-CaCO3 accelerates the hardening progression and diminishes the shrinkage; however, a higher curing humidity of the specimens is considered necessary to enhance the durability of cement composites. On the other hand, the nanoclay in the cement paste was investigated by Chang et al. (2007), who found that a larger enhancement in the drop of the permeability occurred than in the case of compressive strength. It is worth noting that nano-MgO is only just discovered as a chemical nanocomponent that can be used as nanoparticle in the cement matrix. This newly explored nano-product has been supplemented by Shah et al. (2015) into mass concrete because it is competent enough to serve as a shrinkage compensator and is more effectual than other expansion agents that frequently necessitate more amount of water. Moradpour et al. (2013) focused on the alteration with regard to the mechanical performance and attained 80% enhancement in the compressive and 70% in flexural strengths by using 1wt.% of nano-MgO subsequent to curing for 28 days. Jayapalan et al. (2013) discovered that sufficient doses of microCaCO3 and nanoTiO2 can help control not merely the shrinkage but also the ecoimpact of composites enclosing cement.

9.3

Properties

According to Bangert et al. (2003), the durability of cementitious composites can be influenced by susceptibility to damages brought about by external loading together with environmental impacts, viz., actions of abrasive/erosive types, fluctuations in temperature and moisture content, as well as freeze-thaw cycles. Consequently, it is extremely vital to craft certain modifications in the contemporary concrete technology to promote the concrete as a more durable and sustainable

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building material, which can perform reliably with eye-catching cost-effectiveness (Sobolev et al., 2016). Fascinatingly, the ultrafine-sized nanomaterials exhibit unique chemical and physical attributes, and their presence in fresh cementitious composites can induce characteristics that are different from the properties of traditional cement composites (Li et al., 2004a). As a matter of fact, nanomaterials can be induced in the cementitious composites for significantly escalating the resistance against crack initiation and off-putting the propagation of cracks, which are the key outcomes from the physical distresses or damages of cementitious composites, viz., shrinkage, abrasion, and freeze-thaw smash up (Lu et al., 2015).

9.3.1 Shrinkage More often than not, the measured total shrinkage of cementitious composites can be combined with upshots of both, that is, autogenous and plastic shrinkages. The total shrinkage of cementitious composites can be mitigated by the supplements of nano-TiO2, nano-CaCO3, nano-SiO2, and CNFs. During an analytical study by Yang et al. (2015) on the effect of supplementing nano-TiO2 on the total shrinkage of the slag-based geopolymer (GP) paste, they monitored an accelerated cement hydration progression along with a denser microstructure on account of the admixed nano-TiO2, extending a drop in the meso pores quantity, which featured size of 1.25
25 nm. For this reason, nano-TiO2 conspicuously slimmed down the total shrinkage of the paste, which is regarded as a reflection of the meso pores quantity. Notably, the nano-TiO2 works for the most part through the nucleation effect, that is, providing the nucleation sites for the amassing of yields of hydration and, accordingly, regulating the cement-hydration. Liu et al. (2016) drew analogous conclusions and made comparison for the early-age (subsequent to 12 h) shrinkage in the cement paste enclosing nano-CaCO3 and the reference paste. They suggested that there was an optimal quantity for nano-CaCO3. The supplement of nanoCaCO3 encouraged early cement hydration when the dosage of nano-CaCO3 was kept as 1wt.%, accordingly, demonstrating a decline impact on the early-age shrinkage. During the literature review, there was found a conflicting conclusion in the context of the effect of adding nanomaterials on the drying shrinkage of cementitious composites. However, a few studies have supported the concept of reduction in drying shrinkage of cement composites when nanomaterials such as nano-TiO2, synthetic nanofiber, and GO were added during their manufacturing. Yang et al. (2015) and Duan et al. (2016) revealed that the admixing of nano-TiO2 into cementitious composites can trim down the drying shrinkage by means of formation of a compact microstructure with lesser cracks and also by the lesser loss of water via the pore-refining impact and the hydrophilicity-accelerating effect. The quantity of nonreacted phases was found diminished, while the size and the content of the hydration yields were augmented after 5wt.% nano-TiO2 was supplemented into the composites. The dropped porosity and denser microstructure were also monitored, resulting in the enhanced resistance against drying shrinkage. Additionally, more capillary pores, specifically those with a comparatively larger

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size, can be filled up with water in the cement paste possessing an optimal quantity of 3wt.% nano-TiO2. Consequently, the loss of water was dropped on account of the augmented hydrophilicity of the cement paste, which reduced the drying shrinkage thereupon. The report of Lee and Won (2016) revealed that the structural nano-synthetic fiber would dwindle the drying shrinkage by controlling the early crack development in cement composites. The impact of graphene-based nanomaterial (GO) on the drying shrinkage of the cement paste was studied by Lu et al. (2017), who made a suggestion that GO could diminish the drying shrinkage of cement composites via the denser microstructures and declined capillary pores quantity. Nevertheless, Gao et al. (2009) accounted that nano-SiO2 and nano-silica carbide can aggravate the drying shrinkage on admixing in cement-composites because of the water adsorption effect of the said nanoparticles, whereby these nanomaterials absorb free water from the capillary pores in concrete. There was incessant evaporation of surface water from the concrete enclosing nanoparticles after bringing the wet curing to an end. Therefore the drying shrinkage of concrete was found boosted ensuing from the escalating difference in the relative humidity amongst the concrete surface and its interior. In the main, the autogenous shrinkage of cementitious composites takes place as a consequence of self-desiccation through the course of hydration that is primarily generated at early ages and proportional to the quantity of the fine pores. In the context of HPC, the autogenous shrinkage turns out to be more prominent and is a governing feature for controlling the cracks. The cement composites reinforced with nanomaterials, viz., CNT-, nano-MgO-, and nano-Smectite-based clay have displayed inferior autogenous shrinkage as compared to those devoid of nanomaterials (Polat et al., 2017). Konsta-Gdoutos et al. (2010a) studied the influence of the dispersal of CNT on the autogenous shrinkage and pointed out that the admixed CNT can decline the porosity of the cementitious matrix. The referred research study has unearthed that the volume fraction of the fine pores with less than 20 nm diameter was noticeably trimmed down owing to the amalgamation of fine CNT. There exists a close link among the quantity of fine pores and the autogenous shrinkage of the cement paste; both were declined by the admixed CNTs. The analytical research work by Polat et al. (2017) has unveiled the impact of admixed nano-MgO on the autogenous shrinkage of cement paste. They accounted that the autogenous shrinkage of the cement composite can be recompensed by the expansion impact of nano-MgO that can react with H2O to produce extensive yields. Parallel conclusions have also been reported in the case of one more study by Polat et al. (2015), wherein the admixed nano-MgO displayed long-standing expansive effects owing to its slower rate of reaction in the cementitious concrete. Additionally, Hosan and Shaikh (2020) reported that the supplement of 1% of nano-CaCO3 conspicuously slimmed down the early drying shrinkage of higher volume slag concrete (HVSC) as well as high volume slag-fly ash concrete (HVSFAC). It is worth noting that they successfully maintained the shrinkage strain significantly lower in later ages of HVSFAC than control OPC concrete. Nevertheless, the strain diminution gap among the controlled OPC-concrete and

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HVSC enclosing 60% of BFS together with 1% of nano-CaCO3 was found dwindling down in the later on ages. Also, they reported (Hosan & Shaikh, 2021a, 2021b) that inferior drying shrinkage is monitored in both the cases of HVSC and HVSFAC when nano-SiO2 is employed as an addition, and they found that the drying shrinkage of HVSFAC was obviously subordinate to the OPC-concrete in the subsequent ages.

9.3.2 Freeze-thaw damage The damage in cement-composites due to freeze-thaw cycles in cold climate regions is one of the key routes for their deterioration, which can be explained with the help of most important and broadly accepted theories of the hydraulic pressure theory, osmotic pressure theory and crystallization pressure theory (Fan et al., 2015). In general, the pressure mounts due to the volume enhancement coupled with conversion of water, that is, liquid phase to ice, that is, solid phase as well as by means of the water migration in the capillary pores (Gonzalez et al., 2016). This tremendous pressure introduces stress in the internal microstructure of the cementcomposite causing crack initiation in the condition when it surpasses the local strength of the structure. The supplement of nanomaterials such as nano-SiO2, nanokaolinite clay (NKC), nano-Al2O3, and GO contribute the more compacted and denser microstructure resulting in improved resistance against freeze-thaw, that is, frost damage to cementitious composites. Gonzalez et al. (2016) studied the influence of the addition of nano-SiO2 on the damage of cement-concrete exposed to conditions of freeze-thaw cycles. In this case, they found that the nano-SiO2 played a role as an SCM, which can react with a primary product of cement hydration yield—Portlandite, resulting to produce added C
S
H gel. Consequently, an improvement is observed in the context of both, that is, the paste and the interfacial transition zone (ITZ) among the aggregate as well as the paste. What is more to add, a refined pore structure in concrete was monitored indicating a restricted water intrusion, which resulted in lesser accessibility of water to participate in the freezethaw damaging. Hence, the admixing of nano-SiO2 in cement
concrete was found capable of trimming down the frost damage, which was further supported by Quercia et al. (2012), who analyzed the impact of the nano-SiO2 supplement on the durability of cement
concrete. They reported that adding 3.8wt.% of nano-SiO2 enhanced almost all durability parameters even inclusive of the resistance against the freeze-thaw. In this study, the extremely stiff and smaller sized C
S
H gel was generated on account of the pozzolanic activity of nano-SiO2 converting the microstructure denser ensuing the enhanced resistance against frost. On the other hand, GO was added into the cement composites made by Mohammed et al. (2016) for investigating the weight loss of specimens subsequent to exposing them to 540 freeze-thaw cycles. The examination for nitrogen absorption has unveiled that the GO in the cementitious matrix mostly demonstrated the alteration effect of the pore structures that considerably mitigated the quantity of mesopores. Keeping the exposure constant to freeze thaw cycles of 540, a weight loss of only 0.25% was recorded in the samples enclosing GO which is smaller in comparison with weight

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loss reported as 0.8% for the controlled specimens. This is attributed to the fact that water freezes with more difficulty in small pores than in the larger ones.

9.3.3 Abrasion or erosion The resistance against the abrasion/erosion is also one of the major factors for cement composite, especially while it is subjected to abrasive forces in a few definite utilizations such as the pavement surface, in the bridge footing, or the dam structure (Atis & C¸elik, 2002). Normally, the abrasion of the cement
concrete is caused because of the scraping, skidding, rubbing, or sliding of objects on its surface (Siddique et al., 2012). To improve the resistance against abrasion of cement
concrete, nanomaterials, namely, nano-TiO2, nano-SiO2, and nano-silica carbide have been investigated through numerous research studies. Among them, Gao et al. (2017) explored the influence of nano-SiO2 on the wear resistance of fly ash
based cement
concrete. They suggested that there was an optimal dose for nano-SiO2 to display the advantageous effect. The reduction of the wear loss of the cement
concrete by 75% was recorded in comparison with the reference standard cement
concrete when the addition of 2wt.% nano-SiO2 is made. This was assigned to the microaggregate filling effect of the nano-SiO2 and the pozzolanic reaction, which enhanced the allocation of cementitious particles and the orientation degree of Ca(OH)2 in the cement-composite. For these reasons, a denser texture developed, which ultimately contributed to enhanced resistance against the abrasion. Also, Li et al. (2006) found the analogous conclusion while reporting on the improvement in the context of abrasion resistance of cement
concrete achieved through the incorporation of nano-TiO2. They explained the mechanism of this enhancement that if nano-TiO2 is distributed in the cement matrix homogeneously, the development of hydration yields would be controlled to accumulate on the nanoparticles owing to their nucleation effect, ensuing in a more uniform and compact cement matrix. In consequence, the abrasion resistance of cement
concrete was augmented appreciably through the admixing of nano-TiO2.

9.3.4 Nanotechnology for cementitious composites to triumph over their chemical deteriorations During the service life of the cement
concrete structure, it gets greatly affected by the chemical deteriorations when exposed to environments. Nanotechnology for cementitious composites working in the interest of triumph over the chemical damages while exposed to chemical pathways or chemical distresses, which impact adversely on their durability criteria viz., alkali
aggregate reactions (AARs), sulfate attack, acid attack, and thermal degradation, is quite important.

9.3.4.1 Alkali
aggregate reactions AARs take place when the alkalinity in the pore solution goes beyond the threshold value and a presence of active phases in the aggregate. In general, if the active

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phases present are from amorphous silica aggregate, the reaction can be categorized as an alkali
silica reaction (ASR). However, when they are derived from dolomitic limestone aggregate, then they are causing the alkali
carbonate reaction (ACR) in cement
concrete (Locati et al., 2014). The products of AARs are found characteristically extensive and accountable for causing cracks in the cement
concrete when the tensile stress surpasses its local tensile strength owing to the expansion. There are numerous aspects such as alkali content, quantity of water, aggregate activity, and temperature, which influence the extent of AARs as revealed from the past literature (Tang et al., 2015). Not only that, various research studies have indicated that the pozzolanic reaction devours the Ca(OH)2 in cement
concrete, and as a result, it diminishes the alkalinity of the pore solution, which can reduce the impact of ASR (Shafaatian et al., 2013). Consequently, the application of nanomaterials to speed up the pozzolanic reaction will contribute finally to the control of AARs. The utilization of glass powder in order to substitute cement up to 40% as the active phase in the cement matrix and subsequent examination of the ASR of cement composites keeping 3wt.% nano-SiO2 for admixing were carried out by Aly et al. (2012). The outcomes of ASR investigations have uncovered that no damaging impact was detected. Furthermore, the differential thermal (DTA) or thermogravimetric analysis (TGA) and X-ray diffraction (XRD) upshot have exhibited a fall in the content of Ca(OH)2, which was assigned to the pozzolanic activity of nanoSiO2. For this reason, the alkalinity in cementitious composites enclosing nanoSiO2 was found declined to a value lower than the threshold, thereby putting a stop to the ASR episode.

9.3.4.2 Sulfate attack Sulfate attack is one of the important criteria in the context of the durability of cementitious composites. Significantly, it can undermine the durability of cementitious concrete structures. Sulfate attack can form expansive compounds through a series of chemical reactions that take place amongst the hydrates present in the cement paste and aggressive sulfate ions. The expansion impact of sulfate attack in cement
concrete causes not merely the initiation of cracks, but it was also found associated with strength loss and makes softer the cementitious matrix in the long tenure (Atahan & Dikme, 2011). However, the reduction with regard to the permeability of concrete, escalating cement quantity or development of very dense microstructure, and trimming down the ratio for water-to-cementitious materials are a few effectual measures to lessen the sulfate attack damages (Irassar, 2009). In accordance with modern studies (Said et al., 2012; Singh et al., 2013), nano-SiO2 can enhance the resistance of cement
concrete against the sulfate attack by employing its densification impact on the microstructure that slow down the infiltration of sulfate ions and water into the cement
concrete. A comparison of the influence of nano- and microSiO2 on the expansion of cement
mortars exposed to sulfate attack was made by Ghafoori et al. (2016), who reported that the supplement of nano-SiO2 mitigated momentously the expansion of mortars. While nano-SiO2 was distributed evenly in the matrix and agglomeration was absent, it outperformed

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microSiO2 at the equal dose. Arel and Thomas (2017) met with identical conclusions and accounted that the admixing of the nano-SiO2 slimmed down the expansion of cement
mortar subsequent to 23 weeks of exposure to a sulfate environment more than the adding-on of the micro-SiO2. The reduced porosity and enhanced microstructure of specimens enclosing nano-SiO2 were ascribed to be the effect of both the nano-fillers and, of course, the pozzolanic nature of nano-SiO2. Nano-SiO2 is the most efficient nanomaterial in comparison to microsilica, GGBFS or fly ash when the link among the dose of addition and the competence of reducing sulfate-induced expansion are considered (Atahan & Dikme, 2011).

9.3.4.3 Acid attack The cementitious composites deteriorate under acidic environment owing to the attack of chemicals influencing their components or structures. Groundwater, acid rain, and industrial effluents are actively involved in this activity (Deb et al., 2016). The mechanism of the acid attack, that is, acid corrosion—of cementitious composites have been studied in past and found that the deterioration of cementitious composites exposed to sulfuric acid (H2SO4) is mostly because of the presence of H11 and SO422 ions that can pilot to the dissolution of hydration yields and the development of extensive compounds. Fan et al. (2015) have employed the nanomaterial of calcined NKC intending to enhance the resistance of the cementitious mortar against the acidic solution. Their study used specimens of cementitious mortar with or without NKC, which were immersed in a solution of sulfuric acid (H2SO4) and hydrocholoric acid (HCl) displaying a pH value of 1.5. The loss in mass and residual compressive strength was evaluated following the exposure. The optimal dose of NKC demonstrated an advantageous impact on the enhancement in the context of the resistance against the acid attack by the cementitious mortar. They kept the dose of 3wt.% for samples of cementitious mortar with and without supplement of NKC for an exposure of 60 days to the acid solution. Notably, their outcomes revealed that the samples of cement
mortar with NKC proved competent enough to decline the losses in mass by 19% and the compressive strength by 17%, while a comparison was made with the controlled cement
mortar samples not enclosing NKC. Furthermore, the images of backscattered electron (BSE) microscopy exposed that the gel of C
S
H was decalcified owing to the presence of H11 and an extensive crystal of CaSO4  2H2O were formed on the account of the presence of SO422 in the cementitious mortar specimens not containing NKC. Fig. 9.5 illustrates a presence of a larger quantity of C
S
H gel in the specimens with 3 wt.% addition of NKC that was assigned to the filling effect and higher activity of NKC, ensuing in enhanced resistance against the acid attack.

9.3.5 Thermal degradation In the context of the structural cementitious composites such as reinforced cement
concrete, its nonflammable nature and resistance against elevated temperature help shield the reinforced rebar. Nevertheless, the exposure to high temperature

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Figure 9.5 Behavior of concrete under acid exposure (Fan et al., 2015).

inflicts a detrimental impact on the characteristics of cementitious composites in which both physical and chemical alterations take place when subjected to thermal exposure. The nanomaterials, namely, nano-SiO2, graphene sulfonate nanosheet (GSNS), nano-Al2O3, CNT, nano-clay, and GO have displayed the potential to hinder the thermal degradation of cementitious composites. A study by El-Gamal et al. (2018) was carried out with an aim to employ cost-effective nanomaterials in the cement-paste. For that, they obtained the nano-SiO2 from the rice husk ash and further studied its impact on the thermal resistance by cementitious pastes. The referred study pointed out that an optimal dose for the nano-SiO2 is advantageous to recompense the pessimistic impact of elevated temperatures on the attributes of the cementitious paste. The micrographs achieved by means of scanning electron microscopy (SEM) pointed out that the addition of 1wt.% nano-SiO2 in the cementitious paste demonstrated the microfiller effect and pozzolanic activities, which were reflected by its the microstructure showing the densification and compaction.

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Consequently, the residual compressive strength of the cementitious paste subjected to elevated temperatures of up to 800 C was escalated obviously on account of a nano-SiO2 supplement. The conclusions of Horszczaruk et al. (2017) were also found in harmony with them. They analyzed the temperature’s influence ranging from 20 C to 800 C on the thermal resistance of cementitious mortar enclosing 1
5wt.% nano-SiO2. In accordance with the optical microscopic and SEM monitoring, up to 3wt.%, the addition of nano-SiO2 can produce supplementary C
S
H through the pozzolanic reaction contributing to enhancing the microstructure and displayed the capability to bridge over the cracks subsequent to exposure to elevated temperatures. Therefore the addition of nano-SiO2 boosted the thermal resistance of cementitious mortar specifically at temperatures up to 200 C. What is more to add, Heikal et al. (2015) accounted that the amalgamation of 1wt.% of nano-Al2O3 has a speeding up the impact on the hydration of cementitious paste since it played a role of a nano-filler resulting in a superior thermal resistance up to 1000 C to other pastes. Also, the supplement of nano-Al2O3 led to the densification and compaction of the microstructure. The influence of adding nano-clay on the thermal attribute of the cementitious mortar was studied by Irshidat and Al-Saleh (2018), who recorded the elevated residual compressive strength at 200 C as well as superior residual flexural and tensile strengths at 400 C, when the best possible dose of nano-clay, that is, 2wt.% of cement was added to alter the mortar. The width and density of the hairline cracks developed during the exposure to elevated temperature were found reducing due to the presence of nano-clay. In order to produce cost-effective nanomaterials, Sikora et al. (2019) carried out a study wherein they manufactured both nano-SiO2 and CNT from the recycled substrates. Purposefully, they synthesized the CNT/ nano-SiO2 core/shell structures and analyzed the impacts of the very high temperature on the cementitious pastes incorporating the core-shell nanostructure, which they obtained. The monitoring by means of a transmission electron microscope (TEM) has determined that the CNT surfaces were covered productively with a shell of nano-SiO2. This has improved the bonding among the CNT and cementitious paste as well as protected the CNT from calcination in the course of heating ensuing in an expanded temperature range that CNT can display positive effect on the paste. Thus the specimens amalgamating CNT and nano-SiO2 displayed the compressive strength retention up to 600 C, whereas the fresh specimens enclosing CNT have demonstrated a gradual loss in strength subsequent to the exposure at 450 C. On the other hand, the research studies of several researchers (Amin et al., 2015; Zhang, Han, et al., 2016; Zhang, Ma, et al., 2016), have revealed that while CNT is admixed with cementitious composites, no improvement or hindrance found in the cement hydration, but it has performed as a channel to lend a hand in liberating high-pressure steam produced because of higher temperatures or worked as bridges among hydration yields and cracks. The influence of GO on the performance of cementitious concrete at lowering temperature was studied by Mohammed et al. (2017), and they reported that the quantity of capillary pores was found declined while the gel pores were met with augmented numbers ensuing in a more compatible thermal deformation and enhanced resistance against crack development.

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Consequently, the residual compressive strength of the GO-modified cement
concrete samples was recorded as 70% of the original value, while its counterpart reference samples have only 35% subsequent to being subjected to elevated temperatures. Besides GO, GSNS, another kind of derivative of graphene, was explored by Fan et al. (2010) to study its impact on the mechanical characteristics of cement
concrete all through its exposure to very high temperature. They revealed that the sulfonic groups present in GSNS can take part in the reaction with hydration yields resultant in covalent bonding among GSNS and matrix. Hence improved residual strength of cement
concrete can be obtained on account of the improved microstructure when exposed to very high temperatures up to 1000 C.

9.3.6 Compressive strength With regard to compressive strength, the research studies by Shaikh and Supit (2014) have revealed that the concrete incorporating 1wt.% of nano-CaCO3 particles were found to yield the highest compressive strength at all ages. At early age, the compressive strength of concrete amalgamating 1% nanoparticles of CaCO3 was recorded roughly as 146%
148% superior to that of the ordinary concrete. Moreover, the compressive strength of HVFACs is also found enhanced on account of the supplement of 1% nano-CaCO3 at early age. There found the most noteworthy improvement in context of HVFAC incorporating 39% fly ash and 1% nano-CaCO3 figuring to approximately 46%
48%. Also, 1% nano-CaCO3 supplement has improved the long-term, that is, 90 days, compressive strength of ordinary concrete by approximately 40% as well as that of HVFAC incorporating 39% and 59% fly ash by around 57% and 8% correspondingly. In one more study, the same concrete technologists (Shaikh & Supit, 2014) found that the adding nano-SiO2 and nano-CaCO3 in ordinary cement concrete has found with enhanced compressive strength at both early and later ages. Although there is an absence of any noteworthy difference with respect to the compressive strength development of 2% nano-SiO2- and 1% nano-CaCO3-incorporating concretes. On the other hand, the competence of the nanoparticles in the interest to boost the compressive strength of HVFACs is accounted as more prominent in the mixtures whereby 40% replacement of cement is made with them. During the investigations carried out by Shaikh, Steve, et al. (2014) and Shaikh, Supit, et al. (2014) the adding up of nano-SiO2 as a partial substitution of cement has demonstrated higher compressive strength on 7th and 28th day than that of the controlled cement
mortar. The optimum content is reported as 2% of nano-SiO2, which put on a show the best possible compressive strength on 7th day and 28th day among all nano-SiO2 contents used. The indication on the basis of the rate of strength development suggested that the nano-SiO2
mortars were more reactive and have contributed more to the compressive strength on 7th and 28th day. In the case of higher volume, that is, 40% and 50% fly ash incorporation with mortars have shown an increase on the 7th day compressive strength by 5% and 7%, in that order when a supplement of 2% nano-SiO2 was made, but no improvement was observed when the fly ash content escalated to 60% as well as 70%. A noteworthy improvement of 33% and 48% on 28th day compressive strength of higher volume fly ash
mortars enclosing even

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60% and 70% fly ash was recorded through the supplement of 2% nano-SiO2. An insignificant enhancement was reported when the quantity of fly ash was taken as 40% and 50%. At an early age, that is, 3-day period, the compressive strength of HVFAC based on 60% fly ash was found with the boost up of more or less 95% when 2% nano-SiO2 was added, but there was an absence of such enhancement at other ages. Admirably, when a supplement of 2% of nano-SiO2 was made with HVFAC possessing 40% fly ash, the improvement on 3rd-day compressive strength was approximately 24%, which maintained between 15% and 21% in the longer term also. A study by Supit et al. (2014) on the use of ultrafine fly ash (UFFA) put forward a more well-defined impact on the compressive strength of mortars, in particular, at the early ages. The cement
mortar integrating 8% of UFFA as partial substitution of cement displayed 27% improvement with regard to the compressive strength on 7th day in comparison with the controlled cement
mortar. On 28th day, the compressive strength of 8% UFFA-based mortar was found increasing as 23%. This simply implies that the outcomes of strength development of mortars is chiefly determined by the application of UFFA, which is made up of very fine particle sizes in lieu of class-F fly ash, suggesting that UFFA has more prominent influence on compressive strength, in particular, in early ages. Nevertheless, one should consider the correct mixture proportions with a view to obtain the effectual enhancement. In the case of high-volume fly ash (HVFA)
mortars enclosing 40%, 50%, 60%, and 70%, the acceleration in the context of the pozzolanic reaction as well as improvement in the compressive strength between 26% and 63% on 7th day was assigned to the supplement of 8% UFFA. The augment with respect to compressive strength on 28th day was reported between 2% and 31%. It was revealed that UFFA compensates the lower early strength of binary integrated mortars blending cement and class-F fly ash. For this reason, the blended-cement incorporating UFFA and class-F fly ash in HVFA
mortars extends grand potential for its application in concrete constructions wherein both shorter and longer term compressive strength are well thought-out. Furthermore, Shaikh and Supit (2015a) concluded that 8wt.% UFFA-based concrete achieved the optimum compressive strength at all ages. Also, the early age compressive strength of HVFA
concretes is recorded enhanced on the supplement of 8% UFFA. On the top of that, the most momentous improvement of around 200% was monitored in HVFA
concrete blending with 52% fly ash and 8% of UFFA on the 3rd day. The supplement of 8% UFFA boosted the long term, that is, 90 days, compressive strength of ordinary concrete by approximately 55%, while that of 32% fly ash based HVFA-concrete by merely in the region of 10%. In one more study by Shaikh and Hosan (2019a,b), the nano-Al2O3 supplement was found to improve the compressive strength of cement pastes enclosing 70%, 80%, and 90% of blast furnace slag (BFS) by about 16%, 8%, and 2%, respectively. When the combination of BFS and fly ash were employed as 70% and 80%, the enhancements recorded were 9% and 16% in that order because of the supplement of nano-Al2O3. The compressive strength of higher-volume BFS paste comprising 70% BFS and nano-Al2O3 was found with more compressive strength in comparison to controlled OPC paste, but it was not found surpassing when the quantity of BFS was kept as 80% and 90% as well as the combined BFS and FA amounted as 70% and 80%. The optimum mixture

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could be that enclosing 69% slag and 1% nano-Al, whose compressive strength went beyond the strength of controlled OPC paste. Hosan and Shaikh (2020) added nanoCaCO3 in the HVS-paste having 70%, 80%, and 90% slag and augmented the compressive strengths in comparison with the reference HVS paste with no nano-CaCO3. However, the values of compressive strength were reported inferior to that of the controlled OPC paste in almost every mixture. Notably, adding nano-CaCO3 enhanced the compressive strengths of pastes containing of higher volume of slag and fly ash than the reference controlled paste enclosing no nano-CaCO3 and upheld higher compressive strengths than the controlled OPC paste. In a study by Shaikh and Hosan (2019a), high-volume slag (HVS) cement paste possessing 60% slag exhibited approximately 4% more compressive strength than that of controlled cement paste, whereas the HVS cement paste incorporating 70% slag demonstrated identical compressive strength as the controlled cement paste. A noteworthy decline in compressive strength was observed when the slag quantities of 80% and 90% were employed. The HVSFA
cement pastes comprising total slag and fly ash content of 60% demonstrated roughly 5%
16% more compressive strength than controlled cement paste. A considerable cutback in compressive strength is recorded in higher slag
fly ash combinations with rising fly ash quantities. Appreciably, the supplement of 1%
4% nano-SiO2 enhanced the compressive strength by 9%
25%, 11%
29%, and 17%
41% of HVS based cement-paste having 70%, 80%, and 90% BFS, respectively. Also, the nano-SiO2 supplement considerably enhanced the compressive strength of high-volume BFS
FA-based cement-pastes. In accordance with the study by Hosan and Shaikh (2021b), the compressive strength of HVS enclosing 69% of BFS boosted notably figuring 43% on the 3rd day of concrete age because of adding % nanoCaCO3 when compared to their controlled concrete with no nano-CaCO3 and went beyond the compressive strengths of OPC
concrete on 7th day and preserved betterquality strengths in the later ages. A 1% nano-CaCO3 supplement resulted into an increase of the compressive strengths of HVS
fly ash-based concrete incorporating 48.5% BFS and 20.5% fly ash by 28% more on the 3rd day of concrete age than their controlled concrete having an absence of nano-CaCO3 as well as goes beyond the compressive strengths of controlled OPC
concrete on the 28th day, which displayed alike long-term compressive strengths. Hosan et al. (2021) reported that the addition of 1% nano-CaCO3 and 2%
3% nano-SiO2 into both HVS
, and HVSFA
concretes drastically augmented the compressive strengths. Each HVS
and HVSFA
concretes incorporating nano-SiO2 and nano-CaCO3 was found with improved compressive strength of OPC
concrete except the concrete possessing 80% of BFS enclosing 2% nano-SiO2, whose strength reported was just 5% inferior to the OPC
concrete. Shaikh and Supit (2016) observed the highest enhancement in the compressive strength when nanoparticles are dispersed in polycarboxylate etherbased solution of superplasticizer. The ultrasonic dispersion of nano-SiO2 and nanoCaCO3 in the solution of superplasticizer enhanced the early age compressive strength by 26%
28% and 23%
36%, respectively, while the compressive strength of the 28th day was recorded boosted by 17% and 30%, in that order as compared to the ultrasonic mixing of nanoparticles in water and nothing else. According to Hosan and Shaikh (2021b), the compressive strength of HVSC manufactured using 78%

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BFS is found more by almost 34% and 36% on the 3rd and 28th day, in that order, on making the supplement of 2% of nano-SiO2 than their controlled HVSC incorporating with 80% BFS and demonstrated analogous compressive strengths to the OPC
concrete at later on ages. The supplement of 3% of nano-SiO2 into HVSFAC blended by the combination of fly ash plus slag in a quantity of 67% has enhanced the compressive strengths radically by 45% at the 3rd day when comparison made with the controlled concrete without nano-SiO2 as well as also surpassed the compressive strengths of OPC
concrete on 28th day and displayed appreciably elevated compressive strengths than controlled OPC
concrete at later ages. In the research work of Assaedi et al. (2019), the mechanical attributes of PVA fiber-reinforced GP composites manufactured using dissimilar quantities of nano-SiO2 were estimated. The fitting nano-SiO2 quantity was set up as 1.0
2.0wt.%. The PVA containing fiber-reinforced geopolymeric nanocomposite incorporated with 1.0 and 2.0wt.% nano-SiO2 demonstrated enhanced strengths, viz., compressive, flexural, and impact, when compared to the PVA
fiber-reinforced GP-composite with no addition of nano-SiO2. Nevertheless, the boosting of quantity of the nano-SiO2 further than 2.0wt.% negatively influenced the mechanical characteristics of the composites. Shaikh et al. (2015) added nano-SiO2 into concretes possessing 25% and 50% recycled coarse aggregates (RCA) at all ages up to 56 days, which resulted in an increase in the compressive strength. The enhancement with regard to compressive strength was found higher in the case of concrete incorporating 25% RCA than for that blended with 50% RCA. The concrete fabricated with 25% RCA and 2% nanoSiO2 exhibited 92% of the compressive strength of controlled concrete from the 28th to 56th day. This break can be decreased further with the long-drawn-out curing. Shaikh and Supit (2014) revealed that the presoaking of RCA in the solution of nanoSiO2 boosts the compressive strength by around 5% in comparison with direct mixing of nano-SiO2 during the mixing course of action. The RCA
concretes blended with nano-SiO2 mixed by both techniques have displayed roughly 20%
25% enhancement in the compressive strength. The concrete enclosing presoaked RCA in the solution of nano-SiO2 was found only 12% inferior in compressive strength to that of controlled concrete manufactured using natural coarse aggregate (NCA).

9.3.7 Tensile strength In context of tensile strength of nanomaterial-incorporated concretes, a noteworthy research study by Shaikh et al. (2015) revealed that the nano-SiO2 add-on augmented the tensile strength of recycled aggregate concretes. The referred concretes are found displaying the tensile strength on the 28th day figuring 90% superior to that of control concrete.

9.3.8 Water sorpitivity Looking to the investigations on water sorpitivity criteria, Shaikh and Supit (2014) examined it by experimenting on ordinary concrete incorporating 1% nano-CaCO3 and the plain ordinary concrete. They made it public that the earlier one has

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demonstrated approximately 17% and 30% inferior water sorpitivity on the 28th and 90th day, respectively to that of later one. Furthermore, the addition of 1% nano-CaCO3 particles in HVFA
concrete was reported as appreciably effectual in plunging the water sorptivity when a blend with 39% of fly ash was made by partly substituting the cement present in the concrete. In simple words, it is indicative of a finer pore structure with regard to the 1% nano-CaCO3 particles enclosing HVFA system than the plain HVFA paste. Shaikh and Supit (2015a,b) observed the values of water sorptivity of ordinary concrete and HVFA
concrete amalgamated with nano-SiO2, which was recorded notably inferior to that of the plain ordinary cement
concrete. Also, it was observed that the water sorptivity of nano-CaCO3integrated concrete was lesser than the cement
concrete by an average of 17% and 30% on the 28th and 90th day, respectively. The water absorption quantity was found to decline in longer curing time because of an augment in volume of gel from cement hydration which plugged the water-filled spaced and dropped capillary pores in concrete. The supplement of 2% of nano-SiO2 was found to drop the water sorptivity of ordinary concrete by roughly 20%
40% (Supit & Shaikh, 2015). In the case of HVFA
concretes blending with 40% and 60% fly ash, this diminution was found important among 27% and 50% and between 7% and 22%, respectively. The decrease in the volume of permeable voids (VPV) in HVFA containing concretes was also evidenced owing to an add-on of 2% nano-SiO2, though it was in less significant amount. Not only have that, the 2% supplement of nano-SiO2 also displayed considerable lessening in chloride ion infiltration in HVFA
concrete by approximately 28%
38% in concrete amalgamating 40% fly ash and to a smaller extent of 8%
20% in concrete incorporating 60% fly ash. Furthermore, Shaikh and Supit (2015b) studied on the water sorptivity of ordinary concrete manufactured by using 8% UFFA, which was found approximately 50% and 45% inferior to the ordinary concrete respectively on the 28th and 90th day. The employment of 8% of UFFA in making HVFA
concrete has resulted in reduction for around 35% and 50%, respectively in the sorptivity when the curing was done for 28 and 90 days. This indicated that the supplement of 8% UFFA in HVFA system formed a finer pore structure than plain HVFA-paste. Moreover, Shaikh et al. (2015) unearthed that the sorptivity value in both recycled aggregate concretes was much lesser on 28th day than that recorded on 7th day owing to the addition of nano-SiO2 and the prolonged curing period. The increasing supplement of nano-SiO2 particles resulted in the decrease with regard to the sorptivity of recycled aggregate concretes and at the substitution of 25% RCA it exhibited even superior resistance to the controlled concrete, which found with a reduce in the sorptivity by 11% following 7 days of moist curing.

9.3.9 Water absorption The water absorption was studied by Hosan and Shaikh (2021a) by making a supplement of 1% nano-CaCO3 into HVS
and HVSFA
concrete and found the reduced water absorption by 1% and 10% correspondingly subsequent to curing of 28 days. Even so, the greater cutback monitored following 90 days of curing in

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both kinds of concretes demonstrating identical sorptivity to the controlled OPC
concrete. Additionally, Hosan and Shaikh (2021b) accounted that the rate of water absorption with respect to HVS
concrete is met with a reduction of around 27% and 36%, whereas by 38% and 37% in HVSFA
concrete following 28 and 90 days, in that order, when compared with their controlled concretes not enclosing the nano-SiO2. This was credited to the supplement of nano-SiO2 particles.

9.3.10 Chloride ion penetration A research study by Shaikh and Supit (2014) on chloride infiltration criterion revealed that a substitution of 1% of Portland cement with nano-CaCO3 particles declined the permeability of chloride ion in the case of this Portland cementconcrete by something like 20% and 50% on the 28th and 90th day, respectively. As well, the gains of incorporating nano-CaCO3 particles were also achieved in HVFA
concretes, wherein approximately 19% and 12% reductions in penetration of chloride ion taken place in the case of HVFA-concretes enclosing 40% and 60% fly ash, in that order after 28 days of curing which was attributed to the adding together 1% nano-CaCO3 particles. Nonetheless, no momentous enhancement in above referred concretes was monitored when the curing period was kept as 90 days. The outcomes of Shaikh and Supit (2015a, 2015b) for the addition of 2% of nano-SiO2 and 1% nano-CaCO3 in cement concrete reports for a reduce in the chloride ion filtration of ordinary concrete by roughly 27% and 21% on 28th day correspondingly. However, when 2% nano-SiO2 was blended with HVFA
concrete having 38% fly ash, the charge passed declined drastically from 4996 to 3088 coulombs on the 28th day, reporting a fall of about 38% in comparison with the reference concrete manufactured using 40% FA and with no nano-SiO2. In 1% nanoCaCO3 containing HVFA, the charge passed on 28th day is found declined to 4057 coulombs. The application of nanoparticles incorporated with HVFA is found to be more distinct in the concrete which has undergone 40% substitution of cement. Shaikh and Supit (2015b) observed that the substitution of 8% Portland cement with UFFA has resulted in a drop off in context of the permeability of chloride ion in Portland cement-concrete in the region of 18% and 65% on the 28th and 90th day, respectively. What is more, the advantages of supplementing 8% UFFA were also experiential in HVFA concretes, wherein around 49%
68% and 8%
34% diminutions in chloride ion filtration of HVFA
concrete enclosing 40% and 60% fly ash obtained, in that order. Also, Hosan and Shaikh (2021a,b) measured extraordinary resistance against chloride ion permeability in HVS
and HVSFA
concretes with and without the add-on of nano-CaCO3 particles following the curing of 28 and 90 days. The addition of 1% of nano-CaCO3 particles also decreased the charge passed by 14% and 22% of HVS
concrete and by 39% and 45% of HVSFA
concrete than their respective controlled concrete subsequent to curing of 28 and 90 days correspondingly. Analogously, Hosan and Shaikh (2021a, b) also monitored a brilliant resistance against penetration of chloride ion in both the cases of HVS
and HVSFA
concretes possessing nano-silica than their

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respective controlled concretes and the OPC
concrete subsequent to curing of 28 and 90 days. The upshots of Shaikh et al. (2015) have suggested that the amplified nano-SiO2 supplements have a tendency to enhance resistance of the concrete against the chloride ion penetration, given that it performs to encourage the hydration and plug the capillary spaces inside the concrete. A supplement of 2% of nanoSiO2 was found to decline the depth of chloride ion infiltration by 31% and 28% in concretes enclosing 25% and 50% RCA, respectively on the 28th day and it displayed enhanced resistance and surpassed it by 13% and 5%, respectively on the 28th day when a comparison was made with the control concrete. In addition, the curing age influenced for a significant difference to the resistance across all the series, even though the dissimilarity was more momentous for the series integrating the nano-SiO2 particles. In harmony with Shaikh et al. (2018), the resistance against chloride ion infiltration by recycled aggregates concrete Was notably trimmed down on account of the nano-SiO2 addition by both techniques, more than ever, by using presoaked RCA in the solution of nano-SiO2, the resistance against infiltration of chloride ion class alters to “lower” class. Both RCA-based concretes showed evidence of appreciably subordinate penetration of chloride ion than that of controlled concrete enclosing NCA.

9.3.11 Permeability The permeability study by Shaikh and Supit (2014) revealed that the VPV of Portland cement-concrete incorporating 1% nano-CaCO3 particles is found declined considerably up to 46% on 28th day. Additionally, the supplement of 1% nanoCaCO3 particles in HVFA
concrete manufactured using 39% fly ash dropped the volume of permeable voids by almost 30% in comparison with the concrete fabricated with 40% FA. The enhancement has further long-established that the 1% of nano-CaCO3 particles improved the microstructure of HVFA
concretes impacting on the performance of concrete including the augmented strength and decreased VPV. The another study conducted through Shaikh and Supit (2015b) threw lights on VPV of the concrete enclosing 8% UFFA decreased outcomes by about 15%
20% as compared to Portland cement
concrete, whereas the add-on of 8% UFFA into HVFAC possessing 32% fly ash decreased the VPV by roughly 15%
23% in comparison with the concrete that manufactured with 40% FA. Hosan and Shaikh (2021a,b) uncovered that the noteworthy fall in VPV of HVS-concrete composed of 69% BFS and HVSFAC made up of 69% of BFS plus FA was recorded because of the supplement of 1% nano-CaCO3 by more or less 18% and 21% correspondingly than their controlled HVSC and HVSFAC subsequent to 28 days of curing and demonstrated inferior permeable voids to the OPC
concrete and the fall was greater in curing of 90 days.

9.3.12 Drying shrinkage Investigations carried out by Hosan and Shaikh (2021a,b) for influence of add-on of 1% nano-CaCO3 on drying shrinkage of HVSC and HVFAC showed a remarkable

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lowering in the early drying shrinkage value and has also held the shrinkage strain well below in later-on ages of HVSFAC than the controlled OPC
concrete. On the other hand, the strain diminution gap among the controlled OPC
concrete and HVSC produced with 60% BFS and 1% nanoparticles of CaCO3 have been slimmed down in later ages. Also, Hosan and Shaikh (2021a,b) recorded low drying shrinkage in both the cases of HVSC and HVSFAC when a supplement to them is made of nano-SiO2 and the drying shrinkage of HVSFAC is found conspicuously inferior to that of OPC-concrete in the later-on ages.

9.3.13 Chloride diffusion Shaikh and Supit (2014) explored the influence of addition of 1% nano-CaCO3 particles into cement
concretes on the chloride diffusion and found that the coefficient of chloride diffusion has declined by approximately 73%. The chloride diffusion coefficients of the HVFACs amalgamating 1% nano-CaCO3 particles were recorded around 60% and 32% lesser than the concretes enclosing 40% and 60% FA, correspondingly. This is the clear-cut indication that the reactivity and filler impact of nano-CaCO3 particles proved very efficient in plummeting the pore space and its connectivity within the concrete and consequently, the result is lesser penetration of chloride ions. Further, Shaikh and Supit (2015a,b) revealed that the chloride diffusion of concretes incorporating nano-SiO2 and nano-CaCO3 have been found decreased by 26% and 15%, in that order, when compared to the sample of cement-concrete subsequent to the exposure of 60 days. However, the chloride diffusion coefficients of the HVFACs enclosing 2% nano-SiO2 were found roughly 21% and 14% inferior to the concrete manufactured with 40% and 60% FA. On the basis of upshots on the chloride diffusion coefficient, the application of nano-SiO2 is more effectual than the nano-CaCO3 in narrowing down the pore space and its connectivity within the concrete resulting in a lesser amount infiltration of chloride ions. On the other hand, Shaikh and Supit (2015a,b) found that the chloride diffusion coefficient of cement concrete is also diminished by more or less 69% on account of the supplement of 8% UFFA. The chloride diffusion coefficient of the HVFACs blending 8% UFFA were found something like 50% and 25% inferior to the concrete with 40% and 60% FA, respectively.

9.3.14 Corrosion Experimentally, the topic of impact of addition of nanoparticles into concretes is tested by Shaikh and Supit (2015a,b), and they found display of superior corrosion resistance by the concretes enclosing nano-SiO2 and nano-CaCO3 in terms of slighter measured corrosion currents with regard to time when the comparison made with the cement
concrete. Also, the resistance of concretes against the chlorideinduced corrosion was found enhanced owing to the addition of nano-SiO2 and nano-CaCO3 to HVFACs. The weight of rebar corrosion of cement
concrete has lost in the region of 29.67%, whilst in nano-SiO2 and nano-CaCO3 incorporated concretes the steel loss is more or less 18.36% and 7.32%, respectively. In plain

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words, although the loss of steel occurred when corrosion is introduced but the charisma of nanoparticles of SiO2 and CaCO3 in the cement-concretes and HVFACs subordinated the corrosion current and enhanced the period to cracking on account of the enhancement in context of resistance against the chloride permeability and chloride diffusivity. Furthermore, Shaikh and Supit (2015a,b) reported that the concrete incorporating UFFA has also put on show improved resistance against corrosion in terms of smaller measured corrosion currents with regard to time in comparison with cement-concrete. The UFFA
concrete took beyond 1000 h for the corrosion cracks to come into view. Analogous outcomes are also monitored in the context of concrete manufactured using 32% FA and 8% UFFA. The mass loss of rebar resulted on account of the corrosion in cement-concrete is around 29.67% whereas in UFFA-concrete the loss is found in the vicinity of 19.53%. The supplement of 8% UFFA also exhibited healthier resistance against corrosion in terms of nearly 29% and 8% cutback in mass loss in HVFACs possessing 40% and 60% FA, in that order.

9.3.15 Microstructure In context of the microstructural impact of addition of nanoparticles, the mercury intrusion porosimetry (MIP) analysis findings based on the experiments carried out by Shaikh and Supit (2014) made known that the existence of 1wt.% nano-CaCO3 particles in ordinary concrete and HVFACs have lowered the total capillary porosities and pores fine-tuning. Their outcomes on XRD analysis demonstrated that the nano-CaCO3 substitution of cement is effectual for tumbling the calcium hydroxide (CH) and CS in HVFA-pastes; thus the development of supplementary C
S
H gels occurs. The novel peaks of Ettringite are also seen in HVFA-pastes owing to the supplement of 1% nano-CaCO3. The TGA upshots have also corroborated the XRD results exhibiting the reactivity of 1% particles nano-CaCO3 with HVFA in trimming down the CH-content. Additionally, the MIP and DTA/TGA analyses conducted by Shaikh and Supit (2015a) verified that the integration of 2% nanoSiO2 and 1% nano-CaCO3 slimmed down not only the CH quantity but also the total capillary porosity and pores diameter that can put a stop to the detrimental transfer of chloride ions escorting to corrosion of rebar and deterioration to the ordinary concrete and HVFACs. The BSE image analyses by Shaikh, Supit, et al. (2014) demonstrated that the supplement of 2% nano-SiO2 appreciably enhances the microstructure of the matrix of HVFA
mortars. Also, the outcomes of XRD authenticated the result whereby the 2% nano-SiO2 supplement trims down the CH by roughly 58% and 50% in HVFA-mortars enclosing 40% and 60% FA, respectively. The MIP findings of Supit and Shaikh (2015) confirmed a denser microstructure of the paste of nano-SiO2 in harmony with the observation attained from SEM analysis. The application of nano-SiO2 led to a more compact paste with a momentous diminution of the pores among 0.1 and 10 μm. When a comparison is made with the cement paste, the cumulative pore volume of nano-SiO2 paste is found nearly 25% lower. In the context of HVFA pastes with 2% supplement of nanoSiO2, there found a remarkable decline in cumulative pore volumes signifying the

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existence of nano-SiO2, which is profitable for pore modification. The conclusion was made that nano-SiO2 has a momentous influence in declining the total capillary pores and pores diameter of HVFA pastes. The findings of XRD analysis have evidently indicated that the presence of nano-SiO2 in HVFA system offers to the faster pozzolanic reaction than fly ash because of its tremendously finer particle size and larger surface area than class-F fly ash. Owing to the enormously larger surface area and finer particle size, the nano-SiO2 particles react more swiftly with free lime in the hydration reaction than fly ash and produced secondary C
S
H gel and plugged the microvoids in the matrix. In agreement with the BSE image analyses of Supit et al. (2014), the integration of 8% UFFA, as a partly substitution of cement in HVFA pastes created a denser microstructure directing to the augment in compressive strength. It is put forward that the influence of particle packing of UFFA performs a central role in plunging the volume of pores. The XRD analysis leads to report that the UFFA substitution of cement is competent enough to escalate the level of CH consumption of HVFA pastes and thus the supplementary CSH gels produces. It is ascribed to the higher volume of amorphous SiO2 in UFFA. Shaikh and Supit (2015a, 2015b) conducted the MIP analysis the results of which exhibited that the supplement of 8wt.% UFFA into ordinary concrete and HVFACs has diminished the total capillary porosities and shifted the pore distribution towards finer pore sizes. From the phase identification and microstructural point of view, the MIP and DTA/TGA analysis validates that the amalgamation of 8% UFFA abridged the CH amount and declines the total capillary porosities and pores diameter which can put off the destructive chloride ions for transportation escorting to corrosion of rebar and deterioration to the ordinary concrete and HVFAconcretes. The SEM images by Shaikh and Hosan (2019a,b) demonstrated denser microstructure of High-volume BFS and combined BFS
fly ash pastes enclosing nano-Al2O3 than those with that of having an absence of nano-Al2O3. Also, the EDS analysis displays an augment in “Si” and “Al” peaks in EDS spectra around slag particles of pastes of high-volume BFS and combined BFS
fly ash because of supplement of nano-Al2O3 representing formulation of calcium silicate hydrate (CSH)/calcium aluminate hydrate in the matrix. According to Hosan and Shaikh (2020) there found no noteworthy modifications in CH peaks on account of the nano-SiO2 supplement in HVS and HVSFA pastes in XRD analysis. Nevertheless, the enhanced intensity of Ettringite (E), CSH, CAH, and CaCO3were found because of the add-on of nano-CaCO3 in all HVS and HVSFA pastes. The amalgamation of nano-SiO2 resulted in the densification of the internal microstructure in comparison with the reference pastes of HVS and HVSFA with no addition of nano-SiO2 as unveiled in images of SEM and EDS trace analyses. The TGA upshots obtained by Shaikh and Hosan (2019a,b) confirmed the fall of CH in HVS/HVSFA-pastes incorporating nano-SiO2 and thus indicated the development of supplementary CSH gels in the system. The nano-silica supplement is found to reduce the intensity of CH peaks corresponding to 2θ angles of 18.04, 34.11, and 51 degrees of HVS and HVSFA pastes, which indicates the consumption of CH in a reaction of pozzolanic kind. The SEM images and EDS spectra of HVS and HVSFA pastes possessing nano-SiO2 revealed much denser microstructure than those with an absence

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of nano-SiO2. The microstructural investigations of Hosan and Shaikh (2021a,b) long-established a more elevated degree of compacted ITZ of HVS and HVSFAC because of the add-on of 1% nano-SiO2 than the concrete sans nano-SiO2 and owned an identical or even better interfacial bond among aggregates and binder paste than conventional concrete. The XRD analysis conducted by Shaikh and Supit (2016) corroborates the formulation of more C
S
H and Ettringite yields in the course of hydration reaction of pastes enclosing nano-SiO2 and nano-CaCO3 dispersed in solution of polycarboxylate ether (PCE)-based superplasticizer than in other mixing techniques. The XRD findings of Shaikh et al. (2017) unveiled that the supplement of 2% nano-SiO2 plus 1% nano-CaCO3 particles in HVFA
cement paste has amplified the consumption of CH and for this reason the C
S
H gel development takes place. Also, the DTA/TGA validate the outcomes of the XRD, confirming the influences of the reactivity of nano-SiO2 and nano-CaCO3 in narrowing the quantity of CH. The MIP analysis upshots substantiate the competence of nano-SiO2 and nano-CaCO3 to trim down the total volume of capillary pores and pore diameter of HVFA-cement paste. The results bear out a denser microstructure of the HVFA-cement paste with the supplement of 2% nano-SiO2 and 1% nano-CaCO3. The enhanced microstructure of HVFA-pastes was attributed to the presence of nano-SiO2 and nano-CaCO3 particles experiential through nanostructural and microstructural analyses and confirmed that the mechanical and durability attributes of sustainable HVFACs can be enhanced, which will contribute appreciably to the infrastructure sustainability. One more research study of Assaedi et al. (2019) was carried out on SEM micrographs of PVA fiber-reinforced GP composite enclosing 1.0 and 2.0wt.% of nano-SiO2 which demonstrated the denser microstructure of the GP matrix than the one possessing 3.0wt.% nano-SiO2. In summary, all the accessible studies lead to conclude that the addition of nanoparticles to diverse concretes plays an important role for making the microstructure much denser.

9.4

Conclusions and discussion

Nowadays, new-fangled developments in nanoengineering and nanoscience piloted to the nano-modification of cementitious green composites. The said incorporation of nanomaterials like nano-SiO2; nano-Al2O3; nano-TiO2; CNF; CNTs; nanokaolin; nano-clay; nano-CaCO3; synthetic nano-fibers; GO; GSNS; nano-SiO2 carbide; nano-MgO; etc., has the potential to bring about a breakthrough in building material science through their outstanding and unique chemical and physical characteristics such as microdimensions; impeccably ordered presence of the atoms; the capability to get dispersed homogeneously in the binder paste; the superb acting as a filler; enhance packing model structure; refining the intersectional zone in cement; enhanced the electromagnetic interference; acceleration in the hardening progression; owning water adsorption effect; enhanced residual compressive strength and better residual flexural and tensile strengths; augmentation to the split tensile

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strength; improved durability criteria, viz., lending a hand to speed up the pozzolanic reaction, capable to control AARs and ASR episode, an elevated resistances to freeze-thaw conditions, chloride infiltration, the abrasion, sulfate attack, acid attack, and thermal degradation; decrease in the shrinkage, the permeability, the loss of water via the pore-refining impact, and hydrophilicity; escalation in the resistance against crack initiation; off-putting the propagation of cracks; lower down the ratio for water-to-cementitious materials; densification impact on the microstructure; reduced porosity; etc., proving a fantastic route for remarkable improvement in the essential properties like strength, durability, microstructure, thermal, shrinkage, freeze
thaw, abrasion or erosion, as well as chemical deteriorations, viz., AARs, sulfate and acid attacks of resultant novel nano-modified cementitious composites. This modification will boost the efficiency building’s utilized resources—energy, H2O, and materials—while mitigating building impacts on the environment and human health through superior design, construction, and removal. Notably, this strategy is not only technically viable but also strikingly cost-effective. The recent advancements in the context of nano-modification of green cementitious composites generally by using nanomaterials as “performance enhancer” have turned out to be the center of interest for the construction industry and concrete research community in the interest to promote them with ease. Also, this concept may find its way in civil structures exposed to complex dynamic loads as an attractive tool considering sound in both ways—technically and economically. This groundbreaking amalgamation of nano and cement
concrete fields will make it viable to design sustainable edifice materials for their specific applications. However, still, a few challenges are standing in its path of complete promotion such as physical state and proper dispersion into cementitious composites as well as compatibility of nanomaterials in cement, lack of advanced researches on diverse nanomaterials for different essential properties together with mathematical modeling of resulting various green cementitious composites modified with an assortment of nanomaterials, etc., which must be resolved through advanced studies on the subject. Moreover, an introduction of these pioneering most modern composites should also be brought to light in the context of an evaluation and understanding concerning their impact on the environment and health of lives on the planet earth, which is also of key importance prior to accepting them fully. However, their distinguishing characteristics such as elevated compressive strength, straightforward course for the manufacturing, low production cost, expediency to use, denser microstructures, the enhanced durability, and multi-functionality have altogether attributed them a grand center of attention by the industries as well as global concrete researchers considering them as most promising building materials potentially. Thus the application of nanoengineering and nanoscience in concrete technology seems to be unstoppable revolutionary step proving useful to construction and infrastructure industries as future tools. This present state of application of the novel innovative field of nanotechnology with cement-based concrete technology has opened up extensively for new-brand investigations on these avant-garde green building composites. This is a step further with the concept “Go Green, Live Green”!

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3041. Available from https://doi.org/10.1016/j.proeng.2011.07.382. Siahkouhi, M., Razaqpour, G., Hoult, N. A., Baghban, M. H., & Jing, G. (2021). Utilization of carbon nanotubes (CNTs) in concrete for structural health monitoring (SHM) purposes: A review. Construction and Building Materials, 309, 125137. Siddique, R., Kapoor, K., Kadri, E. H., & Bennacer, R. (2012). Effect of polyester fibres on the compressive strength and abrasion resistance of HVFA concrete. Construction and Building Materials, 29, 270
278. Sikora, P., Abd Elrahman, M., Chung, S. Y., Cendrowski, K., Mijowska, E., & Stephan, D. (2019). Mechanical and microstructural properties of cement pastes containing carbon nano-tubes and carbon nanotube-silica core-shell structures, exposed to elevated temperature. Cement and Concrete Composites, 95, 193
204. Available from https://doi.org/ 10.1016/j.cemconcomp.2018.11.006. Singh, L. P., Karade, S. R., Bhattacharyya, S. K., Yousuf, M. M., & Ahalawat, S. (2013). Beneficial role of nanosilica in cement based materials—A review. Construction and Building Materials, 47, 1069
1077. Available from https://doi.org/10.1016/j.conbuildmat.2013.05.052. Sobolev, K., Lin, Z., Flores-Vivian, I., & Pradoto, R. (2016). Nano-engineered cements with enhanced mechanical performance. Journal of the American Ceramic Society, 99(2), 564
572. Available from https://doi.org/10.1111/jace.13819. Stynoski, P., Mondal, P., Wotring, E., & Marsh, C. (1996). Characterization of silica-functionalized carbon nano-tubes dispersed in water. Journal of Nanoparticle Research, 15. Supit, S. W. M., & Shaikh, F. U. A. (2015). Durability properties of high volume fly ash concrete containing nano-silica. Materials and Structures/Materiaux et Constructions, 48 (8), 2431
2445. Available from https://doi.org/10.1617/s11527-014-0329-0. Supit, S. W. M., Shaikh, F. U. A., & Sarker, P. K. (2014). Effect of ultrafine fly ash on mechanical properties of high volume fly ash mortar. Construction and Building Materials, 51, 278
286. Available from https://doi.org/10.1016/j.conbuildmat. 2013.11.002. Tang, S. W., Yao, Y., Andrade, C., & Li, Z. J. (2015). Recent durability studies on concrete structure. Cement and Concrete Research, 78, 143
154. Available from https://doi.org/ 10.1016/j.cemconres.2015.05.021. Taniguchi, N., Arakawa, C., & Kobayashi, T. (1974). On the basic concept of nanotechnology. In Proceedings of the international conference on production engineering (pp. 26
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388). Springer Science and Business Media LLC. ,https://doi.org/10.1007/978-3-642-00980-8_52.. Wen, S., & Chung, D. D. L. (2007). Partial replacement of carbon fiber by carbon black in multifunctional cement-matrix composites. Carbon, 45(3), 505
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246). Elsevier Inc. ,https://doi.org/ 10.1016/B978-1-78242-326-3.00010-5.. Yang, L. Y., Jia, Z. J., Zhang, Y. M., & Dai, J. G. (2015). Effects of nano-TiO2 on strength, shrinkage and microstructure of alkali activated slag pastes. Cement and Concrete Composites, 57, 1
7. Available from https://doi.org/10.1016/j.cemconcomp. 2014.11.009. Yazdani, N., & Mohanam, V. (2014). Carbon nano-tube and nano-fiber in cement mortar: Effect of dosage rate and water-cement ratio. International Journal of Material Sciences, 45. Available from https://doi.org/10.14355/ijmsci.2014.0402.01. Zhang, L., Han, B., Ding, S., Yu, X., Sun, S., & Ou, J. (2016). Nano-scale behaviour and nano-modification of cement and concrete materials. In Advanced research on nanotechnology for civil engineering applications (pp. 28
79). Zhang, L., Ma, N., Wang, Y., Han, B., Cui, X., Yu, X., & Ou, J. (2016). Study on the reinforcing mechanisms of nano silica to cement-based materials with theoretical calculation and experimental evidence. Journal of Composite Materials, 1
12. Zuo, J., Yao, W., Liu, X., & Qin, J. (2012). Sensing properties of carbon nanotube
carbon fiber/cement nanocomposites. Journal of Testing and Evaluation, 40(5), 20120092. Available from https://doi.org/10.1520/jte20120092.

Nano-modified geopolymer and alkali-activated systems

10

Partha Sarathi Deb, Jhutan Chandra Kuri, and Prabir Kumar Sarker School of Civil and Mechanical Engineering, Curtin University, Perth, WA, Australia

10.1

Introduction

The development of an alternative binder to cement can play an important role in the sustainability of concrete. Geopolymer binders can provide a comparable performance to Portland cement with potential for reduction of greenhouse gas emissions (Duxson et al., 2007). The particular precursors to be used depends on the potential for reaction, availability, and cost (Rangan, 2008). Interests on geopolymers have considerably increased in the recent time due to its benefits such as low carbon emission, less processing of the raw materials (Duxson et al., 2007), and favorable strength and structural behaviors (Deb et al., 2014; Khale & Chaudhary, 2007). In the geopolymerization process, a considerable amount of amorphous aluminosilicate phase is transformed into a compact binder by hydrothermal polycondensation (Chen-Tan et al., 2009; Fernandezjimenez et al., 2006). Geopolymer may be considered as an amorphous analog of zeolite or zeolitic precursor (Pal et al., 2003; Partha et al., 2013) that forms in a similar process of zeolites (Davidovits, 2008; Van Jaarsveld et al., 1998). Davidovits (2008) described the geopolymer structure as Si
O
Al
O (polysialate), Si
O
Al
O
Si
O (sialate
siloxo), and Si
O
Al
O
Si
O
Si
O (sialate
disiloxo). The properties of geopolymer depend primarily on the ratio of Si/Al, Na/Al, and the water content (Rangan, 2008). Hajimohammadi et al. (2011) observed analcime formation in the systems with low Si availability, while faujasite development was reported at an intermediate or high Si availability. Duxson et al. (2005) observed a correlation between the microstructure and Si/Al ratio. Specimens with Si/Al # 1.40 showed a relatively porous microstructure. Geopolymers with Si/Al ratio $ 1.65 are considered as more homogeneous with some smaller isolated pores. The effect of Na/Al ratio on geopolymers was studied by several authors (Belkowitz et al., 2015; Rees et al., 2007; Steveson & Sagoe-Crentsil, 2005; Yip & Van Deventer, 2003). Rowles and O’Connor (2003) found that compressive strength increased with the increasing Na/Al molar ratio up to a specific value, above which there was a decline of strength. Xu and Van Deventer (2000) showed that an increase of sodium content promoted high dissolution rates of silica (SiO2) and alumina (Al2O3). Ferna´ndez-Jime´nez and Palomo (2005) stated that the OH2 ion in the alkali activator involves in the dissolution of Si41 and Al31 in fly ash, whereas the Na1 ion involves in the crystallization. Recent Advances in Nano-Tailored Multi-Functional Cementitious Composites. DOI: https://doi.org/10.1016/B978-0-323-85229-6.00001-9 © 2022 Elsevier Ltd. All rights reserved.

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Studies on the use of nanoparticles in OPC binders are widely reported in literature. It is generally reported that the microstructure can be enhanced with the modification of the reaction product by nanoparticles. However, the quantitative evaluation of the different phases and their relationships with the mechanical and physical properties of a product is important to understand the behavior of a binder.

10.2

Use of nanomaterials in cementitious binders

The field of nanoparticles is rapidly maturing into fertile and interdisciplinary research areas from which new multi-functional and smart materials can be developed. Chemical stability, diverse mechanical properties, and high strength of the nano-modified binders have placed them as important materials in the rapidly growing field of nanotechnology. Application of nanoparticles in different types of cementitious binders is receiving ongoing research attention in concrete technology. A good level of understanding exists on the fundamental principles of these materials. Researchers (Sanchez & Sobolev, 2010; Sobolev & Gutie´rrez, 2005; Trtik & Bartos, 2001) have pointed out that using different nanocomposites into conventional building materials can result in enhanced properties required for civil infrastructures. Cementitious binders containing different nanoparticles such as nanoSiO2, nano-Al2O3, carbon nano-tubes, and nano-clay were used to improve the microstructure, rate of hydration, pore structure, strength, and protective barrier in concrete. Among these nanoparticles, nano-SiO2 has been widely used in cement composites owing to its high amorphous SiO2 and high surface area. Nano-SiO2 with particle sizes of 1
50 nm provides more atoms on the surface of each particle because of the high surface area. This factor alters the chemical reactivity of nano-SiO2 with enhanced mechanical properties of cementitious binders (Sanchez & Sobolev, 2010). Though it was shown by researchers that a small amount of nano-SiO2 can provide increase of strength and a denser microstructure, the performance is dependent on its morphology and uniform dispersion of the particles in the mixture (Deb et al., 2014; Khale & Chaudhary, 2007). Dolado et al. (2007) reported that compressive strength of cementitious binders increased with the increase of nano-SiO2 content within the range of 0.2% to 12% by mass of cement. Gaitero et al. (2008) and Jain and Neithalath (2009) reported that the use of nano-SiO2 with cement binders modified the micro- and nano-structure of hydrated paste with an improvement in the chemical stability against aggressive environment. It is well established that the final properties of a hydrated cement composite with nanoparticles relies on the dispersion of nanoparticles. Kawashima et al. (2014) reported that because of the high surface area and energy of nanoparticles, the adhesion between two particles is significantly controlled by van der Waals, electrostatic, and magnetic forces. Jiang et al. (2009) also observed that when nanoparticles are dispersed in a liquid, their hydrodynamic size is usually larger than

Nano-modified geopolymer and alkali-activated systems

349

their primary size and they tend to remain as agglomerates by an electrical double layer, which includes both the inner and outer layers of nuclei. However, previous studies (Chen et al., 2007; Ding & Pacek, 2008; Iler, 1979) showed that nano-SiO2 can be well dispersed in liquid media. Wu et al. (2016) stated that the selfagglomeration of nano-SiO2 in liquid media can be reduced with the single dispersion of nano-SiO2 by increasing the thiolene click reaction on the particle’s surface. Moreover, the hydrophilic nature of silane can also help workability and dispersion of admixtures in the mix (Xu & Van Deventer, 2000). It has been also reported that proper dispersion of nanomaterials accelerates the hydration process of cementitious binders. Vandenberg and Wille (2015) found that proper dispersion of nanoparticles can improve the particle packing, which is important for the enhancement of properties of a composite material. Wu et al. (2016) found that the high specific surface of nano-SiO2 provided a highly reactive siliceous medium in an alkaline environment. The highly reactive SiO2 helps to produce more alkali aluminosilicate gel that increases the strength of the product (Qing et al., 2007). Rees et al. (2007) reported that beyond the maximum specific value of Na/Al ratio, further increase in NaOH reduced the rate of geopolymer reaction.

10.3

Properties of geopolymers and alkali-activated systems incorporating nanomaterials

10.3.1 Fresh properties The fresh properties of nano-modified alkali-activated systems that affect the constructability of the composites are expressed by setting time, flowability, and workability. A workable geopolymer mixture indicates the ease of mixing, placing, and compacting. A reasonable setting time is important to allow for the required haul distance of the fresh mixture and time for transportation, placing, compacting, and finishing. Ambient cured low-calcium fly ash
based geopolymer may have a long time due to its slow reaction (Deb et al., 2015; Nath & Sarker, 2014, 2015). It was shown that the addition of nanomaterials can have a small effect on the setting time of neat fly ash
based geopolymers compared with OPC and GGBFS-blended fly ash geopolymers. Addition of nano-SiO2 to fly ash blended with a calcium rich material decreased setting time of geopolymers by the formation of calcium silicate hydrate (CSH) (Deb et al., 2015). Table 10.1 shows the effects of 1% to 3% nanoSiO2 on the setting times of the fly ash (FA-NS), 15% GGBFS-blended fly ash (FA-S-NS), and 10% OPC-blended fly ash (FA-PC-NS) (Deb et al., 2014; Khale & Chaudhary, 2007). A mixture of sodium hydroxide and sodium silicate solutions was used as the alkali activator. It can be seen that the initial setting time of the fly ash
only geopolymer with 2% nano-SiO2 was 700 minutes, which reduced to 51 minutes for the GGBFS-blended fly ash geopolymer with 2% nano-SiO2. The corresponding final setting times of these mixtures were noted as 1100 and

Table 10.1 Setting times of geopolymer pastes with different nano-SiO2 contents (Deb, 2018). Fly ash Mix id

FANS0


Initial (min) Final (min)


Slag

OPC

FANS1 775

FANS2 700

FANS3 650

FA-SNS0 71

FA-SNS1 62

FA-SNS2 51

FA-SNS3 35

FA-PCNS0 40

FA-PCNS1 34

FA-PCNS2 25

FA-PCNS3 18

1175

1100

950

320

245

180

150

80

62

52

39

Nano-modified geopolymer and alkali-activated systems

351

180 minutes, respectively with 2% nano-SiO2. It can also be noticed that the initial and final setting times of the OPC-blended fly ash mixtures were very low as compared to those of the other mixtures. Overall, the setting time data of Table 10.1 show that the blending of low-calcium fly ash with a calcium-bearing material had a large effect on the reduction of initial and final setting times of the alkali activated binders. The use of nano-SiO2 resulted in further reductions of the setting times in each type of mixture. Table 10.2 shows the flowability of fresh nano-SiO2 incorporated fly ash
based geopolymer mortars in the ASTM C1437
14 (ASTM, 2014) flow tests. It can be seen from the table that the flow behavior of fly ash
based geopolymer mortar is directly influenced by the amount of nano-SiO2 in the mixtures. It is noteworthy that the increase in nano-SiO2 content in the geopolymer mix inevitably increases the cohesiveness and liquid demand of the mixture. As seen from Table 10.2, the flow of fly ash
only geopolymer mortar reduced from 135% to 107% with the use of 3% nano-SiO2 in the mix. This can be mainly attributed to the higher specific surface of nano-SiO2 than that of the fly ash, which implies a higher consumption of alkaline liquid to maintain the same level of workability of an equivalent mix. The increase in nano-SiO2 content showed similar effect on the flowability of OPC- and GGBFS-blended fly ash geopolymer mortars. It is noted from Table 10.2 that with the increase in nano-SiO2 content from 0% to 3%, the slump flow of OPC- and GGBFS-blended fly ash geopolymer mortars decreased by 30% and 34%, respectively. A similar reduction of workability was also observed by incorporating other nanoparticles, such as nano-TiO2 (Duan et al., 2016) and nano-clay (Joshi et al., 2015; Li & Shi, 2020). Incorporating 5% nano-TiO2 in fly ash
based geopolymer reduced the flow value from 215 to 148 mm compared to that of the control mix. Duan et al. (2016) observed reduction of flow by the addition of nano-TiO2 (Fig. 10.1) which is attributed to the van der Waals forces between the fine particles. The agglomeration of particles may reduce the flow. Gu¨l¸san et al. (2019) reported that the addition of nano-SiO2 in self-compacting geopolymer concrete improved the resistance to segregation and bleeding. The mixtures containing nano-SiO2 were found to be more cohesive than those without nano-SiO2. The slump test results of geopolymer concrete mixtures without and with nanoSiO2 are given in Table 10.3. The complete mix proportions of these mixtures can be found in reference (Deb, 2018). It can be seen that slump decreased with the increase in nano-SiO2 content in all the mixture series. This is consistent with the behavior observed in the mortar mixtures of the same geopolymer binders. It is observed from Table 10.3 that fly ash
only geopolymer concrete mixed with higher nano-SiO2 showed similar behavior that was seen in the flow of mortar mixtures. The decrease in workability in the mixes of all three series by the increase in nano-SiO2 can be attributed to the increase of liquid demand by its high specific surface. Similar trend in the workability of geopolymers by using 1%
3% nano-

Table 10.2 Flow of geopolymer mortars with different nano-SiO2 contents (Deb, 2018). Fly ash Mix id Flow (%)

FANS0 135

FANS1 131

FANS2 123

Slag FANS3 107

FA-SNS0 98

FA-SNS1 89

FA-SNS2 77

OPC FA-SNS3 64

FA-PCNS0 80

FA-PCNS1 74

FA-PCNS2 65

FA-PCNS3 50

Nano-modified geopolymer and alkali-activated systems

353

250%

Flow (%)

200%

150%

100%

50%

0% 0.0

1.0

2.0 3.0 Nano - TiO2 (%)

4.0

5.0

Figure 10.1 Flowability of fresh geopolymer mortars with different nano-TiO2 dosage (Duan et al., 2016).

SiO2 was also reported by Luo et al. (2017). Gao et al. (2015) reported less workability of fly ash-GGBFS geopolymers using 1%
3% nano-SiO2. The workability behavior of geopolymer mixes with calcium-rich materials changed significantly with the addition of nano-SiO2. Nath and Sarker (2014, 2015) and Provis et al. (2005) reported decrease of the flow of fly ash geopolymer mortars with the increase of calcium-bearing precursors. Gao et al. (2015) noted that a lower amount of slag yielded a higher slump because of its different morphology. It can be seen from Table 10.3 that slump of geopolymer concrete gradually decreased with the increase of nano-SiO2 for GGBFS- and OPC-blended fly ash geopolymer concretes. As the nano-SiO2 dosage increased from 0% to 3% in the GGBFS-blended fly ash geopolymer concrete, the slump reduced from 190 to 155 mm. Similarly, slump decreased from 180 to 146 mm by 3% nano-SiO2 in the OPC-blended fly ash geopolymer concrete. The GGBFS- and OPC-blended fly ash geopolymer concretes containing 3% nano-SiO2 showed low workability with no seggregation or bleeding.

10.3.2 Mechanical properties 10.3.2.1 Compressive strength of geopolymer concrete containing nano-SiO2 Compressive strengths of the fly ash geopolymer concretes with 0%
3% nanoSiO2 at different ages are given in Table 10.4.

Table 10.3 Slump values of geopolymer concrete mixes with different nano-SiO2 contents (Deb, 2018). Fly ash Mix id Slump (mm)

FAFAFAFAFA-SNS0 NS1 NS2 NS3 NS0 220 210 190 165 190

Slag FA-SNS1 185

FA-SNS2 165

OPC FA-SNS3 155

FA-PCNS0 180

FA-PCNS1 168

FA-PCNS2 152

FA-PCNS3 146

Nano-modified geopolymer and alkali-activated systems

355

Table 10.4 Compressive strengths of fly ash geopolymer concrete containing different percentages of nano-SiO2 (Deb, 2018). Nano-SiO2 (%)

0 1 2 3

Compressive strength (MPa) 7 days

28 days

9.2 18.9 25.2 22.1

17.5 32.1 45.2 37.1

It can be seen from Table 10.4 that the nano-SiO2 addition at a rate of 2% maximised the strength development. The strength enhancement declined when the nano-SiO2 dosage was 3%. The compressive strength of the mixture with 2% nanoSiO2 reached 45.2 MPa at 28 days, which is significantly higher than that of the control mix (17.5 MPa). This shows that strength of the binder increased with the increase of SiO2 content. Though strength development declined for nano-SiO2 content beyond 2%, the strengths of the mixture with 3% nano-SiO2 at different ages were higher than those of the mix without nano-SiO2. The decline in strength for nano-SiO2 content above 2% is attributed to the presence of additional nanoparticles than that was required for the reaction. This means that the available nanoSiO2 in mix FA-NS2 was enough to refine the pore structure and increase strength. Presence of nano-SiO2 beyond this limit remained unreacted and did not contribute to further increase of strength. This suggests that there is a relationship between the amount of nanomaterial and strength development of fly ash
based geopolymer. The strength increase of geopolymers by nano-SiO2 is related to its dissolution rate. Temuujin et al. (2009) observed an increased dissolution rate of the source materials by increasing the fineness. Xu and Van Deventer (2002) concluded that higher dissolution rate of the source material leads to the development of higher compressive strength in geopolymers. It has also been previously observed by Ferna´ndez-Jime´nez and Palomo (2003) that higher amount of aluminosilicate gel can be generated in fly ash
based geopolymers with the addition of highly reactive SiO2 in the mixtures, eventually giving higher strength. Similar behavior was also observed by other researchers (Criado et al., 2007). It was shown that soluble silicates increased the rate of reaction increasing strength (Xu and Van Deventer, 2000). Ferna´ndez-Jime´nez and Palomo (2003) showed that the soluble silicon concentration is an important parameter affecting the geopolymerization process. It appears from Table 10.4 that 2% nano-SiO2 in the fly ash geopolymer concrete reacted almost entirely, and further increase of nano-SiO2 was not effective to increase strength any more. It is believed that the unreacted particles caused selfdesiccation and cracking that reduced the strength, as described by Hajimohammadi et al. (2011). Khale and Chaudhary (2007) noted that the presence of highly reactive silica in the mixtures generated higher alkali aluminosilicate gel giving higher mechanical strengths.

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10.3.2.2 OPC- and GGBFS-blended fly ash geopolymer concrete containing nanomaterials Previous research showed rapid setting of geopolymers using high amounts of OPC ( . 12%) (Nath & Sarker, 2015) and GGBFS ( . 20%) (Deb et al., 2014) with fly ash. For this reason the percentages of OPC and GGBFS were used at 10% and 15%, respectively to avoid the occurence of fast setting. The compressive strengths of GGBFS- and OPC-blended fly ash geopolymer concretes with addition of 0%
3% nano-SiO2 are presented in Tables 10.5 and 10.6, respectively. It can be seen from Table 10.5 that the 28 days compressive strength of the GGBFS-fly ash mixture containing 2% nano-SiO2 reached 37.5 MPa as compared to 15.8 MPa for the control mix. Similar behavior was also observed for OPCblended fly ash
based geopolymer concrete as shown in Table 10.6. Tailby and MacKenzie (2010) and Suwan and Fan (2014) claimed that the use of OPC with fly ash can generate an additional heat, which can further promote the geopolymer reaction. The addition of nano-SiO2 in OPC-blended series might further enhance its geopolymerization process. However, the compressive strength for 3% nanoSiO2 was less than the strength for 2% nano-SiO2. Belkowitz et al. (2015) noted that the unreacted nano-SiO2 caused an excessive self-desiccation and cracking that could reduce the strength. Therefore, the lower strength of the mix with 3% nanoSiO2 can be attributed to the presence of excessive unreacted particles. This means Table 10.5 Compressive strengths of GGBFS-blended fly ash geopolymer concrete containing nano-SiO2 (Deb, 2018). Nano-SiO2 (%)

0 1 2 3

Compressive strength (MPa) 7 days

28 days

7.10 15.20 23.80 16.20

15.80 24.20 37.50 29.20

Table 10.6 Compressive strengths of OPC-blended fly ash geopolymer concrete containing nano-SiO2 (Deb, 2018). Nano-SiO2 (%)

0 1 2 3

Compressive strength (MPa) 7 days

28 days

7.5 16.5 21.5 17.8

15.8 21.5 38.5 25.6

Nano-modified geopolymer and alkali-activated systems

357

that the available nano-SiO2 in mix FA-S-NS2 was enough to enhance the pore structure and increase strength. The presence of extra nano-SiO2 remained unreacted and did not help further increase of strength. Yip and Van Deventer (2003) observed that the presence of calcium increased the strength of geopolymer by the formation of Ca
Al
Si amorphous product. Similar behavior in the enhancement of compressive strength was also observed for other nanoparticles. Zidi et al. (2021) reported that geopolymers synthesized with 5% nano-SiO2 exhibited higher compressive strengths at 20 C and 80 C as compared to the control geopolymer (Fig. 10.2). It is noted from Fig. 10.3 that the use of nano-clay resulted in an enhancement of geopolymer microstructure leading to the increase of compressive strength for the nano-clay dosage of up to 1%. Any further increase in the dosage of nano-clay showed a negative effect on the compressive strength (Khater, 2013). The strength development of the composites was observed until 90 days of age.

Figure 10.2 Compressive strengths of geopolymers with 0% and 5% nano-SiO2 at different curing conditions (Zidi et al., 2021).

Figure 10.3 Compressive strengths of alkali activated slag specimens having various dosages of nano-clay (Khater, 2013).

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Zidi et al. (2019) reported that the use of nano-Al2O3 to geopolymer samples offers more nucleation sites for N
A
S
H gel and resulted in enhancement of the geopolymerization process for nano-Al2O3 content up to 2%. A decline in the strength was observed beyond the nano-Al2O3 dosage of 2%, as shown in Fig. 10.4. However, the compressive strength of the mixture with 3% nano-Al2O3 was still higher than that of the control sample. Assaedi et al. (2020) observed that the optimum content of nano-CaCO3 in fly ash
based geopolymer was 2%. As shown in Table 10.7, higher compressive strengths were found in the geopolymer specimens that contained up to 2% nanoCaCO3 compared with those without nano-CaCO3. Similar trend was reported for flexural strengths of geopolymers containing nano-CaCO3. The addition of nanoparticles up to 2% was considered to improve the bonding at the interface of binder and filler particles that increased compressive and flexural strengths.

10.3.2.3 Nanomechanical properties of fly ash geopolymer containing nano-SiO2 Nano-indentation tests were carried out to determine the hardness and modulus of elasticity by grid indentations of geopolymer pastes with 2% nano-SiO2 using a

Figure 10.4 Compressive strengths of geopolymers with various percentages of nano-Al2O3 (Zidi et al., 2019). Table 10.7 Compressive strength of geopolymer containing nano-CaCO3 at 28 days (Assaedi et al., 2020). Nano-CaCO3 (wt%)

Compressive strength (MPa)

0 1 2 3

16.07 22.40 25.25 22.32

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359

Table 10.8 Proportions of different phases in fly ash
based geopolymer paste obtained from nano-indentation (Deb et al., 2021). Mix

Compressive Strength

FA29 NS0 FA67 NS2

Porous phase (Volume %)

Aluminosilicate gel Partially activated Unreacted phase (Volume %) phase (Volume %) particles (Volume %)

13.0

70.0

8.5

7.5

7.1

84.9

3.0

5.0

similar approach to (Nˇemeˇcek et al., 2011). The indentation data were analyzed to determine the proportions of different phases. The obtained volumetric proportions of the aluminosilicate phase, partially reacted material, unreacted material, and porous phase in the different geopolymer pastes are given in Table 10.8. It was found that the aluminosilicate gel of fly ash geopolymer (FA-NS2) increased from 70.0% to 84.9% volume with 2% nano-SiO2. The porous phase of paste FA-NS2 decreased from 13.0% to 7.1%. The partially reacted phase and unreacted phase decreased from 8.5% to 3.0% and 7.5% to 5.0%, respectively. Therefore, the reaction product increased and porosity decreased by nano-SiO2. Moreover, a relationship could be observed among the compressive strength, Si/Al and Na/Al molar ratios, and volume fractions of different phases (Deb et al., 2021). A correlation between the microstructure and molar ratios was also reported by Rowles and O’Connor (2003). The Si/Al and Na/Al molar ratios increased with the addition of 2% nano-SiO2 that influenced the geopolymer reaction (Deb et al., 2021). Thus, the addition of nano-SiO2 increased the reaction product and reduced porosity, which contributed to the increase of compressive strength from 29 to 67 MPa. Similar changes by nano-SiO2 in the volume fractions of different phases were also found in the GGBFS- and OPC-blended fly ash geopolymers.

10.3.3 Microstructure development 10.3.3.1 Scanning electron microscope images The backscattered scanning electron microscope (SEM) images of the geopolymer samples with and without nano-SiO2 are shown in Fig. 10.5. A compact microstructure was observed in the geopolymers containing nano-SiO2. Though some unreacted fly ash particles can be seen in both the geopolymers, they mostly appear to be well connected by the reaction product. This is mainly attributable to the higher dissolution rate of the source materials due to the higher fineness of the nanoSiO2. Presence of sodium aluminosilicate gel was noticed, which was also reported by other researchers (Oh et al., 2010; Rodrı´guez et al., 2013; Scrivener et al., 2004). The curing condition is known to have an influence in development of the microstructure of fly ash
based geopolymers (Khale & Chaudhary, 2007). The microstructural development is accelerated with increasing curing temperature and

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Figure 10.5 SEM and EDX spectra of geopolymers with 2% nano-SiO2 (A) fly ash only (B) OPC-blended fly ash, and (C) GGBFS-blended fly ash (Deb et al., 2015). EDX, Energydispersive X-ray spectroscopy. Source: From Deb, P. S., Sarker, P. K. & Barbhuiya, S. (2015). Effects of nano-silica on the strength development of geopolymer cured at room temperature. Construction and Building Materials, 101, 675
683. https://doi.org/10.1016/j.conbuildmat.2015.10.044

time. Other researchers (Temuujin et al., 2009) observed a slower microstructure development of class F fly ash geopolymers because of its lower dissolution rate at room temperature. However, it can be seen from Fig. 10.5 that the addition of nano-SiO2 produced a compact microstructure of geopolymer paste at room temperature. Similar observation was noted by Gao et al. (2015) that the microstructure of the geopolymer is more compact with fewer unreacted particles when the SiO2/Na2O ratio is 1.50 with nano-SiO2. Li et al. (2004) found that nano-SiO2 acted as a filler

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material in cement mortar. However, it enhanced the hydration process and thus improved the microstructure when the particles were dispersed well in the mixture. Fig. 10.5 shows the energy-dispersive X-ray spectroscopy (EDX) spectra of the reaction products of geopolymer pastes. The EDX spectra of fly ash geopolymer with 2% nano-SiO2 (Fig. 10.5) indicate a compound in the product which is rich in silicon, sodium, and aluminum. This confirms the presence of sodium aluminosilicate gel products. This is consistent with the observations made by other researchers on fly ash geopolymers (Criado et al., 2007). The presence of silicon, oxygen, and aluminum in the OPC- and GGBFSblended fly ash geopolymer samples indicate the presence of aluminosilicate gel. The presence of calcium and sodium is also found in the EDX spectra (Fig. 10.5B and C). The relatively high concentrations of calcium and sodium in addition to silicon, oxygen, and aluminum suggests the presence of additional CSH, or calcium aluminosilicate (CASH) in these two mixes. These additional products are considered to contribute to the dense microstructures of the geopolymer pastes with GGBFS and OPC. The unreacted particles of these mixes appear to be well connected by the reaction products as shown in Fig. 10.5B and C. Finding of these products are consistent with the those reported in the literature on similar binders (Kumar et al., 2005; Puertas et al., 2000; Temuujin et al., 2009). Duan et al. (2016) reported that the microstructure of fly ash geopolymer with 5% nano-TiO2 was denser and more compact than that of the reference sample. Significant reduction of porosity was reported due to the denser microstructure with high amount of nano-TiO2. Assaedi et al. (2020) showed a denser microstructure of the geopolymer with 2% nano-CaCO3, which is stated as capable of serving as a bridge for load transfer mechanism.

10.3.3.2 X-ray diffraction analysis of geopolymer with nano-SiO2 The peaks of minor crystalline phases such as quartz, mullite, hematite, and magnetite were observed in fly ash based geopolymers. The X-ray diffraction (XRD) pattern of nano-modified geopolymers are similar to that of the control sample, which indicates formation of no new phase from nano-SiO2. However, Portlandite (Ca (OH)2) crystal peaks were observed in OPC- and GGBFS-blended fly ash geopolymer mixtures without nano-SiO2. It appears that the Ca(OH)2 crystals in OPC- and GGBFS-blended fly ash geopolymer control mixes were modified and converted into the hydrated calcium silicate because of the inclusion of 2% nano-SiO2. This is consistent with the findings reported by other researchers (Kumar et al., 2005; Puertas et al., 2000; Temuujin et al., 2009). Yip and Van Deventer (2003) stated that the higher amount of reactive silica reduced the alkalinity of activating solution, which also reduced the precipitation of Ca(OH)2. This eventually led to the formation of more calcium silicate gel. The phase compositions of the fly ash geopolymers, as determined by the Rietveld analysis are given in Table 10.9. The XRD results of the other mixtures

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Table 10.9 Phase abundance (weight %) of fly ash
based geopolymer (Deb, 2018). Phase

Calcite Hatrurite Hematite Magnetite Mullite Quartz Amorphous Content

Weight % FA-NS0

FA-NS2



2.5 3.9 8.6 4.7 80



2.2 3.9 8.7 4.6 81

are available in Deb (2018). Based on the quantitative data, Hematite [Fe2O3] (PDF# 01
076
4579), Magnetite [Fe3O4] (PDF# 04
007
2718), Mullite [Al4.64Si1.36O9.68] (PDF# 01
079
1453) and Quartz [SiO2] (PDF# 00
046
1045) were recorded in all three types of geopolymers. However, Calcite [Ca(CO3)] (PDF# 01
083
0578) and Hatrurite [Ca3 (SiO4) O] (PDF# 01
070
8632) were also found in the OPC- and GGBFS-blended fly ash geopolymers, along with the other crystalline phases. Also, the incorporation of 2% nano-SiO2 reduced the amount of calcite and Hatrurite in the OPC- and GGBFS-blended fly ash geopolymers. Crystalline phases with anatase (A), quartz (Q), albite (AB), and mullite (M) in nano-TiO2
incorporated fly ash geopolymer concrete was observed by Duan et al. (2016). Phoo-ngernkham et al. (2014) reported the same trends of XRD patterns in geopolymer pastes containing nano-Al2O3 and nano-SiO2.

10.3.3.3 Pore structures of geopolymers with nano-SiO2 The skeleton of the differential curve with and without nano-SiO2 exhibited different intensity with respect to various pore diameter. It is observed that in geopolymer control mixes (0% nano-SiO2), two peaks were observed corresponding to the pore diameters of 5.92 and 15 μm (Fig. 10.6A). However, the distribution of the pore sizes was different for the specimen containing 2% nano-SiO2. In fact, the major peak in geopolymers with 2% nano-SiO2 shifted to a diameter of 0.07 μm. Generally, the pore distributions with peak corresponding to the pore diameter of 0.01
10 μm is considered as capillary pores and that with the peak corresponding to 0.001
0.01 μm are considered as gel pores (Holly et al., 1993; Hughes & Amtsbu¨chler, 1986). Feldman and Beaudoin (1976) described the pore-size distribution and porosity as important aspects of the pore structure of cementitious binders. Hajimohammadi et al. (2011) demonstrated that the size of the aluminosilicate gel determines the pore patterns of fly ash geopolymers. The reduction of pore volume and size is attributed to the formation of additional aluminosilicate gel in the mixes with 2% nano-SiO2, which filled the larger pores. The nano-SiO2 also acts as a

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Figure 10.6 Pore structures of fly ash
based geopolymer with and without nano-SiO2 (A) FA-NS0, (B) FA-NS2, (C) FA-S-NS0, (D) FA-S-NS2, (E) FA-PC-NS0, (F) FA-PC-NS2 (Deb, 2018).

filler material in this case where the particles bridge the spaces between unreacted grains. Similarly, as seen from Fig. 10.6(C)
(F), the use of 2% nano-SiO2 in the GGBFS- and OPC-blended mixtures also showed a considerable effect on the pore structure. The total porosity values of the GGBFS-blended fly ash based geopolymers reduced from 14.74% to 12.22% by the inclusion of 2% nano-SiO2. Addition of 2% nano-SiO2 in GGBFS-blended fly ash geopolymer generated more binding gel to fill the pores. Similarly, the porosity value reduced from 15.60% to 8.70% in the

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OPC-blended geopolymer by the inclusion of 2% nano-SiO2. It means nano-SiO2, with its high surface area to volume ratio influenced the geopolymerization process of the different types of geopolymer and the resulting precipitation of higher amount of reaction product in the pores than that of the control mixtures.

10.4

Durability of geopolymers containing nano-SiO2

10.4.1 Carbonation of geopolymers Carbonation test was carried out on nano-SiO2
incorporated fly ash geopolymer specimens. Three 50 mm thick slices were exposed to an accelerated carbonation at 5.0% CO2 concentration (RH 98% and 23oC 6 2oC) until tested (Fig. 10.7). The carbonation depth on the face exposed to CO2 was examined by spraying 1% phenolphthalein pH-indicator after the completion of each exposure period (Fig. 10.7). The depth of carbonation was measured by averaging depths at four points perpendicular to one face of the split mortar specimens. All the values are reported in elsewhere (Deb, 2018). The carbonation depth of fly ash
based geopolymer specimens without nanoSiO2 after 28, 56, and 90 days were recorded as 15, 20, and 26 mm, respectively. However, it was observed that the carbonation depth at the same duration of exposure was significantly reduced with the use of 2% nano-SiO2. The specimens with 2% nano-SiO2 exhibited 5, 9, and 15 mm carbonation depths, which are significantly less than that of the corresponding control specimen. The decrease in the measured carbonation depth is attributed to the reduction of pores and voids by nano-SiO2. Puertas et al. (2006) reported that pores filled with hydrated gel or other precipitated materials usually prevent the ingress of CO2 into deeper layers of the cementitious binder. Deja (2002) observed that the lower value of total porosity and lower average pore diameter enhance the carbonation resistance of cementitious binders. The initial pH values of the specimens of all geopolymer samples before the CO2 exposure were recorded in the range of 11.0
12.0 which is consistent with the findings of other researchers (Davidovits & development, 2005; Deja, 2002). However, it can be noted that pH values of all geopolymer series significantly decreased with prolonged CO2 exposure till 90 days which were in the range of 10.5 to 11.0 (Deb, 2018). Law et al. (2014) also observed lower pH value in fly ash
based geopolymer binder after longer carbonation exposure periods. Silva et al. (2015) found that the pH value of cementitious binder decreased due to the chemical reaction of the product with carbon dioxide. The carbonation depth, pH values, and the SEM images were used to evaluate the effect of nano-SiO2 on carbonation. The SEM images are shown in Fig. 10.8. It is noted from the SEM images that the light areas are particles of unreacted fly ash (Point 1); the gray areas denoted by 2 are geopolymer reaction product, and the dark areas are pores or disintegrated phase (Point 3). It can be seen that existence of pores or disintegrated phase in geopolymer specimens allows carbonation

Nano-modified geopolymer and alkali-activated systems

(A)

(C)

(E)

365

(B)

(D)

(F)

Figure 10.7 Carbonation depth of fly ash
based geopolymer mixtures: (A) FA-NS0 (B) FA-NS2 (C) FA-S-NS0, (D) FA-S-NS2, (E) FA-PC-NS0, and (F) FA-PC-NS2 (Deb, 2018).

to proceed in the normal diffusion process. It is noted from Fig. 10.8 that the voids were mostly filled by geopolymer product when there was nano-SiO2 in the mixture. However, entries of CO2 to the available pores in the geopolymer specimens are still possible, apart from the dense hydrated matrix as carbonation is a continuation process. It is also noted from the figure that no severe deterioration was observed in any of the geopolymer mixes even after 90 days of CO2 exposure.

10.4.2 Sulfate resistance of geopolymers No obvious damage was observed in the geopolymer samples with nano-SiO2 after four months of immersion in 5% sodium sulfate solution. However, minor damages were seen in the OPC- and GGBFS-blended mortar specimens without nano-SiO2.

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Figure 10.8 SEM images of fly ash
based geopolymer mortar after 28 days CO2 exposure: (A) (FA-NS0) (B) FA-NS2 (C) FA-S-NS0, (D) FA-S-NS2, (E) FA-PC-NS0, and (F) FA-PCNS2 (Deb, 2018). SEM, Scanning electron microscope.

The expansions of the geopolymer mortar specimens with continued immersion in sodium sulfate solution up to 120 days are shown in Fig. 10.9. None of the geopolymer specimens showed any sign of expansion in the first few weeks of immersion. It is observed from Fig. 10.9 that the influence of 2% nano-SiO2 in fly ash geopolymer showed less expansion over time than that of its corresponding control specimens. For example, length change of the specimen without nano-SiO2 after 120

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Figure 10.9 Change in length of fly ash-based geopolymer mortar (with and without nanoSiO2) in 5% sodium sulfate solution (Deb, 2018).

days of exposure in sodium sulfate solution was recorded as 0.0025%, whereas the length change of the specimen with 2% nano-SiO2 was only 0.0007%. This reduction of expansion can be attributed to the reduction of interconnected capillary pores with the incorporation of 2% nano-SiO2 in fly ash geopolymer samples that reduced the extent of sulfate ion ingress in the mortar. According to previous research (Clifton et al., 1998) and ACI code (ACI, 2008), the recommended expansion for cementitious binders having a blend of sulfate resisting cement and pozzolanic binders must be less than 0.05% after six months and should not exceed 0.1% after one year. It is noteworthy from Fig. 10.9 that the expansions of all geopolymer mortar specimens were in the range of 0.0007%
0.0031% which are well below the acceptable limit. Bakharev (2005) noted that good performance of fly ash
based geopolymer in sulfate solution was attributed to a more stable cross-linked aluminosilicate polymer structure. In a similar study, Ismail et al. (2013) reported the effect of sulfate attack in fly ash
based geopolymer binder. They concluded that Na2SO4 does not lead to any apparent degradation of the binder with no conversion of the binder phase. Baˇscˇ arevi´c et al. (2014) observed that the atomic ratios of Si/Al and Na/Al of the geopolymer samples treated with the Na2SO4 solution were somewhat lower from its corresponding reference sample’s atomic ratios. However, when OPC- and GGBFS-blended fly ash geopolymer with 2% nano-SiO2 were exposed in 5% Na2SO4 solution, higher expansions were observed than the fly ash
only geopolymer with 2% nano-SiO2. The presence of calcium ion in this case apparently affects the total expansion. Ismail et al. (2013) stated that some Ca21 might be depleted from the gel structure because of ion exchange process and react with the Na2SO4 to form gypsum at the end. Puertas et al. (2002) also observed the formation of gypsum and ettringite in alkali-activated slag specimens.

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Concluding remarks

The following concluding remarks are made on the effect of nanomaterials in fly ash
based geopolymers: 1. The setting times of fly ash
only geopolymers were still long after addition of 2% nanoSiO2. Initial setting time of the GGBFS-blended fly ash geopolymers with 0% to 3% nano-SiO2 varied between 71 and 35 minutes. The corresponding final setting time varied between 320 and 150 minutes. The setting times further reduced in the OPC-blended geopolymers. Thus a range of setting times could be achieved by adding small percentages of nano-SiO2 in fly ash
only and GGBFS- and OPC-blended fly ash
based geopolymers. 2. In general, compressive strength increases with the addition of nanoparticles, up to a limit. The optimum content depends on different factors, such as the type and particle size of the nanomaterial, binder content, curing condition, and curing time. Compressive strength of ambient-cured fly ash geopolymers increased at the early ages by the addition of nanoSiO2, nano-Al2O3, and nano-TiO2. The 7th day strength increased by blending of low calcium fly ash with OPC or GGBFS which further increased with the addition of nanoSiO2. Increase in strength was noted for nano-SiO2 addition of up to 2%. A decline in strength was observed for the addition of 3% nano-SiO2. 3. The reactivity of fly ash
based geopolymer was increased by the addition of nano-SiO2. The incorporation of nanoparticles in both fly ash
only and GGBFS- or OPC-blended fly ash geopolymers showed improvement of the properties which is mainly attributed to the reaction and filling effects of nanoparticles that enhanced the microstructure. The SEM images showed denser products by the addition of nanoparticles. The EDX spectra in nano-SiO2
incorporated fly ash
based geopolymer showed additional reaction products such as CSH or CASH and NASH in the samples containing OPC and GGBFS. These additional reaction products improved the microstructure of geopolymer pastes and increased compressive strength. 4. The incorporation of nano-SiO2 in fly ash
based geopolymer mortar cured at room temperature reduced the carbonation effects. Carbonation depth was reduced from 15 to 5 mm with the addition of 2% nano-SiO2 in fly ash
based geopolymer mortars. 5. The fly ash
based geopolymer mortars showed less expansion after 120 days in sulfate solution as compared to the corresponding OPC- and GGBFS-blended fly ash
based geopolymers without nano-SiO2.

Acknowledgments Most of the works on geopolymers using nano-SiO2 presented in this chapter are taken from the PhD thesis submitted by the first author (Deb, 2018) at Curtin University, Western Australia.

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292. Available from https:// doi.org/10.1007/s10904-014-0105-1. Kawashima, S., Seo, J. W. T., Corr, D., Hersam, M. C., & Shah, S. P. (2014). Dispersion of CaCO3 nanoparticles by sonication and surfactant treatment for application in fly ashcement systems. Materials and Structures/Materiaux et Constructions, 47(6), 1011
1023. Available from https://doi.org/10.1617/s11527-013-0110-9. Khale, D., & Chaudhary, R. (2007). Mechanism of geopolymerization and factors influencing its development: A review. Journal of Materials Science, 42(3), 729
746. Available from https://doi.org/10.1007/s10853-006-0401-4. Khater, H. (2013). Effect of nano-clay on alkali activated water-cooled slag geopolymer. British Journal of Applied Science & Technology, 764
776. Available from https://doi. org/10.9734/bjast/2014/2690. Kumar, K., Bandopadhyay, A.T., Mehrotra, S. (2005). Effect of mechanically activated fly ash on the properties of geopolymer cement. In Proceedings of the world congress geopolymer. Saint Quentin (pp. 113
116). Law, D. W., Adam, A. A., Molyneaux, T. K., Patnaikuni, I., & Wardhono, A. (2014). Long term durability properties of class F fly ash geopolymer concrete. Materials and Structures/Materiaux et Constructions, 48(3), 721
731. Available from https://doi.org/ 10.1617/s11527-014-0268-9. Li, H., Xiao, H. G., Yuan, J., & Ou, J. (2004). Microstructure of cement mortar with nanoparticles. Composites Part B: Engineering, 35(2), 185
189. Available from https://doi. org/10.1016/S1359-8368(03)00052-0. Li, Z., & Shi, X. (2020). Graphene oxide modified, clinker-free cementitious paste with principally alkali-activated fly ash. Fuel, 269, 117418. Available from https://doi.org/ 10.1016/j.fuel.2020.117418.

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Luo, H. L., Lin, D. F., & Chen, S. C. (2017). Improving the properties of geopolymer containing oil-contaminated clay, metakaolin, and blast furnace slag by applying nanoSiO2. Environmental Technology (United Kingdom), 38(13
14), 1619
1628. Available from https://doi.org/10.1080/09593330.2017.1293163. Nath, P., & Sarker, P. K. (2014). Effect of GGBFS on setting, workability and early strength properties of fly ash geopolymer concrete cured in ambient condition. Construction and Building Materials, 66, 163
171. Available from https://doi.org/10.1016/j. conbuildmat.2014.05.080. Nath, P., & Sarker, P. K. (2015). Use of OPC to improve setting and early strength properties of low calcium fly ash geopolymer concrete cured at room temperature. Cement and Concrete Composites, 55, 205
214. Available from https://doi.org/10.1016/j. cemconcomp.2014.08.008. ˇ Nˇemeˇcek, J., Smilauer, V., & Kopecky´, L. (2011). Nanoindentation characteristics of alkaliactivated aluminosilicate materials. Cement and Concrete Composites, 33(2), 163
170. Available from https://doi.org/10.1016/j.cemconcomp.2010.10.005. Oh, J. E., Monteiro, P. J. M., Jun, S. S., Choi, S., & Clark, S. M. (2010). The evolution of strength and crystalline phases for alkali-activated ground blast furnace slag and fly ashbased geopolymers. Cement and Concrete Research, 40(2), 189
196. Available from https://doi.org/10.1016/j.cemconres.2009.10.010. Pal, S. C., Mukherjee, A., & Pathak, S. R. (2003). Investigation of hydraulic activity of ground granulated blast furnace slag in concrete. Cement and Concrete Research, 33(9), 1481
1486. Available from https://doi.org/10.1016/S0008-8846(03)00062-0. Partha, S. D., Pradip, N., & Prabir, K. S. (2013). Strength and permeation properties of slag blended fly ash based geopolymer concrete. Advanced Materials Research (651, pp. 168
173). Available from https://doi.org/10.4028/http://www.scientific.net/ AMR.651.168. Phoo-ngernkham, T., Chindaprasirt, P., Sata, V., Hanjitsuwan, S., & Hatanaka, S. (2014). The effect of adding nano-SiO2 and nano-Al2O3 on properties of high calcium fly ash geopolymer cured at ambient temperature. Materials and Design, 55, 58
65. Available from https://doi.org/10.1016/j.matdes.2013.09.049. Provis, J. L., Lukey, G. C., & Van Deventer, J. S. J. (2005). Do geopolymers actually contain nanocrystalline zeolites? a reexamination of existing results. Chemistry of Materials, 17 (12), 3075
3085. Available from https://doi.org/10.1021/cm050230i. Puertas, F., De Gutierrez, R., Ferna´ndez-Jime´nez, A., Delvasto, S., & Maldonado, J. (2002). Morteros de cementos alcalinos. Resistencia quı´mica al ataque por sulfatos y al agua de mar. Materiales de Construccion, 2002(267), 55
71. Available from https://doi.org/ 10.3989/mc.2002.v52.i267.326. Puertas, F., Martı´nez-Ramı´rez, S., Alonso, S., & Va´zquez, T. (2000). Alkali-activated fly ash/slag cements. Strength behaviour and hydration products. Cement and Concrete Research, 30(10), 1625
1632. Available from https://doi.org/10.1016/S0008-8846(00) 00298-2. Puertas, F., Palacios, M., & Va´zquez, T. (2006). Carbonation process of alkali-activated slag mortars. Journal of Materials Science, 41(10), 3071
3082. Available from https://doi. org/10.1007/s10853-005-1821-2. Qing, Y., Zenan, Z., Deyu, K., & Rongshen, C. (2007). Influence of nano-SiO2 addition on properties of hardened cement paste as compared with silica fume. Construction and Building Materials, 21(3), 539
545. Available from https://doi.org/10.1016/j. conbuildmat.2005.09.001. Rangan, B. V. (2008). Fly ash-based geopolymer concrete. Research Report GC, 4.

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Rees, C. A., Provis, J. L., Lukey, G. C., & Van Deventer, J. S. J. (2007). In situ ATR-FTIR study of the early stages of fly ash geopolymer gel formation. Langmuir: The ACS Journal of Surfaces and Colloids, 23(17), 9076
9082. Available from https://doi.org/ 10.1021/la701185g. Rodrı´guez, E. D., Bernal, S. A., Provis, J. L., Paya, J., Monzo, J. M., & Borrachero, M. V. (2013). Effect of nanosilica-based activators on the performance of an alkali-activated fly ash binder. Cement and Concrete Composites, 35(1), 1
11. Available from https:// doi.org/10.1016/j.cemconcomp.2012.08.025. Rowles, M., & O’Connor, B. (2003). Chemical optimisation of the compressive strength of aluminosilicate geopolymers synthesised by sodium silicate activation of metakaolinite. Journal of Materials Chemistry, 13(5), 1161
1165. Available from https://doi.org/ 10.1039/b212629j. Sanchez, F., & Sobolev, K. (2010). Nanotechnology in concrete
A review. Construction and Building Materials, 24(11), 2060
2071. Available from https://doi.org/10.1016/j. conbuildmat.2010.03.014. Scrivener, K. L., Crumbie, A. K., & Laugesen, P. (2004). The interfacial transition zone (ITZ) between cement paste and aggregate in concrete. Interface Science, 12(4), 411
421. Available from https://doi.org/10.1023/B:INTS.0000042339.92990.4c. Silva, R. V., Neves, R., De Brito, J., & Dhir, R. K. (2015). Carbonation behaviour of recycled aggregate concrete. Cement and Concrete Composites, 62, 22
32. Available from https://doi.org/10.1016/j.cemconcomp.2015.04.017. Sobolev, K., & Gutie´rrez, M. F. (2005). How nanotechnology can change the concrete world. American Ceramic Society Bulletin, 84(10), 14
18. Available from http://www.ceramicbulletin.org/cbpc/Oct05/Sobolev.pdf. Steveson, M., & Sagoe-Crentsil, K. (2005). Relationships between composition, structure and strength of inorganic polymers : PPPart 2 Fly ash-derived inorganic polymers. Journal of Materials Science, 40(16), 4247
4259. Available from https://doi.org/10.1007/ s10853-005-2794-x. Suwan, T., & Fan, M. (2014). Influence of OPC replacement and manufacturing procedures on the properties of self-cured geopolymer. Construction and Building Materials, 73, 551
561. Available from https://doi.org/10.1016/j.conbuildmat.2014.09.065. Tailby, J., & MacKenzie, K. J. D. (2010). Structure and mechanical properties of aluminosilicate geopolymer composites with Portland cement and its constituent minerals. Cement and Concrete Research, 40(5), 787
794. Available from https://doi.org/10.1016/j. cemconres.2009.12.003. Temuujin, J., van Riessen, A., & Williams, R. (2009). Influence of calcium compounds on the mechanical properties of fly ash geopolymer pastes. Journal of Hazardous Materials, 167(1
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88. Available from https://doi.org/10.1016/j.jhazmat. 2008.12.121. Temuujin, J., Williams, R. P., & van Riessen, A. (2009). Effect of mechanical activation of fly ash on the properties of geopolymer cured at ambient temperature. Journal of Materials Processing Technology, 209(12
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112). Van Jaarsveld, J. G. S., Van Deventer, J. S. J., & Lorenzen, L. (1998). Factors affecting the immobilization of metals in geopolymerized flyash. Metallurgical and Materials

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291. Available from https://doi.org/10.1007/s11663-9980032-z. Vandenberg, A., & Wille, K. (2015). Understanding the dispersion mechanisms of nanosilica in ultra high performance concrete. In Nanotechnology in construction. Springer, Cham. ,httpsdx.doi.org/10.1007/978-3-319-17088-6_40.. Wu, J., Ma, G., Ling, L., & Wang, B. (2016). Grafting of hyperbranched polymer onto the nanosilica surface and their effect on the properties of UV-curable coatings. Polymer Bulletin, 73(3), 859
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266. Available from https:// doi.org/10.1016/S0301-7516(99)00074-5. Yip, C. K., & Van Deventer, J. S. J. (2003). Microanalysis of calcium silicate hydrate gel formed within a geopolymeric binder. Journal of Materials Science, 38(18), 3851
3860. Available from https://doi.org/10.1023/A:1025904905176. Zidi, Z., Ltifi, M., Ben Ayadi, Z., & El Mir, L. (2019). Synthesis of nano-alumina and their effect on structure, mechanical and thermal properties of geopolymer. Journal of Asian Ceramic Societies, 7(4), 524
535. Available from https://doi.org/10.1080/ 21870764.2019.1676498. Zidi, Z., Ltifi, M., & Zafar, I. (2021). Synthesis and attributes of nano-SiO2 local metakaolin based-geopolymer. Journal of Building Engineering, 33, 101586. Available from https:// doi.org/10.1016/j.jobe.2020.101586.

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1,2 ˙ Emircan O¨zc¸elikci1,2, Hu¨seyin Ilcan , Gu¨rkan Yıldırım2, and 2 Mustafa Sahmaran ¸ 1 Hacettepe University, Institute of Science, Beytepe, Ankara, Turkey, 2Department of Civil Engineering, Hacettepe University, Ankara, Turkey

11.1

Introduction

In recent decades, the use of nanotechnology in construction industry by the scientists and engineers to develop and characterize multi-functional cementitious composites has become increasingly popular (Du et al., 2019). Application of nanotechnology in research related to cementitious composites is divided into two major branches, namely, nanoengineering and nanoscience (Scrivener & Kirkpatrick, 2008). Basically, the function of nanoengineering in cementitious composites is relevant to the incorporation of nano-sized materials into the matrix to manipulate and enhance the behavior or properties. Nanoengineering is also very useful in developing new generation multi-functional cementitious composites characterized by superior mechanical and durability performance together with novel aspects such as self-healing, self-cleaning, self-leveling, self-sensing, high ductility, etc. through the utilization of different types of nano-sized materials. On the other hand, nanoscience deals with the characterization and measurement of nano-/microscale structure with the use of advanced characterization methods/equipment to reveal relationship between the nanostructure and macroscale properties and performance of the cementitious materials (Sanchez & Sobolev, 2010). In literature studies performed on cementitious composites, nanoengineering approach has been more common until recent decades since nanoscale characterization mostly requires advanced instrumentation such as high spatial resolution and high precision testing equipment/probe. However, nanotechnology-based improvements in cementitious composites are dependent equally on both nanoengineering and nanoscience. Without understanding the matrix structure at nanoscale, development of highperformance cementitious composites incorporated with different nanomaterials becomes time-/labor-/raw material
intensive. Toward this route, the primary purpose therefore should be to consider the changes/improvements in the internal structure at the atomic scale as a result of the ongoing reactions in the presence of nanomaterials. Concrete, which is the most used man-made material in the world, has a complex heterogeneous nano-/microstructure including anisotropic grains, hydration products, interfacial transition zone (ITZ), fibers, voids, cracks, intergranular phases Recent Advances in Nano-Tailored Multi-Functional Cementitious Composites. DOI: https://doi.org/10.1016/B978-0-323-85229-6.00006-8 © 2022 Elsevier Ltd. All rights reserved.

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Recent Advances in Nano-Tailored Multi-Functional Cementitious Composites

with sizes ranging from nano- to micrometer. In literature studies, the microstructure of cementitious composites has continuously been investigated by the researchers to reveal the bridge between the microstructure and macro performance of the ¨ ztu¨rk et al., 2020; Sahin material (O ¸ et al., 2021; Sahmaran ¸ et al., 2011). Although the microstructural characteristics provide some insight into the macro performance of the cementitious composites, the properties of nanoscale structures in the matrix are the key consideration determining the macro performance of cementitious composites. In other words, internal structure of cementitious materials at atomic-scale is critical for the performance at macroscale. This is because of the calcium
silicate
hydrate (C
S
H) gel, which is the most important and least understood Portland cement hydration product, with a high specific surface area and porosity at nanoscale together with some other nanoscale phases. Because these nano-sized hydration products serve as the main building block of the cementitious composites, the intended design of cementitious composites can be better made by taking advantage of the knowledge related to the physical/chemical properties of these products. In order to reach the required knowledge about such reaction products, nanoscale characterization is crucial. For example, in accordance with the microscale analysis performed in the past, C
S
H was considered to have a layered and amorphous structure (Corr & Shah, 2005). However, recent studies focusing on the nanoscale characterization revealed that there are at least two distinct C
S
H forms (named as inner/outer or low-/high-density products) having different strength, porosity, and density characteristics (Constantinides et al., 2003; Tennis & Jennings, 2000; Wei et al., 2017a,b, 2018). Such characterizations performed at nanoscale can therefore improve our understanding and design process of high performance multi-functional cementitious composites, which will be realized at macroscale and requires traditional empirically driven trial-and-error methods to shift toward advanced design methods using and interpreting the fundamental information obtained from the inner structure at nanoscale. In addition to the nanoscale characterization of hydration products of cementitious composites, it is also of great importance to make reliable characterization of the developments in the matrix in the presence of various multiphase additives. Primary condition for the reliable examination of cementitious composites incorporating various multiphase ingredients including those at nanoscale (i.e., nanomaterials) is to ensure the uniform distribution of mixture ingredients, since this has an important effect on the progression of reactions and formation of hydration/pozzolanic reaction products and thus on the development of nanostructure. Nanomaterials, however, tend to agglomerate in the medium, which results in the formation of heterogeneous reaction products, thereby leading to the occurrence of micro weaknesses in the cementitious matrix (Du et al., 2019; Sahin ¸ et al., 2021). Under such conditions, the assessment of nano-tailored cementitious composites in regard to their nano-/micro/macrostructure is not likely to give meaningful results and leads to misinterpretations about the actual performance of the composite material. In order to ensure the homogenous distribution of the ingredients and relatedly, reliable evaluation of cementitious composites at nano-/micro-/macroscales, there exist several approaches such as the use of surfactants, ultrasonication, and specially

Nanoscale characterization of cementitious composites

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functionalized materials. Through such approaches, homogeneous distribution of the ingredients can be assured and proper conditions for the development and characterization of multi-functional cementitious composites can be obtained. Although nanoscale characterization is common in various disciplines, limited tentative studies have been performed in the case of cementitious materials mostly owing to difficulties related to specimen preparation, higher cost, heterogeneous nature of cementitious materials, time-consuming test procedures, and the unawareness of the importance of properties at nanoscale. Recently, nanoscale characterization methods have been utilized progressively to understand the effects of incorporation of nanomaterials on the inner structure of cementitious matrices at nanoscale. Given the limited studies existing in the literature and heterogeneity of the cementitious composites, reliability of the obtained results are one of the main concerns of researchers. In order to overcome such concerns, it is recommended to combine various nano- and microscale characterization test methods to expand knowledge about the nanostructure, as well as select the nanoscale characterization method best suited to the material composition and the property to be determined. Considering the recent developments in the technology and relatedly in the development of advanced characterization techniques together with the absence in the literature, nanoscale characterization branch of nanotechnology will be further discussed/examined within the scope of this chapter in an effort to better illustrate the currently applied characterization methods for the conventional and nanotailored cementitious composites.

11.2

Nanoscale characterization techniques

11.2.1 Nano-indentation Nano-indentation has been one of the most widely used techniques for the nanoand microscale characterization of various materials utilized in different engineering/science disciplines. Invention of the indentation dates back to the 1850s after the publication of Mohs 10-stage hardness scale (Dey & Mukhopadhyay, n.d.). Although Doerner and Nix (1986) revealed principles of the nano-indentation for the first time, Oliver and Pharr (1992) developed the currently used nano-indentation technique in the literature. Researchers have used nano-indentation to determine and evaluate the elastic modulus, hardness, microstrength of the ITZ, creep behavior, and fracture toughness of different phases in heterogenous materials (e.g., cementitious materials) at nano- and microscale (Hu & Li, 2015; Teixeira et al., 2018; Vandamme & Ulm, 2013). The main principle of the nano-indentation method is based on pressing a pyramidal or spherical diamond indenter into the samples with continuously increasing load until reaching to a predetermined level. Thereafter, constant preset load is applied for a while and then removed gradually. Displacement results corresponding to the applied force are also recorded and plotted in load
displacement curves (Fig. 11.1). The slope of the unloading curve, applied load, and corresponding displacement are used in various formulations and

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Recent Advances in Nano-Tailored Multi-Functional Cementitious Composites

Figure 11.1 (A) Typical load
depth curve of nano-indentation test, (B) representative image of indent formed by a pyramidal tip.

techniques for nano- and microscale characterization of the material (Constantinides et al., 2003). Information regarding the microstructural configuration of the materials as obtained from nano-indentation is a function of molecular simulation and macroscale performance of the composites and helps develop the understanding of mechanical properties at macroscale rigorously after using the obtained information in various multiscale analytical and numerical models. Classical nano-indentation test method was enhanced with the grid technique to map the mechanical properties and provide dataset for the statistical analysis through arranging the indentation grid size and spacing by properly taking the sphere of influence into account. Later, statistical nano-indentation technique that utilizes the dataset obtained from the grid technique became more common as it provides more convenient microstructural analysis of the cementitious materials by approximating the data to various analysis methods (e.g., normal or gaussian distribution, or least square or maximum likelihood estimation) (Ulm et al., 2007). To be able to perform the nano-indentation test, some assumptions must be considered, especially for heterogeneous materials such as concrete. One of such assumptions is that the tested materials behave in a linear elastic manner. Secondly, surface of the materials used during the test is considered to be purely flat and perpendicular to the indenter considering that the indentation depth may be affected by the surface roughness and lead to contradictory results. During the nano-indentation test, attention should be paid to the number, depth, and gap of the applied indentation and duration of constant loading since these parameters can influence the nano-indentation test results. Some studies concluded as one of the drawbacks of nano-indentation in regard to cementitious materials that the indenter size and implementation area are relatively large to detect the ITZ between the C
S
H gel and cement grains comprehensively or such small-sized compounds (Luo et al., 2018; Xu et al., 2015). Normally, the ITZ can be analyzed by considering the instantaneous/sudden decreases in the elastic modulus and hardness along a line. In the cases where there are difficulties regarding the analysis of very small regions/compounds with such

Nanoscale characterization of cementitious composites

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interfaces, it is therefore suggested to use the mapping characterization techniques for proper identification after correlating the indentation results (Luo et al., 2018). As another drawback, duration of each indentation typically taking 5
8 minutes is reported to be time-consuming and requires extended periods of time for each tested specimen with comprehensive grid size necessitating high numbers of indent (Baral et al., 2018; Sebastiani et al., 2016). However, with recent technological developments, advanced nano-indentation instruments and techniques, although not yet commonly utilized in research related to cementitious materials, can enable each indent to be performed within the course of a second (Luo et al., 2018; Nˇemeˇcek et al., 2020). In addition to the determination of microstrength of the ITZ, creep behavior, and fracture toughness of different phases in matrix, nano-indentation has also enabled the researchers to identify two different types of C
S
H gels (inner/outer or low/high density) existing in the hardened cement paste as a result of the recordings of two distinct hardness and moduli values from the matrices. Such characteristic information obtained by the nano-indentation method has attracted the attention of researchers recently to characterize nano- and microscale structures and phases, which led to an understanding of microstructural mechanical properties of cementitious composites and created an opportunity for enhancing the macroscopic performances (Hu & Li, 2015). In one of such studies, creep behavior of hardened cementitious paste was investigated by applying a peak indentation load of 2mN via nano-indentation and defining the corresponding contact creep function (Wei et al., 2017a). On different phases, the indents with the holding time of 40 seconds were made to analyze the particular creep activity of clinker, inner and outer C
S
H gels. Results have revealed that the creeping behavior of C
S
H gel is more pronounced than that of clinker, and creeping behavior of outer C
S
H is 30% greater than that of inner C
S
H. It was also concluded that the gel porosity has significant effects on the creep behavior of C
S
H. In another study, fracture toughness of different phases within the cement paste (tri-calcium silicate, di-calcium silicate, and C
S
H) was analyzed via simulated nano-indentation method by using mono- and multiphase model based on finite element analysis utilizing the experimental results of nano-indentation (Gautham & Sasmal, 2019). It was concluded that the evaluation of the fracture toughness of cementitious composites is possible by the computationally simulated nano-indentation technique. In another study, high-speed indentation technique (accelerated property mapping) was applied to three different types of cement paste blends consisting of 100% cement (CEM I 42.5R), 50% fly ash and 50% cement, and 50% slag and 50% cement, separately (Nˇemeˇcek et al., 2020). Results showed that high-speed indentation caused higher elastic moduli for each mixture because of higher strain rate corresponding to fast loading and unloading. In the cited study, creep behavior was not evaluated via this testing method as the load holding period was inadequate. For the tested area of 42 3 42 μm2, the acquisition time for high-speed indentation technique is 21 times shorter than that of classical grid nano-indentation. Overall, such a rapid technique can lead to the performance of fast measurements with reasonable accuracy in comparison to standard nano-indentation test. In a separate study

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Recent Advances in Nano-Tailored Multi-Functional Cementitious Composites

(Konstantopoulos et al., 2020), the effect of carbon nano-tubes (CNTs) on the properties of cementitious pastes and mortars was investigated by nano-indentation method and the results showed that the utilized method is capable of monitoring the pore structure (e.g., capillary, air- and gel-filled pores) in a way to evaluate the stiffness of the materials by quantifying the nanomechanical properties of lower dimension pores and identifying the cement composition (C3S, C2S, inner
outer C
S
H, etc.), which is engineered by the inclusion of each weight ratio of CNTs. It is important to note that although more accurate phase maps of cementitious materials could be drawn with the advanced nano-indentation test techniques, performance evaluation is better be coupled with other nano/microstructure characterization techniques, such as atomic force microscopy (AFM), transmission electron microscopy (TEM), and others, given the lack of any imaging ability of nano-indentation techniques, which can be necessary, especially for the evaluation of heterogeneous materials (Hu & Li, 2015; Luo et al., 2018; Mondai et al., 2008).

11.2.2 Atomic force microscopy AFM, introduced by Binnig et al. (1986), is a nanoscale characterization technique used to image the sample topography in accordance with the interaction of the sharp probe tip with the sample surface (Binnig et al., 1986). AFM is a unique tool for high-resolution atomic measurement of force and imaging used to work with a wide variety of materials such as ceramics, composites, biological macromolecules, polymers, and metals in the field of surface chemistry, molecular biology, and architecture. Fundamentally, AFM works with the principle of characterization of the behavior exhibited on the sample topography by the force which corresponds to laser deviations. As seen in Fig. 11.2, the tip centered on the cantilever creates an attractive and repulsive force as a result of the interactions on the surface. The bending resulting from these forces in the console, which acts as a spring, causes displacement according to the stiffness of the console. The topographic image is obtained by measuring the displacement of the tip with the help of the laser. In AFM measurements, samples to be used are required to be prepared in a way to have a smooth surface without any deterioration to properly evaluate the local mechanical properties. The nanoscale characterization of cementitious composites with the use of AFM technique is becoming growing interest to many researchers. With the use of this technique, characterization of the properties of cementitious composites starting from the surface to the morphology together with the structural analysis of the hydration products can be made. An important nanoscale characterization has been made by using the AFM technique to evaluate the Young’s modulus in different local regions, which resulted in higher Young’s modulus for the paste around the nonhydrated cement particles and the lower Young’s modulus in the ITZ area (Mondai et al., 2008). The modified/equipped peak-force tapping AFM technique has demonstrated a rather superior function among local mechanical methods of measuring the elastic characteristics of the hardened cement paste and helped different phases be described at nanoscale by clear observation of the elastic modulus

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Figure 11.2 Schematic representation of AFM. AFM, Atomic force microscopy.

of heterogeneous structure (Trtik et al., 2012). AFM technique has also been used in recent years to evaluate the mechanism of alkali
silicate reaction (ASR), which is one of the main durability issues having serious negative effects on the serviceability of concrete materials/structures. The nanoparticles of the ASR products under dry and saturated conditions transform from viscous gel to solid state because of water loss, resulting in a complex morphology and surface roughness; thus designs that can mitigate the deterioration of concrete by the ASR mechanism can be better understood at nanoscale by means of AFM technique (Wu et al., 2020). The images in Fig. 11.3 show the ASR products captured by the AFM method in (A) 10 μm and (B) 5 μm scale. According to the AFM measurement, the topography is represented by the darker color as lower height and the brighter color as higher height. Nano-sized ASR products accumulate in the samples with the continuance of reactions, thereby generating latitude differences. In this context, the distribution of ASR products as strips or peaks by creating a latitude difference on the surface was observed. In recent years, the AFM technique has been used to help characterize the hydration kinetics and products of nano-tailored cementitious composites to better/proper incorporation of nanomaterials into the cementitious systems. It has been successfully analyzed using the AFM technique that the nano-sized graphene and graphene oxide addition into the cementitious systems provides an improvement in the hydration process, resulting in the improvement of mechanical properties and significant

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Recent Advances in Nano-Tailored Multi-Functional Cementitious Composites

Figure 11.3 Representative AFM images of ASR products at different scales: (A) 10 μm; (B) 5 μm (Wu et al., 2020). AFM, Atomic force microscopy; ASR, alkali
silicate reaction. Source: From Wu, H., Pan, J. & Wang, J. (2020). Nanoscale structure and mechanical properties of ASR products under saturated and dry conditions. Scientific Reports, 10(1). https://doi.org/10.1038/s41598-020-66262-9.

enhancements in the Young’s modulus (Horszczaruk et al., 2015). The effects of other nanomaterials such as nano-TiO2, nano-SiO2, and multiwalled carbon nanotubes alongside with graphene oxide on the crystallization property and morphology of the C
S
H gels can also be characterized by the AFM technique. It was observed from the literature studies utilizing AFM that the use of nanomaterials changed the morphology of C
S
H gels from sheet-shape to fine structures and formed large granular agglomerations of unequal sizes (Li et al., 2017). The effect of the use of carbon nano-fibers on the local mechanical properties of ITZ in cementitious composites was measured by the AFM technique and found that carbon nanofiber at nanoscale caused significant enhancement of the modulus of elasticity of ITZ (Gao et al., 2019). In 2010, the AFM technique was further improved with the PeakForce Quantitative Nanoscale Mechanical (PF-QNM) mode, which can provide high resolution, perform quantitative measurement, and measure properties such as elastic modulus, energy dissipation, and deformation (Pittenger et al., 2010). In addition to having a high scanning speed, the test can be repeated many times with PF-QNM AFM without sample degradation. In literature studies, it has been reported that it is possible to study both the mechanical characteristics of the cement paste and variations in the mechanical properties of the interface region with the help of PF-QNM, whereas nanoscale adhesion can only be examined by the PF-QNM mode of the AFM (Ren et al., 2019).

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Although AFM is a unique tool for imaging and analysis of morphological characteristics, it is inadequate in terms of chemical analysis. In order to overcome this inadequacy, AFM technique is combined with some other techniques including infrared spectroscopy (AFM-IR), nuclear magnetic resonance (AFM-NMR), tipenhanced Raman spectroscopy (TERS) in recent years (Nguyen-Tri et al., 2020). As can be seen in Fig. 11.4, combining the high resolution topographic imaging capability of AFM with the chemical analysis capability of IR, the AFM-IR technique has been utilized recently for the practical characterization at nanoscale in many areas of application (Centrone, 2015; Dazzi & Prater, 2017; Waeytens et al., 2018). There have also been promising advancements in the use of AFM-NMR hybrid probes to determine both mechanical and chemical properties (Mousoulis et al., 2013). Consequently, it is clear that the AFM technology, which has been very successful in analyzing the traditional cementitious composites at nanoscale at high resolution for many years, can also be used in the analysis of nano-tailored cementitious composites in the future. The combination of the AFM methodology with other newly developed techniques for more accurate analysis and identification of a particular material is very likely to provide substantial advantages over conventional techniques in regard to obtaining more in-depth knowledge about the properties of ultimate reaction products and development of designs for materials/structures with a longer service life.

Figure 11.4 Schematic representation of AFM-IR. AFM-IR, Atomic force microscopyinfrared spectroscopy.

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11.2.3 Transmission electron microscopy TEM has long been used for the nanoscale characterization of several materials such as metals, alloys, glasses, polymers, etc. After the invention of electron microscopy (Knoll & Ruska, 1932), TEM has been evolved into an advanced instrument with scientific development and was benefited significantly in the nanotechnology research as it has the ability to magnify the atomic-scale images of nano-tailored materials with high resolution (Fig. 11.5A and B). In various disciplines, TEM has also been utilized to assess the dispersion of nanomaterials, morphology, lattice type, microstructure of different phases, and crystalline structure at nanoscale through high resolution imaging capability (Hernandez et al., 2008; Jia & Richardson, 2018; Monteiro et al., 2019; Murray et al., 1993; Novoselov et al., 2005; Richardson et al., 2016, 2010; Su, 2017). The main difference between the scanning electron microscopy (SEM) and TEM is that while the former provides a surface imaging and its composition by collecting the reflected electrons from the tested materials, in the latter, electrons transmitting through the specimen are taken advantage of to obtain the abovementioned information (Fig. 11.6). Basically,

Figure 11.5 (A, B) Representative TEM images at different magnification levels (waste sugarcane bagasse ash produced through calcination at 700 C), (C) TEM-SAED image (Jamalludin et al., 2018). SAED, Selected area electron diffraction; TEM, transmission electron microscopy. Source: From Jamalludin, M. R., Harun, Z., Othman, M. H., Hubadillah, S. K., Yunos, M. Z., & Ismail, A. F. (2018). Morphology and property study of green ceramic hollow fiber membrane derived from waste sugarcane bagasse ash (wsba). Ceramics International, 44 (15), 18450
18461. https://doi.org/10.1016/j.ceramint.2018.07.06.

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Figure 11.6 Scheme showing the representative working principle of SEM and TEM techniques. SEM, Scanning electron microscopy; TEM, transmission electron microscopy.

during TEM analysis, beam of electrons transmits through the thin cross-section of the specimen and images of the section related to the transmitted electrons are formed. However, it needs to be pointed out that in order to obtain reliable/accurate high-resolution imaging by TEM, ultra-thin specimens are required to be prepared to increase the transparency, and the specimens need to withstand the rapid transmitted beam damage. Moreover, while working with TEM, experimenter should be aware of the detrimental effects of ionizing radiation. Similar to other nano characterization techniques, only a limited space can be analyzed by using the TEM analysis. Latest technological developments allow standard TEM to be equipped with several imaging, spectroscopy, and diffraction techniques to enhance the image resolution and characterization ability. For example, scanning transmission electron microscopy (STEM), X-ray energy dispersive spectroscopy (EDS), electron energyloss spectroscopy (EELS), and selected area electron diffraction (SAED) (Fig. 11.5C) can be used singly (TEM-EDS and TEM-EELS) or in combination (STEM-EELS or STEEM-EDS) with TEM. With such enhanced equipment, information regarding the micro and crystalline structure, morphology, and chemical composition can be acquired in much more detail. Utilization of TEM with enhanced equipment has attracted the attention of the researchers for nanoscale characterization of cement-based materials, although the preparation of specimens is elaborative. In the work of Zhan et al. (2021), TEM and SEM were used for the microstructural characterization of microbially induced mineralization products and loose sand particles available in the new type of cementitious materials. TEM morphology and diffraction pattern of calcite in sand particles were investigated within the scope of the cited study. The results revealed that calcite produced through microbially induced mineralization surrounded the sand particles and filled the gaps/voids between the sand particles resulting in a more compact structure. In another study by Karthick et al. (2019), characteristics of graphene oxide
manganese oxide (GO
MnO2) nanomaterials which were used as a corrosion monitoring sensor in the reinforced concrete structures, were investigated with TEM analysis (Karthick et al., 2019). TEM images demonstrated that GO is having an exfoliated very thin layers, and MnO2 nanorods are well dispersed in the medium. Images also confirmed MnO2 nanomaterial crystal growth on the GO surface. Moreover, higher magnified TEM images revealed similar results with

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X-ray diffraction (XRD) analysis in the conducted study. In the works of He et al. (2018, 2020), TEM-SAED was used to characterize the microstructure of magnesium oxychloride cement (MOC) paste incorporating glass powder (GP), pulverized fuel ash (PFA), and sewage sludge ash (SSA) cured under CO2 and air condition (He et al., 2018, 2020). TEM-SAED results demonstrated that the hydration products of air cured MOC paste had crystalline structure while the hydration products of CO2 cured MOC paste had amorphous hydration products. Incorporation of GP, PFA, and SSA was found to be influential on the hydration and microstructure of the system, which led to the formation of extra amorphous phases in the medium and could be the reason for better water resistance of MOC paste. It was also noted that the TEM-SAED results, which were related to the crystalline structure of MOC paste were concordant with the results obtained from quantitative XRD results. In another study by Boehm-Courjault et al. (2020), microstructure, crystallinity, and composition of ASR products in concrete were examined with STEM, STEM-EDS, and TEM-SAED as TEM provides higher spatial resolution for imaging and chemical analysis compared to SEM-EDS (Boehm-Courjault et al., 2020). TEM images and related experimental technique/analysis revealed that there formed two different types of ASR products in the cement-based materials considering the morphology, crystallinity, and volume, namely, untextured and platey. Scho¨nlein and Plank (2018) investigated the impacts of polycarboxylate superplasticizer (PCEs) on the nucleation and crystal growth of C
S
H with TEM. Images proved that the existence of PCEs has a significant impact on the kinetics of conversion of globular C
S
H into nanofoil-like C
S
H. This conversion is significantly delayed in the presence of PCEs, when a layer develops around the globular C
S
H particles, resulting in core-shell morphology. In another study, TEM-SAED was used for the characterization of modified nano GO (by tetraethylorthosilicate), capable of reinforcing and functionalizing cement (Lin et al., 2020). The TEM images and SAED pattern revealed that the coated silica is amorphous with a dispersed scattering pattern, while the modified GO retains/preserves the initial basal crystalline structure of GO sheet. In the work of Li et al. (2020), TEM-EDS was utilized to characterize high- and low-calcium C
S
H seed precursor (synthesized by diluted hydration of pure triclinic C3S) in regard to the morphological features. High-resolution TEM images revealed that the morphology of nanocrystalline C
S
H synthesized through the lime
silica reaction is similar to that of the foil-like C
S
H seeds developed in the study. The morphological characteristics regarding fiber-foil features of hydrated C3S were verified by the high-resolution TEM photographs, which demonstrated that the fibers are hundreds of nanometers long and the nano-foils are crumpled. It is also stated that the morphology of C-S-H from hydrated C3S is affected by the amount of lime present. In another research (Zhan et al., 2020), TEM was used for the characterization of carbon nano-tubes-coated fly ash, which was developed for the purposes of improved piezoresistivity (outstanding strainsensing capability) and dispersion ability in cement-based matrix. TEM images demonstrated that a few micrometers of extremely dense and fluffy CNTs were successfully grafted onto the surface of the fly ash. Besides, the hollow inert existence and well-developed graphitic sheets of the synthesized CNTs were verified by

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high-resolution TEM images. In the work of Avet et al. (2019), measurement of the composition and morphology of C
A
S
H in the paste mixtures with limestone calcined clay cement blends incorporating different amounts of kaolinite clay, was carried out by using STEM and STEM-EDS since the utilization of SEM-EDS was reported to be challenging for the measurement of blends containing fine particles (e.g., metakaolin, silica fume, etc.) due to the volume of interaction of electron beam and phase mixing. It was concluded this study that STEM can provide the composition of C
A
S
H free from interaction with other phases and more information on the C
A
S
H morphology. STEM analysis demonstrated the fibrillar morphology of C
A
S
H and revealed that the calcined clay or the calcined kaolinite of the calcined clay have no noticeable effect on the C
A
S
H morphology. However, it was also stated that SEM-EDS is a much more common and practical method for obtaining the composition of C
A
S
H. As the above-detailed research works conclude, with the advancements in technology, TEM having a high spatial resolution capability can be utilized as a promising technique in characterization of cementitious composites at nanoscale. Given the advanced characterization capability of the TEM technique, it is also used in the analysis of cementitious composites with nanomaterials recently. In addition to its mapping capability, with the combination of other techniques, TEM can also provide characteristic information about the micro and crystalline structure, morphology, and chemical composition of nano-tailored composites.

11.2.4 Nuclear magnetic resonance Since its discovery by Isidor Rabi et al. (1938), NMR has become one of the most important techniques in nanoscale and nanomaterial characterization and awarded by the Nobel Prize (Rabi et al., 1938). In Fig. 11.7, schematic representation of NMR testing is shown. The nuclei in a molecule of the sample placed in the magnetic field tend to form magnetization as a result of the NMR signal generation by the excitation of the nuclei with radio waves. The observation of specific magnetic resonance frequencies (RF) in a molecule by the NMR enables the identification of significant chemical and structural characteristics of the molecule (Fig. 11.7). In addition to the conventional methods used for the microstructural analysis of cementitious composites, the NMR technique is becoming more common recently. It has provided valuable knowledge at nanoscale in the realization of structural analysis (Cong & Kirkpatrick, 1996) and nature (Richardson, 1999) of C
S
H gels, in determining the porosity and pore distribution (Pipilikaki & Beazi-Katsioti, 2009), and measuring the reaction kinetics and structure of hydroxyl groups available in cement plus pozzolanic systems (Richardson, 2004). 29Si magic angle spinning NMR (MAS NMR) technique in cementitious composite research has been one of the most widely used methods for identifying the main hydration reactions of C3S and C2S. Depending on the spectral method used, the classification and characterization of silicate species and Qn structures were studied widely with the 29 Si MAS NMR technique (Cuesta et al., 2018; Li et al., 2019).

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Figure 11.7 Schematic representation of NMR testing. NMR, Nuclear magnetic resonance.

Besides its characteristic measurement capability, NMR has been developed continuously by combining it with some other measurement techniques for more consistent analysis in specific studies. In order to evaluate the densification capability of C
S
H gels and nonlinear growth of gel porosity was determined by using 1H NMR relaxometry, which is also one of the modified NMR measurement techniques (Muller et al., 2013). The analysis of silicate species and dilute absorbed organic molecules has been quite challenging until today due to several reasons including low sensitivity of NMR, low particle surface area, etc. The improved signal sensitivity provided by the dynamic nuclear polarization NMR (DNP NMR) method has played a major role in overcoming these challenges (Sangodkar et al., 2015). As shown in Fig. 11.8, the bond structures of C
S
H particles at high Ca/Si ratios were characterized by density functional theory (DFT) calculations at approximately 400 nm with the high sensitivity DNP NMR method (Kumar et al., 2017). Recently, the nature, structure and interactions of hardened cement matrix at atomic size were investigated by solid-state NMR (SS NMR) and significant findings were established in regard to the characterization of the hydration mechanism (Walkley & Provis, 2019). However, considering the experimental and interpretative difficulties, it became clear for different disciplines to come together and collaborate. In recent years, the early hydration chemistry of the use of biodentine in cementitious composites was investigated with the 1H-29Si CP MAS NMR technique and concluded that, owing to the weak crystalline nature, the C
S
H

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Figure 11.8 C
S
H analysis by DNP NMR (Kumar et al., 2017). DNP NMR, Dynamic nuclear polarization nuclear magnetic resonance. Source: From Kumar, A., Walder, B. J., Kunhi Mohamed, A., Hofstetter, A., Srinivasan, B., Rossini, A. J., Scrivener, K., Emsley, L. & Bowen, P. (2017). The atomic-level structure of cementitious calcium silicate hydrate. Journal of Physical Chemistry C, 121(32), 17188
17196. https://doi.org/10.1021/acs.jpcc.7b02439.

formation mechanism of dental cements can be determined to a large extent by the NMR technique, unlike techniques such as XRD and Fourier transform infrared spectrophotometry (FTIR) (Li et al., 2019). Recently, considerable attention has been given to the understanding of the effect of nanomaterials used in cementitious composites on the hydration kinetics by NMR method. According to the quantities of Q0 and Q3 1 Q4, which represent different silicon
oxygen combinations, observed by the 29Si MAS NMR analysis, it was interpreted that the addition of nano-silica promotes the cement hydration degree and the pozzolanic reaction kinetics of fly ash (Liu, Ma, et al., 2019). In addition, the calculation of the mean chain length value from the data provided by 29 Si MAS NMR spectra played an important role in the determination of the degree of silicate polymerization of system with nano-silica and the effect of C
S
H gel formation of metakaolin (Jamsheer et al., 2018). 27Al NMR spectrum was used to observe the effect of chloroaluminates on the chemical bonding grades of nanoalumina particles and positive effects of chloroaluminates on the chemical bonding were reported (Liu, Tan, et al., 2019). Consequently, the NMR technique, which provides important information in analyzing the structure of hydration products and observing the nuclear speciation, performs well in understanding the atomic-scale material chemistry in binding phases. Additionally, examination of cementitious composites through methods such as

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1

H NMR, 29Si NMR, 27Al NMR, and 23Na NMR and development by combining with other characterization techniques continue in the progress of time.

11.2.5 Small-angle neutron scattering Scattering methods are among the most important techniques used for microstructural characterization in field of chemistry, biology and materials science. Among scattering methods, there are methods with different wavelength and scattering angle capabilities such as neutron scattering, X-ray scattering and light scattering. Small-angle neutron scattering (SANS) is advantageous compared to other methods due to its suitable wavelength for nano-size characterization, chemical and physical nondestructiveness and scattering at low angles such as 5 and 10 degrees (Squires, 1996). As shown in Fig. 11.9, after the incident beam has touched the sample, SANS instruments are able to measure the structure on a length scale (d) as a function of the angle between the scattered beam with the 2D neutron detector. Fundamentally, the neutron wavelength (λ) and scattering angle (θ) determine the length scale probed through the following formula: d

θ

λ ðwavelengthÞ ðscattering angleÞ

(11.1)

The most important ability of the SANS is that it is probably the only way to quantify the heterogeneity in a mesoscopic real-space length scale of 1
300 nm. The widespread use of SANS in the evaluation of gel properties and crystal morphology in the field of polymers is common due to its ability to prevent inconsistent scattering by simply delivering scatter contrast to the sample and perform

Figure 11.9 Schematic representation of SANS measurement. SANS, Small-angle neutron scattering.

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high-pressure measurements and shear deformation measurements via strong penetration of neutron beams (Shibayama, 2011). Properties of C
S
H gel, which controls all the mechanical and durability properties behind the scenes throughout the service life of cementitious systems, have been analyzed increasingly by the SANS method in recent years. Contrary to methods like TEM and gas sorption, SANS provides great benefits for the nanoscale analysis of cementitious systems due to the measurement technique that is nondestructive and does not require drying process. In the measurements made with the SANS technique, three different regimes of interest can be mentioned considering the scattering vector (Q) values. The smallest microstructural features are determined at high Q values, while volume fractal and surface fractal scaling behaviors are determined at lower Q values. These regimes are generally sized to include the arrangement of C
S
H gel and properties of calcium hydroxide. The volume and surface fractal densities determined by the SANS method are related to the matrix structure, which develops with the progression in hydration reactions. These data allow the determination of the properties such as matrix density and gel voids by interpreting the strength development and heat of hydration during hydration (Jennings et al., 2007). Additionally, with the combination of SANS and X-ray scattering, it is possible to determine the bulk density and mean formula associated with the specific drying conditions of the C
S
H gel. In addition to a classification of how tightly water is bound to C
S
H, it is possible to make inferences to define the chemically active surface area in the cement and classifications in accordance with the location of the water to estimate the concrete properties (Allen et al., 2007). In a study, authors used the combination of SANS and 29Si MAS NMR to analyze the microstructural improvements of the dissolved C
S
H gel and reported that the SANS specific surface area increased over time and that the C
S
H dissolution observed in the 29Si MAS NMR results was consistent with the transformation to the tobermorite structure (Trapote-Barreira et al., 2015). In recent years, studies on the analysis of cement hydration products, which are quite complex at nanoscale, by SANS method have rapidly increased in number. In a molecular dynamics study on the C
S
H gel morphology, the results of parameters related to the SANS method such as the short-range structures of calcium and silicon atoms at nanoscale, their local structures and depolymerization of silicate structures during hydration were found to be quite consistent with the experimental results (Hou et al., 2015). In order to monitor the structural changes in the C
S
H gel during dissolution, SANS method was used and concluded that as the C
S
H gel dissolved, the high-density C-S-H structure transformed into a lowdensity structure and the measured specific surface area increased. The SANS data also proved that the SANS total internal surface has a tendency to increase as the Ca/Si ratio decreases (Trapote-Barreira et al., 2015). In order to evaluate the microstructural developments as a result of the cement hydration in the presence of volcanic ash, a review was carried focusing on the parameters including the total surface area, fractal surface area, and volume fractal morphology by the use of SANS technique. According to the results, the increase in the total surface area was related to the gelation effect, while the increase in the specific surface area was related to the

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nucleation effect due to the addition of volcanic ash. In addition, the transition to a dense microstructure as a result of an increase in the nucleation and growth of hydration products was observed because of the increase in the volume fractal component (Kupwade-Patil et al., 2020). The use of small-angle scattering methods such as SANS has been very successful in the analysis of the complex microstructure of cementitious composites and used frequently to determine the total surface area, specific surface area and volume fractal components of hydration products. Significant advances have been made in the microstructural interpretation at nanoscale by combining such scattering methods with other common characterization techniques in analyzing the cementitious composites.

11.2.6 X-ray computed nano-tomography Interpretation of the two-dimensional (2D) nano characterization test results (e.g., nano-indentation, TEM, etc.) may be complicated and misguiding while evaluating the morphology and distribution of particles, since the characterization of materials (especially those having heterogenous nature) may vary along a third direction. In order to overcome such a problem, characterization in the third direction is necessary in an identical specimen. In order to obtain three-dimensional (3D) nanoscale structure of materials nondestructively, X-ray computed tomography has been widely used in various disciplines for a long time (Brenner & Hall, 2007; Cnudde & Boone, 2013; Dierolf et al., 2010; Masad et al., 2002; Mo¨bus & Inkson, 2007). Invention of the tomography dates back to the 1970s by Godfrey Hounsfield and Allan Cormack who shared the Nobel Prize in 1979. In time, the technique was slowly improved in regard to data acquisition times and resolution quality. Initially, X-ray computed micro-tomography was utilized to evaluate the leaching, alkalisilica reaction, reinforcement corrosion, cracking formation, pore network, early hydration products, insulation properties, density, morphology of cementitious materials up to half a micron scale (Bossa et al., 2015; Brisard et al., 2020; du Plessis & Boshoff, 2019; Erdem et al., 2012; Monteiro et al., 2019). However, spatial resolution obtained by the microtomography technique was found to be insufficient to represent the gel pores, capillary pores and nanomaterials accurately. Recently, with the advancements in the technology, new generation nanotomography has been developed enabling nanoscale resolution up to 10
20 nm. Utilization of X-ray computed nano-tomography in cementitious and other materials have provided clearer and more reliable information about the cement hydration products, density and packing density of hydration products, transport properties, pore structure/shapes/size/distribution/connectivity/tortuosity, fiber distribution, crack width, fracture characteristics, morphology, and particle distribution (Bossa et al., 2015; Brisard et al., 2020; Monteiro et al., 2019; Withers, 2007; Zhao et al., 2018). Moreover, the method can be used to analyze the products formed as a result of self-healing existing in voids and cracks (Brisard et al., 2020). Effects of freezing-thawing cycles on nano and microstructure can be investigated via computed nano-tomography by following the changes in micro-porosity.

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During the analysis of X-ray computed nano-tomography, condensed X-Ray radiation is projected onto the sample, creating a 2D projection images on the detector. The sample is rotated regarding each predetermined angular position to provide series of projection images as represented in the Fig. 11.10. Then, obtained 2D projection images are reconstructed to form 3D computed nano-tomography images (Bossa et al., 2015; Brisard et al., 2020; Monteiro et al., 2019; Withers, 2007; Zhao et al., 2018). As computed nano-tomography provides information about the inner and outer morphology and structure mainly, nano-tomography is coupled with the X-ray fluorescence for the evaluation of distribution of chemical constituents. By this way, high-resolution 3D specimen images with detailed chemical information can be reached (Hu et al., 2014). In one study, nano-tomography is also combined with XRD (XRD-nCT) to acquire more accurate information about the inner structure of especially featureless amorphous or nanostructured metallic alloys with fast acquisition times (Stoica et al., 2021). Although nano-tomography has an advantageous ability of nondestructive nanoscale characterization up to 10
20 nm scale by visualizing 3D structure, operator dependency for the interpretation of the data, lack of worldwide accepted protocols for the operation due to variety of sample size/shape/ composition and rather long testing periods can be regarded as limiting factors for this technique (Cnudde & Boone, 2013). Despite its several drawbacks, in recent studies dealing with the cementitious materials, X-ray nano-tomography was used to obtain information about the nanoscale 3D structure of samples without causing destruction. In a study by Chen et al. (2019), hydration of pure C3S and internal pore microstructure were investigated with various time intervals by monitoring the changes in the pore shapes/sizes (Fig. 11.11) (Chen et al., 2019). In the cited study, X-ray nano-tomography has been confirmed to be an effective method for nondestructive analysis of the porosity of cement-based materials. The study provided extensive morphological details on the structure, size/volume, and spatial distribution of the pores inside the hydrating C3S paste via nano-tomography. According to the results, during hydration, the volumes of sealed pores inside the hydrating paste tend to grow larger, while the open pores tend to shrink, confirming that the growth of hydrates within the

Figure 11.10 Schematic representation of the working principle of X-ray computed nanotomography.

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Figure 11.11 Rendering of the 3D volume images of the hydrated C3S (A) surface rendering of the specimen after 5 days of hydration, (B, C) transparent rendering of the segmented 3D image of “a” from two different directions (D) surface rendering of the specimen after 28 days of hydration, (E, F) transparent rendering of the segmented 3D image of “d” from two different directions (Chen et al., 2019) (Each color/shape represents pores with different sizes and shapes in paste specimen). Source: From Chen, B., Lin, W., Liu, X., Iacoviello, F., Shearing, P. & Robinson, I. (2019). Pore structure development during hydration of tricalcium silicate by X-ray nano-imaging in three dimensions. Construction and Building Materials, 200, 318
323. https://doi.org/ 10.1016/j.conbuildmat.2018.12.120.

hardened C3S paste would fill to “empty” space of the open pores throughout continuing hydration. In another study (Hu et al., 2016), combined analysis of utilizing nano-tomography and nano-X-ray fluorescence (nano-tomography assisted chemical correlation) to investigate the early hydration products of C3S was made by evaluating the 3D structure, chemical composition and mass density of hydration products forming during the induction period of cement. It was stated that nano-tomography could provide one-to-one analysis of the same area before and after a reaction, allowing researchers to observe exactly how and when the reaction occurs. These datasets later can be converted into 3D models to reveal reactions on the particle’s surface and interior and volumetric datasets may be generated from these observations as well. Overall, nano-tomography assisted chemical correlation results revealed that the reactions of individual particles were not uniformly distributed either on the surface or in the interior of the particles. Shirani et al. (2020) used X-ray ptychographic nano-tomography to investigate the hydration of CaAl2O4 at various temperatures (4 C, 20 C, and 50 C to simulate field condition) as information about porosity formation as a result of ongoing reactions of calcium aluminate

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cement-based matrix during service life was not clear. Particularly, in the research, ptychographic nano-tomography has been used to analyze CaAl2O4 hydration with the aim of understanding the water porosity formation after heterogeneous conversion reactions, since conversion reactions have a direct effect on the porosity, thereby on mechanical strength and durability of cementitious composites. It was revealed in accordance with the tomography analysis that the secondary formed porosity because of the progression in hydration at mesoscale is with average pore size of 140 nm. In the work of Netinger Grubeˇsa et al. (2019), the resistance of cementitious mortars against freeze-thaw cycles was investigated with X-ray microcomputed tomography analysis (conducted by using X-ray nano-tomography equipment) in addition to SEM observations and mercury intrusion porosimetry measurements by focusing on the evaluation of pore systems. Obtained tomography data demonstrated that the pore connectivity is the most significant parameter affecting freeze/thawing resistance, the higher the connectivity of macropores, the greater the freeze/thawing resistance of cementitious materials. Above-listed studies from the literature clearly show that X-ray computed nano-tomography is a useful technique and can be used successfully in nanoscale characterization of homogenous and heterogeneous matrices in 3D enabling detailed observations of the samples. Moreover, the technique can be coupled with several other testing equipment to improve the quality of test results and provides better insight into the matrix composition and morphology.

11.2.7 Other characterization techniques In addition to abovementioned techniques, there are several other test methods which are less common and have been of interest only to limited tentative research in literature to acquire information from and characterize the cementitious matrices at nanoscale. One of such techniques is the quantitative nanoscale modulus mapping in the form of scanning probe microscopy (SPM), which was newly introduced to the literature to evaluate the mechanical properties of materials with homogenous or heterogenous nature with less contact force and lower depth of indentation. Basically, storage and loss modulus of materials are obtained by the modulus mapping test methods, which record the displacement amplitude and phase lag during the scanning process (Luo et al., 2018; Zlotnikov et al., 2017). During SPM modulus mapping measurement, standard indenter is used to provide low static contact force on the samples. Such a force is utilized to scan the surface of prepared specimen with high frequency sinusoidal oscillation to obtain elastic response of applied force and height profile, creating 2D topographical image that can be effectively used to investigate the different phases existing on the surface of the specimen (Xu et al., 2015). The displacement of the indenter tip and applied periodic force are used to reach local storage and loss moduli of the tested materials. During the test, since the indenter tips penetrate just a few nanometers into the surface of materials, nanoscale modulus mapping can be counted as a nondestructive test method and provide a superior resolution up to 20 nm (Wei et al., 2017b). Fundamentally, SPM modulus

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mapping test equipment is a combination of nanoindenter with a piezo-scanner, lock-in amplifier, and a force modulation system (Gao et al., 2018; Li et al., 2015, 2016). SPM modulus mapping has been applied to various material science disciplines to characterize materials at nano- and microscale. Recently, in cementitious matrices, modulus mapping technique has started to be used for the examination of various phases such as cement grains, interface and C
S
H gel (outer and inner type) by evaluating the obtained modulus value and 2D topographical images. In one of such studies (Wei et al., 2018), mechanical properties of different types of C
S
H gels (inner and outer C
S
H) were investigated with SPM modulus mapping, in addition to nano-indentation technique. Results obtained from both techniques were concordant with each other and proved that modulus mapping is adequate to characterize and distinguish the multiphases existing in cementitious matrix nondestructively. In another study (Xu et al., 2015), both nano-indentation and modulus mapping were used for the investigation of nanomechanical properties of the interface between C
S
H gels and cement grains (Xu et al., 2015). Although nano-indentation yielded interface width of less than 5 μm, modulus mapping resulted in an interface of approximately 200 nm. Although there are several studies related to the utilization of modulus mapping in cementitious matrices, the application of the technique has not been fully understood and is not common among researchers for the characterization of cementitious matrices having multiscale/multiphase properties. For nanoscale characterization with the help of imaging technique, helium ion microscopy (HIM) is also utilized by several researchers, although rarely, in various science disciplines (Hlawacek et al., 2012; Joens et al., 2013). HIM is a highperformance alternative to SEM with superior resolution ability up to 0.3
0.4 nm scale, as helium ions can be concentrated within a small-size probe and yield a much smaller volume of interaction on the surface of the specimen (Morgan et al., 2006; Notte et al., 2007; Ward et al., 2006). Basically, working principle of HIM is similar to SEM in terms of detection of secondary electrons, yet it uses a focus beam of helium ions instead of electrons for imaging and analysis. It provides better contrast and enhanced depth of focus compared to electron microscope (Rodenburg et al., 2014). Besides, it does not require part coating on the surface of the materials as opposed to SEM. Mainly, HIM images can provide information about the nanoscale surface morphologies and pores formed the tested specimens (Emmrich et al., 2016). In a study by Song et al. (2019), characterization of the pore structure of hardened cement paste has been made with the help of HIM in addition to other direct and indirect test methods. HIM was capable of detecting the gel pores (1
10 nm), small capillary pores (10
100 nm) and large capillary pores (0.1
1 μm) thanks to high resolution imaging capability. In another study (Morandeau et al., 2016), HIM was used to investigate the nano- and microscale morphology of alkali-activated slag, focusing on several types of gels formed. Two different types of C
(N)
A
S
H gel were identified during the HIM analysis (like two types of C
S
H gels) after observing foil-like and globular shaped morphology. Although there are several studies focusing on the use of HIM technique, the equipment used for the test is not common and cost-effective, which make the

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HIM a less common nanoscale mapping technique compared to other techniques (e.g., TEM, AFM) and not very widely used technique in the branch of cementitious materials for the time being. Synchrotron-based X-ray radiation has been recently brought to the fore in the investigation of the morphologic characteristics of cementitious products, as it is a less harmful technique and provides high resolution at a scale of 10 nm. This allows an important nanoscale analysis, especially when combined with the scanning transmission X-ray microscopy (STXM) (Li et al., 2019). STXM has a working principle based on the recording of a function of X-ray beam intensity at the sample location, which is transmitted from the zone plate to the sample. In the STXM technique, the measured signal is a function of energy and position and is converted to parameters such as sample density, sensitivity, and thickness (Ha et al., 2012). STXMmeasured electron binding energies provide the near-edge X-ray absorption fine structure (NEXAFS) spectra and give information about the chemical composition of the material. In assessing the microstructure of cementitious materials, the interactions of the calcium and chloride ions with C
S
H gels can be interpreted by the use of STXM together with NEXAFS data (Li et al., 2015). Another advantage of STXM is that it enables detection by providing high energy resolution of bond changes of the material composition, especially when characterization of the Ca particles is unable due to its weak spectral sign (Herna´ndez-Cruz et al., 2014). Another technique used for the high resolution material characterization at submicron level is laser scanning confocal microscopy (LSCM) with 3D imaging capability. LSCM works with the principle that the beam emitted by the laser passes through the pinhole formed between the sample and the detector, forming an optical slice and receiving a 3D data set with these optical slices at different focus levels. The technique has been successfully used in 3D imaging of fine pores and void sections of cementitious composites at various depths and in determining their matrix properties (Head & Buenfeld, 2006). Although this method has higher resolution than the X-ray micro-computed tomography and the ability to display a wider area than the focused ion beam-nanotomography (FIB-nt), it is known that the imaging depth is lower (Yio et al., 2015). This disadvantage requires samples to be cut if the volumetric region to be observed is deep, which limits its use as it causes loss of material layer in cementitious systems.

11.3

Challenges and future perspectives

Advances in nanoscale characterization over the past few decades have provided significant opportunities in the analysis of materials (especially for heterogeneous ones) for many industries by enabling information regarding nano- and microscale structure. Although the integration of nanoscale characterization into the construction industry has a recent history, significant breakthroughs have been made in a short period. Along with the development of multi-functional cementitious composites with superior mechanical and durability properties in the presence of

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Recent Advances in Nano-Tailored Multi-Functional Cementitious Composites

nano-sized materials, equipment/techniques capable of characterizing such composites at nanoscale have also been developed recently, which allows us to understand the fundamental behavior at macroscale more clearly. Although there have been significant advances in the nanoscale characterization of cementitious composites, there are still limitations for the researchers who want to use. In particular, factors such as the limited number of testing facilities, elaborate specimen preparation, time-consuming test procedure, heterogeneous inner structure of cementitious materials, higher cost and unawareness of the importance of properties at nanoscale restrict the widespread use of nanoscale characterization techniques in cementitious composites. These limitations therefore have led to the emergence of only limited number of studies related to nanoscale characterization of cementitious composites. The absence of a standardized correlation and low comparability criteria between the findings obtained from the studies focusing on the nanoscale characterization of cementitious composites are also issues that need to be overcome in the field. Considering the challenges on the basis of nanoscale characterization, researchers should have a high level of experience and knowledge in regard to the principles of nanoscale characterization and techniques. On the other hand, rather than the full endorsement of exactness of research findings obtained by a single method, greater accuracy of the obtained results can be reached with the help of concurrent performance of other nanoscale characterization techniques. Hereby, studies involving nanoscale characterization will always lead to meaningful and accurate analyses for subsequent studies in the literature. Moreover, collected cumulative knowledge from the nanoscale characterization findings will provide a base for test standards/protocols to obtain more accurate/reliable/interpretable test results, which may lead to widespread usage of nanoscale characterization test methods on cement-based materials. As the incorporation of nano-sized materials into the matrix in an effort to manipulate and enhance the behavior/properties of cementitious materials has been among the trending topics recently, the challenges related to nanoscale characterization should be overcome while developing nano-tailored multi-functional cement-based composites.

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Low CO2 reactive magnesia cements and their applications via nano-modification

12

Tien-Dung Nguyen1,2, Cise Unluer 3, and En-Hua Yang1 1 School of Civil and Environmental Engineering, Nanyang Technological University, Singapore, Singapore, 2Department of Mechanical, Aerospace and Civil Engineering, University of Manchester, Manchester, United Kingdom, 3School of Engineering, University of Glasgow, Glasgow, United Kingdom

12.1

Introduction

MgO was used as binder for masonry construction and as stabilizer for soil bricks in ancient times. It is usually obtained by calcining natural magnesite rock (MgCO3) (Shand, 2006). It can also be produced from the magnesium silicate rocks (e.g., forsterite or serpentines) (Garcia et al., 2010; Giammar et al., 2005; Montserrat et al., 2017; Sanna et al., 2014) as well as extracted from seawater or brine (Dong et al., 2017, 2018a,b). The reactivity of MgO highly depends on its degree of crystallinity, which is associated with its crystalline size. Lower calcination temperature usually results in lower crystallinity degree (i.e., smaller crystalline size) and higher reactivity (Kuenzel et al., 2018). As such, MgO can be classified into three grades depending on the calcination temperature during production (Shand, 2006): Light burned MgO is produced at a temperature of B700 C
1000 C. It has the least crystallinity and the highest reactivity, and as a result, it has been used as reactive magnesia cement (RMC). Hard-burned MgO is manufactured at a temperature of B1000 C
1400 C and has been used as an expansive additive to compensate shrinkage in concrete (Mo et al., 2014; Lingling & Min, 2005; Mo et al., 2010). Dead-burned MgO is produced at a temperature of 1400 C
2000 C. It has the highest crystallinity and the least reactivity and has been used in refractory applications. The formation of magnesium hydroxide (Mg(OH)2, brucite) from the hydration of MgO provides a mechanical strength sufficient for masonry construction. In the 1860s, the blending of RMC with various magnesium salts such as magnesium chloride or magnesium sulfate to form magnesium oxy-chloride (MOC) cement (Urwongse & Sorrell, 1980) or magnesium oxy-sulfate (MOS) cement (Walling & Provis, 2016; Beaudoin & Ramachandran, 1978) with better binding properties was patented (Sorel, 1866). The hydration products of MOC and MOS produce Recent Advances in Nano-Tailored Multi-Functional Cementitious Composites. DOI: https://doi.org/10.1016/B978-0-323-85229-6.00004-4 © 2022 Elsevier Ltd. All rights reserved.

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better binding properties than brucite. MOC and MOS cements have been used to make fire-resistant and lightweight insulation products (Sorre & Armstrong, 1976; Beaudoin & Feldman, 1977). However, the use of MOC and MOS is limited because of their poor water resistance. Alternatively, the blend of RMC, either lightly burned or dead-burned, with acid phosphate salts to form magnesium phosphate (MOP) cements can produce good mechanical strength and water resistance (Mestres & Ginebra, 2011; Gartner & Hirao, 2015). The rapid hardening of MOP makes it useful for fast repairs (Seehra et al., 1993). Despite technical advantages, its scarce natural resource limits its application in the large scale (Gartner & Hirao, 2015). In 1889, magnesium silicate hydrate (M
S
H) cements produced by mixing calcined MgCO3 and finely pulverized silica were patented (Cummings & Earize, 1889). Generally, low-carbon M
S
H cements are formed from RMC and a source of highly reactive silica from industrial by-products such as silica fume, steel slag, or coal fly ash (FA) (Jin & Al-Tabbaa, 2014a,b; Zhang 2011). The M
S
H cements possess high compressive strengths (i.e., up to 70 MPa) and good heat resistance (i.e., up to 1500 C) (Walling & Provis, 2016; Zhang 2011). Another approach focusing on the blending of RMC and hydrated magnesium carbonates (HMCs) (xMgCO3 yMg(OH)2 zH2O) to obtain a low-carbon hydraulic cement was patented in 2008 (Vlasopoulos & Robert Cheeseman, 2008). The resulting RMC
HMCs pastes with up to 50% HMCs exhibit rapid hardening, significant strength gain, good water resistance, and low embodied carbon (Vlasopoulos & Robert Cheeseman, 2008; Rani Devaraj et al., 2010; Flatt et al., 2012; Gartner & Sui, 2018). It is known that RMC hardens by hydration and carbonation, which sequestrates a significant amount of CO2. Good progress has been made in recent years in improving the hydration and carbonation of RMC-based composites (Dung & Unluer, 2016; Dung & Unluer, 2018a,b,c; Liska & Al-Tabbaa, 2009; Liska et al., 2012; Morrison et al., 2016; Unluer & Al-Tabbaa, 2014; Liska et al., 2008; Unluer & Al-Tabbaa, 2013). When compared to the ordinary Portland cement (OPC), RMC has more advantages because of its significantly lower calcination temperature, that is, 750 C
900 C versus 1400 C (Walling & Provis, 2016), and it permanently absorbs atmospheric CO2 in the form of HMCs (Liska et al., 2008; Unluer & Al-Tabbaa, 2013; Vandeperre & Al-Tabbaa, 2007; Morrison et al., 2016), and the ability to be fully recycled (Sonat et al., 2017a). Global climate change induced by increased anthropogenic CO2 emissions is the most urgent and critical global issue of our time. This led to a growing interest in developing an RMC-based concrete, which can potentially replace OPC with a product of a much low carbon footprint (Gartner & Hirao, 2015). This chapter presents the recent development of RMC and its applications via nano-modification. Details of research work up to date on the production of RMC, the hydration and carbonation mechanisms of RMC-based formulations, and their improvement via nano-modification, the mechanical properties of nano-tailored RMC-based composites, as well as the applications of RMC in developing novel cementitious binders are presented.

Low CO2 reactive magnesia cements and their applications via nano-modification

12.2

409

Production of reactive magnesia cements

RMC can be obtained via the dry route from the calcination of magnesite (MgCO3) or the wet route from magnesium-bearing solutions such as seawater or brine. RMC is mostly produced through the dry route because of its lower energy requirements than the wet route (Canterford, 1985).

12.2.1 Dry route The dry route for MgO production typically requires the crushing of magnesite before calcination through the dissociation of MgCO3 into MgO and CO2. MgCO3 ! MgO 1 CO2

(12.1)

The reactivity of RMC is greatly determined by the source of magnesite, presence of impurities, and most importantly the calcination conditions, that is, temperature and residence time (Liu et al., 2007). RMC is typically calcined in the range of 700 C
1000 C (Thomas et al., 2014). During the calcination, the applied heat is transferred from the combustion gases to the surface of the magnesite particle toward the micropores between the particle surface and the reaction interface. The transferred heat at the interface leads to the dissociation of MgCO3 into MgO and CO2. The escape of CO2 from the interface to the particle surface leave a porous calcined layer. The porosity of this layer determines the crystal size and the hydraulic reactivity of the resulting MgO (Shand, 2006). At low calcination temperature (i.e., ,900 C), the loss of CO2 leaves a very porous structure with high specific surface area (SSA), which results in RMC with high reactivity. In contrast, increased calcination temperature (i.e., .900 C) and/or residence time leads to enlarged crystallize size with low SSA, which ends up with low reactivity (Liu et al., 2007; Eubank, 1951; Birchal et al., 2000). The structure of MgO evolved from mesocrystal at 400 C
600 C to polycrystallite at 700 C, pesudomorphous MgO at 800 C
900 C, and cubic single crystal at . 1000 C. Accordingly, its surface morphology changed from smooth to fractured structures followed by the structure composed of uniform nanoparticles resulting from the sintering of MgO at a higher temperature (Zhang et al., 2015). Impurities such as Fe2O3 and SiO2 promote the sintering by increasing the contents of the vitreous phase (Eubank, 1951). Higher-grade RMC often requires the pretreatment for the removal of Fe2O3 and SiO2 impurities prior to the calcination (Walling & Provis, 2016). The production of RMC from the natural magnesite rock involves the emission of 1.1 g CO2 and the enthalpy requirement of 2.9 kJ/g of RMC (Gartner & Sui, 2018). Alternatively, RMC can be produced from magnesium silicate [e.g., forsterite (Mg2SiO4) and serpentines (Mg3Si2O5(OH)4)], which could be a more favorable route as it requires lower energy (Walling & Provis, 2016). In this method,

410

Recent Advances in Nano-Tailored Multi-Functional Cementitious Composites

magnesium silicates under the supercritical carbonation (100 C
225 C, .75% CO2, 7.1
9.7 MPa) give magnesite and quartz as demonstrated in (Eq. 12.2). Mg2 SiO4 1 2CO2 ðgÞ ! 2MgCO3 1 SiO2

(12.2)

The magnesite can be further calcined to produce MgO and the CO2 can be recycled, while the quartz can be either utilized or discarded. The theoretical enthalpy requirement for the decomposition of forsterite and serpentine are 0.86 and 2.25 kJ/g of RMC (Gartner & Sui, 2018), respectively, which are significantly lower than the production of RMC from the natural magnesite. Moreover, the use of magnesite silicates in the production of RMC does not release fossil CO2. Despite these advantages, this process has not been implemented because of the lack of industrial-scale energy-efficient facility for high temperatures and CO2 pressures manufacturing (Winnefeld et al., 2019; Walling & Provis, 2016). Some progress has been made to enable the implementation of this process for the development of a low-carbon RMC cement (Gartner et al., 2014).

12.2.2 Wet route Seawater, natural brine, and reject brine contain significant amount of magnesium; for example, Mg21 concentration in seawater is around 1.29
1.35 g/L (Boyd, 2015; Wright & Colling, 1995). Unlike the dry route, the extraction of RMC via the wet route involves two main steps: (1) the precipitation of brucite (Mg(OH)2) and (2) the calcination of brucite to obtain RMC. In the first step, an alkaline base [e.g., slaked lime (Ca(OH)2) or dolime (CaMg (OH)4)] is introduced to the Mg-containing solution to enable the precipitation of Mg(OH)2, as shown in Eqs. (12.3) and (12.4). MgCl2 1 CaðOHÞ2 ! CaCl2 1 MgðOHÞ2

(12.3)

MgCl2 1 CaMgðOHÞ4 ! CaCl2 1 2MgðOHÞ2

(12.4)

In addition to Ca(OH)2 and CaMg(OH)4, NaOH and NH4OH can also be used to enable the precipitation of Mg(OH)2 from brine (Dong et al., 2018a,b), as shown in Eqs. 12.5 and 12.6. The morphologies of Mg(OH)2 (i.e., crystal size, shape, and structure), which later determine the characteristics of MgO, strongly depend on the type of solvent and base used during its precipitation, and the pH and temperature of the reaction (Alvarado et al., 2000). Mg21 1 2Na1 1 2OH2 ! MgðOHÞ2 1 2Na1

(12.5)

Mg21 1 2NH4 1 1 2OH2 ! MgðOHÞ2 1 2NH4 1

(12.6)

Low CO2 reactive magnesia cements and their applications via nano-modification

411

In the second step, the precipitated Mg(OH)2 slurry in the first step is filtered, washed, and calcined at 500 C
700 C for 2
12 h to form MgO (Eq. 12.7). MgO can also be obtained via the pyrohydrolysis of magnesium chlorides in superheated steam at up to 1000 C, as shown in Eq. 12.8 (Shand, 2006). MgðOHÞ2 ! MgO 1 H2 O

(12.7)

MgCl2 6H2 O ! MgO 1 2HCl 1 5H2 O

(12.8)

Some researchers studied the feasibility of producing MgO from brine and seawater via the wet route. Al Mutaz and Wagialia (1990) produced MgO by using brine obtained from desalination activities in the Arabian Gulf. Ivanov et al. (1973) synthesized MgO from Black Sea water and Bosnian dolomite with a purity of 96.7%. Alkathili (2016) also produced MgO (B95% purity) from the Arabian Gulf seawater and dolomite. The precipitation of Mg(OH)2 was achieved via the addition of NaOH into the reject brine in Barcelona, Spain (Casas et al., 2014). MgO was also obtained from the calcination of Mg(OH)2, which was precipitated by the addition of NH4OH into Tunisian natural brine (Behij et al., 2013). The production of RCM with a high reactivity from reject brine of the Tuaspring desalination plant, Singapore was studied with a similar procedure (Dong et al., 2017; Dong et al., 2018b). Similar to the dry route, increased calcination temperature and residence time reduce the SSA and increase the crystal size, which result in low reactivity of MgO (Eubank, 1951; Hirota et al., 1992; Green, 1983). This enables a range of MgO products to be produced, depending on the degree of reactivity required. The feasibility of producing RMC from reject brine collected from a local desalination plant in Singapore, via the addition of base materials such as NaOH and NH4OH at different calcination condition was studied (Dong et al., 2018a,b). These studies revealed that the addition of NH4OH into reject brine resulted in a more porous, flake-like morphology of Mg(OH)2 and consequently a higher SSA and reactivity of MgO than the use of NaOH. The final product with the highest reactivity and SSA (B79 m2/g) was obtained from the calcination of Mg(OH)2 at 500 C for 2 h (Dong et al., 2018b) This research has currently identified potential routes that lead to the production of high reactivity MgO, which can be used in various applications in the construction sector.

12.3

Hydration and carbonation of reactive magnesia cements

The biggest advantage of RMC over OPC is its ability to sequestrate CO2 permanently in order to lower carbon footprints associated with the construction activities. In the RMC-based materials, the hydration of RMC forms Mg(OH)2(aq,s) (Eqs. 12.9
12.12), associated with an increase in the pH of the pore solution, which facilitates the dissolution and diffusion of CO2 in the pore space. The

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Recent Advances in Nano-Tailored Multi-Functional Cementitious Composites

dissolution of CO2 enables the carbonation of Mg(OH)2(aq,s) to form a range of HMCs, as shown in Eqs. 12.13
12.16. MgO-alkaline oxide plays an electron donator role in water: 2 MgOðsÞ 1 H2 OðlÞ ! MgðOHÞ1 ðsurfaceÞ 1 OHðaqÞ

(12.9)

OH2 anions are adsorbed on the positively charged surface: 2 1 2 MgðOHÞ1 ðsurfaceÞ 1 OH ðaqÞ ! MgOH OHðsurfaceÞ

(12.10)

OH- anions are desorbed from the surface, releasing Mg21 and OH2 ions into the solution: 21 2 MgOH1 OH2 ðsurfaceÞ ! MgðaqÞ 1 2OHðaqÞ

(12.11)

Ion concentration reaches supersaturation, at which point the hydroxide starts to precipitate as brucite on the oxide surface: 2 Mg21 ðaqÞ 1 2OHðaqÞ ! MgðOHÞ2ðsÞ

(12.12)

The carbonation of Mg(OH)2(aq,s) to form HMCs: 2MgðOHÞ2ðaq;sÞ 1 CO2ðaqÞ 1 2H2 O ! MgCO3 MgðOHÞ2 3H2 O ðartiniteÞ

(12.13)

5MgðOHÞ2ðaq;sÞ 1 4CO2ðaqÞ ! 4MgCO3 : MgðOHÞ2 : 4H2 O ðhydromagnesiteÞ (12.14) 5MgðOHÞ2ðaq;sÞ 1 4CO2ðaqÞ 1 H2 O ! 4MgCO3 : MgðOHÞ2 : 5H2 O ðdypingiteÞ (12.15) MgðOHÞ2ðaq;sÞ 1 CO2ðaqÞ 1 2H2 O ! MgCO3 : 3H2 O ðnesquehoniteÞ

(12.16)

The hydration and carbonation are the critical processes as they determine the formation of HMCs, which enables the RMC-based samples to harden and gain strength. However, the hydration and carbonation degrees of RMC are quite low, limiting the subsequent formation of HMCs and the strength development of the final products. The hydration mechanism of RMC is related to a dissolution
precipitation process, which is controlled by the dissolution of MgO, as shown in Eq. (12.9)
(12.12) (Amaral et al., 2010). The low dissolution of MgO under ambient conditions (Rocha et al., 2004; Fruhwirth et al., 1985) and the low solubility of CO2 in the pore solution of Mg(OH)2 with a pH value of B10.5 inhibit the formation of HMCs and strength gain of RMC-based materials. In addition to the slow dissolution of MgO and CO2, the precipitation of hydration and carbonation

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products on the surface of unreacted MgO/Mg(OH)2 particles prevents them from further contacting with H2O and CO2. This mechanism leads to a physical barrier around the unreacted phases, which results in the low hydration and carbonation degrees of RMC and low compressive strength of RMC-based materials (Dung & Unluer, 2016; Dung & Unluer, 2017a,b). Furthermore, the rapid formation of HMCs under accelerated carbonation covers the surfaces of unreacted MgO grains and prevents their further contact with water for further hydration. Accordingly, the use of higher CO2 concentrations also resulted in a lower conversion of MgO to Mg(OH)2. The unhydrated MgO of RMC-based concretes under the ambient (B0.04% CO2), 5%, 10%, and 20% CO2 were 16.8%, 19.7%, 27.4%, and 34.9%, respectively (Dung et al., 2021). The RMC-based concretes hydrating and carbonating under the ambient conditions gained a 28-day compressive strength of less than 4 MPa. The accelerated carbonation conditions (i.e., 5%
20% CO2 concentration) stimulated the rapid formation of HMCs and enabled RMC-based samples to gain a 28-day compressive strength of 28 MPa (Dung & Unluer, 2016; Unluer & Al-Tabbaa, 2014). Numerous studies have focused on the improvement in the hydration and carbonation mechanisms to enhance the mechanical performance of RMC-based materials.

12.3.1 Improving the hydration mechanism and mechanical performance of reactive magnesia cement-based materials via nano-modification The limitation in the dissolution, as well as hydration of MgO, was enhanced by the use of hydration agents (HAs). The most common HAs in the literature were magnesium chloride (MgCl2) and magnesium acetate ((CH3COO)2Mg) (Filippou et al., 1999). The hydration of MgO in the presence of MgCl2 or (CH3COO)2Mg takes place with a similar mechanism as shown in Eqs. (12.17)
(12.20). The complex magnesia acetate ions (CH3COOMg1) shown in Eq. (12.18) migrate away from their original MgO grains to enable the precipitation of Mg(OH)2 in the bulk solution. This alternation of the hydration mechanism increases the space available for the continuous hydration of MgO. Moreover, the increased formation of Mg(OH)2 in the bulk system increases the amount of brucite available for carbonation. The introduction of HAs increased the hydration degree of RMC at 14 days of hydration by B54%, which results in 107% higher compressive strength and 74% lower water absorption as compared with the control sample (Dung & Unluer, 2017a,b). Dissociation of magnesium acetate: 21 ðCH3 COOÞ2 MgðaqÞ 22CH3 COO2 ðaqÞ 1 MgðaqÞ

(12.17)

Dissolution of magnesia: 1 2 MgOðsÞ 1 CH3 COO2 ðaqÞ 1 H2 OðlÞ 2CH3 COOMgðaqÞ 1 2OHðaqÞ

(12.18)

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Dissociation of magnesium complexes: 2 21 CH3 COOMg1 ðaqÞ ! CH3 COOðaqÞ 1 MgðaqÞ

(12.19)

Precipitation of magnesium hydroxide due to supersaturation: 2 Mg21 ðaqÞ 1 2OHðaqÞ ! MgðOHÞ2ðsÞ

(12.20)

Alternatively, the introduction of nucleation nano-seeding (0.5% hydromagnesite) in the mix design can alleviate the low hydration and carbonation degrees of MgO, which are caused by the precipitation of brucite and HMCs on the surface of unreacted MgO and Mg(OH)2. When hydromagnesite nano-seeds are well-dispersed in the pore space, they enable the formation of brucite and HMCs on their surfaces, away from the MgO grains, thereby increasing the contact surface for the further hydration and carbonation. As hydration and carbonation proceed, the nucleation of hydration and carbonation phases not only form on the surface of the MgO and Mg (OH)2 but also around the nano-seeds dispersed within the pore space. This additional formation of hydration and carbonation products in the pore space increases the 28day strength of RMC-based concretes by 33% (i.e., 48 vs 64 MPa) (Dung & Unluer, 2017a,b). Moreover, the synergistic combination of HA (magnesium acetate 0.05 M) and hydromagnesite nano-seeds (0.5%) led to an increase in the carbonation degree by 96% associated with 56% lower water absorption and 46% higher 28-day compressive strength (i.e., 48 vs 70 MPa) (Dung & Unluer, 2018a,b,c).

12.3.2 Improving the carbonation mechanism and mechanical performance of reactive magnesia cement-based materials via nano-modification The carbonation of RMC-based materials was improved via the introduction of carbonation agents, for example, sodium bicarbonate (SBC, NaHCO3) or sodium chloride (SC, NaCl). The presence of carbonation agents accelerated the dissolution of CO2 because of the increase in initial pH within the pore solution, which improved the carbonate network with improved HMC content and morphology and increased the 28-day compressive strength by B107% (24 vs 50 MPa) (Dung & Unluer, 2018a,b,c). The simultaneous implementation of high-temperature precuring to accelerate the hydration and SBC (NaHCO3 0.1 M) to enhance the carbonation further improved the microstructure and 28-day compressive strength of RMC-based materials by B129% (24 vs 56 MPa) (Dung & Unluer, 2018a,b,c). Alternatively, the synergistic inclusion of nucleation nano-seeds (0.5% hydromagnesite) and of SBC (NaHCO3 0.1 M) led to the dense microstructure composed of interconnected carbonate networks and a 142% increase in 28-day compressive strength of RMCbased concretes (24 vs 58 MPa). This combination facilitated the high rate of carbonation throughout the curing and enabled the samples to obtain 72 MPa at 56 days (Fig. 12.1) (Dung & Unluer, 2019). The obtained results highlighted the role

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Figure 12.1 Strength development and morphology of RMC concrete under accelerated carbonation. Compressive strength and microstructure of carbonated RMC-based concrete including nano-seeds and SBC. RMC, reactive magnesia cement; SBC, sodium bicarbonate.

of (1) nucleation nano-seed in facilitating the nucleation of brucite away from the original MgO grains and extending the area for reactions between hydrated MgO and dissolved CO2 and (2) SBC in increasing the initial pH of pore solution to enhance the dissolution of CO2 and facilitate the continuation of the carbonation reaction at a later stage.

12.3.3 Limitations of carbonation diffusion Fig. 12.2 reveals X-ray micro CT images of the RMC pastes after 3 and 28 days of accelerated carbonation (i.e., 20% CO2, 80% 6 5% relative humidity, and 30 C 6 1.5 C) (Dung et al., 2019). The paste samples (diameter of 5 mm, height of 13 mm) were covered by plastic molds during the carbonation process; thus their carbonation was mostly observed on sample surfaces. The variations observed in the grayscale was a representation of the changes in the density of the paste. The formation of HMCs with a gray tone was observed on the top of the sample, while almost all of the sample was the uncarbonated phases with a dark tone (Fig. 12.2). The 1D CO2 diffusion was reflected by the carbonation depth within these

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Figure 12.2 Carbonation depth observed via X-ray CT. X-ray micro computed tomography (CT) images of RMC paste samples carbonated for: (A) 3 days and (B) 28 days. RMC, reactive magnesia cement.

cylindrical paste samples. Fig. 12.3 presents the development of carbonation depth of RMC paste samples under 20% CO2 with carbonation time. The carbonation depth increased quickly at early ages and reached a plateau of B1.25 mm after 7 days of carbonation, indicating the limitation of CO2 diffusion within RMC samples. The pore distribution of the two regions of interest (ROI, including the carbonated region at the top layer and the uncarbonated region at the inner sample) in RMC samples is visualized in Fig. 12.4. The results demonstrated a decrease in pore density in the top layer when compared with the inner layer. The cumulative void frequency confirmed the denser microstructure of the carbonated region than the uncarbonated region (Fig. 12.5). The denser microstructure of the carbonated region at the top layer was explained by the filling of pores via the expansive formation of HMCs. The progressive reduction of the pore density at the top of the samples inhibited the further diffusion of CO2, explaining the plateau observed in the carbonation depth. Fig. 12.6 presents the influence of CO2 concentrations on 1D carbonation depth of RMC pastes at the age of 7 days (Dung et al., 2021). The carbonation depths of RMC paste under 5%, 10%, and 20% CO2 were 0.7, 1.9, and 1.2 mm, respectively. The 3-D carbonation diffusion of the RMC-based concretes was also monitored via the color change of phenolphthalein due to the change in pH of the pore solution of samples (Chincho´n-Paya´ et al., 2016). Fig. 12.7 demonstrates the

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Figure 12.3 Development of carbonation depth with time. Carbonation depth of RMC paste samples at different carbonation durations. RMC, reactive magnesia cement.

influence of CO2 concentrations on the progress of the carbonation depth in RMC samples during 28 days of curing (Dung et al., 2021). The obtained results clearly indicated the influence of CO2 concentration on the carbonation depth during the initial stages of curing. Consequently, higher CO2 concentrations stimulated the diffusion of CO2 and resulted in the highest carbonation depth in the sample under 20% CO2 on the first day (11 mm vs 0.5
8.8 mm). This higher carbonation depth translated into the densification of the exterior layers, which partially inhibited the further diffusion of CO2 at greater depths within this sample. Therefore this sample revealed a lower carbonation depth than that under 10% CO2 after 3 days (18 mm vs 17 mm). While the curing under 20% CO2 resulted in the plateau of carbonation depth after 7 days at 18 mm, that under 10% CO2 reached 21 mm after 28 days of carbonation. This limitation of carbonation diffusion led to the lower compressive strength of sample under 20% CO2 than 10% CO2 after 28 days (i.e., 60 vs 63 MPa). Although the curing under 5% CO2 revealed fast progress of the carbonation depth during the first 7 days of curing (from 3 to 16 mm), this was followed by a relatively stable level of carbonation afterward. The curing under 5% CO2 demonstrated the lowest carbonation depth when compared with that under 10% and 20% CO2 after 28 days (15.6 mm vs 18
20.7 mm). On the other hand, the porous structure of this sample, which was cured under ambient conditions, enabled the continuous diffusion of CO2 to reach 9 mm of carbonation depth at 28 days. The results under indicated the curing under 10% CO2 as the best CO2 diffusion condition, thereby suggesting this environment as the most favorable condition for the development of microstructural and mechanical properties of RMC-based materials.

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Figure 12.4 Comparison of the top and inner layers of Aviso pore representations in RMC paste samples showing (A) ROI locations, and samples cured for: (B) 1 day, (C) 3 days, (D) 7 days, and (E) 28 days under 20% CO2. ROI, regions of interest.

12.4

Durability of reactive magnesia cements

12.4.1 Nitric acid resistance of reactive magnesia cement-based concretes RMC concretes with and without 50% FA substitution and RMC-FA were prepared to investigate the nitric acid resistance (Muthu et al., 2020). These concretes were initially cured under accelerated carbonation (10% CO2, 70% relative humidity, and 25 C) for up to 28 days to gain strength. The cured samples were first saturated in distilled water for 7 days and then directly immersed in nitric acid (Mg(NO3)2) 0.5 M for up to 14 days. Table 12.1 reveals the changes in the pH and composition

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Figure 12.5 Cumulative pore volume obtained from X-ray micro computed tomography (CT) images. Cumulative pore volume variation within RMC samples cured for 28 days under 20% CO2. RMC, reactive magnesia cement.

Figure 12.6 Relationship between CO2 concentration and 1D carbonation depth of RMC pastes. 1D carbonation depth of RMC pastes cured under different CO2 concentrations for 7 days. RMC, reactive magnesia cement.

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Figure 12.7 Development of 3-D carbonation depth with time. 3-D carbonation depth of RMC-based concretes at different carbonation durations. RMC, reactive magnesia cement.

Table 12.1 Changes in the pH and composition of the external solution where samples were immersed. Characteristics

pH Magnesium, Mg (mg/L) Silicon, Si (mg/L) Total carbon, C (mg/L)

First in water

Second in HNO3 0.5 M

Third in HNO3 0.5 M

RMC

RMCFA

RMC

RMCFA

RMC

RMCFA

9.5 (6.9) 159 (0.2) 3 (0) 77 (0)

9.7 (6.9) 198 (0.2) 1 (0) 93 (0)

8.5 (1.2) 5633 (0.4) 5 (0) 236 (1.2)

8.6 (1.2) 5969 (0.4) 6 (0) 237 (1.2)

8.1 (1.2) 7481 (0.4) 5 (0) 231 (1.2)

7.8 (1.2) 7158 (0.4) 10 (0) 138 (1.2)

Notes:  Mg and Si were measured in diluted 1000 folds and acidified using 2% HNO3. Values in () indicate the original values in the external solution. FA, fly ash; RMC, reactive magnesia cement.

of the external solution following the immersion of RMC and RMC-FA concretes. The results indicated the leaching of Mg21, CO322, and OH2 ions when these samples were immersed in distilled water. The additional hydration of RMC within the submerged samples and the leaching of OH2 ions led to an increase in the pH of the external solution from 6.9 to 9.5
9.7, despite the leaching of CO322 ions from the pore solution.

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When exposed to Mg(NO3)2 0.5 M, periclase, brucite, and HMCs within the samples would be dissolved in the low pH of HNO3 solution, as seen in Eqs. (12.21)
(12.25). The dissolution of these phases and the leaching of dissolved magnesium nitrate (Mg(NO3)2) solution led to the coarsening of the pore structure over time. This resulted in a hostile environment suitable for the rapid movement of the acid solution into the cement matrix and the faster leaching of dissolved ions, thereby explaining an increase in the concentration of Mg21 ions in the external solution by .30 times after the immersing these samples in the acid for 7 days. A similar trend was observed in the concentration of CO322 ions. The further immersion of these samples in acid until 14 days increased in the concentration of Mg21 ions in the external solution by nearly 40 times. MgO 1 2HNO3 MgðNO3 Þ2 1 2H2 O

(12.21)

MgðOHÞ2 1 2HNO3 MgðNO3 Þ2 1 2H2 O

(12.22)

MgCO3 1 2HNO3 MgðNO3 Þ2 1 CO2 1 H2 O

(12.23)

MgCO3 3H2 O 1 2HNO3 MgðNO3 Þ2 1 CO2 1 4H2 O

(12.24)

MgCO3 : MgðOHÞ2 : 3H2 O 1 10HNO3 5MgðNO3 Þ2 1 4CO2 1 10H2 O

(12.25)

The changes in the mass of RMC samples were monitored at each step of water saturation and acid exposure (Fig. 12.8). The RMC samples were dried by wiping off each surface with a dry towel prior to measuring sample mass. Following an

Figure 12.8 Relationship between mass change and compressive strength of RMC concrete when subjected to acid solution. Changes in the mass and strength of concrete specimens at different durations of water saturation and acid exposure. RMC, reactive magnesia cement.

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increase in mass during saturation underwater, both RMC and RMC-FA concretes experienced a continuous decline in their mass under acid exposure. The dissolution of Mg phases and leaching of Mg21 ions were attributed to this reduction in mass of samples. The higher mass loss of RMC-FA concrete could be due to the erosion of FA particles in addition to the leaching of dissolved Mg phases. The lower hydration and carbonation of the RMC-FA concrete could lead to a higher porosity than the RMC concrete, which would make the RMC-FA concrete more vulnerable to acid attack and hence present a higher mass loss. The mass loss associated with the dissolution of Mg phases and leaching of Mg21 ions caused the reduction in compressive strength by 56% after 14 days of acid exposure (Muthu et al., 2020). While a reduction in the contents of some HMC phases and brucite within RMCFA concrete was observed after acid expose, the RMC concrete maintained a relatively comparable composition before and after exposure.

12.4.2 Chloride, sulfate, freeze-thaw, and seawater resistance of reactive magnesia cement-based concretes Pu and Unluer (2018) studied the durability of carbonated RMC-based concretes with/without the inclusion of FA and ground-granulated blast-furnace slag (GGBS) under chloride, sulfate, and seawater attacks and freeze-thaw condition for up to 6 months. Four samples were prepared for the investment. The binder of the control sample (CS) contained 100% RMC, while that of FA and GGBS samples included 50% FA and 50% GGBS, respectively, in addition to 50% RMC. The performance of these samples was compared with the PC sample, which used 100% PC. The replacement of RMC by other binders led to the reduction of the water/binder ratio of the CS from 0.6 to 0.45, 0.45, and 0.35 in samples FA, GGBS, and PC, respectively. The RMC samples were cured under the accelerated carbonation (i.e., B30 C, B80% relative humidity, and 10% CO2), while the PC sample was cured under the ambient condition (i.e., B30 C, B80% relative humidity, and B0.04% CO2). After 28 days of curing, all samples were subjected to four different aggressive conditions: (1) sodium sulfate (10%), (2) sodium chloride (10%), (3) seawater, and (4) freeze-thaw (i.e., saturated specimens were frozen at 220 C for 16 h, followed by thawing in a 10% sodium chloride solution at 28 C for 8 h, for 180 cycles). The obtained results revealed that sodium sulfate and freeze-thaw were the most aggressive environments leading to the severe deterioration of RMC-based concrete samples. Samples subjected to sodium sulfate (10%) or freeze-thaw experienced a strength loss by 10.7%
17.8% after 6 months, whereas seawater showed the lowest reduction in strength (5.8%
6.9%). The quantitative analysis of hydrate and carbonate phases obtained by X-ray and thermogravimetric analysis indicated that hydromagnesite was the major strength source in the RMC sample. The possible reaction between RMC samples and the aggressive solutions could be attributed to the strength loss. RMC-based samples showed higher strength results both before and after subjecting to aggressive environments when compared with PC-based samples. The microstructural analysis indicated that the stability of magnesium

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hydration and carbonation products contributed to the higher resistance of RMC in sodium sulfate and freeze-thaw environments. The decomposition of hydrate and carbonate phases (e.g., hydromagnesite) was responsible for the strength loss of RMC samples at longer exposure durations. In addition to a lower water/binder ratio, the inclusion of FA and GGBS samples with a filling effect and the formation of hydrate phases (e.g., hydrotalcite) led to a denser microstructure than the CS.

12.4.3 Corrosion resistance of reactive magnesia cement-based pastes Hay and Celik (2020) studied the corrosion resistance of RMC and PC paste samples. Accordingly, a rebar of 50 mm in length was embedded in the center of a 5 3 H10 cm cylinder sample. PC paste samples were cured under the moisture condition, while one set of RMC paste samples was cured under the moisture and another was cured under the accelerated carbonation (20% CO2, B80% relative humidity, 30 C). After curing for 28 days, samples were subjected to corrosion cycles of 14-day air drying and 14-day immersion in NaCl solution (3.5%). The research results indicated that the low pH level of pore solution in RMC samples under both moisture and accelerated carbonation conditions (9.9
10.5) could not enable the formation of passive films on the rebar surface to protect the substrate from corrosion. On the other hand, the formation of protective layers on the rebar surface was observed in the PC sample, whose pore solution’s pH ranged from 12.5 to 12.7. As a result, the rebars in the carbonated RMC samples exhibited a corrosion rate of Btwo orders of magnitude higher than those in the PC samples during the early cycles. The denser microstructure due to the formation of HMCs resulted in higher corrosion resistance of carbonated RMC samples than noncarbonated RMC samples. A corrosion rate of embedded rebars in samples measured by the polarization resistance is shown in Fig. 12.9. After 28 days of curing, the polarization resistance of PC sample was 3385 kΩ/cm2, while that of noncarbonated and carbonated RMC samples was 2.7 and 9.3 kΩ/cm2, respectively. The significantly lower polarization resistance of noncarbonated and carbonated RMC samples than PC sample was attributed to the lack of passivation due to the low pH of the pore solution within these samples. Subjecting the samples to the NaCl solution led to the ingress of chloride into the samples, resulting in the subsequent activation of the rebar surface and a reduction in the polarization resistance due to an acidic environment at the steel
concrete interface (Neville, 1995). A noticeable drop in polarization resistance was observed in the PC sample after three cycles, indicating the initiation into de-passivation and corrosion, during which there was a relatively small reduction in polarization resistance in noncarbonated and carbonated RMC samples. However, the polarization resistances of the PC sample were significantly greater than those of noncarbonated and carbonated RMC samples up to six cycles. Afterward, there was no significant change in polarization resistance of all samples. This period was responsible for the time required for the system to reach an equilibrium state.

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Figure 12.9 Polarization resistance of concretes after 12 cycles of corrosion. Corrosion resistance of PC, noncarbonated (MG), and carbonated RMC (MGC) samples over 12 cycles. RMC, reactive magnesia cement. Source: From Hay, R. & Celik, K. (2020). Hydration, carbonation, strength development and corrosion resistance of reactive MgO cement-based composites. Cement and Concrete Research, 128. https://doi.org/10.1016/j.cemconres.2019.105941.

Fig. 12.10 displays the corrosion density (corrosion rate) of samples over 12 cycles. In line with the polarization resistance results, the corrosion rates of the PC sample were lower than those of noncarbonated and carbonated RMC samples. The corrosion rates of the PC sample remained below 0.1 μA/cm2 for 3 cycles, reached 1.4 μA/cm2 after 6 cycles, and stabled afterward. On the other hand, the initial corrosion rate of the carbonated RMC sample was 2.1 μA/cm2, which was B225 times higher than that of PC sample but B2.5 times lower than that of the noncarbonated RMC sample (B5.3 μA/cm2). While the corrosion rates of carbonated RMC samples slightly increased over 12 cycles, those of noncarbonated RMC samples increased by Bsix times after six cycles (B32.5 μA/cm2).

12.5

Nano-tailored strain-hardening reactive magnesia cementitious composites

12.5.1 Mechanical properties To improve the application of RMC-based materials within the construction industry by enhancing their tensile ductility, Ruan et al. (2018) developed new RMC strain-hardening composites involving the inclusion of a small amount of polyvinyl

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Figure 12.10 Corrosion rate, corrosion density of PC, noncarbonated (MG), and carbonated RMC (MGC) samples over 12 cycles. RMC, reactive magnesia cement. Source: From Hay, R., & Celik, K. (2020). Hydration, carbonation, strength development and corrosion resistance of reactive MgO cement-based composites. Cement and Concrete Research, 128. https://doi.org/10.1016/j.cemconres.2019.105941.

alcohol (PVA) fibers. To improve the performance, a nanoscale surface coating on the PVA fiber surface was adopted. The resulting PVA-RMC composites demonstrated strain-hardening behavior as their tensile stress continued to increase even in the presence of a crack. The improved tensile ductility of fiber-reinforced samples was achieved via the formation of multiple fine cracks (,100 μm) with very small spacing (1
5 mm) (Li et al., 1995; Li et al., 2001; Yang & Li, 2014). Three mix proportions with different water/binder (W/B) ratios were prepared to investigate the tensile performance of RMC strain-hardening composites (Table 12.2). Sodium hexametaphosphate (Na(PO3)6) was used as a water reducer. To obtain desirable fiber dispersion, 30% RMC was replaced by FA, which reduced the yield stress without compromising viscosity in all samples. Paste samples were cast into cubic (50 3 50 3 50 mm) and dogbone molds (Fig. 12.11A), then stored in a sealed container for three days prior to demolding. After demolding, samples were cured under an accelerated carbonation condition (10% CO2, 30 C 6 1.5 C, and 85% 6 5%) for 7 days. The effect of curing duration was studied by extending the curing of sample RMC-0.41 until 28 days. The compressive strength of cubic samples was measured in accordance with (The Standard ASTM, 2013), while the uniaxial tensile test was performed at a loading rate of 0.02 mm/min (Fig. 12.11B), and two linear variable differential transformers were used to determine the extension of gauged length (60
70 mm). After the uniaxial tensile test, the crack width and spacing on each specimen were determined by a microscope at a magnification of 420 3 .

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Table 12.2 Mixture proportions of the prepared samples. Sample

RMC0.53 RMC0.47 RMC0.41

Mixture proportions (kg/m3)

W/ B

Curing duration (days)

RMC

FA

Water

PVA fiber

Na (PO3)6

0.53

799

342

601

26

59

7

0.47

853

365

573

26

53

7

0.41

864

370

510

26

51

7, 28

RMC, reactive magnesia cement.

Figure 12.11 Sample dimension and test setup. Uniaxial tensile test: (A) dimension of the dogbone specimen and (B) test setup.

Table 12.3 shows the compressive and uniaxial tensile strengths of samples. The tensile stress recorded at the occurrence of the first crack is referred to as the “first cracking strength,” whereas the tensile stress and tensile strain approaching specimen failure are referred to as the “ultimate tensile strength” and “tensile strain capacity,” respectively. The tensile strain capacity, crack spacing, and average crack width of samples are also presented in Table 12.3. The linear relationship between tensile stress and tensile strain was observed in all RMC-based fiber-reinforced samples in the elastic stage immediately after

Table 12.3 Mechanical test results. Sample

RMC0.53 RMC0.47 RMC0.41 RMC0.41

Curing duration (days)

Compressive strength (MPa)

7

Average crack width (μm) Frist cracking strength (MPa)

Ultimate tensile strength (MPa)

Tensile strain capacity (%)

Crack spacinga (mm)

4.6 6 0.07

1.09 6 0.11

1.32 6 0.09

1.40 6 0.66

12.80 6 3.50

146.80 6 22.90

7

16.27 6 0.47

1.60 6 0.13

2.89 6 0.26

2.13 6 1.02

2.66 6 1.10

49.74 6 12.88

7

18.97 6 0.47

2.19 6 0.48

2.61 6 0.48

2.64 6 1.22

1.45 6 0.14

47.21 6 15.08

28

-

2.38 6 0.24

3.67 6 0.35

2.70 6 0.96

1.74 6 0.27

63.77 6 10.38

RMC, reactive magnesia cement. a Crack spacing (mm) 5 Gauged length extension/crack number; a smaller crack spacing is usually associated with a higher degree of strain-hardening.

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applying uniaxial tensile load (Fig. 12.12). The increase in tensile load led to the introduction of the first crack, which was an indication of the start of the strainhardening stage. The progressive generation of multiple fine cracks was responsible for the slow increase in tensile stress as compared with tensile strain during the strain-hardening stage. Tensile stress dropped at the end of the strain-hardening stage because of the deterioration of fiber bridging followed by damage localization with increasing load. The spring effect of fiber bridging enabled the shrink of a majority of the cracks after the release of the tensile load upon specimen failure. The distribution of multiple fine cracks on a failed specimen is shown in Fig. 12.13 The layout of fibers at the location of the crack at failure is seen in Fig. 12.14, while the pulled-out fiber and the leftover fiber tunnel at a fracture point are shown in Fig. 12.14B and C. The smooth surface of fibers with a very little paste attached to it indicate that a majority of fibers were pulled out instead of ruptured, which implies the fiber strength was not fully utilized in the RMC-based materials and the control phase was the interface between fiber and paste.

Figure 12.12 Tensile stress versus strain curves of RMC concretes. Tensile stress versus strain curves of samples: (A) RMC-0.53
7d, (B) RMC-0.47
7d, (C) RMC-0.41
7d, and (D) RMC-0.41
28d. RMC, reactive magnesia cement.

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Figure 12.13 The crack pattern of RMC sample. Crack pattern of a typical sample (RMC0.41 at 7 days). RMC, reactive magnesia cement.

Figure 12.14 Failed specimen using fibers under tensile test. Illustration of a typical failed specimen (RMC-0.41 at 7 days): (A) fracture surface with several pulled fibers, (B) SEM image of a pulled-out fiber, and (C) SEM image of a leftover tunnel from the pulled fiber. RMC, reactive magnesia cement.

The dense microstructure obtained by reducing w/b ratio led to the higher compressive and tensile strengths. The decrease in w/b ratio also resulted in the improvement in the tensile strength and ductility, which were mainly determined by the fiber bridging. The reduction in crack spacing and crack width in samples with

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lower w/b ratio was an indication of more robust strain-hardening within these samples due to the enhanced fiber bridging. The increase in HMC formation over the longer CO2 curing duration (28 vs 7 days) strengthened the interface between fiber and pastes, resulting in 40% higher ultimate tensile strength in sample RMC-0.41. However, the increase in curing duration did not cause noticeable changes in the tensile ductility, crack space, and crack width. These results indicated the feasibility of enhancing the ultimate tensile strength of RMC strain-hardening composites via the curing duration without the compromise of strain-hardening robustness.

12.5.2 Self-healing performance After the tensile test at the age of 7 days, the autogenous crack healing properties of sample RMC-0.41 were investigated by Qiu et al. (2019). Accordingly, the samples were subjected to tensile strain levels of 0% (control group), 0.5%, and 1% and then conditioned to engage in autogenous healing. Four different environmental regimes for conditioning included (1) the ambient conditions (A, 22 C 6 3 C and 75% 6 9% relative humidity) for 20 days, (2) alternative water and ambient conditions (WA) for 20 days (i.e., 10 cycles of one day in the water followed by 1 day under the ambient conditions), (3) alternative water and accelerated carbonation conditions (WC) for 20 days (i.e., 10 cycles of one day in the water followed by 1 day under the accelerated carbonation, 10% CO2, 30 C, and 90% relative humidity), (4) accelerated carbonation conditions (C) for 10 days. Table 12.4 provides the details of three prestraining levels and four conditioning regimes prepared for the investigation of autogenous healing performance of 10 groups of specimens. Fig. 12.15 highlights the role of accelerated CO2 concentration in reducing the crack width on the surface of samples. While the ambient conditions could not reduce the surface crack widths (Fig. 12.15A) and the water/ambient conditions slightly reduced the surface crack widths (Fig. 12.15B) and could just seal those less than 30 μm (Fig. 12.15B and Fig. 12.16B), the use of elevated CO2 concentration enabled the surface self-healing of cark widths over 50 μm (Fig. 12.15C and D). However, while self-healing was observed on the surface, the cracks deep inside remained unhealed (Fig. 12.16C). The lack of water provision would limit the continuous hydration of unhydrated MgO and subsequent formation of Mg(OH)2(aq,s) available for further formation of HMCs, inhibiting the surface self-healing of some samples under accelerated CO2 conditions (Fig. 12.15D). The uniaxial tensile stress
strain curves (i.e., prestraining before conditioning and reloading after conditioning) of RMC-based strain-hardening composite samples were shown in Fig. 12.17. Table 12.5 summarizes the postconditioning tensile properties of specimens, and Fig. 12.18 demonstrates the recovery ability of elastic stiffness, tensile strength, and strain of samples after subjecting different conditioning regimes. Fig. 12.18 highlights the role of water and/or CO2 in the recovery of mechanical tensile properties of RMC-based strain-hardening composite samples. While the elastic stiffness recovery of A-5 sample was 11% of its control, that of other conditioning samples ranged from 46% to 105%, and of these the elastic stiffness recovery of WA-5 sample was the most noticeable (Fig. 12.18A). Samples A-

Table 12.4 Details of prestraining levels and conditioning regimes. Group

A-0a A-5 WA0a WA-5 WA10 WC-0b WC-5 WC10 C-0 C-5

No. of specimen

5 5 5

No. of conditioning cycles

Total duration (days)

2-day ambient

10

20

1-day water/1-day ambient

10

20

Conditioning regimes

6 5

b c

Prestrain level (%)

No. of cracks

Crack widtha (μm)

0 0.5 0


3.4 6 2.2



47 6 28


0.5 1

4.7 6 2.3 8.6 6 2.7

36 6 30 62 6 15

5 6 5

1-day water/ 1-day CO2

10

20

0 0.5 1


4.3 6 2.5 7.4 6 4.9


33 6 12 64 6 50

5 6

1-day CO2c

10

10

0 0.5


5.6 6 3.6


32 6 26

For individual specimen, crack width 5 residual tensile displacement/crack number. Included as the control group cured under the same conditioning regime. Water was sprayed on the specimen surface once a day.

a

Preloading

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Figure 12.15 Crack widths of RMC specimens. Crack widths of RMC-based strainhardening composite specimens before and after (A) ambient, (B) water/ambient, (C) water/ CO2, and (D) CO2 conditioning; for each data point. RMC, reactive magnesia cement.

5, WA-10, and C-5 showed similar ultimate tensile strength to their control samples, while the water and CO2 conditioning regimes increased the ultimate tensile strength of WA-5, WC-5, and WC-10 by 23%, 15%, and 15%, respectively (Fig. 12.18B), and their total strain capacity by 73%, 44%, and 66%, respectively (Fig. 12.18C). Figs. 12.19 and 12.20 show microstructural analyses of healed crack regions of samples conditioned under WA and WC, respectively. The healing products were seen in both the surface and the interior regions at the crack of samples under WA conditioning (Fig. 12.19). The WC conditioning led to denser healing products, enabling complete healing at the surface region (Fig. 12.20A). The mapping element (Fig. 12.19B and Fig. 12.20A) indicated the healing products consisting of Mg, C, and Si. While the presence of Mg and C implied the formation of HMCs, the presence of Si was attributed to the use of FA. The rapid formation of HMCs at the surface region would inhibit the diffusion of water and CO 2 and restrain the further formation of healing products in the

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Figure 12.16 Crack sealing of specimens subjected to different conditions. (A) No apparent crack sealing of specimens subjected to ambient conditioning, (B) complete crack sealing of specimens subjected to water/ambient conditioning, and (C) crack sealing on the surface of specimens subjected to water/CO2 conditioning.

interior region (Fig. 12.20B). The thin-flake morphology in the sample under WA conditioning and the rod-like morphology in the sample under WC indicated the formation of hydromagnesite/dypingite and nesquehonite within these samples, respectively. The assessment of healing performances of samples under A, WA, WC, and C is presented in Table 12.6. The A conditioning gave no healing, while the high CO2 concentration of the C condition only led to the seal at the surface of the crack region but no mechanical recovery. The results indicated the need for water for the continuous hydration of MgO and further formation of HMCs (Vandeperre & Al-

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Figure 12.17 Preloading and reloading uniaxial tensile stress
strain curves. Preloading and reloading uniaxial tensile stress
strain curves of (A) A-5, (B) WA-5, (C) WA-10, (D) WC-5, (E) WC-10, and (F) C-5 specimens.

Tabbaa, 2007; Russell et al., 2001). The WA and WC led to noticeable crack sealing and mechanical recovery. However, the healing performances under WA were sensitive to the prestrain level. While WA-5 achieved more recovery than WC-5 and WC-10, the healing of WA-10 was limited.

Table 12.5 Postconditioning tensile properties of reactive magnesia cement-based strain-hardening composites. Samples

Elastic stiffness (GPa)

First cracking strengtha (MPa)

Ultimate tensile strength (MPa)

Reloading strain capacity (%)

Total strain capacityb (%)

Crack spacing after failure (mm)

A-0c A-5 WA-0c WA-5 WA-10 WC-0c WC-5 WC-10 C-0c C-5

8.91 6 3.76 1.01 6 0.37 7.85 6 3.16 8.26 6 1.52 4.13 6 1.43 9.13 6 5.92 6.78 6 3.13 6.02 6 1.50 8.39 6 1.52 3.82 6 1.11

2.76 6 0.60 2.58 6 0.24 2.46 6 0.26 2.36 6 0.48 1.94 6 0.33 2.43 6 0.38 2.52 6 0.39 2.38 6 0.45 1.71 6 0.67 1.52 6 0.14

3.08 6 0.34 3.01 6 0.13 3.02 6 0.15 3.71 6 0.27 3.03 6 0.77 2.96 6 0.45 3.41 6 0.50 3.41 6 0.20 2.56 6 0.44 2.85 6 0.24


1.33 6 0.76
1.92 6 0.95 1.14 6 1.12
1.41 6 0.77 1.41 6 0.99
1.88 6 0.93

1.69 6 0.40 1.47 6 0.36 1.33 6 0.37 2.30 6 0.49 1.66 6 0.58 1.11 6 0.26 1.60 6 0.44 1.84 6 0.50 1.89 6 0.76 2.01 6 0.29

7.3 6 5.5 5.8 6 4.2 6.9 6 5.8 3.2 6 1.7 4.3 6 2.7 9.7 6 6.1 4.2 6 3.4 7.6 6 4.0 11.3 6 4.4 9.0 6 8.9

a

For group A-5, WA-5, WA-10, WC-5, WC-10, and C-5, the value was taken as the tensile stress at the end of elastic stage. For group A-5, WA-5, WA-10, WC-5, WC-10, and C-5, the value equals to the residual strain from preloading plus the strain because of reloading. Included as the control group cured under the same conditioning regime.

b c

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Figure 12.18 The recovery ability of elastic stiffness, tensile strength, and strain of samples. (A) Elastic stiffness ratio, (B) ultimate tensile strength ratio, and (C) strain capacity ratio of RMCCbased strain-hardening composites subjected to different conditioning regimes normalized by the properties of corresponding control group (i.e., A-0, WA-0, WC-0, and C-0). RMC, reactive magnesia cement.

12.6

Other applications

12.6.1 Reactive magnesia as alkali activator The use of RMC as an alkali activator has been widely studied. The hydration of RMC results in the formation of Mg(OH)2(aq,s) and the increase in pH of pore solution. The impurities in RMC (commonly CaO) increases the equilibrium pH of the dissolved RMC paste from 10.5 to 11.3, which is sufficient to break the Si
O and/or Al
O bonds in by-products. The dissolution of silica and alumina depends on the OH2 concentration per unit surface of silica/alumina. The absorption of OH2 ions increases the coordination number of the silicon/aluminum atom on the surface of the silica/alumina particles, which consequently weakens the oxygen bonds to the underlying silicon/aluminum atoms. Silicon/aluminum atoms go into the solution and readily complex with metallic cations in solution to allow the further polycondensation of magnesium silicate hydrate (M
S
H) (Zhang 2011; Jia et al., 2016; Jin & Al-Tabbaa, 2014a,b), and/or calcium (aluminum)

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Figure 12.19 Microstructure of healed crack regions of samples conditioned under WA. Microstructural characterization of healing products formed under WA conditioning: (A) the microstructure of near-surface region of the healed crack; (B) the microstructure and EDX mapping (the entire image b1) of interior region of the healed cracked. The dashed lines show the original crack width.

silicate hydrate (C
(A)
S
H), and hydrotalcite (Yi et al., 2014; Jin, Gu, & AlTabbaa, 2015). The reaction between brucite and amorphous silica also forms M
S
H (Vandeperre & Al-Tabbaa, 2007; Zhang et al., 2014). The most byproducts used in RMC-activated materials are (1) silica fume (i.e., a byproduct of the silicon and ferrosilicon alloy production) (Zhang et al., 2011; Jia et al., 2016; Jin & Al-Tabbaa, 2014a,b; Zhang et al., 2014; Lothenbach et al., 2015; Nied et al., 2016), (2) GGBS (i.e., a byproduct of the steel and iron industry) (Jin & Al-Tabbaa, 2014a,b; Yi et al., 2014; Jin, Gu & Abdollahzadeh, & Al-Tabbaa, 2015; Jin, Gu, & Al-Tabbaa, 2015; Zheng et al., 2019), (3) FA (i.e., a byproduct

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Figure 12.20 Microstructural analysis at the near-surface region of the healed crack. Microstructural characterization of healing products formed under WC conditioning: (A) the microstructure and EDX mapping of the near-surface region of the healed crack. The dashed lines show the crack width, (B) no healing products were identified in the interior part of the crack.

from coal combustion in electric power plants) (Vandeperre & Al-Tabbaa, 2007; Dung et al., 2020), (4) rice husk ash or RHA (i.e., a byproduct from the burning of rice hulls) (Sonat & Unluer, 2019).

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Table 12.6 Qualitative assessment of autogenous healing subjected to different conditioning regimes. Conditioning

Crack width reduction

Stiffness recovery

Tensile strength/strain capacity enhancement

A WA

None Medium

WC C

Excellent Good

None Excellent but sensitive to prestrain level Good and robust Limited

None Excellent but sensitive to prestrain level Good and robust None

Since the actual ratio of Mg/Si in the synthetic M
S
H ranges from 0.67 to 1 (Zhang et al., 2011; Brew & Glasser, 2005), RMC and silica are often blended by a ratio of 40 wt.% RMC and 60 wt.% silica fume (Zhang et al., 2014). The formation of M
S
H enabled RMC
silica fume pastes to gain a compressive strength of 16 MPa (Jin & Al-Tabbaa, 2014a,b). The use of sodium hexametaphosphate (NaHMP) as a dispersant agent allowed the reduction in water/binder ratios of 0.8 to 0.4, thereby increasing the compressive strength of RMC
silica fume pastes to 70 MPa (Lothenbach et al., 2015). In addition to the reduction in w/b ratio, the absorption of phosphate species on the MgO inhibits the nucleation of the Mg(OH)2, increasing the concentration of Mg21 ions and the pH of pore solution (pH .12). The improved dissolution of silica due to the increase in pH coupled with the increase in concentration of Mg 21 ions enhanced the formation of M
S
H gel (Jia et al., 2016). Despite the initially accelerated hydration and strength development at the elevated temperature (60 C), the sealed conditions at 30  C yielded the highest strengths for RMC
silica fume pastes at the later ages (Sonat et al., 2017b). The use of hydromagnesite as nucleation nano-seeding improved the hydration of RMC
silica fume systems and their compressive strengths (Singh et al., 2020). RMC-activated GGBS more effectively than hydrated lime (Yi et al., 2014). Hydrotalcite and C
(A)
S
H were the main hydration products of RMC
GGBS systems. The replacement of less than 12% GGBS with FA improved the workability of RMC
GGBS
FA systems without the compromise of the compressive strength. The further carbonation of RMC
GGBS
FA concretes converted the unreacted brucite into the HMCs and increased their compressive strengths by up to B100% (i.e., from 10
15 to 20
27 MPa) (Dung et al., 2020). On the other hand, RMC was also used as an alkali activator to activate RHA, which mainly contains amorphous silica (Sonat & Unluer, 2019). The RMC
RHA samples achieved highest compressive strength at early ages (7 days), but their strength development was slower than the RMC
silica fume samples at the later stage.

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12.6.2 Reactive magnesia to accelerate the activator and hydrated magnesium carbonates as nano-seeding materials Although the use of sodium carbonate (Na2CO3) as an alkali activator offers many advantages for alkali-activated GGBS materials such as cost-effective, less caustic, and low drying shrinkage (Roy et al., 2000; Duran Ati¸s et al., 2009), it has not been widely reported because of the long setting time and low strength development that it causes (Yuan et al., 2017; Ke et al., 2016). The initially low alkalinity of Na2CO3 solution prolongs the weakening and breaking of the bonds of Si
O and Al
O, thereby delaying the setting time of Na2CO3-activated GGBS materials. The introduction of RMC accelerated the consumption of CO322 ions from Na2CO3 solution to form HMCs (e.g., hydromagnesite (Fig. 12.21)), which subsequently increased the pH of pore solution of the Na2CO3-activated GGBS samples and improved the dissolution of GGBS. As a result, the inclusion of RMC significantly shortened the setting times and improved the compressive strengths of Na2CO3-activated GGBS samples (Dung et al., 2019). On the other hand, the introduction of hydromagnesite as nucleation nano-seeding (S) stimulated the formation of the hydrate and carbonate phases (e.g., hydrotalcite, gaylussite, and calcite), which further increased the pH of pore solution of Na2CO3-activated GGBS samples. Moreover, the simultaneous inclusion of RMC and S further enhanced the hydration kinetics and mechanical performance of Na2CO3-activated GGBS concretes (Dung et al., 2019; Dung et al., 2021). Table 12.7 presents the proposed mixture proportions using RMC and/or S to improve the hydration kinetics and mechanical performance of Na2CO3-activated GGBS concretes (Dung et al., 2019). RMC and/or S were used by 10% and 0.5% of the precursor, respectively. Fig. 12.22 indicates the higher pH of pore solution of samples M, S, and S.M than the CS during the first 24 h of hydration. This increase

Figure 12.21 Hydromagnesite formation within Na2CO3-activated GGBS samples using M. The formation of hydromagnesite within M sample. GGBS, ground-granulated blast-furnace slag.

Table 12.7 The mixture proportions of Na2CO3-activated ground-granulated blast-furnace slag concretes using reactive magnesia cement (RMC) and/or S. Mix

CS M S S.M

Precursor (%)

Mixture composition (kg/m3)

W/B

Slag

S

RMC

100 90 99.5 89.5

0 0 0.5 0.5

0 10 0 10

0.55

Slag

S

RMC

Na2CO3

Coarse aggregates

Water

600 540 597 537

0 0 3 3

0 60 0 60

48

1050

356.4

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Figure 12.22 pH of pore solution of Na2CO3-activated GGBS samples. pH values of Na2CO3-activated GGBS during the first 24 h. GGBS, ground-granulated blast-furnace slag.

in pH values of the pore solution accelerated the hydration of these samples. Fig. 12.23 shows the heat flow of samples over 210 h of hydration. All samples had three main stages of hydration. The first stage of the hydration was the preinduction period which was associated with a heat release during the first 2 h and corresponded to the increase in pH of the pore solution (Fig. 12.22) because of the dissolution of GGBS and M and the subsequent formation of carbonates (e.g., calcite and gaylussite) (Ke et al., 2016; Ferna´ndez-Jime´nez & Puertas, 2001). The preinduction period was followed by the induction period, characterized by ta very low heat release and a decrease in pH of the pore solution. The last stage of the hydration was the acceleration and deceleration period, which was associated with a high heat release from the nucleation, growth, and precipitation of the reaction products. The introduction of M or S led to exothermal peaks that appeared noticeably earlier with higher intensities and higher initial slopes during the preinduction and acceleration and deceleration periods (Fig. 12.23). The earlier and more intense acceleration and deceleration peaks of M, S, and S.M samples indicated the rapid

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Figure 12.23 Heat flow of samples over 210 h of hydration. Heat flow of Na2CO3-activated GGBS paste samples. GGBS, ground-granulated blast-furnace slag.

growth of their reaction products, which was responsible for their earlier setting times and higher compressive strengths. The CS demonstrated slow initial (94.3 h) and final (148.2 h) setting times and low compressive strength (14 MPa after 28 days). The use of RMC and/or S significantly shortened the setting times of M, S and S.M samples to 12.2, 13.1, and 7.8 h (initial setting) and to 19.2, 19.7, and 12.8 h (final setting), respectively; and increased their 28-day compressive strength to 32, 35, and 38 MPa, respectively (Dung et al., 2019). The use of M and S not only improved the hydration kinetics, subsequent setting times, and mechanical performance, but it also enhanced the carbonation resistance of Na2CO3-activated GGBS samples (Dung et al., 2021). Fig. 12.24 shows the compressive strength of Na2CO3-activated GGBS concrete samples cured under accelerated carbonation (i.e., 10% CO2, 80% RH, and B30 C) after 28 days of curing under the ambient conditions. The decalcification of C
(A)
S
H, associated with the expulsion of Al and Si, led to the loss of 53% (i.e., by mass) of hydration products and 57% loss of compressive strength (6 vs 14 MPa). The higher alkalinity of the pore solution within M, S, and S.M samples provided a higher CO2 buffering capacity, leading to the partial conversion of the crystalline C
(A)
S
H into amorphous phases during carbonation instead of decomposing. Furthermore, the increase in the formation of hydrotalcite-like with layered double hydroxides within samples incorporating M allowed the sequestration of CO2 via the formation of huntite, thereby retarding the decalcification of C
(A)
S
H. Moreover, the formation of HMCs as

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Figure 12.24 Compressive strength of Na2CO3-activated GGBS samples subjected to accelerated carbonation. Compressive strength of Na2CO3-activated GGBS concrete samples under accelerated carbonation after 28 days of ambient curing. GGBS, ground-granulated blast-furnace slag.

a result of the reaction between Mg phases provided by M and CO2 during the carbonation process (Fig. 12.25) contributed to the formation of a continuous network of binding phases. These advancements in the hydration and carbonation reactions facilitated further strength gain and resulted in higher compressive strengths of Na2CO3activated GGBS concretes after the carbonation (Dung et al., 2021). As seen in Fig. 12.24, the compressive strengths of M and S.M samples increased from 32 and 38 MPa to 42 and 54 MPa, respectively after 90 days of accelerated carbonation. The use of S reduced the strength loss due to the decalcification of C
(A)
S
H by 21% (38 vs 30 MPa) when compared with the CS (57%).

12.6.3 Hydraulic binders of MgO
hydromagnesite A low-carbon hydraulic binder based on RMC was created by mixing MgO and hydromagnesite (HY) (Vlasopoulos & Robert Cheeseman, 2008; Rani Devaraj et al., 2010; Flatt et al., 2012). When compared with the hydration of pure MgO, the use of up to HY 50% in MgO
HY blends resulted in a more rapid and extensive dissolution of RMC (Fig. 12.26) and significantly improved morphology of hydrated products (Fig. 12.27). Accordingly, the use of MgO
HY blends gave noticeably strong hydraulic binder compared to MgO hydrate alone, which had almost no binding capacity (Kuenzel et al., 2018). These MgO
HY binders

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Figure 12.25 HMCs formation within samples including M. Formation of HMCs after 28 days of carbonation of samples: (A) M and (B) S.M. HMCs, hydrated magnesium carbonates.

Figure 12.26 Heat released from the hydration of MgO paste. Isothermal conduction calorimetry data for the pure MgO and 9:1 MgO:HY pastes. Heat output is per kg of total sample mass. Source: From Kuenzel, C., Zhang, F., Ferra´ndiz-Mas, V., Cheeseman, C. R., & Gartner, E. M. (2018). The mechanism of hydration of MgO-hydromagnesite blends. Cement and Concrete Research, 103, 123
129. https://doi.org/10.1016/j.cemconres.2017.10.003.

containing high levels of carbonate would significantly reduce the carbonation footprint. Moreover, their applications would be wider than the carbonated RMC as their hardening and strength gain do not require a high concentration of CO2, which is not available in many locations.

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Figure 12.27 Morphology of hydrated M SEM images of (A) as-received MgO; (B) asreceived HY sample; (C) and (D) 9:1 MgO: HY pastes after 7 days of hydration; (E) and (F) 9:1 MgO: HY pastes after 28 days hydration. Source: From Kuenzel, C., Zhang, F., Ferra´ndiz-Mas, V., Cheeseman, C. R., & Gartner, E. M. (2018). The mechanism of hydration of MgO-hydromagnesite blends. Cement and Concrete Research, 103, 123
129. https://doi.org/10.1016/j.cemconres.2017.10.003.

Fig. 12.26 demonstrates the inclusion of 10% HY greatly accelerated the early hydration of MgO compared with the pure MgO sample. This early accelerated hydration of MgO would be attributed to the role of HY acting as growth sites for hydrates (Kuenzel et al., 2018). After the early accelerated hydration, the rate of heat evolution is lower for the MgO
HY blend, implying the higher unhydrated MgO in this blend than the pure sample at the later ages. This suggests that the additional hydrates produced in the blend sample might act as surface blocking or diffusion barriers, thus inhibiting further hydration of MgO (Kuenzel et al., 2018).

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Despite the lower hydration degree of MgO, the MgO
HY pastes produced significantly higher compressive strengths (i.e., B24 MPa after 28 days) than the pure MgO paste (insufficient strength to demold). During the hydration, CO322 ions released from HY could react with Ca21 ions released from the hydration of CaO (an impurity in the MgO source) to form a small amount of calcite at early ages, which partially contributed to the strength development of MgO
HY pastes. Furthermore, a new poor crystallinity or amorphous phase corresponded to a broad peak of brucite was seen in the X-ray diffraction pattern of MgO
HY pastes (Fig. 12.28) (Kuenzel et al., 2018). This poor crystallinity or amorphous phase would have a high degree of cohesion, which contributed to significant strength gain of MgO
HY samples. The pure MgO sample did not show any dehydration (Fig. 12.29A), and HY gave only dehydration at B270 C (Fig. 12.29B and Fig. 12.30B), whereas the MgO
HY sample produced a significant amount of additional hydrate phase with a thermal decomposition peak B100 C (Winnefeld et al., 2019). This hydrate phase presented after 6 h of hydration and did not increase significantly afterward. It is

Figure 12.28 X-ray diffraction patterns of hydrated MgO and MgO-HY pastes. Powder Xray scans of hydrated MgO and MgO-HY pastes. B: brucite, C: Calcite, H 5 hydromagnesite, M: Magnesite, and P: MgO. Source: From Kuenzel, C., Zhang, F., Ferra´ndiz-Mas, V., Cheeseman, C. R., & Gartner, E. M. (2018). The mechanism of hydration of MgO-hydromagnesite blends. Cement and Concrete Research, 103, 123
129. https://doi.org/10.1016/j.cemconres.2017.10.003.

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Figure 12.29 Thermogravimetric analyses for samples with and without HY. Thermogravimetric analyses after different hydration times of (A) MgO, (B) MgO-HY (70:30). Source: From Winnefeld, F., Epifania, E., Montagnaro, F., & Gartner, E. M. (2019). Further studies of the hydration of MgO-hydromagnesite blends. Cement and Concrete Research, 126. https://doi.org/10.1016/j.cemconres.2019.105912.

hypothesized that this hydrate phase corresponded to the poor crystallinity or amorphous phase, which led to cohesive binding and strength gain in these blends (Winnefeld et al., 2019).

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Figure 12.30 TG-IR curves of samples with and without HY. Thermogravimetric analysis coupled with FT-IR of (A) pure hydromagnesite and (B) MgO-HY (70:30) hydrated for 28 days. Source: From Winnefeld, F., Epifania, E., Montagnaro, F., & Gartner, E. M. (2019). Further studies of the hydration of MgO-hydromagnesite blends. Cement and Concrete Research, 126. https://doi.org/10.1016/j.cemconres.2019.105912.

12.6.4 Reactive magnesia cement for 3D printing RMC demonstrated as a suitable material for 3D printing. In addition to magnesium acetate (0.1 M) used to accelerate the hydration and carbonation degrees, caustic MgO (3 wt.%) was employed to improve the hydration and buildability of RMC mortar (i.e., accelerated layer hardening and strength development at early stages) (Khalil et al., 2020). Furthermore, a superplasticizer (i.e., polycarboxylate ether), a suspension
aid additive (i.e., hydroxyethyl
cellulose) and a de-foamer (i.e., a nonionic surfactant) were used to reduce the plastic viscosity, improve the paste homogeneity and reduce the air entrapment, respectively (Khalil et al., 2020). The resulting mixture performed excellent extrudability, followability, and buildability, which enabled the 3D printing with complex geometries for a duration of up to 60 min without any flow interruption or structural collapse (Fig. 12.31). The 3Dprinted RMC samples exhibited excellent shape retention and overall integrity even after accelerated carbonation. The cylindrical cast and 3D-printed RMC samples (Fig. 12.32A) were cured for 3 days under the ambient condition and further 7 days of accelerated carbonation condition (20% CO2, 30 C, and 80% relative humidity) prior to the compressive strength tests. Despite higher porosity (59.1% vs 57.8%), the 3D-printed sample had significantly higher compressive strength than the cast sample (31 vs 16 MPa). The higher strength gain of the 3D-printed sample could be attributed to the improved carbonation that was obtained by the better CO2 diffusion due to the higher porous nature of the 3D-printed sample (Khalil et al., 2020). However, the 3D-printed sample displayed delamination of the outer wall which is formed by the successively deposited perimeters (the exterior boundary of each layer), followed by the failure of the sample core which is formed by the successively deposited infills of each layer. This failure indicated the limited strength gain of the 3D-printed sample due to the perimeter/infill adhesion (Khalil et al., 2020).

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Figure 12.31 3D-printed structures of RMC samples. 3D-printed structures with dimensions in mm. RMC, reactive magnesia cement. Source: From Khalil, A., Wang, X., & Celik, K. (2020). 3D printable magnesium oxide concrete: towards sustainable modern architecture. Additive Manufacturing, 33, 101145. https://doi.org/10.1016/j.addma.2020.101145.

RMC-activated slag paste demonstrated as a sustainable material for spray-based 3D printing (Lu et al., 2020). The initial setting time of RMC-activated slag paste (i.e., 40% RMC and 60% slag, and water/binder of 0.32) was 67 min, which was suitable for the working window of 3D printing as it was not long to limit the printing height and not too short to narrow the working window for offline 3D printing. The addition of 40% FA cenosphere (FAC) increased the initial setting time of RMC-activated slag to 100 min. Furthermore, the use of FAC reduced plastic viscosity and dynamic yield stress, but increased static yield stress the fresh RMCactivated slag mixtures, resulting in good delivery and deposition performance for 3D printing. The RMC-activated slag mixtures including FAC presented better spray-printing quality and the buildup thickness remained almost uniform in the single or multiple layers. The use of more than 70% industrial by-products and less than 30% RMC significantly reduced the CO2 emission and environmental impact of the RMC-activated slag
FAC mixtures when compared with the current spraybased 3D printing mixtures in the literature (Lu et al., 2020; Lindemann et al., 2019).

12.7

Future outlook

The low pH of pore solution and the demand for accelerated CO2 curing for strength gain limits the application of RMC in concrete structures. Besides, the inhibition of CO2 diffusion into the sample core leads to the low conversion of MgO and brucite into HMCs. The high contents of unreacted MgO and brucite reflect the low utility of RMC in carbonated RMC-based materials. Moreover, the further contact of MgO and brucite with water and CO2 during the entire service life of materials can result in volume stability problems.

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Figure 12.32 3D-Printed and cast RMC samples. Compressive strength and failure mechanism of cast and 3D-printed RMC samples. (A) Photograph showing cylindrical samples that are nearly 25 mm in both diameter and height, (B) Compressive strength curves, and (C) state of samples after the compression test. RMC, reactive magnesia cement. Source: From Khalil, A., Wang, X., & Celik, K. (2020). 3D printable magnesium oxide concrete: towards sustainable modern architecture. Additive Manufacturing, 33, 101145. https://doi.org/10.1016/j.addma.2020.101145.

To improve the utility and performance of carbonated RMC-based materials, RMC should be partially replaced by industrial by-products such as GGBS or FA. This replacement not only reduces the amount of RMC use but also enables the formation of C
(A)
S
H, M
S
H and hydrotalcite at the sample core, where the formation of HMCs is limited due to the inhibition of CO2 diffusion. In the

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presence of GGBS and FA, the uncarbonated brucite will be consumed in the formation of M
S
H and hydrotalcite to improve the mechanical performance and long term durability of carbonated RMC-based materials. The use of GGBS or FA also enhances the sustainability of carbonated RMC-based materials. Despite the significant improvement in mechanical performance, the hydration of RMC
hydromagnesite binders displays the noticeable contents of unhydrated MgO and brucite. The further accelerated carbonation of MgO and brucite is suggested to enhance the utility of RMC and the mechanical performance of RMCbased materials.

Acknowledgments The authors acknowledge the financial support from the Singapore MOE Academic Research Fund Tier 1 (RG 95/16) and the Singapore MOE Academic Research Fund Tier 2 (MOE2017-T2
1
087 (S)).

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504. Available from https://doi.org/ 10.1111/j.1151-2916.1980.tb10752.x. Vandeperre, L. J., & Al-Tabbaa, A. (2007). Accelerated carbonation of reactive MgO cements. Advances in Cement Research, 19(2), 67
79. Available from https://doi.org/ 10.1680/adcr.2007.19.2.67. Vlasopoulos, N., & Robert Cheeseman, C. (2008). Binder composition. Walling, S. A., & Provis, J. L. (2016). Magnesia-based cements: A journey of 150 years, and cements for the future? Chemical Reviews, 116(7), 4170
4204. Available from https:// doi.org/10.1021/acs.chemrev.5b00463. Winnefeld, F., Epifania, E., Montagnaro, F., & Gartner, E. M. (2019). Further studies of the hydration of MgO-hydromagnesite blends. Cement and Concrete Research, 126. Available from https://doi.org/10.1016/j.cemconres.2019.105912. Wright, J., & Colling, A. (1995). Seawater: its Composition, Properties and Behaviour. Available from https://doi.org/10.1016/C2013-0-10208-5. Yang, E. H., & Li, V. C. (2014). Strain-rate effects on the tensile behavior of strainhardening cementitious composites. Construction and Building Materials, 52, 96
104. Available from https://doi.org/10.1016/j.conbuildmat.2013.11.013. Yi, Y., Al-Tabbaa, A., & Liska, M. (2014). Properties and microstructure of GGBS-magnesia pastes. Advances in Cement Research, 26(2), 114
122. Available from https://doi.org/ 10.1680/adcr.13.00005. Yuan, B., Yu, Q. L., & Brouwers, H. J. H. (2017). Time-dependent characterization of Na2CO3 activated slag. Cement and Concrete Composites, 84, 188
197. Available from https://doi.org/10.1016/j.cemconcomp.2017.09.005. Zhang, T., Cheeseman, C. R., & Vandeperre, L. J. (2011). Development of low pH cement systems forming magnesium silicate hydrate (M-S-H). Cement and Concrete Research, 41(4), 439
442. Available from https://doi.org/10.1016/j.cemconres.2011.01.016. Zhang, T., Vandeperre, L. J., & Cheeseman, C. R. (2014). Formation of magnesium silicate hydrate (M-S-H) cement pastes using sodium hexametaphosphate. Cement and Concrete Research, 65, 8
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86112. Available from https://doi.org/10.1039/c5ra17031a. Zheng, J., Sun, X., Guo, L., Zhang, S., & Chen, J. (2019). Strength and hydration products of cemented paste backfill from sulphide-rich tailings using reactive MgO-activated slag as a binder. Construction and Building Materials, 203, 111
119. Available from https:// doi.org/10.1016/j.conbuildmat.2019.01.047.

Future developments and challenges of nano-tailored cementitious composites

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Arslan Akbar and K.M. Liew Department of Architecture and Civil Engineering, City University of Hong Kong, Kowloon, Hong Kong, China

13.1

Background

Cement composites are the most widely used materials in the world because of their excellent properties and low cost as compared to other composites (Akbar et al., 2021a,b; Farooq et al., 2020a,b). The use of cement composites has been tremendously increased in the past decade, and China has consumed about 60% of the total consumption per capita. Cement, being the primary component of cementitious composites, consumes a significant amount of energy and resources during its production and causes a significant environmental impact in terms of CO2 emissions (Akbar & Liew, 2020). However, cement composites consume less energy as compared to other composites. Similarly, cementitious composites’ production accounts for about 5% of total greenhouse gas emissions around the globe. On the other hand, cement composites can absorb CO2, which is termed as carbon sequestration. Studies showed that from 1930 to 2013, about 4.5 Gt of carbon was absorbed by cement composites owing to carbonation reaction in cement composites, and this accounted for about 43% of CO2 emission during the calcination process of cement production during that period (Power et al., 2017). Another fundamental reason for the high demand for cement composites lies in their composition, as earth crust comprises about 98% of the primary components of cement composites. Owing to the increase in urbanization, the demand for cement composites has rapidly increased in recent years, which has confronted serious challenges to meet modern society’s sustainability goals. Cement composites are believed to possess the most complex structure owing to their multiscale, multiphase, and multicomponent nature (Kai et al., 2021a, 2021b). Owing to the multiphase structure, the thermodynamic instability can cause crack initiation in the cement composites. Similarly, the cement composites are vulnerable to cracks due to tensile load application (Liew & Akbar, 2020). Cement composites show minimal deformation and are susceptible to cracks even at small strain levels. These cracks weaken the structural integrity, affect structures’ serviceability, and pose more severe challenges in extreme environments. The generic problems of Recent Advances in Nano-Tailored Multi-Functional Cementitious Composites. DOI: https://doi.org/10.1016/B978-0-323-85229-6.00007-X © 2022 Elsevier Ltd. All rights reserved.

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cement composites related to low tensile strength against cracks can be addressed by reinforcing the cement matrix at the nanoscale. The inclusion of nanoreinforcement into the cement composites results in excellent resistance against crack initiation and propagation; therefore a new generation of cement composites can be developed, which possesses significant energy-absorbing capacity before failure.

13.2

Introduction

Nanotechnology is an emerging field to understand the material properties at the nanoscale. Remarkabledevelopments in nanotechnology have resulted in a significant increase in the use of nanomaterials in various fields owing to their excellent mechanical and multi-functional properties. Nanomaterials have also found their application in cement composites as nano-reinforcement, and significant improvements have been achieved in the mechanical properties of cement composites. To date, significant advancements have been made in terms of nano-engineered cement composite, and basic research ideas are becoming apparent. In cement composites, significant progress has been reported, but many issues still need dedicated attention by the researchers. Specifically, issues related to the mechanism of reinforcement at the nanoscale, practical approaches for the modification in the properties of cement composites, and the design process of nano-tailored cement composites still need much attention to fully understand these aspects of nano-tailored cement composites (Khitab, 2016). To date, several nanomaterials have found their application into the cement composites as effective reinforcement, that is, nano-clay, graphene, nanosilica, carbon nano-fibers, and carbon nano-tubes. The possibility of synthesizing new nano-reinforcements offers the possibility of advancement in nano-engineered cement composites that posses higher mechanical strength, durability, lower energy consumption, and longer service life of structures to their resistance against crack initiation and propagation. However, because of the extremely small size of nanomaterials, their handling during the production of nano-tailored cement composites poses severe threats to human health. Studies have shown that inhalation or physical contact of the human body with nanomaterials is supposed to cause significant damage to human organs. A significant increase in nanomaterials’ use increases the risks of mishandling that can cause the release of these materials into the atmosphere, thus contaminating the air, water, or soil, which ultimately triggers severe harmful impacts on the environment. This chapter includes the recent advancements in the field of nano-tailored cement composites. The resultant implications and risks related to nano-tailored cement composites are also discussed, and potential prevention measures are proposed.

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13.3

461

Future developments

13.3.1 Factors affecting the design of nano-tailored cement composites Owing to nanotechnology and nanoscience advancements, nano-modification of cement composites can solve many of the problems faced by the construction industry. Nanotechnology can help in understanding the composition, design, and fabrication of cement composites. Therefore the basic design methodologies in nanoscience can be implemented in the design of nano-tailored cement composites. The general idea is to design the inherent characteristics and fabricate the cement composites keeping in mind the specific application. The construction industry demands simple, cost-effective, sustainable cement composites; therefore all these aspects should be considered during the design of nano-tailored cement composites. Nanotechnology relies on two basic design approaches (1) the “bottomup” approach and (2) the “topdown” approach (Baoguo et al., 2019). In the former, atoms and molecules are assembled to design the materials for a specific application, while in the latter, larger structures are disassembled into smaller components without altering their parent properties. The illustration of both design approaches is shown in Fig. 13.1. In the design of nano-tailored cement composites, “topdown,” “bottomup,” or a combination of these two design approaches can be effectively adopted. A large number of experimental variables are involved in the design of nano-tailored cement composites, which increases the complexity of the design process. Therefore incremental improvement in the properties of nano-tailored cement composites can be made. With the emergence of computer technology, the computational time for complex engineering problems has been significantly reduced. Therefore the advanced computational tools and experimental methodologies can be integrated to design nanotailored cement composites. Artificial intelligence (AI) has recently been successfully employed in the design of cement composites; therefore large research databases can be used with AI to estimate the desired properties of cement composites in accordance with the initial design composition (Ahmad et al., 2021; Huang et al., 2020; Javed et al., 2020). In addition to this, knowledge gained from other design methods such as metamaterials, bio-inspired design, and digital twins can be employed to design nano-tailored cement composites (Baoguo et al., 2011; Liqing et al., 2016).

13.3.2 Production of nano-tailored cementitious composites Production of cement composites involves dry and wet mixing of the raw ingredients. However, conventional mixing does not effectively disperse the nanomaterials into the cement composites because of their small size. The uniform dispersion of nanomaterials into the cement composites is the key criterion to effectively gain the benefits from these nanomaterials into the cement composites. Therefore mechanical (ultra-sonication) and chemical (functionalization) effort is needed to uniformly disperse nanomaterials into the cement composites. However, efforts are needed to develop such techniques to help large-scale, energy-efficient uniform dispersion of

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Figure 13.1 “Bottomup” and “topdown” design approach. Illustration of “bottomup” and “topdown” design approach in nanoscience. Source: From Sobolev, K., & Ferrada-Gutie´rrez, M. (2005). How nanotechnology can change the concrete world: Part 1; American Ceramic Society Bulletin, 84(10), 14
17. https://doi.org/ 10.1002/9780470588260.ch16.

nanomaterials into the cement composites (Liew et al., 2020). The high cost of nanomaterials is also one of the issues that limits their large-scale application into cement composites. In the last two decades, the price of nanomaterials has been significantly reduced, and a very small quantity of nanomaterials can provide significant improvement in the properties of cement composites; therefore the price of nanomaterials will no longer be a critical issue. Till now, efforts have been made to develop nano-tailored cement composites by employing nano-cements, nanoreinforcements, and nano-mineral admixtures.

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Much effort has been made to manufacture nano-tailored cement composites by the direct addition of nanomaterials to the cement composites. Fine and coarse aggregates are key ingredients of cement composites; efforts can be directed to the modification of aggregates at the nanoscale level to improve the aggregates’ inherent properties. Surface coating and treatment with nanomaterials or in-situ growth of nanomaterials on the surface of aggregates can provide effective improvement in cement composites’ mechanical properties. The interfacial transition zone (ITZ) between cement matrix and aggregates is vulnerable to crack upon loading. Therefore nano-modification of ITZ targeted by surface modification of aggregates can be an appropriate fabrication technique for the nano-tailored cement composites. Similarly, efforts can be expanded toward the addition of nano-polymers into cement composites. Nanomaterials are mostly susceptible to release into the environment during the fabrication process of nano-tailored cement composites. Therefore the development of an efficient and clean fabrication process is of great importance, and efforts are needed in this domain. 3D printing, grafting, in-situ growth, and self-assembly are some of the potential techniques that can be adopted in the efficient, large-scale, and environment-friendly fabrication of nano-tailored cement composites.

13.3.3 Experimental techniques for characterization of nanotailored cementitious composites The properties of nano-tailored cement composites are significantly influenced by the performance of nanomaterials at the nanoscale (Kai et al., 2021b). The modification of intrinsic properties of nano-tailored cement composites desires special techniques and methodologies for complete understanding. Therefore the characterization of nano-tailored cement composites involves interdisciplinary techniques from other fields of science. Owing to the complex structure of nano-tailored cement composites, basic characterization techniques for cement composites cannot be applied. Similarly, some of the techniques used for the characterization of nanotailored cement composites are not needed in the case of conventional cement composites, for example, UV-spectroscopy, which is used to characterize the extent of dispersion of nanomaterials into the aqueous solution. To observe the microstructure of nano-tailored cement composites, scanning electron microscope has been widely used. Similarly, some of the studies employed transmission electron microscopy to deeply study the structure of nanomaterials used in the production of nano-tailored cement composites. Some of the other widely used techniques to characterize the nano-tailored cement composites are thermogravimetry, X-ray diffraction, rheometer, mercury intrusion porosimetry, and energy dispersive spectroscopy. All these characterization techniques are limited to the specific small area of the nano-tailored cement composites. Therefore efforts are needed to develop such techniques capable of characterizing the nano-tailored cement composites in bulk form not limited to specific areas. Though different microscopic approaches as discussed above have shown an excellent way to

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investigate the interaction between the added nanomaterial and cement matrix at a laboratory scale for small samples, its applicability for the bulk of the material at an industrial scale is one of the major concerns. Moreover, a couple of researchers have also criticized the approaches mentioned above for a complete understanding of nanomaterial dispersion in the bulk of the material. Neutron radiography is a type of nondestructive testing usually employed to visualize the internal structure of the samples. Neutron radiographic technology has been rarely employed to study the behavior of nano-tailored cement composites. Therefore this technology has the potential to characterize the internal cracks of the nano-tailored cement composites successfully. Similarly, low-field nuclear magnetic resonance probing is also a nondestructive characterization technique that can be employed to study the age-related changes in the pore structure of nano-tailored cement composites. On the other hand, the mechanical properties of cement composites reinforced with nanomaterials can be studied using nano-indentation technology. This technology is capable of characterizing the mechanical properties of cement composites at the nanoscale. Therefore nanoscale properties can be effectively studied to understand the overall behavior of the nano-tailored cement composites. In addition to this, atomic force microscopy and acoustic emission techniques have the potential capability to study the properties of nano-tailored cement composites. With the advancement in computer technology, now it is possible to simulate the complex behavior of cement composites. Molecular dynamic simulation can successfully provide insight into the behavior of nano-reinforcements in the cement composites, and their interaction with the cement matrix can be studied at the nanoscale level (Kai et al., 2020). However, owing to cement composites’ heterogenous structure, it is sometimes difficult to fully relate the simulation results with the experimental findings. Therefore efforts are needed to develop optimized numerical simulation techniques that can predict nano-tailored cement composites’ long-term behavior.

13.3.4 Multi-functional properties of nano-tailored cementitious composites Early investigations showed that nanomaterials significantly influence the hydration process of the nano-tailored cement composites (Makar et al., 2005). The type and amount of nanomaterials significantly influence the mechanical properties of nanotailored cement composites, and dispersion of nanomaterials into the cement composites and composition of binding material also affect the properties of such composites. The desired multi-functional properties of nano-tailored cement composites can be achieved by employing appropriate design and fabrication techniques together with such a nanomaterial that possesses those properties. For example, if self-cleaning properties are desired, it is appropriate to fabricate nano titanium dioxide
based cement composites (Li et al., 2018a). On the other hand, if piezoelectric properties are required, it is feasible to add carbon nano-tubes into the cement composites (Li et al., 2018b; Kai et al., 2021a). Therefore it is desired to extensively

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study the multi-functional properties of various nano-tailored cement composites. In the past, several studies have shown significant improvement in the mechanical properties of cement composites by the addition of very small quantities of nanomaterials. However, sometimes it is challenging to achieve the same enhancement level by refabricating similar nano-tailored cement composites. Therefore extensive research is needed to validate the correctness of such reported properties of nanotailored cement composites. The mechanical properties such as compressive strength, flexural strength, splitting tensile strength, and fracture toughness of nano-tailored cement composites have been widely studied in the past. However, efforts are needed to fully understand the elastoplasticity, dynamic splitting tensile strength, shear strength, and mechanical behavior of nano-tailored cement composites under complex loading conditions. Similarly, alkali
aggregate reaction, heat resistance, thermal diffusivity, self-healing, and pyroelectric properties of nano-tailored cement composites need much attention to adopt the multi-functional cement composites in real-life applications. In addition to this, efforts can be made to develop nano-tailored cement composites with hydrophobic properties. Previous studies have shown that enhancement in the properties of nano-tailored cement composites was more evident in the case of cement paste or mortar; however, such a level of enhanced properties cannot be achieved in concrete. This can be justified by the heterogeneity introduced by the addition of large aggregates that significantly alter the concrete properties. The properties of nano-tailored cement composites significantly affected the pore structure; therefore efforts should be made to study the effect of coarse aggregates on the pore structure and interaction of these aggregates with the surrounding cement matrix and nano-reinforcements to develop generic properties of nano-modified cement composites with coarse aggregates.

13.3.5 Enhancement mechanism of nano-tailored cementitious composites The enhancement in properties of nano-tailored cement composites mainly depends on the extensive uniform distribution of nanomaterials into the cement matrix. Generally, the mechanical properties of nano-tailored cement composite are significantly dependent on the aspect ratio, nano-size effect, nucleation effect, and surface effect of the nanomaterials. Studies have shown that carbon nano-tubes could effectively increase the growth of hydration products by providing a nucleation effect (Makar et al., 2005). The nano-size effect of nanomaterials provides an excellent filling effect and fills the nano-size pores in the cement matrix and thus provides dense structure resulting in enhanced mechanical properties of nano-tailored cement composites. The self-sensing and electrical conductivity can be introduced to the cement composites by employing such nanomaterials that possess field emission conduction, tunneling conduction, contacting conduction, and intrinsic electrical conductivity. In any nano-tailored cement composites, the conductivity of the electric charges mainly depends on all of the above-mentioned mechanisms. The electrical

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conductivity of nano-tailored cement composites can be enhanced by employing highly conductive nanomaterials, improving the bonding of nanomaterials with the surrounding cement matrix, decreasing the distance between the nanoparticles, and altering the contact between the nanoparticles. The presence of space charges at the interface of insulative cement matrix and conductive nanomaterials results in an increase in dielectric constant, which ultimately improves the shielding capability of nano-tailored cement composites against electromagnetic waves. Similarly, two-dimensional nanoparticles such as graphene nanoparticles enhance the mechanical properties of cement composites by providing the surface effect. The large surface area of graphene nanoparticles provides strong interfacial bonding with the surrounding cement matrix by strong Van der Waals forces. Large surface area and two-dimensional shape of graphene nanoparticles provide effective reinforcement against crack initiation and helps in diverting the cracks, thereby improving the energy-absorbing capability of nano-tailored cement composites.

13.3.6 Potential use of nano-tailored cementitious composites Nanomaterials have been successfully used in cement composites because of their excellent mechanical properties capable of delaying the crack initiation and propagation in the cement composites. Owing to improved mechanical properties, nanotailored cement composites can be practically employed to construct high-rise buildings, tunnels, nuclear powerplants, and high-speed rails. Owing to the high cost, nano-tailored cement composites remain at the research phase, and their practical application is limited. Therefore efforts are needed to promote the practical application of nano-tailored cement composites. Apart from excellent mechanical properties, nanomaterials also possess multi-functional characteristics. The intrinsic multi-functional properties of the nanomaterials can be utilized to develop specific properties in the cement composites as carbon nano-tubes are electrically conductive. Therefore, cement composites reinforced with carbon nano-tubes can pass an electric charge through them, and this property can be utilized to design selfsensing cement composites. Several similar applications of nano-tailored cement composites can be achieved by employing proper nanomaterial into the cement composites. Recently nanomaterials are successfully utilized to develop self-healing properties in epoxy nanocomposites (Nie et al., 2019). However, limited information is available on the development of self-healing cement composites by the application of nanomaterials. Therefore nanomaterials have great potential to be utilized in the development of self-healing cement composites. Nanomaterials can be added as fillers into the cement composites that can provide effective nucleation sites for the growth of hydration products and can thus promote the long-term hydration process to heal any susceptible gaps in the cement matrix. Similarly, active nanomaterials such as nano-silica can react with calcium hydroxide to promote the growth of calcium silicate hydrate gel, filling the pores by providing self-healing capability to cement composites.

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Researchers have found that under ultraviolet light, spherical nano-titanium dioxide particles can generate free radicals that are capable of reacting with the toxic gasses in the environment and can thus purify the air (Shen et al., 2012). Therefore the application of nano-titanium dioxide into the cement composites can introduce self-cleaning properties. The structures made with nano-titanium dioxide
based cement composites have significant potential to purify the surrounding air by degrading harmful gases into innoxious elements by the process of oxidation. Similarly, it has been found that nano-titanium dioxide particles can convert themselves into a transparent layer of moisture that acts as a protector against dust, thus introducing the self-cleaning property to the infrastructures. Nanomaterials such as carbon nano-tubes possess thermoelectric and piezoelectric properties, resulting in electric charge production upon the change in temperature or pressure, respectively. These properties can also be achieved in the cement composites by the addition of such nanomaterials. Such induced properties in the cement composites can be further utilized for energy harvesting purposes. Similarly, chemo-thermo-piezoresistive innovative cement composite was developed for the real-time detection of gas leakage (Vipulanandan et al., 2020). It is proposed that further investigation on the application of nano-tailored cement composites should be carried out to understand the mechanism and performance at different scales. In addition to this, the focus should be concentrated on the practical application of various multi-functional properties of nano-tailored cement composites.

13.4

Challenges

Nano-tailored cement composites have gained massive interest of researchers; however, these nano-tailored composites still confront challenges during the design, fabrication, mechanism understanding, and practical application process. Despite the availability of high-end fabrication and characterization techniques, heterogeneity in nano-tailored cement composites sometimes results in conflicting findings among various researchers. Nanomaterials have been widely used in cement composites because their excellent strength and design of nano-tailored cement composites usually follow the bottomup approach. The currently employed design process of nano-tailored cement composites requires the production and handling of each raw ingredient separately, including nanomaterials. Therefore during the production and handling, the chances of exposure of such nanomaterials to humans and the environment are significantly high. Such uncontrolled high exposure of nanomaterial to the environment and human poses significantly severe threats to human health. Recently, it was estimated that by 2020, about 6 million people would be exposed to nanomaterials (Shafique & Luo, 2019). Therefore there is a significant need for a systematic database on the occupational exposure limit and toxic effects of nanomaterials being used in the construction industry. Owing to the extremely small size of nanomaterials, nanoparticles get suspended in the air and can cause harmful impacts

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to human health. Some of the most widely used nanomaterials in the development of nano-tailored cement composites such as carbon nano-tubes, nano-silica, and nano-titanium dioxide are susceptible to cause lung damage, inflammatory and immune responses, and liver damage, respectively. Significant effort has been made to study the effect of nanomaterials on the mechanical performance of resultant cement composites. However, very little importance is given to study the environmental impacts associated with the production of nanomaterials and resulted in nano-tailored cement composites. Therefore it is of great challenge to study the impacts of nanomaterials posed to the environment. Therefore it is suggested that a life cycle assessment of nano-tailored cement composites should be conducted to enlighten this darken side of the nano-tailored cement composites. Further efforts should be made to mitigate those environmental impacts associated with the nano-tailored cement composites. To meet modern society’s sustainability goals, it is necessary to develop more sustainable, energyefficient methods to develop nano-tailored cement composites. Production techniques in nanotechnology have been significantly improved over time, and more significant amounts of nanomaterials are being developed on the commercial scale resulted from the increase of nanomaterials in various industries. However, no efforts have been made, and no such technologies are available to recycle such larger amounts of nanomaterials. The cost and environmental impacts associated with the production of virgin nanomaterials can be significantly reduced if recycling technology for the nanomaterials can be introduced. Therefore it is another challenge for researchers to develop such a technique that can be employed to recycle nano-tailored cement composites. Although nanomaterials cost has been significantly reduced over the past decade, the practical application of nano-tailored cement composites is still limited. Therefore there is a need to develop cost-effective nanomaterials to accept their applications in real infrastructures. Apart from cost, uniform dispersion of nanomaterials into the cement composites is still a significant challenge. Owing to strong forces among nanoparticles, they tend to make bundles, which makes it difficult to uniformly disperse them into the cement composites. Therefore, efforts can be made to neutralize these strong forces among the nanoparticles, which will result in easy dispersion of these particles into the cement matrix. For such treatment, nanoparticles can be mixed in the aqueous solution of dispersant, which helps in releasing the strong forces among particles. Another reason for the poor dispersion of nanomaterials into the cement composites is the size of cement particles. Larger cement particles result in the nonuniform dispersion of nanoparticles. Therefore, reducing the size of cement grains can effectively improve the uniform dispersion of nanoparticles into the cement matrix. By employing conventional dispersion techniques such as mechanical dispersion and chemical treatment, new methods can be integrated to further improve the dispersion of nanomaterials in cement composites. One potential technique can be the in-situ growth of nanoparticles on the cement grains. This can resolve the issues related to the uniform dispersion of nanoparticles into the cement composites. Large-scale dispersion of nanoparticles into the aqueous solution can also be employed to

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develop uniformly dispersed nano-tailored cement composites (Parveen et al., 2013). Doing so will be a step forward toward the application of nano-tailored cement composites in the construction of real-life structures. Another critical challenge in the development of nano-tailored cement composites is the optimum dosage of nanomaterials. Various nanomaterials behave differently at different scales. Therefore there is a significant need to develop an optimum dosage for each nanomaterial to obtain the best possible properties using the least nanomaterials into the cement composites. After the fabrication of desired nano-tailored cement composites, the next challenge arises during the characterization of such materials. Owing to the multiscale heterogeneity of nano-tailored cement composites, it is challenging to study nanocomposites using a single experimental technique. Therefore the combination of various techniques is employed to fully understand the interaction nanoparticles with the surrounding cement matrix. However, these techniques only characterize a specific limit area or point. Therefore it is difficult to fully integrate the properties to complete a cementitious structure. It is of great need that such characterization techniques should be developed that are capable of studying the cement composites in bulk. Therefore the properties of the whole cement composite can be studied at once. Although molecular dynamic simulation of nano-tailored cement composites has been actively studied in recent years, some challenges still need to be addressed for adequate progress of molecular dynamic, multiphysics simulation, and computational prediction of nano-engineered cement composites (Zhang et al., 2020). Indepth observations of cement composites are needed through experiments to resolve the issues in constructing the accurate atomistic models of complex cement matrix and its interaction with nano-reinforcements. Similarly, appropriate knowledge on the force fields, which describe the atomistic structure of material components and their interaction with each other, is lacking. The study on the long-term performance of nano-tailored cement composites through molecular dynamic simulation is also a great challenge.

13.5

Summary

First, a general overview of nano-tailored cement composites was given, and then future perspectives and challenges in the field of nano-engineered cement composites were discussed. Advancement in nanotechnology has demonstrated the potential of nanomaterials in introducing various beneficial properties to cement composites. Nano-tailored cement composites can possess multi-functional properties such as durable, self-healing, self-cleaning, thermoelectric, piezoelectric, and strong and can be applied in various applications such as high-rise infrastructures, dams, bridges, and nuclear power plants. Nano-tailored cement composites are evolving with time, and with the new nanomaterials, this field is further expanding. With the increase in the use of nanomaterials in the construction industry, the exposure-related risk to human health and environmental impacts of such

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nanomaterials are also increasing. Therefore efforts are needed to increase the benefit to risk ratio to an optimal level. Nano-tailored cement composites require knowledge from various science fields to fully understand the behavior such compounds at different scale levels. The latest technologies to get new nanomaterials, various design and fabrication techniques, followed by modern technology to characterize the behavior of nano-tailored cement composites, are essential to increase the use of multi-functional properties of nano-tailored cement composites in real infrastructures. The eco-efficient design of nano-tailored cement composites can help in achieving the sustainability goals of modern society. Till now, a deep understanding of the behavior of nano-tailored cement composites at the laboratory level has been made. Still, a lot of efforts are needed to integrate laboratory studies with real-field applications. It is expected that revolution in nanotechnology can impose considerable effects on society and the environment by introducing multi-functionality to our structures by applying nano-tailored cement composites.

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379. Available from https://doi.org/10.1016/j.aej.2019.12.048. Huang, J. S., Liew, J. X., Ademiloye, A. S., & Liew, K. M. (2020). Artificial intelligence in materials modeling and design. Archives of Computational Methods in Engineering. Available from https://doi.org/10.1007/s11831-020-09506-1. Javed, M. F., Farooq, F., Memon, S. A., Akbar, A., Khan, M. A., Aslam, F., Alyousef, R., Alabduljabbar, H., & Ur Rehman, S. K. (2020). New prediction model for the ultimate axial capacity of concrete-filled steel tubes: An evolutionary approach. Crystals, 10(9), 1
33. Available from https://doi.org/10.3390/cryst10090741. Kai, M. F., Zhang, L. W., & Liew, K. M. (2020). Carbon nanotube-geopolymer nanocomposites: A molecular dynamics study of the influence of interfacial chemical bonding upon the structural and mechanical properties. Carbon, 161, 772
783. Available from https:// doi.org/10.1016/j.carbon.2020.02.014. Kai, M. F., Zhang, L. W., & Liew, K. M. (2021a). Atomistic insights into structure evolution and mechanical property of calcium silicate hydrates influenced by nuclear waste caesium. Journal of Hazardous Materials, 411, 125033. Available from https://doi.org/ 10.1016/j.jhazmat.2020.125033. Kai, M. F., Zhang, L. W., & Liew, K. M. (2021b). New insights into creep characteristics of calcium silicate hydrates at molecular level. Cement and Concrete Research, 142, 106366. Available from https://doi.org/10.1016/j.cemconres.2021.106366. Khitab, A. (2016). Advanced research on nanotechnology for civil engineering Applications. Li, Z., Ding, S., Yu, X., Han, B., & Ou, J. (2018a). Multifunctional cementitious composites modified with nano titanium dioxide: A review. Composites Part A: Applied Science and Manufacturing, 111, 115
137. Available from https://doi.org/10.1016/j. compositesa.2018.05.019. Li, G., Yuan, J. B., Zhang, Y. H., Zhang, N., & Liew, K. M. (2018b). Microstructure and mechanical performance of graphene reinforced cementitious composites. Composites Part A: Applied Science and Manufacturing, 114, 188
195. Available from https://doi. org/10.1016/j.compositesa.2018.08.026. Liew, K. M., & Akbar, A. (2020). The recent progress of recycled steel fiber reinforced concrete. Construction and Building Materials, 232. Available from https://doi.org/10.1016/ j.conbuildmat.2019.117232. Liew, K. M., Pan, Z., & Zhang, L. W. (2020). The recent progress of functionally graded CNT reinforced composites and structures. Science China: Physics, Mechanics and Astronomy, 63(3). Available from https://doi.org/10.1007/s11433-019-1457-2. Liqing, Z., Siqi, D., Shengwei, S., Baoguo, H., Xun, Y., & Jinping, O. (2016). Nano-scale behavior and nano-modification of cement and concrete materials. IGI Global. Available from https://doi.org/10.4018/978-1-5225-0344-6.ch002. Makar, J., Margeson, J., & Luh, J. (2005). Carbon nanotube/cement composites-early results and potential applications. Proc. 3rd Int. Conf. Constr. Mater. Performance, 1
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Power, I. M., Dipple, G. M., & Francis, P. S. (2017). Assessing the carbon sequestration potential of magnesium oxychloride cement building materials. Cement and Concrete Composites, 78, 97
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883. Available from https://doi.org/ 10.1016/j.conbuildmat.2012.04.097. Sobolev, K., & Ferrada-Gutie´rrez, M. (2005). How nanotechnology can change the concrete world: Part 1; American Ceramic Society Bulletin, 84(10), 14
17. Available from https://doi.org/10.1002/9780470588260.ch16. Vipulanandan, C., Aldughather, A., Panda, G., & Wong, G. (2020). Real-time gas leak detection and quantification using smart cement. In International Petroleum Technology Conference 2020, IPTC 2020. International Petroleum Technology Conference (IPTC). Available from https://doi.org/10.2523/iptc-20248-ms. Zhang, L. W., Ji, W. M., Hu, Y., & Liew, K. M. (2020). Atomistic insights into the tunable transition from cavitation to crazing in diamond nanothread-reinforced polymer composites. Research; A Journal of Science and its Applications, 2020. Available from https:// doi.org/10.34133/2020/7815462.

Index

Note: Page numbers followed by “f” and “t” refer to figures and tables, respectively. A Abrasion of cement-concrete, 320 Accelerate activator carbonates as nanoseeding materials, 440
444 Acid attack, 322 Acid rain, 322 Acrylic acid, 146 Additive manufacturing, 251
253 Aerogels, 288 Air pollution, 161 Air purification, utilization of TiO2 for, 164
165 Alkali activator, reactive magnesia as, 436
439 Alkali-activated systems properties of geopolymers and alkaliactivated systems incorporating nanomaterials, 349
364 fresh properties, 349
353 mechanical properties, 353
359 microstructure development, 359
364 pore structures of geopolymers with nano-SiO2, 362
364 scanning electron microscope images, 359
361 X-ray diffraction analysis of geopolymer with nano-SiO2, 361
362 Alkali
aggregate reactions (AARs), 320
321 Alkali
carbonate reaction (ACR), 320
321 Alkali
silica reaction (ASR), 320
321 Alkali
silicate reaction (ASR), 380
381 Alternating current (AC), 121 Alumina (Al2O3), 150, 292, 347 Aluminum alloys, 293 Ambient conditions (A conditions), 430

Anatase, 163
164, 170 Antigraffiti coatings, 293 Argon (Ar), 289 Artificial intelligence (AI), 461 Assembled methods, 18
19 Atomic force microscopy (AFM), 379
383 AFM-infrared spectroscopy (AFM-IR), 383 AFM-nuclear magnetic resonance (AFM-NMR), 383 Attapulgite, 257 Autogenous healing, 142
143 Autonomous healing, 143
145 B Bacillus subtilis, 146 Backscattered electron (BSE), 322 Bentonite, 257 Bingham model, 215, 220
222 Bio-inspired design, 461 Bismuth (Bi), 164
165 Blast-furnace slag, 143 “Bottomup” approach, 461 Brookite, 163
164 BTRHEOM rheometer, 222
223 Buildability assessment, 253
254, 255f Building(s), 277 construction, 277 conventional insulation materials and methods, 283
286 energy-efficient buildings, 282
283 fundamentals of building physics, 277
282 nano-technology role for building insulation, 286
293 Bulk cement paste phase, nano-core effect in, 5
10

474

C Cadmium oxide (CdO), 162
163 Calcination process, 141
142 Calcined nano-clays, 84
86 Calcium aluminate hydrate (C
A
H), 12, 150 Calcium aluminosilicate (CASH), 361 Calcium carbonate, 145 Calcium hydroxide (CH), 149
150, 333
335 Calcium ions, 145 Calcium
silicate
hydrate gel (C
S
H gel), 1, 333
335, 349
351, 361, 375
376, 379
380 properties, 391 types, 395
396 Carbon black (CB), 84
86, 262, 306
307 nanoparticles, 314
315 Carbon dioxide (CO2), 141
142 emissions, 408 Carbon monoxide (CO), 161 Carbon nano-fibers (CNFs), 2
3, 39
41, 84, 86, 105, 233
234, 258, 262, 305, 313
314 durability, 41 functional properties, 41 self-sensing properties, 41 hydration, 39 mechanical properties, 40
41 rheology, 39 workability, 39
40 Carbon nanomaterials, 305 Carbon nano-tubes (CNTs), 2
3, 31
39, 83
84, 86, 142, 149, 233
234, 258, 261
262, 305, 309
313, 310f, 322
324, 348, 379
380, 465, 467
468 durability, 36 functional properties, 36
39 damping properties, 38 electromagnetic properties, 38 self-healing properties, 39 self-sensing property, 36
38 thermal properties, 38 hydration, 33 mechanical properties, 33
36 dynamic mechanical properties, 36 static mechanical properties, 33
36

Index

rheology, 33 workability, 33 Carbon-based nanomaterials (CBNMs), 105, 111
113, 123
124, 129 Carbonation (C), 430 conditions, 430 of geopolymers, 364
365 limitations of carbonation diffusion, 415
417 carbonation depth observed via X-ray CT, 416f carbonation depth with time, 417f comparison of top and inner layers, 418f cumulative pore volume, 419f mechanism and mechanical performance of RMC-based materials, 414
415 of reactive magnesia cements, 411
417 Casson model, 217
218 Cellulose, 284 cellulose-ether-derived VMAs, 257
258 nanomaterials, 84, 86 Cemagref-IMG rheometer, 222
223 Cement, 68 composites, 459
460 Cementitious binders, nanomaterials in, 348
349 Cementitious composites, 1, 5f, 305
306, 375. See also Nano-tailored cementitious composites applications of cementitious composites with nanomaterials, 48
50 autogenous shrinkage of, 318 basic principles of tailoring cementitious composites with nanomaterials, 5
13 components, 3f dispersion of nanomaterials, 14
19 durability of, 316
317 nano-engineering in, 375 nanomaterials, 4t prospects of cementitious composites with nanomaterials, 51
52 tailoring cementitious composites with 0D nanomaterials, 19
31 tailoring cementitious composites with 1D nanomaterials, 31
41 tailoring cementitious composites with 2D nanomaterials, 41
48

Index

Cementitious materials, 209, 215, 277 Cementitious matrix, 141
142 Cementitious systems, 84
85 Challenges, nano-tailored cementitious composites, 467
469 Characterization of nano-tailored cementitious composites, 463
464 Chemical grafting, 18
19 Chemical oxygen demand (COD), 27 Chloride diffusion, 332 ion penetration, 330
331 Chloroform (CHCl3), 167
168 Classical nano-indentation test method, 378
379 Coaxial cylinder rheometer, 219
222 Composites, nano-tailored strain-hardening RMC, 424
435 Compressive strength, 325
328 of geopolymer concrete, 353
355 Computer technology, 461, 464 Concentric cylinder rheometer. See Coaxial cylinder rheometer Concrete, 141
142, 375
376 nano-additions effects on fresh and hardened state of, 258
262 technology, 251 Contaminated clays, 257 ConTec
BML rheometer, 222
223 Control sample (CS), 422 Conventional insulation materials and methods, 283
286 cellulose, 284 cork, 284 EPS, 284 mineral wool, 283 nZEB, 285
286 polyurethane, 285 XPS, 284 Cork thermal insulations, 284 Corrosion, 332
333 corrosion-resistant coatings, 293 resistance of RMC-based pastes, 423
424 Crack healing, 143 propagation of nano-tailored HPFRCC, 90
91 Crystalline admixtures, 143 Crystallization pressure theory, 319
320

475

Curing age/condition of cement-based systems and NOx degradation capability, 186
187 Cyclohexane (C6H12), 285 D De Kee model, 217 Dead-burned MgO, 407 Deflection-hardening cementitious composites (DHFRCC), 69
70 Density functional theory (DFT), 388
389 Development of nano-tailored cement composites, 467
468 of self-healing cement composites, 466 Differential thermal analysis (DTA), 320
321 Digital concrete manufacturing, 251
253 challenges with using nanomaterials as additives, 263
265 implementation of nanomaterials in extrusion-based 3DCP, 253
258 nano-additions effects on fresh and hardened state of concrete, 258
262 techniques for digital fabrication with concrete, 252f Digital fabrication, 251
253 Digital twins, 461 Direct current (DC), 121 Dispersants, 88 Dispersing agents, 17 Dispersion of nanomaterials, 14
19, 263
264 assembled methods, 18
19 functional modification, 17
18 in-situ growing method, 18 surface coating method, 19 traditional methods, 14
17 Dissolution
precipitation process, 412
413 Dry route for MgO, 409
410 Drying shrinkage of HVSC and HVFAC, 331
332 Ductility, 68 Durability of cementitious composites, 316
317 of concrete, 67 of geopolymers containing nano-SiO2, 364
367

476

Dynamic nuclear polarization NMR method (DNP NMR method), 388
389 Dynamic yield stresses, 253
254 E eBT2 rheometer, 222
223 Electrical conductivity cement composites, 465
466 Electrochromic materials, 294 Electron energy-loss spectroscopy (EELS), 385 Electrostatic self-assembly, 18 Energy-dispersive X-ray spectroscopy (EDX), 361 Energy-efficient buildings, 282
283 Energy-efficient coatings, 293
296 electrochromic materials, 294 nano-coating categorization, 296 PCMs, 293
294 photovoltaic coatings, 294
296 Engineered cementitious composites (ECC), 73, 104, 107
108, 143 Erosion of cement-concrete, 320 Ethylene glycol dimethacrylate, 146 Expanded polystyrene (EPS), 283
284 Extensional flow, 209
210 Extruded polystyrenes (XPS), 284 Extrusion-based 3DCP, 251
253 nanomaterials implementation in, 253
258 Eyring model, 216 F Fabrication technique, 463
465, 470 Fiber reinforced cementitious composite (FRCC), 69 Fiber-reinforced concretes (FRCs), 67
68, 107
108 Fibers, 68, 233
234 Filler concentration, 210
211, 212t First crack, 426 First cracking strength, 426 Flame-retardant coatings, 292 Flow, 209
210 Fluids, 209 Fly ash (FA), 143, 306
307, 311
313, 407
408 fly ash
based geopolymers, 349
351 Fly ash cenosphere (FAC), 450

Index

Focused ion beam-nano-tomography (FIB-nt), 397 Formwork printing, 251
253 Fourier transform infrared spectrophotometry (FTIR), 388
389 Fourier’s law, 280 Fractional change of resistivity (FCR), 121
123 Freeze-thaw damage, 319
320 resistance of RMC-based concretes, 422
423 Fuel combustion, 141
142 Functional modification, 17
18 G Gas-filled panels (GFPs), 289 Gauge factor, 121
123 Gaussian distribution, 378
379 Geopolymer (GP), 317, 347 durability of geopolymers containing nano-SiO2, 364
367 carbonation of geopolymers, 364
365 sulfate resistance of geopolymers, 365
367 matrix, 355 properties of geopolymers and alkaliactivated systems incorporating nanomaterials, 349
364 fresh properties, 349
353 mechanical properties, 353
359 microstructure development, 359
364 pore structures of geopolymers with nano-SiO2, 362
364 scanning electron microscope images, 359
361 X-ray diffraction analysis of geopolymer with nano-SiO2, 361
362 Geopolymerization, 347 Na/Al ratio effect on, 347 Glass fibers, 143 Glass powder (GP), 385
387 Glass-reinforced concrete (GRC), 68 Graphene (C140H42O20), 41
46, 305 durability, 44 functional properties, 44
46 damping properties, 46 electromagnetic properties, 44
45

Index

self-sensing properties, 44 thermal properties, 45 thermoelectric properties, 45 graphene-based nanomaterial, 318 hydration, 42 mechanical properties, 43
44 dynamic mechanical properties, 44 static mechanical properties, 43
44 nano-fibers, 89 rheology, 42 workability, 42 Graphene oxide (GO), 84, 86, 108
111, 258, 262, 305, 322
324 Graphene oxide
manganese oxide (GO
MnO2), 385
387 Graphene sulfonate nano-sheet (GSNS), 322
324 Graphite nano-fibers (GNF), 108
111 Graphite nano-platelets (GNPs), 84, 86, 107, 145, 315 Green cementitious nano-composites, 306
307 Ground granulated blast furnace slag (GGBFS), 306
307 GGBFS-blended fly ash geopolymer concrete, 356
358 Ground-granulated blast-furnace slag (GGBS), 422 H Halloysite, 257 Hard-burned MgO, 407 “Hatschek” process, 67 Heat transfer, 278, 279t transmission, 277
278 Helium ion microscopy (HIM), 396
397 Herschel
Bulkley model, 215
217, 221
222 High volume slag-fly ash concrete (HVSFAC), 318
319 High-cycle fatigue tests, 77 High-performance concrete (HPC), 67, 307
308 High-performance fiber-reinforced cementitious composites (HPFRCC), 67
83 evaluation of mechanical properties of, 74
78

477

evaluation of other properties of, 78
79 field applications of, 79
83 influence of using nanomaterials in, 88
93 early and hardening stages of nanotailored HPFRCC, 89 mechanical and durability properties of nano-tailored HPFRCC, 89
90 strain-hardening and crack propagation of nano-tailored HPFRCC, 90
91 nanomaterials in, 83
88 production and design parameters of, 70
74 High-pressure homogenizers methods, 88 High-speed homogenizer methods, 88 High-speed indentation technique, 379
380 High-volume fly ash (HVFA), 325
328 Higher volume slag concrete (HVSC), 318
319 Hollow fibers, 151 Hydrated magnesium carbonates (HMCs), 407
408 as nano-seeding materials, 440
444 Hydration mechanism and mechanical performance of RMC-based materials, 413
414 of reactive magnesia cements, 411
417 Hydration agents (HAs), 413 Hydraulic binders of MgO
hydromagnesite, 444
448 Hydraulic pressure theory, 319
320 Hydrofluorocarbon (HFC), 284 Hydromagnesite (HY), 444
445 Hydrophilic coatings, 290
292 Hydrophobic coatings, 290
292 I IBB rheometer, 222
223 In-situ growing method, 18 Inorganic materials, 283 Insulation, 277 Interfacial transition zone (ITZ), 1, 83, 319
320, 375
376, 463 nano-core effect in, 10
12 Iron oxide (Fe2O3), 164
165 J Japan Concrete Institute (JCI), 74
75

478

K Kaolin (Al2Si2O5[OH]4), 146
147 Kaolinite, 257 Krypton (Kr), 289 L Laser scanning confocal microscopy (LSCM), 397 Latent heat storage (LHS), 293
294 Layer extrusion, 251
253 Layered double hydroxides (LDHs), 292 Least square estimation, 378
379 Light burned MgO, 407 Light scattering, 390 Long-term hydration process, 466 Low CO2 reactive magnesia cements applications, 436
450 accelerating the activator and hydrated magnesium carbonates as nanoseeding materials to, 440
444 as alkali activator, 436
439 hydraulic binders of MgO
hydromagnesite, 444
448 RMC for 3D printing, 449
450 durability of RMC, 418
424 hydration and carbonation of RMC, 411
417 nano-tailored strain-hardening RMC composites, 424
435 production of RMC, 409
411 M Magnesite, 407, 410 Magnesium acetate ([CH3COO]2Mg), 413 Magnesium chloride (MgCl2), 407
408, 413 Magnesium hydroxide (Mg[OH]2), 407
408, 411 Magnesium nitrate (Mg[NO3]2), 421 Magnesium oxide (MgO), 407 MgO-alkaline oxide, 412 Magnesium oxy-chloride cement (MOC cement), 385
387, 407
408 Magnesium oxy-sulfate cement (MOS cement), 407
408 Magnesium phosphate cements (MOP cements), 407
408 Magnesium salts, 407
408 Magnesium silicate hydrate cements (M
S
H cements), 407
408

Index

Magnesium sulfate. See Magnesium chloride (MgCl2) Maximum likelihood estimation, 378
379 Mechanical properties of nano-tailored cement composites, 465 Mercury intrusion porosimetry (MIP), 333
335 Metakaolin, 306
307 Metal coatings, 293 Metamaterials, 461 Microcracks, 103 Microencapsulation, 145 Microscale characterization test method, 377 Mineral wool, 283 Modified Bingham model, 216
217 Multifunctional cementitious composites, 375 Multiwalled carbon nano-tubes (MWCNT), 31
33, 91, 104, 309 MWCNT90, 108
111 MWCNT99, 108
111 N Nano carbon black (NCB), 2
3, 105
107 Nano-additions effects on fresh and hardened state of concrete, 258
262 carbon nano-fibers, graphene oxide, and carbon black, 262 carbon nano-tubes, 261
262 nano-alumina, 260 nano-clay, 259
260 nano-silica, 258
259 nano-titania, 259 other mineral additions, 260
261 Nano-admixtures, 103 Nano-alumina (nano-Al2O3), 150, 230
231, 260, 305, 322
324, 348 Nano-BN, 46
48 durability, 46
47 functional properties, 48 oil
water separation properties, 48 hydration, 46 mechanical properties, 46 Nano-calcium carbonate (nano-CaCO3), 84
85, 232, 260, 305 Nano-carbon fibers (CNFs). See Carbon nano-fibers (CNFs) Nano-carbonate, 305

Index

Nano-clay, 84
86, 146
147, 234
236, 257, 259
260, 305, 322
324, 348 Nano-coatings for buildings, 290 categorization, 296 types, 290
293 antigraffiti coatings, 293 corrosion-resistant coatings, 293 flame-retardant coatings, 292 hydrophilic and hydrophobic coatings, 290
292 wear-resistant coatings, 292 Nano-core effect in bulk cement paste phase, 5
10 filling or bonding effect, 10 intrinsic effect, 8, 9t large surface area, 6 nucleating effect, 8
10 pinning effect, 10 small size, 6 factors affecting, 12
13 in interfacial transition zone, 10
12 zone, 12 Nano-cracks, 91 Nano-engineering, 375 in cementitious composites, 375 Nano-Fe3O4, 150 Nano-fibers, 151 Nano-graphene oxide (nano-GO), 236
238 Nano-inclusions, 142 Nano-indentation, 377
380 method, 379
380 tests, 358
359 Nano-iron, 149
150 Nano-iron oxide (nano-Fe2O3), 305 Nano-kaolin, 305 Nano-kaolinite clay (NKC), 319
320 Nano-magnesia (nano-MgO), 261 Nano-metals, 84
85 Nano-modification of rheological properties of cementitious composites rheology of nano-modified cementitious composites, 223
238 test methods, 218
223 theoretical background, 209
218 Nano-modified green cementitious composites, 305 nanomaterial types for modification of green cementitious composites, 306
316

479

properties, 316
335 abrasion or erosion, 320 freeze-thaw damage, 319
320 nano-technology for cementitious composites to triumph over chemical deteriorations, 320
322 shrinkage, 317
319 Nano-paraffin, 293
294 Nano-piezoresistive materials in cementitious composites, 105
108 Nano-plates, 234
238 Nano-scale characterization techniques, 377
397 AFM, 380
383 challenges and future perspectives, 397
398 nano-indentation, 377
380 nuclear magnetic resonance, 387
390 other characterization techniques, 395
397 small-angle neutron scattering method, 390
392 TEM, 384
387 X-ray computed nano-tomography, 392
395 Nano-scale particles, 223
232 Nano-science, 375 Nano-seeding materials, hydrated magnesium carbonates as, 440
444 Nano-silica (nano-SiO2), 2
3, 19
23, 84
85, 89, 147
149, 223
225, 258
259, 305, 307
308, 322
324, 348, 466
468 compressive strength of geopolymer concrete containing, 353
355 durability, 21 durability of geopolymers containing, 364
367 functional properties, 21
23 self-healing properties, 23 self-sensing properties, 21
23 thermal properties, 23 hydration, 20 mechanical properties, 20
21 dynamic mechanical properties, 21 static mechanical properties, 20
21 nanomechanical properties of fly ash geopolymer containing, 358
359 pore structures of geopolymers with, 362
364

480

Nano-silica (nano-SiO2) (Continued) rheology, 20 workability, 20 X-ray diffraction analysis of geopolymer with, 361
362 Nano-sized superabsorbent polymers, 146 Nano-tailored cement composites, 466
468 Nano-tailored cementitious composites, 460 challenges, 467
469 future developments, 461
467 bottomup and topdown design approach, 462f enhancement mechanism of, 465
466 experimental techniques for characterization of, 463
464 factors affecting design of, 461 multifunctional properties of, 464
465 potential use of, 466
467 production of, 461
463 nano-piezoresistive materials in cementitious composites, 105
108 parameters influencing sensing ability of, 108
121 cementitious matrix properties, 116
120 concentration of nanomaterials, 111
113 dispersion, 113
115 intrinsic properties, 108
111 surrounding conditions, 120
121 perspectives, 129
131 use of nanomaterials in self-sensing cementitious composites, 121
129 Nano-tailored HPFRCC, early and hardening stages of, 89 Nano-tailored strain-hardening RMC composites, 424
435 mechanical properties, 424
430 crack pattern, 429f mechanical test results, 427t mixture proportions of prepared samples, 426t tensile stress vs. strain curves, 428f self-healing performance, 430
435 crack widths of RMC specimens, 432f microstructure of healed crack regions, 437f postconditioning tensile properties, 435t

Index

preloading and reloading uniaxial tensile stress
strain curves, 434f prestraining levels and conditioning regimes, 431t qualitative assessment, 439t recovery ability, 436f Nano-tailored TiO2-based photocatalytic cementitious systems TiO2 as photocatalyst, 163
173 utilization of TiO2 in cementitious systems for air purification purposes, 173
189 Nano-technology, 104, 277, 305, 375, 460
461 for cementitious composites, 320
322 acid attack, 322 alkali
aggregate reactions, 320
321 chloride diffusion, 332 chloride ion penetration, 330
331 compressive strength, 325
328 corrosion, 332
333 drying shrinkage, 331
332 microstructure impact of nanoparticles addition, 333
335 permeability, 331 sulfate attack, 321
322 tensile strength, 328 thermal degradation, 322
325 water absorption, 329
330 water sorpitivity, 328
329 nano-technology-based materials, 283 role for building insulation, 286
293 aerogels, 288 and construction industry, 286 energy-efficient coatings, 293
296 GFPs, 289 nano-coatings for buildings, 290 nano-technology applied on thermal insulation, 286
287 phase change materials, 290 types of nano-coatings, 290
293 VIP, 288
289 Nano-thermal insulation materials, 287
288 Nano-titania (nano-TiO2), 2
3, 24
29, 83, 151, 225
228, 259, 286, 305, 308
309, 467
468 durability, 27 functional properties, 27
29

Index

electromagnetic shielding and absorption properties, 28 photocatalysis properties, 27 self-healing properties, 29 self-sensing properties, 29 hydration, 24 mechanical properties, 25
27 dynamic mechanical properties, 27 static mechanical properties, 25
27 rheology, 24 workability, 24
25 Nano-titanium dioxide. See Nano-titania (nano-TiO2) Nano-tomography combined with XRD (XRD-nCT), 393 Nano-tubes, 233
234 Nano-zinc oxide (nano-ZnO), 228
230 Nano-zirconium oxide (nano-ZrO2), 2
3, 29
31, 231
232 durability, 29
30 functional properties, 31 self-healing properties, 31 mechanical properties, 31 dynamic mechanical properties, 31 static mechanical properties, 31 workability, 29 Nanomaterials, 1, 305, 460
463, 466 applications of cementitious composites with, 48
50 basic principles of tailoring cementitious composites with, 5
13 in cementitious binders, 348
349 challenges with using nanomaterials as additives, 263
265 dispersion of nanomaterials, 263
264 safety issues, 264
265 concentration, 111
113 dispersion of, 14
19 in HPFRCC, 83
88 advantages of using, 85
86 application challenges of using nanomaterials in, 86
88 influence of using, 88
93 effect of nanomaterials on properties of cementitious systems, 87t types of, 84
85 implementation in extrusion-based 3DCP, 253
258

481

comparison between polymeric VMAs and nanomaterials, 257
258 nanomaterials as thixotropic agents, 256
257, 256t printing processes and required material behaviors, 253
255 properties of geopolymers and alkaliactivated systems incorporating, 349
364 prospects of cementitious composites with, 51
52 for self-healing purposes, 145
151 carbon nano-tubes, 149 nano-alumina, 150 nano-clays, 146
147 nano-fibers, 151 nano-iron, 149
150 nano-silica, 147
149 nano-sized superabsorbent polymers, 146 nano-titania, 151 types used for modification of green cementitious composites, 306
316 carbon black nanoparticles, 314
315 CNFs, 313
314 CNTs, 309
313 nano-silica, 307
308 nano-titania, 308
309 relevant lines of study using other nanoparticles, 315
316 use of nanomaterials in self-sensing cementitious composites, 121
129 Nanomechanical properties of fly ash geopolymer containing nano-SiO2, 358
359 Nanoparticles, 256
257, 348 in OPC binders, 348 NASH. See Sodium aluminosilicate (NASH) Natural fibers, 143 Near-edge X-ray absorption fine structure (NEXAFS), 397 Neutron radiography, 464 Neutron scattering, 390 Nickel oxide (NiO2), 293 Nitrogen oxides (NOx), 161
162 degradation abrasion/wearing/weathering of surface of cement-based systems and, 185
186

482

Index

O One-dimension (1D) nanomaterials carbon nano-fibers, 39
41 carbon nano-tubes, 31
39 tailoring cementitious composites with, 31
41 nanoparticles, 263 One-factor tests, 223 Ordinary cementitious concretes, 108
111 Ordinary Portland cement (OPC), 315
316, 408 OPC-blended fly ash geopolymer concrete, 356
358 Organic materials, 283 Osmotic pressure theory, 319
320 Oxygen (O2), 166
167

PeakForce Quantitative Nano-scale Mechanical mode (PF-QNM mode), 382 Pentane (C5H12), 284 Permeability of Portland cement-concrete, 331 Phase change materials (PCMs), 290, 293
294 Photocatalysts, 151, 162
163, 165
166 Photocatalytic activity, 162
163, 169
171, 178
179 of anatase, 170 of semiconductive materials, 165 of TiO2 particles, 167, 171
172 Photocatalytic degradation capability of TiO2, 163 Photocatalytic effect, 308 Photocatalytic reaction, 165 Photogenerated holes (h1), 166
167 Photons, 166 Photovoltaic coatings (PV coatings), 294
296 Piezoresistivity, 108
111, 113
115 Pollutants purifying, 50 Polycarboxylate superplasticizer (PCEs), 385
387 Polycarboxylate-based superplasticizer (P-SP), 113
115 Polycarboxylates, 305 Polyethylene fibers (PE fibers), 73 Polymeric VMAs and nanomaterials, 257
258 Polypropylene fibers (PP fibers), 73 Polysialate (Si
O
Al
O), 347 Polyurethane, 285 immobilization, 145 Polyvinyl alcohol (PVA), 18
19, 73, 309, 424
425 Pore structures of geopolymers with nanoSiO2, 362
364 Portland cement, 141
142, 277 Pozzolanic reaction, 143 Pozzolans, 145 Printing processes and required material behaviors, 253
255 Pulverized fuel ash (PFA), 385
387

P Parallel rotating plates rheometers, 222 Particle bed printing, 251
253

Q Quantitative nano-scale modulus mapping, 395
396

Nitrogen oxides (NOx) (Continued) amount of TiO2 in cement-based systems and, 178
179 combined presence of metal/non-metals and TiO2 in cement-based systems and, 180
182 curing age/condition of cement-based systems and, 186
187 final surface texture of cement-based systems and, 187 mechanism of TiO2, 165
167 mixture composition of cement-based systems and, 182
185 operation-related parameters and, 187
189 particle size ofTiO2 in cement-based systems and, 177
178 quality of distribution of TiO2 particles in cement-based systems and, 174
177 type of TiO2 in cement-based systems and, 179
180 Non-Newtonian fluid, 210 Nonconventional approaches, 251 Nonzero energy buildings (nZEB), 285
286 Nuclear magnetic resonance (NMR), 387
390

Index

R Reactive magnesia cement (RMC), 407 applications, 436
450 durability of, 418
424 chloride, sulfate, freeze-thaw, and seawater resistance of RMC-based concretes, 422
423 corrosion resistance of RMC-based pastes, 423
424 nitric acid resistance of RMC-based concretes, 418
422 hydration and carbonation of, 411
417 carbonation mechanism and mechanical performance of, 414
415 hydration mechanism and mechanical performance of, 413
414 limitations of carbonation diffusion, 415
417 nano-tailored strain-hardening RMC composites, 424
435 production of, 409
411 dry route, 409
410 wet route, 410
411 RMC-based concretes, 416
417 RMC
GGBS
FA systems, 439 Reduction and oxidation reactions (redox reactions), 162
163 Regions of interest (ROI), 415
416 Reiner
Riwlin equation, 221 Resistance (R-value), 278
280, 281t Resonance frequencies (RF), 387 Rheology, 209, 253 of concrete, 256
257 of nano-modified cementitious composites, 223
238 nano-plates, 234
238 nano-scale particles, 223
232 nano-tubes and fibers, 233
234 Rheometers, 219 test, 218
223 coaxial cylinder rheometer, 219
222 parallel rotating plates rheometers, 222 Rheopexy, 210 Risks related to nano-tailored cement composites, 460 Robertson
Stiff model, 217 Rutile, 163
164, 170

483

S Scanning electron microscopy (SEM), 359
361, 384
385 Scanning probe microscopy (SPM), 395
396 Scanning transmission electron microscopy (STEM), 385 Scanning transmission X-ray microscopy (STXM), 397 Scattering method, 390 Seawater, 410 resistance of RMC-based concretes, 422
423 Selected area electron diffraction (SAED), 385 Self-assembly concept, 290 Self-cleaning, 162
163 concrete, 259 Self-compacting concrete (SCC), 215 Self-healing cementitious composites nanomaterials for self-healing purposes, 145
151 toward self-healing concrete, 142
145 Self-healing concrete, 142
145 autogenous healing, 142
143 autonomous healing, 143
145 Self-sealing capacity, 147
148 Self-sensing cement composites, 465
466 Self-sensing cementitious composites sensing of deformation and cracking under mechanical loading, 121
127 sensing of dynamic actions for traffic monitoring, 127
129 special self-sensing applications, 129 use of nanomaterials in, 121
129 Sensing of deformation and cracking under mechanical loading, 121
127 of dynamic actions for traffic monitoring, 127
129 Sepiolite, 257 Set-on-demand printing, 254
255 Sewage sludge ash (SSA), 385
387 Shape-memory alloys, 144 Shear flow, 209
210 Shear stress, 220
221 Shrinkage of cement
concrete, 317
319

484 29

Si magic angle spinning NMR technique (MAS NMR technique), 387, 389 Sialate
disiloxo geopolymer (Si
O
Al
O
Si
O
Si
O), 347 Sialate
siloxo geopolymer (Si
O
Al
O
Si
O), 347 Silica (SiO2), 347 Silica fume (SF), 143, 306
308, 311
313 Silica nanoparticles. See Nano-silica (nano-SiO2) Silicon dioxide. See Nano-silica (nano-SiO2) Single shear stress
shear rate model, 215 Single-walled CNTs (SWCNTs), 31
33, 105
106, 309 Sisko model, 217
218 Slag-based geopolymer, 317 Slag-blended fly ash geopolymer, 351
353 Slump test, 351 Slurry infiltrated fibrous concrete (SIFCON), 68 Small-angle neutron scattering (SANS), 390
392 Smart/multi-functional properties, 1
2, 18 Sodium aluminosilicate (NASH), 361 Sodium bicarbonate (SBS), 414
415 Sodium carbonate (Na2CO3), 440, 441t Sodium chloride (SC), 414
415 resistance of RMC-based concretes, 422
423 Sodium deoxycholate (SDC), 17 Sodium dodecyl sulfate (SDS), 113
115 Sodium dodecyl sulfonate (SDS), 17 Sodium dodecylbenzene sulfonate (SDBS), 17 Sodium hexametaphosphate (Na[PO3]6), 425, 439 Sodium sulfate resistance of RMC-based concretes, 422
423 “Soft” matrix, 38 Solid-state NMR (SS NMR), 388
389 Specific surface area (SSA), 256
257, 409 Stannic oxide (SnO2), 294
296 Static yield stresses, 253
254 Statistical nano-indentation technique, 378
379 Steel, 277 Steel slag powder (SS), 121
123 Strain-hardening of nano-tailored HPFRCC, 90
91

Index

Strategic Highway Research Program (SHRP), 67 Structural health monitoring, 48
50 Sulfate attack, 321
322 resistance of geopolymers, 365
367 Sulfur dioxide (SO2), 161 Sulfuric acid (H2SO4), 322 Superabsorbent polymers (SAPs), 143 Superoxide radical, 166
167 Superplasticizers (SP), 17, 88, 113
115, 251, 257
258 Supplementary cementitious materials (SCMs), 215, 251, 306
307 Surface coating method, 19 Surfactants, 88, 264 Suspension rheology and rheological models for cement-based systems, 209
218 Synchrotron-based X-ray radiation, 397 Synthetic microfibers, 143 T Tattersall Mk-II and-III rheometer, 222
223 Tensile strain approaching specimen failure, 426 capacity, 426 Tensile strength of nanomaterialincorporated concretes, 328 Theoretical background of cementitious composites, 209
218 Thermal capacity (C-value), 281
282, 282t Thermal conductivity, 280 Thermal degradation, 322
325 Thermal insulation materials, 283 nano-technology applied on, 286
287 Thermal insulation of buildings through classical materials and nanomaterials conventional insulation materials and methods, 283
286 energy-efficient buildings, 282
283 fundamentals of building physics, 277
282 heat transmission, 277
278 resistance, 278
280 thermal capacity, 281
282 thermal conductivity, 280 thermal transmittance, 281 nano-technology role for building insulation, 286
293

Index

Thermal transmittance (U-value), 281 Thermogravimetric analysis (TGA), 320
321 Thixotropic agents, nanomaterials as, 256
257, 256t Thixotropy, 210 Three-dimension (3D), 392 nano-scale structure, 392 printing RMC for, 449
450 technology, 70 3D concrete printing (3DCP), 251
253 nanomaterials implementation in extrusion-based 3DCP, 253
258 Tip-enhanced Raman spectroscopy (TERS), 383 Titania, 151 Titanium dioxide (TiO2), 151, 162
163, 286, 293, 308 amount, 170
171 crystal structure, crystallite size and crystalline phase, 169
170 humidity, 173 irradiation, 172
173 as photocatalyst, 163
173 photocatalytic property, 165
173 factors affecting photocatalytic activity, 167
173 NOx degradation mechanism, 165
167 pollutant concentration, 171
172 specific surface area and particle size, 167
169 structure, 163
164 temperature, 172 utilization of TiO2 for air purification, 164
165 utilization of TiO2 in cementitious systems for air purification purposes, 173
189 abrasion/wearing/weathering of surface of cement-based systems and NOx degradation capability, 185
186 amount of TiO2 in cement-based systems and NOx degradation capability, 178
179 combined presence of metal/non-metals and TiO2 in cement-based systems and NOx degradation capability, 180
182

485

curing age/condition of cement-based systems and NOx degradation capability, 186
187 final surface texture of cement-based systems and NOx degradation capability, 187 mixture composition of cement-based systems and NOx degradation capability, 182
185 operation-related parameters and NOx degradation capability, 187
189 particle size ofTiO2 in cement-based systems and NOx degradation capability, 177
178 quality of distribution of TiO2 particles in cement-based systems and NOx degradation capability, 174
177 type of TiO2 in cement-based systems and NOx degradation capability, 179
180 “Topdown” approach, 461 Torque
velocity cycle, 219
220 Traditional methods, 14
17 Traffic detection, 50 Transmission electron microscopy (TEM), 322
324, 379
380, 384
387 Travertine, 309 Trichloroethylene (TCE), 167
168 Triton X100, 113
115 Tungsten oxide, 293 Tungsten trioxide (WO3), 164
165 Two-dimension (2D) nano-characterization test, 392 nanomaterials graphene, 41
46 nano-BN, 46
48 tailoring cementitious composites with, 41
48 nanoparticles, 263, 466 U Ultimate tensile strength, 426 Ultra high-performance fiber cementitious composites (UHPFRCC), 67
68. See also High-performance fiberreinforced cementitious composites (HPFRCC) Ultra-high performance concrete (UHPC), 2
3, 104
105, 145, 251

486

Ultra-high-performance fiber-reinforced composites, 143 Ultrasonication process, 88 Uniaxial tensile stress
strain curves, 430
432 UV-spectroscopy, 463 V Vacuum insulation panel (VIP), 288
289 Vanadium oxide (V2O5), 293 Vane rheometer, 222
223 Vascular healing system, 151 Vascular system, 143
144 Viscosity modifying admixtures (VMAs), 215, 257
258 Volcanic ash, 306
307 Volume of permeable voids (VPV), 328
329 of Portland cement-concrete, 331 Vom Berg model, 216 W Water and accelerated carbonation conditions (WC conditions), 430 Water and ambient conditions (WA conditions), 430 Water sorpitivity, 328
329 Water to binder ratio (w/b ratio), 215, 425 Wear-resistant coatings, 292 Weight-to-binder ratio (w/b ratio), 315 Wet route, 410
411 Williamson model, 218

Index

Workability behavior of geopolymer, 353 World Health Organization (WHO), 161 X X-ray computed nano-tomography, 392
395 energy dispersive spectroscopy, 385 micro CT images, 415
416 scattering, 390 X-ray diffraction (XRD), 320
321, 361, 388
389 analysis, 385
387 of geopolymer with nano-SiO2, 361
362 Xenon (Xe), 289 XPS. See Extruded polystyrenes (XPS) XRD-nCT. See Nano-tomography combined with XRD (XRD-nCT) Z Zero-dimension (0D), 263 nanomaterials nano-SiO2, 19
23 nano-TiO2, 24
29 nano-ZrO2, 29
31 tailoring cementitious composites with, 19
31 nanoparticles, 263 Zeta potential, 175
177 Zinc oxide (ZnO), 162
163 Zirconium dioxide (ZrO2), 292 Zirconium oxide nano-particles, 231