Application of Waste Materials in Lightweight Aggregates (Innovations in Environmental Engineering) [1 ed.] 1032321539, 9781032321530

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Application of Waste Materials in Lightweight Aggregates Application of Waste Materials in Lightweight Aggregates presents the current state of knowledge on aggregates production methods, their characteristics, current standards and legal regulations. In addition, the book briefly discusses the issue of the presence of different types of waste in the environment (including municipal, agricultural, energy and mining industries), their characteristics and uses for the production of lightweight aggregates. This book serves as a source of academic information on the course and conditions of using various waste treatment processes for academics, engineers, professionals and students interested in environmental engineering, as well as for companies dealing with recycling and disposal of waste. Franus Małgorzata, PhD, ScD (habilitation) was born in 1974 in Dębica, Poland. In 1998 she received MSc at the AGH University of Science and Technology in Krakow. Since 2000 she has been working in the Lublin University of Technology, Faculty of Civil Engineering and Architecture. She received a PhD in the discipline of environmental engineering (2009) and a DSc (habilitation) in 2016 at the Lublin University of Technology, Faculty of Environmental Engineering. She is an associate professor at the Department of General Construction, Lublin University of Technology. Now she is working on the utilization of selected waste for the production of sintered lightweight aggregates and concrete, (i) and (ii) the use of microwave radiation for the production of construction materials, synthesis, modification and characterization of sorbents used to remove pollutants from water and sewage, (iii) catalytic carriers for air purification manufacturing, (iv). She has published 79 papers, including 40 papers in the list of Journal Citation Reports, three books and is co-author of six Polish patents. She participated in the implementation of 17 research projects, three of which she led.

Innovations in Environmental Engineering Editor-in-chief: Lucjan Pawłowski Associate Editor: Małgorzata Pawłowska Editorial Board: Andrei Victor Sandu, Artur Pawłowski, Christopher G. Uchrin, Corrado Sarzanini, Czesława Rosik-Dulewska, Ewa Klimiuk, Guomo Zhou, Hanna Obarska-Pempkowiak, Irena Wojnowska-Baryła, Katarzyna Juda-Rezler, Katarzyna Majewska-Nowak, Kazimierz Banasik, Marek Gromiec, Maria Wacławek, Maria Włodarczyk-Makuła, Marzenna Dudzińska, Sergio Orlandi, Serhii Kvaterniuk, Tomasz Winnicki and Vladimir S. Soldatov. The ‘Innovations in Environmental Engineering’ Series is devoted to the publication of monographs that aim to integrate environmental engineering and the concept of sustainable development, by counteracting negative changes in the environment with technological methods of neutralizing pollutants into the environment. Books in the series typically cover the following topics: chemical/biological treatment of liquid and solid wastes, waste mineralization, acid rain, air pollution neutralization, indoor air pollution prevention, and risk analysis. Particular attention is paid to the selection of new engineering methods enabling the neutralization of pollutants in particular parts of the environment: air, water, soil, as well as modification of production process in the direction of reducing the emission of pollutants in to the environment. The Vehicle Diesel Engine Start-up Process Operational and Environmental Aspects Paweł Droździel Application of Waste Materials in Lightweight Aggregates Franus Małgorzata

For more information about this series, please visit: www.routledge.com/Innovations-in-EnvironmentalEngineering/book-series/ENV

Application of Waste Materials in Lightweight Aggregates

Franus Małgorzata

Designed cover image: © Shutterstock Images First published 2023 by Routledge 4 Park Square, Milton Park, Abingdon, Oxon OX14 4RN and by Routledge 605 Third Avenue, New York, NY 10158 Routledge is an imprint of the Taylor & Francis Group, an informa business © 2023 Franus Małgorzata The right of Franus Małgorzata to be identified as author of this work has been asserted in accordance with sections 77 and 78 of the Copyright, Designs and Patents Act 1988. All rights reserved. No part of this book may be reprinted or reproduced or utilised in any form or by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying and recording, or in any information storage or retrieval system, without permission in writing from the publishers. Trademark notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation without intent to infringe. British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library ISBN: 978-1-032-32153-0 (hbk) ISBN: 978-1-032-32155-4 (pbk) ISBN: 978-1-003-31309-0 (ebk) DOI: 10.1201/9781003313090 Typeset in Times by Apex CoVantage, LLC

Contents Preface 1 Classification of lightweight aggregates 1.1 Introduction 1.2 The history of lightweight aggregate 1.3 Section of lightweight aggregate 1.3.1 Natural aggregates 1.3.2 Artificial aggregates 1.3.3 Recycled aggregate

ix 1 1 2 3 4 6 9

2 Regulations and standards 2.1 Introduction 2.2 Review of standards and legal provisions applicable to aggregates 2.3 Engineering properties of lightweight aggregates 2.3.1 Apparent density, dry particle density and water absorption 2.3.2 The void percentage, porosity and bulk density 2.3.3 The compressive strength 2.3.4 Individual aggregate crushing strength 2.3.5 The frost resistance 2.3.6 The bloating index 2.3.7 Shape and texture 2.3.8 Classification parameters of lightweight aggregates

13 13 14 18 19 19 20 20 21 21 21 22

3 Alkali – silica reactivity of aggregate 3.1 Introduction 3.2 Types and mechanism of the alkaline reaction 3.3 The mechanism of expansion 3.4 Effects of the Alkaline-Silica Reaction (ASR) 3.4.1 Cracking 3.4.2 Expansion that causes movement and deformation 3.4.3 Pop-outs 3.5 Test methods for alkaline reactivity 3.6 Reactivity of lightweight aggregate 3.6.1 Expanded glass 3.6.2 Expanded shale 3.6.3 Expanded clay 3.6.4 Expanded perlite 3.6.5 Microsphere 3.6.6 Volcanic rock 3.6.7 Sintered fly ash 3.6.8 Other aggregates/artificial aggregates

24 24 25 27 28 29 30 30 30 31 32 34 35 35 36 37 38 38 v

vi Contents 4 The production process of lightweight aggregates 40 4.1 Introduction 40 4.2 Preparation of raw materials 41 4.3 Cold bonding approach 41 4.3.1 Agitation granulation 42 4.3.2 Pressure or compaction granulation 46 4.4 Sintering approach 48 4.4.1 The role of the liquid phase in the sintering process 52 4.4.2 Chemical reactions occurring in the course of the pre-sintering process53 4.5 Autoclaving (hydrothermal) approach 55 4.5.1 Accelerated carbonation approach 58 4.6 Microwave radiation approach 65 4.6.1 The basics – theoretical 65 4.6.2 Microwave generators 67 4.6.3 Comparison of sintering and microwave heating 68 5 Influence of production parameters on properties of lightweight aggregates 5.1 Introduction 5.2 Cold bonding method 5.2.1 Effect of raw materials 5.2.2 Effects of binders and additives 5.2.3 Effects of production parameters on the granulation 5.2.4 Effect of water content and rotation speed on the granulation 5.3 Sintering method 5.3.1 Effects of raw materials 5.3.2 Factors affecting the lightweight aggregate expansion 5.4 Accelerated carbonation method 5.4.1 Effect of raw materials 5.4.2 Effect of carbonation conditions 5.5 Comparison of physico-mechanical properties of lightweight aggregate produced with different hardening methods 5.5.1 Macro- and microstructure 5.5.2 Water absorption 5.5.3 Loose bulk density and particle density 5.5.4 Individual crushing strength 5.6 The impact of the lightweight aggregates production on the environment 5.6.1 Leaching behaviour 5.6.2 Gas emissions 6 Useful waste in the production of lightweight aggregates 6.1 Introduction 6.2 Municipal sewage sludge 6.2.1 Sewage sludge 6.2.2 Sewage sludge ash 6.2.3 Water treatment sludge

69 69 70 70 72 74 76 77 77 78 88 89 89 90 90 90 92 94 96 96 97 100 100 103 103 113 122

Contents vii 6.3 Coal power plant residues 6.3.1 Coal fly ashes 6.4 Mineral transformation industry 6.4.1 Glass cullet production 6.4.2 Types of cullet 6.4.3 Glass cullet characteristics 6.4.4 Application of glass cullet in civil engineering 6.4.5 Glass cullet in lightweight aggregate 6.5 Mining and quarrying residues 6.5.1 Mining and quarrying residues production 6.5.2 Hard coal-mining wastes 6.5.3 Wastes from lignite mining 6.5.4 Waste from mining of non-ferrous metal ores and chemical raw materials 6.5.5 Drilling fluids and other drilling wastes 6.5.6 Wastes from mining of rock raw materials 6.6 Construction and demolition wastes 6.6.1 Construction and demolition wastes (CDWs) production 6.6.2 Handling construction and demolition wastes 6.6.3 Main types of recycled aggregates from construction and demolition waste (CDWs) 6.6.4 Application of CDWs in civil engineering 6.6.5 Application of CDWs in lightweight aggregate 6.7 Sediments dredging 6.7.1 Harbour sediment 6.7.2 Reservoir sediments References Index

132 132 152 152 154 155 157 160 164 164 166 171 172 178 180 186 186 187 189 191 195 199 199 212 223 273

Preface The constant economic development in the world and the continuous improvement of living standards generate a huge amount of waste, the amount of which increases proportionally to the population growth and the consumption scale. Waste is a key problem for the natural environment as well as for human life and health, since it introduces hazardous substances into the environment that are more and more difficult to decompose. Increasing concentrations of heavy metals and pesticides in soils can be observed. Additionally, the content of hazardous organic substances in groundwater and surface waters is increasing. The general assumptions of the waste management strategy include preventing their generation as a starting point (along with minimizing the waste generation) and preferring their reuse, treating the disposal (neutralization) as a last resort when no other way is possible. Both international policy and actions undertaken by countries in their own territories are increasingly focused on the development and implementation of such mechanisms that will allow reducing the amount of waste which deteriorate an environment. On the other hand, a significant increase in the amount of generated waste by various industries can lead to a waste disposal catastrophe. Strict environmental regulations, high transport costs to the disposal site, taxes on landfills and the limited availability of landfills are cause of concern. Hence, alternative ways of using waste are constantly searched for. A lively and huge interest in ecotechnology is fuelled by the growing concern for sustainable development. A systematic search for new methods of waste valorization was undertaken – in particular, in the civil engineering sector, which prompted many studies on the use of municipal and industrial waste as possible raw materials for the production of aggregates. The process of accumulating published worldwide literature on recycled and secondary used materials with undertaking systematic analysis and data evaluation is undoubtedly an effective tool for characterizing materials and determining their application potential in various disciplines as well as for addressing important environmental issues and sustainable development. This approach has been adopted in the development of this book on lightweight aggregates from various types of waste. The goal of sustainable development is to move towards renewable resources and away from non-renewable resources in order to avoid generating waste in favour of its recycling. Increased interest in artificial and recycled aggregates is observed mainly in the places where significant infrastructure investments are located within the potential resource base for their production. They are usually cheaper than natural aggregates, with comparable physical and mechanical properties. Often, an additional advantage of artificial aggregates is their much lower bulk density than natural ones. For the recipient, this is a significant advantage because, in a road embankment or road foundation, the used volume is important, while payment is done in terms of weight of raw material. Thus, the density, which is approximately 50% lower, directly affects the cost of the construction investment as well as the cost of transport. Waste recovery also reduces the fees associated with their storage. In addition to the presented “economic” advantages, environmental and social benefits should also be distinguished. The environmental benefits include, first of all, the protection of natural resources (minerals) for the production of aggregates, reduction of emissions from transport, smaller amounts of waste disposed of in landfills and the recovery of land taken by them. The extraction of natural aggregates has a significant impact on the environment in terms of CO2 emissions. During its production, CO2 is emitted through various processes such as the extraction, crushing and transportation. Social benefits include supporting the local labour market, reducing the intensity of heavy traffic, ix

x Preface and thus increasing road safety. The depletion of natural aggregate resources is another major problem towards sustainable development worldwide. In addition, the natural aggregate resources are preserved for future generations. The source of unconventional raw materials, completely or partially replacing raw materials for aggregate production, are municipal solid waste, sediments from water reservoirs and port dredging, residues from coal power plants, mining and quarrying, processing of mineral resources and the metallurgical industry. These new types of raw materials are often characterized by significantly different chemical and mineralogical composition in comparison to clays and shales traditionally used for the production of expanded clay aggregates. In consequence, these materials show, in most cases, different technological properties, particularly expansiveness with respect to the present industrial batches utilized to produce the lightweight aggregate. Thus, the use of waste may result in significant modifications of the production process in order to obtain the desired technical parameters of aggregates. These changes mainly concern the shaping and firing processes due to the different plasticity and thermal behaviour of unconventional raw materials. The aim of this study was to review waste and other unconventional raw materials for the production of lightweight aggregates using various methods of hardening them in order to ensure ecological balance and an appropriate quality of life in societies. The need to manage these wastes and use them in a way that increases the possibility of saving primary raw materials will ensure their safe disposal and reduction of newly generated waste.

Classification of lightweight aggregates

1

1.1 INTRODUCTION The history of lightweight aggregate dates back to the ancient Greeks and Romans. In 1913, an incident in the Nathan Thomas Hayde brick factory in the USA contributed to the creation of ceramsite. The aggregate was named haydite. The first plant producing this aggregate was launched in 1917 and is now manufactured all over the world. Lightweight aggregate, or ceramsite, is a building material that is created as a result of thermal treatment by burning mineral raw materials at a temperature of up to 1300°C (Sokolova and Vereshagin 2010). According to the European standards, mineral lightweight aggregates for concrete, mortar and grout should have a bulk density lower than 1200 kg/m3 and a grain density not greater than 2000 kg/ m3 (PN-EN 13055). In the current standard, four types of aggregates are proposed for lightweight aggregates: natural, artificial, waste and recycled, which are a group of artificial aggregates. Natural aggregates are the aggregates from natural sources (deposits) that can only be processed mechanically. This group mainly includes light volcanic rock crushed aggregates, which were formed from cooling magma during volcanic eruptions. Among them there are natural pumice, scoria, volcanic tuff, volcanic ash, diatomites, carbonate rock and silica aggregate (Brown and Calder 2005). Natural aggregates, as a rule, are characterized by high water absorption, water demand and quite low strength, which makes it impossible to use them in high-strength concretes. Another group of lightweight aggregates are artificial aggregates. Initially, they were defined as the aggregates of mineral origin produced from natural resources such as clays, loams and shales, which were subjected to heat treatment in rotary kilns at a temperature of 900–1300°C. According to the new classification, artificial aggregates can also be produced from secondary waste materials generated in the power industry (ashes, slags), iron and non-ferrous metals, heat engineering, ceramics and mining. The artificial aggregates from a thermally treated mineral raw material are ceramsite (clay) and aluminoporite, included in lightweight aggregates (density below 1000 kg/m3). Heat-treated artificial aggregates also include perlite, which is a naturally occurring acidic rock of volcanic origin, and expanded vermiculite, which results from high-temperature treatment of vermiculite mineral. If the ashes are exposed to high temperatures of 1000–1350°C, it is possible to sinter them and obtain an aggregate called ash-porite (Góralczyk and Kukielska 2013) The artificial aggregates from waste materials that have undergone other than thermal treatment are mainly the aggregates produced from post-smelting slag, power plant ashes, underground mining waste materials and other mineral waste materials, which are not subjected to additional thermal DOI: 10.1201/9781003313090-11

2  Application of Waste Materials in Lightweight Aggregates treatment. The main production processes are similar to the technologies used in opencast mines of rock raw materials, especially gravel and crushed aggregates. These include metallurgical slag, shale rock, granulated blast furnace slag and metallurgical pumice. There is also an aggregate called Aardelite, which is produced by infusing with water vapor at a temperature of 75–85°C of fly ash from the combustion of hard coal with hydrated lime and chemical additives. Another type of artificial aggregate is elporite, obtained from foamed slag generated in dust furnaces of power boilers. Recycled aggregates are the aggregates resulting from the processing of inorganic material previously used in construction, such as demolition works or road reconstruction. Another group of aggregates is the waste from mineral resources generated during underground mining of hard coal and ores, and wood that is particularly easy to use in households and small facilities (e.g. in coal-fired central heating boilers) (Salgado and Silva 2022). The group of artificial and recycled aggregates can be described as alternatives to natural aggregates.

1.2  THE HISTORY OF LIGHTWEIGHT AGGREGATE The ancient Greeks and Romans mainly used natural pumice or other porous volcanic rocks such as scoria and tuffs as lightweight aggregate. The aggregate obtained from these deposits was ground and then mixed with sand and lime. As a result, they obtained the building material of considerable strength and water resistance, which was used to build walls, water channels and many other monumental structures. At the request of Emperor Justinian in the 4th century AD, two engineers – Isidore of Milctus and Anthemius of Tralles – built the St. Sofia Cathedral, widely known as Hagia Sofia, in Istanbul, Turkey, using this type of lightweight aggregate. Other buildings include the prestigious Pont du Gard, built in 70–82 AD. In addition, in order to reduce the weight of structures, the Romans used natural lightweight aggregates and hollow vases in the technique known as “Opus Caementitium”. This method of building was also used in the construction of the pyramids in the Mayan period in Mexico. In the USA, pumice has been used in construction since 1851, and in 1908–1918, it was employed for the construction of aqueducts in Los Angeles. Since 1935, it has been used as a light insulating building material in the USA and its popularity in this sector continued to grow (Kumar and Arunakanthi 2018). A coincidence contributed to the formation of expanded clay. It happened in 1913 in the Nathan Thomas Hayde brick factory in the USA, when a smoker poured too much fine coal into the furnace and fell asleep. Excessive temperature inside the furnace caused a brown mass to be accidentally obtained in the furnace instead of the burnt red brick. The frightened smoker began smashing it with a hammer, and the pieces that were split off were hard and very light at the same time. The material was found to have application potential. During subsequent experiments, it was noticed that, during the firing process, a material in the shape of round or oval lumps of various fractions was obtained. Each of the porous “balls” inside was covered with a hard ceramic cover. The light aggregate (LWA) created in this way is extremely durable, and at the same time, has a much lower density than sand and gravel and is a good thermal insulator. The process of manufacturing a synthetic LWA in a rotary kiln was patented by Stephen J. Hayde in 1908 (Tang et al. 2011). He named the new product haydite, after his name, and the first plant producing this aggregate was launched in 1917. It was used to build the hulls of ships during World War I. One of the first and the best known was the 7,500 Mt tanker Selma, launched in 1919. With time, the demand for expanded clay became so great that the plants producing this material began to be established all over the world. The aggregate is used not only in construction but also in geotechnics and gardening. In 1920, a large manufacturing facility was built in Kansas City, and in 1927, the first factory in Canada was established. Until the outbreak of World War II, small expanded clay factories were established in Sweden, Norway and Denmark. After the war,

1  •  Classification of lightweight aggregates  3 the demand for building materials grew dynamically. New factories, roads, bridges and airports were needed. Factories producing lightweight aggregate began to be built around the world. In the years 1935–1939 in Rüdersdorf, Germany, the first production line of lightweight aggregates from expanded clays was launched, according to Philip Holzmann. In 1938, a production plant for lightweight aggregates from expanded clays was established in Denmark under the trade name LECA from the first letters of Lightweight Expanded Clay Aggregate. It is produced in many countries such as Portugal, Italy, Finland, Norway, Iran, Germany and Poland. The resulting sintering technology using rotary kilns was a tremendous achievement, and the LWA industry in the United States and European countries continued to advance until the 1950s. In the USSR in the 1930s, light aggregate was produced on an industrial scale, and in the 1960s and 1980s, they were the world’s largest producer of lightweight aggregates under the name ceramsite. In Europe in the following years, the production of aggregates based on sintered fly ash was started. The first plant producing ash-rock aggregate was established in Great Britain in 1960 (Lytag), in Germany in 1973 and in the Netherlands in 1983. Since 1995, Poland has been producing aggregate under the Lytag license (EuroLightCon 2000), and since 1979, expanded clay has been produced in the factory in Gniew. It is currently the most modern LECA production plant in Poland and Europe.

1.3  SECTION OF LIGHTWEIGHT AGGREGATE Lightweight aggregate, or expanded clay, is a building material that is created as a result of thermal treatment by burning mineral raw materials at a temperature of up to 1300°C. The natural raw materials for the production of aggregates are usually expanded clays, shales (Sokolova and Vereshagin 2010) and natural volcanic materials, such as pumice, perlite, vermiculite, volcanic scoria and slag. According to PN-EN 13055–1 “Lightweight aggregates – Part 1: Light aggregates for concrete, mortar and thin mortar” and PN-EN 206–1: 2002 “Concrete. Requirements, properties, production and compliance”, the light mineral aggregates for concrete, mortar and cement slurry should have a bulk density lower than 1200 kg/m3 and a grain density not higher than 2000 kg/m3. The current standard for lightweight aggregates presents four types of aggregates: natural, artificial, waste and recycled, which are a group of artificial aggregates (Figure 1.1).

FIGURE 1.1  Classification of lightweight aggregates for concrete according to PN-EN 13055.

4  Application of Waste Materials in Lightweight Aggregates TABLE 1.1  Groups and assortments of lightweight aggregates according to PN-86/B-23006. LIGHTWEIGHT AGGREGATE GROUP

ASSORTMENT NAME

Crushed stone mineral aggregates

carboporite from light limestone

Artificial aggregates from heat-treated minerals

expanded clay, aluminoporite

Artificial aggregates from heat treated industrial waste

graphite, schistosity, ashstone

Artificial aggregates from industrial waste without further heat treatment

elporite, furnace slag

According to the previous national standard PN-86/B-23006, “Aggregates for lightweight concrete”, depending on the type of raw material used for the production of aggregate and the method of its production, they were divided into four groups and ten basic assortments: • • • •

crushed stone mineral aggregate, artificial from heat-treated mineral raw materials, artificial from industrial waste subjected to thermal treatment, artificial from industrial waste not subjected to additional thermal treatment, which indicates differences in their division (Table 1.1).

In the PN-86/B-23006 standard, sintered aggregates belonged to two groups. Currently, all types of aggregates are included in the group of artificial aggregates. However, this division seems less clear than the previous one. It may be questionable to distinguish between artificial, waste and recycled aggregates. According to the previous PN-86/B-23006 standard, all three groups of aggregates were covered by the term “artificial aggregates”. According to PN-EN 12620, PN-EN 13043, PN-EN 13055 part 1 and 2, PN-EN 13193, PN-EN 13242, PN-EN 13383–1 and PN-EN 13450, the definition of aggregates was established as follows: • natural aggregates are the aggregates from natural sources (deposits) that can be processed only mechanically, • artificial aggregates are the aggregates that have mineral origin and they are the result of industrial processes involving thermal or other modification, • recycled aggregates are the aggregates that result from the processing of inorganic materials previously used in construction.

1.3.1 Natural aggregates Natural aggregates are the aggregates of mineral origin. This group mainly comprises light volcanic rock crushed aggregates, which were formed from cooling magma during volcanic eruptions (Mboya et al. 2011). These include natural pumice stone, scoria, volcanic tuff and lava ash. They have very diverse physical properties and variable sizes, from particles with the size in the range of submillimetres (ashes) to large boulders (Brown and Calder 2005). Their structure is dense (volcanic tuffs) or spongy (volcanic scoria and pumice). Therefore, they are materials rich in silica, alumina and iron oxide, as well as containing small, variable amounts of alkali metal and alkaline earth oxides (Hossain 2006). Pumice is formed as a result of the rapid cooling of the lava; it is a fine-porous rock; it has a whitegray external color, which largely depends on its chemical composition. As a result of its high porosity,

1  •  Classification of lightweight aggregates  5 the pumice obtains unique properties, such as good thermal insulation. In addition, despite the porosity, it has adequate resistance. Scoria is a lava crust that is also very porous and rough; it looks like furnace slag. It consists of volcanic glass fragments and may contain phenocrysts (Bryan 2004). Scoria and pumice are among the most abundant pyroclastic materials (Best 2002; Alemayehu and Lennartz 2009). They are generally used in civil engineering as aggregate or supplementary cementitious materials in the manufacturing of potential structural materials through geopolymer synthesis, lightweight, thermal or acoustic insulating panels, ceramic materials, low-grade refractory concretes, adsorbents and filter materials. Fine and coarse aggregates are mainly used for concretes, mortars and building blocks (Top and Vapur 2018). Pumice, as a fine aggregate, improves the flow properties, workability and cohesion of self-compacting concrete (Ardalan et al. 2017), while scoria, as a fine and coarse aggregate, was used for non-structural concrete due to weight reduction, thermal and acoustic insulation (Demirdag and Gunduz 2008) as well as for mortars and cements (Bogas and Cunha 2017; Tchamdjou et al. 2017, 2018). Another type of pyroclastic rock is volcanic tuff, which is light, dense, usually porous sedimentary rock. Volcanic tuff is a dense volcanic rock formed by the cementation of ash, scoria, lapilli and other ejecta from an erupting volcano (Hong et al. 2019). Volcanic tuff with the addition of marble, travertine (Tekin 2016) and silica (e.g. nanosilica, micro silica and Styrene-Butadiene Latex) was used to obtain a geopolymer composite (Ekinci et al. 2019). An innovative method of activating the geopolymerization process of building materials is the modular use of volcanic tuff with calcium-based materials, fly ash and with the addition of various chemical activators such as calcium oxide (CaO, 99.9%), calcium sulfate dihydrate (CaSO4 · 2 H2O, AR), sodium hydroxide (NaOH, AR) and sodium metasilicate nonahydrate (Na2SiO3 · 9 H2O, AR) (Toprak and Arslanbaba 2016; Song et al. 2021). Volcanic ash is a non-generic term which refers to fine fragments of pyroclastic materials. Typical volcanic ashes are pyroclastic debris with the size below 2 mm (Dingwell et al. 2012). However, crushed (powdered) volcanic scoria/slags are often referenced as volcanic ashes (Tchakouté et al. 2013). The properties of volcanic ashes show significant differences because their chemical and mineral composition is related to the different types of magma they come from as well as to the eruption conditions (Siddique 2011). Volcanic ash is used for cement and concrete, for partial replacement of cement, paste and mortar or for the production of cement and concrete mixtures (Scrivener et al. 2016; Al-Fadala et al. 2017), geopolymeric and ceramic materials, lunar soil stimulants and adsorbents. Due to their pozzolanic properties, they can be used for the production of masonry elements (al-Swaidani et al. 2016). Diatomites – i.e. organogenic rocks made mainly of diatom shells (unicellular algae) – are also natural aggregate. As the diatoms died, their tiny shells sank and formed thick layers over the centuries. The fossilized rocks have been pressed into a soft, chalky rock, which is now called diatomaceous earth (Korunic 1998). Diatomaceous earth, or diatomite, is light because of its cellular structure and high porosity (Pimraksa and Chindaprasirt 2009). It has a low thermal conductivity, but it is rather soft and not very reactive (Degirmenci and Yilmaz 2009). Uncalcined, lightweight diatomite aggregate was used to produce lightweight concretes with strength of 2.5–8 MPa and a specific weight of 0.90–1.19 g/cm3 (ACl 213). On the other hand, the calcination of fine-grained diatomite at a temperature of 600°C results in a fine and medium lightweight aggregate. Higher calcination temperature up to 1000°C allows obtaining a stable and highly durable coarse aggregate, owing to which the strength of concrete blocks is higher (Posi et al. 2013). This group of aggregates also includes carboporite made of calcareous tuffs, necrosis, light organogenic limestones and other materials, mainly made of calcium or calcium-magnesium carbonate, as well as diatomite aggregate obtained from diatomaceous earth, diatomite, spongiolite and other rocks composed mainly of amorphous silica.

6  Application of Waste Materials in Lightweight Aggregates Natural aggregates, as a rule, are characterized by high water absorption, water demand and quite low strength, which makes it impossible to use in high-strength concrete.

1.3.2 Artificial aggregates Initially, the definition of artificial aggregates was understood very narrowly as aggregates of mineral origin produced from raw materials, such as clays and slates which were heat treated in rotary kilns at a temperature of 900–1300°C. According to the new classification of artificial aggregates, they can also be produced from secondary waste materials generated in the power industry (ash, slag), iron and non-ferrous metallurgy, heating, ceramics, mining (European Commission Document 2007), which is included in the CEN/TC154/TG10/N736 document called (Table 1.2). This document proposes a new classification of artificial aggregates, produced from different types of secondary raw materials.

TABLE 1.2  Secondary materials. Artificial aggregates. Final report for aggregates from secondary deposits. Document CEN/TC154/TG10/N736. TYPE P

A

B

C

D

E

F

SOURCE natural aggregates

construction and recycling

municipal solid waste incineration plant

energetics

iron and steel industry

non-ferrous metal industry

other metallurgical industry

SUBTYPE

SPECIFIC/CHARACTERISTIC MATERIALS

P

all petrographic types included in PN EN 932–3

A1

reclaimed asphalt (rubble)

A2

crushed concrete

A3

crushed masonry brick

A4

mix A1, A2 and A3

B1

coarse-grained ashes

B2

fly ash

C1

fly ash from coal combustion

C2

fly ashes from fluidized bed furnaces

C3

slag from power plant boilers

C4

bottom ash from coal combustion

C5

bottom ash from fluidized bed boilers

D1

steel slag

D2

crystalline blast furnace slag (air-cooled)

D3

glassy blast furnace slag

D4

electric arc furnace slag

D5

stainless steel waste

E1

copper slag

E2

molybdenum slag

E3

galvanizing slag

E4

phosphorous slag

F1

foundry sand

F2

foundry furnace slag

1  •  Classification of lightweight aggregates  7 TYPE

G

H

I

SOURCE

coal mining

dredging

other

SUBTYPE

SPECIFIC/CHARACTERISTIC MATERIALS

G1

coal slate burned

G2

waste from hard coal (coal shale)

G3

previously selected waste from coal and rock mining

G4

used shale oil

H1

dredging sand from rivers and reservoirs

H2

dredging clay

I1

soil from excavations

I2

ash from paper industry

I3

waste incineration ash

I4

biomass combustion ash

I5

glass cullet

I6

expanded clay

Depending on the volume density, rock aggregates are divided into three types: • heavy aggregate with a dry density above 3000 kg/m3, • normal aggregates with a dry density of 2000–3000 kg/m3, • lightweight aggregates with a dry density below 2000 kg/m3. The waste from the energy, heating, metallurgy, ceramics and mining industries, which can be a raw material for the production of artificial aggregates, accounts for over 80% of the waste produced in Poland. After processing, this waste can be used as aggregates. The scope of using waste for the production of aggregate is unlimited, as long as the produced aggregate meets the standard requirements, depending on the intended use. All groups of aggregates have equal prospects with regard to their application to concrete, bituminous mixtures, bound and unbound hydraulic mixtures. The only criterion for the use of aggregate is its properties. In the PN-78/B-01101 standard (which does not apply anymore) named “Artificial aggregates”, the division includes group D – aggregates of organic origin in which, first of all, the aggregates resulting from the processing of waste plastics were taken into account. According to the currently adopted standards, such raw material is not an aggregate. Artificial aggregates made of thermally treated mineral raw material are ceramsite (claydite) and aluminoporite classified as lightweight aggregates (density below 1000 kg/m3). They arise as a result of firing easily fusible, swellable raw clay materials, ranging from clay shale through compact and plastic clays to clays swelling at high temperatures from 1000–1200°C, which meet the appropriate requirements (swelling/sintering ability, appropriate grain, chemical and mineral composition). For the production of ceramsite, it is recommended to use the clay with a swelling factor of at least 2.5 and to conduct firing in several stages, in which the temperature in the furnace ranges from 150 to 1250°C. The obtained product has a spherical shape, size of 0–16 mm, relatively high porosity, with the predominance of closed pores and a bulk density of 0.20 to 1.0 g/cm3. Low density is the main advantage of ceramsite aggregates, owing to which they are used in numerous applications, including road construction. They are used primarily to relieve buildings and structures (replacement of the native soil with ceramsite aggregate), construction of road surfaces on weak surfaces but also for mortars, renders

8  Application of Waste Materials in Lightweight Aggregates and many other applications. The regular shape, the content of fine pores, which are usually closed, the tight outer layer preventing the penetration of the cement slurry and the high mechanical strength of the grains make it possible to obtain high-strength concrete from this aggregate, well workable, with correspondingly lower cement consumption, and at the same time, lighter than other types of lightweight aggregates. Thermally treated artificial aggregates also include perlite and expanded vermiculite. Perlite is a naturally occurring acidic rock of volcanic origin. Hundreds of millions of years ago, during the eruptions of submarine volcanoes, lava quickly cooled and closed the water drops inside. The trapped water droplets, constituting only 2 to 5% of the volume, are the most important component of perlite, which is responsible for its specific properties. Expanded vermiculite is produced by high-temperature treatment of the vermiculite mineral. During the process, it releases the water accumulated in inter-packet voids and increases its volume 10–25 times, taking the form of swollen, mineral accordions. There are no natural deposits of vermiculite in Poland. Worldwide extraction is concentrated in South Africa, the Americas, Asia and Eastern Europe. Vermiculite, in exfoliated form, is light, non-flammable, compressible, very absorbent and non-reactive; moreover, it can have high cation exchange capacity and constitutes a good sound as well as heat insulator. Perlite ore milled to the appropriate granulation swells when subjected to sintering for a few seconds at the temperature of 900–1000°C, increasing its volume up to 20 times. The product of the swelling (expansion) process is expanded perlite with the structure of porous, white granules. Artificial aggregates from waste materials subjected to thermal treatment are aggregates from fly ash after the combustion of hard coal or lignite and aggregates from cooled steel slag. Hard coal and lignite largely consist of non-flammable substances, which remain as fly ash as a result of combustion. This material is waste, which is largely disposed of in landfills. Nevertheless, by subjecting the fly ashes to high temperatures of 1000–1350°C, it is possible to obtain an aggregate called sintered fly ash. It is characterized by a relatively low bulk density of 650–850 kg/m3 but high porosity and water absorption at the same time. In Poland, Pollytag is a well-known sintered fly ash aggregate (Kostrzewski et al. 2018). This type of aggregate includes aluminoporite, obtained by sintering non-swellable clay raw materials, such as loess, moraine clays, alluvial clays, clays and stagnant silts with the addition of process fuel, which are then crushed and sorted. Artificial aggregates from waste materials (by-products) subjected to other than thermal treatment are mainly the aggregates produced from metallurgical slag, power plant ashes, underground mining waste materials and other mineral waste materials. The main production processes are similar to the technologies used in opencast mines of rock raw materials, especially gravel and broken aggregates. Nevertheless, a significant part of the raw materials included in this group had previously been subjected to thermal treatment during their creation or storage. These include metallurgical slags that end up in a liquid landfill and go into a solid state there, and shales formed during the exploitation and hard coal enrichment processes, which are under the influence of high temperature due to the processes of self-heating and combustion of coal, which is, especially in older landfills, a large part of the stored rock masses (Kozioł and Kawalec 2008). Another type is sintered and crushed coal shale, or slate rock. Coal shales are separated from it during enrichment of the basic mineral which is present as gangue near coal deposits. They are a valuable raw material for the production of sintered aggregates because, in addition to the characteristics of clay raw materials (chemical and mineral composition), they contain carbon, which is the technological fuel necessary for the sintering process. Granular blast-furnace slag is obtained by rapidly cooling the liquid blast-furnace slag in during smelting of pig iron in large furnaces. This method is used to obtain a granular, porous material with a glass structure. For the production of granulated slag, mainly basic slags with a basicity modulus higher than 1.15 are used. It is usually used in the production of cements as filler for lightweight concretes and mortars. The difference between the production of metallurgical pumice and granulated slag is the

1  •  Classification of lightweight aggregates  9 more rapid cooling in the granulation process and the using highly basic slags (usually above 43% CaO) for this purpose. Blast furnace pumice is the product of the interaction of a small amount of water on the liquid slag. A small amount of water on which the slag is poured turns into steam and penetrates from below into the slag layer. This causes the slag to blow out, creating a porous mass. Depending on the bulk density in the compacted state, there are two classes of aggregate: I and II. Artificial aggregates also include fly ash from hard coal combustion hardened with hydrated lime, with chemical additives, where the agglomerating substance is water. The granulate is steamed at a temperature of 75–85°C. Aardelite is an aggregate used, in particular, for the production of LB-30 class concrete prefabricates, but the low strength and high water-absorption of the aggregate itself limit its other applications. The Polish equivalents of the earlier-described technology are those which produce the Cegran, Megran and Pregran aggregates (Sokołowski 2005). Elporite is another type of artificial aggregate (BN-75/6722–08), obtained from foamed slag generated in dust furnaces of power boilers. The grains of the mineral substance remaining after the combustion of the coal dust, which are in the state of fire ductility, collide and stick together into larger conglomerates and fall to the bottom of the furnace chamber. The chamber is closed with a bath filled with water, in which the incandescent slag agglomerates undergo pore creation. This material, after dehydration and grinding, can be used as a lightweight aggregate for concrete production. Depending on the grain size distribution, elporite is divided into three groups of fractions: 0.45, 0.41 and 0.42 mm, and, depending on the bulk density in the loosely packed state, it is divided into six classes. It is often used as a supplement to fine fractions in ceramsite concrete material. The value of many physical and mechanical properties of natural aggregates is closely related to the petrographic characteristics and structure of the material from which the aggregate was produced; therefore, it cannot be corrected in a process. The production of artificial aggregates from waste materials makes it possible to change the properties of aggregates by modifying the process conditions or the composition of the starting materials. This allows changing properties such as density, strength and frost resistance. In turn, it is possible to obtain the aggregate adapted to the intended use. Artificial aggregates can be used both as a full replacement of natural aggregates or together with them in mixtures. Such a variant makes it possible to obtain the desired level of properties when using materials from local or nearby raw materials.

1.3.3 Recycled aggregate Recycled aggregates are the aggregates resulting from the processing of an inorganic material previously used in construction. The group of artificial and recycled aggregates can be described as an alternative to natural aggregates. They are increasingly used in the construction sector. They are usually cheaper than natural aggregates, with comparable physical and mechanical properties. Often, an additional advantage of artificial aggregates is their much lower bulk density, compared to natural aggregates. From the recipient’s point of view, this is a significant benefit because, in a road embankment or road foundation, the used volume (cubic meters) is important, while payment is done in terms of weight of raw material. Thus, a 50% lower density directly affects the cost of the construction investment. Recycled aggregates are produced in places of construction sites, such as demolition sites and road reconstruction. Demolition works consist in the destruction of unnecessary buildings and infrastructure with the use of explosives. The same effect is achieved by mechanical destruction of individual elements of the objects. Currently, more and more works of this type are carried out in Poland. They are related to the reconstruction and liquidation of outdated road and industrial infrastructure. Due to the development of the technology used to carry out these works, dismantling – rather than demolition – remains the more frequent solution. The raw material obtained during the demolition works is crushed

10  Application of Waste Materials in Lightweight Aggregates and sorted. In this process, elements of reinforcement and other metallic materials are separated magnetically in order to obtain scrap and to clean the resulting aggregate (Kozioł and Kawalec 2008). The number of aggregates that are formed in a given place is relatively small. In the case of reconstruction or expansion of the road surface, the mineral material obtained from the demolition of the old surface is crushed and used to build a new road. There are a couple of reasons to be interested in demolition waste: necessity of waste management, clean up the demolition area, development of construction engineering, demolition scale (potential resources of secondary raw material), as well as greater respect for the natural environment and its resources. An important argument is the inevitable search for more favourable economic relations. Demolition materials cannot be treated in the same way as other wastes, especially non-recyclable “disposable” wastes. According to The Act on wastes from 27 April 2001, a producer of construction waste is obliged to recover, neutralize or transfer it to other entities. The reasons for the increase in the amount of construction waste from dismantling are: • a large number of unusable buildings requiring dismantling, • dismantling of the buildings that do not meet the current design and functional requirements, • creating construction waste resulting from destructive natural phenomena, such as earthquakes or hurricanes. In such cases, the composition of the waste depends mainly on the type of structure affected and on the event itself. Hurricane waste is usually mixed, while earthquake waste is mostly stone or brick. For example, Hurricane Katrina and Hurricane Rita together severely damaged or destroyed more than 275,000 homes, more than the total number of housing units demolished in the entire year in the United States (Faleschini et al. 2017). Such considerations make the estimation of the construction and demolition wastes (CDW) more and more complicated. However, according to EPA (2016b), the amount of CDW produced in the United States in 2014 can be estimated at around 480 million tonnes, followed by concrete (70%), asphalt concrete (14%), wooden products (7%), drywall and renders (3%), brick, clay and roof tiles (2%) and steel (1%). At the same time, in 2017, the United States produced approximately 390 million tonnes of sand and construction gravel, which was used as aggregate for concrete (USGS 2018), as well as 1 billion tonnes of crushed stone used as construction material (USGS 2018). In Europe, however, the CDW production in 2014 can be estimated at approx. 868 million tonnes, which is about 34.7% of the total waste stream, and the composition differs significantly from country to country. Currently, the level of CDW material recycling and recovery also varies between the members of EU (from less than 10% to over 90%), although the recycling rate is expected to be about the same for all European countries in the near future (Pellegrino et al. 2019). In addition to the earlier-mentioned aggregates, the group of aggregates produced from waste mineral raw material, formed during underground mining of hard coal and ores, is becoming more and more visible on the market. According to the previously applicable standards (before 2004), these aggregates were classified as artificial; however, according to the applicable standards, they are natural aggregates because the mineral raw material from which they are produced has not undergone any processing, except a mechanical one. Nevertheless, these aggregates are not included in the statistics on natural aggregates due to the raw material from which they are produced. Therefore, it is proposed to introduce a new division of non-natural aggregates (Figure 1.2). Each new concrete structure has the potential to create concrete waste, which will have to be managed in the future. The problem of management and storage of construction waste is a growing, troublesome issue generating high costs. Demolishing a large number of buildings and engineering structures contributes to the acquisition of concrete aggregate from crushed concrete elements. The components of recycled concrete aggregate are, apart from natural aggregate, also cement mortar and impurities.

1  •  Classification of lightweight aggregates  11

FIGURE 1.2  Proposed division of alternative aggregates.

FIGURE 1.3  Contact zone in concrete with concrete recycling aggregate (reprinted from publication We˛gliński et al. 2017).

Natural aggregate, as a component of concrete, has a significant impact on the physical and chemical properties of recycled concrete aggregate. The strength is determined by the aggregate-cement slurry contact zone, which has a significant impact on the recycling material crushing method, as well as the amount of mortar remaining (Zając and Gołębiewska 2010, 2012; Węgliński et al. 2017). The figure shows a diagram of the contact zone in concrete with recycled concrete aggregate (Figure 1.3).

12  Application of Waste Materials in Lightweight Aggregates Concrete and brick rubble, after mechanical processing into “recycling” aggregate, can be reused in construction or road construction, which entails specific economic effects, such as the use and liquidation of the existing (growing) rubble heaps and limitation of the use of natural resources of aggregates. As a result, the degradation of the natural environment is limited. Aggregate of the “recycling” (R) type can be used for concrete works, shaping and compacting the ground (brick rubble with lime), making the initial foundation and binding layers of roads, hardening squares, parking lots and access roads. The international association of experts and laboratories for construction materials and structures RILEM distinguishes three types of recycled aggregate: • Type I – material from masonry elements, • Type II – material from concrete elements, • Type III – a material consisting of a mixture of at least 80% natural aggregate, maximum 20% recycled aggregate. Recycled concrete aggregate (RCA) is divided into three classes depending on the application possibilities: • Class A – recycled aggregates for concrete with a wide range of applications, including marine and environmental structures, • Class B – covering most combinations of recycled aggregate with natural aggregate, suitable for concrete with moderate exposure class conditions, • Class C – recycled aggregate intended for concretes only in the mildest exposure classes conditions (Parekh and Modhera 2011). Wood is another type of recycled aggregate. Even when obtained from dismantling, it is a clean, ecological energy resource, particularly easy to use in households and small facilities (e.g. in coal-fired central-heating boilers). Contamination with veneers, paints or impregnants is not a problem for thermal utilization until special furnaces with combustion process control are applied. For this purpose, it is necessary to crush this material by special crushers, which operate mainly on the principle of hammer mills. The wood, which is not chemically contaminated after being crushed, can be used as an additive for the production of compost.

Regulations and standards

2

2.1 INTRODUCTION Directive 2008/98/EC of the European Parliament and of the Council of 19 November 2008 on waste defines the objectives and tasks for proper waste management. The basic idea of the directive is to create legal measures promoting the concept of “a recycling society striving to eliminate waste generation and to use waste as a resource”. The obligation to test and assess the content of heavy metals in aggregates results directly from the provisions of Directive 89/106 EEC (Directive 89/106 EEC Construction Products) and Mandate M125 (Mandate M125 Aggregates). The testing of hazardous substances is carried out for the water extract obtained in accordance with the standard (PN-EN 1744–3: 2004 Part 3: Preparation of extracts by washing out aggregates), and the testing of the content of individual harmful substances in the aggregate is conducted on the basis of the PN-EN 1233: 2000 standards and PN-82 C-04570.05. Evaluation of the obtained results is carried out according to the criteria included in the Ordinance of the Minister of the Environment of 29 November 2002 on conditions that should be meet when discharging sewage into water or soil and in the matter of substances especially harmful to the aquatic environment. With regard to the use of waste in construction, there are currently a number of standards in Poland in the field of methodology for testing lightweight aggregates. The properties of the aggregate are determined by the characteristics of both the individual particles and the combined material. They can be described by physical, chemical and mechanical properties. Depending on the grain size, there are fine aggregates with a grain size of up to 4 mm, coarse aggregates with a grain size of 4 to 63 mm and filler with a grain size of less than 0.063 mm. The division of aggregates into fine and coarse ones is justified by the role they play in concrete – i.e. for the appropriate water demand, and thus for the consistency of the concrete mix. The aggregates for construction purposes, depending on their properties, are divided into categories (PN-EN-1260). This division proves their different quality and is an indicator of usefulness. The parameters characterizing the physical properties of the aggregates are (ρd) – apparent particle density and (ρa) apparent dry particle density and water absorption after 24 hours of immersion of aggregate in water (WA24h). They are defined on the basis of the procedure set out in the EN-ISO-1097–6:2000b standard. The void percentage (H) that is the relative, volume share of the voids (cavities) between the grains in the volume unit of the material, porosity (P) and bulk density (ρb) is determined according to EN-ISO 1097–3:2000a. Aggregate strength must always be taken into account when designing lightweight structural concretes due to the fact that it is the weakest strength element of the composite. Compressive strength (crushing resistance) was measured according to the EN-ISO 13055–1 standard, while the crushing strength of individual aggregates was tested by means of California bearing ratio (CBR) testing apparatus. The frost resistance of lightweight aggregates is DOI: 10.1201/9781003313090-213

14  Application of Waste Materials in Lightweight Aggregates determined on the basis of the EN-ISO 1367–1 standard. It is the maximum allowable percentage of loss in mass of the aggregate soaked with water and subjected to cyclic freezing to −17.5°C (10 cycles) and thawing to 20°C. Expansion of lightweight aggregates is determined through the diameter (measured by using pachymeter) of aggregates granules before and after process their firing (pre-firing + firing steps) (González-Corrochano et al. 2014). The parameters that have a significant impact on the properties of the concrete mix as well as the hardened concrete are also the shape or flatness index and the texture of the aggregate surface. They are described by such parameters as the mean thickness, particle volume, the flakiness ratio, the elongation ratio, sphericity, shape factor, convexity ratio and fullness ratio (Barksdale et al. 1991; Kuo et al. 1996).

2.2  REVIEW OF STANDARDS AND LEGAL PROVISIONS APPLICABLE TO AGGREGATES At the level of EU law, the basic legal act setting the objectives and tasks regarding the proper handling of waste is currently Directive 2008/98/EC of the European Parliament and of the Council of 19 November 2008 on waste. The basic idea of the directive is to create the legal measures promoting the concept of “a recycling society striving to eliminate waste generation and to use waste as a resource”. Taking these assumptions into account, the Directive establishes a hierarchy of proceedings, the diagram of which is shown in Figure 2.1. It indicates the order of priorities in the policy and regulations regarding waste prevention and the methods of waste management, which relate to: • prevention, • preparation for re-use, • recycling, • other recovery methods, for example energy recovery, • disposal through safe storage.

FIGURE 2.1  Scheme of waste handling according to Directive 2008/98/EC on waste.

2  •  Regulations and standards  15 In recent years, the ecological aspect of this economic activity has been gaining more and more importance in the construction industry. The legal basis of this aspect is the dynamically developing set of regulations, shortly referred to as environmental law. It is the Waste Act of 27 April 2001 that, in Art. 1, defines “the principles of waste management in a manner ensuring the protection of human life and health and the protection of the environment in accordance with the principle of sustainable development, in particular the principles of preventing waste generation and its negative impact on environment, as well as recovery or disposal of waste”. With regard to the use of industrial waste in construction, the basic regulations resulting from the Environmental Protection Law (Act – Environmental Protection Law of 27 April 2001, Journal of Laws No. 62, item 627, with amendments) concern: • waste management and handling hazardous waste (Waste Act of April 27, 2001, Journal of Laws No. 62, item 628, with amendments; Regulation of the Minister of Environment of September 27, 2001 on the waste catalogue (Journal of Laws No. 112, item 1206)), • air protection (Waste Act of April 27, 2001, Journal of Laws No. 62, item 628, with amendments; Regulation of the Minister of the Environment of September 27, 2001 on the waste catalogue (Journal of Laws No. 112, item 1206), land, soil, agricultural and forest crops [Act of February 3, 1995 on the protection of agricultural and forest land, Journal of Laws No. 16/95, item 78, with amendments] and surface and ground waters (Regulation of the Minister of the Environment of 29 November 2002 on the conditions to be met when discharging sewage into water or soil and on substances particularly harmful to the aquatic environment) and Journal of Laws No. 212, item 1799; Act of July 18, 2001, Water Law, Journal of Laws 2001, No. 115, item 1229, with amendments) against contamination from waste. • environmental protection in investment activities; under the construction law (of 7 July 1994, Construction Law, Journal of Laws No. 89/94, item 414, Journal of Laws of 2000 No. 106, item 1126) and the Environmental Protection Law at every stage of the process investment, which also applies to road investments, an environmental impact assessment (EIA) is required; the scope of this assessment is set out in the Environmental Protection Law Chapter 2 Art. 46–57. When classifying waste, the Regulation of the Minister of the Environment (Journal of Laws No. 112, item 1206) on waste catalogues applies. It specifies the catalogue of waste together with a list of hazardous waste and the method of their classification. The waste catalogue divides waste into 20 groups, depending on the sources of its generation. The secondary raw material (material) from which the aggregate was produced in accordance with the standards for aggregates (Annex A to the standards according to Chapter 2) should be thoroughly identified and tested at the stage of preliminary tests, taking into account the presence of released hazardous substances – e.g. heavy metals. The obligation to test and assess the content of heavy metals in aggregates results from the requirements of the PN-EN standards mentioned. The standards do not define the scope of these tests – i.e. they do not specify which types of heavy metals should be determined for the aggregate. The explanation should be found in the content of the Annex ZA of the PN-EN standards on aggregates. The obligation to test the content of released heavy metals results directly from the regulations of Directive 89/106 EEC (Directive 89/106 EEC Construction Products) and the Mandate M125 (Mandate M125 Aggregates). The products placed on the market must be safe and only such products can be marked with the CE symbol. Hence, these tests should be performed as part of the Factory Production Control system, providing the necessary knowledge about the raw material for the production of aggregates. The main difficulty in carrying out the research is the lack of European standard regulations in the field of test methods and criteria for the evaluation of hazardous substances. In such a situation, the methods and criteria of

16  Application of Waste Materials in Lightweight Aggregates evaluation applicable in individual European Union countries should be used in the research. In Poland, in the field of testing hazardous substances, the determination of radioactive elements and harmful substances introduced into water or soil is performed. The testing of hazardous substances is carried out for the water extract obtained in accordance with the standard (PN-EN 1744–3: 2004 Part 3: Preparation of extracts by washing out aggregates), and the testing of the content of individual harmful substances in the aggregate is determined on the basis of the PN-EN 1233: 2000 standards. Water quality. Determination of chromium. Methods of atomic absorption spectrometry, PN-ISO 8288: 2002 Water quality. Chemical properties analysis. Determination of cobalt, nickel, copper, zinc, cadmium and lead. Methods of atomic absorption spectrometry with flame atomization and PN-82 C-04570.05 Water and sewage. Investigation of metals content by atomic absorption spectrometry. Determination of barium in water and PN-EN 12457 Characterization of waste-Leaching-Compliance testing for leaching of granular waste materials and sludge. The evaluation of the obtained results is carried out according to the criteria included in the Ordinance of the Minister of the Environment of 29 November 2002 on conditions that should be met when discharging sewage into water or soil and in the matter of substances especially harmful to the aquatic environment. The EU raw materials policy in the Raw Materials Initiative (EC 2008) is one of the pillars providing for the maximum use of recyclable materials in eco-investment technologies to manufacture the products of a quality that is not inferior to those from natural resources. This approach is also dominant in the new European 2020 strategy replacing the Lisbon Strategy. One of the key areas in the assumptions for the national strategy for innovation and efficiency of the economy (Assumptions for the strategy of innovation and efficiency of the economy, Ministry of Economy, Warsaw, 19th May 2010), developed by the Ministry of Economy, is the use of all raw materials, including secondary raw materials (Góraczyk and Kukielska 2013). They will be a necessary supplement to natural resources, and it will also be a pro-ecological action for the environment. An important argument for the use of waste in the production of building materials, apart from economic and ecological benefits, are often also the technical benefits of using them in the production of certain materials (Monzo et al. 1999a; Ferreira et al. 2003; Kosior-Kazberuk 2008). A full assessment of the possibilities of using waste should take the following factors into account: physicochemical properties determining the possibility of its processing, impact on the technical properties of the building material and environmental impact. Standardization work in this direction was also carried out in the European committee CEN TC 154 “Aggregate”. They are aimed at standardizing the raw materials that can be the basis of aggregate production. The standards which are currently in force in Poland in the field of using of waste in construction for methodology of testing lightweight aggregates are presented in Table 2.1. TABLE 2.1  Standards for aggregates. STUDY OF THE BASIC PROPERTIES OF AGGREGATES TITLE

NUMBER

Sampling methods

PN-EN 932–1: 1999

Procedure and terminology of simplified petrographic description

PN-EN 932–3: 1999

Methods of reducing laboratory samples

PN-EN 932–2: 2001

Basic equipment and calibration

PN-EN 932–5: 2001

Definitions of repeatability and reproducibility. Study of the geometric properties of aggregates

PN-EN 932–6: 2002

Determination of the grain composition. Sifting method

PN-EN 933–1: 2000

2  •  Regulations and standards  17 STUDY OF THE BASIC PROPERTIES OF AGGREGATES Determination of the grain composition. Nominal dimensions of test sieves openings

PN-EN 933–2: 1999

Determination of grain shape using the flatness index

PN-EN 933–3: 1999

Determination of grain shape. Shape indicator

PN-EN 933–4: 2001

Determination of the percentage of grains with surfaces resulting from crushing or breaking of coarse aggregates

PN-EN 933–5: 2000

Study of the aggregate flow coefficient

PN-EN 933–6: 2002

Tests of thermal properties and resistance of aggregates to weather conditions

PN-EN 933–7: 2002

Assessment of the content of fine grains. Sand index study

PN-EN 933–8: 2001

Assessment of fine particles content. Methylene blue method

PN-EN 933–9: 2001

Assessment of fine particles content. Graining of fillers (air stream sieving)

PN-EN 933–10: 2002

Research on resistance to freezing and defrosting

PN-EN 1367–1: 2001

Magnesium sulfate test

PN-EN 1367–2: 2000

Examination of solar basalt gangrene by cooking method

PN-EN 1367–3: 2002

Determination of drying shrinkage

PN-EN 1367–4: 2000

TESTS OF MECHANICAL AND PHYSICAL PROPERTIES OF AGGREGATES Determination of abrasion resistance (micro-Deval)

PN-EN 1097–1: 2000

Methods for determining the resistance to grinding

PN-EN 1097–2: 2000

Determination of bulk density and void

PN-EN 1097–3: 2000

Marking voids for dry, thickened filler

PN-EN 1097–4: 2002

Determining the water content by drying in a dryer

PN-EN 1097–5: 2001

Determination of particle density and water absorption

PN-EN 1097–6: 2002

Determination of grain density. Pycnometric method

PN-EN 1097–7: 2001

Determination of the polishability index

PN-EN 1097–8: 2002

RESEARCH ON THE CHEMICAL PROPERTIES OF AGGREGATES Chemical analysis

PN-EN 1744–1: 2000

Determination of hazardous substances in water extracts of aggregates

PN-EN 1744–3: 2004/Ap1

Tests for chemical properties of aggregates – Part 8: Determination of metal content in municipal incineration plant bottom ash-derived aggregates (MIBA) as a classification test

PN-EN 1744–8: 2013

TESTS OF FILLING AGGREGATES USED FOR BITUMINOUS MIXES, CONCRETE AND MORTAR Lightweight aggregates – Part 1: Lightweight aggregates for concrete, mortar and grout

PN-EN 13055–1–2004

Light aggregates – Part 2: Light aggregates for unbound and hydraulically bound bituminous mixes and surface treatments

PN-EN 13055–2: 2006

Aggregates for unbound and hydraulically bound materials used in construction objects and road construction

PN-EN 13242: 2004

Aggregates for bituminous mixes and surface treatments

PN-EN 13043: 2004

Tests of filling aggregates used for bituminous mixes. Bituminous number

PN-EN 13179–2: 2002

18  Application of Waste Materials in Lightweight Aggregates

2.3  ENGINEERING PROPERTIES OF LIGHTWEIGHT AGGREGATES The properties of the aggregate are determined by the characteristics of both the individual particles and the combined material. They can be described by (i) physical properties, (ii) chemical properties and (iii) mechanical properties. The physical and durability properties include: water absorption and density, frost resistance tested directly (three methods), grain size, alkali-silica reactivity, abrasion resistance (three methods), polishing, grinding resistance, shrinkage during drying. The listed features are essential for the quality of concrete in the environments with a fixed aggressiveness. According to the PN-EN 12620 standard, the size of aggregate for construction purposes are determined by the dimensions of the lower (d) and upper (D) sieves and expressed as d/D. This marking allows the presence of a certain number of grains that remain on the upper sieve (oversize grains) and a certain number of beans that can pass through the lower sieve. Depending on the grain size, aggregate is distinguished: • • • •

fine – aggregate with grain size up to 4 mm (D ≤ 4 mm, d = 0 mm), coarse – aggregate with grain size from 4 to 63 mm (D ≥ 4, d ≥ 2 mm), fillers – most of the grains pass through the sieve 0.063 mm; it can be added to building, materials to achieve the demanded properties.

Aggregates for construction purposes, depending on their properties, are divided into categories, defined as the level of aggregate properties expressed by its value range or limit value, and there is no relationship between the categories specified for different properties (PN-EN 12620+A1: 2010 Aggregates for concrete). The categories of aggregates are distinguished, among others, in terms of: graining (Table 2.2), flatness index value, shape index value, shell content in coarse aggregate, dust content, Los Angeles index value, resilience, abrasion resistance, polishability, frost resistance or chemical requirements. The division of aggregate into fine and coarse aggregate is justified by the role it plays in concrete. While fine aggregate may act as a stabilizing concrete mix, affecting its viscosity, workability and segregation, coarse aggregate is mainly a volumetric fill. The grain size is also related to the basic

TABLE 2.2  Basic requirements for aggregate particle size according to the PN-EN-1260 standard. PERCENTAGE FROM MASS PASSING THROUGH AGGREGATE Coarse

SIZE [mm]

2D

CATEGORY G

1.4 D

D

D

D/2

GC 85/20 GC 80/20

D/d ≤ 2 or D ≤ 11.2

100

98–100

85–99 80–99

0–20

0–5

D/d> 2 or D> 11.2

100

98–100

90–99

0–15

0–5

Gc 90/15

Fine

D ≤ 4 and d = 0

100

95–100

85–99

–

–

GF 85

Natural 0/8

D = 8 and d = 0

100

98–100

90–99

–

–

GNG 90

Continuous grading

D ≤ 45 and d = 0

100

98–100

90–99 85–99

–

–

GA 90 GA 85

2  •  Regulations and standards  19 property of the aggregate, which is water demand (water absorption); that is, the need for water to obtain a specific consistency of a concrete mix. It depends on the size of the grain surface, although it is also influenced by its roughness and shape. The finer the aggregate, the more grains there are in a given unit of mass or volume, the greater the total surface area of these grains and the greater the water demand of the pile. The remaining geometrical properties of the aggregate strictly depend on the type of raw materials from which it was made.

2.3.1 Apparent density, dry particle density and water absorption The parameters characterizing the physical properties of the aggregates are (ρd) dry particle density and apparent density (ρa) and water absorption after 24 hours of immersion of aggregate in water (WA24h). They are defined on the basis of the procedure set out in the EN-ISO-1097–6: 2000b standard. The parameters were calculated according to equations 2.1, 2.2 and 2.3.

pa =

M4 (2.1) M 4 - ( M 2 - M3 )

pd =

M4 (2.2) M1 - ( M 2 - M3 )

WA24 h =

100 ( M1 - M 4 ) (2.3) M4

where: ρd – dry particle density (kg/m3), ρa – apparent density (kg/m3), M1 – mass (g) of saturated pellets on the dry surface (after immersing in water for 24 hour), M2 – mass (g) in the pycnometer containing saturated pellets and water M3 – mass (g) in the pycnometer containing water M4 – mass (g) of the dry sample, prior to immersion in water. The European Standard specifies that the aggregate of mineral origin should not exceed the bulk density in a dry state of 2000 kg/m3, while the specific density of sintered aggregates is different. The compact structure of the aggregate and the higher specific density value result from the excessive formation of the glass formation at higher sintering temperatures (Al-Bahar and Bogahawatta 2006).

2.3.2 The void percentage, porosity and bulk density Other parameters that characterize aggregates are the void percentage (H) that is the relative, volume share of the voids (cavities) between the grains in the volume unit of the material, porosity (P) and bulk density (ρb). They are determined on the basis of equations 2.4, 2.5 and 2.6, according to EN-ISO 1097–3: 2000a:

pb = H=

m2 - m1 (2.4) V 100·( p d - pb )

pd

(2.5)

20  Application of Waste Materials in Lightweight Aggregates P=

pd - pa (2.6) pd

where: H – voids (%), P – porosity (%), ρb – bulk density (kg/m3), m1 – weight of the empty container (g), m2 – weight of the container and the test sample (g), V – volume of the container (cm3). The bulk density of aggregates affects the volume of the mortar required for the concrete matrix, and thus determines the economy of the mixture (Alexander and Mindess 2005). The European Standard specifies that mineral aggregate should not exceed a loose bulk density above 1200 kg/m3 (prEN 13055–1). The porosity of light aggregates is in the range of 20–50%. Greater porosity increases water absorption by increasing infiltration – i.e. the ability of its gravitational drainage, which increases frost resistance. As the porosity of the ceramsite increases, the diffusivity of water vapor in its pores also increases. Besides, negative temperatures do not change the properties of the aggregate because the open structure of pores and their relatively large dimensions do not reduce frost resistance. Due to its high porosity, ceramsite is also a good acoustic insulator. It can be used for the production of lightweight ceiling blocks and low conductivity wall blocks.

2.3.3 The compressive strength Compressive strength was measured according to the EN-ISO 13055–1 standard. The force needed to recess the piston to a predetermined depth in a cylinder filled with compacted aggregate is a measure of the resistance. Compressive strength was calculated according to equation 2.7: Ca = L +

F (2.7) A

where: Ca – the bulk crushing resistance (N/mm2), L – force exerted by the piston (N), F – force needed to the piston cavity (N), A – piston area (mm2).

2.3.4 Individual aggregate crushing strength The crushing strength of individual aggregates is tested by means of California bearing ratio (CBR) testing apparatus. The crushing strength of pellets was determined by placing the pellet between the two corresponding plats and loaded diametrically until failure occurred. An average of 20 randomly chosen pellets was tested to calculate the average crushing strength for each type of lightweight aggregate. The crushing test was performed on the pellets of different sizes such as 20, 16, 12, 10, 8 and 6 mm by means of a 28 KN capacity load-ring. The individual crushing strength ‘σ’ was calculated by means of strength index formula as given in equation 2.8: s=

2.8 . P (2.8) Y .X2

where P – failure load and, X – distance between the two plate of the pellet or diameter of pellet (Kockal and Ozturan 2010; Niyazi and Turan 2010). Aggregate strength must always be taken into account when designing lightweight structural concretes due to the fact that it is the weakest strength element of the composite. The type of porous aggregate used largely determines the potential strength of lightweight concrete. Both the apparent density of

2  •  Regulations and standards  21 the aggregate, resulting from its porosity, and the structure of this porosity are important here. Most of the artificially produced by sintering lightweight aggregates (e.g. ceramsite, aluminoporite, sintered fly ash) are characterized by a different structure of the sintered coating, dominated by fine closed pores, and grain interiors with larger and often open pores (Domagała 2005). Thus, the crushing strength of aggregates is primarily determined by their porosity (Wasserman and Bentur 1997; Ramamurthy and Harikrishnan 2006) but also other factors, such as a change in mineralogical composition, binder melting point, margin of densification occurs during sintering, bloating of the aggregate, internal defects caused by thermal stress, size and sintering temperature. Aggregates of smaller dimensions have a higher crushing strength of aggregates than larger ones (Gomathi and Sivakumar 2015). Thermal and polymer treatment of the aggregate also increase its crushing strength (Wasserman and Bentur 1997).

2.3.5 The frost resistance Frost resistance of lightweight aggregate was determined according to the EN-ISO 1367–1 standard. It is the maximum allowable percentage of loss in mass of the aggregate soaked with water and subjected to cyclic freezing to −17.5°C (10 cycles) and defrosting to 20°C. Frost resistance was calculated according to the equation 2.9:

F=

M1 - M 2 .100% (2.9) M1

where: M1 – is the initial, total mass of dried test samples (g), M2 – is the final, total mass of dried test samples, which remained on a given sieve (g), F – loss of mass of test samples after cyclic freezedefrosting (%).

2.3.6 The bloating index Bloating index (BI), or swelling rate, is determined from the diameter (measured by using pachymeter) granules of aggregates before and after the firing process (pre-firing + firing steps). BI is obtained by the following formula (equation 2.10): BI =

D-d . 100% (2.10) d

where: D refers to the diameter (mm) after sintering while d refers to the presintering diameter (Fakhfakh et al. 2007; González-Corrochano et al. 2014). The bloating mechanism of lightweight aggregate is known to be the formation of a black core caused by the reduction of the inside of the aggregate (Park et al. 2005). It depends on the reactivity of the material, the binder content and the sintering conditions (temperature, time and atmosphere).

2.3.7 Shape and texture The grain size, shape or flatness index and texture of the aggregate surface have a significant influence on the properties of the concrete mix as well as the hardened concrete (Jamkar and Rao 2004). They are

22  Application of Waste Materials in Lightweight Aggregates responsible for its workability, water-tightness, tightness and voids, as well as susceptibility to segregation, influencing its strength and durability through these parameters. The shape of the aggregate significantly modifies the properties of the computationally determined curve of the crumb pile. Flat and elongated grains are characterized by a larger surface than isometric grains, affecting the water demand and workability of the concrete mix. They are more difficult to compact, have a greater tendency to segregate, voids and honeycombing can easily form beneath them. The shells present in the aggregates of sea origin and the surface of crushed grains have a similar negative effect. Crushing the grains, however, can also be beneficial, as the rough surface allows for better physical adhesion of the cement grout. Several parameters are a quantitative measure of the shape and texture of aggregates. The problem is that different researchers use varying parameters in the form of shape indices to describe the same aggregate property. Therefore, there are also different definitions of the same shape index (Barksdale et al. 1991; Kuo et al. 1996). Majority parameters such as sphericity, shape factor and convexity of coarse aggregate for concrete can be determined using digital image processing (DIP) (Mora et al. 1998; Kwan et al. 1999). Unlike other methods, DIP enables the estimation of particle thickness and volume, and therefore can be used to measure the thickness-dependent shape parameters and to evaluate the weighted average values of the shape parameters of individual particles in an aggregate sample. Since only two-dimensional particle projection is recorded for image analysis, the thickness and volume of the particles cannot be directly obtained from the DIP results. Nevertheless, a method for estimating particle thickness and volume has been developed earlier (Mora et al. 1998). This method is based on the assumption that the aggregate particles from the same source should have more or less the same shape. With this assumption, the average particle thickness can be estimated from the particle width as (equation 2.11):

mean thickness = l × breadth (2.11) where: λ is a parameter dependent on the flakiness of the aggregate. This equation shows that the particle volume can be estimated as (equation 2.12):

volume = meanthickness x area = V x breadth x area (2.12) The aggregate sample consists of many particles. To determine the shape parameter of an aggregate, the shape parameter of each particle has to be measured first. The shape parameters of aggregates can be assessed on the basis of the following parameters such flakiness ratio, sphericity, shape factor, convexity ratio, fullness ratio (Krumbein 1941; Barksdale et al. 1991; Kuo et al. 1996; Yue and Morin 1996).

2.3.8 Classification parameters of lightweight aggregates Summary of the three basic properties of LWA (dry particle density [ρd], water absorption [WA24], crushing strength [S]) taken from the European standard EN-13055–1, two LWA commercial datasheets (Argila 2019; Arlita 2019) and from the relevant literature (Rossignolo 2009) enable the categorization of lightweight aggregates (Table 2.3). The adopted criteria allowed distinguishing five groups of LWA, applicable in various engineering fields such as construction, geotechnics, horticulture, as well as acoustic and thermal insulation.

2  •  Regulations and standards  23 TABLE 2.3  Classification parameters of lightweight aggregates (Souza et al. 2020). PARTICLE DENSITY (pd, g/cm3) < 2.00 g/cm

3

WATER ABSORPTION (WA24,%)

COMPRESSIVE STRENGTH (S, MPa)

APPLICATION

0–20

>5

High-strength concrete (HSC)

0–20

> 2.3

Structural lightweight concrete

0–34

> 1.8

Non-structural lightweight concrete, lightweight mortars

10–38

> 1.8

Geotechnical applications

10–38

> 1.0

Gardening and landscaping, thermal and acoustic insulation

3

Alkali – silica reactivity of aggregate 3.1 INTRODUCTION

Alkaline reactivity is defined as the tendency of some aggregate components to react with the alkali derived from the cement. Alkali-aggregate reactions are divided into three main types: alkali-silica reaction (ASR), alkali-silicate reaction, and alkali-carbonate reaction (ACR). The alkali-silica reaction takes place between the hydroxyl ions occurring in the pore solution as well as certain types of silica present in the aggregate (ACI 1998). The reaction product is a hydrated alkali silicate gel, characterized by a high water absorption capacity, which leads to a significant increase in its volume (Thomas 2011; Thomas et al. 2013). The resulting gel, absorbing water, exerts pressure on the cement slurry that limits it, which leads to its expansion. Many studies reported that significant expansion takes place only when sufficient calcium in the form of Ca(OH)2 is available. Apart from expansion the effects of the alkali-silica reaction (ASR) include cracks in concrete elements, local crushing of concrete, chipping, pop-outs, extrusion of the joint material (sealant), gel exudation, surface discoloration, efflorescence, stains and dripstones (Du and Tan 2014). The methods of testing the alkaline reactivity of aggregates are divided into groups, which include chemical (“quick”) methods for determining the potential reactivity, based on the chemical reactions between the aggregate and the alkaline solution; direct methods for the determination of reactivity based on the measurement of elongation/expansion of concrete beams and petrographic methods, indicating the presence of potentially reactive minerals. Some studies indicate that lightweight aggregates are not prone to ASR; moreover, ASR gels – which are produced by reactive aggregates found in the concrete – may even be accommodated on their porous structure. Nevertheless, in some cases, lightweight aggregates are reactive and cause expansion. Expanded glass aggregate constitutes a promising material, compared to certain types of lightweight aggregates. Contrary to an ordinary aggregate, in a mortar with glass aggregate, ASR products are not formed at the aggregate-matrix interface, but they may be stored in a porous aggregate. In the production of lightweight concrete, expanded slate is one of the most commonly used lightweight aggregates. The concretes with the addition of this aggregate show acceptable values in terms of the results of the alkali-silica reactivity test. No detrimental expansion of the mortar containing sintered expanded clay aggregates was found, despite the presence of a glassy phase, which may indicate a potential alkaline reactivity. Ground perlite rock as well as ground expanded perlite with opal and monzodiorite aggregate in the slurry are effective in inhibiting ASR. Powdered expanded perlite used as a cement substitute significantly inhibits the reaction of the aggregates containing active silica with 24

DOI: 10.1201/9781003313090-3

3  •  Alkali – silica reactivity of aggregate  25 sodium and possibly potassium hydroxides. Moreover, the use of the fly ash microsphere in the concrete does not show harmful ASR expansion, in line with ASTM C227 as well as ASTM C1260. The addition of 30% pumice and leather as a replacement for the reactive aggregate may reduce expansion to the values below the maximum limit. The mortar bar prepared using tuff aggregate with a partial replacement of fine aggregate amounting to 25% causes expansion which approximates the maximum limit; in turn, increasing the share of tuff aggregate up to 100% results in an expansion value of about 0.15–0.20% (Kazantseva et al. 2018). The use of ash from palm oil fuel ash reduces expansion and increases its content in the bar. Other lightweight aggregates that exhibit an alkali-silica reactivity below the acceptable limit include sintered fly ash aggregate, expanded slate aggregate, the aggregate from sewage sludge and reservoir sediment artificial aggregate, artificial sintered microcrystalline silica aggregate, sintered silica fume and the cold boned aggregates based on cement or oil palm shells as bio-solid waste in the ASR from the palm oil industry (Awal and Hussin 1997; Juenger and Ostertag 2004; Chiou and Chen 2011; Le et al. 2018; Vandanapu and Muthumani 2019).

3.2  TYPES AND MECHANISM OF THE ALKALINE REACTION The mechanical and physical properties of aggregate, which constitute the basic concrete component, significantly affect the concrete behaviour in engineering structures. This property is believed to have a significantly impact the durability and strength of concrete. Alkali-aggregate reaction (AAR) constitutes a reaction that occurs between the alkali hydroxides in concrete. These are predominantly derived from the portland cement as well as certain aggregate types. There are three types of alkaline reactivity. These constitute reactions of alkali with amorphous silica, silicates or carbonates, which are often the main or significant component of the aggregates. Alkaline-silica reactivity (ASR) is the tendency of some aggregate components – i.e. reactive silica – towards reacting with alkali in the form of sodium and potassium hydroxides derived from portland cement (Munir et al. 2017). American Concrete Institute provides the following definitions (ACI 1998): • alkali-aggregate reaction, AAR – a chemical reaction occurring in concrete or mortar between hydroxyl ions (OH-) of the hydroxides (Na and K) originating from hydraulic cement (or other sources) as well as constituents of certain aggregates, • alkali-silica reaction, ASR – a chemical reaction occurring between alkali hydroxide ions (-OH) (Na+ and K+) originating from hydraulic cement (or other sources) and certain silica rocks as well as minerals, including microcrystalline quartz, opal as well as acid volcanic glass which are found in some aggregates, • alkali-carbonate reaction, ACR – chemical reaction between hydroxyl ions derived from sodium and potassium alkali hydroxides in a cement slurry solution as well as some carbonate rocks; in particular, dolomitic limestones and calcitic dolostone. Reactive silica contained the aggregate undergoes a reaction with the hydroxyl ions found in cement slurry to form an expansive sodium-potassium-calcium silicate gel. Ions of alkali metals (sodium and potassium) contribute to the high levels of hydroxyl ions, followed by expansive gel formation (Thomas 2011). The resulting hygroscopic gel swells by absorbing water, and the swelling pressure – when the

26  Application of Waste Materials in Lightweight Aggregates tensile strength of the concrete is exceeded – causes cracks and damage to the concrete structure elements. A characteristic feature of the alkali-aggregate reaction is that shells are formed around the aggregate grains, cracks pass through the aggregate grains, grain detach and change their volume, the phase composition changes, micro-cracks and fissures are formed, efflorescence and stains appear on the surfaces and, as a result, the destruction of concrete elements occurs (Lindgård et al. 2012). The concrete in road construction (pavements, bridge elements, etc.) is more susceptible to alkali-aggregate reactions than the concretes protected against moisture and/or fluctuating temperatures (e.g. concrete inside buildings, ceilings, floors) during operation. Some of these reactions (especially with certain forms of silica) are difficult to see and identify because they are very slow. A diagram showing the sequence of events is shown in Figure 3.1. The factors influencing the course and speed of the ASR reaction are (Thomas 2011): • hydrothermal conditions (humidity, temperature) affecting concrete, • the amount of alkali in the concrete, • type of aggregate and the amount of potentially reactive components it contains (mineral composition, grain size, structure and texture), • the extent of internal and external restraint to movement for instance, amount and distribution of reinforcing steel, • the degree of orderliness of the reactive silica structure.

FIGURE 3.1  Alkali-silica reaction scheme (reprinted from publication Thomas et al. 2013).

3  •  Alkali – silica reactivity of aggregate  27 The influence of aggregate size on ASR has often been studied. When reactive aggregate particles are of smaller size, faster expansion occurs (Multon et al. 2010; Gao et al. 2013). This phenomenon can be explained by the diffusion theory (Ichikawa 2009; Poyet et al. 2007). However, some studies show that the use of a sufficiently small aggregate fraction inhibits the ASR expansion (Cyr et al. 2007; Idir et al. 2010). In turn, certain experiments demonstrated that very fine fractions contribute either to significant expansion (Dhir et al. 2009) or complete lack thereof (Stanton 2008). The alkali-silica reaction involves the hydroxyl ions found in the pore solution as well as some types of silica (SiO2) present in the aggregate. SiO2 mainly comprises siloxane groups (≡Si-O-Si≡); however, even crystalline silica is characterized by disorder at the surface. Surface oxides are hydroxylated (also in pure water), forming silanol groups (≡Si-OH). Silica tends to dissolve when hydroxyl ions (OH-) are present in high concentration. This process occurs first when the silanol groups are neutralized, followed by acting on the siloxane groups. These reactions occur in the following way (equations 3.1, 3.2) (Dent Glasser and Kataoka 1981a): ≡Si-OH + OH- → ≡Si-O- + H2O(3.1) ≡Si-O-Si≡ + 2OH- → 2≡Si-O- + H2O(3.2) The Si-O- ions, characterized by negative charge, attract the elements such as sodium (Na+) and potassium (K+), which are positively charged and occur in the concrete pore solution. Initially, a gel or an alkali-silicate solution is formed, governed by the content of moisture. However, if calcium is present, silica precipitates out of solution in the form of an alkali silicate gel (CaO-Na2O/K2 O-SiO2-H2O), mainly comprising potassium, sodium and silica, as well as calcium, albeit in a small amount. As the silica dissolves, the concentration of hydroxyl ions (in addition to the pH value) decreases (Dent Glasser and Kataoka 1981b). The final concentration of silica is dependent upon the initial ratio of SiO2/Na2O. The situation becomes more complex in concrete, possibly since a large amount of calcium is present, which reduces the concentration of silica in the solution as well as additionally provides hydroxyl ions. Consequently, the equilibrium conditions in mortar and concrete are reached slowly. The morphology and chemical composition of the gel (reaction product) (Moranville-Regourd 1989) as well as the physical properties of the gels are very diverse, also in terms of their ability to swell freely.

3.3  THE MECHANISM OF EXPANSION Despite the common knowledge of the chemical reactions taking place, many different expansion mechanisms have been offered: • according to Hansen (1944), the cement slurry which surrounds the reactive aggregate grains constitutes a semi-permeable membrane which can be penetrated by water (or pore solution), but larger complex silicate ions are retained. Water is drawn into the reacting grain, characterized by the lowest chemical potential. Afterwards, formation of an osmotic pressure cell followed by growing hydrostatic pressure exerted on the cement slurry, results in cracking of the surrounding mortar, • McGowan and Vivian (1952) challenged the classical osmotic theory, stating that the fracture surrounding the cement slurry in the form of “membrane” caused by ASR would result in reduction of hydraulic pressure as well as prevention of further expansion. The authors

28  Application of Waste Materials in Lightweight Aggregates offered an alternative mechanism which is based on the physical absorption of water by the alkaline silica gel as well as gel swelling that occurs afterwards, • Powers and Steinour (1955a, 1955b) suggested that imbibition and osmotic pressures can both be generated, which is dependent upon the solid or liquid state of the alkali-silicate complex. According to the proposed hypothesis, the reaction product with appropriate composition may constitute a semi-permeable membrane. Irrespective of the mechanism, swelling is thermodynamically the same – i.e. it results from water that enters the region where the action of the solvent or adsorption decreases its free energy. While the exact role of calcium in gel expansion is still not clear, several mechanisms have been proposed, as presented below: • calcium contributes to “alkali recycling” as it replaces the alkali in the reaction product, thus generating an alkali for further reactions (Thomas 2001), • Ca(OH)2 provides OH- ions while maintaining a high level of OH- within the solution (Wang and Gillott 1991), • calcium, which is found in the pore solution in high concentration, prevents the silica diffusion from the reacting particles of the aggregate (Chatterji 1979; Chatterji and ClaussonKass 1984), • if calcium is unavailable, the reactive silica is dissolved in the alkali hydroxide solution causing no damage (Thomas 1998), • calcium-rich gels are required to induce expansion directly or indirectly through the formation of a semi-permeable membrane around the reactive grain particles of the aggregate (Thomas 1998; Thomas et al. 1991; Bleszynski and Thomas 1998), • pozzolans effectively control the expansion of mortars and concretes immersed in solutions of alkali salts (including alkali hydroxide), which results from usage of Ca(OH)2 by a pozzolanic reaction reducing the calcium availability for the alkalisilica reaction (Kawamura et al. 1988; Alasali and Malhotra 1991; Bleszynski and Thomas 1998). Swelling of the concretes or mortars that contain reactive aggregate may be prevented via eliminating Ca(OH)2 prior to sample immersion in alkaline solutions. Removal of Ca(OH)2 from concrete by leaching was carried out by Chatterji (1979), and Thomas (2001) carbonated Ca(OH)2 via exposing mortar samples to an environment enriched with CO2. While the exact role of Ca(OH)2 is ambiguous, the presence of calcium clearly causes destructive reactions. Thus, a reduction in calcium availability – for example, via consumption of Ca(OH)2 in pozzolanic reactions – should reduce the expansion caused by ASR.

3.4  EFFECTS OF THE ALKALINE-SILICA REACTION (ASR) In order to initiate ASR in concrete, three conditions must be met: • it is imperative that reactive mineral forms occur in aggregate materials, • alkali hydroxides (Na+, K+, OH-) should be present in high concentration in the liquid phase of the slurry, • sufficient humidity must be ensured.

3  •  Alkali – silica reactivity of aggregate  29 The concrete parts affected by ASR react differently. The most common ASR symptoms are: • cracking, • swelling resulting in deformation, displacement and relative motion, • local concrete crushing, • squeezing out the joint material (sealant), • protrusions from the surface – splinters, pop-outs, • surface discoloration as well as gel exudation – eruptions, stains, drips.

3.4.1 Cracking Alkaline silica gel generates tensile stresses in the concrete and causes cracks perpendicular to the concrete surface. These cracks extend from the surface but usually do not penetrate below the reinforcement (American Concrete Institute 1998). The ASR fractures are affected by various factors, including: the geometry or shape of the concrete element, environmental conditions, the presence as well as distribution of reinforcement, in addition to the stress fields or loads applied to the concrete. Map fracture (also known as a fracture pattern), which constitutes randomly-oriented relatively free (not restrained) cracks on the surface of concrete elements that can travel in all directions, is a classic ASR symptom. However, shrinkage, freeze/thaw cycles, as well as sulfate attack may also cause random orientation crack patterns. Steel reinforcement can reduce the ASR expansion in the concrete due to the applied compressive stress. However, the surface cracks caused by ASR are often insignificantly decreased through the usage of external or internal reinforcements. If the expansion is constrained in at least one direction, more significant expansion occurs towards the smallest constraint and cracks become oriented towards the same direction as the constraining stresses. For instance, in concrete pavements, in which the expansion occurs longitudinally, the more significant expansion occurs transversely, with cracks developing predominantly longitudinally. Conversely, in reinforced concrete columns, cracks usually align vertically as a result of the constraint of the primary reinforcement and dead load. In the case of coupled bridge girders, cracks tend to have a horizontal alignment as a result of the constraint that is imposed by prestressing tendons located parallel to the axis of the beam. The concrete elements subjected to ASR and cyclically exposed to the effect of rain, wind and sun as well as parts of concrete piles in tidal zones frequently exhibit more severe surface cracks as a result of stress cracking induced in “less expansive” surface layer (resulting from variable humidity conditions or alkali leaching/dilution processes) under the expansive internal concrete core pressure (Thomas et al. 2013; ACI 1998) (Figure 3.2).

FIGURE. 3.2  Concrete elements subjected to ASR as well as exposed to cyclical action of wind, rain and sun (reprinted from publication Thomas et al. 2013; ACI 1998).

30  Application of Waste Materials in Lightweight Aggregates Discoloration frequently appears around cracks, caused by gel exudate. Cracks in concrete are generally the largest in the areas of the structure with a constant flow of moisture, e.g. from the ground behind retaining walls, near the waterline in piers, under paving slabs, as well as the elements of the structure exposed to rain or by wick action in columns or piers.

3.4.2 Expansion that causes movement and deformation The scope of ASR is frequently varied in different elements of the affected concrete structure, resulting in such disturbances as: • relative displacements of adjacent structural units or concrete elements, • deflection, in addition to closure of the joints, extrusion of the sealing materials, ultimately leading to concrete chipping at the joints. It should be remembered that deformation of concrete structures may result from several mechanisms – e.g. shrinkage, gravity, load, hydraulic pressure, thermal moisture movements, shock creep and vibration (BCA 1992).

3.4.3 Pop-outs The main factor behind the occurrence of loosening is probably the expansion of single unstable or frost-prone aggregate particles (e.g. laminated, argillaceous and schistose, porous or clayey particles or some varieties [porous] of ironstones and chert) caused by frost on the concrete surface. Pop-outs may also result from poor bonding between dusty aggregate particles and the cement paste. Moreover, reactive alkali-silica aggregates that expand in vicinity of the concrete surface may detach parts of the concrete surface layer, leaving a reactive aggregate in the lower part of concrete (Dolar-Mantuani 1983; American Concrete Institute 1998).

3.5  TEST METHODS FOR ALKALINE REACTIVITY Since Stanton (1940) described the Alkaline Aggregate Reaction (AAR), there has been a great deal of interest in laboratory research aimed at: • forecasting if expansion and resulting cracking in concrete will be caused by the employed aggregate, • assessing the preventive measures for the safe usage of the aggregates that have been identified as potentially reactive. The phenomenon of alkaline reactivity of aggregates is complex; therefore, the selection of test methods to determine whether an aggregate is reactive is not simple and requires long-term, multi-stage tests. ASR in building structures was first discovered by Stanton (1940), who also developed a test method for evaluating reactivity of aggregates and use it for assessing the application of pozzolans for ASR expansion control. The method developed by Stanton, which, in principle, is very similar to

3  •  Alkali – silica reactivity of aggregate  31 the currently used ASTM C227 technique, is still used, but since the groundbreaking Stanton’s ASR research, various new test methods have been devised and implemented. Of the methods for evaluating the alkaline reactivity of aggregates, the following types are distinguished: • direct – measurement of the expansion of mortar or concrete samples, • chemical – leaching alkali from concrete components, • petrographic – stating the presence of potentially reactive minerals. In the absence of any European standards for testing the alkaline reactivity of aggregates, the procedures according to the American ASTM standards are applied. The majority of the standards describing the tests and used in various countries are based on the ASTM standards. The most popular methods for testing expansion of mortar bars immersed in a 1M solution of sodium hydroxide at 80°C for 14 days include ASTM C1260, RILEM TC 106-AAR and DD 249:1999. The RILEM Committee recommended the following methods for testing alkaline reactivity: general principles for assessing the potential alkaline reactivity (AAR-0, 2003), petrographic description (AAR-1, 2003), accelerated mortar bar method (AAR-2, 2002), bar method at 38°C for mixed aggregates (AAR-3, 2003), bar method at 60°C for mixed aggregates (AAR-4.1, 2004), quick bar method for carbonate aggregates (AAR-5, 2003). The combination of high temperature and alkaline environment results in a rapid, measurable expansion, also in the case of the aggregates characterized by low reactivity. In these methods, sodium ions are the source of alkali; therefore, the alkali content in the cement used in the production of mortars is not specified in the standard (Thomas et al. 2013). The identification and quantification of the reactive phases in the aggregates is carried out according to ASTM C 295 by applying optical methods (involving transmitted and reflected light), which can be assisted with the additional usage of X-ray diffraction (XRD) as well as scanning electron microscopy (SEM). The majority of the minerals causing ASR may be identified and quantified; however, certain tiny quartz forms (finely divided) cannot be found using a petrographic microscope. Certain silica-siliceous limestones comprise up to 5% of undetectable (by means of a petrographic microscope) finely divided quartz particles, capable of inducing concrete expansion ASR (Fournier and Bérubé 1991). Therefore, one should be careful when classifying an aggregate as one that is non-reactive.

3.6  REACTIVITY OF LIGHTWEIGHT AGGREGATE The study of the influence of mineral additives on the course of the aggregate reaction with sodium and potassium hydroxides was discussed by Thomas (Thomas 2011). He showed that pozzolanic additives reduce the concentration of OH- ions on the liquid phase of the cement slurry. This is due to the binding of sodium and potassium ions by the C–S–H phase. Low sodium and potassium contents reduce the aggregate reaction. The low calcium content is also significant (Thomas 2011), and thus, the influence of mineral additives with not only pozzolanic but also hydraulic properties. Aluminum oxide contained in mineral additives has a positive effect on reducing the aggregate reaction with sodium and potassium (Rajabipour et al. 2015). Numerous studies on the impact of individual lightweight aggregates, primarily expanded glass, expanded shale, expanded clay and expanded perlite, confirmed their beneficial effect. According to some opinions, lightweight aggregates are not vulnerable to ASR; moreover, the porous structure thereof may even accommodate the ASR gels produced by other reactive aggregates found within concrete. Nevertheless, in some cases, it has been found that lightweight aggregates are

32  Application of Waste Materials in Lightweight Aggregates reactive and cause significant expansion. Although the research in this area is limited, it is important to summarize the state of the art in order to define the guidelines for the safe use of lightweight aggregate in concrete.

3.6.1 Expanded glass Expanded glass may be employed as a lightweight aggregate. The method of obtaining aggregate consists in grinding glass waste to a fine fraction and then forming small balls (with the addition of a foaming agent) from it. Then, they are fired in the furnace at a temperature greater than the glass-softening point in order to obtain a viscosity lower than 106.6 Pa·s. The next stage is cooling, where the obtained granulate is separated into fractions due to grain size (Limbachiya et al. 2012). The swelling agents constituting at least one of the following compounds: MnO2, SrCO3, CO2, O3, CaSO4, water glass and talc, degas at a temperature ranging between the glass softening point and the maximum firing temperature; thus, these gases become trapped in the glass structure. Regarding the carbon-containing agents such as SiC, starch, sugar or organic waste, these react with the glass or the atmosphere to form gases (Köse and Bayer 1982). Compared to some other lightweight aggregate types, expanded glass aggregate seems to be a very promising material. It is characterized by relatively low water absorption, high mechanical strength, as well as significant frost and chemical resistance. Pen or a closed cell structure may be obtained, depending on the selected firing method and additives (Köse and Bayer 1982). Nowadays, glass foam manufacturers use about 98% of various glass waste and only 2% of pure glass (Jasaitiene et al. 2010). Given that glass is composed of a large amount of amorphous silica (about 70%), this has an influence on the ASR alkali-silica reaction even at low alkali concentrations (Ducman et al. 2002). The combination of the high silica content and the amorphous structure of glass means that, as an aggregate, it is potentially harmful and can react with the alkali (even at a low concentration) present in the cement (Dolar-Mantuani 1983; Mladenovič et al. 2004). The expanded glass occurring in the pore solution increases the concentration of Na+ (from the value of 171 mg/L to 315358 mg/L), in addition to Si (from the value of 0.30 mg/L to 57271.7 mg/L), which means favourable conditions for the ASR reaction (Kazantseva et al. 2018). Scanning electron microscopy (SEM) studies have shown that calcium silicate in the form of a gel is formed on the expanded glass surface, which confirms the potential aggregate reactivity with the alkali contained in the cement. Soaking expanded glass in a solution of NaOH causes the formation of a coating on the expanded clay resulting from the reaction between the expanded clay glass and the alkaline solution (Udvardi et al. 2020). The tests of the alkaline-silica reactivity of the lightweight aggregate indicate discrepancy in the test results determined on the basis of different standard testing methods. A chemical method which is based on ASTM C289 suggests high reactivity of expanded glass, which also generates alkali. Nevertheless, the mortar tested according to ASTM C227 showed no expansion or cracks (Ducman et al. 2002). This may be attributed to the porosity of the aggregate that is capable of absorbing a large amount of the produced gel. The behaviour of the aggregate resembles that of rhyolitic pumice. Expansion characterizing the mortar comprising an aggregate of expanded glass, determined by the accelerated mortar bar test method did not exceed the permissible maximum limit (approx. 0.045%). The mortar with a density equal to 1569 kg/m3 and compressive strength amounting to 52.3 MPa showed a similar expansion to the mortar using ordinary fine-grained aggregate, according to RILEM (Rumsys et al. 2018). The durability of structural lightweight concrete (LWC) comprising expanded glass and silica fume the addition as fine aggregate replacement is similar. Expansion equal to 0.04% after 14 days according to accelerated mortar bar test conducted on the basis of ASTM C1260 was below the safe limit of 0.08–0.10% (Carsana and Bertolini 2017). Gorospe

3  •  Alkali – silica reactivity of aggregate  33 et al. achieved a comparable expansion value for the mortar containing the expanded glass. The ASR expansion characterizing a mortar bar that comprised expanded glass aggregate was significantly lower compared to that obtained in the case of crushed glass aggregate (Gorospe et al. 2019) and other kinds of lightweight aggregates such as ceramsite, perlite and vermiculite (Collins and Bareham 1987; Ducman et al. 2002). It is believed that firing glass at high temperature may decrease the ASR expansion (Maragechi et al. 2012). Cracks in the expanded glass aggregate can be observed, indicating that a reaction had occurred between the alkaline solution and the expanded glass, limited solely to the expanded glass voids and caused no harmful expansion (Carsana and Bertolini 2017). Contrary to ordinary aggregate, in the mortar with glass aggregate, ASR products are not formed at the aggregatematrix interface but may be stored within a porous aggregate (Figure 3.3). The presence of ASR products confirms the compressive strength decrease (by approx. 20–30%) as well as the density increase (by approx. 28%) of the samples subjected to identical conditions in the accelerated mortar bar test (Popov et al. 2015). The expansion due to the reactivity of alkali-silica mortar with glass aggregate may be close to or exceed the maximum limit value (Mladenovič et al. 2004; Limbachiya et al. 2012; Bumanis et al. 2013a). The research according to accelerated mortar bar test which was based on RILEM TC 1062 of mortar with foam glass granules showed that its density after hardening was 490555 kg/m3 with a 28-day compressive strength of 2.84 MPa and employing various cement types with different content of alkali. In this study, the 14-day expansion varied from 0.160 to 0.205% and was above the maximum expansion limit (0.054%).

FIGURE 3.3  ASR products in the concrete comprising (a) dense normal aggregate and (b) porous lightweight aggregate (reprinted from publication Popov et al. 2015).

34  Application of Waste Materials in Lightweight Aggregates Due to the harmful effects of ASR, the bending strength may drop to 73% and the compression strength, to 43%. The decrease in strength proves that the occurrence of expansion products causing cracks deteriorates the quality of lightweight aggregate mortar comprising the expanded glass aggregate addition. Cracks on the sample surfaces were not observed visually, which can be attributed to stress relaxation resulting from the application of porous expanded glass aggregates (Bumanis et al. 2013b). The cement slurry between the foam glass granules is not very compact and rich in pores. Porous calcium silicate hydrates (C–S–H) as well as large particles (< 100 µm) were found within the microstructure of a sample prepared with the CEMEX CEM I 42.5R cement. Ettringite crystals may occur inside the pores as well as within the interaction zone between hydrated cement paste (HCP) and foam glass granules. Lowering the pH value through alkali leaching or other reactions – e.g. ASR contributes to promoting the acicular ettringite recrystallization in hardened concrete. The crystals of this type are found within the open pores of foam glass granules as well as on the surface of hydrated cement minerals. Concrete interaction in any form or disintegration via ASR increases the rate of ettringite leaving its original location in the paste, moving into solution and recrystallizing in larger spaces, including open pores of aggregate, cracks or voids. Cracks in the cement slurry occur in some places inside the aggregate pores. This can fill the pores of the aggregate granules and enhance the contact zone between the foam glass and cement slurry, as well as affect the compressive strength due to the change in the spherical coating of the granules. ASR can potentially take place even in deeper layers of foam glass granules due to the widening of the contact zone area. As far as ASR reduction in concrete with glass aggregate is concerned, it was found that changing the cement type is characterized by lesser effectiveness than using pozzolanic materials (Bumanis et al. 2013b). Comparing to fly ash, microsilica gave the best ASR inhibition effect when used as a cement replacement material in the concrete with expanded glass aggregate (Popov et al. 2015; Vaganov et al. 2017).

3.6.2 Expanded shale Expanded shale constitutes one of the most common lightweight aggregates used in lightweight concrete production. The concretes with the addition of this aggregate show acceptable values in terms of the results of the alkali-silica reactivity test. Both dried and pre-wetted, expanded shale can be used as replacement of reactive fine aggregate to reduce the damage to concrete/mortar from ASR (Grotheer 2008). The greater the exchange rate, the greater the expansion reduction, showing the lack of expanded shale aggregate reactivity. Replacing the reactive fine aggregate by means of expanded shale in the amount up to 50% is insufficient to reduce the expansion below a certain limit. The complete application of the reactive aggregate with pre-wetted, expanded shale as a lightweight aggregate is the only way of reducing the swelling of the mortar to as little as 0.02% (Li et al. 2019). Similar results were obtained when twothirds of the fine aggregate was replaced by means of expanded shale. As a result, the mortar expansion was reduced to 0.06% (Wang et al. 2009b). The reduction of damage due to ASR in the concrete comprising reactive aggregate is based on the following mechanisms: • improvement of the interphase zone, • the dilution effect induced through partial replacement of the reactive aggregate by lightweight aggregate, • retaining ASR products within the lightweight aggregate voids (Shin et al. 2010).

3  •  Alkali – silica reactivity of aggregate  35 The absence of cracks in the voids of expanded shale that passed from the reactive fine aggregate indicates that the second mechanism was largely irrelevant in the case of expanded shale aggregate (Beyene et al. 2017). Partial replacement (30% by volume) of expanded shale aggregate and non-reactive quartz aggregate with reactive, coarse aggregate indicated a marked difference in terms of concrete expansion, which was determined on the basis of accelerated ASR test for 26 weeks. In the concrete that comprised quartz aggregate as partial replacement of the reactive aggregate, an expansion reaching about 0.35% was observed, while, in the case of the analogous concrete containing shale aggregate, it reached 0.05% (Teramoto et al. 2018). In concrete, partially replacing non-reactive fine aggregate by expanded shale, which also contains reactive coarse aggregate, may decrease the concrete expansion caused by ASR. On this basis, it was found that there are other mechanisms that reduce the expansion, in addition to the effect of dilution resulting from the replacement of reactive aggregate. The analysis of the pore solution shows that the presence of expanded shale reduces the concentration of Na+ and K+, and thus, the alkalinity of the pore solution, which results in a reduction of expansion (Li et al. 2018). Petrographic studies have shown that the pre-wetted, expanded shale addition that partially replaces the reactive fine aggregate can decrease the occurrence of cracks on the surface of a 2.5-yearold concrete sample. The decreased occurrence of ASR cracks can be linked to the intrinsic hardening effect characterizing the expanded shale aggregate that has improved the interface and slurry quality. It is believed that the dense microstructure of the cement slurry hinders the movement of alkaline solutions from the slurry into the reactive aggregate as well as makes it more resistant to crack propagation (Beyene et al. 2017).

3.6.3 Expanded clay Expanded clay constitutes a lightweight aggregate obtained via heating clay in a rotary kiln to about 1200°C. The arising gases form porous and light structure, which is commonly used as lightweight aggregate in concrete. No detrimental expansion of the mortar containing sintered swelling clay aggregates was found, despite the presence of a glass phase, which may indicate potential alkali reactivity based on the ASTM C289 chemical method (Collins and Bareham 1987). However, the sensitivity of swelling clay aggregate to alkali is lesser than those in glass aggregate (Mladenovič et al. 2004). An accelerated test of mortar bars (ASTM C1260) shows that the expansion characterizing the mortar bar containing reactive fine aggregate is lesser after partially replacing it with expanded clay fine aggregate (Li et al. 2018). The expansion of mortar bars produced using only ceramsite aggregate was lower than the maximum allowable value (Mladenovič et al. 2004). The oven-dried expanded clay reduced the expansion of the mortars more effectively than the prewetted aggregate. This is related to a decrease in the effective water/cement ratio and an improvement in the microstructure, thus reducing the permeability (Li et al. 2018).

3.6.4 Expanded perlite Perlite constitutes a volcanic glass with the content of SiO2 of approx. 70%. When it is heated to 1000°C, it swells forming a porous aggregate of expanded perlite. Expanded perlite is a very low-density material, used primarily as filler in lightweight cement composites with thermal insulation properties. During the production and processing of expanded perlite, a fine material fraction is formed, which cannot be used in typical applications due to the unfavourable grain size distribution. However,

36  Application of Waste Materials in Lightweight Aggregates as a result of its composition and glassy structure, this material, after grinding, has a high pozzolanic activity (Kotwica et al. 2016) and can be used as a mineral additive to cements and concretes (Urhan 1987). Milling destroys the porous microstructure of the waste perlite grains and produces a material with a large specific surface area. The reaction intensity of aggregate containing opal and monzodiorite aggregate with sodium and potassium hydroxides was seen to significantly decrease in the mortars with the addition of ground perlite as well as ground expanded perlite. Expanded perlite turned out to be more effective. It has a very low calcium and high aluminum content, which should be considered beneficial from the point of view of the effect on the reaction of silica in aggregate containing potassium and sodium (Bektas et al. 2005). Powdered, expanded perlite used as a cement substitute significantly inhibits the reaction of aggregates containing active silica with sodium and, possibly, potassium hydroxides. This is confirmed by the results of the tests carried out both in a model system, comprising quartz glass with a grain size of 1–4 mm as well as the tests of mortars with the addition of natural reactive aggregate. In both cases, a decrease in the expansion of the mortar samples treated with sodium hydroxide solution at elevated temperature was found. The tests of mortars and slurries with the addition of ground perlite confirmed its pozzolanic properties. The greater content of the C–S–H phase, formed in the perlite slurry, as well as the reduction of permeability and the calcium hydroxide content in the slurry, is most likely the reason for the greater resistance of the mortars to aggregate corrosion caused by the reaction of silica with potassium and sodium hydroxides (Kotwica and Fular 2018). The C–S–H phase formed as a product of this reaction is very important in terms of using pozzolanic additives. It binds sodium and potassium ions in the interlayer areas. This causes immobilization of some of the alkaline cations and reduction of their share in the reaction with active forms of silica in the aggregate (Thomas 2011). The ASR tests of the mortar with perlite aggregate indicate that, despite exceeding the expansion limit in the ASTM C227 test, no cracks were visually observed. This results from the ASR product incorporation into the voids found within the expanded perlite aggregate (Urhan 1987). The reactivity characterizing the expanded perlite aggregate is lesser than that exhibited by expanded glass aggregate, which is probably due to the lower content of alkali, despite the similar content of SiO2. Partial dissolution and cracking occurred inside the perlite aggregate, which was also covered with reaction products (different crystalline products and calcium-alkali-silica gel) that had raised the alkali and Ca content (Mladenovič et al. 2004).

3.6.5 Microsphere Microspheres, also called cenospheres or aluminosilicate microspheres, are formed in some types of coal-burning, pulverized boilers. The fly ash microsphere usage in the concrete shows no harmful ASR expansion. Its value, as determined by ASTM C227 and ASTM C1260, is below the established limits. In fact, as indicated by the accelerated mortar bar test, replacing fine aggregate with the fly ash microsphere can decrease the expansion. The reason may be the pozzolanic nature of the fly ash microsphere that reduces the pH of the pore solution as well as mortar permeability. This is confirmed via the SEM image indicating plate-shaped crystals – i.e. the reaction products – which were placed in the space that was previously occupied by the fly ash microsphere (Wang et al. 2012). Similarly, Kazantseva and colleagues (2018) reported that the concentration of Ca2+ within the pore solution decreased markedly, which indicates a pozzolanic reaction characterizing the fly ash microsphere (Kazantseva et al. 2018). Typically, hollow glass microsphere comprises finer particles with size up to 0.15 mm, in comparison with expanded glass aggregate. Hollow glass microsphere is frequently employed in wellbore cement for lightweight cement slurry production. The mortar comprising hollow glass microsphere

3  •  Alkali – silica reactivity of aggregate  37 produced according to the ASTM C227 standard showed no expansion during 172 days, although the SEM image confirmed showed a thin, white layer (considered an ASR product), which surrounded the hollow glass microsphere (Thibodeaux et al. 2003). The ASR products appeared in wellbore cement mortar containing hollow glass microsphere, in which the shell/wall of the microsphere has been reacted. Addition of nitrate lithium in the amount of < 5% caused a reduction reaction on microsphere glass, as the surface of the microsphere shell/wall was still intact. Thus, it can be inferred that lithium nitrate is capable of reducing ASR within wellbore cement mortar. Because the accelerated mortar bar test showed the reactivity of hollow glass microsphere, conducting additional long-term tests has been suggested in order to confirm its alkali-silica reactivity (Thibodeaux et al. 2003; Albers 2017).

3.6.6 Volcanic rock The volcanic rocks with a porous structure that can be used as lightweight aggregates in concrete production include tuff, scoria and pumice aggregates. The accelerated mortar bar test conducted in accordance with ASTM C1260 showed that the addition of 30% pumice and scoria as a replacement for reactive aggregate can decrease the expansion to the values that do not exceed the maximum limit. The pumice aggregate application exhibited superior performance, since the SEM image observation did not reveal any ASR product (Oyan et al. 2013). In turn, the use of 30% scoria may limit the expansion below the limit value threshold. Addition of 10% pumice aggregate in place of the reactive fine aggregate within the mortar bar was also satisfactory (Tapan 2014). When pumice and scoria are a partial substitute of cement (40% replacement rate), the expansion of mortar bars comprising borosilicate-reactive aggregate is decreased to safe levels (Hossain 2005, 2006). The mortar bar produced using tuff aggregate with a partial replacement of fine aggregate amounting to 25% achieved expansion rate that almost reached the maximum limit. In turn, further increasing the share of tuff aggregate up to the value of 100% resulted in an expansion value ranging from 0.15 to 0.20%. Therefore, the usage of palm oil fuel ash (POFA) was recommended as a partial substitute of cement at 50% in order to effectively decrease the expansion characterizing the mortar bar comprising the tuff aggregate as a result of to the pozzolanic reactivity exhibited by POFA (Awal and Hussin 1997). The decrease in expansion was followed with the increase in ash content in the bar. After 12 days of exposure, the maximum expansion for a control sample (0% POFA) was 0.166%. Replacing 10% with POFA resulted in a 25% reduction in expansion, so the expansion value after 12 days was 0.126%. A further reduction, albeit slight, was seen in the samples containing 30% POFA, suggesting that such amount is not enough to decrease the ASR expansion to a level considered acceptable. However, in the case of a 50% change, there was a significant reduction in expansion (0.058%) that is well below the limit value. It is noteworthy that the bars containing 50% POFA experienced a delayed expansion as well. POFA meets the requirement of pozzolanicity and can be classified in between Class C and Class F pozzolana, in line with ASTM C618–92a. The influence of pozzolanic materials exerted on ASR expansion depends to a varying extent on the pozzolanicity, calcium content, alkali content and degree of fineness. The content of alkali in the fly ash varies greatly (typically 1.0 to 8.0% Na2Oeq) and is higher than in ordinary portland cement (0.2–1.4% Na2Oeq). Most of these alkalis are primarily bound to the glassy phase of the ash, whereas the content of soluble alkali is typically less than 0.1% Na2Oeq (Thomas 1994). This means that only a fraction, commonly called “available alkali”, which remains effectively accessible to ASR, rather than all the alkali, participate in the alkali-silica reaction. However, the views on the subject are divided (Hobbs 1986). The POFA effectiveness, similarly to that of other fly ash types, increases with the level of tuff aggregate replacement (Awal and Hussin 1997; Demirboga and Khatib 2022).

38  Application of Waste Materials in Lightweight Aggregates

3.6.7 Sintered fly ash The lightweight aggregate comprising sintered fly ash constitutes a commercial solution for the manufacturing of lightweight structural concrete. The coarse aggregate with sintered fly ash, replacing the partially non-reactive coarse aggregate, achieves greater expansion compared to the control sample. This extensibility was small and considered non-harmful. The expansion exhibited by the concrete comprising sintered fly ash is higher at lower water content, which indicates the correctness of the dilution theory. However, the value of expansion was not even close to the results obtained in the case of the low porosity aggregate that constitute the control. This means reducing the pressure and amount of gel around sintered fly ash due to the decrease of the water to cement ratio. Gel was found in the fine-grained micrite matrix of certain aggregate particles; however, the porosity in oolites was largely unaffected. Thus, the pressure of the gel can likely only be released by the existence of voids in the aggregate without the need for the gel to grow into its pores. The expansion of concrete containing coarse aggregate from sintered fly ash was slightly increased at lower water content, showing no gel or concrete damage. Due to the lack of a gel, the low expansion values result from the potential reactivity of the low porosity coarse aggregate also present in concrete, competing for the available alkali. This can be observed if the minimum proportion of reactive aggregate is exceeded. The usage of sintered, coarse fly ash aggregate is advantageous because it reduces the expansion of concrete, comprising reactive aggregate that mainly resulted from the reduced alkalinity of the pore solution (Collins 1989). Replacement of sand with sintered fly ash aggregate (pulverised fly ash) in the amount of 5% reduces the expansion of mortars by 30%, in comparison with the sample containing sand only. The data was based on the modified accelerated mortar bar method, ASTM C 1260. This results from the porosity of sintered fly ash. The porous structure of the aggregates facilitates the access for the gel, in which it can expand freely (Juenger and Ostertag 2005).

3.6.8 Other aggregates/artificial aggregates The chemical method carried out on the basis of the ASTM C289 standard did not show the alkaline reactivity characterizing the vermiculite aggregate. Moreover, the 14-day expansion, which was obtained on the basis of on accelerated mortar bar test, was similar to the expansion of mortar prepared from ceramsite aggregate (Mladenovič et al. 2004). Expanded slate aggregate showed a decreased ASR in the concrete that also contained reactive aggregate but was less effective than expanded clay or expanded shale aggregates (Li et al. 2019). Artificial aggregate from sewage sludge as well as reservoir sediment artificial aggregate has less ASR potential in comparison with expanded clay aggregate, which was confirmed by the Sc/Rc ratio achieved in ASTM C289 (Mo et al. 2021). Replacement of reservoir sediment artificial aggregate to the pulverized waste glass powder at 20% showed an increase in the ratio of Sc/Rc. The waste glass powder causes a decrease in alkali-silica reactivity, probably as a result of the pozzolanic reaction (Chiou and Chen 2013). The aggregate containing artificial sintered microcrystalline silica reduces the expansion exhibited by mortar bars even more than sintered fly ash aggregates. The expansion after 14 days was 80% lower compared to the mortar with sand alone (Juenger and Ostertag 2005). According to accelerated testing conditions for ASR, sintered silica fume (i.e. amorphous silica) is reactive. In turn, agglomerated silica fume particles in the same size range decreased ASR expansion of sand. Variations in alkaline reactivity between the aggregates with the same chemical composition result from the different bonding ways between the silica particles. They modified the surface inside the particles as well as created a matrix with a differing pore structure. In the course of sintering, the

3  •  Alkali – silica reactivity of aggregate  39 spherical silica fume particles merged, forming a solid matrix with a pore structure characterized by discontinuity. In turn, agglomerated silica fume exhibits high porosity and has lower strength because of silica fume particles with a diameter of about 0.1 mm that are bound only by weak van der Waals forces. It has been hypothesized that the silica fume comprised in the agglomerates is involved in the pozzolanic reaction with the cement slurry during hardening. The pore solution, containing hydroxyl and calcium ions, easily enters the porous network structure of the agglomerates. Moreover, the particles of silica fume favour the pozzolanic reaction, as they provide multiple nucleation sites available for calcium hydroxide (CH). A non-expansive calcium silicate hydrate (C–S–H) having a lower CaO/SiO2 ratio (C/S) than the C–S–H resulting from the cement hydration reaction is produced in the course of the pozzolanic reaction. The product, resembling C–S–H, is capable of acting as an alkali sink. Therefore, it can prevent hydroxyl and sodium ions from attacking the sand in mortar as well as expansive ASR gel formation. The pozzolanic reaction is unable to occur in the aggregates comprising sintered silica fume. Their low particle surface area does not provide the sites for CH nucleation. These aggregates react with alkali (1N NaOH), forming a silica-alkali gel that is comparable to natural aggregates containing amorphous silica (Juenger and Ostertag 2004). To determine the alkali-silica reactivity of the fly ash-based aggregate, GGBS and cement, as well as the geopolymer (fly ash + alkali + GGBS), an accelerated mortar rod test was conducted in accordance with ASTM C1260. The reactivity did not exceed the harmful expansion limit (0.20%). The cement mortar bars comprising geopolymer granular aggregate exhibit slightly greater expansion compared to the mortars containing cement-based aggregate (Ul Rehman et al. 2020). Oil palm shell (OPS) constitutes a bio-solid waste originating from the palm oil industry in tropical countries. It could potentially be employed as aggregate in concrete mixture. OPS has been studied as natural lightweight aggregate since 1984 (Islam et al. 2016). As it is known, ASR may be caused by the reactive silica in the aggregates as well as the available silica in the pore solution of concrete. However, the content of silicon dioxide in oil palm shell exceeds 65% (Amer et al. 2015). In the course of the hydration process, sodium alkali and potassium hydroxide are released as well as interact with the concrete components. This contributes to the ASR development in the concrete. Expansion occurs when the pore solution reacts with the hydroxyl ions in the concrete. The mortars and concrete with coarse aggregate were checked in the prism bar and mortar bar tests. The mortar bars elongation was 0.038 mm after 16 days and 0.039 mm following 90 days of testing, while the expansion of the concrete sample was 0.041 mm following 16 days as well as 0.0425 mm following 90 days. It follows that OPS, despite the significant content of silica, may be used as fine and coarse aggregate in concrete and show comparable expansion to concrete with normal aggregate (Vandanapu and Muthumani 2019).

4

The production process of lightweight aggregates 4.1 INTRODUCTION

Artificial aggregates, including lightweight aggregates, are man-made building materials; thus, their properties largely depend on the manufacturing process and the raw materials used. The basic raw materials in the lightweight aggregate technology are clay raw materials – i.e. clays, loams, silts, loess and various types of waste. In the lightweight aggregate technology, hardening methods such as cold-bonding, autoclaving, sintering, accelerated carbonation, and microwave radiation are used. The preparation of raw materials involves crushing them with crushers, followed by sorting, dosing, mixing, granulating and drying. A distinction is made between agitation granulation and compacting. Agitation granulation corresponds to the consolidation of fine, wet solid particles, 1–500 µm in size, into larger agglomerates as a result of rolling in a rotating drum or disc granulator without the use of external compaction force. Compaction granulation is a method which consists in compressing a specific batch of granular material as a result of which the air is displaced from the inter-grain space, individual grains are brought closer to each other and thus, the forces connecting these grains are created. After densification, the extruded fresh pellets are cut into small lump pieces and rolled to obtain spherical granules (Tajra et al. 2019b; Ren et al. 2021). Production of lightweight aggregates using the cold bonded pelletizing technique is realized at the temperature from 0–100°C. It consists in consolidation of fine particles of the raw material into larger granules. Water or a liquid binder is sprayed onto the powder in order to stick it together in the pelletizer. The hardening of the granules is over 28 days. In order to increase their mechanical strength, cement (up 20%) or lime is added (Ul Rehman et al. 2020). Most lightweight aggregates are produced by sintering natural raw materials that expand at temperatures from 1000 to 1300°C in rotary kilns, shaft kilns or sintering grates. During the sintering of raw materials at high temperature, a liquid phase is formed, in which the mechanisms of transport and mass exchange occur much faster than in the solid phase. In addition, there are oxides which increase the viscosity and surface tension in solutions such as SiO2, Al2O3, as well as those that lower these parameters: CaO, Na2O. The influence of the mentioned oxides is strongly correlated with the temperature of the solution. The effect of the liquid phase on the sintering mechanisms depends, to a large extent, on its quantity. The high temperature causes various chemical reactions to take place, which may take place within the liquid phase or crystallization from the liquid phase, between a solid 40

DOI: 10.1201/9781003313090-4

4  •  The production process of lightweight aggregates  41 and a liquid component, between a gas and a solid component or a liquid in a solid phase between two solid components, by resublimation from the gas phase. The synthesis reactions creating new phases are most often exothermic and can therefore reduce the energy of the sintering process. The resulting products may change in volume – i.e. shrinkage or expansion may occur (Lis and Pampuch 2000; Ramamurthy and Harikrishnan 2006; Rahaman 2008; Lakshmanan 2012). Another process for hardening lightweight aggregates is autoclaving. Green pellets are gradually heated up to 200°C and steam under pressure in autoclaves. The particles of the mixture obtain activation energy and condensed moisture from water vapor and undergo rapid and gradual hydration (Ma et al. 2011a; Lin et al. 2000). The accelerated carbonation technology is a method of obtaining lightweight aggregates and effective carbon dioxide sequestration. The freshly prepared pellets may be turned into a solid compound of aggregates via accelerated carbonation. Under the conditions involving high concentration of CO2, the development of pellet strength can be accelerated to a certain degree (Scrivener et al. 2018; Jiang et al. 2018). The microwave technology yielding lightweight aggregates uses microwave radiation, which is electromagnetic radiation between infrared radiation as well as ultra-short waves, having a wavelength ranging from 1 mm to 1 m, corresponding to a frequency of 300 MHz-300 GHz (Komarov 2012). As a result of the low radiation quanta energy, the interaction involving raw materials takes place at the molecular level. It is based on the interaction that occurs between the particles and electromagnetic field. Microwave curing can replace the conventional curing techniques in order to implement energy-saving, environmentally friendly and economical production of lightweight aggregates (El-Feky et al. 2020; Hanif et al. 2021).

4.2  PREPARATION OF RAW MATERIALS The basic raw materials in the lightweight aggregate technology are clay raw materials of various origins and geological age, exploited mainly by the opencast method. These include plastic raw materials: loams, clays, clay shales and loess. Their main minerals are clay minerals (illite, montmorillonite, chlorite, kaolinite), which give them the properties that allow them to be formed into granules by means of the plastic method. The preparation of raw materials involves their crushing and sorting. Crushing is carried out using cone crushers, jaw crushers, gyratory crushers, roller crushers, cone mills, hammer mills or impact mills. The type of crusher used is dependent upon the properties characterizing the material and the required size, with the optimal shape of the crushed particles being a cube. This is a key parameter because it determines the shape of the resulting aggregate. Crushing may take place in one go or it may be combined with sieving before the desired size is achieved. Vibrating screens are usually used. The jaw crusher is the most popular crusher for shale and relatively dry clay. Raw materials with high moisture content can block the crusher. For shale as a raw material, crushing and screening operations are usually sufficient; these are required to prepare the kiln feed. This also applies to some clays; however, most clays are too soft or brittle to withstand processing, and the resulting material is much too “fine” material (Wilson 1976).

4.3  COLD BONDING APPROACH Production of lightweight aggregates using the cold-bonded pelletizing technique is a method realized at a temperature of 0–100°C. This method consists in consolidating fine, moistened raw material

42  Application of Waste Materials in Lightweight Aggregates particles into larger granules – i.e. a liquid binder or water are most often sprayed onto the powder in order to stick it together. The formed granules are then consolidated in a pelletizer and hardened to increase their mechanical strength (Tajra et al. 2019b). In order to ensure the appropriate strength of the aggregates, hydraulic raw materials in the form of cement (up 20%) or lime are used, and the hardening time of the granules is over 28 days (Ul Rehman et al. 2020). The cold-bonded method is an energy-saving alternative to the sintering process. It has lower energy consumption as well as decreases the formation of secondary pollutants (Thomas and Harilal 2015). Recently, many studies have considered the manufacturing of lightweight aggregates by means of the cold-bonded method with the use of various waste materials (Ferraro et al. 2020). Production of green pellets is related to maintaining the appropriate conditions of the production process. After the process of crushing and sorting the raw materials, they are dosed, mixed, granulated and dried. The process conditions and the type of apparatus used may vary. Therefore, there are following types of granulation: • agitation granulation: • drum granulation, • disc (or pan) granulation, • mixer granulation, • cone granulation; • compacting: • extrusion • roll pressing • pellet mills. The selection of the granulation method and agglomerating devices is determined by several factors, among others: the capabilities of the agglomerating devices, properties and quantity of the material fed, binder properties, requirements for the final product quality, energy consumption, size and shape, moisture content, production speed requirements and production environment (Ahmed and Jay 2015).

4.3.1 Agitation granulation Agitation granulation involves consolidating fine, wet, solid particles with a size of 1–500 μm into larger agglomerates with specific physico-mechanical properties as a result of rolling in a rotating disc or drum granulator without the participation of external compaction force (Ennis and Litster 1997). The palletizing disc is inclined (at approximately 45⁰), rotates around a central axis and has a rim extending above the surface of the disc (Figure 4.1). The rim may be perpendicular to the target, sloping outwards or may rise gradually. As the disc rotates, it works by alternately wetting the particles with a stream of water and covering them with a dry material. This creates a snowball effect as the particles gradually increase in size. The largest spheres move to the periphery of the disc and fly through the rim when they reach the desired size. This value depends on the angle and speed of the wheel, the height of the rim and the points at which the dry material and water enter. Granules are formed as a result of the action of binding forces between the particles of raw materials and at the time of liquid-bridge-creation or via a combination of surface tension, viscous forces and capillary pressure until the formation of permanent bonds (Suresh and Karthikeyan 2016). If powdered raw materials are subject to granulation, regardless of whether it is pressure granulation or nonpressure granulation, a binding liquid is used in the agglomeration process. Water is the most common

4  •  The production process of lightweight aggregates  43

FIGURE 4.1  Drum and disc granulation (reprinted from publication Ren, P., Ling, T.-Ch., Mo, K.H., Recent advances in artificial aggregate production, Journal of Cleaner Production 291, 125215, Copyright [2021] with permission from Elsevier).

binder, but also, other materials are applied, such as cement, lime, bentonite, water glass, starch solution, mineral oil-in-water emulsions or paper mill wastewater. They facilitate the approximation of the primary particles only during granulation or induce permanent cohesive forces which increase the strength of the connections between the primary particles, also after drying the product. The dosing of the binder is an important factor. Insufficient or excessive volume thereof in the raw material mass means the development of insufficient cohesive forces. Process pelletization is characterized by three stages: • pendular state (water at the grain contact), • funicular state (water filling certain pores), • capillary state (water filling all inter-grain spaces) (Harrison and Munday 1975; Baykal and Döven 2000). A schematic representation of the pellet formation and the liquid saturation state achieved with increasing intermolecular forces is presented in Figure 4.2. The pelletization process begins with using water to wet the mixture. The nucleation phase enables to form loosely packed granule nuclei. Depending on the resistance of the granule nuclei and intensity of mixing, there may be a consolidation phase through the collision of different nuclei. As a result of collisions, the granules fuse forming larger particles by coalescence. To achieve effective particle formation, strong bonding between the granules should take place (Hapgood et al. 2007). The formation of granules in a granulation disc involves three pathways: seed formation, seed growth and, ultimately, granulation formation. In the case of the fly ash pellets, formation of fly ash seeds will occur at the beginning (path 1); they increase in size with the rotation of the disc or drum (path 2), assuming a ball shape (path 3). The growth path of the fly ash pellets in the granulating disc is shown in Figure 4.3. The pellet production efficiency is affected by such factors as the angle and speed of rotation of the granulator disc, the amount of water and the duration of the granulation (Bijen 1986). The finished granulation product is generally referred to as fresh pellet. Due to the lack or very low strength of the granules, particular care should be taken at this stage, when hauling and stockpiling. The granulation process is primarily driven via gravity/centrifugal forces. The operating parameters controlling the course of the process include the amount of water in the raw material mixture as well as the inclination and rotational speed of the granulator disc. The rotating plate, which produces

44  Application of Waste Materials in Lightweight Aggregates

FIGURE 4.2  Diagrammatic representation of pellet formation (reprinted from publication Bijen, J.M., Manufacturing processes of artificial lightweight aggregates from fly ash, The International Journal of Cement Composites and Lightweight Concrete, 8(3), 191–199, Copyright [1996] with permission from Elsevier).

FIGURE. 4.3  Growing path of pellets (reprinted from publication Bijen, J.M., Manufacturing processes of artificial lightweight aggregates from fly ash, The International Journal of Cement Composites and Lightweight Concrete 8(3), 191–199, Copyright [1996] with permission from Elsevier).

4  •  The production process of lightweight aggregates  45 the effect of particle rotation, constitutes the primary element of a pelletizer. The speed controller adjusts the rotation speed of the plate, and the slope of the plate can be also controlled. The addition of water can be manual or performed in automatic mode, in which the pelleting machine is fitted with a nebulizer as well as suitable nozzles used to spray water onto the mixture. In addition, at least one scraper is typically installed in various locations on the rotary disk. The scrapers remove the material that adheres to the surface as well as the sides of the rotating plate, thus preventing the formation of unused particles (Ferraro et al. 2020). Various pelletizer machines may be employed for the agglomeration process by granulation, including drum type, disc or pan type, mixer type and cone type (Sivakumar and Gomathi 2012).

4.3.1.1 Pin mixer Pin mixer constitutes a high-speed micropelletizing and conditioning device converting fine ­materials – e.g. ash, dust or other raw material with the addition of a liquid binder – into microgranules by means of a high-speed, central rotor-shaft as well as a radially elongated pin assembly. The pin mixer features a cylindrical shell having a replaceable inner liner as well as a shaft with pins that are radially extended. A small gap is left between the pin tips as well as the inner mixer shell lining. The shaft rotates at a speed of several hundred revolutions per minute. Depending on the needs, the arrangement of the pins is variable. The material is introduced at one end of the stationary cylindrical housing, followed by addition of a fine stream of liquid. The spindles, rotating at high speeds, mix the raw materials and the liquid binder, eliminating air and reducing the volume of water between the particles, which leads to the thorough mixing as well as compaction of the materials moving to the lower outlet. A moistened micropellet, with a shape that is spherical to irregular, constitutes the end product. A pin mixer may be employed as a stand-alone agglomerating device as well as a pre-processing device for wetting and mixing (conditioning) the material before the final agglomeration process using a disk or drum granulator (FEECO 2015a).

4.3.1.2 The paddle mixer The paddle mixer, or screw mill, constitutes a horizontal trough in a shape of a barrel, with two counter-rotating shafts as well as a series of blades attached on these shafts at a predetermined angle, along the trough length. A slight gap is left between the blade ends and the inner mixer walls. The paddles move the material from the top of the trough down on each side and subsequently force the material upwards between the shafts. Thus, a turning and kneading effect occurs, resulting in a high capacity of mixing. A binder or a liquid may be employed for conditioning (wetting) and agglomeration (FEECO 2015b) as well.

4.3.1.3 Rotary drum agglomerator A rotary drum agglomerator, also called a spherical drum, comprises a rotating, inclined, open cylinder which, in the presence of a liquid or binding substance, ensures rotation and growth to form spherical granules. The process consists of screening and continuous recirculation of the undersized spheres. The undersized spheres are returned to the drum two or three times, including the raw fine particles. The pattern of material movement in the drum depends on amount of material inside, the drum speed, as well as the structure of the inner drum surface. Commercial rotary drums have varying dimensions, depending on the required capacity. The diameter of the drum can range from 1 to 3.5 metres, whereas the length may range from 5 to 9.5 metres; the drum inclination amounts to 2–10 degrees (Srb and Ruzickova 1988).

46

Application of Waste Materials in Lightweight Aggregates

4.3.1.4 Disc pelletizer Disc granulator, or pan pelletizer, comprises a rotating, tilted disc which is driven by a reducer, a binder spray system and a scraper frame, including scrapers, supported on a base and a heavy construction frame. The disc angle and rotation speed can be adjusted to control the size of the pellets for different applications. Spherical or nodular pellets are formed in the disc granulator when a binder or liquid are present. The material is introduced to a rotating disc. The moisture droplets gather into a few particles, while rotation hits and compacts these loosely formed kernels or seeds. Compaction forces the water to come to the surface, where more particles can be collected. Centrifugal force results in classification of granules based on their size. The first stream of seeds or nuclei is on one side of the disc, and the granules increase in size as they move from right to left. The granules remain on the wheel until they are of the desired size; afterwards, they are removed from the wheel. The scrapers remove the build-up forming on the side walls and bottom of the disc as well as create a smooth surface, helping to guide the granules into their separate streams (FEECO 2015c).

4.3.2 Pressure or compaction granulation Compaction granulation is a method which consists in compressing a specific batch of granular material, which results in the displacement of air from the inter-grain space, bringing individual grains closer to each other and, as a result, creating the forces connecting these grains (Liao and Huang 2011a; Huang and Wang 2013). In turn, the pelletization is a well-known and common method of achieving non-pressure agglomeration. The moisturized fines are enlarged into spherical pellets via collision as well as coalescence, which occur due to their rolling movement within the pelletizer disc (Bijen 1986). After compacting, the freshly extruded pellets are cut into 15-mm-long pieces, followed by rolling to obtain spherical granules with approximate diameter of 8–10 mm, in order to prevent the corner effects of cylindrical pellets (González-Corrochano et al. 2009a; Liao and Huang 2011a; Tsai 2012). Generally, compaction stress and material moisture constitute the primary factors influencing compaction granulation. Optimum humidity varies with the type of materials used, as a result of their varying plasticity (Tsai 2012). González-Corrochano et al. (2009a, 2016) reported that the amount of water added to the mix should remain at 2–3% or 5–6% over the yield point. An insufficient amount of water leads to incorrect granulation of the aggregate; in contrast, excessive humidity has a great effect on the pellet durability. The particle density of compacted pellets is generally higher due to the lower porosity, as a result of compaction, than in the case of agitation granulation. The applied compaction pressure and compressive strength of the aggregate are characterized by a linear relationship (Chang et al. 2010a). The optimum value of pressure approximates 35–42 MPa, while lower or higher value will drain the binding liquid or result in poor compaction. The formation of granules is related to the compacting of the structure of the powder substance; therefore, one of the indicators of the granulation capacity is the ability of the powder to increase density under the action of a certain pressure as well as its ability to maintain the shape obtained during the granulation process. Regardless of the granulation method used, the formation of granules takes place as a result of the compaction of the structure of the substance, determined by the forces acting between the particles or crystals (Tsai 2012). The compaction agglomeration can performed using the following methods: pellet mills, extrusion, roll pressing or Punch-and-Die presses (unidirectional piston-type compaction).

4  •  The production process of lightweight aggregates  47

4.3.2.1 Extrusion press agglomeration In the course of the extrusion process, a material or mixture thereof is forced through a die – i.e. a special metal tool which causes the material to assume the specific required shape. The end product, called extrudate, constitutes a material characterized by a fixed profile that may be processed or cut to create the end product. Various kinds of extrusion presses are available, predominantly ram extrusion presses and screw extruders. In the screw extruder, material is transported by rotating a bolt in a tight-fitting housing with barrel shape that creates the pressure required to overcome the friction in the open die plate sealing the barrel end. There is a horizontal extrusion channel, which tapers slightly, enabling the mass to build up sufficient pressure for initial setting. Materials are pressed by the reciprocating punch against the briquettes, in previous strokes, which then remain wedged in the channel. Fresh materials and all the other briquettes found in the channel are compressed in each stroke up to the point when the axial force is sufficient to overcome the wall friction as well as the potential back pressure acting at the channel mouth. Then, right before each stroke is completed, the entire column of briquetted material is moved forward, whereas a briquette is discharged from the machine (Pietsch 2002). A pellet mill is another device used for agglomeration. In the production process, the materials are forced through an open die, forming granules or cylindrically-shaped briquettes having a variable length as well as constant diameter. Various pellet mill types are available, including ring-die mills for large-scale pellet production and flat-die mills employed for smaller-scale production. In roll presses, large amounts of finely-divided solids have to undergo conversion into agglomerated, larger pieces. Roller presses are equipped with two equally-sized rolls that rotate in opposite directions at the same speed. Compaction is achieved by squeezing the material in the gap area. A grinding machine can be employed to granulate the compacted product to the appropriate particle size. The majority of materials can undergo agglomeration using this technique with a binder, very high pressure and/or heat, if necessary. Generally, a lesser amount of binder is required in this method; therefore, little or no drying of the agglomerates occurs. Punch-and-Die presses (unidirectional piston type compaction) are the oldest pressure agglomerating devices used for compacting materials. As a result of the powder compaction, the resulting products are characterized by very good uniformity in terms of shape, size as well as density. The methods employed in this agglomeration type are classified according to the movement of individual tool elements (die, upper punch, and lower punch) against each other. Solid die compaction can be divided into one-sided and two-sided pressing. In one-sided pressing, both the die and the lower punch are stationary. Compaction is performed by an upper punch moving in a stationary die. As far as bidirectional pressing is concerned, die is the only element permanently fixed within the press. The lower and upper punches simultaneously move up and down into the die (Ahmed and Jay 2015) Figure 4.4 shows an innovative, two-stage pelletization introduced in the cold-bonded aggregate method that involves a core structure comprising bonded waste power-binder which was covered with a pure binder, forming a dense outer shell layer (Colangelo et al. 2015; Tajra et al. 2018).

FIGURE 4.4  Scheme of core-shell-structured AAs-double-step granulation (reprinted from publication Almadani et al. 2022).

48  Application of Waste Materials in Lightweight Aggregates The waste incorporated into the binding matrix in the case of one-step pelletization ranges from 50 to 70%. In two-stage granulation, the aggregate from the one-stage process is enclosed in the outer coating of the binder, which is able to enhance the technological properties characterizing aggregates and the leaching of toxic substances. The “core-shell” structure enables the internal sealing of the granules, reducing water absorption and improving the mechanical properties of lightweight aggregates. This type of lightweight aggregate was manufactured via encapsulating expanded perlite powders of various sizes in the range of 2–4 mm and < 125 mm in a matrix comprising fly ash and cement. Encapsulation was carried out by means of a pelletizer disc, with a diameter of 40 cm and depth of 10 cm. First, 25 g of expanded perlite was placed in the granulator disc. Then, it was sprinkled with water and 1000 g of a dry mixture containing cement and ash was given. The use of powdered, expanded perlite in the amount of up to 40% as a substitute of fly ash greatly enhances the properties of the produced aggregates, like compressive strength and hydration heat flow. A lightweight aggregate with a low particle obtained the density in the range of 0.88–1.44 g/cm 3 with the expanded perlite content up to 50% and the crushing strength amounting to 2.04–2.66 MPa. The high specific surface area of perlite and its amorphous structure demonstrate its function as a pozzolanic material increasing the strength of aggregates and the transition zone compaction (Tajra et al. 2018). The innovative double-step pelletization approach enables to improve the physico-mechanical as well as stabilization properties of binding matrices of any studied mixture. In the pelletization process, lime, cement as well as coal fly ash in the amount from 50–70% are used as binding systems. Coal fly ash/lime and cement systems are characterized by good engineering performance; in contrast, the lime-based systems were found not to be unsuitable as binder of granules. Density ranges from 1000 to 1600 kg/m 3; water absorption, from 7 to 16%; and crushing strength, from 1.3 to 6.2 MPa. In the second step of granulation, the stabilization of the aggregates is increased (Colangelo et al. 2015).

4.4  SINTERING APPROACH Sintering is a complex, thermally activated physical and chemical process that takes place at elevated temperatures. Its kinetics are controlled by the process conditions, the most important of which are sintering temperature and time, the type of atmosphere and the type of furnace. It is a technological process consisting in the transformation of a powdered material into a polycrystalline body with a certain porosity and appropriate mechanical strength. The essence of this process involves the mass transfer mechanisms that lead to macroscopic changes in the material. In the sintering process, reactions can take place in the solid and liquid phase as well as polymorphic transformations. Most of the lightweight aggregates are produced by sintering natural raw materials that swell at temperatures from 1000 to 1300°C in rotary kilns. The aggregates are most often sintered with the use of sintering grates as well as using other devices such as shaft kilns and rotary kilns. Although sintering is a process with high energy consumption, the aggregates obtained by sintering have favourable engineering properties, in comparison with other artificial aggregates. In the laboratory scale, the production of lightweight aggregates consists in taking appropriate steps. The raw pellets were subjected to thermally treatment in line with the four steps (Qi et al. 2010): Step 1: Pelletization.

4  •  The production process of lightweight aggregates  49 At this stage, the raw materials are prepared, dosed, mixed and dried, which should be conducted very slowly to avoid cracking. Step 2: Preheating treatment. Prior to thermal treatment, raw pellets were settled for 24h at room temperature (22°C). Then, dried raw pellets, which were held by porcelainous crucibles, were settled in muffle. Subsequently, they were preheated for 10 to 30 minutes at a certain pre-determined temperature (the preheating time and preheating temperature varied in different experiments). At this stage, the residual water contained in the product and volatile components gradually evaporate, whereas the organic substances present in the material are decomposed and “burned out”. Preheating also aims to make the bonds between the material particles in the pellet stronger, thus increasing its coherence. Step 3: Hardening of the green pellets by sintering. At this stage, the substrate particles are fused at the contacts, and the strength of the pellets increases with the growth of the molten contacts. The formation of particle bonds during the sintering process is shown in Figure 4.5. In the initial phase, powders combine without shrinkage and diffusion over free

FIGURE 4.5  Formation of particles bonding during the sintering process (reprinted from publication Rivera 2011).

50  Application of Waste Materials in Lightweight Aggregates surfaces, rearrangement of grains as well as the formation of necks at the points of contact of particles are important. Grain boundaries are formed between two adjacent particles in the contact plane. The material from the particles is transferred to the necks via diffusion as well as other mechanisms of mass transport, enabling their growth, as the bonding of the particles enters an intermediate stage during which the pores between the particles start rounding. When the mass is transported further, the rounding of pores becomes more pronounced, and some of them become isolated from the particle grain boundaries. This is known as the final setting stage (Rivera 2011). Step 4: Cooling treatment. Following the sintering process, the pellets become settled in draft cupboard until they cool to room temperature – i.e. 22°C. Step 5: After-treatment, which may consist of crushing, sieving and/or wetting. On an industrial scale, the production of lightweight aggregate includes the following stages: Step 1: Extraction of the raw material in a mine or quarry. Step 2: Crushing the material with jaw crushers, cone crushers, auger mills or hammer mills and screening. Then, the material that is oversized is fed back to the crushers, whereas the material passing through the screens is sent to the warehouse. Step 3: Feeding the material from the warehouse to the rotary kiln that is fired with natural gas, coke, coal, coke or fuel oil up to a temperature of around 1200°C. Step 4: Material liquefaction as heating, gas bubble formation by the carbon compounds present in the material, contributing to material expansion; this process releases volatile organic compounds (VOCs). Step 5: Moving expanded product (clinker) from the furnace, conveyor, to the cold room where it is air-cooled to form a porous material. Step 6: Size-sorted, chilled, lightweight aggregate is crushed if necessary, collected (stored) and shipped (Figure 4.6). After placing the product in the oven and heating it at a certain rate of temperature increase, gradual changes begin to take place in it, the macroscopic manifestations of which are changes in dimensions and possibly weight and colour (Ramamurthy and Harikrishnan 2006). During the sintering of raw materials, chemical phenomena occur, accompanied by a change in the thermodynamic potential as a result of polymorphic transformations as well as reactions in the solid and liquid phase (Lis and Pampuch 2000). In the initial stage, in the low temperature range, the residual water contained in the product (raw pellets) and the organic substances present in the material are decomposed and “burned out”. These phenomena can cause a slight shrinkage, which is continuously compensated by the increase in dimensions due to thermal expansion of the system. The expansion of the sintered product is stopped and the system begins to contract. It is one of the most important moments of the sintering of the so-called sintering initiation temperature. The material is sintered to form a ceramic matrix made of aluminum silicates, mainly mullite and amorphous phases. The swelling processes also take place in the system, and the characteristic temperature for this process is the swelling start temperature at which the sample, during thermal treatment, reaches the volume of the initial sample after the softening process and another increase in volume.

4  •  The production process of lightweight aggregates  51

FIGURE 4.6  Manufacturing of expanded shale, clay and slate (reprinted from publication Rao and Darter 2013).

If no phase changes – i.e. melting or evaporation – take place in the system, then further keeping the product at high temperature does not lead to further changes in linear dimensions. In this case, the maximum temperature is called the sintering temperature, and the holding time of the material in the furnace, at this temperature, is called the sintering time. These parameters, together with the rate of temperature increase during heating and decrease during cooling, belong to the basic data of the sintering process. Macroscopic changes in the material during sintering are the result of the changes in the material known as microscopic sintering symptoms. When analyzing the microstructure characterizing the sintered material, the shape and size of the grains, their phase composition, the shape and size of the pores, the presence and distribution of amorphous phases are taken into account. Up to the sintering start temperature, there are no significant changes in the density and porosity of the material. However, when the temperature is high, even before the contraction starts, in the image of raw pellets, changes are observed in the shape, size of grains and pores. A manifestation of such changes in the microstructure is an increase in the specific surface area, accompanied by a reduction in density and a gradually increasing the total porosity. In the microstructure, there is a clear reduction in the packing density of the grains. The number and size of pores increases and individual grains gradually form a frame. However, regular changes in density and porosity do not always correspond to identical changes in individual microareas of the material. The state obtained by sintering differs substantially from the initial state before sintering. Macroscopic and microscopic observations show that the characteristic symptoms of sintering are the transformation of the material from the compacted powder into a porous material, which is accompanied by: • first, the volume shrinkage of the material and then the shape retention swelling due to pore growth, • changes in material properties (mainly physical – i.e. density, porosity, strength, colour).

52  Application of Waste Materials in Lightweight Aggregates The changes taking place in the system at elevated temperatures are irreversible. The actual macro- and microscopic behaviour of each sintered system – i.e. the sintering start temperature, the nature, speed and time of compaction as well as the final density and microstructure achieved, and hence, the product properties – depend on many factors, related both to the physico-chemical properties of the system and external process parameters (Lakshmanan 2012). Sintering is a process influenced by many quantitative and qualitative factors. This is mainly due to the complexity and multi-directional nature of the processes that may take place. The properties of the final product are influenced by starting material factors (grain size, phase composition, chemical composition) and sintering factors such as temperature, time, pressure.

4.4.1 The role of the liquid phase in the sintering process During the sintering of raw materials at high temperature, a liquid phase is formed in which the mechanisms of transport and mass exchange take place much faster than in the solid phase because the diffusion and mass transfer mechanisms are much faster in the liquid than in the solid (German 1985). However, this influence is not clear-cut, as it depends on the type, quantity, chemical composition and temperature. The chemical composition and temperature of the liquid phase affect the viscosity, surface tension and solid liquid wettability of the grains of the sintered raw material. The higher the temperature of the liquid, the lower the cohesive forces is – i.e. viscosity and surface tension. There is also a regularity indirectly resulting from Gibbs’ law that the more components a liquid phase contains, the lower the cohesion forces are (Nadachowski et al. 1999). In addition, there are oxides that increase the viscosity and surface tension in solutions, such as SiO2, Al2O3 and also those that lower these parameters: CaO, Na2O. The effect of these oxides is strongly correlated with the temperature of the solution (Pampuch et al. 1992). When the liquid has a large contact angle – i.e. it poorly wets the grains permanently – then it actively participates in the mass exchange but it poorly penetrates the solid grains, often creating isolated losses, which may hinder the compaction processes of the material. On the contrary, when the liquid has a small contact angle, it intensely penetrates the solid powder grains and accelerates the sintering mechanisms. The viscosity of the liquid affects the rate of mass transfer; the lower it is, the more intensive the exchange of components (Rahaman 2008). The influence of the liquid phase on the sintering mechanisms depends, to a large extent, on its quantity. If there are small amounts of the liquid phase, it acts as a dispersed substance. It creates capillary forces at the grain boundary (in the necks), which bring the grain centres closer and reduce the pore volume. The more wettable the liquid is, the more effective this process. Moreover, the liquid settling at the grain boundary facilitates their sliding and grain rearrangement. The lower the viscosity of the liquid, the less the friction and the greater the possibility of slippage. The liquid intensifies the process of grain rearrangement; also by the fact that, by etching the grains, it facilitates breaking the existing necks. In summary, a small amount of liquid during sintering intensifies the processes that take place during sintering in the solid phase. A liquid in small amounts always has a positive effect on sintering, but the more wettable it is and the lower its viscosity, the greater the intensity of the sintering processes. In the case of sintering with a high proportion of the liquid phase, the situation is more complicated and the sintering processes depend on the wettability of the liquid on the solid grains. When the liquid is well wettable, then it is a substance that disperses the solid grains. In extreme cases, the grain boundaries disappear and the mass is transported mainly through the liquid phase. If the sintering takes place with a large amount of liquid phase with poor wettability, the liquid exists in a dispersed form – i.e. droplets. The liquid droplets can, in this case, be treated as grains of a separate phase. The poorly wettable liquid additionally “pushes” the solid grains, thus hindering the process of the

4  •  The production process of lightweight aggregates  53 approach of the grain centres. After cooling, capillary pores form between the droplets of liquid and the solid grains, which cannot be virtually eliminated (Kingery et al. 1976; Pampuch et al. 1992; Lis and Pampuch 2000).

4.4.2 Chemical reactions occurring in the course of the pre-sintering process The reactions taking place during the initial sintering influence the development of the microstructure in the aggregates and, consequently, have a significant effect on their properties. Aggregate sintering has gained wide recognition on an industrial scale because the energy in this process is used very efficiently. In terms of energy cost, lowering the time of firing temperature significantly affects the production costs. Undoubtedly, there is a maximum heating rate for each raw material composition which causes thermal reactions to achieve the desired properties of the products. Heating which is slower than maximum accounts for a safe margin for the burnout process. The knowledge of microstructural changes and the chemical reactions occurring in the course of the process is required in sintering. High temperature causes a variety of chemical reactions that may take place: • • • • •

in the liquid phase or crystallization from the liquid phase, between a solid and a liquid component, between a gas and a solid or liquid component, in the solid phase between two solid components, by resublimation from the gas phase.

The synthesis reactions creating new phases are most often exothermic and can therefore reduce the energy of the sintering process. The resulting products may change in volume – i.e. shrinkage or swell may occur. The sintering profile of the ceramic body representing structural changes that depends on temperature and time can be divided into three parts (Reed 1995): • heating during which green pellets are rather fragile and unstable, • formation of a liquid phase, having the viscosity that decreases with the annealing temperature; the viscous properties of the liquid phase are lost at the glass transition temperature (glass transition temperature); the applied stress may result in body deformation, • a final part of the cooling curve below the temperature of glass transition, where the product is characterized by relatively high strength and brittleness. Designing an optimal sintering profile requires determining the physical and chemical reactions which take place during the sintering process. When designing the sintering profile, the data on reversible and irreversible thermal expansion should be taken into account as a first step. In this procedure, the shape of the sintering profile is determined in three steps. The rate of heating should be specified in each section, separately. The data on irreversible thermal expansion should be employed in the heating step, while the data on reversible expansion should be employed in the design cooling step. The differential thermal analysis of DTA enables the study of the thermal effects accompanying the processes occurring during the heating of the test substance. These can be endo- or exothermic chemical reactions (decomposition, oxidation, reduction) and phase transformations (recrystallization,

54  Application of Waste Materials in Lightweight Aggregates melting). It consists in measuring the temperature difference between the samples of the test substance and the reference substance during their controlled heating. Since the reference substance does not undergo the transformations accompanied by thermal effects, the measured temperature difference depends on the rate of heat absorption or release of heat by a sample of the test substance (Paulik et al. 2004). Differential scanning calorimetry TG-DSC constitutes an instrumental analytical method, recording the difference in heat flux flow between the environment (heating system) and test substance as well as between the environment (heating system) and reference as a function of temperature. The result of the measurement is the DSC curve, representing the amount of heat which was exchanged by the sample with the environment per time unit (ordinate) as a function of time or temperature (abscissa) – i.e. dH/dt = f(T) (Drzeżdżon et al. 2019). High temperature microscopy provides the data for aggregate firing and sintering characteristics. It allows the behaviour of the aggregate sample to be monitored from room temperature to the sintering stage. During the measurement, the image is analyzed in the form of marked values of characteristic temperatures and photographic documentation of changes in the shape and size of the sample surface observed in the subsequent stages of heating. Using this technique for sintering testing various ceramic substrates has been described by many researchers (Aldo et al. 1998). This method is necessary to determine the safe and effective sintering temperature of ceramic masses. It allows assessing the temperature and the degree of thermal swelling of the mass as well as the temperature and the degree of liquid phase formation. During the measurement, changes in the size of the shape as a function of temperature are recorded, on the basis of which the characteristic temperatures of the raw materials are determined: the sintering start temperature, the maximum sintering, softening, the beginning and maximum of swelling and melting, as well as the temperature intervals in the subsequent heating phases. Knowledge of these parameters is the basis for assessing the range of the lightweight aggregate sintering temperature interval: • the temperature at which the swelling begins is the temperature at which the sample, during thermal treatment, reached the initial sample volume after the softening process and another increase in volume, • the temperature of the maximum increase in volume at which the sample reached its highest value, • the sample swelling temperature interval, which is the difference between the maximum expansion temperature as well as the initial expansion temperature, • the swelling factor of the sample at maximum expansion temperature. The changes in the shape of clay samples with waste in the form of sewage sludge, recorded using a high-temperature microscope, are shown in Figure 4.7. Changes in the size and shape of the clay sample and sewage sludge at the temperature of the beginning and maximum sintering are the result of thermal transformations of the minerals included in the raw material mass (dehydration of clay minerals, dehydroxylation, combustion of organic matter, dissociation of carbonates). The product of these changes is the solid phase and the gas phase, freely released outside the area of the porous sample. Clay with the addition of sewage sludge is a medium-swelling raw material with a temperature at which it swells (the temperature at which it increases 1.5 times its volume) of about 1105°C and the maximum thermal expansion coefficient Smax≈1.24 at 1300°C. The interval is 250°C, increasing the maximum sintering temperature to about 1180°C. Sewage sludge intensifies the sintering process (reducing the volume by almost 20%) and significantly reduces pyrogenic swelling compared to the mass of clay alone (Franus 2016).

4  •  The production process of lightweight aggregates  55

FIGURE 4.7  Changes in the shape and size of the surface of clay and sewage sludge determined by a high-temperature microscope (reprinted from publication Franus 2016).

4.5  AUTOCLAVING (HYDROTHERMAL) APPROACH In the autoclave hardening process, green pellets are gradually heated up to 200°C and treated with steam under pressure in airtight tanks called autoclaves (Bijen 1986). The water vapour is forced into the inner regions of the material through the micropores. The molecules of the mixture obtain activation energy and condensed moisture from water vapour undergo rapid and gradual hydration (Xie et al. 2021). The materials cured in an autoclave at a temperature of 160–200°C, a saturated vapour pressure of 0.6–1.6 MPa have higher strength than standard cured products (20 ± 3°C, RH > 95%) (Lin et al. 2000). Autoclaves are available in multiple sizes as well as diverse pressure and temperature capacities. However, the majority have a cylindrical shape in order to transfer internal pressure as well as provide maximum usable space inside. Figure 4.8 presents a schematic of a heater, controller, an airflow system, a vacuum system and a pressure system. Carbon steel is used for the production of the tanks. Generally, at one end, there is a door where parts and tools are loaded or unloaded. In order to achieve effective and profitable production, using an autoclave of the right size, maximum pressure, temperature and production capacity is essential. The gas flow and the resultant heat transfer capacity are often clearly defined. Original Equipment Manufacturers (OEMs) usually state the heating capacity of an autoclave via a minimum heating rate for a given thermal load. Although an autoclave is capable of ensuring uniform pressure, a uniform heat flow cannot be guaranteed, whereas determination of the thermal regime for a given production

56  Application of Waste Materials in Lightweight Aggregates

FIGURE 4.8  Scheme of an autoclave processing system (reprinted from publication Fernlund, G., Mobuchon, Ch., Zobeiry, N. Autoclave processing, in book: Reference Module in Materials Science and Materials Engineering; Copyright (2018) with permission from Elsevier).

load is still challenging. Autoclaves, however, involve significant purchase, operating and tooling costs, especially for large parts (Centea et al. 2015). However, supporting energy-efficient construction and reducing carbon dioxide emissions have given autoclaved products wide application perspectives. Usually, commercialized, autoclaved construction products are prepared from limestone (e.g. quicklime and cement), siliceous materials (e.g. fly ash or quartz sand) and pressurized H2O at a temperature of 125–200°C. The autoclaved products from sand/quicklime (the main chemical component is SiO2) are mainly produced on the basis CaOSiO2-H2O (C-S-H) phase formation, whereas, when the starting raw materials are fly ash/quicklime (the main chemical components are SiO2 and Al2O3), the C-A-S-A-H as well as CaO-Al2O3-SiO2-H2O (C-A-S-H) phases are formed. Due to the presence of these phases (C-A-S-H and C-S-H) in the raw materials, various autoclaved materials can be obtained; for example, brick (Zhao et al. 2012; Li et al. 2021) or aerated concrete (Fan et al. 2014). Autoclaved lightweight aggregate was obtained from the mixture of propylene oxide, powdery quartz sand, portland cement and fly ash. Propylene oxide (PO) constitutes an essential chemical material, mainly used in the production of polyols polyether, propanediol and various non-ionic surfactants (Centea et al. 2015). After 24 hours of hardening at room temperature, granulated raw materials were autoclaved at 180°C under pressure for 8 hours and subsequently cooled to room temperature over 2 hours. The primary mineral components of the propylene oxide are calcium hydroxide Ca(OH)2 as well as calcium carbonate CaCO3. In the autoclaving process, when the hardening time and temperature of the hydrated calcium silicates are kept constant, phases are separated, which depends mainly on the ratio of Ca/Si. In turn, the aggregate strength is determined by the types as well as number of phases. High-strength tobermorite constitutes the main hydrated type of calcium silicate when the Ca/Si ratio is close to 0.83 (Ma et al. 2011a).

4  •  The production process of lightweight aggregates  57 The strength of the aggregates is primarily determined by the shell thickness, shell constituents, Ca/Si ratio and the quantity of powdered quartz sand. As a result, the dual effect of core and shell must be considered to obtain high-strength lightweight aggregates. The specific strength and the compressive strength of a cylinder both greatly increase along with the powdery quartz sand dosage. This can be related to the double effect that powdery quartz sand has on aggregate. Thus, dosing quartz sand in order to obtain high compressive strength should be considered. Sand acts as a reinforcing phase; therefore, the greater the amount, the greater the crack resistance. In addition, powdered quartz sand can react with the hydrated calcium silicate, thereby increasing the bond strength between the components. Compressive strength, and thus the specific strength, increase at a faster rate than the apparent density with increasing dosing of powdered quartz sand. In the case of autoclaved treatment, the formed phase type primarily depends on the hardening time, original ratio of Ca/Si and autoclaving temperature. The types and number of phases determine the strength of the aggregate core; moreover, the strength of mono-alkaline hydrated calcium silicate present in the raw material mass is higher than that of the dual alkaline. When the ratio of Ca/Si ranges from 0.70 to 0.96, tobermorite is the major product of hydration of the core of aggregate. Other minerals are mullite, α-quartz, calcite. Crystal intergrowth generated by them is characterized by multiple points of contact; hence, POSA exhibits high specific strength as well as compressive strength of the cylinder. When the ratio of Ca/Si amounts to 0.96 to 1.1, initially, the concentration of Ca2+ in the liquid phase is relatively higher, whereas the concentration of SiO2 in the liquid phase is relatively lower. Then, the dual alkaline C2-S-H(A) is firstly formed. As the ratio of Ca/Si increases, the glassy phase content, constituting an active component of the fly ash, decreases and the silica reactions are significantly reduced, thus hindering the C2-S-H(A) transformation into tobermorite. This results in an insufficient amount of tobermorite. However, extending the hardening time of the aggregates increases the amount of tobermorite. The reaction time for the conversion of a dual alkaline hydrous calcium silicate to a non-alkaline hydrous calcium silicate increases along with the Ca/Si ratio. However, excessive amounts of tobermorite are not advisable, as this may worsen the degree of operation of the equipment and, at the same time, the consumption of steam will increase. The ratio of CaO/SiO2 was thus determined as an experimental variable with a lower limit of 0.3 and an upper limit of 0.8. The crust layer and its thickness play govern the strength of the autoclaved aggregate; therefore, the thickness of the crust (the ratio of the mass of the crust and the core) as well as the crust component (the ratio of the mass of fly ash and cement) are determinants of the strength of the aggregates. At the initial stage of aggregate formation, the reaction that occurs between fly ash and calcium hydroxide in the core is slow, while the core strength is very low. Therefore, the early hardness of the aggregate surface can be improved by the coating. Secondly, it determines the strength of the coating itself. When it is too thick, it weakens the bond strength between the core and the shell – i.e. a thicker shell is unable to constrain the core and improve the core strength. Thus, the coating can effectively perform the core strengthening functions only when the components of the coating, the thickness of the coating, the core size and the core strength show a better fit with each other (Ma et al. 2011). The analysis of the literature showed that few researchers studied autoclaved lightweight aggregates containing the CaO-SiO2-H2O system of oxides in their chemical composition. Silicates show variable activity and influence on the reaction rate, which results in the formation of various mineral phases responsible for the properties of lightweight aggregates. Wang et al. (2014) obtained the lightweight aggregate using the autoclaving method with fine river sand, quartz tailings, cement and quicklime. The quartz waste in its chemical composition mainly contains silicon dioxide (SiO2). It constitutes a typical by-product which is generated in the glass industry in the course of hydraulic sorting and sieving of quartz sand. Mixed raw materials with water containing 20–25% wt. were sealed in the chamber for 4 hours in order to achieve full digestion of quicklime.

58  Application of Waste Materials in Lightweight Aggregates Then, the mixture was placed on the granulator disc, granules 5–16 mm in size were formed, which were hardened at room temperature 20°C for 24 hours; subsequently, it was placed in an autoclave. The cure temperature was increased to 195°C in 3 hours with an autoclave pressure amounting to 1.38 MPa. Afterwards, the temperature kept constant at 195°C for another 10 hours before being lowered to room temperature. Thus, the loose bulk density reached 1008–1087 kg/m3 while apparent density was 1598–1818 kg/m3. The absorbability of aggregates after 1 h is in the range of 7.1%–21.05%, and after 24 h, in the range of 13.77%–21.93%. When the CaO/SiO2 ratio is increased from 0.3 to 0.8, the compressive strength increases; afterwards, its slight decrease is observed. The optimal ratio of CaO/SiO2 equals 0.4. When the CaO/SiO2 ratio is low, less calcium silicate hydrates are generated in the course of the hydrothermal reaction between SiO2, Ca(OH)2 and H2O, which results in reduced strength. In turn, greater CaO/SiO2 ratio reduces quartz microaggregates, which also results in a reduction in strength (Wang et al. 2014). According to previous studies (Isu et al. 1995, Kikuma et al. 2011), fine particle size quartz is characterized by high reactivity and solubility, in comparison with coarse quartz. In turn, fine quartz forms tobermorite as well as C-A-S-A-H gel more easily. It has been found that quartz with a particle size lower than 5 µm is capable of forming C-A-S-A-H gel a tobermorite in 0.5 h. In contrast, quartz having particle size >10 µm is capable of promoting the formation of a C-A-S-A-H gel characterized by a high CaO/SiO2 ratio via continuous dissolution. Despite the relatively slow dissolution rate, the continuous conversion of the C-A-S-A-H gel to tobermorite is favoured, which is beneficial to the strength of the autoclaved aggregates (Isu et al. 1995).

4.5.1 Accelerated carbonation approach Carbon dioxide (CO2) constitutes the primary greenhouse gas of anthropogenic origin. The atmospheric concentration of CO2 has increased from the level of 280 ppm before the Industrial Revolution to 420 ppm in the year 2020 (Yuan 2020). The increasing amount of CO2 and other greenhouse gases (GHGs) in the atmosphere caused an increase in the mean global surface temperature of the Earth by about 2°C (Bodman et al. 2013). It is estimated that 40% of greenhouse gas emissions are connected with the CO2 concentration. The cement, ethylene oxide, metallurgy, biogas industries, coal-fired power plants and oil refineries are recognized as the main emission sources of this gas (Styring et al. 2011; Markewitz et al. 2012). Therefore, reducing the CO2 emissions is one of the priority environmental actions, especially taking into account the principles of sustainable development. The technologies of carbon capture and utilization (CCU) are a novel way of reducing the CO2 emissions to the atmosphere as well as achieving economic benefits by employing CO2 as a raw material in industry (Qiu et al. 2020). The accelerated carbonation technology is a method of obtaining lightweight aggregates and effective carbon dioxide sequestration. Accelerated carbonation may be employed to transform freshly prepared pellets into a solid compound of aggregates. If the concentration of CO2 is high, the strength development of pellets can be accelerated to some extent.

4.5.1.1 Reaction processes of accelerated carbonation The carbonation process takes place at different rates, which is determined by a number of aspects related to the structure and environment, the most important of which are the CO2 concentration in the air, humidity and temperature, and the chemical composition of the binders (Scrivener et al. 2018). Under natural conditions, the atmospheric CO2 may slowly infiltrate into materials and transform into CO32- if water is present, reacting further with alkaline products to yield carbonates as well as other substances. The natural process of carbonation takes place very slowly as a result of the formation of

4  •  The production process of lightweight aggregates  59

FIGURE 4.9  Reaction processes of accelerated carbonation treating RCAs (reprinted from publication Pu, Y., Li L., Wang, Q., Shi, X., Luan, Ch., Zhang, G., Fu, L., Abomohra, A. El-F., Accelerated carbonation technology for enhanced treatment of recycled concrete aggregates: A state-of-the-art review; Construction and Building Materials 282, 122671; Copyright (2021) with permission from Elsevier).

carbonate layers that hamper the further CO2 diffusion within the matrix. Thus, to accelerate it, special curing conditions are used in which the atmosphere contains a high concentration of CO2 (Liang et al. 2020). In the hardened concrete surface layer, natural carbonation takes place slowly (about 10–8 cm2/s) (Jiang and Ling 2020), whereas accelerated carbonation constitutes an accelerated natural carbonation process in which CO2 undergoes a reaction with fresh or hardened concrete under controlled carbonation conditions (Fernández-Bertos et al. 2004). Treating recycled concrete aggregates (RCA) comprises eight key steps, as shown in Figure 4.9. At the beginning, CO2 infiltrates into the loosely bound mortar via cracks or pores; then, it dissolves in the pore water, yielding carbonic acid in the process. Calcium ions that decompose from calcium silicate hydrates (C-S-H), calcium hydroxide (Ca(OH)2), dicalcium silicate (C2S), tricalcium silicate (C3S), non-hydrated cement clinker minerals and hydrated calcium aluminate phases (ettringite (AF)), as well as calcium sulfoaluminate hydrates, undergo a reaction with carbonate ions, forming silica gel and calcium carbonate (CaCO3) (Fernández-Bertos et al. 2004; Phung et al. 2015). Calcium carbonate, which constitutes a reaction product, constitutes a poorly soluble and thermodynamically stable compound, which eventually precipitates as vaterite, aragonite and calcite in the pores as well as crevices of the aggregate (Peter et al. 2008). In the course of the carbonization reaction, the outside of the artificial aggregate is carbonized gradually, whereas the inside remains substantially unchanged due to the calcium carbonate precipitation, which hinders the carbon dioxide penetration. Because the volume and hardness of the solid calcium carbonate phase are greater than those of calcium hydrate silicate or calcium hydroxide, porosity is reduced and the cracks are filled, thus improving the quality characterizing the artificial aggregate (Fang and Chang 2015). In addition, the carbonation processes constitute highly exothermic reactions (Fernández-Bertos et al. 2004) – i.e. these reactions are favourable as a result of the low energy consumption. Calcium hydroxide carbonation Due to the fact that the solubility of calcium hydroxide in water is the greatest out of calcium compounds (Thiery et al. 2007), it undergoes a reaction with CO2 most easily (equations (4.1) and (4.2)), resulting in the calcium carbonate precipitation in fissures and pores (Castellote and Andrade 2008). CO2 + H2O → H2CO3(4.1) Ca(OH)2 + H2CO3 → CaCO3 + 2H2O(4.2)

60

Application of Waste Materials in Lightweight Aggregates

Carbon dioxide penetrates into the pore water and dissolves in it, forming carbonic acid (Eq. (4.1)) (Johannesson and Utgenannt 2001) that reacts with Ca(OH)2, forming calcium carbonate precipitate, due to its low solubility (Eq. (4.2)) (Zivica and Bajza 2001). Such an irreversible reaction leads to a decrease in the calcium ions concentration in the pore solution, contributing to further calcium hydroxide decomposition (Garrabrants et al. 2004). Calcium hydroxide dissolution is also dependent upon the temperature found inside the carbonation chamber (Lekakh et al. 2008). In addition, it is worth noting that CO2 permeation is a parameter controlling the rate of calcium hydroxide carbonation (Castellote and Andrade 2008). In the course of the reaction, the surface of calcium hydroxide crystals becomes covered with a thin calcium carbonate layer, which inhibits further carbon dioxide penetration, and the rate of carbonation is gradually slowing down (Cizer et al. 2012; Galan et al. 2015). In the course of the reaction, the type of calcium carbonate polymorph depends primarily on the dominance thermodynamic and kinetic factors (Arandigoyen et al. 2006). In the case the kinetic factors prevail, then calcium carbonate precipitates first as vaterite or aragonite, eventually transforming into calcite that constitutes a more stable polymorph. In contrast, if thermodynamic factors dominate the reaction process, calcium carbonate will directly precipitate in the form of calcite. On the basis of the equation (4.2), following accelerated carbonation, the volume of artificial aggregate increases by 11.8% (Zhang et al. 2015b). Calcium silicate hydrates carbonation Many studies have shown that calcium silicate hydrates (C-S-H) can also react with CO2 (Suzuki et al. 1985; Goto et al. 1995). The amorphous C-S-H gel has the largest share in cement hydration products (approx. 70%) (Zhang et al. 2015b). The carbonation of C-S-H occurs when the majority of calcium hydroxide is consumed, whereas the reaction rate of C-S-H finally exceeds that of Ca(OH)2 (Jang et al. 2016). C-S-H carbonation leads to the formation of amorphous silica gel and calcium carbonate crystals (equation (4.3)) (Morandeau et al. 2014). xCaO·ySiO2·zH2O + xCO2 → xCaCO3 + y(SiO2·tH2O) + (z-yt)H2O

(4.3)

The degree of carbonation largely dependent upon the initial ratio of Ca/Si in the C-S-H phase (Black et al. 2007). As the C-S-H phase carbonates, the ratio of Ca/Si decreases and the phase becomes porous, resembling amorphous silica (Black et al. 2008). The rate of decomposition of C-S-H increases as the Ca/ Si relationship decreases (Sevelsted et al. 2015). This is not the case with the carbonation of other calcium compounds – e.g. ettringite and calcium hydroxide, which are essentially not affected by the concentration of CO2 during the carbonation process. Overall, C-S-H carbonation produces calcite, vaterite and aragonite (Groves et al. 2005; Morandeau et al. 2015). While the molar volume characterizing the non-carbonized C-S-H is greater than that of silica gel, calcium carbonate precipitation may compensate for the loss in volume. Therefore, the total volume of aggregates remains essentially unchanged (Morandeau et al. 2014). Carbonation of other substances Cement clinker minerals (Castellote and Andrade 2008) – e.g. dicalcium silicate (C2S), tricalcium silicate (C3S), tetracalcium aluminoferrite (C4AF), tricalcium aluminate (C3A) as well as tetracalcium aluminate (C4A) – are also susceptible to carbonation. In most cases, carbonation occurs before the completion of the hydration reaction. Due to the carbonation reaction between C3S, C2S and CO2, calcite and calcium silicate hydrates can be formed at an early stage, which eventually convert to silicate gel and calcite as indicated in the equations (4.4) and (4.5) (Young et al. 2006; Jang and Lee 2016): 3CaO·SiO2 + 3CO2 + nH2O → SiO2 NH2O + 3CaCO3 2CaO·SiO2 + 2CO2 + nH2O → SiO2 NH2O + 2CaCO3

(4.4) (4.5)

4  •  The production process of lightweight aggregates  61 It was found that ettringite as well as other aluminates may also be carbonated effectively at relatively low concentration of carbon dioxide, besides calcium hydroxide (Hyvert et al. 2010). Ettringite decomposes by reacting with carbon dioxide to form aluminum gel and gypsum, as shown in the equation (4.6): 3CaO·Al2O3·3CaSO4·32H2O + 3CO2 → 3CaCO3 +3(CaSO4·2H2O) + Al2O3 xH2O + (26-x)H2O(4.6) On the condition of accelerated BA carbonation in a rotating drum batch reactor with an automated CO2 supply under the pressure close to atmospheric: • rotational speed and degree of reactor filling, • temperature in the reactor, • time, • gas pressure, • relative humidity, • gas flow rate, • CO2 concentration (Brück et al. 2018). There are four ways in which accelerated carbonation of cement-based materials may be carried out: • “Standard Carbonation Method”, involving a temperature of 20±2°C, relative humidity of 70±5%, as well as CO2 concentration of 20%±3%, • “Pressurized Carbonation Method”, consisting in a temperature of 25±2°C and relative humidity of 50±5%. This method has been employed by numerous authors, achieving pressures of 0.1, 1.48 and 4 atm CO2, respectively (Shi and Wu 2008; Monkman and Shao 2010; Kou et al. 2014), • “Flow-through CO2 Curing Method” – in which a CO2 gas and air mixture is initially injected from one side of the chamber, followed by its release from the opposite side. In this method, the relative humidity value amounts to 50±5%, whereas the temperature approximates 23±2°C (Zhan et al. 2016), • “Water-CO2 Cooperative Curing Method” – in which samples are immersed in water, to which a mixture of N2, CO2, and O2 is subsequently injected (Ghacham et al. 2017). Accelerated carbonation was used to harden uncemented, artificial aggregates from the basic oxygen furnace slag (BOFS). The BOF process generates basic oxygen furnace slag, accounting for approximately 70% of the annual output of steel slag (Jiang et al. 2018). Low hydration reactivity and a high content of free calcium oxide (f-CaO) prevent the use of BOFS in cement-based building materials. Nevertheless, on the basis of the high CO2 reactivity characterizing BOFS from the high content of CaO (Ghouleh et al. 2017; Liu et al. 2016b), comprehensive studies pertaining to direct carbonation on BOFS were performed under varying processing conditions. BOFS carbonation enables to utilize the CO2-activated BOFS in the process of manufacturing cold-bonded aggregates, according to eqs. (4.7) and (4.8) (Bertos et al. 2004; Rostami et al. 2012): C3S + (3-x)CO2 + yH2O CxSHy + (3-x)CaCO3(4.7) C2S + (2-x)CO2 + yH2O CxSHy + (2-x)CaCO3(4.8) A customized pan granulator having a diameter of 700 mm and a flange height of 200 mm, with a scraper attached to the bottom surface as well as a charging hopper, was employed for granulation. The rotation speed of the disc was 15 rpm with a 45° angle of inclination. The dry BOFS powder

62  Application of Waste Materials in Lightweight Aggregates was wetted with a sufficient amount of water and nuclei were formed, which grew by coalescence or layering. To determine the influence of CO2 on the process of BOFS granulation as well as the properties of the aggregate produced, two carbonation methods were used: synchronous carbonation CC (used during and after granulation) and post carbonation AC (only after granulation). To compare the properties of the aggregates, representative aggregates were also produced as a result of the granulation and hardening process in atmospheric air AA (without carbonization). During synchronous carbonation CC, a stream of 99.9% CO2 was introduced into the granulator at atmospheric pressure, while post-carbonation was carried out under controlled conditions (temperature = 20°C, relative humidity = 65%, CO2 concentration = 20%). Post carbonation AC is an effective method of improving the properties of aggregates. Their strength, compared to the control samples, increases by 220%, reaching the optimal value of 5.24 MPa after 14 days. The control samples of aggregates cured in atmospheric air (without carbonation) and aggregates carbonated only after AC granulation are dark grey and have unevenly spaced bumps caused by insufficient layering in the agglomeration process. Their loose bulk density is similar and in the range of 1250 kg/m3–1280 kg/m3, whereas water absorption is about 11–12%. A two-layer structure was observed, with the absorption of CO2 in the shell and core being 10.99% and 7.38%, respectively. In the area of the aggregate core, a fibrous C-S-H phase and large, hexagonal portlandite (CH) crystals were observed. The aggregate shell was composed of rhombohedral calcite as well as aragonite. As a result of the formation of carbonate minerals, the microhardness value was 40% higher. Synchronized carbonation CC is a promising method which can be used to obtain only lightweight aggregates. By combining synchronized and post-carbonation, the aggregate with a loose bulk density was obtained that was much lower and amounted to about 900 kg/m3, meeting the requirements for lightweight aggregates. Water absorption was also higher, reaching 27% in wt. The aggregate obtained a low strength of 0.55 MPa after 14 days. Such a large difference between CC and AC resulted from the granulation of CO2, which influenced the microstructure development. The compact and dense structure was formed by the hydration products in the form of the C-S-H phase. After carbonation, the presence of aragonite and calcite was detected. The sizes of the obtained granules are much smaller compared to the aggregates obtained by using the post carbonation method. About 86 wt.% are aggregates with a diameter of less than 9.5 mm. Their surface is smooth, which was influenced by the longer granulation time, and thus, the appropriate layering of the material. The disadvantage of the method, however, is the release of a significant amount of heat during the granulation process, as a result of which a large amount of bridging water is evaporated, resulting in unfilled voids. The type of method influences the CO2 absorption. The main differences characterizing post-­ carbonation and synchronous carbonation are duration, CO2 concentration and temperature. Synchronized carbonation is carried out under the gradually self-heating conditions at ∼ 100% CO2 concentration and time approx. 30 minutes, while in the post carbonation method, CO2 is kept constant at 20% concentration and temperature of 20°C for a 4-day period. Temperature changes affect the growth rate of reaction products, CO2 solubility, calcium dissolution rate as well as nucleation (Yadav and Mehra 2017), while prolonged exposure to CO2 and greater concentration of CO2 enable higher conversion of reactive minerals to calcium carbonate (Castellote et al. 2009). The AC method obtained the CO2 absorption at the level of 10.03%, while the CC method is more effective and shows absorption at the level of 15.70% (Jiang and Ling 2020). Shi et al. (2019) employed post-carbonation in production of aggregates. The aggregates made of cement powder and CO2 cured concrete waste have been found to have higher crushing strength compared to normally cured aggregates. As a result of action CO2 on cement materials initially forms silicate gel, C-S-H and calcite. The carbon dioxide CO2 reacts with the products of cement hydration as well as other phases found in the non-hydrated cement. In these reactions, CO2 binds to the man-made aggregate, which leads to carbon sequestration. In turn, the water absorption of the artificial aggregate

4  •  The production process of lightweight aggregates  63 following 24 hours of CO2 curing was much lower (by an average of 19.2%) than the value obtained in the case of normally cured aggregates. This may stem from the fact that when CO2 cures, various carbonation products are formed that favourably fill the pores within artificial aggregates, decreasing the intrinsic porosity. In the course of the carbonation process, numerous calcium carbonate crystals were formed following the reaction between C-H and CO2, which form a compact microstructure when deposited in the pores. Thus, the total porosity was reduced, broke the connections between the pores were broken and the absorption rate of artificial aggregates was lowered (Shi et al. 2019). Treatment by Accelerated Carbonation Technology (ACT) may be employed in the case of calcium- and magnesium-rich thermal residues; for instance, paper incineration and municipal ashes, steel slags, pulverized fuel ashes, wood ashes, etc. (Li et al. 2007; Fernández-Bertos et al. 2004; Johnson 2000). Carbonization was carried out under controlled conditions via waste exposure to CO2 at an increased concentration. Combination of ACT with the method of producing agglomerates from waste such as biomass ash, cement kiln/bypass dusts, municipal solid waste incineration bottom and fly ashes, sewage sludge ash, pulverized fuel ash, paper wastewater sludge incineration ash and wood ash enables the creation of lightweight aggregates (Gunning et al. 2008; Padfield et al. 2004). The waste that reacts significantly with CO2 is defined as having absorbed 5% or more carbon dioxide by dry weight. The aforementioned materials had the potential to self-cement as well as bind the nonreactive quarry fines into pellet from. Portland cement was mixed with quarry fines as reference binder. The latter were added in an amount of 10, 20, 30, 40 and 50% on a dry basis, followed by water addition and mixing until a firm consistency was obtained. The material was pelletized in a drum pelletizer at ambient pressure and temperature under a carbon dioxide gas flow. After pelletizing, the aggregates were kept in a curing chamber for 72 h under a dry carbon dioxide flow. Dry CO2 aids in overcoming the adverse effect of the saturated pore network resulting from agglomeration. Carbonation ceases with material saturation and only proceeds when it has dried sufficiently to enable the infiltration of CO2 (Cultrone et al. 2005). On an industrial scale, a pilot plant was also implemented, which can produce 100 kg of aggregates per hour (Figure 4.10). Water, thermal residue binder and quarry fines, were pre-mixed in a 50 L mixer and transported to a rotary carbonation reactor. Afterwards, they were discharged into a 1100 L curing chamber circulated for 7 days with dehumidified CO2. Dehumidifying the CO2 enables its circulation in a closed loop, without its wastage by release into the atmosphere. The produced aggregate was characterized by a bulk density under 1000 kg/m 3 as well as high absorption capacity. The crushing strength of the aggregate was 30% to 90% higher than that available on the market in the UK. The concrete blocks comprising carbonized aggregate obtain the compressive strength of 24 MPa. Carbonated aggregate blocks would be ideal as an alternative

FIGURE 4.10  Scheme of carbonated aggregate production process (reprinted from publication Ren, P., Ling, T.-Ch., Mo, K.H., Recent advances in artificial aggregate production, Journal of Cleaner Production 291, 125215, Copyright (2021) with permission from Elsevier).

64  Application of Waste Materials in Lightweight Aggregates to foamed concrete for building internal partition walls. The accelerated carbonated aggregate product was used for green roofs, which are covered with plants, providing better aesthetics and insulation properties of the building as well as great environmental benefits. The low-energy coldbonding method reduces energy consumption and greenhouse gas emissions to the atmosphere (Gunning et al. 2009). Artificial aggregate in the carbonation process was also obtained from steel slag, which is an alkaline residue in the metallurgical industry produced during the refining of iron. The worldwide steel production currently stands at 1.3 billion tonnes per year, producing around 400 million tonnes of “steel” and “iron” slag by-products (Kuwahara and Yamashita 2013). Iron slag is widely used in construction; in contrast, steel slag is recycled to a limited extent. It comes from a later stage of steel refining, and little reuse is partially since there are not hightemperature, hydraulic calcium-silicate phases (Shi and Qian 2000). The main reason, however, is the large free content of magnesium oxide (MgO) and lime (CaO), which cause adverse expansive effects as well as volumetric instability (Geiseler 1996). Powdered steel the slag was mixed with water. The water-to-slag ratio (w/s) was 0.05, 0.10, 0.15, 0.20 and 0.25. The wetted powder was then compacted to obtain individual cylindrical specimens 15 mm in diameter by means of a stainless-steel mould as well as a uniaxial load amounting to 16 MPa. Immediately after forming the aggregates, they were placed in an airtight tank, which was exposed to CO2 (pressure 1.5 bar) at various times (2, 4, 12 and 24 hours). High reactivity of steel slag has been proven relative to CO2. After 2 hours of carbonation, the aggregate strength increased significantly. The average compressive strength was 81.1 MPa, whereas the CO2 absorption was 14%. Extending the carbonation to 4, 12 and 24 hours resulted in only a minor increase in the average strength. The reaction rate was the highest during the first 2 hours of carbonization. After carbonization, the aggregate structure is compact and less porous. The granules showed excellent freeze-thaw resistance. Slag particles were consumed during the process to produce C-S-H and CaCO3, which simultaneously nucleate at the sites previously occupied by the pore fluid. Steel gel is particularly rich in dicalcium silicates (C2S) and may react rapidly with CO2, forming CaCO3 and calcium silicate bicarbonate (C-S-H), as generally shown in equation (4.9): Ca2SiO4 + (2-x)CO2 + yH2O → xCaO·SiO2·yH2O [C-S – H] + (2-x)CaCO3(4.9) The C-S-H phase formed during cement hydration and is common in hardened concrete. It contributes to the increase of strength and other physical and mechanical changes in concrete. It is largely governs the binding properties characterizing the hydrated cement (Chatterji 1995). The C-S-H production using steel slag by carbonation is a prospective valourization opportunity for binder applications. The use of steel slag activated with carbon dioxide is an effective method of waste disposal, resource conservation, CO2 utilization and sequestration and may be employed to obtain artificial carbonate aggregates that can be used for concrete. The slabs of concrete prepared by means of slag granules as an aggregate substitute obtained the compressive strength similar to the concrete prepared from commercial granite aggregates as well as Litex lightweight aggregates (Ghouleh et al. 2017). In accelerated carbonation, it is possible to permanently store CO2 within the aggregate, whereas the use of hydraulic materials such as cement in the raw material composition is not required. Therefore, low-grade waste – e.g. steel slag, waste cement, incinerated ashes, including paper ash and wood ash – may be fully used for the production of carbonized aggregates (Gunning et al. 2009; Jiang and Ling 2020; Tang et al. 2019). The value-added application is unique, being aimed at green concrete development.

4  •  The production process of lightweight aggregates  65

4.6  MICROWAVE RADIATION APPROACH 4.6.1  The basics – theoretical The microwave technology uses microwave radiation, which is electromagnetic radiation that occurs between infrared radiation as well as ultra-short waves, characterized by a wavelength in the range of 1 mm–1 m that corresponds to a frequency 300 MHz–300 GHz. In this part of the electromagnetic spectrum, there are frequencies used for medical, scientific and industrial purposes. The frequencies available or industrial processing of various materials are as follows: 0.915 GHz, 2.45 GHz, 5.8 GHz and 24.124 GHz (Komarov 2012). The energy of radiation is transferred in the form of specific portions called quanta. A single quantum has energy proportional to the frequency of the radiation (wave). The effect of electromagnetic radiation on bodies depends on the amount of energy transferred by energy quanta (depends on frequency or wavelength). The energy carried by the quanta of microwave radiation is too small to be absorbed by electrons; therefore, it does not excite them and thus knock them out of the atom. The influence of microwave radiation on particles does not contribute to the changes in their structure. It is related to the dielectric properties of the medium. In the case of microwave radiation, the low energy of the radiation quanta cause the interaction with the body to take place at the molecular level; moreover, it is based on the interaction between the particles and electromagnetic field. The influence of the electromagnetic field at the microwave frequency, which increases the temperature of a given body, is related to its dielectric properties; namely, the ability to conduct electricity. Placing a dielectric in an external electric field leads to its polarization. If the frequencies are higher, permanent dipoles cannot keep up with the field by making halfturns. If the frequencies are high, the rotation of the dipoles decreases to zero; their orientation disappears; therefore, their dielectric permittivity decreases (Zieliński 2019). Supporting chemical syntheses/reactions with wave radiation mainly consists in “microwave dielectric heating”, resulting from the capacity of a reagent to absorb the energy assigned to microwave radiation as well as convert it into thermal energy (Kumar et al. 2020). Microwave heating, induced by the electrical component of electromagnetic radiation, may occur in two ways: via ionic conductivity and rotation of dipole molecules. Rotation of dipole molecule is the phenomenon of dielectric losses, primarily caused by the orientation (dipole) polarization. If dipole particles are present in the material, they attempt to align themselves with the sense and direction of the alternating electromagnetic field, making them move. When rotating, the microwave radiation energy is converted into kinetic energy, transmitted via the colliding particles. The result is even heat distribution in the heated material (Jones et al. 2002; Motasemi and Afzal 2013). The second mechanism of microwave radiation absorption is based on ionic conductivity. The ions present in the material move in the direction of an alternating electric field. Migrating ions collide with the ions that move in the opposite direction, causing a thermal effect the magnitude of which increases with the mobility and concentration of ions (Meredith 1997). Generally, the greater the induced polarity, the stronger the influence of microwave is, indicating volumetric, uniform and selective heating characteristics of microwave irradiation (Fernández and Menéndez 2011). The process of heating the sample begins from its inside, owing to which the final energy is reached, and it also enables the synthesis of a material with a more uniform, porous structure (Jones et al. 2002; Motasemi and Afzal 2013).

66  Application of Waste Materials in Lightweight Aggregates

FIGURE 4.11  Physical modelling of microwave sintering in stages (reprinted from publication Rybakov, K.I., Olevsky, E.A., Krikun, E.V., Microwave sintering: fundamentals and modeling. Journal of American Ceramic Society 96, 1003–1020, Copyright (2021) with permission from Elsevier).

The dielectric interactions between microwaves and materials are described using such parameters as depth of microwave penetration (D) and absorbed power (P). The average absorbed power – i.e. volumetric absorption of microwave energy – is dependent upon the “loss tangent”, indicating the material ability to be heated and polarized. In turn, the material ability to transfer microwave energy into heat is measured by the loss factor, whereas the material ability to be polarized is measured via the dielectric constant. Microwave provides rapid and volumetric heating, as well as conducting; thus, highly homogeneous temperature distribution into the material is achieved (Bykov et al. 2001; Mishra and Sharma 2016). The use of microwave energy for the heat treatment of materials is a well-known idea, which involves depositing the energy directly in the product as well as possibly creating uniform temperature distribution. Volumetric heating contributes to the processing time minimization, improvement of the diffusion rate and lowering of the power consumption (Leonelli et al. 2008). Heat is obtained via conversion from the electromagnetic energy in the material. Therefore, the external surface of the materials will directly receive the heat produced from the sample core, thus providing uniform and selective heating (Singh et al. 2015). Microwave sintering constitutes an interdisciplinary issue combining the methods from a few areas of physics as well as materials science. Creation of a comprehensive microwave sintering process model (Figure 4.11) would allow for the spatial distribution and temporal evolution of temperature, the electromagnetic field in the processed material as well as the variables reflecting the occurrence of processes in the material, including grain size, porosity, mass flux and stress. Generally, these variables depend on changes in temperature; however, they may be affected by direct (non-thermal) electromagnetic field effects as well. The earlier-mentioned recommendations are interrelated and, moreover, generally require extending the model space well beyond the powder object to be sintered. For instance, the electromagnetic field distribution in an object is generally dependent upon: • effective magnetic as well as dielectric properties of the material, which are dependent on temperature; structural variables – e.g. porosity, • properties and dimensions of the applicator in which the processing takes place, • matching conditions between the applicator, transmission line, and microwave source.

4  •  The production process of lightweight aggregates  67 In a similar manner, the temperature distribution in the heated material is determined via: • electromagnetic field distribution, • heat capacity, thermal conductivity of the material (depending on porosity and temperature) and effective absorption properties, • heat dissipation conditions, which may include thermal insulation properties, material emissivity, convection, etc. The following interrelated factors may influence the properties of the sintered material, including shape and overall compaction: • • • •

temperature distribution within the material as well as its evolution during sintering stress tensor components in the object, spatial distribution of porosity and average particle size, electromagnetic field structure, provided that its direct, non-thermal influence on sintering is taken into account (Rybakov et al. 2013).

4.6.2 Microwave generators Microwave ovens are used to sinter the samples, consisting of three main elements: an applicator, transmission lines and the source. A microwave source in the form of vacuum tubes produces electromagnetic radiation; then, the transmission lines transmit the electromagnetic energy from the source to the applicator, in which energy is reflected or absorbed by the material. Certain vacuum tubes which have been employed for microwave heating involve traveling wave tubes (TWT), klystrons and magnetrons. Being mass-produced, magnetrons are the cheapest and most readily-available source. Magnetron tubes apply resonant structures for electromagnetic field generation; hence, they can only generate an electromagnetic field with constant frequency. In the microwaves with variable frequency of the electromagnetic field, the TWT design allows for the amplification of a wide range of microwave frequencies within the same tube. The microwave source energy is coupled to the applicator via transmission lines. In the case of low-power systems, coaxial cables are often used as transmission lines. At high power output and frequencies, the losses occurring in coaxial cables are high, and waveguides, constituting hollow tubes in which electromagnetic waves propagate, are frequently employed as the preferred transmission lines in microwave heating systems. In waveguides, there are two possible modes of microwave propagation: transverse magnetic and transverse electrical. Apart from waveguides, several other transmission line components are applied for coupling microwaves to the material in the applicator, detection purposes and device protection. These include tuners, directional couplers and circulators (Thostenson and Chou 1999). During microwave interaction, materials are frequently subjected to structural and physical changes affecting their dielectric properties. Therefore, the capacity of microwaves for heat generation varies during the process and can make it difficult to model and control the process. The design of microwave devices is especially vital, since the electromagnetic field is determined by the processing equipment. Recently, microwave radiation has attracted interest as a result of its potential application in different fields of modern industrial technologies and science, including concentration and extraction of samples, fusion and incineration of materials, drying and digestion samples, mineralization, radioactive and industrial waste remediation (Oda 1992), organic synthesis (Rao et al. 1999), thermal utilization

68  Application of Waste Materials in Lightweight Aggregates of waste (Fukasawa et al. 2018), production of metals, powder metallurgy (Mondal 2011), composite materials sorbents and ceramics (Clark et al. 2005; Yang et al. 2020; Pang et al. 2021). Many researchers have shown unexpected results of research into obtaining lightweight aggregates through the application of microwave radiation as an alternative source of energy.

4.6.3 Comparison of sintering and microwave heating In the conventional sintering process, high temperatures and high rates of heating the furnace are very often required. These furnaces employ a great number of refractory materials, fuel and expensive heating elements for achieving and maintaining high temperature over long periods of time. In conventional heating, heat is generated and transferred to samples of external heating elements via convection, conduction and radiation. The surface of the material is heated first and the heat will be transferred to the core when the temperature increases. Thus, internal stresses and temperature gradient are formed (Mishra and Sharma 2016) between the cold interior and the hot surface, leading to inhomogeneous microstructure and compact distortion. As far as convectional heating is concerned, the core is at lower temperature compared to the outer surface; in contrast, the core in microwave heating is at higher temperature, in comparison with the outer surface (Gupta and Wong 2008). Conventional sintering constitutes a relatively slow process and requires time to reach thermal equilibrium. In microwave sintering, the ramp rate may even reach 100°C/min, while the standard ramp rate in conventional sintering is 10°C/min (Ramesh et al. 2018). Owing to microwave radiation, the energy losses related to the simultaneous heating of the walls of the furnace or other, even largesized elements of the heating device or heat carriers are eliminated and the heating rate is quick. This process is characterized by a considerably fast start-up and stop. Due to the shortened firing time of the materials by up to 10 times or more, energy is saved (Ramesh et al. 2018). Therefore, the methods using microwaves significantly reduce energy consumption, especially compared to high-temperature processes in which heat loss increases dramatically along with process temperature. In addition, they decrease processing cost and the time required for process completion. Significantly lesser consumption of energy is because in the case of microwaves, the sample itself becomes the heat source. Consequently, effective thermal reduction of mass reduces the required energy input. Other advantages of this method include, first of all, even heat distribution in the heated material, unique microstructure and properties of the materials obtained, improved product performance, energy saving and high reaction selectivity (Sutton 1989; Rybakov et al. 2013; Yan and Li 2016). The energy in conventional heating systems is transmitted via radiation, convection and conduction. In the case of microwave heating, it is delivered through molecular interaction with the generated electromagnetic field directly to materials. Hence, the components or elements of the material become heated instantaneously and individually, overcoming the constraints resulting from the heat transfer properties characterizing the material. The method has many advantages over others, as it emits lesser amounts of hazardous gases, does not necessitate using high temperature as well as contributes to uniform heating and physical and mechanical properties of aggregates (El-Feky et al. 2020). Moreover, it shortens the time needed to build up strength, in comparison with the cold-bonding and autoclaving process. The results show that the produced aggregates have the physical and mechanical properties that are suitable for the design of lightweight concretes, both for insulation and construction purposes. The satisfactory results obtained from the experimental studies have also confirmed that microwave curing can replace conventional techniques for the realization of economic, energy-saving and environmentally friendly production of artificial aggregate (Hanif et al. 2021).

Influence of production parameters on properties of lightweight aggregates

5

5.1 INTRODUCTION The lightweight-aggregate production process affects their physical and mechanical properties. Raw materials used in cold-bonding technique are mostly low- and high-calcium fly ash, binders, water and chemical activators. In addition to ash, the raw materials used are bituminous and lignite, municipal solid waste incinerator fly ash, cement kiln dust, rice husk ash, wastewater treatment sludge. The binders added for the production of aggregates are usually bentonite, metakaolin, cement, glass powder, clay and chemical additives like Na2SiO3, NaOH, Na2SO4, NaOH, CaCl2, CaSO4, KCl, K2O3Si. The pelletization process of the aggregates depends on the gradation and humidity of the waste material, physicochemical and rheological properties, the type and amount of binder and activator used, the parameters of the granulation process – i.e. the angle of inclination and rotational speed of the granulator disc – temperature and time of the hardening process and volume of added water. The formation of granules occurs much earlier when the addition of any type of binder is increased. Generally, binders facilitate the granulation of aggregates but also determine the final pellet characteristics. They affect the green and dried strength of balls and fired strength of pellets, chemical, mineralogical composition and quality of fired pellets (Tajra et al. 2019a). In the sintering process, clays, loams and clay shales are usually used to obtain lightweight aggregate. The diversified chemical composition of clay and waste materials determines the expansion of lightweight aggregates. To obtain the optimal expansion of the aggregates, it is necessary to create a liquid phase with sufficient viscosity and the release of gases as a result of the decomposition of organic or mineral materials such as carbonates, oxides, hydrates and sulfates. Most of the minerals present in raw materials are transformed by thermal treatment. The high content of SiO2 and Al2O3 oxides in the raw materials ensures the formation of a vitreous phase of high viscosity and also causes the formation of a hard, vitrified outer layer of lightweight aggregate, favouring the increase of its strength and reduction of water absorption. To determine the suitability of clay raw materials for the production of sintered aggregate, the mass ratio of SiO2/Σflux (Σflux is the sum of Fe2O3 + MgO + CaO + Na2O + K2O), which should be greater than 2 and (SiO2 + Al2O3)/Σflux, which should range from 3.5 to 10. If the chemical composition of the raw materials is within the given ranges, the pellets expand. The appropriate chemical composition of raw materials for obtaining sintered aggregate should be within the “expandable” region or bloating area in the diagrams Riley’s and Cougny. On the basis of the chemical composition of the raw materials – in particular, the presence of SiO2, Al2O3 and flux oxides – the bulk density of lightweight aggregates can be assessed, which ranges from 0.3 to over 2.5 g/cm3. Therefore, the expansion DOI: 10.1201/9781003313090-569

70  Application of Waste Materials in Lightweight Aggregates of lightweight aggregates depends on the physical and chemical properties of the raw materials, physical and chemical properties of the sintering conditions of the green granules (i.e. temperature, time, sintering rate), the type of atmosphere in the furnace and the additives used. The properties of carbonated aggregates are influenced by the conditions during the granulation of raw materials – i.e. the angle and speed of the granulator disc, the amount of added water, cement and the hardening time of lightweight aggregates. Excessive or insufficient water can lead to a sharp drop in the carbonation degree. The size of the waste aggregate gradually increases along with speed and angle of the granulator disc. Depending on the type of the lightweight aggregate producing method from waste, their microand macrostructure, physical and mechanical properties differ in terms of their porosity, type and number of binders, granulation conditions and thermal treatment.

5.2  COLD BONDING METHOD 5.2.1 Effect of raw materials For the production of lightweight aggregates in cold-bonding technique, low- and high-calcium fly ashes, binders, water and chemical activators are most commonly used (Table 5.1). Fly ash (FA) is a readily available waste material which is produced in significant quantities worldwide. Class C fly ash is more effective in terms of development because it contains calciumaluminum-silicate glass, which is highly reactive (Siddique and Khan 2011). The glassy component of the fly ash reacts with Ca(OH)2 to form a hydration product that acts as a cementitious material (Malhotra and Mehta 1996). The properties of cold-bonded aggregate depend mainly on the matrix bonding strength caused by cement hydration and/or the pozzolanic reaction. Therefore, the reactivity of the raw materials used in the production process is considered to be a key factor in influencing the properties of cold-bonded aggregate. The silicon dioxide contained in raw material reacts with calcium hydroxide to form hydration products which make the denser and stronger cold-bonded aggregate (Gesoǧlu et al. 2012a). Apart from fly ashes, for production of aggregates, local industrial waste, including bituminous and lignite fly ash (Vasugi and Ramamurthy 2014a), wastewater treatment sludge (WTS) and desulfurization device sludge (DDS) (Ferone et al. 2013), granite quarry dust (GQD) (Harilal and

TABLE 5.1 Raw materials, binders, chemical additives, water content use in production lightweight aggregate by researchers (Tajra et al. 2019a) AUTHORS

RAW MATERIAL

BINDER

WATER CONTENT WT-%

ADDITIVES

Tajra et al. (2018)

Class-F fly ash

Ordinary Portland cement CEM I 42.5 R

12

expanded perlite

Güneyisi et al. (2015), Gesog˘lu et al. (2014)

Class-F fly ash

OPC

22

–

Reddy et al. (2016)

Class-C fly ash

CEM I 52.5 and lime

–

–

5 • Influence of production parameters on lightweight aggregates

AUTHORS

RAW MATERIAL

BINDER

WATER CONTENT WT-%

71

ADDITIVES

Videla and Martinez (2002)

Class-C fly ash

OPC, lime and Pozzolan cement

–

–

Geetha and Ramamurthy (2010a)

Class-F fly ash

Cement and lime

30–33

Ca(OH)2, Na2SO4

Kockal and Ozturan (2010, 2011))

Class-F fly ash

Ordinary Portland cement CEM I 42.5 R

22–25

–

Kockal and Ozturan (2011)

Class-F fly ash

Bentonite and glass powder

22–25

–

Terzic´ et al. (2015)

Class-F fly ash

Water glass

–

Vijay (2015)

Class-C fly ash, copper slag

Ordinary Portland cement and lime

49

–

Palma et al. (2015)

Automotive shredder fluff, fly ash

Ordinary Portland cement CEM I 32.5 R

92

–

Palma et al. (2015)

Class-F FA

lime

70

–

Vasugi and Ramamurthy (2014a)

Bituminous and lignite fly ash

Ordinary Portland cement and lime

28–33

Kumar et al. (2010)

Class-F fly ash

Ordinary Portland cement

30

–

Baykal and Döven (2000)

Class-F fly ash

CEM I 32.5 N and lime

29–33

–

Manikandan et al. (2009)

Class-F fly ash

Bentonite

31–35

–

Tangtermsirikul and Wijeyewickrema (2000)

Lignite fly ash

Ordinary Portland cement CEM I

–

–

Gesog˘lu et al. (2014)

Class-F fly ash

Ordinary Portland cement CEM I 42,5 R

18–20

–

Geetha and Ramamurthy (2010a)

Pulverized fly ash

Clay binders, Ordinary Portland cement and lime

25–33

Thomas and Harilal (2015)

Class-F fly ash and quarry dust

Ordinary Portland cement

–

–

Gesog˘lu et al. (2012a)

Class-F fly ash, ground granulated blast furnace slag (GGBFS)

Portland cement CEM I 42.5

18–20

–

Gesog˘lu et al. (2012b)

Ground granulated blast furnace slag (GGBFS)

Portland cement CEM I 42.5 R

22

–

Ca(OH)2

Ca(OH)2, Na2SO4

Ca(OH)2

72

Application of Waste Materials in Lightweight Aggregates

Thomas 2013; Thomas and Harilal 2015), ground granulated blast furnace slag (GGBFS) (Gesoǧlu et al. 2012a, 2012b; Hwang and Tran 2015), rice husk ash (RHA) (Bui et al. 2012) municipal solid waste incinerator fly ash (MSWI) (Colangelo et al. 2015; Tang et al. 2017), cement kiln dust (CKD) and marble sludge (MS) (Colangelo and Cioffi 2013), paper sludge ash (PSA) and washing aggregate sludge (WAS) (Tang and Brouwers 2018) and fresh concrete slurry waste (CSW) (Tang et al. 2019) are used as well.

5.2.2 Effects of binders and additives The binders added to the raw material mixture are an essential element in the pelletization process, especially when raw material with low cementation properties is granulated. They affect the formation of additional hydration products like calcium silicate hydrate (C-S-H) and ettringite. As a result, the density and crushing strength of the aggregates increase with low water absorption. A binder with high, free lime content and low calcite content improves the engineering properties of lightweight aggregates (Colangelo and Cioffi 2013). The table shows the raw materials, binders and additives used by researchers for the production of lightweight aggregates using the cold-bonding method. Binders and chemical additives acting as activators are designed to: • • • • •

improve the efficiency and performance of pellets, maintain the appropriate size and shape of the aggregates until they harden, increase the granulation efficiency and the strength of granules, lower the water absorption of aggregates (Sengul et al. 2011), shorten the granulation process (Vasugi and Ramamurthy 2014a; Nor et al. 2016).

The most common binding materials used in the production of aggregates are bentonite, metakaolin, cement, glass powder and clay, Na2SiO3, NaOH (Vasugi and Ramamurthy 2014b; Güneyisi et al. 2015). The pelletization process depends on the production efficiency, gradation of raw materials, the type and content of the used binder, the angle of the granulator disc setting, the disc rotation speed and the pellet formation time. The formation of granules occurs much earlier with increasing addition of any type of binder. The physical and mechanical properties of the aggregates are definitely more favourable, along with the increase in the cement content in the raw material mixture, reaching the efficiency of up to 95% with the cement content of 20 wt %, the angle of inclination of the granulator disc of 40° and the rotational speed of 40 rpm. However, with the same wheel angle and speed but with a cement content of 5%, the efficiency drops to around 83%. Increasing the cement content helps to bind the fly ash particles efficiently. The main factor influencing the crushing resistance of aggregates is the cement content. For example, at a constant angle of 35° and a speed of 30 rpm, increasing the cement content from 5 to 20 wt % significantly increases the strength of the aggregates (from 0.34 to 2.04 MPa). This can be explained by the formation of calcium silicate hydrate (C-S-H) during cement hydration. Moreover, hydration results in the additional formation of about 15–25 wt % calcium hydroxide (CH) (Taylor 1998; Siddique and Khan 2011). Moreover, the addition of 14% bentonite as a binder increases the pelleting efficiency from 12% to 98%, while, for the same kaolinite content, only 68% increase was recorded, which was related to a lower plasticity index of kaolinite (35%) than bentonite (370%) (Manikandan and Ramamurthy 2007). The addition of 5% and 10% of cement to fly ash increases the strength of the aggregates by 27 and 37%, respectively. This is due to the formation of ettringite in addition to the hydration products in

5 • Influence of production parameters on lightweight aggregates

73

the form of the C-S-H phase (Narattha and Chaipanich 2018). The addition of cement and lime to the fly ash, which is a component of the aggregate initial mixture, also causes even a fourfold increase in the 28-day strength, compared to the control fly ash aggregates without the addition of binders (Baykal and Döven 2000). For waste materials, such ground granulated blast furnace slag (GGBS) and/or rice husk ash (RHA) and fly ash the alkaline activator solution (combination of sodium silicate and 10 M sodium hydroxide) was added by the pelletization process (Bui et al. 2012). As a result of alkaline activation, GGBS increases the precipitation of the C-S-H phase, which improves the geopolymer bonding and hardening as well as the strength of the aggregate (Kumar et al. 2010) Sodium sulfate (Na2SO4) accelerates the pozzolanic reaction of fly ash and the compaction of the structure of the aggregate produced, thus reducing water absorption and increasing the strength of cold-bonded aggregate. Alkaline activator caused increases of bulk density and 10% fines value of the aggregates compared to the aggregate with cement. The research on the influence of binders in the form of metakaolin, furnace slag and bentonite as well as an alkaline activator in the form of sodium hydroxide with concentration of 8, 10 and 12 M on the process of fly ash lightweight aggregates production showed that the production depends on the type and dosage of the binder in the pelletizer. Granules are formed in the granulator disc after just 7 minutes and become stable after 10 minutes. The maximum pellet production efficiency was achieved by using a bentonite binder at the level of 20% of fly ash mass with the addition of an alkaline activator (10 M NaOH). As the activator concentration increased (to 12 M), it was found that the formed granules became very sticky and cohesive. The furnace slag binder exhibits higher efficiency of the granulation process, compared to bentonite and metakaolin (Gomathi and Sivakumar 2014, 2015). Improvement of the mechanical properties of the aggregates was also obtained by combining the basic oxygen furnace (BOF) steel slag with a binder in the form of sodium silicate and sodium hydroxide solution, both in the granulation test and in the combined granulation-carbonization test. The granules produced with the use of an alkaline activator have an average size of 1 to 5 mm and appropriate mechanical properties, owing to which the aggregate can be used for applications in civil engineering (Morone et al. 2017). The addition of Ca(OH)2 and Na2SO4 to bituminous and lignite pond ash causes the physical properties of both types of aggregates, such as bulk density, water absorption, open porosity and strength, to significantly improve. This is due to the fact that Na2SO4 increases the concentration of SO42- ions, which enables the production of ettringite in higher amounts. The formation of ettringite increases the volume of the solid by 164%, and the formation of the C-S-H phase increases the volume by 17.5%, which considerably densifies the matrix structure (Vasugi and Ramamurthy 2014a). Owing to the surface treatment method, in which an alkaline solution (combination of water glass and sodium hydroxide with a molar concentration of SiO2/Na2O equal to 2.5) was sprayed onto the surface of the foamed cold-bonded aggregate in a pelletizer disc, this increased the unit weight and crushing strength by 4.5–6% compared to untreated aggregates (15–27%) with lower water absorption by about 2–33% (Hwang and Tran 2015). The double pelletization technique was carried out on the already-produced aggregate, which was covered with a dense outer coating consisting of cement/coal fly ash with 1:1 ratio. The water absorption of aggregates decreases by 12–18%, compared to the absorption of aggregates made only in one granulation stage (Colangelo et al. 2015). As a result of the surface treatment method consisting in spraying the aggregates with a mixture of cement and silica during the granulation process in the pelletizer disc, the water the absorption of aggregates is reduced by approximately 27%, while the particle density, loose bulk density and crushing strength are improved by approximately 13%, 11% and 14%, respectively, compared to untreated aggregate (Tajra et al. 2019b). The strength of the aggregates is positively influenced by special additives, such as crumb rubber (CR) and nanosilica (NS). In order to reduce the density of aggregates, it is possible to successfully

74  Application of Waste Materials in Lightweight Aggregates use hydrogen peroxide (HP) as a foaming agent (Hwang and Tran 2015) and expanded perlite powder (EPP) as a mineral admixture (Tajra et al. 2018).

5.2.3 Effects of production parameters on the granulation The factors influencing the physical and mechanical properties of fresh pellets are, first of all, the type and characteristics of the material used in the granulation process (humidity, degree of grinding, type of binder) and the conditions during the mechanical process, such as the angle of inclination and rotational speed of the granulator disc as well as the granulation time (Baykal and Döven 2000; Geetha and Ramamurthy 2010b; Kockal and Ozturan 2011). The granulation process is influenced both by the structure of the granulator – i.e. its diameter, length, rim height, angle of inclination – as well as the operating parameters – i.e. the filling factor, the frequency of rotation of the apparatus and the residence time of the material in the granulator. These factors influence the number of collisions of the particles that make up the granules. The greater their number, the greater the efficiency of the granulation process is. The granulator disc should therefore be set at a certain angle and with a certain rotational speed to avoid the domination of gravitational or centrifugal forces on the movement of the particles in the pellet mill (Tajra et al. 2018). In the granulation process, in addition to the design of the device and its operating parameters, the physicochemical and rheological properties of the granulated materials play an important role. The basic parameter is the internal friction coefficient, which affects the nature of the movement and mixing in granulating devices. Other parameters, such as the flowability or adhesive properties of powders, depend on the moisture content of the material, its particle size composition, density, shape and surface condition of the particles. Plasticity is a mechanical property that determines the ability of a material to resist deformation and destruction under the action of external forces (Kłassien and Griszajew 1989). This feature depends on its structure and dispersion and changes with humidity and temperature (Harikrishnan Ramamurthy 2006). The course of the granulation process is also influenced by the wettability of the material, which depends on the intensity of the interaction between the surface of the solid and the liquid. When the wetted particles contact each other, bridges are formed at the points of contact (Baykal and Döven 2000). Granulator speed is the main factor affecting strength and porosity, while humidity influences granule size (Harikrishnan and Ramamurthy 2006). The strength of fresh pellets depends on the combination of the cohesive force (surface tension generated by the height of the liquid column) and the blocking effects between the particles (generated by the existence of a surface texture), which is related to the water content of the pellets (Arslan and Baykal 2006). It was found that most of the pelletizer disc angles used were adjustable in the range of 35°~50° and rotational speeds of 35~55 rpm (Table 5.2). Aggregate with cement kiln dust, marble sludge and blast furnace slag, cold-bonded at different rotational speeds of the granulator disc (35, 45 and 55 rpm) and a constant angle of 50°, obtained the highest density and strength at a speed of 45 rpm (Colangelo and Cioffi 2013). Tajra et al. (2018), in their research, using three rotational speeds (20, 30 and 40 rpm) and the inclination of the disc of 35° and 40°, observed that at each constant angle of the disc, the produced aggregate showed a clear increase in particle density and bulk crushing strength with an increase in the speed of rotation. This confirms the thesis that increasing the rotational speed promotes densification of the cover matrix (Tajra et al. 2018). These observations were confirmed by Harikrishnan and Ramamurthy, who used granulator wheel speeds of 20–40 rpm and inclination angles of 40–70°. The rotational speed had a decisive influence on the strength and water absorption of the aggregates due to the formation of more compacted aggregates. The higher the rotational speed, the lower the water absorption of the aggregates (Harikrishnan and Ramamurthy 2006).

5  •  Influence of production parameters on lightweight aggregates  75 TABLE 5.2  Production parameters used in the literature. PELLETIZER DIMENSIONS

MECHANICAL PARAMETERS OF PELLETIZER

DIAMETER DEPTH (mm) (mm)

MATERIAL

ANGLE (⁰)

SPEED (RPM)

PELLETIZATION DURATION MINUTES

AUTHORS

Perlit, cement, fly ash

400

100

35 and 40

20, 30 and 40

15

Tajra et al. (2018)

Fly ash

400

150

43

45

20

Kockal and Ozturan (2010)

Fly ash

800

250

–

–

20

Gesog˘lu et al. (2014), Güneyisi et al. (2015)

Fly ash

560

250

55

55

–

Manikandan and Ramamurthy (2008)

Fly ash with cement

500

270

36

55

15

Gomathi and Sivakumar (2015)

Fly ash

570

250

40 and 70

20 and 40

10 and 20

Harikrishnan and Ramamurthy (2006)

Combined industrial solid wastes

1000

150

45

15

15

Tang and Brouwers (2018)

Cement kiln dust, marble sludge, blast furnace slag

900

–

50

35, 45, 55

–

Colangelo and Cioffi (2013)

Lignite fly ash, Portland cement CEM

1000

–

45

35–45

–

Tangtermsirikul and Wijeyewickrema (2000)

Class-F fly ash, water glass

600

300

40

35

20

Terzic´ et al. (2015)

Coal combustion wastes

800

–

50

45

–

Ferone et al. 2013

Class-F fly ash, Ordinary Portland cement CEM I 32.5 N and lime

400

–

35–50

35–55

20

Baykal and Döven (2000)

There is a dependency between the speed, pelletizer angle and diameter of disc. That is the critical revolution, which can be defined as a function of drum diameter and angle, using the following equation:

n cr =

42.3 D

sin a (5.1)

76

Application of Waste Materials in Lightweight Aggregates

where ncr is the critical revolution per minute (rpm), D is the diameter of the disc (m) and a is the inclination angle of the disc in degree (Baykal and Döven 2000). To obtain sufficient consolidation and granule size, it is important to use the correct agitation time. Studies by Baykal and Döven have shown that the formation of granules occurs within a time range of 6 to 9 minutes and the saturation point is reached after 20 minutes, after which the strength gain was only 2%, although the operating time of the granulator increased by 50% (Baykal and Döven 2000). These results are consistent with the majority, where the adopted granulation time does not exceed 20 minutes. The granulation efficiency also depends on the grain size of the raw material. The use of finer powders has a positive effect on the granulation efficiency and fresh properties pellets. Lower amounts of fresh pellets were obtained during pelletization of fly ash with a fineness of 287 m2/kg than when using fly ash with a fineness of 570 m2/kg (Gesoǧlu et al. 2012b). However, it should be noted that, when organic substances are involved in the granulation process, incompatibility between the materials usually leads to a reduction in granulation efficiency (Harikrishnan and Ramamurthy 2004).

5.2.4 Effect of water content and rotation speed on the granulation The amount of water, the type of raw material used – i.e. its properties – affect the effects of aggregate granulation. It happens that the water content in the granulation process plays a greater role than the rotational speed of the pelletizer disc and therefore should be controlled. It has a direct impact on the efficiency of the process as well as the properties of the obtained lightweight aggregates. The amount of water required in the production of aggregates also depends on the properties of the raw materials used. If too little water is added, it is difficult to obtain strong granules, while too much water results in slurry (Chiou et al. 2006). The amount of water to be used in the process must be predetermined with regard to the desired void ratio of the final product with respect to process efficiency. Moisture represents an optimal state or a capillary state. The most suitable state for pellet formation is the capillary state, in which all inter-grain spaces are completely filled with water and there is no water film on the pellet surface (Srb et al. 1988). Even small changes in optimal humidity lead to the destruction of the capillary force, which, in turn, causes large variations in the size and technical properties of the produced pellets (Baykal and Döven 2000; Harikrishnan and Ramamurthy 2006). It was found that, with the same parameters of the granulation process of different clays (rotation speed = 45°), uniform spheres or those with a large size distribution are formed. If the same type of clay is used and the same rotation speed of 45° but with a different amount of water (12% and 16%), lightweight aggregates with a significantly different fraction are obtained (Le et al. 2020). For difficulty of granulation analysis, the granulated fraction parameter can be used. It is calculated using the equation: Granulated fraction =

The amount of round ball larger than 0.9mm The amount of raw material + water added

(5.2)

Clay granulation is influenced not only by the process parameters but also by its chemical composition, mineral composition, powder properties, the volume surface mean diameter (VMD), reduction ratio, angle of repose and collapse angle. Among all the factors, VMD and the reduction ratio have

5 • Influence of production parameters on lightweight aggregates

77

the most significant influence on the obtained fraction in the granulation process. Thus, factors like the volume surface mean diameter (VMD) and reduction ratio can be used to control and optimize the granulation of different clays. If the raw material particle size is closer to the optimal particle size from the beginning of the pretreatment stage, the production efficiency will be improved (Li et al. 2020).

5.3 SINTERING METHOD 5.3.1 Effects of raw materials Clays, loams and shales are commonly used raw materials for the production of lightweight aggregate (Bernhardt et al. 2014b). Therefore, the raw materials for the production of lightweight aggregates differ significantly from each other in terms of chemical composition due to the presence of different minerals (Liu 2004; Shuguang et al. 2010; González-Corrochano et al. 2010). The mineral composition of clays from which lightweight aggregates are obtained includes clay minerals, such as smectite, illite, kaolinite, chlorite, and quartz, plagioclase, calcite and dolomite. Other phases associated with clay mineral such as muscovite and vermiculate have also been identified. The main chemical components of the raw materials are silica, alumina and iron oxides, but also CaO, which is derived from carbonate minerals. The content of Na2O and K2O is mainly attributed to clay minerals and feldspars (Fakhfakh et al. 2007). The dominant ingredient is silicon oxide and alumina, also present in silicates such as feldspar. Iron occurs in the form of Fe2O3 or FeO in chlorites, mica and other iron silicates as well as in pirite, marcasite and siderite. Calcium and magnesium oxides (CaO and MgO) are found in carbonates, silicates or gypsum. Sodium and potassium oxides (Na2O and K2O), on the other hand, are present in feldspars, in illite and montmorillonite (Handke 2005). In most expanded clays, the only gases responsible for expansion are carbon and sulfur dioxides. The dominant source of carbon dioxide is calcite; less often, dolomite and ankerite; while the source of sulphur dioxide may be pyrite or, in some cases, marcasite, which are present in the raw materials. The presence of organic matter also influences the expansion of the aggregates, as reducing the oxidation state of the iron Fe2O3 produces CO and CO2 (Decleer and Viaene 1993). Kaolinite minerals, rich in alumina, do not expand because the glassy phase is formed only after heating them to a temperature of 1400°C. Illites, montmorillonites and chlorites have the ability to expand because the high content of alkaline or alkaline earth elements promotes the formation of a glassy phase at 950 to 1050°C. These minerals also retain water up to a temperature where expansion usually occurs. Thus, two important processes take place in clay raw materials – i.e. the formation of a glassy phase in a wide temperature range and the release of gases resulting from the dissociation of minerals, such as carbonates, oxides, hydrates and sulphates. Most of the minerals present in raw materials are transformed by thermal treatment (Ayati et al. 2018). The amorphous phase may come from the opal phase present in the diatomite (Fragoulis et al. 2004) or from secondary materials (Soltan et al. 2016). Its amount increases along with sintering temperature (Liao and Huang 2011a). Of the three main representatives of the plagioclase group, the anorthite is assigned a high content of CaO, which remains after decomposition of carbonates. Calcium ions (Ca2+) are capable of substituting in the aluminosilicate matrix, and this enables the formation of an anorthite.

78

Application of Waste Materials in Lightweight Aggregates

Albite, the Na end-member of the plagioclase division, has also been reported to form in lightweight aggregate manufactured from reservoir sediment as a result of the addition of NaOH (Liao et al. 2013). The amorphous phase may come from the opal phase present in the diatomite (Fragoulis et al. 2004) or from secondary materials (Soltan et al. 2016). Its amount increases along with sintering temperature (Liao and Huang 2011a). Of the three main representatives of the plagioclase group, the anorthite is assigned a high content of CaO, which remains after decomposition of carbonates. Calcium ions (Ca2+) are capable of substituting in the aluminosilicate matrix, and this enables the formation of an anorthite. Albite, the Na end-member of the plagioclase division, has also been reported to form in lightweight aggregate manufactured from reservoir sediment as a result of the addition of NaOH (Liao et al. 2013). The main minerals found in lightweight aggregates are quartz, anorthite, silimanite, ringwoodite, hematite and mullite (Zhang and Gjörv 1990a), with mullite being the main phase responsible for strength (Tenorio et al. 2004; Huang 2006). Some minerals affect the physical properties of lightweight aggregates. The forsterite phase, formed as a result of sintering high-carbon ferro-chromium slags belonging to the olivine group of nesosilicates, affects endurance (Zhang et al. 2015b). Forsterite has poor thermal-shock resistance; therefore, its presence causes internal stresses during cooling, leading to microcracks, which reduces the compressive strength (González-Corrochano et al. 2011).

5.3.2 Factors affecting the lightweight aggregate expansion The factors influencing the expansion mechanism can be divided into two classes (Ozgüven and Gündüz 2012): (A)

(B)

Factors depending on the physical and chemical properties of the body: • chemical composition of the mass, • phase composition, including the amount of glassy phase, • viscosity of the glass phase at high temperature, • particle size distribution, • use of additives. Factors depending on sintering conditions: • temperature, • firing schedule, including time, pre-heating rate and duration, • the atmosphere in the oven, • furnace structure (design).

These factors ensure the presence of a sufficient amount of liquid phase to transform the body into a viscous (pyroplastic) state with a suitable viscosity in which the gas bubbles causing expansion will be separated and trapped. Taking these factors into account influences the obtaining of a product with appropriate density, shrinkage, mechanical strength and porosity. Sintering factors and product parameters after sintering can be related by mathematical relationships, which can be the basis for controlling the sintering technology. The role of technology is to define these operations and their technical parameters, which will allow for control and repeatability of the process to obtain a specific quality of the product.

5  •  Influence of production parameters on lightweight aggregates  79

5.3.2.1 Effect of SiO2 – Al2O3 – flux-ratio change on the expansion characteristics of lightweight aggregate The chemical composition determines the expansion of lightweight aggregates. In order to obtain optimal expansion of aggregates, two basic conditions must be met: • formation of a liquid phase with sufficient viscosity, • the release of gases from the decomposition of organic or mineral materials during the formation of a liquid phase (González-Corrochano et al. 2009a). Raw materials heated to the point of incipient fusion generate gases during sintering, which is characterized by the loss on ignition (LOI) parameter of the material. In order to explain the expansion mechanism of the raw materials (wastes) used to obtain aggregate, one should take into account not only their chemical composition but also a number of variables, such as the viscous behaviour at high temperatures, gas generation and process variables. According to the liquid sintering theory, the green body softens above the softening point and liquid phase appears under the influence of alkali metal oxides. At lower temperatures, only a small amount of the highly viscous liquid phase appears and a small volume of gas is released, which can lead to expansion and formation of small pores in the body. As the temperature rises, more of a lowviscosity liquid phase is formed and the produced gas causes an increase in volume under the influence of surface tension, which leads to the formation of large pores in the granules. However, when the surface tension between the gas and the liquid phase reaches a certain level at a certain temperature, gas is released in the body and interconnected pores are formed. The expansion behaviour of the samples is shown in Figure 5.1.

FIGURE 5.1  The expansion process in the granule (Pg: gas pressure. ɣl: surface tension) (reprinted from publication Jiang et al. 2019).

80  Application of Waste Materials in Lightweight Aggregates During this process, the most critical moment is the degree of match between the liquid phase and the gas emission. There is a variant balance between the gas pressure (Pg) and the surface tension of the liquid phase (ɣ1). In fact, ɣ1 is greater than Pg at the corresponding temperature, where an ideal pore structure is obtained. Consequently, the evolution of pore morphology indicates that there is a close correlation between sintering temperature and pore structure based on the degree of match between the liquid phase and gaseous emission (Jiang et al. 2019). Expansion of the aggregates occurs when the gases released during sintering are trapped by the silicate phase of appropriate viscosity. At high temperature, SiO2 and Al2O3 act to form a network, which has a positive effect on stickiness. The high content of SiO2 and Al2O3 in the raw materials ensures the formation of a vitreous phase of high viscosity and also causes the formation of a hard, vitrified outer layer of lightweight aggregate, favouring the increase of its strength and reduction of water absorption (Yue et al. 2011). Oxides SiO2 and Al2O3, present in the raw materials – compared to flux elements such as Fe2O3, Na2O, K2O, CaO and MgO – have a higher melting point. The higher the content of SiO2 and Al2O3, the higher the sintering and softening temperature is. To determine the suitability of clay raw materials towards the production of sintered aggregate, the weight ratio of SiO2/Σflux oxides is used; (Σflux is the sum of Fe2O3+MgO+CaO+Na2O+K2O), which should be greater than 2 and (SiO2+Al2O3)/Σflux, which should be in the range from 3.5 to 10 (Chen et al. 2010; de Gennaro et al. 2004; González-Corrochano et al. 2009a). If the chemical composition of the raw materials is in a given range, adequate expansion of the granules is obtained. To determine the expansion properties of raw materials, Riley (1951) and Cougny (Cougny 1990) proposed percentages of SiO2, Al2O3, Fe2O3 and other oxides such as CaO+MgO+Na 2O+K 2O (Table 5.3). The appropriate chemical composition of raw materials for obtaining sintered aggregate should be within the “expandable” region or bloating area in the diagrams (Figures 5.2, 5.3). Riley and Cougny diagrams are a kind of oxide diagrams in which one of the oxides corresponds to SiO2, Al2O3, Fe2O3 and (MgO+CaO+Na2O+K2O). When the metal is in different degrees of oxidation referred to oxygen, its oxides are treated as separate components (independent components) – e.g. FeO and Fe2O3. The raw materials that create air bubbles in the raw material mass causing them to expand are pyrite, hematite, calcite, dolomite, siderite, magnesite, pyrite with calcite and hematite with calcite. It has been shown that only 14 out of 52 raw materials expand well; the others have little or no expanse. The gas-producing compounds added also acted as fluxes. Usually, the silica and alumina/flux content ratio (Riley 1951) is the key, without identifying the most important swelling chemicals by their chemical composition. However, the proportion of TABLE 5.3  Chemical composition of raw materials according to Riley (Riley 1951) and Cougny (Cougny 1990). CHEMICAL COMPOSITIONS

(%)

DIAGRAM OF RILEY (%)

DIAGRAM OF COUGNY (%)

SiO2

48

48

(~ 50)

Al2O3

27

27

52

Fe2O3

13

∑25

25

Inne

12

Total

100

23 100

100

5  •  Influence of production parameters on lightweight aggregates  81

FIGURE 5.2  Riley diagram of bloating area (reprinted from publication Jiang et al. 2019)

FIGURE 5.3  Cougnay diagram of bloating area (reprinted from publication Wie and Lee 2019).

aluminum, iron and alkaline earth metal oxides, regardless of the silica content, is also considered to be a factor controlling aggregate swelling (Utley et al. 1965; Cougny 1990). The chemical composition of the raw material mixture must meet the requirement of at least 60% content of the glass-forming silicon oxide. In the case of a lower SiO2 content, the raw material package

82  Application of Waste Materials in Lightweight Aggregates must be modified by introducing an additional high silica component. In the case of a SiO2/Al2O3 ratio of at least 4, only an additional alkaline component is required. In order to obtain the optimal composition of the raw material composition formed at a temperature not higher than 900°C, the amount of the liquid phase should be at least 70% (Rattanachan and Lorprayoon 2005; Kažmina et al. 2009). The “chemical design” is still a popular procedure in lightweight aggregate formulations (Hung and Hwang 2007; de Gennaro et al. 2008). There are no standard procedures for assessing the expansion potential and firing behaviour of the ceramic, in addition to assessing the factors influencing the expansion process. However, it is known that the addition of various substances may increase the expansion of the aggregates during firing. Due to the use of waste materials to obtain aggregates, the raw materials, apart from the eight main components in the form of oxides included in the predictive schemes, may also contain significant amounts of elements that are not normally considered for clays and shales because they are present in trace amounts. These chemical components include P2O5, B2O3 and heavy metals (Ba, Cr, Mn, Pb, Sr, Zn). Phosphorus is present in sewage sludge (Bhatty and Reid 1989a; Lin 2006; Mun 2007), post-flotation sludge (Loutou et al. 2013) as well as some harbour and reservoir dredging sediments (Tay and Show 1992a; Wei et al. 2008; Tang 2014). Boron is present in mining residues (Kavas et al. 2011, Christogerou et al. 2014), whereas heavy metals are abundant in the waste from the metallurgical industry (Kim et al. 2005; Huang et al. 2007; Lee et al. 2007), sewage sludge and fly ash. As P2O5, B2O3 and heavy metals can have a significant influence on the technological behaviour of lightweight aggregates, the original Riley and Cougny parameters were modified to include these additional chemical components: • Al2O3 ⁄and fluxing⁄ for Riley, • Fe2O3 ⁄Al2O3 ⁄and (MgO+CaO⁄+Na2O+K2O+P2O5+B2O3) for Cougny where fluxing⁄ (Fe2O3 ⁄+MgO+CaO⁄+Na2O+K2O+B2O3+P2O5); Fe2O3 ⁄ stands for the sum of heavy metal oxides (Fe2O3, Cr2O3, MnO, PbO, ZnO); Al2O3 ⁄ is the sum of (Al2O3+TiO2); CaO⁄ means the sum of alkali-earth oxides (CaO+SrO+BaO). Other elements in the raw material mass are carbon, especially present in fly ash and other combustion residues (Kim et al. 2005; Aineto et al. 2005; González-Corrochano et al. 2009a; Ducman and Mirtič 2011), sulfur found mainly in municipal solid waste (Hwang et al. 2012; Tuan et al. 2013), chlorine present in marine sediments (Laursen et al. 2006; Wei et al. 2011; Wei et al. 2014) and in the fly ash from solid waste incineration plants (Chen et al. 2010; Hwang et al. 2012). However, in the manufacturing of lightweight aggregates, these elements volatilize partially or almost completely in the furnace atmosphere during firing at various temperatures.

5.3.2.2 Effects of sintering temperature The sintering temperature and time are factors that have a significant influence on the properties of the aggregate (Table 5.4). Fresh and wet granules should be oven dried (105~110°C) before sintering to prevent cracking and crushing due to uneven heating (Huang and Wang 2013). The sintering temperature influences the formation of the glassy phase, the production and the containment of the evolved gases. A simplified model of aggregate expanded in the sintering process in an industrial rotary kiln is shown in Figure 5.4. First, the granules are rapidly heated in the initial sintering stage, which causes the surface to vitrify. Then, in the zone of maximum temperature, the aggregate expands due to the entrapment of the evolved gas (resulting from the appropriate viscosity of the liquid). This process apparently follows the rate of sintering by increasing the pore diameter, and eventually, expansion. A schematic representation of these concepts was proposed by Cougny (Figure 5.5).

5  •  Influence of production parameters on lightweight aggregates  83 TABLE 5.4  Temperature and materials used during the sintering process by researchers. PRE-HEATING STAGE MATERIALS

TEMPERATURE (OC)

DURATION (MIN)

SINTERING STAGE TEMPERATURE (OC)

DURATION (MIN)

AUTHORS

Sludge, fly ash

300~450

–

1150~1225

–

GonzálezCorrochano et al. (2009a)

Sewage sludge

400

20

900~1100

10

Wang (2013)

Sludge, fly ash, clay

400

10

1100

12

Liu et al. (2013)

Sewage sludge

420

20

950~1080

15, 30, 45

Wang et al. (2008)

Sewage sludge, coal ash

420

20

1050~1100

30

Wang et al. (2009a)

Incineration fly ash, reaction ash

500

5, 10

1100~1200

10, 15

Chen et al. (2010)

Sludge

600

10

900~1100

10, 20

Deng and Li (2013)

Clay, sewage sludge

750~800

–

1050~1150

10, 15

Mun (2007)

Mining residues, heavy metal sludge, incineration fly ash

750~950

–

1050~1250

5, 15, 25

Huang et al. (2007)

FIGURE 5.4  Conditions for sintering lightweight aggregates (reprinted from publication Dondi, M., Cappelletti, P., D’Amore, M., de Gennaro, R., Graziano, S.F., Langella, A., Raimondo, M., Zanelli, Lightweight aggregates from waste materials: reappraisal of expansion behavior and prediction schemes for bloating; Construction and Building Materials 127, 394–409, Copyright (2016) with permission from Elsevier).

84

Application of Waste Materials in Lightweight Aggregates

FIGURE 5.5 Conditions necessary for lightweight aggregate expansion, modified after Cougny (reprinted from publication Dondi, M., Cappelletti, P., D’Amore, M., de Gennaro, R., Graziano, S.F., Langella, A., Raimondo, M., Zanelli, C., Lightweight aggregates from waste materials: reappraisal of expansion behavior and prediction schemes for bloating; Construction and Building Materials 127, 394–409, Copyright (2016) with permission from Elsevier).

This expansion model also works for lightweight aggregates containing waste, as confirmed by numerous studies (Aineto et al. 2005; Korat et al. 2013; Ducman et al. 2013; Velis et al. 2014). The pre-firing stage also plays an important role, as it prevents the deterioration of pellets quality under “flash heat” conditions and aggregate explosion during the final sintering process (Lo et al. 2016; Wang 2013). In the process of producing aggregate from high carbon fly ash at the pre-firing temperature of 830°C, it was shown that the amount of unburned carbon was completely reduced at this stage of sintering, and further sintering resulted in the release of energy in the form of heat, which allowed for its savings in the sintering process. The authors suggest different pre-firing times from 7.5–20 minutes. Longer process duration reduces expansibility and improves the homogeneity of pellets, which contributes to increasing particle density and reducing their water absorption (Huang and Wang 2013). The heating rate adopted during sintering has a significant role in the densification and contraction of the aggregate and therefore its properties (Adell et al. 2008). The rate of aggregate cooling is also important. The strength of the aggregates is about five times lower with rapid cooling due to the formation of microcracks (Zhang et al. 2015a). The conditions of the sintering process affect the properties of lightweight aggregate, primarily bulk density, strength and water absorption (Liao et al. 2011a).

5.3.2.3 Effect of additives Various types of aggregate additives and their specific dose are aimed at increasing the expansion of the aggregates and improving their mechanical properties. One of the aggregates additives is CaO, used as the main flux oxide on the properties of lightweight aggregate. CaO is considered as a glass-forming oxide that can promote the formation of a porous structure and, sometimes, to trigger a pozzolanic reaction (Liao et al. 2011a).

5  •  Influence of production parameters on lightweight aggregates  85 The addition of Na2CO3 is used to reduce the softening point of the glass phase, which is formed during the production of the aggregate. Na2O is a widely used flux in the production of soda-lime glass because of its ability to lower both the glassy temperature and the softening point (Shelby et al. 2004). Lowering these temperatures potentially promotes expansion at even lower temperatures, which saves energy and costs. Sodium carbonate melts at 850°C and decomposes into Na 2O and CO2 at roughly the same temperature (Kim and Lee 2001; Liao and Huang 2012; Bernhardt et al. 2014b). The addition of SiO2 to the aggregates may increase the viscosity of the vitreous phase, the formation of smaller pores and their homogeneous distribution in the volume of the granulate. It generally acts as a network former in glasses, thus increasing the glass viscosity and temperature (Tg) (Molinari et al. 2020). During the sintering process, the right amount of SiO2 in lightweight aggregates increases the formation of the liquid phase, closing the pores in the solid particles. As a result, the water adsorption of aggregates decreases with increasing amount of SiO2 additive. The appropriate amount (even 35%) can improve the strength of lightweight aggregates because, as a result of heating the raw materials, crystalline phases in the form of quartz and mullite may form and the structure becomes more compact (Cao et al. 2019). The addition of SiO2 in the form of quartz sand and powdered glass pestle (amorphous SiO2) has a positive effect on the sintering temperature, expansion and mechanical properties of various types of lightweight aggregates (Tsai et al. 2006; Fakhfakh et al. 2007; Bernhardt et al. 2014b). Another important additive in lightweight aggregates is iron, being one of the main components responsible for gas formation at elevated temperatures (Decleer and Viaene 1993). The glassy temperature (Tg) and the viscosity of the vitreous phase mainly depend on the oxidation state of the iron, which was observed in the case of iron-containing silicate alloys, where an increased amount of Fe3+ leads to an increase in both the viscosity and the glass transition temperature (Dingwell 1991; Liebske et al. 2003). Studies on the softening point of the matrix of clay-based lightweight aggregate show that, depending on the iron oxidation state, the softening points range from approximately 700°C for the reduced matrix phase (essentially Fe2+) to approximately 950°C for the oxidized phase matrix (essentially Fe3+) (Bernhardt et al. 2014a). Accordingly, depending on the oxidation state of the iron, the viscosity and softening points of the glassy phase may be increased or decreased, and the expansion may result from the presence of additional gas-forming components (iron oxide). Fe2O3 and metallic iron are used as additives to study the influence of the oxidation state on the iron properties of lightweight aggregates (Bernhardt et al. 2014b). (Decleer and Viaene 1993). Red colour indicates a large amount of Fe3+, a gray-green colouration indicates a large amount of Fe2+ and the black colour indicates an intermediate oxidation state. The differentiation in the degree of oxidation is a consequence of the strongly reducing atmosphere (mainly CO) inside the granule, caused by the carbothermal reduction of Fe2O3, and the increasingly oxidizing atmosphere towards the surface of the granule, caused by oxygen diffusing inside the granules as a result of burning in the air Bernhardt et al. 2014b).

5.3.2.4 Bloating agents: the role of evolved gases The expansion process is associated with the presence of gases, mainly oxygen and/or carbon oxides, which are trapped by the liquid phase generated at high temperature (Tsai et al. 2006; Kang and Lee 2010). These gases (e.g. CO2, CO, H2O, H2, O2, SO2 or Cl2) are released due to the transformation and decomposition of organic phases (e.g. organic matter) and/or minerals such as carbonates, sulfides, phyllosilicates, chlorides, ferrous minerals, water-glass, Fe2O3, MnO2 and SiC (Chopra et al. 1964; Ducman et al. 2002, 2013; Korat et al. 2013).

86  Application of Waste Materials in Lightweight Aggregates The development of porosity in the structure of the material depends on the following factors: • gas inflow that is the result of a chemical reaction and a heat stream, • pore creation and bonding by internal pressure, • transition of small bubbles into larger ones due to the pressure differences between them (Köse and Bayer 1982). Black Core Bloating Mechanism Typical lightweight aggregate is foamed by the black coring phenomenon caused by the reduction of Fe2O3. The oxidation of organic substances and carbon inside the aggregate creates a reducing atmosphere. The chemical reactions (5.3 to 5.5) of this phenomenon are as follows: Fe2O3 · FeO · Fe2O3 +

1 O2(5.3) 2

2 Fe2O3 + C →2 Fe3O4 + C 2 Fe3O4 + C → 3 FeO + CO

(5.4) (5.5)

CO gas produced in the reducing atmosphere increases the internal pressure. The resulting FeO also lowers the viscosity of the inside of aggregate to form a liquid phase and promotes viscous behaviour. The Fe2O3 reduction temperature depends on the oxygen partial pressure. When the oxygen partial pressure is normal, the Fe2O3 reduction temperature is 1400°C, while, when the oxygen partial pressure is low, the Fe2O3 reduction temperature is lowered to 1000°C. In turn, the high partial pressure of oxygen favours the reduction of Fe2O3 at 1400°C and not at the sintering temperature (1100–1200°C). Consequently, the black core bloating mechanism is not activated and Fe2O3 as well as carbon increase the sintering temperature and hinder expansion. In other words, controlling the speed of these reactions determines the expansion of the artificial lightweight aggregate. During rapid sintering, the surface is rapidly sealed due to the presence of the liquid phase and the carbon is burnt inside the aggregate with a low oxygen partial pressure. Therefore, even if the aggregate is obtained by fast sintering at high oxygen partial pressure, a black core is observed inside the aggregate. Finally, FeO is formed because the black core reaction lowers the melting point of the core and promotes viscous behaviour and the pores are intensely distributed inside the granules (Liao et al. 2013; Lee et al. 2019). The expansion process can be effective only when the amount of iron oxide Fe2O3 in organic matter exceeds 5% wt. in the batch (Ehlers 1958). This is important, as iron oxide is usually present in small amounts in many wastes. In his research, Lee described the effect of Fe2O3 and Fe3O4, the amounts of which ranged from 5 to 30% wt. in various wastes, such as the bottom-ash, reject-ash and dredged soil. The aggregates containing 10–15% of iron oxide show the lowest specific gravity, but the more iron oxide, the higher the bulk density due to sintering in liquid phase (Lee 2014). Various additives are also used to increase the expansion of the aggregates. Silicon carbide (SiC), which is obtained from the mud from polishing of ceramic tiles, is also an expanding additive to aggregates (Xi et al. 2012). The expansion mechanism seems to be related to the oxidation of SiC and the formation of CO and CO2 gases at high temperature: SiC + 2 O2 → SiO2 + CO2↑(5.6) 2 SiC + 3 O2 → 2 SiO2 + 2 CO↑(5.7) The expansion process taking place in the mixture comprising silica precipitate and fly ash with the addition of SiC as a foaming agent was tested at various temperatures and sintering times

5  •  Influence of production parameters on lightweight aggregates  87 (Korat et al. 2013). Fly ash contributes to the formation of a liquid phase, and SiC acts as a blowing agent, which decomposes at 1220°C, whereby the gases are trapped in the structure, creating porosity. The development of porosity in the presence of a liquid phase is related to the oxidation reaction (eq. 5.6). A mixture of natural zeolite rocks (clinoptilolite or chabazite) and SiC-containing sediments derived from polishing stoneware tiles were used for the production of lightweight aggregate. It turns out that the expansion is strongly dependent on the presence of SiC in industrial waste (de Gennaro et al. 2008). The expansion-supporting material is manganese oxide. Its addition in the amount of 3–7% by mass to waste glass and silica mud contributes to the decomposition of the oxide at high temperature, so that the released gases are also trapped inside the glass matrix. The expansion efficiency depends on the course of redox reactions occurring in the range of 500–1050°C, as a result of which the Mn 5O8, Mn 2O3, Mn3O 4 and MnO oxides are formed. This process implies the release of oxygen for each reduction reaction of Mn4+ to Mn 3+ and Mn 2+ with intermediate steps of mixed valence. Longer sintering times for green pellets favour expansion (with increasing pore size (Ducman et al. 2013).

5.3.2.5 Effect of furnace atmosphere Controlling the furnace atmosphere is essential for the production of high-quality sintered materials. The furnace atmosphere plays an important role in the various steps of the sintering process, as shown in Figure 5.6. The atmosphere in the furnace plays an essential role in preventing the over-oxidation of the raw materials in the event of the formation of residual oxides. In particular, regulation of the furnace atmosphere is key to eliminating the decarburization phenomena that can occur in carbon-containing dense particles (Tan et al. 2021). Thus, qualitative sintering refers to a process that is carefully controlled, and it is especially important to achieve appropriate carbon content. Selected furnaces use nitrogen (N2) and hydrogen (H2) as inputs to create a sintering atmosphere. The equilibrium approach to the sintering atmosphere is difficult due to the following factors: • temperature differences throughout the furnace lead to different activities of carbon and oxygen (O2),

FIGURE 5.6  The atmosphere functions in a sintering furnace (Dionne, B.G., McCalla, P., Malas, A, Rothstein, J., Moroz, G: An approach to carbon control of sintering furnace atmosphere: theory and practice; Metal Powder Report 70(5), 247–252, Copyright (2015) with permission from Elsevier).

88  Application of Waste Materials in Lightweight Aggregates • some components of the raw material mixture may react with the atmosphere, causing decarburization, • sintering furnaces usually do not have circulation fans to assist with convection, • the evaporation of the binder produces the gas mixtures that can affect the carbon potential of the entire atmosphere of the furnace, • there are differences in the carbon potential between the local pores and the free atmosphere inside the furnace (Dionne et al. 2015). Sintering takes place according to present conditions, including the rate of temperature rise and fall as a function of time, the type of atmosphere in the furnace chamber. Temperature graph, the so-called the sintering curve, characterizes the rate of temperature increase during heating, the time of keeping the load at the maximum temperature and the rate of temperature decrease during cooling. The furnace atmosphere is a gaseous environment in the working space furnace. It is described by chemical composition, temperature and pressure. An oxidizing atmosphere is created when “fresh” oxygen and air enter the furnace throughout the sintering process. The mixing of gases and exhaust air, which is in the furnace, is automatically removed from the furnace on the basis of the temperature difference. Hot air is light, so it automatically escapes from the furnace, drawing new air into this place with natural oxygen content. If the burner system does not provide an increase in the amount of combustion air, the effect is the opposite. The fuel burns incompletely; then, a reducing atmosphere is created in the furnace chamber and the temperature is lowered. The presence of a reducing atmosphere in the furnace can be determined subjectively – by a characteristic smell or objectively – by the analysis of the exhaust gases collected directly from the furnace chamber. For the correct furnace atmosphere to be achieved, it is necessary to install a so-called high temperature oxygen probe. This allows following the process in the oven. The aggregates sintered in a reducing atmosphere (with a reducing CO gas flow [at a CO flow rate of 10 l/min]) showed only a black core in cross-section. The black core of lightweight aggregates sintered under an inert atmosphere was more elongated as the N2 gas flow rate increased (Kim et al. 2009).

5.4  ACCELERATED CARBONATION METHOD Accelerated carbonation is used to transform freshly prepared granules into solid aggregates. During the carbonation process, crystals of calcium carbonate join together into an interlocking lattice, forming a bond between grains (Arandigoyen et al. 2006). Under the conditions of high CO2 concentration, the strength development of pellets can be accelerated to some extent. After 4 days’ carbonation curing of fresh pellets, bottom slag strength (BOFS) increases 3.2 times, compared to that of normally cured aggregates (Jiang and Ling 2020). Compared to cold bonding, due to the accelerated carbonation, CO2 can be permanently stored in the aggregate without the need for plumbing materials such as cement. Therefore, waste such as waste cement, steel slag, incinerated ashes like wood ash and paper ash can be fully used for carbonation (Gunning et al. 2009; Jiang and Ling 2020; Tang et al. 2019). The accelerated carbonation reaction is a highly exothermic and diffusion-controlled process. It depends on the physical properties and chemical composition of raw materials as well as on the carbonation conditions – i.e. CO2 concentration, pressure and temperature (Morone et al. 2014).

5  •  Influence of production parameters on lightweight aggregates  89

5.4.1 Effect of raw materials Industrial residues with high alkaline earth metal content are used for the production of carbonated aggregate. So far, air pollution control residues (APCr) (Gunning et al. 2011a), cement kiln dust (Lake et al. 2016), basic oxygen furnace slag (Jiang and Ling 2020), recycled coarse aggregates (Li et al. 2019), concrete waste powder (Shi et al. 2019) containing a high content of CaO or quarry fines (Gunning et al. 2009) have been utilized. If inactive CO2 wastes are used, the addition of binders – for example, cement, cement kiln dust, paper ash and wood ash – is necessary to increase the strength of the aggregates (Gunning et al. 2011b). Water also plays an important role in the carbonation reaction, as it allows Ca2+ ions to dissolve and react with CO2. To maintain a balance between carbonation and granulation, the optimum moisture content should be between 20 and 22%, depending on the highest amount of calcite in the final product. Excessive or insufficient water can lead to a sharp drop in the degree of carbonation (Morone et al. 2014; Melton et al. 2020). Too little water can cause the formation of smaller granules (1–3 mm in diameter) that disintegrate quickly. Gradual increase in the moisture content of the sprayed material leads to agglomeration of the small particles and increased size of the granules. However, when the amount of water is slightly higher, it sometimes leads to caking of the material to the machine (Bardin et al. 2004).

5.4.2 Effect of carbonation conditions Carbon dioxide curing is the reaction between carbon dioxide and alkali metal hydroxides, oxides and silicates (Fernández-Bertos et al. 2004). The properties of carbonated aggregates are influenced by several factors, including the conditions during the granulation of raw materials – i.e. the angle and speed of the granulator disc, the amount of added water, cement and the hardening time of light aggregates. The size of the artificial aggregate obtained from crushed concrete waste and cement gradually increases along with speed and angle of the granulator disc (30–55°). When the tilt angle is too small (30°, 35°), the corresponding rotation speed is also too low. Under these conditions, the low friction causes an undesirable slipping motion. Increasing the angle of inclination (e.g. 40°, 45°) is accompanied by material slippage, the friction being sufficient to partially transfer the raw material up the wheel before gravity causes an “avalanche” back down to the base of the drum. Under such conditions, the diameter of the formed aggregates gradually increases under the influence of gravity to roll down the disk and form “fresh granules”. When the slope angle is higher (50° and 55°), both the weight and size of the artificial aggregates increase due to rotation on the turntable. However, it often happens that, at higher angles of inclination and rotational speeds, the formed aggregates are prone to disintegration and smaller particles are created. The degree of decay increases with time, and also, fine particles are formed (Sherritt et al. 2003; Shi et al. 2019). The amount of water also has a significant influence on the granulation of carbonated aggregates due to the fact that some wastes are very sensitive to water. The hardening time of carbonated aggregates has a significant impact on their strength. The longer the hardening time of aggregates, the higher their strength. The strength of the artificial aggregate obtained from crushed concrete waste and cement reached the highest value of 2.3 MPa after 28 days. Moreover, twice greater CO2 absorption was obtained when the carbonization hardening time was extended to 28 days (Brück et al. 2018). The carbonation reaction increases rapidly in the first 2 hours and slows down gradually over time (Melton et al. 2020). The improvement in strength and CO2 absorption by carbonized PCM aggregates is more pronounced after 3 days and then remains constant (Drissi et al. 2020). The reason may be the early carbonation of the cement

90  Application of Waste Materials in Lightweight Aggregates and the thickening of the outer layer, which reduces the diffusion of CO2 to the inner parts of the aggregate and hence lowered the development of the later strength gain. Carbon dioxide curing enables the formation of phases responsible for the strength of the aggregate in the first 3 days (Jiang and Ling 2020).

5.5  COMPARISON OF PHYSICO-MECHANICAL PROPERTIES OF LIGHTWEIGHT AGGREGATE PRODUCED WITH DIFFERENT HARDENING METHODS 5.5.1 Macro- and microstructure The macro- and microscopic structure of lightweight aggregates varies depending on the type of production method. All aggregates have a spherical and rounded shape and a rough surface. Sintered aggregates have greater porosity as a result of expansion of raw materials during heating and lower water absorption due to the disconnected pore structure and dense outer coating, compared to the aggregates obtained by means of the cold-bonding, accelerated carbonation method (Huang and Wang 2013). In sintered aggregates, the pore distribution is heterogeneous. The pores in the core are large and evenly spaced, while fine pores form in the outer layer. The size of the internal pores depends on the sintering temperature and is usually proportional to the sintering duration. The dominant form of pores with diameters below 20 nm begins to form between 1000–1050°C and grows above 1050°C. Ultimately, they stabilize their pores in the range of 20–160 nm at 1100°C. In the case of the outer layer, increasing the pre-sintering time favours its development and the strength of the aggregate (Huang and Wang 2013). Already, by changing the pre-sintering time from 1.5 to 5 minutes, the outer aggregate layer increases in thickness (González-Corrochano et al. 2014). For other types of aggregates (cold-bonded, carbonized), the difference in the structure between the core and the shell is not great, compared to sintered ones, although the cores also have larger pores than the coatings. For cold-bonded, lightweight aggregates, the matrix mainly consists of hydration and pozzolanic reaction products, such as C-S-H, ettringite. In contrast, the carbonation reaction usually contributes to the formation of CaCO3 in the outer layer, resulting in a relatively denser carbonated crust. Cold-bonded and carbonized aggregates have a relatively looser microstructure than sintered ones.

5.5.2 Water absorption The absorbability of lightweight aggregates depends on their porosity, used additives and sintering temperature. It is an indicator of aggregate frost resistance. It is assumed that the water absorption not exceeding 1% guarantees frost resistance of the aggregates. A significant value of porosity generates high absorbability of aggregates. Hydrophobic additives can reduce the water absorption of aggregates up to 5.4%, and hydrophilic additives increase it. The water absorption of fly-ash-derived aggregates ranges from 0.7% to 34%, while commercial fly ash aggregates are 10–25% (EuroLightCon 2000; Clarke 2005). Coating the aggregate surface with polymers (Wasserman and Bentur 1997) and an increase in sintering temperature (Ma et al. 2011) reduces the absorption capacity. At higher sintering temperatures, a glassy texture forms on the surface of the aggregates, which may impede pore communication (Chiou

5  •  Influence of production parameters on lightweight aggregates  91

FIGURE 5.7  Water absorption of artificial aggregate (reprinted from publication Ren, P., Ling, T.-Ch., Mo, K.H, Recent advances in artificial aggregate production, Journal of Cleaner Production 291, 125215, Copyright [2021] with permission from Elsevier).

et al. 2006). The reduction in water absorption potential of the aggregates was also observed as a result of adding a binder in the process of cold-bonding aggregate production, regardless of its type. Sintered aggregates are most often characterized by water absorption below 20% (Figure 5.7). The water absorption values of sintered fly ash aggregates are slightly higher than those of sintered clay and sewage sludge aggregates. This is due to the formation of a glassy layer on the surface of the aggregates, which prevents water penetration (Kockal and Ozturan 2011). The water absorption of the cold-bonded fly-ash-derived aggregates ranges from 10 to 30%, and for most of them, it remains at the level of approx. 20%. This is related to the reactivity of raw materials and the dosing of binders, such as ground granulated blast furnace slag (GGBS), cement kiln-dust (CKD) and fine-grained (PFA) cement (Padfield et al. 2004). The water absorption of cold-bonded and ground granulated blast furnace slag aggregates is the lowest and amounts to 7.5%, while the fly-ash-derived aggregates are approximately 14.2%. The use of GGBS makes it possible to improve the conditions of hydration and pozzolanic reactions, thus facilitating the production of a compact structure with lower porosity. Similarly, the choice of binder plays an important role in the case of carbonated aggregates. Carbonated aggregates without binder or with low-reactivity materials such as (CKD) and PFA show high water absorption (> 20%), while this value drops to about 10% when the aggregates contain cement and Ca(OH)2 as binder (Jiang et al. 2020; Padfield et al. 2004; Shi et al. 2019). Although carbonation treatment contributes to the formation of a denser structure on the outer pellet layer, this does not result in an obvious decrease in the water absorption value (Drissi et al. 2020).

92  Application of Waste Materials in Lightweight Aggregates

5.5.3 Loose bulk density and particle density Figure 5.8 shows that most of the artificial aggregates belong to lightweight aggregates, except for carbonated ones, the particle density of which is often greater than 2000 kg/m3. The sintered aggregates show the lowest density. The value of the specific density is influenced by type of raw material, type and amount of binder, sintering temperature and time. Binders are most often used in the cold-bonding method (Jaroslav and Zdenka 1988; Li et al. 2001; Yoo and Jo 2003). Regardless of the type of binder used the specific gravity of the aggregates increases along with the quantity and sintering temperature thereof (Kockal and Ozturan 2011). The surface of aggregates containing more binder is more vitrified and will resist the volatilization of the produced CO2 gas (Huang et al. 2007). The specific density of lightweight aggregates obtained from waste in the form of fly ash ranges from 1.33 to 2.35 g/cm3. It is 13–46% lower compared to the density of natural aggregates. It grows when the aggregate, during heat treatment, is subjected to an increasing sintering temperature from 1100 to 1200°C (Kockal and Ozturan 2010). The specific density of aggregates without the addition of a binder increases along with the sintering temperature, and in the presence of a binder, the specific gravity decreases at a higher sintering temperature of 1200°C (Kockal and Ozturan 2011). This may be due to the bloating effect caused by the production of more gas during the sintering process (Adell et al. 2008). The density of sintered fly-ash-derived aggregates is higher than that of clay and sewage sludge aggregates because the fly ash contains fewer flux elements and has a higher melting point of minerals (e.g. SiO2 and Al2O3), which means that it exhibits lower expansion properties (Yue et al. 2011). Compared to sintered aggregates, the granules hardened by other methods show a slightly higher density, probably due to the addition of binders, which usually have a higher specific gravity and create fewer air voids in the system. The type of raw material used affects the increase in aggregate density. The apparent particle density of carbonized BOFS aggregates ranges from 2900 to 3200 kg/m3, which probably results from the significant amount of iron in the slag (Morone et al. 2014). The aggregates classified as lightweight aggregates with a bulk density in the range of 560–1120 kg/m3 are usually used in lightweight concrete (Kosmatka et al. 2002). In addition, according to Huang

FIGURE 5.8  Loose bulk density and particle density artificial aggregate (reprinted from publication Ren, P., Ling, T.-Ch., Mo, K.H, Recent advances in artificial aggregate production, Journal of Cleaner Production 291, 125215, Copyright [2021] with permission from Elsevier).

5  •  Influence of production parameters on lightweight aggregates  93 and Wang (2013), the artificial aggregate used in structural and non-structural concrete should have particle densities of 1200–1800 kg/m3 and less than 1000 kg/m3, respectively. Loose bulk density of lightweight artificial aggregates hardened with cold bonding or sintering methods ranges from 769–1017 kg/m3 and is slightly higher than in some natural aggregates such as pumice, scoria and diatomite, in which this parameter ranges from 475–500 kg/m3 (Gökçe and Koc 2012). Table 5.5 shows the physical and mechanical properties of lightweight aggregate from literature. The strength is comparable to natural and many other synthetic aggregates (Saleem et al. 2020). TABLE 5.5  Physical and mechanical properties of lightweight aggregate briefed in literature.

SIZE (mm)

AUTHORS MATERIALS

LOOSE APPARENT PARTICLE AGGREGATE BULK SPECIFIC WATER TOTAL CRUSHING IMPACT DENSITY DENSITY ABSORPTION POROSITY STRENGTH VALUE (kg/m³) (g/cm3) (%) (%) (MPa) (%)

Cold Bonding Kockal and Fly ash Ozturan (2011)

9.5–19

Bui et al. (2012)

4.75–9.5 768.5

Fly ash, Rice husk ash

789.0

1.63

25.5

–

3.70

–

20.5

–

6.00

–

Gomathi Fly ash, 12 and Metakaolin Sivakumar (2014)

794

1.50

29.75

–

2.07

–

Tang et al. Bottom ash 2–8 (2017) fine particles

25%) compared to those determined by ASTM C1585–04. The apparent density of the granules drops with the increase in the content of both the foaming agent and the hardener, which indicates increased volume of both closed and open pores. In addition, a greater foaming agent quantity results in a higher proportion of the coarser fraction. The particle-crushing strength is influenced by various factors, such as surface texture, shape, volume and distribution of internal pores. It is enhanced when the amount of foaming agent is increased from 49 to 54%, and then it decreases with its subsequent addition to the raw mix. A significant amount of Na2SiO3 is necessary for geopolymerization and full fly ash activation. Thus, the strength increases by raising the amount of Na2SiO3; however, large pores are formed with further foaming agent addition, resulting in a decreased strength value. The highest value of strength, equal to 1.10 MPa, was found for the FLWA comprising 54% foaming agent and 1% hardener, which results in the formation of small, evenly spaced pores. The decrease in strength is attributed to the random distribution and connection of macropores (Alqahtani et al. 2021). The obtained FLWA aggregates from the fly ash

6  •  Useful waste in the production of lightweight aggregates  151 were added to the concrete. The concrete density was 1666 kg/m3; the compressive strength, 7.5 MPa; flexural strength, 4.29 MPa; sorpitivit index, 0.145 mm/min1/2. Due to the higher density of concrete, it was proposed for use as masonry elements (< 1680 kg/m3) appropriate for both insulation and nonload-bearing walls, in line with ASTM C129. The high bending-strength indicates the possibility of using concrete for insulating boards. Capillary water absorption increasing linearly along with the square root of time indicates no microcracks or internal damage to the concrete. Therefore, the concretes comprising fly ash aggregates sintered with the use of microwave radiation can be an ecological, energy-saving and environmentally friendly material for non-structural or insulation applications. Quick and immediate heat curing using microwaves has transformed small-sized, spherical granules into larger and irregular, porous, lightweight aggregates recommended for use on an industrial scale (Alqahtani et al. 2021). Similar aggregates in microwave technology were obtained by Saleem et al. (2020), with the difference that the raw material mixtures contained different amounts of fly ash (FA) and silica fume (SF). Fly ash and silica fume were applied in amounts of 90% and 10% of total weight of solid materials, respectively. A total of three raw material mixtures with a variable number of binding materials, an alkaline activator, were obtained. One of them contained 89% fly ash, 10% silica fume (by the total solid), 80% Na2SiO3 + 20% NaOH (% by total liquid). The other two raw material mixtures contained a different amount of activator – i.e. 70% Na2SiO3 and 30% NaOH as well as 60% Na2SiO3 and 40% NaOH. Before microwave annealing, the granules were shiny and had a smooth surface structure, while, after microwave hardening, they remained smooth but with visible pores on the surface and inside the aggregate. The bulk density of aggregates in a loose and compacted state – as a parameter determining the unit mass of concrete and, consequently, the own load of concrete structures – was 699 kg/m3 and 738 kg/m3, respectively. An example of a lightweight aggregate formed by the action of microwave radiation is geopolymer lightweight aggregate from class F fly ash and alkaline activators in the form of NaOH and Na2SiO3, to which NaHCO3 powder was added in the amount of 1% wt. The formed granules with a diameter of 14–17 mm, after 2–3 hours at room temperature, were dried in a microwave oven for 5 minutes. The properties of aggregate directly influence the performance properties of concrete; therefore, it is necessary to evaluate these properties in order to obtain the concrete with the desired properties. The mechanical properties of artificial LWA primarily depend on the curing conditions, dosage and type of the binder, and the shape and dimensions of the proportion of water or alkaline activator. The clear expansion (76.53%) and porous structure of the aggregates indicate an active foaming effect of Na2SiO3, which, consequently, also results in a significantly low loose bulk density (443 kg/m3) and compacted bulk density (443–485 kg/m3), higher water absorption (22.3%) and porosity (54.9%). Despite the fact that the mechanical strength of the aggregates (1.6 MPa) was reduced as a result of the porous internal structure and was lower compared to other lightweight aggregates hardened with the cold-bonded method, sintered or natural, the aggregate obtained as a result of a 5-minute microwave exposure can be used for foam concrete, both for insulation and structural applications. Such activities enable the insulation and structural applications to support energy saving, low cost and sustainability (Hanif et al. 2021). When heating the granules in the microwave, moisture and gases, this causes expansion and the formation of pores. In earlier studies, 8.8% and 8.7% expansion was reported for basic furnace oxygen slag autoclaved aggregates (Brand and Roesler 2015). The LWA produced from shale and slates showed an expansion of 61–68% (Qureshi et al. 2017). Natural lightweight aggregates such as perlite expand on heating, up to 4–20 times of the original volume (Jedidi et al. 2015). Both cold-bonded and sintered synthetic aggregates are nearly twice as dense as natural aggregates. The aggregates heated in the microwave also have a lower density than natural aggregates. This primarily results from the presence of pores in the internal structure due to foaming agent addition (Hanif et al. 2021).

152  Application of Waste Materials in Lightweight Aggregates

6.4  MINERAL TRANSFORMATION INDUSTRY 6.4.1 Glass cullet production Glass constitutes one of the oldest artificial materials; it is a type of ceramics, and its use dates back over 9,000 years (Alaani et al. 2012). Glass is made with quartz sand, calcium and soda; therefore, it is commonly known as soda-lime-silica glass. Since glass properties may be changed by modifying its chemical composition, there are more than 400,000 different types of glass recipes (Sciglass 2014). Glass is a highly versatile material; with its uniqueness in mechanical properties, chemical durability and transparency, it is used in a wide variety of industrial applications, including packaging, construction, electronics and transportation. Globally, approximately 101 million tonnes of glass were produced in 2015. In the European Union, glass production accounts for approximately one-third of the total world production (Glass Alliance Europe 2015). The production of container glass, including flasks, jars and bottles used in drink, food, medicine and cosmetics packaging constitutes the largest sector of the glass industry, accounting for 63% of total production (Figure 6.10). Next in line is flat glass, accounting for 29% of total production, primarily used in the automotive and construction industries. The rest of the sectors, including household glassware for tableware and kitchenware, fiberglass for composite materials and special CRT glass, are relatively small in comparison, accounting for less than 10% of the total glass industry.

FIGURE 6.10  Glass production by sector in the European Union-28 in 2015 (reprinted from publication Glass Alliance Europe 2015).

6  •  Useful waste in the production of lightweight aggregates  153 The main material used in the glass production of is quartz sand. It is mixed with fluxes, stabilizers, dyes and other ingredients to ensure an efficient melting and forming process and the desired properties of the product. European Union Directive 94/62/EC establishes the legal framework for packaging waste (EUROPA 2007). It encapsulates all packaging waste – e.g. glass, paper and cardboard, plastics and wood – setting out the prerequisites that must be met by all EU Member States. It also requires the measures, such as national programs, to prevent the formation of packaging waste. Moreover, it encourages developing packaging reuse systems. Glass has been included in the EU Packaging Directive and specific recycling rates have been established for Member States. It also includes the restrictions on the amount of glass cullet which may be reused in glass production due to its contamination. Recycling rates differ in various countries. In the US, only 26% of the 11.48 million tonnes of glass waste produced is recycled, whereas 61.3% is landfilled (USEPA 2014). Due to the EU Directive on Packaging and Packaging Waste (84/62/EC), which aims to recycle materials, Europe has the highest glass recovery rate. However, it varies greatly from region to region. In 2016, the European Union achieved 74%, the highest recycling rate, with Denmark, Belgium, Sweden and the Netherlands at over 95% (FEVE 2019). Therefore, it is necessary find a way of reducing glass waste in the countries where there is still no waste management, since the possibility of achieving high levels of glass recycling has been proven. The glass cullet recovery rate in other regions is less than 50% (Jiang et al. 2019). In 2018–19, Australia generated a total of 1,160,000 tonnes of glass waste, or 46 kg per capita, of which only 59% was recycled and the rest went directly to landfills (Dong et al. 2021). In some countries, glass waste is recovered and used as a backfilling material (primarily the Czech Republic, Estonia and Germany); most of them choose to recycle and reuse it to manufacture new glass products (Eurostat 2016). Waste-glass treatment processes In the recycling process, glass is collected, sorted, crushed and utilized to manufacture bottles again (glass cullet). However, if sorting the glass is not possible, it is crushed and mixed with glass of various types, the regeneration of which is uneconomical or unprofitable (fine glass). It is not possible to reuse such fine glass for manufacturing of glass containers, since there are differences in melting points and chemical incompatibilities. For instance, only 5 g of fine glass is sufficient to prevent one ton of glass from being recycled (Afshinnia and Rangaraju 2015). Hence, they most often end up in landfills, presenting no economic value, and besides, they only raise serious environmental concerns. Melting of glass cullet provides environmental benefits more than its use otherwise. Often, the method is limited due to the presence of impurities such as metals, organics, ceramics and ceramic glass, mixing of glass colours (contamination caused by dyes, especially in the production of silicate glass) (Robert et al. 2021). The collected glass waste must undergo a number of processes, primarily involving the removal of impurities before the glass cullet will be available for re-release to the market. The processes to which the waste glass is subjected typically include visual inspection, crushing and screening steps as well as the removal of ferrous and non-ferrous metals. There are several collection methods for glass waste; for example, commercial collection, kerbside collection, bring-site collection, flat-glass collection and electric-glass collection. A recycling system based on collection from households is considered to be a better solution than collection at collection points (Dacombe et al. 2004). After arrival at the recycling plant, glass waste is subjected to a selection/ segregation, prior to the pollutant-removal process. Assuming that the input material mainly comprises glass waste, selection is made based on the physical appearance of the glass waste, such as colour and lamination condition (applies to flat glass waste), to obtain product of a high quality. Non-glass contaminants, such as organic materials, ferrous and non-ferrous metals as well as ceramics and other inorganic materials, are separated from the glass aggregate by vibrating screens and discharged into a separate container. Metallic impurities are removed by magnetic separation

154  Application of Waste Materials in Lightweight Aggregates techniques, and non-ferrous metal is removed by non-magnetic techniques. In turn, organics may be removed by washing or burning. Glass separation by colour is achieved using a high-energy laser light system and a CCD (chargecoupled device) (Mayer 2004). The next step is crushing of glass cullet, a technological process aimed at bringing the material to the appropriate grain size composition. Glass crushers allow the glass to be pulverized to a size not exceeding 5 cm, which is a safer and easier to process form for further recycling. To ensure the greatest value of glass cullet, some plants use separate crushers for each colour of glass – i.e. colourless, brown, green (Wartman et al. 2004).

6.4.2 Types of cullet Glass is a multifunctional material, so its use is diverse. Due to its chemical composition, it is broadly grouped into three categories: • soda-lime glass (Na2O, CaO, 6SiO): it is mainly a mixture calcium silicate and sodium silicate. It is colourless and dissolves at low temperatures. It is used in the production of window panes, laboratory cylinders and instruments (Gopi 2009; Karazi et al. 2017), • potassium-calcium glass (K2O, CaO, 6SiO): it is mainly a mixture of calcium silicate, otherwise known as hard glass. It is dissolved at high temperatures and is used to produce the items that must withstand extreme temperatures (De Bardi et al. 2015; Karazi et al. 2017), • potassium lead glass (K2O, PbO, 6SiO): in most cases, it is a mixture of lead silicate and potassium silicate, which retains shine and an unusual ability to refract light. It is used in the production of synthetic gemstones, crystals, focal lenses, electric bulbs, prisms (Karazi et al. 2017). Due to its different forms of occurrence, glass waste takes various forms, like containers and flat glass, lead glass and Pyrex, fiberglass and kinescope (CRT) (Table 6.9). TABLE 6.9  Types waste glass (Dhir, 2018; Kang and Schoenung 2005; Stickel and Nagarajan 2012; Turner 2018; Dov et al. 2021; Robert et al. 2021). TYPE OF GLASS WASTE

PROPERTIES

Containers

Soda glass is made of quartz sand with the addition of sodium carbonate (Na2CO3) and limestone (CaCO3) with a variable amount, depending on the type of application; these include bottle containers, jars, porcelain tableware and fluorescent lamps

Lead glass

It contains significant amounts of lead oxide (PbO). Used for the production of decorative objects, ceramic glazes and vitreous enamels, optical glass with high light diffusion ability (flint), radiation shields and glass for insulating metal parts of lamps

Borosilicate glass (Pyrex)

It contains boron trioxide; exhibits high chemical and thermal shock resistance; used for the production of glasses, laboratory and kitchen utensils, fluorescent lamps, for storing nuclear waste

Fiberglass

Produced with glass and then travels through a channel out to different forehearths after which the fibres are formed, used in fibre optic cables, reinforced items and insulation

Kinescopes (CRT)

CRT glass is made of various glasses containing SiO2, Na2O, CaO and other components for colouring, oxidizing and protecting against X-rays (K2O, PbO, BaO, ZnO, MgO); used in computer monitors and television screens

6  •  Useful waste in the production of lightweight aggregates  155 Most of the produced glass waste originates from flat glass and containers. They have a similar chemical composition (soda-lime glass); however, their processing at the end of their life cycle and preparation for recycling requires two various collection and treatment methods. Packaging glass, due to its various shapes, sizes, colours and dirt, is significantly more difficult to recycle than flat glass. Regardless, the amounts involved in recycling both types of glass are significant. Pursuant to the applicable law (Directive 2002/96/EC of the European Parliament and of the Council, 2003. No. 119/2005), suppliers, importers and manufacturers are obliged to demonstrate a certain level of recycling and recovery of materials from waste electronic equipment. As far as electronic devices with cathode ray tubes (kinescopes) are concerned, this level should amount to 75%, and the recycling rate should equal 65% of the weight of waste equipment (Mrowiec et al. 2011). In connection with the foregoing, undertaking the actions allowing the effective management of CRT glass cullet – for instance, for application in concrete – is justified (Najduchowska et al. 2016).

6.4.3 Glass cullet characteristics Chemical and physical characteristics The glass properties, including thermal expansion, optical clarity and chemical durability, may be influenced by altering the chemical composition. Table 6.10 shows the oxide composition of waste glasses based on a literature review. Silicon dioxide, known as silica (SiO2), constitutes the primary component of glass, accounting for approximately 60–75% of its total composition on average. The basic raw materials for the production of soda-lime glass are sand (main source of SiO2), soda ash (source Na2O), limestone (source CaO), dolomite (source CaMg[CO3]2) (McLellan and Shand 1984). After calcination, soda acts as a flux that lowers the melting point of sand, whereas limestone is a stabilizer, increasing the chemical durability of the glass. In some soda-lime glass products, part of the Na2O and CaO are in certain cases replaced with magnesium oxide (MgO) and potassium oxide (K2O) (Scalet et al. 2013). Aluminum oxide (Al2O3) increases the viscosity of the molten silica glass mass (De Jong et al. 2011). The Al2O3 content in the aluminosilicate glass is the highest (on average 16.7%), in the soda-lime glass there is usually up to 3% of Al2O3. Both borosilicate and lead glass contain less than 3% Al2O3 in their total composition. The Fe2O3 content is only approximately 0.5%. The average content of Fe2O3 in the aluminosilicate glass is 9.4%. Fluxing oxides are the sum of Fe2O3 MgO+Na2O+K2O+ CaO, which, during thermal treatment, ensure the development of glass phases (Bouachera 2021). Other additives, including transition metal oxides, are used at a 0.5–5% concentration to add colour to the final products (De Jong et al. 2011). Examples of the commonly used colouring elements are Cd(II) giving the red-orange and yellow colour; Co(III) giving the green colour; Fe(II), imparting a blue-green colour; and Cu(II), imparting a light-blue colour (Scalet et al. 2013). In borosilicate glass production, the addition of boron oxide (B2O3) as an alkali replacement (Na2O and K2O) enhances the thermal resistance of glass (De Jong et al. 2011) Lead glass usually comprises more than 20% lead oxide (PbO), an essential additive for increasing the refractive index of decorative glass items. Additionally, lead glass is utilized in TV kinescopes due to the ability of PbO to shield radiation. Soda-lime glass usually comprises significant CaO quantities, with a mean percentage of almost 10% – i.e. greater than in typical Class F fly ash. The CaO content of aluminosilicate, lead and borosilicate glasses is similar, usually not exceeding 5% (Pascual 2015).

SIO2

AL2O3 FE2O3 CAO MGO NA2O

K 2O

ZNO

PBO

B2O3 P2O5 TIO2 BAO

SO3

LOSS ON IGNATION

AUTHORS

Boro-silicate glass 74.25 1.65

0.16

2.09

–

3.82

0.93

–

–

16.63 –

22.3

7.17

–

–

–

–

Korjakins et al. (2009)

52.89 8.14

0.08

6.69

3.75

11.6

11.62 1.33

0.269 3.16

1.56

6.41

7.55

0.188 20.89 0.3

–

0.10 1.60 0.07

–

Serniabat et al. (2014)

Otunyol and Okechukwu (2017)

Potash-lead glass 51.39 4.47

Soda-lime-silica glass 38.4

6.40

29.2

6.60

2.90

3.90

0.80

9.50

2.2

–

–

–

–

–

–

Romero-Perez et al. (2001)

71.5

1.50

0.024 9.50

2.00

15.5

–

–

–

–

0.08 –

–

–

–

Aktas et al. (2017)

72.26 1.46

0.47

9.32

3.16

12.65 0.54

–

–

–

0.13

73.10 2.20

0.3

14.3

1.40

6.90

0.90

–

–

–

–

–

0.40

0.50

73.2

0.02

11.00 0.04

13.5

–

–

–

–

–

–

0.21

–

–

–

0.15 –

2.1

–

Chindaprasirt et al. (2021) Bouachera et al. (2021) Toya et al. (2007)

Soda-lime glass 70.49 0.35

2.80

24.40 1.56

–

70–74 1–3

–

5–11 1–3

12–16

71.7

0.4

8.9

13.2

0.32

10.79 1.57

2.5

72.22 1.63

2.1 2.94

Kourti and Cheeseman (2010) 0.8

13.12 0.64 7.62

Loryuenyong et al. (2009)

68.2

10.1

0.242 9.90

64.3

2.90

–

18.80 –

13.03 1.53

68.21 1.85

0.86

10.53 1.03

71.40 2.54

0.37

11.20 1.60

–

–

–

0.1

–

–

0,10 0.07

–

–

–

–

0.229

0.1

Petrella et al. (2007)

–

Du and Tan (2014)

0.366

1.1

Kim et al. (2015)

–

–

Letelier et al. (2019)

16.41 0.47

0.22

–

Rzepa et al. (2020)

12.25 0.36

0.16

0.82

Aliabdo et al. (2016)

–

–

156  Application of Waste Materials in Lightweight Aggregates

TABLE 6.10  Oxide composition by weight (%) of different recycled waste glass in literature.

6  •  Useful waste in the production of lightweight aggregates  157

6.4.4 Application of glass cullet in civil engineering Cullet is a raw material in many applications, including building materials (brick), concrete (as source of aggregate and additive), paving (as additive to asphalt and aggregate), construction aggregate (base course, landfill cover, highway fill, drainage), water filters, artwork, fiberglass, sandblasting and much more (Amran et al. 2022). Glass shredded and sieved through a 9.5 mm sieve is similar to a natural aggregate and exhibits the engineering properties that are also comparable to natural aggregate materials. Moreover, glass cullet can be used for structural applications (backfill, embankments, base course, subbase course, pavement), drainage (drainage blankets, sand filters, foundation drainage,). Glass cullet may also be used as an unbound substitute for aggregate in substructures and load-bearing layers of pavements, as well as a material bound in asphalt (“glasphalt”) and concrete (“glascrete”) (Landris and Lee 2007). Cement substitute in concrete and mortars Glass waste, which contains large amounts of silicon and calcium, may be described as amorphous, thus showing a pozzolanic or even cementitious character (Aliabdo et al. 2016; Islam et al. 2017). In concretes or mortars, they behave similarly to cement, which brings environmental benefits (Letelier et al. 2019). Adding glass cullet to portland cement does not significantly affect setting time and cement expansion. In the fresh state, the stability and consistency of the concrete are virtually unchanged, but density is somewhat decreased. In addition, the hydration heat of the concrete is reduced. Depending on the degree of fragmentation and content, its usage may maintain or enhance the compressive strength of concrete (Kamali and Ghahremaninezhad 2015; Al-Zubaidi et al. 2018). The relationship between tensile strength and compressive strength is also unchanged. The powdered glass cullet addition increases shrinkage and decreases the modulus of elasticity of the concrete. Overall, it improves the durability and permeation properties characterizing concrete. The concrete containing powdered glass cullet shows no risk of deleterious expansion in the Alkaline Silica Reaction (ASR) due to the high presence of silica. Indeed, it has been shown that the mortar with powdered glass cullet shows lesser ASR expansion compared to the reference mortar without waste (Dhir and Dyer 2001; Shi and Zheng 2007). The type of the analyzed glass powder and its colour affect the compressive strength. Lower strength values were found for the addition of green-coloured glass, which replaced the cement in 15%. The compressive strengths of concrete with the addition of neon-coloured powder glass and brown powder glass in the amount of 15% were also insignificant. Neon glass contains high amounts of calcium carbonate (CaCO3), which has an effect on the compressive strength of concrete (Al-Zubaidi et al. 2018). Nevertheless, it has been found in many studies that a mixture containing 10 and 15–30% of glass waste in concrete, as a fine aggregate or a binder, respectively, will not negatively affect the splitting tensile and compressive strength, respectively (Arulrajah et al. 2015; Atwan 2017; Du et al. 2017; Elaqra and Rustom 2018; Tho-In et al. 2018). The use of 10 and 15% glass powder as a cement substitute increases the compressive strength of the mortar by about 9.0% and concrete by 16%. Moreover, absorption, water absorption, void ratio and density are improved as a result of using a 10% cement substitute in the form of glass powder (Aliabdo et al. 2016). When lowering the compressive strength with the share of glass powder in an amount greater than 15.0%, it is recommended to reduce the w/c ratio by even 0.03 to obtain concrete with strength of 45 MPa (Aliabdo et al. 2016). The mortars with the addition of glass waste are characterized by a lower obstruction than standard sand mortars. The mortar with the addition of 15% of the mixture of glass waste is the closest to the conventional mortar with the addition of portland cement. The mortars with the addition of glass

158  Application of Waste Materials in Lightweight Aggregates waste demonstrate better strength after 28 days of maturation, while they show lower early strength but higher than in the case of mortar containing fly ash. The rate of use of calcium hydroxide in the waste glass paste was significant, which indicates its reactivity and the possibility of using it as a pozzolanic admixture in cement composites (Kim et al. 2015; Cabrera-Covarrubias et al. 2018). A substitute for natural aggregate Glass cullet can be used in concrete, paving stones and asphalt as aggregate (Du and Tan 2014; Chung et al. 2017). The use of glass cullet (GC) admixture has no effect on fresh concrete properties, except for the consistency, which, with a high waste content, may reduce it. When the water/cement ratio is constant, the GC addition enhances the compressive strength of the concrete. Most of the data show that adding waste glass as a fine aggregate has a positive effect on flexural strength, modulus of elasticity and concrete shrinkage. It results in improvement or lack of significant changes in the permeation and durability. Using a small amount of waste glass in concrete due to its high silica content can undoubtedly always cause an ASR concern. The addition of glass cullet for concrete in the form of a replacement for fine aggregate in an amount of 20% reduces the expansion of concrete at an early age, but at a later age, it is higher than in the case of control concrete. Regardless, the quantity of the GC filler added may be excessive, possibly affecting the overall packing of the aggregate as well as the concrete properties. It was therefore found that the optimal level of glass cullet should be 5%, yielding acceptable ASR expansion and compressive strength of the concrete are acceptable (Tang et al. 2005). Application of the glass cullet as a fine aggregate in concrete (up to 4%) does not cause significant changes in strength, but increasing the amount to 30% enhances compressive strength, in comparison with to normal concrete (Guatam et al. 2012). Using waste glass to partially replace fine aggregate in structural concrete in the proportions of 15, 20, 25, 30 and 40% is highly advantageous. The compressive strength increases to 30%, while the strength after 7 and 28 days was 9 and 6% higher, respectively, than in the case of the control concrete. This proves that the concrete comprising up to 30% of fine glass aggregate shows a greater compressive strength development compared to traditional one (Adaway and Wang 2015). When glass waste is added in the amount of 15, 25, 35, 45 and 50%, partially replacing fine aggregate in concrete, it increases the compressive strength by 3 and 7% after 7 and 28 days, respectively, as the content of glass waste is increased to level of 15%, after which a negative effect on the compressive strength was observed. In turn, flexural strength does not change when 15% glass cullet is added. On the other hand, it decreases by 25 and 47% at a 25% addition exchange level on days 7 and 28 and increases by 33 and 37.5% at a 35% exchange level. Strength decreases with increasing content of glass waste. Initial and final setting times decrease as the content of glass waste increases to 15%, which means that fine waste glass aggregate could act as retarder in concrete. The workability of concrete increases, but with a 35% exchange, it decreases, and the water absorption decreases as the amount of waste increases. Partial replacement of fine aggregate by waste glass is profitable because the amount of expensive sand (fine aggregate) can be reduced to 15%, which will decrease the cost of concrete production (Otunyol and Okechukwu 2017). The ability to partially or completely replace glass as an aggregate in concrete has many environmental benefits, including improved engineering design for concrete mixes (Chung et al. 2017) Glass cullet in geopolymers Soda-lime glass can be used for the production of geopolymers (Cyr et al. 2012). Waste glass containers in the amount of 10, 20, 30% weight were mixed with fly ash and sodium alkaline activator silicate (Na2SiO3) and sodium hydroxide solution (NaOH) in liquid form. The curing of samples occurred at a constant temperature of 20°C and humidity of 95%. The prepared geopolymer paste was fired at 800, 1000 and 1200°C for 2 hours. Due to the smooth surface of the glass particles and low water

6  •  Useful waste in the production of lightweight aggregates  159 absorption, the workability of the geopolymer increases with the content of waste glass powder. The polycondensation process is positively influenced by the highly reactive silicon and aluminum oxide excited by waste glass powder, which reduce the setting time of fresh samples of geopolymer paste. The compressive strength and bonding strength would improve after increasing the waste glass powder content. The optimal values of the strength parameters were obtained with the content of 20% waste glass. At the melting point of the waste, glass powder fills the geopolymer pore system, increasing the integrity of the geopolymer gel, as a result of which the fire resistance of geopolymeric gels is improved (Jiang et al. 2020) Glass cullet in the ceramic and other areas The appropriate amount of glass waste and the temperature of its firing indicate that it is possible to obtain bricks, tiles, porcelain, glaze, glass fibres with appropriate physical and mechanical properties. The porosity of bricks is influenced by the chemical composition characterizing the raw material, especially the presence of clay minerals, the size of the particles, the degree of their packing and the method of heat treatment. Highly porous bricks of high porosity are produced for thermal insulation. Water absorption and apparent porosity of the bricks fired at various temperatures from 900 to 1200°C, comprising up to 45% glass cullet, increase rapidly. Water absorption amounting to 2–3% and compressive strength ranging from 26 to 41 MPa were obtained for the bricks comprising 15–30% by mass wasted glass from structural glass walls and fired at 1100°C (Loryuenyong et al. 2009). Drying shrinkage decreases with increasing glass cullet content, while the firing shrinkage may be slightly greater. Linear shrinkage, which is a key factor in determining the degree of compaction during firing, decreases with increasing amounts added to the glass cullet mix. On the other hand, water absorption remains almost unchanged when wasted glasses was added in small amount to the composition of bricks, but after exceeding 30% by mass, these values increase rapidly. This is due to the increasing number of open pores formed on the surface of the brick. However, the vitrified phases cause the fired clay to fuse with each other. The brick with the increase in the amount of waste glasses becomes more compact. The effect is a decreased bulk density as well as increased apparent density between the content of the wastes glass 30 and 45 wt.% (Loryuenyong et al. 2009). A mixture of fly ash (FA) and GC glass cullet from recycled window glass (soda-lime glass) (FA:GC = 1:1) was added for clay in the amount of 0, 2, 4, 6 and 8% by weight of the clay weight; afterwards, it was fired at the temperature of 950, 1050 and 1100°C. Increasing the content of FA-GC reduces water absorption and porosity as well as increases strength, bulk density and firing shrinkage. The addition of ash improved the brick strength as a result of increased content of MgO, SiO2, and Al2O3 oxides in fly ash, and glass cullet acted as a flux and aided in the formation of the fused-bonding of clay and glass phase as well as helped in the synthesis of crystalline quartz as well as sintering at decreased temperature (Chindaprasirt et al. 2021). Another material using this type of glass includes tiles made of various ceramic materials. Depending on the intended use, they must meet the requirements. The addition of LCD glass to the raw material mass had a positive effect on such tile properties as water absorption and thermal expansion coefficient (Kim et al. 2016). The addition of glass waste from thin-film transistors (TFT-LCD) in the amount of 0–50% indicates that the apparent weight loss increases. The addition of 50% of the waste glass significantly increased in the porosity coefficient. Increasing the firing temperature improves the flexural strength and the abrasion resistance of the tiles (Lin 2007). LCD can be used as a fluxing agent, substituting feldspar. The viscosity of LCD glass at the sintering temperature of ceramic plates is optimal for a dense solid obtained by sintering glass powder. Sintered body has a dense microstructure owing to the rich liquid. Even when the feldspar is fully replaced, pyroplastic deformation and liquid outflow do not occur. Replacing waste LCD glass had a

160  Application of Waste Materials in Lightweight Aggregates positive effect on such properties as water absorption and thermal expansion coefficient (Kim et al. 2015). Glass cullet can be used as a raw material for the production the glaze (da Silva et al. 2012), foam glass ceramic (Fernandes et al. 2009), porcelain (Wannagon et al. 2012), filtration medium (Bove et al. 2015) layers foundations for the road surface (Meland and Dahl 2001), sound-proof and warm composite construction materials – for example, stone, slag, foam glass blocks, glass fibres, porous insulating materials (Pavlushkina and Kisilenko 2011; Portnyagin et al. 2011; Silva et al. 2018), bituminous mineral mixes (Chuprova and Mishurina 2016; Al-Fakiha et al. 2020), glass-asphalt (Wu et al. 2005), catalyst (Alfaro et al. 2011) and magnetic glass (Konon et al. 2019).

6.4.5 Glass cullet in lightweight aggregate Lightweight aggregate with waste glass is most often obtained with the addition of natural resources, such as clay and waste such as fly ash. Glass and fly ash are the potential resources which currently constitute waste materials in many countries. The treatment of the waste material, including pelletizing and rotary kiln sintering processes, is similar to those required for the treatment of commercial lightweight aggregates produced from shale and clay, and therefore, the processing costs are similar. However, avoiding the cost of importing lightweight aggregate or using pumice stone and environmental impact makes the production of lightweight FA/glass aggregate a feasible alternative. The properties of lightweight aggregates are dependent on the processing conditions, the content of glass waste and sintering temperature. Lightweight aggregate should be of low density and porosity and strong, with a sintered, ceramic core, which is the top layer to prevent water ingress and should have a pozzolanic character to create a durable aggregate-cement bond in the concrete. The shape should be close to spherical in order to enhance the properties characterizing fresh concrete (Cheeseman and Virdi 2005). The waste glass addition lowers the sintering temperature of the aggregates because it promotes the flux content in their green pellets (Wei et al. 2017). Structural lightweight concrete (LWC) is employed to make a structure lighter and to improve thermal properties and energy efficiency (Real et al. 2016). The LWC density may be from 1400 to 2000 kg/m3, whereas its compressive strength can exceed 20 MPa (Ali et al. 2018). It contains lightweight aggregates which, owing to their high porosity and cellular structure, significantly reduce the thermal conductivity and density of concrete, simultaneously reducing its mechanical strength properties – e.g. compressive strength and modulus of elasticity (ACI 213R 2014; Remesar et al. 2017). Such aggregate can be obtained from recycled glass foam. Commercial GF-LWAs are characterized by densities in the range of 300–850 kg/m3, a thermal conductivity of 0.07 Wm/K, governed by the grain size (Chung et al. 2017) as well as compressive strength, measured in foam blocks, in the range of 1.4–2.8 MPa (Zegowitz 2010). Glass foam lightweight aggregate (GF-LWA) is a fine powder, obtained by grinding the glass cullet, the size of which is usually less than 100 µm (König et al. 2018). The powder is then mixed with an expansion agent – e.g. manganese dioxide (MnO2), silicon carbide (SiC), calcium sulfate (CaSO4), followed by a heating cycle to expand and bind the glass powder (Ducman et al. 2002). The heating cycle is carried out until the optimal maximum temperature is reached, governed by the expansive agent used (Souza et al. 2017), which can range from 750 to 1200°C (Kourti and Cheeseman 2010; Chung et al. 2017). Glass foam is typically produced from 98% glass cullet obtained from kinescopes (CRT) from computer monitors, laboratory containers (boron silicate) and bottles (soda-lime) (Yousefi et al. 2016). This material is mainly produced by sintering glass powder and a foaming agent, releasing gas (CO2, CO) during thermal treatment (Nikitin et al. 2013). Foaming agents may act through thermal

6  •  Useful waste in the production of lightweight aggregates  161 decomposition or a redox reaction. Redox materials comprise carbon (silicon carbide [SiC], carbon, organic compounds) and their action is based on the oxidation reaction of the foaming agent (Lakov et al. 2013). Foaming agents are sometimes combined with oxidation catalysts, yielding GF with a finer pore size distribution, owing to which better mechanical properties are achieved. One of the best combinations when it comes to cost and availability is manganese dioxide (MnO2), used as an oxidizing agent (Petersen et al. 2015; König et al. 2016) with activated carbon (König et al. 2017). The production factors of glass foam lightweight aggregate determine its cellular structure. Scarinci et al. (2005) found that the particle size and consolidation parameters of powders (i.e. foaming agents and ground glass cullet) affect the pore fineness and total porosity. However, controlling the overall final porosity is challenging, since the arrangement, gradation and consolidation of powders influence the GF porosity development for each individual powder (Scarinci et al. 2005). Two types of glass foam lightweight aggregate GF-LWA have been developed, differing in terms of production factors, including fraction of foaming agent, particle size distribution of ground glass and heat-treatment temperature. It was done in order to obtain statistically significant effects as well as interaction thereof in pore structure development, in addition to determining the effect of these results on their thermal and mechanical properties. After taking into account six production factors, such as type (C or S), fineness (f) and proportion (D) of the foaming agent, the maximum thermal process temperature (T), the packing degree of the glass powders (E) and the compaction pressure of the powder mix (C) prior to the thermal process, it was found that the production of lightweight aggregates based on glass foam (GF) is a viable option to control the expansion process. It is possible to modify the pore structure, and thus, the required mechanical and thermal properties of lightweight aggregate can be obtained. Taking into account the influence of the earlier-mentioned factors on the porosity, it has been found that higher porosity can be obtained by increasing the fraction of the foaming agent, its fineness as well as the thermal process temperature. The interactions between temperature and the fraction of the foaming agent as well as between the degree of disintegration of the foaming agent are also important in the formation of porosity. On the other hand, the glass-powder packing and the powder-mixture compaction pressure do not have a significant effect on the pore structure (Arriagada et al. 2019). Expanded-glass lightweight aggregate was also made up of glass cullet and the addition of silicon carbide (SiC) as an expansion agent, which was added in proportions of 2.5 to 6.1% wt. The maximum production process temperature was 850°C. The cooling rate and the content SiC substantially alter the microstructure and expanded properties of glass lightweight aggregate. Greater SiC contents and slower cooling rates increased absorption; in contrast, the absorption rate was accelerated with faster cooling rates. Faster cooling increases desorption (water release), absorption rate and specific surface area, which improves the total water delivery capacity of the EG-LWA. The specific strength decreases with faster cooling and greater SiC content. EG-LWA is weakened with increasing pore diameter and porosity and with growing cracks and microcracks resulting from rapid cooling. EG-LWA is effective in reducing autogenous shrinkage (Cuevas and Lopez 2021). The glass waste from window glass and bottles was subjected to crushing, followed by grinding to a fineness of less than 100 mm with the addition of an expansive agent and methylcellulose solution. The firing of granules was carried out in a rotary kiln at a temperature of 880°C for 10 to 15 minutes (Ducman and Kovačevič 1999). The obtained aggregates have an average apparent density – 0.18 kg/m3. For the fractions below 4 mm, the unit weight is 0.22 kg/m3 and the water absorption is 11.0% by mass. They are highly reactive and provide an additional source of alkali. This results from the porous aggregate-structure, capable of absorbing large amounts of formed, conchoidally cracked gel. The aggregate behaves like rhyolite pumice stone (St John et al. 1998).

162  Application of Waste Materials in Lightweight Aggregates The obtained aggregates belong to the group of expanded-glass lightweight aggregates (EG-LWA), which is obtained by combining glass cullet and various types of expansive additives and subjected to thermal treatment, contributing to the expansion of aggregates from two to 10 times, in relation to the original volume (Nemes and Józsa 2006; Arriagada et al. 2019). The production of high-performance lightweight fillers (LWFs) is carried out by quick sintering at high temperatures using the additives capable of generating gas that provides the appropriate level of residual porosity. To obtain this type of material, two requirements must be met during sintering: • evolution of gases from thermally unstable components, • presence of a liquid phase of high viscosity, enabling the air-tight sealing of gases. Recycled glass produces a sticky phase. It is characterized by an amorphous structure, high silica content and a large surface-area after grinding. As a result of the high silicon dioxide (SiO2), calcium oxide (CaO) and sodium oxide (Na2O) content, glass has a fairly low sintering temperature, which shortens the firing time, thus reducing energy consumption (Matteucci et al. 2002; Bernardo et al. 2007). The sodium which is contained in the glass enables the formation of a low-viscosity alloy which is capable of encapsulating the evolving gases. Preparation of an expanded-glass particle involves mixing finely ground glass and a suitable expansion agent. Then the mixture is fired at a temperature above the glass softening point, which has a viscosity of 106.6 Pas (Kingery et al. 1976). Among the various expanding agents, paper sludge ash is a source of CO2 derived from the CaCO3 decomposition (WRAP: Paper Sludge Ash 2007). To facilitate sintering in the liquid phase, waste glass was added to the paper sludge ash, which is characterized by a low sintering activity below 1200°C, ensuring gas evolution, the formation of large pores and the production of foamed materials. Optimized process parameters, including an appropriately selected amount of waste, particle size and sintering conditions guarantee obtaining the products comparable to high-performance lightweight fillers (LWFs) available on the market (Spathi et al. 2015). Compared to lightweight aggregates, LWF has lower density and water absorption. They provide acoustic insulation, soundproofing properties, low thermal conductivity and potentially increased fire resistance (Blengini et al. 2012). High performance LWFs containing 80% wt. glass and 20% wt. PSA sintered at 800°C for 15 min are characterized by a density of 1 g/cm3 and a water absorption of 17%, in comparison with commercial products having the water absorption of 115% (Spathi et al. 2015). A lightweight aggregate based on raw materials from glass cullet in the amounts of 20 and 30% wt. can be successfully obtained with the addition of sewage sludge and clay (Bouachera et al. 2021). Fired at different temperatures of 700, 900, 1030 and 1030°C, they are characterized by the apparent porosity values from 14.4 to 37.6%. The highest value is for the aggregates that contain 30% sewage sludge, while a lower value characterizes aggregates with the addition of 20% glass waste. The inclusion of sewage sludge in the raw-material mixture promotes the formation of pores as a result of to their chemical composition with high organic-matter content, while waste glass promotes the creation of the melting phase that decreases porosity. Water absorption increases along with porosity. The lightest aggregate contains 30% sewage sludge and 30% glass, 40% clay and was fired at 1030°C. Its density is influenced by the content of SiO2, especially the SiO2-Al2O3-fluxing oxides ratio. In the course of the thermal treatment, the low melting of the additives is increased along with the SiO2 content. The accumulation of glass waste in the aggregate composition results in an increased the vitreous phase on the surface, which increases the volume of gases trapped in the aggregates (Tsai et al. 2006). The aggregates containing 20 and 30% wt. of waste glass with only clay as an additive (no sewage sludge) show high values of porosity, reaching approximately 1.2 g/cm3, resulting from the lower amount of gases emitted in the course of thermal treatment. The aggregates containing 20% glass waste and sintered at 1030°C are characterized by higher compressive strength. Glass waste creates the

6  •  Useful waste in the production of lightweight aggregates  163 melt phase, during the firing of which the volume of pores is reduced, whereas compressive strength is increased (Kavas et al. 2011; Bouachera et al. 2021). Moreover, soda-lime glass in the form of a powder in a similar amount – i.e. 20, 30, 40% wt. – can be obtained by mixing kaolinitic waste, schist fines and sewage sludge, which were fired in a kiln at temperatures up to 1200°C for a few minutes. The thermal expansion test results show that it is possible to produce light granules at temperatures between 1050 to 1100°C using only waste materials. Thermal treatment of glass waste leads to partial devitrification of glass at temperatures higher than 700°C and the formation of new crystalline phases – i.e. quartz, plagioclase and wollastonite. Needle-like structures of wollastonite have been found to be the mineral responsible for improving the strength of the fired product (Kanari et al. 2016). Another type of lightweight aggregate is the one obtained from a mixture of glass cullet (40%) of fly ash (60%), which was subjected to quick sintering at a temperature of 1040 to 1120°C in a rotary kiln. The addition of glass containing silica (SiO2), lime (CaO) and sodium oxide (Na2O) lowers the sintering temperature as well as enhances the mechanical and physical properties of sintered lightweight aggregate. The aggregate contains isolated, spherical gas voids, in which the glassy phase is of low viscosity. There is a black coring effect in the inner part of the lightweight aggregate, which forms crystalline phases such as albite, cristoballite, augite, wollastonite and diopside. High background level in the XRD data also indicates the presence of amorphous glass (Chimenos et al. 1999; Eusden et al. 1999). The fracture surface of the sintered material comprising 40% waste glass is characterized by a dense matrix that contains several isolated, relatively spherical pores with a diameter of 10–50 mm. These are formed when the viscosity of glass debris is such that the gases emitted in the course of inorganic decomposition reactions are trapped inside the granulate and form voids. A lightweight aggregate made of waste glass and fly ash exhibits the properties comparable to those available on the lightweight aggregate available on the market, such as Optiroc, Aardelite and Lytag. The average density of the Lytag aggregate is 1.34 g/cm3; water absorptivity, ∼17%; and the crushing strength is 6.3 MPa (Kourti and Cheeseman 2010). To obtain the desired expansion and physical properties of the aggregate, ceramic tile polishing sludge and screen glass were employed. Lightweight, expanded aggregates were manufactured using both rotating pilot kiln and static laboratory kiln, fired at maximum temperatures in the range of 1150–1200°C. Mechanical properties, water absorption and bulk density are fully comparable to their commercially available counterparts. The material which performed the best was utilized as coarse aggregate in lightweight construction concrete and pilot-scale cellular concrete (corresponding to construction and thermal/acoustic insulation applications). It turns out that the technical properties comply with the standard requirements for thermal conductivity (18–24 W/m·K for cellular concrete) and compressive strength (> 25 MPa for lightweight structural concrete). Thus, it was proven that it is possible to obtain and use the aggregates from waste in the design of lightweight concrete, in line with the vision of a circular economy (Graziano et al. 2022). Fresh pellets constituting a raw material mixture of glass waste, coal fly ash and calcium compounds were fired at 1050–1175°C. Gypsum and calcite have been determined to be good fluxes, as opposed to calcium chloride. This is because of the poor solubility between calcium chloride and the mixture of fly ash and glass, and therefore, the formed glassy layer cannot effectively surround the gases emitted from inside the pellets. Moreover, the lightweight aggregates containing calcium chloride fired at temperatures of 1050 and 1100°C retain significantly more water compared to the aggregates enriched with the other two calcium compounds. The densities of all particles generally decrease as the firing temperature increases. This downward trend means, firstly, that, at higher temperatures, more gases are released and, secondly, that the pellets sinter at higher temperatures, favouring the formation of a glassy film that retains more gases, which, in turn, leads to a greater expansion degree. For comparison, in the case of the pellets fired from a mixture of CFA and glass waste (75% CFA + 25% glass waste), their density

164  Application of Waste Materials in Lightweight Aggregates was reduced from 1.77 g/cm3 (at a temperature of 1050°C) to 1.15 g/cm3 (in temperature 1175°C) with increasing temperature. The addition of CaCl2 has no significant effect on the density of the aggregate, while the CaCO3 and CaSO4·2H2O additions contribute to lighter weight of the fired granules lighter, particularly when firing at a temperature of ≥ 1150°C (Chen et al. 2010; Liao and Huang 2011a). The density of the lightweight aggregates with the addition of CaCl2 is the highest at 1100°C. This could be explained by the fact that two different factors determine particle density: expansion and sintering. Sintering is widely employed in the metallurgical industry to decrease porosity in order to make the sintered materials a more compact mass. Greater temperature results in increased shrinkage and higher greater particle density. On the other hand, the particle density decreases as a result of the expansion caused by the release of gases inside the sintered solid if a sticky layer which surrounds the gases is formed in due time. This fact indicates why the granules comprising CaCl2 have a higher particle density at 1110°C compared to those annealed at 1050°C. It has been observed that the granules heated at 1100°C have the most volumetric shrinkage, making them the densest. Nevertheless, if the firing temperature exceeds 1100°C – i.e. 1150 and 1175°C – the expansion of pellets is increased, which counterbalances the shrinkage effect due to sintering and consequently the reduction of aggregate density. Addition of calcium species to the comprising fly ash and waste glass has no significant effect on the microstructure of the produced pellets, unless they are fired at a temperature of ≥1150°C. At this temperature, formation of larger pores (> 400 μm) occurs only in the granules comprising CaSO4 and CaCO3, rather than CaCl2 (Wei et al. 2017).

6.5  MINING AND QUARRYING RESIDUES 6.5.1 Mining and quarrying residues production The extractive industry has been defined as a branch of the economy dealing with opencast, underground or borehole extraction of minerals from deposits or their processing. The most important element of the mining industry corresponds to mining plants, including: • underground mining plants exploiting hard coal, copper, zinc and lead ores, salt, ceramic clays, gypsum and anhydrite, • opencast mining plants exploiting lignite, rock and clay materials, foundry and glass sands, sulfur, • borehole mines exploiting crude oil and natural gas, salt, sulfur, healing and thermal waters, coal bed methane, as well as underground gas storage facilities, opencast mines exploiting common minerals (Dulewski 2007). The exploitation of hard coal generates waste, the source of which are mining works related to the opening and mining of seams as well as the processes of its enrichment. The large amount of generated mining waste is due to the fact that the mineral extraction processes produce the waste that usually does not have the properties that allow its direct use. During the extracting and processing minerals processes, part of the rock material qualifying for further treatment (or direct use) is separated, and parts not suitable for use constitute waste. The scale of mining and processing of minerals also remains high, which generates huge amounts of waste (http://geoportal.pgi. gov.pl; Galos and Szlugaj 2014).

6  •  Useful waste in the production of lightweight aggregates  165 The management of mining waste is the responsibility of the producer, in accordance with the Act on waste and the Act on mining waste. This obligation is connected with the necessity to bear additional costs, the amount of which depends on the amount and properties of the generated waste and possible directions of its management. Mining tailings include the waste that is generated while extracting, enriching and processing minerals. The majority of mining and tailings from rock mining (mining of phosphate rocks and metal ores), in addition to 20 specific types of waste from mineral processing, are classified by EPA as “special waste” and are exempted under the Mining Waste Exemption from Federal Hazardous Waste Regulations under Subheading C The Resource Conservation and Recovery Act (RCRA). The growing public awareness of the consequences of inappropriate waste management forced the creation of a number of legal acts. The basic legal act regarding the extractive waste management is the Act of 10 July 2008 on extractive waste (Journal of Laws of 2008, No. 138, item 865), which transposes the Directive of the European Parliament into the national legislation and of the Council 2006/21/EC of 15 March 2006 on management of industrial waste mining industry and amending Directive 2004/35/ EC (Journal of Laws UE L 102 of 11 April 2006). The Waste Act defines the principles of mining waste management, running a mining waste disposal facility as well as the procedures related to obtaining the permits related to mining waste management as well as preventing major accidents in mining waste facilities, category A. Extractive waste is defined as the waste from exploration, recognition, extraction, processing and storage of minerals from deposits. In turn, a mining waste treatment facility is defined as a facility intended for the storage of extractive waste in solid, liquid, solution or suspension form (including heaps and settling ponds), including dams or other structures used to contain, retain, restrict or strengthen such a facility; a mining excavation filled with mining waste for technological and reclamation purposes is not regarded as a mining waste treatment facility. A producer of extractive waste is obliged to use such methods of searching, identifying, extracting, processing and storing that prevent extractive waste from being produced or allow keeping its quantity as low as possible, simultaneously reducing the negative impact on the environment or threat to human life and health. Mining constitutes the first phase of rock mining and consists in the removing ore from the ground. It is followed by enrichment, which is the first attempt to release and concentrate valuable minerals from the mined ore. Upon completion of the enrichment step, the residual material frequently chemically and physically resembles the material (mineral or ore) that has been mined, except for the reduced particle size. The enrichment processes comprise crushing, milling, rinsing, dissolving, filtration, crystallization, sizing, sorting, drying, sintering, granulating, briquetting, calcining, roasting to prepare for leaching, gravity concentration, electrostatic separation, magnetic separation, flotation, solvent extraction, ion exchange, electrolysis, precipitation, amalgamation and leaching in heaps, dumps, vats, tanks and in situ methods. The extraction and enrichment of minerals generate large amounts of waste. The processes for processing minerals usually follow the enrichment and involve the techniques which frequently alter the physical structure and chemical composition of the mineral or ore. Mineral processing techniques include, for instance, electrolytic refining smelting and etching or acid treatment. The waste streams resulting from the processing of minerals usually have little or no similarity to the materials introduced into the process, resulting in non-rock product and waste streams. Each of the stages of ore extraction and processing can generate mining waste. These wastes generally have various chemical and physical properties, resulting in varying potential environmental impacts. The appropriate amount of generated waste depends primarily on the deposit type and alternative technological solutions used in the extraction and processing of the ore. Demolition of deposits

166

Application of Waste Materials in Lightweight Aggregates

FIGURE 6.11

Different types of mining activity waste (reprinted from publication BRGM 2001).

in opencast quarries frequently constitutes one of the stages generating most debris in ore mining operations. The main types of mining waste, in addition to topsoil and overburden, can be divided into two categories (Figure 6.11): • gangue or waste rock, • tailings (flotation waste). Two further types of “waste” can be mentioned due to their environmental need management: • temporary ore storage, • slags (from a later stage of metal utilization). It is worth mentioning that, in some EU Member States, the term “waste” is not used for residues (coarse or fine) resulting from the extraction and processing of rock aggregates. In most cases, these are saleable products, provided that local market conditions are favourable. Moreover, both coarse and fine remains are needed for land reclamation and landscaping (BRGM 2001). Waste mineral raw materials are created during extracting minerals from the deposit (post-mining waste) or also in the processing of the already extracted raw material (waste processing). The main sources of mineral secondary raw materials are mining and energy. Mineral waste raw materials constitute about 90% of all post-production waste and about 58% in the fuel and energy industry. Reducing the environmental impact of mining activities is recognized as a key element of low-carbon economic development. That is why it is so important to conduct the research on the effectiveness of green mining and sustainable development strategies for the mining industry (Wang et al. 2020c).

6.5.2 Hard coal-mining wastes Hard coal mining generates significant amounts of waste. Hard coal-mining waste is usually grouped into mining waste, and the waste is due to the source of its formation. Mining wastes are generated in the course of enabling and reparatory works. They are referred to as gangue or waste rock (Kotowski

6  •  Useful waste in the production of lightweight aggregates  167 2006). It constitutes 6% of the total mass of mining waste (Central Statistical Office of Poland). The tailing-waste generation occurs on the surface of the earth and is associated with hard coal enrichment – coal sorting, washing and flotation (Mirkowski and Badera 2015). In terms of weight, it constitutes approximately 94% of the waste generated from hard coal mining (Central Statistical Office of Poland). It includes: • unburned coal shale (raw) taken directly from the drilling fluid, • unburned coal shale stored for at least one year in a heap, • decarburized shale from mechanical processing of unburnt shale, fresh or stored for at least one year in a heap, • brown coal shales commonly known as red shales or slate cuttings, • wastes from access works and processes in the form of dolomite, quartz, feldspar and other rocks (Sybilski et al. 2004). Hard coal-mining waste is diversified in terms of the petrographic and chemical composition and depends on the geological conditions characterizing the exploited deposit. The content of particular carboniferous rocks in hard coal-mining waste determines its mineral composition. They are dominated by clay minerals, including illite and kaolinite, which amount to 28–82% and 9–65%, respectively. There are also quartz (3–37%), chlorite (< 10%) and pyrite. Other metal sulfides occur locally – i.e. marcasite (Fe), chalcopyrite (Cu), sphalerite (Zn) and galena (Pb). The waste also includes potassium feldspars, plagioclases and muscovite (Bzowski 2013; Mirkowski and Badera 2015). From a chemical viewpoint, the hard coal waste produced in Poland comprises silica, which occurs in the range of 34.7–66.9%, and the presence of aluminum oxide (Al2O3) – with a content of about 21%. The waste also includes iron oxides – from 0.9 to 12.9% – potassium oxides – from 2 to 4.2% – as well as titanium, sodium and calcium oxides in the amount of 1 to 2% (Pyssa 2016). In the hard coal-mining waste, slightly elevated contents of radioactive isotopes of thorium, potassium and uranium can be observed (Pyssa and Rokita 2012; Mirkowski and Badera 2015). The potassium content in the hard coal-mining waste is sufficiently high (Bojarska and Bzowski 2012), while the remaining elements are found in insufficient amounts. The calcium content is particularly low (Mirkowski and Badera 2015). The average carbon content of the waste is 9.4%. Carbon may potentially threaten the thermal activity of waste during storage or reclamation works (Bojarska and Bzowski 2012; Mirkowski and Badera 2015). The main components of the coal waste mentioned earlier are clay rocks called loam or clay shales, mudstones and sandstones. Most often, in the immediate vicinity of the coal seams, there are claystones. The share of mudstone in the waste is variable. They constitute the basic mass of mining waste, and their share in processing waste is often equal to that of clay. Sandstone crumbs are found almost exclusively in mining waste (Sokół and Tabor 1996). A characteristic feature of the waste from hard coal mining is a large mineral and petrographic diversity. Individual rocks are characterized by different physicochemical properties, which mainly determine their functional properties. The properties of the raw material are determined by lithological formation, place of extraction and carbon content (Góralczyk and Baic 2009).

6.5.2.1 Waste rock Waste rock constitutes one of the by-products of hard coal mining connected with the processing and extraction of this raw material. Waste rock is a permanently unused mining product. The main type of gangue is formed by opencast rock removal to expose shallow ore. It is a rock which is weathered to varying degrees, although with depth it becomes increasingly fresher and shows the geological features of the local surroundings. It has a similar composition to the rocks of the sector. The largest amount

168  Application of Waste Materials in Lightweight Aggregates of waste (in terms of tonnage) comes from demethanation in opencast mines. In underground mines, waste rocks are formed in corridor workings (shafts, crosscuts). These materials show the characteristics of the accompanying mineral (Galos and Szlugaj 2010). Their petrographic composition is highly variable. The share of clay, sandstone, mudstone and overgrowth is varied and depends on the access works carried out (Klojzy-Karczmarczyk et al. 2016).

6.5.2.2 Clay rocks Clay rocks lie in the immediate vicinity of the coal, forming the top and bottom of the seams, and appear as inserts (overgrowths) of some of its seams. The grain diameter of these rocks is 0.01 mm. They constitute about 70% of the total amount of waste and are the basic component of tailings. Depending on the mineral composition and the degree of diagenesis, these rocks have different physical and technological properties, such as washability, plasticity, expansion, refractoriness. In terms of minerals, the most common are kaolinite-illite and kaolinite-sericite-quartz rocks (RosikDulewska 2016).

6.5.2.3 Mudstones Mudstones are sedimentary rocks with a diameter of 0.1–0.01 mm. There are two basic types of mudstone: • normal mudstones, the mineral composition of which is similar to that of fine-grained claybinder sandstones, • carbonate mudstones made of dusty quartz with a carbonate binder. They are sideric, ankaritic or dolomitic mudstones. Mudstones, similarly to clay rocks, are usually found in tailings, but their share usually does not exceed 40% (Rosik-Dulewska 2016).

6.5.2.4 Sandstones These are light grey and grey sedimentary rocks with different grain diameters and different binders. They constitute 13–15% of the amount of waste and consist mainly of quartz, kaolinite, illite and admixtures of carbonaceous substance. The most common are arc and quartz sandstones. Quartz dominates in quartz sandstones; feldspar and mica are sporadic. Due to the type of binder, sandstones with clay, carbonate and silica binders are distinguished. Carboniferous series sandstones are characterized by variable mechanical strength and resistance to washing in water (Rosik-Dulewska 2016).

6.5.2.5 Ways of managing the resulting coal waste The main directions of coal-waste management include: • hydrotechnical construction (embankments, river embankments, dams, etc.), engineering and road construction, including the manufacturing of mineral aggregates, • manufacturing of slate rock aggregates from spontaneously fired coal slate, • manufacturing of raw materials for mortars, cements, concrete, mineral-asphalt mixtures and construction ceramic,

6  •  Useful waste in the production of lightweight aggregates  169 • coal recovery and manufacturing of low-energy raw materials for combustion in power plants, • the use of waste as a filler in underground workings (Galos and Szlugaj 2016). An important direction in the use of coal waste are also, among others, land levelling and reclamation of degraded areas (Baic and Witkowska-Kita 2011), soil fertilization and melioration (Rosik-Dulewska 2015), sewage and waste neutralization (Klojzy-Karczmarczyk and Mazurek 2017) as well as other activities in mining technologies (Rosik-Dulewska 2015). Aggregates used in civil engineering, hydrotechnical and road construction Mudstones, clay shales, clays and sandstones are compact sedimentary rocks; therefore, their natural use is for the production of aggregates. The necessary condition to obtain aggregate from a given raw material is meeting the standard requirements. For the purposes of road construction, the aggregate should not be heavily carbonized and should contain significant amounts of clay substances that affect plasticity and washability (Galos and Szlugaj 2010). In accordance with the Ordinance of the Ministry of the Environment of 11 May 2015 on the recovery or disposal of waste outside installations and devices, it may be used for the construction or reconstruction of railway structures or tracks, railway and road embankments, road and motorway substructures, foundations, provided that that they will not harm the environment and that the planned solutions will be in line with the provisions of spatial planning and development and construction law. Production of slate-stone aggregates Slate stone is an artificial lightweight aggregate. It can be obtained either as a result of mechanical processing of self-burned coal shale from old dumps or heat treatment (sintering) of raw coal shale (Galos and Szlugaj 2013, 2016). For the production of slate, coal shale is used, which is characterized by a significant content of clay rocks and mudstones (the so-called coal shale), which also contains carbon. As a result of environmental problems (dust and gas emission) and high production-costs (mainly energy costs), the use of thermal processes for the production of slate rock burnt gas was abandoned in Poland. The material with similar characteristics of slate rock (so-called red, burnt slate) may be obtained from self-burned coal shale, deposited in old coal-waste dumps. After subjecting this material to simple grain grading processes, a slate rock aggregate is obtained with a grain size that ranges from 0–3 mm to 16–32 mm, and sometimes even over 200 mm. Slate rock aggregate is used not only in construction (e.g. construction storage yards for industrial plants, parking lots at commercial facilities and roads). It can also be used in the construction of the lower layers of embankments – the freezing zone (without restrictions in all soil and water conditions) and the upper layers of the embankments (in the freezing zone, for hardening roadsides, except for highways) (Galos and Szlugaj 2012, 2013, 2016). Manufacture of cement and ceramics Another important direction in the usage of hard coal mining waste is the production of cement and construction ceramics. A significant user of waste materials from the hard coal mining industry is the cement industry, which annually uses from 150 to 200 thousand Mg of this type of raw materials (Galos and Szlugaj 2016; Żymła 2018). Coal waste is used as the so-called low raw material, being a carrier of aluminum, silicon and iron and also a low-energy raw material. In the context of their use by cement plants, attention should be paid to the reduction of fuel consumption in the kiln during the firing of portland clinker, which is their significant advantage Żymła 2018). Some coal shales have a chemical composition similar to clay in construction ceramics. The limiting factor in the production of wall ceramics is their calorific value, related to the content of carbonaceous substance. In the case of coal shale, the calorific value is different and depends on the place of origin but also the method of obtaining them or their granulation (Góralczyk et al. 1996). The high

170  Application of Waste Materials in Lightweight Aggregates calorific value limits the use of some varieties of slate because the thermal energy they contain is several times higher than the energy that can be used in the production of building ceramics. In small amounts, coal shale is also used as a complementary component, slimming and reducing the sensitivity to drying of the ceramic mass and also used in the technology of plastic forming of construction ceramics. The presence of a moderate amount of coal in this raw material reduces the fuel consumption for firing construction ceramics, which significantly increases the value of this raw material (Taras and Szabat 2005; Galos and Szlugaj 2009). Backing and sealing material Another direction of economic use of coal waste is its addition to the hydraulic or dry-pneumatic backfill material, where this waste is an addition to the basic backfilling material, which is sand (Kuczyńska and Pomykała 2011; Cała et al. 2016). The share of coal waste in the filling mixture in the amount of up to 50% does not reduce the quality of backfilling (Sokół and Tabor 1996). It is one of the few technologies that allows for the management of sludge waste without the need for dewatering. It has a number of advantages, among which one of the most important is the effective reduction of fire hazard in underground mines (Plewa and Mysłek 2001; Kozioł et al. 2011).

6.5.2.6 Preparation of lightweight aggregates Lightweight aggregate is produced in the process of sintering (thermal treatment) of raw coal shale. The coal slate is characterized by a high content of clay minerals and mudstones, which, at the same time, contains about 10% of carbon. After subjecting this material to simple grain grading processes, slate rock aggregate is supplied with a grain size that ranges from 0–3 mm to 16–63 mm, reaching even up to 125–350 mm. These are mostly utilized for constructing parking lots, storage yards and internal roads of industrial plants. As a result of increased water absorption values, their use in the construction of public roads is limited (Koperski and Lech 2007; Galos and Szlugaj 2013, 2014). Lightweight aggregate was obtained from waste in the form of coal sludge (40%), clay from washing raw limestone bulk (40%) and fluidized-bed power-plant fly ash (20%) through sintering at above 1200°C for 30 minutes. The DTA analysis of coal sludge shows intense CO2 emission in the range of 300–700°C. This is certainly connected with the carbon contained in the structure which undergoes intense oxidation. The thermal decomposition of clays and fly ash and coal sludge produces calcium and magnesium silicates (diopside forms) and aluminosilicates such as sanidine and mullite. Additionally, iron-rich hedenbergite (Ca,Fe(Si2O6)2) phases were formed due to the sintering of coal sludge. Apart from silicates, there are also potassium aluminosilicates (K(Si3Al)O8), corresponding to the composition of sanidine and mullite (3Al2O3·2SiO2). The aggregates have a compact grain structure; the difference in colour between the outer and inner surround surfaces is clearly visible. The porosity is mostly fine, there are few large pores and large cracks are absent. The produced aggregates are characterized by crushing resistance of 5.9–7.5 MPa. The aggregates with a predominance of clay exhibit the greatest strength, which results from their structure characterized by higher compactness and minerals formed in the course of sintering (additionally wollastonite [CaSiO3]). In turn, strength decreases with the share of coal sludge. An essential parameter used to determine the strength of sintered materials involved production and emission of gaseous compounds in the course of pellet sintering and, consequently, an increase in porosity, as evidenced by the decreased bulk density of the sinters produced from pellets with the greatest content of carbon sludges. The drop in strength is considerable, with the coal sludge content increased by 20%, which results in the resistance reduced by over 23%; however, they can be successfully used to manufacture lightweight aggregates. For example, the crushing strength of the commercial tested aggregates is as follows: Aardelite – 0.09 MPa; Matrix geoceramzite – 0.8 MPa; LECA Gniew – 0.7–4.0 MPa. Moreover, they exhibit good resistance to aggressive environments (salts, acids, bases) and high

6  •  Useful waste in the production of lightweight aggregates  171 freeze-defrost resistance. These aggregates meet the basic requirements established for the materials used in construction and road construction, as evidenced by the obtained properties (Skotniczny et al. 2022). An innovative direction in the management of hard coal-mining waste in the form of post-flotation slime is the use them for the manufacturing of lightweight aggregates. The produced lightweight aggregates have a volume density of 1.93 g/cm3, a bulk density of 0.64 g/cm3, water absorption of 34.69% and crushing strength of 3.6 MPa (Góralczyk et al. 2009). Sandstones, mudstones and claystones were used for the production of lightweight aggregate. The mineral composition primarily includes the minerals of kaolinite, halloysite, muscovite, illite, limonite, siderite and quartz in relation to carbon. It was found that the quality and ecological properties of the aggregate allow it to be used in the production of insulating or insulating-construction lightweight concretes. The main advantage of use is the reduction of sulfur dioxide emissions (Kapuscinski and Pozzi 1993).

6.5.3 Wastes from lignite mining The exploitation of lignite is accompanied by the extraction of large amounts of various overburden rocks. The overburden rocks, seam layers and layers between brown coal seams include accompanying minerals, which include clays, kaolin raw materials, sands and gravels, basalts and basalt decay, limestones, peat, humic formations, silty clays and rock rubble. The overburden, treated as waste rock, consists mostly of non-selectively collected waste, placed on external dumps, together with ashes and slag from power plants or internally dumped in inactive workings. Some mines, however, exploit part of the overburden in a selective manner, collecting the accompanying minerals and mineral waste materials in separate landfills (Fajfer et al. 2010).

6.5.3.1 Ways of managing the generated waste The kaolinite clays from mine area can be used for ceramics (porcelite and faience) in the refractory industry for the production of building ceramics. Natural aggregate (sand and gravel) may be used in construction and road construction (Stryszewski 1995); sub-coal miocene sands, as a slimming material in the manufacturing of building materials. The Poznań clays were used in the ceramics and drilling industries. Peat and lake chalk can be used in agriculture (Stryszewski 1995; Kulczycki et al. 2004).

6.5.3.2 Preparation of lightweight aggregate The mining residues with a composition similar to shale in combination with incinerator fly ash and heavy metal sludge were used to obtain lightweight aggregate. Residues mainly consist of silicon dioxide (SiO2), followed by and iron oxide (Fe2O3) and aluminum oxide (Al2O3) and traces of sodium oxide (Na2O), potassium oxide (K2O), magnesium oxide (MgO) and calcium oxide (CaO). The production process involves preparation of raw material, followed by its drying, grinding, mixing, granulating, sintering and cooling. The process is conducted in a tunnel kiln, which is a counter-flow kiln, as the sintering conveyor loaded with pellets moves (irregularly) through the kiln in the opposite direction. There are four main zones in the kiln: pre-heating, feeding, sintering and cooling zones. The length of each zone is 0.5–3 m. The pellets that enter the pre-heating and feeding zones come into contact with hot gases from the sintering zone located below. In this way, they are heated to 750, 850 and 950°C, and then, sintered at temperatures of 1050, 1100, 1150, 1200 and 1250°C for 5, 15 and 25 minutes. After the sintering zone, the resulting granulate passes into the cooling zone, where some of its heat is transferred to the cooling air, which flows in the opposite direction. Through this sintering method,

172  Application of Waste Materials in Lightweight Aggregates the granules undergo rapid vitrification of their surface, resulting in an absorption rate of less than 5% and a low compressive strength equal to 4.3 MPa; moreover, fewer heavy metals are washed away with increasing sintering temperature. When the sintering temperature exceeds 1150°C, the aggregates produced from the sludge containing heavy metals become harmless and the requirements of the Taiwan Environmental Protection Agency are met (Huang et al. 2007).

6.5.4 Waste from mining of non-ferrous metal ores and chemical raw materials Mining of non-ferrous metal ores is the second largest waste producer after coal mining. The amount of waste produced by the non-ferrous metal industry in relation to the volume of mined minerals is the highest in the entire mining industry. This is due to the low content of useful metals in the excavated material and the need to remove waste rock in the enrichment processes. The diagram of waste generation in this branch of the mining industry is presented in the Figure 6.12. This group includes the waste of copper, zinc, lead ores and chemical raw materials – sulfur, rock salt and barut.

6.5.4.1 Wastes from mining of copper ores The mining waste from the extraction as well as processing of copper ores consists almost exclusively of the waste generated during the flotation treatment of these ores. Copper ores are mineralized copper shales accompanied by sandstones, limestones and dolomites. During the processing of copper ore, it undergoes the following technological processes: • preparatory: crushing, grinding, grading – sieve and flow, preparing it for the enrichment process, • the main one: flotation as the primary process for the re-treatment tasks.

FIGURE 6.12  Scheme of waste generation in the mining and processing industry of non-ferrous metals (reprinted from publication Reference Document on Best Available Techniques for Management of Tailings and Waste-Rock in Mining Activities 2004).

6  •  Useful waste in the production of lightweight aggregates  173 The primary method of copper ores enrichment is flotation. Particular types of copper ores differ in the character of their enrichment. The flotation process is carried out in several stages: initial flotation and main or primary, sands and main flotation. The main mass of flotation waste comes from processing and only a small part of rock waste, from mining. Mining waste arises during the development of the deposit, preparatory works and consumables. It mainly comprises copper shale as well as sandstones, limestones and dolomites (Fajfer et al. 2010). Flotation waste constitutes more than 90% by weight of the mass of the ore mined. In terms of waste, their character is decided by the most abundant components; namely, carbonate rocks (dolomites and limestones) and sandstones as well as clay rocks and marls. Flotation waste is finely grained (less than 0.3 mm), poorly differentiated in terms of graining; the majority of them were grain-size, below 0.06 mm (Mizera 1990). The nature of the mineralization of non-ferrous metal ores, including most of those mined in the world copper ores, ensures that, as a result of their processing, 94–96% of the extracted raw material mass becomes fine-grained flotation waste. The management of such an amount of waste material has a strong impact on the processing costs of the extracted raw material. Landfills (settling tanks) of such waste are the objects which, due to their size and location, become a permanent element of the environment in which they are located and may pose a serious threat to this environment (Fajfer et al. 2010). Ways of managing the generated waste Previous studies have confirmed the possibility of using flotation waste in ceramics with the use of thermal methods. In combination with other ingredients, brown and black pigments are obtained for ceramic enamel and glaze (Ozel et al. 2006). It was also found that the addition of flotation waste to ceramic mixtures reduces both the shrinkage and water absorption of ceramic products. Interesting results have been obtained from the study of the use of flotation wastes as a source of iron compounds for the production of hydraulic binder – i.e. portland cement. These include cements containing ground portland clinker, referred to as common cements, and special cements with high resistance to sulfates, low heat of hydration and low-alkaline cements. Hydraulic binders are also clay cements, Roman cements and hydraulic lime. The possibilities of using specific wastes are determined mainly by the chemical composition of basic raw materials, the type of cement produced, the technical equipment of a given plant and economic conditions (Kudełko and Nitek 2011).

6.5.4.2 Wastes from mining of zinc and lead ores About 130 lead minerals and about 50 zinc minerals are known. The main ore minerals of these metals are sulfides: sphalerite (ZnS) and galena (PbS), yielding 90% of global Zn and Pb production. In addition to Zn and Pb, ore also contains: gold, silver, cadmium, antimony, bismuth, tin, indium, gallium and others (Takuski 1980). In the mining of zinc and lead ores, 70% of waste is managed and 30% is landfilled or stored. However, in connection with the limitation of extraction and liquidation of mines, a clear decrease in the amount of generated waste is observed (Dulewski and Madej 2007). Mining waste includes mainly dolomites and limestones, subsoil clays, sand and gravels. Characteristic for operational waste is that the mineralization is related mainly to the fine-grained class. Large dolomite crumbs are less mineralized and the mineralization is more dispersed. The residual material is weathered, and its physicochemical parameters vary. Of the ore-bearing minerals that occur in dolomites, it is necessary to mention zinc minerals: blende, smithsonit, less frequently, hemimorphite; lead minerals: galena, cerussite; iron minerals: marcasite; and hydrated iron oxides: goethite and limonite (Girczys and Sobik-Szołtysek 2002). The tailings from pre-enrichment processes are coarse-grained dolomite blocks with granulation from 5–14 mm to 25–80 mm the so-called lump dolomite (Szczęśniak 1990). The introduced

174  Application of Waste Materials in Lightweight Aggregates enrichment process in heavy suspension liquids leads to the separation of waste of a narrow particle size class and with a low metal content. The wash waste mainly contains oxidized Zn-Pb minerals. Sulfide minerals end up in waste when the grain is highly porous with a reduced volumetric weight. Flotation waste mainly contains dolomite (70%) and calcite, limonite, gypsum, SiO2, sulfides and Zn and Pb oxides and Fe sulfides (markasite) (Ślusarek 1995). The zinc-lead ores are recovered in the flotation process mainly sulfides. The zinc and lead oxidation minerals and their adhesions pass largely to waste. The concentration of metals contained in wastes (sulfides, oxides) is dependent on the flotation technologies used (variable in time) and the waste storage method (Weryński 1994). Of all the primary wastes of the zinc-lead industry, only flotation wastes are problematic due to their large amounts and they require solutions to the issue of their management (Jennett and Wixson 1972). Ways of managing the generated waste Mining waste (dolomites, sand, gravel, clays) are mainly used in road and construction industries. However, they are a greater problem in flotation waste, the management of which is difficult. These wastes are treated sometimes as a potential deposit of zinc and lead ores or useful raw materials. They are used also for the construction of sediment ponds. New technological solutions in the processes of enrichment of Zn-Pb ores in domestic conditions boil down to the improvement of the efficiency of the enrichment processes and the quality parameters of the concentrates by reducing the magnesium content and the economic use of waste. Technological solutions concern, among others, collective flotation from the silt fraction of ore, the use of large-size flotation machines and the modernization of the ore classification system before the flotation process. The most advantageous solution corresponds to the methods that allow the use of waste at the place of its generation. Therefore, research was conducted on the preparation of proppant composites for filling post-mining voids in underground-mine workings. Backfilling primarily serves to control the harmful impact of the rock mass pressure resulting from the selection of a useful mineral deposit. It does not completely prevent the surface above the depleted deposit from lowering, but it significantly reduces it, allowing the objects to be kept afloat. This solution is supported by high demand for filling, substitution of natural resources with waste materials and maintaining the profitability of Zn-Pb ore extraction (Dydecki et al. 1980, 1984). Another direction of using post-flotation waste is to obtain magnesia binder, which, due to its physical properties, can be used, inter alia, in mining (the so-called Sorela cement). It is obtained by reacting magnesium oxide or caustic dolomite (CaCO3·MgO) with a solution of a divalent salt (e.g. magnesium chloride) (Żelazny et al. 2005). Studies are also conducted on the recovery of magnesium from flotation waste and usage of waste gypsum generated in the production of high-quality zinc concentrate with reduced magnesium content. It should be noted that fertilizing magnesium sulfate can be produced from waste containing magnesium. In addition, magnesium sulfate is used in other industries, such as food, pharmaceutical, construction and glass industries (Ullmann’s 1987; Cichostępska 2004; Fela et al. 2005; Kowalski et al. 2006d). Preparation of lightweight aggregates There is a great potential in the application of mining tailings polluted with heavy metals for conversion into lightweight aggregate. The mining waste was obtained from a Pb-Zn sulfide mine. Sand particles from the waste sample were removed, while the fraction with the size lesser than 63 µm was dried in an oven at 60°C for lightweight aggregate production. The carbon fibre waste, sepiolite and thermoplastic

6  •  Useful waste in the production of lightweight aggregates  175 were also added to the waste in various proportions. Lightweight aggregate was obtained from various raw material mixtures: 90% Pb-Zn sulfide mine + 10% sepiolite, 97.5% Pb-Zn sulfide mine and 2.5% thermoplastic, 97.5% Pb-Zn sulfide mine + 2.5% carbon fibre waste. The mixtures were mixed with the optimal amount of distilled water (30%) and stored for 72 hours under air-tight conditions, from which pellets with a diameter of 9.3 mm were then formed. Following drying, the finished pellets had the diameter of about 8.9 mm, indicating shrinkage of about 4.5% with respect to the wet pellets. The granules were fired for 4 minutes at a temperature of 1175°C. The addition of sepiolite to the Pb-Zn sulfide mine improved the workability of the raw material mixture; hence, the issue of low plasticity of waste was solved. In this case, the use of the carbon fibre waste did not bring any additional advantages; thus, only the neo-formation of microspheres connected with FC decomposition may be emphasized. Depending on the presence and decomposition potential of the mineral and organic components, the volume of the gas released for pore formation indicates that the mixture 90% Pb-Zn sulfide mine + 10% sepiolite has a high gas release potential (LOI firing – 11.32%). In aggregates (90% Pb-Zn sulfide mine + 10% sepiolite), the loss of mass approximating 7% in temperature range of 475–950°C can be attributed to the siderite decomposition (first decrease in mass from 475°C) and dolomite (second decrease to 950°C). The exothermic peak observed at 542.5°C results from the complete oxidation of a small pyrite amount in a reaction yielding SO3 and/or gases (Huang et al. 2007). The carbon fibre and thermoplastic waste additions are in turn potential sources of H2O, CO and/or CO2, as evidenced by the high reported LOI scores (LOI approximating or equal to 100%) (Huang et al. 2007). The loose bulk density of aggregates is less than 1.20 g/cm3 and the particle density ρa < 2.00 g/cm3; thus, they constitute lightweight aggregates, according to EN-13055–1 (2002) (Moreno-Maroto et al. 2019).

6.5.4.3 Wastes from iron ore mining Iron ore is mined by means of the opencast and underground methods. The method of mining is dependent on the geological and mining conditions of the ore. The choice of the exploitation method depends on the form of the deposit, mineralization variability, nature of the surrounding rocks, water content. The mining of many iron ore deposits was started with the opencast system, and as the shallowly deposited resources are depleted, further access is made by means of vertical or inclined shafts, tunnels or an appropriate system of cross members. Folded or steep deposits and igneous deposits are selected by the sub-story heading system or by a caving-type excavation system. There are many variations of bed removal in any system. As shallow decks are depleted, the method of their exploitation changes as well. The vast majority iron ore deposits contain the ores that require a processing before use. Iron ores are enriched mainly by magnetic and gravity methods as well as flotation (Bolewski 1979). This group of waste includes mining (exploitation) and processing waste (waste from roasting furnaces). The mining waste from mine heaps (formerly resulting from opencast and shaft works, and later, from deep excavations) is mainly clays and shales and the sub-layers of clay, sandstones and sands with admixture siderites. Post-mining dumps contain very fine material, mainly with grain size below 0.2 mm (silt fraction – 50%). The mineral composition is dominated by quartz, muscovite and clay minerals. The tailings are mainly raw seedings from the initial screening of the ore and its crushing. They are mainly siderite crumbs and fine ore crumbs with clay (up to 30 mm). Quartz, chalcedony and pyrite are secondary minerals (Ratajczak 1998). Ways of managing the generated waste The possibilities of using the waste from iron ore mining are quite wide. Part of siderites, which ended up on heaps (natural and roasted seedlings), may be used after appropriate processing in metallurgy.

176  Application of Waste Materials in Lightweight Aggregates Research was carried out on the possibility of using raw materials from heaps in the construction ceramics industry (Ratajczak and Korona 2000). Raw seedlings and those roasted by cement plants are used in the cement production. Part of ivory sand meets the requirements of suitability for construction (production of concrete). The clays from the heaps can also be used for the production of road clinker. There were also studies on the use of clay for the improvement of sandy soils. One can also apply them as the lowest class mineral sorbents. Preparation of lightweight aggregates Iron ore waste (IOTs) can be successfully used for manufacturing lightweight aggregates. The iron contained in the waste significantly affects the physical properties of the aggregate. Increased Fe content results in higher density of expanded clay and decreased sintering temperature (Xu et al. 2009). In the course of sintering, the aggregate is exposed to a weak reducing atmosphere; hence, the particles close to the micropores primarily consist of iron oxide. Higher Fe2+ content makes olivin (Fe2SiO4) the main binding phase in the sinter (Lv et al. 2010). Since olivine has a low melting point, LECA also has reduced sintering temperature, which makes the process more energy-efficient. Iron ore waste (IOTs) in the amount of 80% wt. and slag from coal gasification (CGS) in the amount of 20% wt. were used to obtain LECA in the high-temperature sintering process. IOT was activated with NaOH, forming a geopolymer that was sintered at temperatures of 650, 850 and 1050°C. The present Fe3+ is involved in geopolymerization, while, for Fe2+, it was not confirmed. The Fe2+/ Fe3+ ratio increases with the sintering temperature, thus facilitating nucleation inside the granules. The increased porosity is due to coal combustion and calcium carbonate decomposition. The variable temperature and humidity during sintering affect the internal temperature field and humidity, which results in the formation of appropriate stress and deformation fields, which favour the formation of microcracks in geopolymers. The porosity and pore surface increase during sintering at 850°C. In turn, at 1050°C, the internal cracks disappear, the porosity and pore area decrease. Heating below their melting point causes a number of chemical and physical changes to their structure, including increases in density and crystal growth as well as liquid phase formation. The pores and cracks were filled with crystals, thus enhancing the mechanical properties of the geopolymer. The improvement in the strength of the expanded clay occurs due to the mullite-pyroxene network formation at 1050°C. The compactness of its structure increased. In particular, the compressive strength increased following sintering at 1050°C. These aggregates can therefore be used for high-strength lightweight concrete (Li et al. 2022). The optimal compressive strength equal to 10.53 MPa, bulk density of 917.84 kg/m3, water absorption after 1 hour 9.9% and porosity of 14.33% of ceramsite were obtained by mixing 60% by weight of IOTs, 30% by weight of bentonite and 10% by mass bauxite, which was burned out at 1120°C. The compressive strength was dependent on the number of vitreous elements present in the structure. An important role in the formation of glassy components is played by the increase in the content of Fe2+ from IOT. High-strength ceramsite may be employed for the production of light partition plates, which are light, durable, high-strength, shock-resistant, characterized by good heat preservation and moisturizing properties (Li et al. 2020). Sewage sludge (SS) and coal fly ash (CFA) constitute other types of waste added to iron ore tailings to make lightweight aggregate. The dried pellets were heated for 5–25 minutes at 550°C and then sintered at the designated temperature of 1050–1250°C for 60–100 minutes. The sintered products were slowly cooled to approximately 240°C over the course of 3 hours. A heating time of 15 minutes was considered optimal to yield the aggregates with low water absorption rate. Following 15 minutes of heating at 550°C, the granules were subjected to sintering. The bulk density ranges from 1.02 to 1.04 g/cm3. Wear rate and water absorption rate decrease with increasing sintering time and then remain almost constant after 80 minutes sintering. In turn, as the sintering

6  •  Useful waste in the production of lightweight aggregates  177 duration increases, the bulk density first increases and then stabilizes. The sintering temperature significantly affects the properties of lightweight aggregates. When the temperature is increased from 1050 to 1150°C, the wear rate and water absorption rate decrease rapidly, but the bulk density increases significantly. A further increase in sintering temperature has little effect on bulk density, wear rate and water absorption rate. This means that the particle consolidation mechanism (e.g. liquid phase and solid state sintering) is still ineffective at the sintering temperature lower than 1150°C, and the sintering duration is shorter than 80 minutes. Conversely, at higher temperatures and longer durations, the liquid phase sintering is an efficient mechanism for rapidly reducing open porosity and propagating proximity between particles (Monteiro et al. 2008). The cold-bonding technique is a widely used method of obtaining lightweight aggregates from iron ore tailings. Pioneering research (Dutta et al. 1992, 1997) shows that the mixture which is most suitable for the production of cold-bonded granules comprising iron ore tailings (IOT) should contain cement/cement clinkers, in addition to other ingredients. Cold-bonded granules comprising portland cement clinker and GGBS have a greater crushing strength, in comparison with the pellets comprising higher quantities of clinker (Dutta et al. 1992). Likewise, cold-bonded pellets with appropriate mechanical properties may be obtained via fine silica addition to the iron ore tailings. Granules with favourable mechanical properties may also be produced with a decreased amount of binder (from 10% to 4–6%) by increasing the binder surface area to approximately 4100 cm2/g (Dutta et al. 1997). High-alumina cement, which is added in the amount of 7% by mass, constitutes an alternative to calcium silicate cements (e.g. portland cement). It also yields iron ore pellets with greater strength (Aota et al. 2006). The iron ore fine concentrate was mixed with US high volatile bituminous carbon and high alumina cement. The remaining ingredients included water, plasticizer, silica fume and bentonite. The granules with the addition of binders of aluminum cement have greater area strength, both at room as well as elevated temperatures (800, 1000, 1200, 1250°C). The pellets produced using silica cement exhibit lower strength at room temperature, which further decreases at elevated temperatures. When using alumina cement as binder in the cold bonding process of iron ore, it also improves strength at higher temperatures (Aota et al. 2006). As a result of the high degree of fragmentation (Blaine number) and induration temperature requirement, significantly degrading the properties during reduction, it is difficult to produce highquality pellets from hematite ore (IOT). Improving the properties of iron ore pellets occurs with optimal Blaine number a (2150 cm2/g), but the index of reductive degradation turned out to be low. The addition of a flux containing MgO – i.e. pyroxenite – contributes to decreased degradation index and improved pellet swelling index for identical Blaine number as well as other optimized process parameters (Pal et al. 2015a). Ca(OH)2 and bentonite are also used as inorganic binders in the manufacturing of cold-bonded pellets comprising IOT. Bentonite dosed in the amount of approx. 0.5% gives positive results, especially in the case of high Blaine numbers (degree of fragmentation) of hematite iron ore pellets (Pal et al. 2015a). Ca(OH)2 added in an amount from 10 to 14% (w/w) gives significantly increases compressive strengths for silica IOT (McDonald et al. 2016). High compressive strength can be obtained by adding an organic binder in the form of molasses to the raw material mixtures, in an amount from 20 to 50% (Cevik et al. 2013). Increasing the dose adversely affects the porosity of the pellets. In turn, combining molasses with quicklime added in the amount of 4% and 10% by weight improves the strength of pellets (Pal et al. 2015b). In particular, the mixtures used to produce cold-bonded IOT granules with large carbon content (38% wt.) were investigated. Favourable parameters were obtained in the case of a mixture containing slaked lime in combination with dextrin. The produced cold-bonded pellets exhibit increasing compressive strength along with an increase in the amount of dextrin in the mixture to 10% (Sah and Dutta 2010). The coldbonded IOT pellets containing dextrin in an amount from 1 to 5% were characterized by very high

178  Application of Waste Materials in Lightweight Aggregates compressive strength (Agrawal et al. 2000). However, carboxymethylcellulose, added in 1–2%, gave better results compared to todextrin, even when the latter was combined with bentonite or replaced by calcium lignosulfonate (Nikai and Garbers-Craig 2016). The process of cold bonding was also conducted on gold mill tailings and pyrrhotite tailings (Amaratunga 1995; Amaratunga and Hmidi 1997). A few mixtures were tested for both wastes with varying compositions of gypsum β-hemihydrate and portland cement, added at varying percentages. It was found that, in order to the reduced consumption of portland cement and the high strength of the pellets, the optimal solution is a 10% binder dosage, comprising portland cement and dihydrate gypsum in a ratio of 60:40 for pyrrhotite tailing (Amaratunga 1995). As far as gold mill tailings are concerned, the optimal results were obtained with a 4% binder dose comprising portland cement and β-hemihydrate gypsum in a ratio of 80:20 (Amaratunga and Hmidi 1997).

6.5.5 Drilling fluids and other drilling wastes Exploration drilling work includes anthropogenic action, which may worsen state of the environment. They pose a significant threat to the environment exploration and production wells for exploration and exploitation of hydrocarbons deposits – crude oil and natural gas – because of their number and the properties of the raw material sought. The drilling process, like every production process, has its own specific technology. One of its elements is the necessity for oil exploration wells and gas, using drilling fluids of various compositions in the drilling process. Considering the environmental impact of drilling work, one must mainly take under consideration the possibility of soil, surface and ground waters contamination with the contaminants contained in drilling wastes. Drilling wastes consist essentially of drill cuttings and spent drilling mud but often contain they also technological water, remains of cement slurries, petroleum substances, packer and reaction fluids, possibly crude oil and oils from bathtubs used in instrumentation works (Steczko and Krasińska 1998). Thus, waste is a complicated, multicomponent mixture and the substances it contains may exert negative impact on soil, surface and underground waters (Steczko et al. 1994). The main component of waste is spent drilling fluid, the chemical composition of which is very diverse, often including they are chemical reagents that are not neutral to the environment and are toxic (Getliff and James 1996). The waste that is forming during drilling work in the oil and gas mining industry is a threat to the environment due to the pollutants they contain: • salt from drilled intervals and introduced to the drilling fluid to counteract caverning of walls drilled in salt formations, • the fats, oils and greases they need to be used in the drilling process, • petroleum substances from deposits, • heavy metals (Cr, Pb, As, Zn) derived from the minerals or chemical additives used in the preparation of drilling mud, from materials used for cementing holes, from elements of casing holes or drilled rock layers, • agents used to prevent the fermentation processes of some scrubber components at high temperatures, • alkaline ingredients, • organic compounds with high reduction potential, • surfactants, the products of decomposition of numerous chemical components of washers (Bilan 2000; Bjørlykke 2015).

6  •  Useful waste in the production of lightweight aggregates  179 The contaminants contained in gas and oil drilling wastes may, in the event of infiltration into the environment, cause contamination as a result of excessive salinity of waters and soil, the penetration of oil derivatives to soil and waters, ion imbalance, unfavourable pH changes, migration to water and heavy metal soils, increased consumption of oxygen disturbing the biological balance. Drill cuttings management greatly increases the total cost incurred during drilling operations (Minton and McGlaughlin 2003).

6.5.5.1 Ways of managing the generated waste The management options for drill cuttings involve landfilling, discharge at sea or re-injection stabilization/solidification with binders – e.g. portland cement (CEM I) (Leonard and Stegemann 2010), solvent extraction (Chen et al. 2017a), bioremediation (Yan et al. 2011), thermal treatment via microwave heating (Petri Junior et al. 2015) and portland cement replacement in concrete products (Mostavi et al. 2015) and grouts (Aboutabikh et al. 2016). Submarine discharges pose a significant threat to the marine environment; the majority of the earlier-mentioned land-based management options fail to decrease the hazardous properties of the waste, as drill cuttings contain highly concentrated chloride salts. There has been little research into how drilling waste is transformed into valuable products and therefore no commercially viable recycling options for this type of waste are available.

6.5.5.2 Preparation of lightweight aggregates Drill cuttings were used for the production of lightweight aggregate. They were taken from an oil field in the North Sea, dried, milled, pelletized and fired at a temperature of 1160 to 1190°C. The drill cuttings were characterized by a typical evaporite composition, having high content of chloride salts. Thus, their usage as raw material for the manufacturing of lightweight aggregate is limited, as the resulting products exhibit a high level of leaching. A pre-treatment of rinsing is necessary to decrease the leaching of chloride ions. The washing process contributed to the decrease of the initial sintering temperature, also improving the properties of the lightweight aggregate. Drill cuttings tend to expand because they contain minerals such as montmorillonites that improve gas retention and sintering (Huang and Wang 2013). As a result of sintering, new crystalline phases are formed; CaMgSi2O6 constitutes a major neo-formed phase. The BaSO 4 present in the drill cuttings remains unreacted in the aggregate produced as a result of its thermal stability in the adopted firing temperature range. It may successfully inhibit the expansiveness of alkali silica gel between cement and amorphous silica occurring in numerous other types of waste (Tajuelo Rodriguez et al. 2018). The production of lightweight aggregate using drill cuttings contributes significantly to landfill diversion and material saving (Ayati et al. 2019). The shale cuttings were added to a lightweight aggregate, constituting an alternative binder and, prospectively, as a bloating agent at 2%. Fly ash was also added to the waste (2%). For comparison, other aggregate was produced (98% fly ash and 2% bentonite). The obtained granules were fired at the temperature of 1200°C. The bulk density of the obtained aggregates was lower than 1200 g/cm3, while the dry particle density was above 2000 kg/m3. The apparent and bulk density values were decreased, in comparison with the aggregate comprising bentonite. The aggregates with shale cuttings added have much lower water absorption compared to the aggregates with bentonite. Low water absorption is probably due to the presence of a tight, vitrified coating on the surface of the aggregates. Interestingly, the aggregates with the addition of shales are characterized by high porosity (up to 50%) compared to a product containing bentonite and a commercial product. The lower bulk and apparent density of the aggregates as well as the lower crushing resistance were attributed to higher porosity. When shale cuttings were used to replace bentonite, the mechanical properties of aggregates were slightly changed but still within the values declared by commercial producers, ranging from 2 to 10 N/mm2. The results

180  Application of Waste Materials in Lightweight Aggregates confirm that the aggregates with the addition of slate are suitable for the use of high classes of lightweight concretes as well as other insulation and construction systems (Piszcz-Karaś et al. 2019).

6.5.6 Wastes from mining of rock raw materials 6.5.6.1 Mining of phosphate rock According to the definition, “phosphate rock” constitutes unprocessed ore and processed concentrates containing some form of apatite, a calcium phosphate mineral. It is the basic source of phosphorus in phosphorus fertilizers, essential for agriculture. Phosphate deposits can be igneous or sedimentary; however, over 80% of the global phosphate rock production originates from sedimentary deposits resulting from the deposition of phosphate-rich materials in the marine environment. The United States, North Africa, the Middle East and China contain large sedimentary deposits. Igneous deposits originate from carbonatite and silica poor intrusions. They are mined in Zimbabwe, South Africa, Russia, Finland, Canada and Brazil. Phosphate rocks are mined mainly by means of surface methods, using bucket wheel excavators and raglines for larger deposits as earthmovers or power shovels for small deposits. The room-and-pillar method is employed in underground mines, similarly as in the case of coal mining. In 2013, phosphate rock was mined almost exclusively in opencast mines, and there was a single underground mine in the world (Jasinski 2017). The main user of phosphorites is the fertilizer industry (production of phosphorus fertilizers); on a much smaller scale, they are used by the chemical industry to produce various types of phosphorus compounds. Phosphate rock occurs in various types of sediments in the form of concretions, rich in calcium phosphates. They are used in the production of natural phosphorus fertilizers. Much of the phosphate mining work involves removing large swaths of plants and digging the soil beneath to reach the phosphate-ore. This material is then transported via a pipeline to a nearby plant, where the phosphate ore is separated from sand and clay in a process called “enrichment”. After the enrichment process, the separated phosphate rock ore is treated with sulfuric acid yielding phosphoric acid, which is used in fertilizers. The process also produces phosphogypsum, a radioactive by-product that is stored in large heaps. The extraction of sedimentary phosphate ores in opencast mines involves drilling and shooting large amounts of various intermediate layers consisting of limestone, marl, clay and flint. They are usually treated as mine tailings and stored in waste rock heaps. The re-use of the waste rock from phosphate mines (in particular red clay) is becoming a very important issue, aimed at reducing the surface of the heap and its negative impact on the environment (Amrani et al. 2019). Depending on the composition, gangue can have the properties similar to the raw materials – clay/sand/aggregates used in civil engineering and hydraulic engineering. Taking the mineralogical composition of the phosphate rocks into consideration (Bilali et al. 2005; El Ouardi 2008), the sludge should comprise clays, carbonates and quartz. For this reason, phosphorite gangue should be treated as secondary raw materials for building materials production (Taha et al. 2016). Ways of managing the generated waste Moroccan rock phosphate mines extract yellow clays along with other waste rock, which are then deposited in large heaps on site. They constitute an effective by-product as a source of aluminosilicates for the production of geopolymers. Geopolymers are mainly used in the construction industry due to their thermal and chemical stability in highly aggressive environments as well as favourable mechanical properties (Zhang et al. 2011). They are also applied to immobilize hazardous and toxic materials (El-Eswed et al. 2017).

6  •  Useful waste in the production of lightweight aggregates  181 The yellow clays collected from the mines consist mainly of montmorillonite, dolomite and quartz. The raw clays were subjected to calcination at various temperatures from 500 to 900°C to achieve thermal activation; then, the calcined powder was used to synthesize geopolymers by mixing it with an alkaline solution having a varying NaOH/Na2SiO3 ratio. Thermal treatment at 900°C destroyed the crystalline structure of montmorillonite, which resulted in the formation of new phases – e.g. gehlenite (Ca2Al2SiO7) and periclase (MgO) – in addition to two cement gels – i.e. N-A-S-H (sodium aluminum silicate hydrate) and C-A-S-H (calcium aluminum silicate hydrate). The geopolymer had the compressive strength of about 25 MPa after 28 days, which indicates that the yellow clay from Moroccan phosphate mines can be used in construction (Mabroum et al. 2020). The waste rocks resulting from the exploitation of phosphates comprise a mixture of lithological formations that occur within the sequence of phosphates. These by-products include common and abundant flint. It can be successfully used as an aggregate to replace natural gravel in concrete mixtures. Flint aggregates are primarily composed of quartz as well as fluorapatite and dolomite in trace amounts. Their physical and geotechnical properties are comparable to the natural aggregates utilized in concrete production. When natural aggregates were fully replaced with the flint aggregates from phosphate waste during the B25 concrete production, significant results were obtained. Flint aggregates have the geotechnical properties that mostly meet the B25 concrete requirements, yielding very good compressive strength properties, which averaged 29 MPa after 28 days. Moreover, the flint-based concrete achieved higher flexural strength of 4.90 MPa, on average (El Machi et al. 2021). Ceramic membrane filters produced by means of industrial by-products may constitute an alternative to valorize phosphate mine waste, especially if the membranes are used in the treatment of industrial waste water. They are characterized by good thermal, chemical and mechanical resistance. Ceramic membrane filters are made of natural clay and waste from phosphate mines (phosphate sediments). Mixtures of the earlier-mentioned materials and sawdust (blowing agent, up to 20% wt.) were fired at the temperature of 900–1100°C for 4 h. The filtration results are quite interesting, which allows the use of these membranes in industrial wastewater treatment (Loutou et al. 2019a). The red clay from phosphate mines, containing quartz, dolomite and palygorskite in its mineral composition, was used for the production of bricks fired at a temperature of around 1050°C for 2.5 h (Loutou et al. 2019b). Preparation of lightweight aggregates An example is the preparation of lightweight aggregate from a red clays sample, which was extracted from the interlayers of the phosphate mines. The mineral composition is composed of dolomite, quartz and – to a lesser extent – illite and palygorskite. Clay in the amount of 70–100% was mixed with organic waste in the form of coffee grounds and sawdust in the amount of 5–30% wt. in order to increase the porosity of the aggregates. The formed granules were dried 12 hours at 105°C and then fired at 1100°C for 1 hour. The recipe was based on the use of 25% by weight of sawdust as a porosity enhancer. The bulk density of aggregates decreased with the increase in the amount of organic waste. The most favourable results were obtained for the composition of the raw material mixture containing 25% by weight of a porosity increasing agent – i.e. sawdust. The density of the aggregates was 0.8 and the compressive strength was 7.4 MPa. The water absorption of aggregates gradually increases with the amount of organic constituents, which increasing porosity and subsequently creating accessible water paths (Gu et al. 2016). Adding these components to a matrix that already contains a pore-forming source (in this case, dolomite in red clay) enhances the pore formation process (Demir 2008). The thermal effects of red clay at 100, 240, 725 and 770°C is related to the dehydration of free water (Pramono et al. 2018), followed by the removal of constitutional water and decomposition of dolomite. The earlier-mentioned endothermic features are vital in the oxidation process of the gasogenic agent that enables to develop microstructural pore channels (Bayoussef et al. 2021).

182  Application of Waste Materials in Lightweight Aggregates Lightweight aggregate was obtained from expanded clay (up to 30% by weight) and phosphate sludge. The mineral composition of the waste is characterized by smectite clay mineral, fluorapatite, dolomite, quartz and calcite. The used binder corresponded to a swelling clay material obtained from a clay stratum at the phosphate basin. It consists of dolomite, quartz and montmorillonite. Mixtures of dried sludge (105°C) and clay were moistened with water (40% wt.); granules (< 3 cm diameter) were formed, which were dried and heated for 4 hours at 900–1200°C in an electric furnace. It turns out that aggregate shrinkage increases along with firing temperature, especially in the case of granules containing more than 5% wt. clay. These changes are mainly due to the elimination of the pores and the partial melting of the solid. Water absorption increases with the addition of clay, and the main increase occurs at 1200°C. This results from the change in the number and size of pores. Burnt pellets comprising 5% wt. clays were composed of macro- and mesopores. Conversely, the granules containing 30% wt. the clays show only macropores with a relatively narrow distribution range and a clear increase (32%) of the pores having a diameter of 5 mm. The density of the fired pellets does not indicate clear and even changes depending on the clay admixture. However, the aggregates fired at 1000 and 1200°C have a density above 2 g/cm3. Lightweight aggregates may be considered fired at 900 and 1100°C. The compressive strength changes, depending on the clay admixture; it increased along with temperature, except for 1100°C. The greatest values were recorded at the temperature of 1200°C (13.5–19.5 MPa). Due to the high fluorapatite content, of porous microstructure and low density, the aggregates fired at a temperature of 900 and 1100°C may be successfully used in hydroponics and horticulture (Loutou et al. 2013). Another way of obtaining the lightweight aggregate from phosphorus waste is adding it to the raw material mixture containing cement kiln dust and pottery red clay. The dried granules were fired in an oven in the range of 900–1100°C for 4 hours. The most important factor having a positive effect on the properties of aggregates is temperature. Aggregates can be heated in fast firing cycles (Loutou et al. 2016). The red clay and the phosphorus waste are composed of quartz, carbonates and clay minerals, while the cement kiln dust consists of mullite and quartz. The phosphorus waste contains fluorapatite and is a poor silica-alumina material. In the process of thermal treatment, the gehlenite formed was considered a metastable phase, which presumably formed because the potentially reactive silica exhibited low activity. The fluxes, primarily of clay, in addition to quartz and structured silica, cause intense melting. Under these conditions, the mullite partially melts. Aggregates are suitable for construction/insulation concretes due to their particularly high density and compressive strength (up to 19 MPa). On the other hand, the lightweight aggregate obtained from a mixture of phosphate rock waste and cement dust (phosphorus waste-cement kiln dust) is characterized by a compressive strength value of 9–30 MPa and a density of 1.68–1.98 g/cm3. The heated pellets can be successfully used as a lightweight insulating product. In the case of the aggregates made of a mixture of red clay and phosphate rock waste, the compressive strength increased by an average of 4 MPa/100°C, whereas the water absorption decreased by 5%/100°C. The strength-to-density ratio of the sintered granules (up to 12 MPa/g/cm3) is so high that these materials can be classified as lightweight structural aggregates. The phosphorus waste is a beneficial by-product for the structural/insulating lightweight aggregate (Loutou and Hajjaji 2017). The process of preheating and proper sintering was performed to obtain the aggregates from phosphate tailings, black shale and soft interlayer. First, the granules were subjected to drying at 105°C for 3–5 h and then preheated in an oven at 300–500°C for 10–90 min, followed by sintering at 800–1050°C for 10–90 min. Following sintering, the aggregate was cooled to room temperature via natural convection. The one-factor experiment involved six groups of studies. It turns out that the content of phosphorus, the strength of the particles and the degree of water absorption show varying correlations between the effect of the calcination process and material proportion.

6  •  Useful waste in the production of lightweight aggregates  183 In the course of the firing process, the alumina and silicon oxide melt, resulting in the formation of mullite, and the internal porosity of the ceramsite is reduced. As a result, the 1-h water absorption index is reduced, whereas the strength of the aggregates is increased. The content of phosphorus and the 1-h water absorption coefficient continuously decrease with increasing the content of black shale. At the black slate content of 10 to 20%, strength is increased along with the proportion of black slate proportion. When the share of black slate is from 30 to 50%, the strength of aggregates does not change much. Further optimization tests show the importance of the influence factors in the following order: sintering temperature>preheat temperature>sintering time>preheat time. On the other hand, the optimization analysis of the calcination parameters indicates that the preheating time and temperature and are 9.6 min and 350°C, respectively, whereas the sintering time and temperature are 60 min and 943°C, respectively (Yang et al. 2017).

6.5.6.2  Extraction of mineral resources Construction and road aggregates are made using sedimentary (carbonate), igneous rocks (deep water, effusive) and metamorphic rocks. Natural aggregates are produced from crushed rocks (sand, gravel), and the remaining ones are crushed aggregates (obtained by mechanical crushing of rocks). Rock raw materials are: • sedimentary rocks (crumb rock, clay), • igneous rocks (predominance of effusive rocks – basalts, melaphyres, porphyres), • metamorphic rocks. Rock raw materials are mined open-cast, which has a negative environmental impact. The scale of this phenomenon is dependent on: • • • •

the type of mineral extracted, the size of its extraction, mining and geological conditions of its presence, operating systems used.

Mining of rock raw materials is dominated by mass exploitation, rather than selective exploitation, which leads to an increase in the amount of waste and causes significant losses of accompanying minerals. The waste from the rock raw materials mining includes the waste generated in the following processes: • mechanical processing of dolomites, limestones, quartzite sandstones and igneous rocks into crushed aggregate for construction and road construction, • wet processing of glass sands and kaolin raw materials, • screening and washing of natural aggregates, • mechanical processing of limestone and dolomite rocks for the lime, chemical, food and metallurgical industries, • processing of stone building and road elements. In the extraction and processing of natural aggregates, the sands with a grain size of smaller than 2 mm are waste. Waste sands can be used for the production of concrete, mortars, sand-lime bricks or construction ceramics. The waste from the production of crushed aggregates mainly corresponds to the grains smaller than 4 mm, but also those up to 25 mm and even 30 mm thick. Most of this waste is used for land

184  Application of Waste Materials in Lightweight Aggregates reclamation and construction of local roads. However, the use is small and amounts to 10–20%. The stone waste produced during mining and processing of blocks, slabs and other stone elements is generally processed into crushed aggregate. In the processes of glass sand processing, a large amount of sludge is formed, which, apart from quartz dust, contains significant amounts of kaolinite that can yield kaolin. In processing plants, gravel and sand undergo washing and classification by particle size; the finest particles are separated in a slurry. The mixture of water and solids – i.e. the rinsing sludge – is discharged mainly to nearby retention ponds (Schmitz et al. 2009). Preparation of lightweight aggregates Sand sludge can be successfully used to obtain lightweight aggregates. The high dispersibility of sand sediments indicates the possibility of low-temperature frits production. Sand sediments constitute a mixture of oxides, carbonates, various clay minerals, feldspar and quartz. The preliminary studies yielded promising results, which confirmed the possibility of applying the raw material of this type for lightweight aggregates production. In turn, such aggregates may be used as heat insulation filler or in concrete production (Volland et al. 2014). Sand sludge with a particle size of 2–63 µm was mixed with ash soda to reduce the melting point of the mixture, and served carbon black type 220 was added as a foaming agent. Afterwards, the mixture was sintered at temperatures of 800, 825, 850 and 900°C with isothermal holding times amounting to 1 h. The properties of lightweight aggregates from sand sediments are dependent on the foaming temperature and pre-firing temperature of the green granules (Volland et al. 2014). Lightweight aggregate was obtained from zeolitic rocks and sand sludge. Sand slime constitutes a waste product originating from crushing and screening plants for gravel and sand production. Its mineral composition comprises carbonates, various clay minerals, feldspar and quartz. Grains of 2–63 µm were used for the aggregate. Raw material mixtures contain a variable amount of zeolitic rock (20, 30, 40%) and sand sludge (40, 50, 60%) as well as a constant amount of soda ash (20%), the task of which is to lower the melting point of the mixture. The three mix compositions were fired at 800, 850, 875 and 900°C for 1 hour. As a result of the interaction between silica (SiO2) and soda ash (Na2CO3), sodium silicate (Na2O·nSiO2) is formed and CO2 is released. The DTA of the raw mix compositions at different temperatures shows endothermic and exothermic effects, corresponding to the melting of the glass phase and oxidation of gas forming agents. When firing temperature is raised, the temperature of the exothermic effect is changed. Endothermic effects are related to the silicate formation processes. The temperatures depend on the variable phase composition. When 20% zeolite rocks are introduced to the composition of the raw mixture lowers the temperature needed for lightweight aggregates produce by 50°C, in addition to enhancing their mechanical and physical properties. Optimal results were obtained for a crude mix containing 30–40% zeolite rocks, 40 and 50% sand sludge at a pre-firing and foaming temperature of 875–900°C and 900°C, respectively. The obtained lightweight aggregates are characterized by a relative density of 721–940 kg/m3, bulk density ranging from 170–400 kg/m3, compressive strength 1.4–4.22 MPa and water absorption of 1–3%. When the zeolite content is increased in the raw material mixture, the content of the vitreous phase is enhanced, the viscosity of the mixture is lowered and a material with a more porous structure is created, while the water absorption does not increase significantly. The lightweight aggregates with zeolite rock and sand sludge addition are characterized by a porous structure having a total porosity value of 60–70%. This material is suitable as thermal insulation filler and lightweight aggregate for concrete (Volland and Brötz 2015). Lightweight aggregate was also obtained from granite-marble sludge which was added to the raw material mixture 90%, sepiolite rejection in the amount of 10%, with the addition of glass fibres

6  •  Useful waste in the production of lightweight aggregates  185 in the amount of 0, 2.5, 5 and 10% (w/w). The material was fired in a rotary kiln for 4, 8 and 16 minutes a temperature of 1100, 1125 and 1150°C. The addition of glass fibres promotes aggregate expansion and the formation of an internal structure showing both unburned carbon fibres and pores. The mechanical properties are also enhanced. The values of most parameters are varied, governed by the firing conditions and the content of glass fibres. Increased content of glass fibres content raises LOI and lowers density values, which can be related to the earlier-mentioned presence of light unburned carbon fibres in the aggregate structure. The aggregates without the addition of carbon fibres have a higher density and thus improved compressive strength with increasing firing time and temperature, while water absorption decreases. Part of the aggregates does not expand, even if the raw material has the chemical composition in the Riley area. Although the statistical research by Dondi et al. (2016) shows that the Riley chart is characterized by high percentage of erroneous predictions, the lack of expansion in this aggregate is probably related to an inappropriate grain size distribution, rather than its chemical composition (Cougny 1990). It was proven that the expansion effect and lightness of granite-marble waste improve with the increased clay content in the raw material mixture, which is also affected by the chemical composition of the added clay. The reason for this is that the gases produced during firing can quickly escape through the pores with almost no obstruction before a sticky matrix is formed. This is probably the main reason why some aggregates of this type shrink (Moreno-Maroto et al. 2017b). Similar raw material composition and firing conditions were also used to obtain lightweight aggregates, with the difference that polyethylene-hexene thermoplastics was added to marble and granite waste in the form of sludge and sepiolite rejections to test its suitability as a raising agent. The expansion effect of the aggregates was not found, nor was the typical cellular structure consisting of a shell and a core with relatively large pores; instead, a structure comprising microchannels and micropores was obtained. Increases in firing time and temperature resulted in greater sintering, yielding greater compressive strength, density and shrinkage but reduced water absorption and porosity. The addition of polyethylene-hexene thermoplastics did not cause any enhancement; instead, a significant decrease in compressive strength was observed. The lightweight aggregate without the addition of polyethylene-hexene thermoplastics, sintered in a minimum time of 4 min and at a temperature of 1100°C, can be used in water filtration systems and/or hydroponics. Granite sludges from cutting and polishing as well as from the production of ceramics (sludges from polishing stoneware tiles) were used to produce lightweight aggregates. Both types of waste exhibit suitable burnout properties due to the presence of SiC, which decomposes at high temperature with gas evolution, acting as an expansion promoter, yielding the aggregates having a particle density < 1 kg/m3. Nevertheless, slight changes in the composition of the mixture make it possible to obtain the aggregates with varying properties – from the values similar to typical expanded commercial clays to the products having good mechanical properties particle strength 6.9 MPa. The recipe of the porcelain stoneware polishing sludge with 50% wt. of granite sawing sludge raw material mixture was used to produce lightweight construction concretes (de Gennaro et al. 2009). Hazardous ornamental stone granitic waste was also added in various proportions to the clay. The content of alumina, silica and the sum of the fluxes controls the viscosity and amount of the liquid phase within the granules in the course of their firing. Expanded aggregates exhibit a reduced bulk density with increasing clay content and temperature. The main component of lightweight aggregates is the glassy phase. Some remnant/neogenic phases are randomly dispersed in the dominant vitreous mass. The molten material formed during firing, which is characterized by high viscosity, is primarily responsible for the occurrence of small, uniform pores. Conversely, larger bubbles are caused by lower melt viscosity in the sample. The prepared aggregates, following firing at a temperature of 1200°C, constitute the lightweight aggregates that can be employed as acoustic and thermal insulators in lightweight concrete (Soltan et al. 2016).

186  Application of Waste Materials in Lightweight Aggregates

6.6  CONSTRUCTION AND DEMOLITION WASTES 6.6.1 Construction and demolition wastes (CDWs) production The continuous development of investments resulting from economic growth leads to a global increase in the demand for construction aggregates (Freedonia 2012); hence, the continuous exploitation of natural resources, with the risk of serious consequences for the environment, is favoured. The construction industry consumes significant amounts of natural resources. Statistics show that, by the end of 2025, it is expected that the global demand for aggregates will increase from 45 billion Mt (2017 data) to 66 billion Mt (PMR 2017), which proves their significance in the construction sector. On the other hand, the construction industry also generates the most extensive and heaviest waste stream, in comparison with other economic activities (de Brito and Silva 2016; Silva et al. 2017). The environmental problems associated with construction and demolition waste (CDW) are increasing worldwide (Islam et al. 2019; Mehrjardi et al. 2020), with many countries trying to create different laws and raise awareness in order to promote environmental protection (Akhtar and Sarmah 2018). On a global scale, the urbanization rate was 54.3% in 2016, 55% in 2018 and is expected to increase to 68% worldwide by 2050 (UN 2018; Aslam et al. 2020), which will result in a continuous increase in the number of generated CDWs. Wastes arise as a result of construction, reconstruction, extension, destruction, road works, maintenance and demolition (Duan et al. 2019). China and the United States are considered to be one of the world’s largest construction and demolition waste producers. Figure 6.13 shows the countries that produce the most of this type of waste (Aslam et al. 2020). From the estimated annual CDWs amount in the world, approx. 30% is generated by the USA (US EPA 2018), whereas, in the case of China, it is approx. 30–40% (Yuan et al. 2012; Jin et al. 2017b). In

FIGURE 6.13  The most CDW waste generator countries (million tonnes) (reprinted from publication Aslam et al. 2020).

6 • Useful waste in the production of lightweight aggregates

187

2014, the estimated amount of CDWs approximated 534 million Mt in the US (US EPA 2016a) and around 1130 Mt in China (Lu et al. 2017; Menegaki and Damigos 2018). In turn, the estimated amount of CDWs increased in 2015, as it was around 548 Mt in the US (US EPA 2018) and 2,500 Mt in China (Duan et al. 2019). A report published in 2010 stated that the amount of construction and demolition wastes produced annually in India is 1,012 million Mt, corresponding to 8.31 kg per capita per year. In the USA, the quantity of CDW generated in 2014 was estimated by the Environmental Protection Agency at approximately 484 million Mt (USEPA 2016a). In the EU, CDW is one of the largest waste streams in terms of volume and weight. In 2018, the total quantity of waste produced by all households and economic activities (EU-27) was 2,317 million Mt; it was the highest amount recorded in 2008–2014. In accordance with the Eurostat data, the construction sector waste corresponds to approximately 37% of total waste production, and its value in 2018 amounted to 972.6 million Mt. Italy, producing approx. 60.5 million Mt, is the fourth European country in terms of construction and demolition wastes production, after France, Germany and the Netherlands, accounting for around 34% of all waste generated domestically (i.e. total waste generated by all households and economic activities) in the same year (ISPRA 2019, 2020). However, some of this data might be inconsistent as a result of inadequate waste policy, uncontrolled activities or undeclared values, leading to misrepresented indicators. Nonetheless, CDWs are indeed produced worldwide in significant amounts, indicating the urgent need for further action aimed at integrating more CDWs into the current construction practice and closing the supply chain loop (de Brito and Silva 2016). The regulations and laws issued by governments of different countries worldwide have organized and created a market for products and building materials from CDWs. According to da Rocha and Sattler (2009), the emergence of construction and demolition wastes takes place in all the main phases of a building life cycle – i.e. construction, renovation and demolition. There is a tendency in many countries to consider waste as a by-product or resource which may have different useful applications. Construction Waste corresponds to relatively clean, heterogeneous building materials resulting from different construction activities. It arises in connection with the construction and renovation of buildings, resulting from an excess of materials (surplus supplies), the formation of damaged or destroyed materials (hence unusable), processing waste (metal output, saw dust), cut pieces, dismantled formwork, used accessories and tools, garbage and packaging produced or discarded on construction sites. Demolition waste arises as a result of the demolition of buildings, roads, bridges, etc., and their complete renovation or removal. Demolition waste also comprises demolition debris resulting from natural disasters (tsunamis, hurricanes and earthquakes), social conflicts, explosions, fires and vandalism and the collapse of weak structures (Tam and Lu 2016).

6.6.2 Handling construction and demolition wastes Typical steps for processing construction and demolition wastes to obtain appropriate recycled aggregates (RA) involve the following steps (Hiete 2013): • separating various waste fractions (e.g. removal of metals, plastics, wood, waste residues, and paints from aggregates), • disposing of polluted waste (e.g. heavy metals, gypsum, asbestos), • conducting the pre-screening process (segregation of fine fractions comprising light particles and soil), • sieving to obtain the required particle size, • sorting using air separators removing light particles,

188  Application of Waste Materials in Lightweight Aggregates • metal separation using electromagnets, • manual selection lines for the elimination of materials difficult to remove by other techniques (Ambrós et al. 2017; Hollstein et al. 2017). Different combinations of the earlier-mentioned processes may be observed in various plants. The quality of input construction and demolition wastes governs the implementation or removal of these steps. If the waste is of high quality, fewer treatments can be carried out to achieve the correct quality of recycled aggregates (RA). Waste treatment processes vary, depending on the type of construction and demolition wastes delivered to the recycling plant. If they consist mainly of concrete blocks, recycled concrete aggregates (RCA) of good quality are easily obtained with fewer processing steps and therefore lower cost. A simplified workflow for the recycled production concrete aggregates (RCA) from selected concrete blocks is shown in Figure 6.14.

FIGURE 6.14  Process treatment of construction and demolition wastes stream on base concreteo (reprinted from publication de Brito, J., Agrela, F., Silva, R.V., Construction and demolition Waste, In.: New Trends in Eco-efficient and Recycled Concrete; Woodhead Publishing Series in Civil and Structural Engineering, Copyright [2018] with permission from Elsevier).

6  •  Useful waste in the production of lightweight aggregates  189 After the initial screening step to remove possible contamination, the material is fed to a jaw crusher. Mobile crushers and sorters are frequently installed on construction sites, enabling on-site processing. Otherwise, special processing sites are created that typically produce higher quality aggregate (CSI 2009). Then, by means of an electromagnet, ferrous metals are separated from the ground material, followed by the final stage of screening, in order to obtain the particles in the form of course, fine or all-in RCA. Highly differentiated batches will require more processing steps to obtain recycled concrete aggregates (RCA) of suitable quality for concrete, contributing to higher process costs. Following prescreening, which probably eliminates more contaminants compared to the previous type of construction and demolition wastes, the material passes through a jaw crusher and then through electromagnetic separation. The second step of screening might additionally include an air-screening process for the removal of light particles (for instance, gypsum). Additional steps may be implemented, governed by the contamination level of the primary batch and the intended product quality. When coarse particles are present, secondary and tertiary crushing steps can be used. Fragments are obtained with a rounder shape and with less adhering mortar content. The eddy current separator may also be utilized for the removal of non-ferrous metals from construction and demolition wastes, after passing through an electromagnetic separator (de Brito et al. 2018). The pressure resulting from increasing costs of disposal, increased consumer and public awareness of environmental protection – as well as stricter legal regulations – have made the waste management sector a key position in the long-term sustainable development of the construction, demolition and manufacturing industries. Optimally, this would mean that all project-lifecycle stages (design, planning, production, construction, demolition and reconstruction) become an integrated operation (Hurley et al. 2001).

6.6.3 Main types of recycled aggregates from construction and demolition waste (CDWs) Construction and demolition waste usually disposed of in treatment plants may be divided into three categories, considering their composition and nature: • concrete blocks – the construction and demolition waste of this type is most often analyzed and used for the production of high-quality recycled aggregates (RA); it is obtained from the demolished concrete structures – e.g. walls, breakwaters, bridges, broken pavements and bricks from buildings, • mixed and clean construction and demolition wastes – the waste mainly consists of concrete, masonry or bituminous material; usually obtained via separation of different wastes in a demolition process, • mixed demolition rubble – it is often connected with a lack of source selection and requires special pre-treatment to eliminate some components. Recycled aggregates (RA) of three main types are used for concrete production: • recycled concrete aggregates (RCA), which usually consists of at least 90% of the mass of natural stone fragments and concrete (Ru and Rc, respectively, according to EN-933–11 [2009]), • recycled freemasons aggregate (RMA): rubble is an umbrella term for different mineral building materials. This kind of recycled aggregates (RA) consists of a minimum of 90% by weight of such materials (Rb in line with EN-933–11 [2009]),

190

Application of Waste Materials in Lightweight Aggregates • MRA: this type of material comprises different quantities of sorted and crushed concrete (Ru and Rc) and masonry rubble (Rb) within the limits of the two above-mentioned RAs (Silva et al. 2014).

Construction and demolition wastes are grouped into five fractions, including wood, concrete and mineral, metal, unsorted mixed fractions and miscellaneous fractions. Specifically, they can include: metals (ferrous and non-ferrous), bricks concrete, ceramics and tiles, glass, plastics, wood, tar and bituminous mixtures, stones and soils (contaminated), gypsum-based materials (i.a. plasterboards), insulation materials (i.a. asbestos), waste electrical and electronic equipment, packaging materials, hazardous substances and chemicals (i.a. solvents) (USEPA 2016a; Arisoy and Sgem 2016). Each new material must meet certain environmental and economic requirements. In other words, a given by-product is an acceptable substitute for a natural material if it does not cause any loss of performance (or little loss) and its use has limited environmental or economic effects. Some materials, if not managed in a responsible manner, may contaminate the environment, threaten public health and create problems. Building materials often comprise hazardous substances. These include asbestos (present in roofs and roof tiles, insulation and fireproofing), lead-based paints (used on roofs, roof tiles and electric cables), phenols (in polychlorinated biphenyls [PCBs], adhesives and resin-based coatings) that may be present in fireproof materials and joint seals as well as polycyclic aromatic hydrocarbons (PAHs) (often found in floor coverings and roofing felt). Hazardous waste must be segregated at source, as even small quantities contained in the construction and demolition wastes may threaten the environment and workers; in addition, they may hinder recycling. Regulations of recycled aggregate Article 11.2 (b) of the Waste Framework Directive (2008/98/EC) of the European Union and the European Parliament states that Member States shall take the necessary measures to achieve, by 2020, a level of at least 70% (by weight) of non-waste hazardous, excluding naturally occurring materials specified in category 17 05 04 (soils and stones other than those mentioned in 17 05 03) on the waste list and should be prepared for re-use, recycle or other material recovery.

This is also applicable to backfill operations replacing other materials with waste. One of the first and most simplified methods of regulating the use of recycled aggregates (RA) in production of concrete corresponded to setting strict incorporation limits and imposing specific physical properties enabling the attainment of the concrete exhibiting the properties comparable to traditional material (RILEM 1994). Despite the significant number of studies published over the previous 20 years, there has been little development in the existing specifications and standards pertaining the influence of recycled aggregates on concrete properties. Specifications and standards have been published in several countries to further cover construction and demolition wastes (Gonçalves and de Brito 2010). These normative documents greatly differ because of the numerous types of building materials utilized in various regions and the refining processes used in different countries, which may lead to recycled aggregates (RA) with significantly changing properties. Nevertheless, the main types of particles in RA were established in existing specifications and standards; they include recycled masonry aggregates (RMA) derived from aerated concrete blocks and lightweight concrete, blast furnace slag blocks, ceramic bricks and bricks and sand-limestone bricks as well as recycled concrete aggregates (RCA) derived from crushed, decommissioned concrete structures. In the EN-12620:20021A1:2008 standard (2008), recycled aggregates (RA) are evaluated and certified in accordance with their primary components;

6  •  Useful waste in the production of lightweight aggregates  191 pieces of crushed mortar and concrete are marked as Rc; unbound natural aggregate, as Ru; ceramic bricks and tiles, calcium silicate freemasons units, as Rb; glass, as Rg; bituminous materials, as Ra; and other particles, as X (clay, soil, metals, plastic, rubber and gypsum plaster). However, this standard includes a number of categories significantly differing in the content of individual components. This effectively prevents the wider usage of recycled aggregates in concrete production because producers and designers of concrete are not aware of the actual impact of these differences on the performance of concrete. According to the Polish regulations on waste classification, there are two types of construction rubble. One type does not pose a threat to the environment and is not listed on the list of hazardous waste, while the second type, containing hazardous substances, is included on that list. Construction waste has been classified into group 17 under the name “Waste from construction, renovation and dismantling of construction objects and road infrastructure (including soil and soil from contaminated areas)”. In the field of waste management (including on-site), the Act obliges the planning, design and handling waste, taking into account the following hierarchy: • waste prevention, • preparation for re-use, • recycling, • other recovery processes, • neutralization, and taking into account the principle of proximity – i.e. processing waste in the first place at the place where it is generated. The classification of construction rubble according to the Ordinance of the Minister of the environment of 27 September 2001, which defines: • • • •

groups of waste depending on the source of their formation, subgroups and types of waste and their codes, list of hazardous waste, the way of classifying waste.

The average recovery rate by 2020 in the EU (27) was 47% (Vegas et al. 2015). In Italy, 77% of CDWs were recycled in 2018 (ISPRA 2020); therefore the 70% target seems achievable. However, the amount of construction and demolition wastes earmarked for landfilling is still significant (24%). Simultaneously, other sources reported a lower recovery rate approximating 10% (Legambiente 2017). This is because 77% relates to construction and demolition streams wastes processed and stored but not yet recovered and unused in actual applications. The processed materials are usually collected in treatment plants with no specific outlets. Large storage areas in treatment facilities are often temporary landfills (Gálvez-Martos et al. 2018).

6.6.4 Application of CDWs in civil engineering A high recycling rate was achieved when comprehensive selective demolition measures were adopted in certain countries, such as Germany, the Netherlands and Denmark. Recycling was also favoured by the development of a strict environmental policy, and thus, facilitated the wider use of recycled aggregates (RA) in construction applications. Overall, there is a market for reusing the aggregates from construction and demolition wastes in drainage, road construction and various other construction projects. CDWs segregation and recovery

192  Application of Waste Materials in Lightweight Aggregates technologies are well developed, readily available and generally inexpensive. Despite its potential, recycling and material recovery rates vary widely across countries around the world (from < 10% to over 90%).

6.6.4.1 Application of recycled aggregates (RA) Recycled aggregate (RA) is typically utilized as a bulk backfill in the surface or base material in road construction (Jia et al. 2015), in lean concrete foundations, in new concrete production and hydraulically bound materials. Recycled materials utilized in pavement construction may be grouped into the following categories: • • • • • • •

reclaimed asphalt pavement (RAP), recycled concrete aggregate (RCA), waste rock (WR), crushed brick (CB), fine recycled glass (FRG), industrial by-products (e.g. slag and fly ash), industrial and domestic waste (e.g. broken glass).

RAP and RCA constitute the most frequently utilized recycled materials, used as an unbound granular substructure and underlay for road paving. The load-bearing layers and substructures are the main structural elements of the pavement. Elevation of the traffic loads and distributing them over the framework and substrate, the stresses are significantly reduced and the substrate does not have significant deformations. The load-bearing layer supports the pavement and is a stable platform for paving the road. The substructure also helps to drain excess water. Recycled concrete aggregate (RCA) may be used utilized as a 100% replacement for natural aggregate in the bearing layer and substructure (CCAA 2013). Many researchers have recently studied geotechnical properties, including moisture, physical, plastic and mechanical properties and the load-bearing capacity of recycled construction and demolition wastes aggregates to assess their suitability for road applications. Arulrajah et al. (2014) claimed that waste rock (WR), crushed brick (CB) and recycled concrete aggregate (RCA) met the strength parameters and can be used in substructures/pavement sleepers. However, the recommendation was to mix recycled medium glass, fine glass and reclaimed asphalt pavement (RAP) with aggregates or higher quality additives. Geotechnical and geo-environmental assessment of road embankments constructed from processed construction and demolition wastes indicated that the quantity of pollutants emitted from construction and demolition wastes is significantly below the limits established by the European Directive 2003/33/EC. By conducting triaxial compression tests, it was shown that the mechanical parameters of recycled materials are comparable to those for natural aggregates with a similar particle size distribution (Cristelo et al. 2016). Using 100% recycled waste in road bases reduces the maximum dry density and increases the moisture content of the subbase materials. California Load Index (CBR) for construction and demolition wastes and recycled material was low compared to natural aggregates. Plain, crushed concrete has also been found to have superior properties compared to the construction and demolition wastes with the addition of clay bricks (Poon and Chan 2006). The research on the geotechnical properties of five main categories of construction and demolition wastes, including fine recycled glass (FRG), reclaimed asphalt pavement (RAP), waste rock (WR), crushed brick (CB) and recycled concrete aggregate (RCA), indicates that recycled concrete aggregate RCA and WR have the geotechnical properties that are very similar or better than typical quarry

6  •  Useful waste in the production of lightweight aggregates  193 granular subbase materials. It was found that CB, with a decreased target moisture content equal to 70%, also meets the requirements of typical materials for granular stone frameworks. In contrast, FRG and RAP failed to meet the requirements for paving materials (Arulrajah et al. 2014). Granular construction and demolition materials can also be used to build a compound stabilized framework using binding materials, such as portland cement. By examining the efficiency of using recycled concrete aggregates of treated cement in a road construction in Malaga, Spain, it has been found that the increasing cement content enhances compressive strength (Perez et al. 2013). Depending on its quality, recycled aggregate may only be utilized in concrete production or hydraulically bound materials if the waste aggregate does not contain hazardous contaminants. In most cases, they only occur on the surface layers of old concrete and thus present in trace amounts. In other circumstances, potentially hazardous pollutants can be water-insoluble, effectively making them harmless in concrete with the addition of recycled aggregate (de Brito and Saikia 2013). The possibility of using concrete with recycled aggregate (RAC) has been demonstrated in relation to its mechanical, strength and construction properties (Xiao et al. 2012; Pacheco et al. 2015; Tošić et al. 2021). When recycled aggregate is used as a direct replacement for virgin aggregate at the same water/ cement (w/c) ratio, the strength of the resulting concrete is usually lowered. However, Dhir with coworkers (1999) showed that a certain amount (20–30% of the weight of coarse aggregate) can be added to concrete without affecting its performance. In the case of higher content of recycled aggregate, the strength can be easily compensated by reducing the water/cement ratio. A method of designing a mixture was developed, the obtained strengths of which exceed 80 N/ mm2 (Dhir et al. 2003). The lower strength of concrete with the addition of recycled aggregate concrete is usually associated with a weaker transition zone between the mortar and aggregate due to the fact that a layer of mortar is already adhered to the aggregate (Ryu 2002; Etxeberría et al. 2006), which additionally increases the porosity of concrete (Gomez-Soberon 2002; Sanchez de Juan and Gutierrez 2009). In addition, recycled concrete freemasons aggregate (RMA) often has even lower strength due to the presence of weak and porous aggregate particles; for example, bricks and low-strength plaster. However, MRA with a high stone content shows a higher strength than with recycled additives concrete aggregate (RCA) (Paine and Dhir 2010). Processed construction and demolition waste is frequently used as recycled aggregate, replacing natural aggregate in mortars. The CEM I-42.5R cement was used for the mortars, in addition to washed natural sand (NS), commercial lightweight aggregate, recycled concrete aggregate (RCA), crushing of unusable precast aggregates and mixed recycled aggregate (RMA). The bulk densities of the recycling aggregate are within the limits of the EN 1015–10 standard. Recycled, fine-grained concrete performs better than mixed, recycled aggregate. Following addition of recycled aggregates, the water absorption of the mortars rises. As a result of porous structure, the lightweight mortar with the addition of expanded clay exhibits weaker mechanical properties. Differences in the strength of the mortar between recycled and reference mortars are insignificant. A direct relationship was found between the mechanical strength of the design of mixtures and their water/cement ratio, the former decreasing with increasing water content in the mixture. Hence, the quality of the recycled aggregate used has a negligible influence on this behaviour (Muñoz-Ruiperez et al. 2016).

6.6.4.2 Application of recycled masonry aggregates (RMA) Building rubble is a relatively weak material in terms of mechanical properties. However, this material has the properties that allow it to be used in road engineering and geotechnics. First of all, construction rubble is a non-shed, frost resistant material. Moreover, British research shows that building rubble is characterized by a high CBR load-capacity index. These features enable it to be used in layers of improved flooring and as reinforcement layers on weak soils.

194  Application of Waste Materials in Lightweight Aggregates

FIGURE 6.15  Use of recycled masonry aggregates (reprinted from publication Pourkhorshidi et al. 2020).

This will allow for the preparation of load-bearing substrate for road surface structures, eliminating the use of mineral materials, the resources of which are decreasing. The second direction of using construction rubble is the construction of road embankments. Here, a very large application of this material can be envisaged (Figure 6.15). Relevant application areas only arise if the material contains more than 80% of bricks by weight, being screened into fractions prior to use. The necessary high content of brick may only be achieved by pre-separation, selective demolition or collecting the material from roof reconstruction. The sand with particles < 5mm, which is produced from pure-brick rubble from a brickyard or from a mixture of used and unused debris, may be utilized on sports fields or tennis courts. In this case, the layers on the underside constitute a load-bearing layer and a drainage layer, respectively. The top layer is a wear layer, which consists of brick sand with a thickness of 0–1, 0–2 and 0–4 mm. The coarse aggregates with a share of ceramic tiles and brick > 80% by weight may be utilized as a substrate for roof gardens. Finer components, such as pumice stone, concrete and mortar residues, stone and aerated concrete have no effect on the quality. The material should be free of iron and other metals. Such properties as finely divided grains or high water-absorption are desirable in these kinds of applications, as they improve shear strength and nutrient storage. Depending on the type of plants planned and the angle of the roof, it is possible to implement designs with one or more layers. The use of recycled masonry materials in cement-bonded systems is under development. The use of brick chippings is proposed as lightweight aggregate or as aggregates for slabs used on terraces and pavements (Mueller and Stark 2002).

6  •  Useful waste in the production of lightweight aggregates  195 Made of construction rubble (RMA) and concrete aggregates (CTMiGr) and variable cement content, it is possible to obtain the mixtures with good workability, which are particularly influenced by the moisture content of the RMA. However, dry density decreases as the RMA content increases. This results from the low density of RMA and its high water-absorption. The content of RMA in CTMiGr constitutes another parameter that influences the compressive and indirect tensile strengths of the mixture, in addition to the water/cement ratio and dry particle density (Xuan et al. 2012).

6.6.5 Application of CDWs in lightweight aggregate Recycling industrial by-products as well as construction and demolition waste is an effective method of obtaining lightweight aggregate. Masonry rubble in the composition, which includes concrete rubble, fired bricks and mortar as well as other industrial by-products, including ground granular blast furnace slag (GBFS) and fly ash, are suitable materials for the manufacturing of lightweight aggregates. The fired clay brick found in the masonry rubble has similar chemical composition to that of the clay. Then, crushed clay – i.e. brick and mortar with a particle size from 10 to 40 mm – is ground and added to granulated blast furnace slag (GBFS) and fly ash having particle size below100 μm. In order to obtain the expansion effect of the granules, the SiC blowing agent was added in the amount of 3% by mass. The influence of the process parameters – i.e. temperature (Series 1) and burning time (Series 2) – on the expansion of the aggregates was investigated. In series 1, ground brick/mortar in the amounts of 20/80, 40/60 60/40, 80/20% were mixed together, while, in series 2, different mixtures were obtained with the addition of industrial waste – i.e. brick, mortar, fly ash, GBFS. The formed granules were subjected to drying and firing in an oven for 6 minutes, 9 minutes and 12 minutes at a temperature of 1160 to 1250°C. It turns out that waste materials – e.g. fly ash and brick – have a chemical composition in the ternary diagram of SiO2 – Al2O3 – (Fe2O3+CaO+MgO+Na2O+K2O) described by Riley and Wilson. Other materials – i.e. GBFS and mortar – are comparatively far from the clay expansion area. The sintering conditions depend, to a small extent, on the chemical composition of the materials. Optimally, sintering temperature should range from 1220 to 1250°C, and the obtained lightweight aggregates from the combination of industrial waste materials and construction and demolition waste and have a density below 1000 kg/m3, a strength of 1–8 N/mm2 and water absorption of 10–16%. Recycled lightweight aggregates can be used as a substitute for natural aggregates in lightweight concrete or as a porous material for indoor planting (Nguyen et al. 2021). A mixture comprising autoclaved aerated concrete rubble and sand fractions from masonry rubble may also be used as a raw material to manufacture lightweight aggregates. Silicon carbide (SiC) expansion agent was added to the comminuted raw materials with particle size < 100 µm. Other additives, like NaCl, CaSO4·2H2O, coke powder, beet sugar, did not meet the expectations due to aggregate expansion. The masonry powder may be used in the amount up to 100% of the mix, whereas the content of autoclaved, aerated concrete should be kept below 50%. During the thermal treatment, gas is generated inside the granules in the temperature range at which the melting phase is formed. Firing temperature ranges should be in the range of 1260–1290°C. When firing temperature is below 1260°C, complete decomposition of SiC is impossible, whereas the melting phase is formed in insufficient amount. When the temperature is too high, the granules shrink and, as a result, their density increases. With regard to the amount and degree of grinding of the expansion agent, its amount is important. If too small amounts of higher fraction than the raw material are added, granules with high density are obtained. Conversely, larger amounts and finer SiC grains increase the pore formation, contributing to lower density. The light granules have a density of 0.62 g/cm3 after the addition of 3% SiC. Compact and low-porosity granules are made without the addition of SiC. Therefore, the density of granules can

196  Application of Waste Materials in Lightweight Aggregates be modelled by appropriate dosing of the agent responsible for the porosity of the aggregates (Mueller et al. 2008). Lightweight aggregate was also produced from by-products, such as clay bricks separated from masonry waste with a diameter of 0/4 mm and the waste from the aerated concrete industry (powder 0/1 mm). The proportions of the main components ranged from 33 to 67%. The aggregates were obtained in two variants. Aerated concrete with a fraction of 4/8 mm was covered with a layer of masonry powder; thus, the porous grains were sealed and stabilized under the influence of water and burnt afterwards. Taking into account the large share of the sandy fraction from the process of crushing aerated concrete, the second version of aggregates was developed – i.e. from aerated concrete with a fraction of 0/2 mm, granules with a size of 4/8 mm were formed, which were covered with masonry powder and then fired. The porosity and particle size distribution of aerated concrete sand are disadvantageous for granulation; hence, it is necessary to use various binders and a significant amount of water. These facts make the production process more complicated. Version 2 granules achieve water absorption of approx. 10% and bulk density of 1.6 g/cm³. On the other hand, in version 3, both wastes were ground and thoroughly mixed, which resulted in a significant improvement in the granulation process. It turns out that the solidification of thick, aerated concrete fractions or the use of stabilizing layers combined with thermal treatment in a rotary kiln that corresponds to versions 1 and 2 is challenging. Firing aerated concrete to a temperature > 1200°C results in thermal decomposition of C-S-H phase and calcite, consequently causing reduced strength and shrinkage of aerated concrete grains. Comminution of the raw materials (version 3), contributes to good dispersion of aerated concrete grains in the melting phases which are formed by the masonry powder. The good obtained-aggregate properties and high granulation-capacity of the material make method 3 the most effective. The properties of granules from fine-grained raw materials are dependent on the firing process parameters and mixture composition. The required firing temperature is determined by the melting points of the raw materials (1200–1280°C for aerated concrete and 1190–1300°C for brick). The temperature must be high enough to form a sufficient quantity of the melting phase, binding the particles together and enabling effective work of the expansion agent (SiC). However, the melting phase must be low enough to ensure the stability of the granulate. Up to the heating temperature < 1265°C, SiC does not decompose completely and the melting phase is sufficiently formed in the mixture, whereas at the furnace temperature exceeding 1290°C, the granulate shrinks, resulting in an increase in its density. The temperature range of 1260–1290°C yields the granules having the lowest density of approximately 0.6 g/cm3. The second important factor affecting the lightweight aggregate properties corresponds to the expanding agent content. Silicon carbide (SiC) added to the raw material mixture gives off gas in a broad temperature range from 800 to 1150°C. By changing its content and particle size, the pore size and the structure of the granules may be modified. In the case of the tested mixtures, the highest expansion effect was achieved with the SiC content in the range of 3–6%. Aggregates show the lowest bulk density, while excessive concentration of SiC causes the granules to collapse. The water absorption values are inversely proportional to the bulk density value. It is possible to control the properties of the produced granulate in a wide range by the SiC proportion, the degree of its grinding and changes in the furnace temperature. Increasing the firing temperature reduces the aggregate density (Reinhold and Mueller 2002). The content of bricks in masonry rubble also affects the density of lightweight aggregate. With a content of 20% by mass, the density of the aggregates is below 1000 kg/m3, while the lowest density values were achieved when the brick content was 40–70% by mass. In terms of strength and water absorption, the produced lightweight aggregates are not different from commercial expanded clay. They meet the requirements for chemical properties and environmental parameters. Lightweight concrete was obtained from lightweight aggregates with a diameter of less than 2 mm, replacing sand, as well as with a diameter of 2/4, 4/8 mm with the CEM I 32.5 R cement addition and a water/cement ratio equal to 0.45. The carbonation rate of the concretes comprising lightweight aggregate made of masonry rubble is similar to that of the reference concretes. The compact aggregate/cement slurry interface contributes to the high durability characterizing lightweight concrete.

6  •  Useful waste in the production of lightweight aggregates  197 There is a significant potential for the application of the lightweight aggregates made of masonry rubble in the construction industry. They may be utilized as aggregate for structural lightweight concrete in addition to the production of blocks or concrete elements. As a result of low thermal conductivity, rubble aggregates can also be used as loose insulating material. The lightweight aggregates from masonry rubble may be manufactured using the LECA production technology, in accordance with the basic stages of the process presented in the Figure 6.16.

FIGURE 6.16  Simplified process of producing lightweight aggregates from masonry rubble (reprinted from publication Mueller, A., Schnell, A., Ruebner. K., The manufacture of lightweight aggregates from recycled masonry rubble; Construction and Building Materials 98, 376–387, Copyright [2015] with permission from Elsevier).

198  Application of Waste Materials in Lightweight Aggregates The waste material is pre-stored and homogenized. The production process begins with shredding construction debris to which an expansion agent is added. The raw materials mixed in the mixers are then granulated on presses and granulation discs. Thermal stabilization is performed in a rotary kiln. In the auxiliary cooler, the heat of the pellets is used to pre-heat the combustion air. A neutral release agent must be used. Expanded granules are sorted upon leaving the cooler. The different steps in the process can be separated and spread out in several different places. Production of green granules need not take place on the site of the rotary kiln. The optimal approach depends on the adaptation to local constraints. The lightweight aggregate from masonry rubble is advantageous in that its production requires few primary resources. Only the release and aggregate expansion agents are partially used in the thermal process. Industrial production of lightweight, recycled aggregates can replace expanded clays and natural pumice as well as decrease the consumption of natural resources. Energy savings are achieved because the masonry material used to make the aggregate needs less water for sharpening compared to the clay. Preliminary cost assessment shows that an energy saving of about 15% is possible, compared to the production of aggregate from expanded clays. Obtaining aggregates from recycled materials can effectively reduce the sulfate content of demolition waste because gypsum may be decomposed thermally, followed by its recovery from flue gases (Mueller et al. 2015). The multi-stage method of thermal and hydrothermal hardening was used to produce lightweight aggregate from masonry rubble containing 50% brick. In the thermal method, 3% SiC was added as a blowing agent to the raw material mixture containing powdered rubble, and the formed granules were fired in a rotary kiln at a temperature of 1180°C. In the hydrothermal hardening method, caustic lime powder (CaO) was used in an amount of 5, 7 and 9% as a binder, and green pellets were processed in an autoclave at 200°C and a steam pressure of 1.6 MPa. In terms of mineralogy, quartz is the primary component of the rubble, followed by calcite (Rübner et al. 2013). The strength of aggregates is enhanced by calcium silicate hydrates (C-S-H), formed via the reaction between quartz and hydrated lime (Eden 2011). All aggregates satisfy the requirements for lightweight aggregates with a bulk density below 2000 kg/m3, particle strength of more than 1 MPa. The morphology of these two types of aggregates is clearly different. Thermally hardenable granules consist of partially fused areas; there are macropores and voids ranging from micro- to millimetres. The shape of pores is irregular. Conversely, the hydrothermal, curable granules are characterized by a microstructure of greater density, created by agglomerates of mineral phases and sintered particles. There are fewer pores, usually with a size of less than 20 µm. The earlier-mentioned qualitative observations may be connected with the total porosity of aggregates, which, in the case of thermally hardened aggregates, exceeds 50%, and in the case of the aggregates obtained as a result of hydrothermal treatment, is lower than 40%. Thermally hardened granules are characterized by partially fused glassy areas and large internal macropores which are connected through narrow throats. They achieve the desired properties, typical for lightweight aggregates. In contrast, hydrothermally cured pellets have a uniform mesoporous system containing ink-bottle and plate-like pores. Such properties are demonstrated by the cement slurries containing a C-S-H gel with a macroscopically dense microstructure (Rübner and Hoffmann 2006). The size of most pores is below 100 µm. The variation in the pore volume distribution indicates that the most common pore diameters are around 20 µm, remaining constant as the bulk density of the aggregate changes. Owing to two production methods and variable process parameters, it is possible to adapt lightweight aggregates to individual needs (Rübner et al. 2015).

6  •  Useful waste in the production of lightweight aggregates  199

6.7  SEDIMENTS DREDGING 6.7.1 Harbour sediment 6.7.1.1 Harbour sediment production An important process related to the pollution of marine sediments is the dredging of bays/ports as a consequence of the activities aimed at their maintenance. The growing importance of seaports and the increase in container handling, resulting from the globalization of the global economy, is associated with the need to maintain the navigability of fairways, canals and port basins – i.e. carrying out systematic dredging works of varying intensity, which result in significant volumes of spoil necessary for management (Bray and Cohen 2010; Chen et al. 2016). The annual amount of sediments dredged in France exceeds 56 million m3, compared to 300 million m3 in the USA, 200 million m3 in Western Europe (CEDA 2008) and 100 million m3 in China (Oh et al. 2011). During these works, the living conditions may deteriorate for bottom-dwelling organisms, but also for fish, birds, mammals and humans. The current, frequently occurring problems related to dredging works in the coastal zone include: • poorly developed activities concerning the practical use of the excavated material, including the criteria for the use of the material for artificial shore supply, • lack of specificity in the legal aspects containing the criteria for assessing the degree of contamination of the extracted spoil in the Polish legislation, • no rules for designating new sea lap sites and monitoring of the existing ones. The deposit of spoil can generate negative effects and disrupt the naturally occurring balance in the aquatic ecosystem. Dredging is a necessary process for many reasons, which include preventing flooding, facilitation of navigation, maintaining port functionality and accommodating all uses of the water system in question (maintenance and extension work). However, this process is also associated with the problem of management of the excavated sediments. The extracted, dredged spoil (depending on its type and degree of pollution) can be economically used, deposited on silting fields or dumped in separate areas of the seabed – the so-called sea lap sites (London Convention 2002; Heise 2007; Bortone and Palumbo 2007; Barcelo and Petrovic 2007; Bray 2008; Uścinowicz 2011; HELCOM 2015). Due to the fact that sediments are the final reservoir of many pollutants, their re-placement in the sea poses a potential threat to biological life in a given reservoir because, under favourable conditions, pollutants may be released from the sediment into the waters and, as a result, their circulation in the environment can be restarted (Barcelo and Petrovic 2007; Uścinowicz 2011). The current maritime policy, regulations as well as research efforts undertaken constitute an attempt to account for various aspects of the sediment pollution problem, but these regulations and actions are not consistent and do not contribute to efficient management (Bortone and Palumbo 2007). So far, no comprehensive and uniform strategy papers have been developed at European level for sludge contamination and in particular for spoil management. As regards the handling of extracted spoil and its storage at sea and on land, general guidelines in international law are contained in three Conventions: the Convention on the Prevention of

200  Application of Waste Materials in Lightweight Aggregates Pollution of the Sea by Dumping of Wastes and Other Substances (London 1972), the Convention on the Protection of the Marine Environment the Baltic Sea Region (Helsinki, April 9, 1992), the OSPAR Convention for the Protection of the North-East Atlantic Marine Environment (1972) and the European Directives based on them. Detailed conditions, necessary data and the procedure for issuing permits for disposal of dredging spoil into the sea and dumping of waste or other substances in the sea are described in the Regulation of the Minister of Transport and Construction of 26 January 2006 (Journal of Laws of 2006, No. 22, item 166). The Helsinki Convention describes the priority groups of substances that are commonly considered harmful – i.e. heavy metals as well as their compounds, organic tin and phosphorus compounds, organic halides, pesticides including myxomicides, insecticides, herbicides and fungicides, wood-preservative chemicals, paper, cellulose, cellulose pulp, textiles and leather, hydrocarbons and petroleum oils, other organic compounds having adverse effect on the marine environment, phosphorus and nitrogen compounds, radioactive substances, radioactive waste, persistent materials capable of floating, remaining in suspension or sinking and substances that seriously affect the taste and/or odour of seafood products, as well as the colour, clarity, smell, taste or other properties of the water. In terms of dredged material management, the European Union Member States must conform to the applicable European directives – e.g. Marine Strategy Framework Directive 2008/56/EC, Water Framework Directive 2000/60/EC, Natura 2000 sites under the Birds and Habitat Directives (2009/147/ EC and 92/43/EEC). In addition to the previously used practice in the field of excavation testing and determining its properties, one of the most important international guidelines that could become a proposal for Polish legislation is the HELCOM Guidelines for the Disposal of Dredged Material, Adopted in June 2007, Helsinki Commission, Baltic Marine Environment Protection Commission. Technologies of dredging Extraction, transport and sludge disposal constitute three stages related to the dredging works (Highley et al. 2007; Manap and Voulvoulis 2015). These steps are repeated until the target amount of sludge is extracted; each step requires the application of varying technologies. The development of the dredging industry caused different types of dredgers to now be available, depending on the type of sediment and their depth of occurrence. Dredging is initiated via extracting the sediments at the site using a mechanical and/or hydraulic cutter (Antipov et al. 2006; Du and Li 2010). Both maintenance and capital dredging may necessitate extraction methods. All hydraulic dredgers transmit the fluffed material from the in-situ state in slurry form via a pipe system which is connected to a centrifugal pump. If the sediment is very loose in a natural state, it may be enough to use suction alone; however, mechanical loosening or water jet application may be necessary for harder material. In the case of fine materials, hydraulic dredging is characterized by highest effectiveness because they are easy to keep suspended. Thicker materials – and even gravel – can be machined, but this requires more pump power and contributes to faster wear of pipes and pumps. A suction dredger is a stationary device employed for sand extraction. The suction tube is driven vertically into the sand bed. Sand is loaded onto barges or pumped directly to the cultivation site through a pipeline. In its simplest form, a profile or common suction dredger comprises a pontoon on which a suction pipe and a pump can be attached, enabling to connect them to the discharge pipe. Primarily, these types of dredgers are employed to obtain filler material for reclamation, where the material is deposited ashore by a floating pipeline. Modern suction dredgers are capable of extracting material from great depths as well as lifting sand from beneath a clay overburden. This type of dredger, known as a submersible suction dredger, is capable of extracting filler material from a depth of up to 100 m. The capacity largely depends on

6  •  Useful waste in the production of lightweight aggregates  201 the permeability characterizing the dredged material, being the best on clean sands (https://europeandredging.eu/Hydraulic_dredger). Finally, the dredging sludge is disposed of at the selected site. Storage of extracted spoil in the sea and on land Contaminated sediment is generated at increasingly higher rate as a result of the rapid expansion of industrialization and urbanization (Wang et al. 2018). Direct ways of disposal include: • storage in landfills, • use in agriculture and forestry, • combustion, • sea dumping (Weilin et al. 2016). Generally, open storage is not allowed when dealing with heavily contaminated sediments (Morton 2001; Su 2002). Remediation is frequently necessary in the case of contaminated dredging sediments – e.g. by mechanical agitation as well as aeration (Manap and Voulvoulis 2015). Other techniques used for remediation also involve chemical oxidation, vitrification, solidification/stabilization, electrokinetics, bioremediation, thermal extraction, isolation, physical separation processes and sequential extraction techniques (Mulligan et al. 2001; Pensaert et al. 2008). In justified cases (economic, environmental), the spoil is stored on land, on the so-called silting fields – that is, appropriately located and arranged (in accordance with the spatial development plan), mostly in coastal areas. The silting fields contain both uncontaminated and contaminated sediment. If contaminated spoil is found, it must be separated from the uncontaminated one and stored in a separate part of the silting field, protected against the migration of pollutants into the groundwater as well as against the infiltration of atmospheric precipitation. The storage of dredged material may not deteriorate the quality of the soil and the requirements of the Regulation of the Minister of the Environment of 9 September 2002 on soil quality standards (Journal of Laws No. 165, item 1359) (Boniecka et al. 2008, 2010b, 2013). Disposal of dredging sludge by landfilling is becoming an issue of increasing public concern due to the impact of negative effects on the quality of the environment and ecosystems (Ju et al. 2016; Chen et al. 2017b, 2018). Substantial areas of land are required for landfills (Sayadi et al. 2015), and other disposal methods can result in a range of environmental hazards related to the presence of heavy metals, pathogens as well as persistent organic pollutants found in dredging sludge (ThanhBinh and Kazuto 2019; Ferrans et al. 2020). Along with high land prices and growing public awareness concerning environmental protection, dredging sludge disposal constitutes a broad alternative to cost reduction and protection of natural resources (Qi et al. 2016). The main problems of onshore sludge storage include the high cost of the equipment used; for example, pipelines, drainage pumps, field embankments, transport from the place of their collection to storage. In addition, while dredging is in progress, huge land areas are necessary for drying the spoil of excavated sediments. One should also add the aspect of the small capacity of such fields, which, in the places of intensive dredging works (mainly in the vicinity of Szczecin), are insufficient (Boniecka et al. 2008, 2010b, 2013). Depositing of dredging spoil and port basins in the aquatic environment may have negative effects and disrupt the naturally occurring balance in the aquatic ecosystem. Moreover, the displacement of contaminated sediments poses a significant threat to the environment, forcing them to be treated in a special way defined by legal standards (Dembska et al. 2012).

202

Application of Waste Materials in Lightweight Aggregates

Offshore spoil storage is economically beneficial, but it can pose a real threat to the environment. The places intended for this purpose are not specially protected. Only places in the sea are chosen for their location, where the natural depth of the bottom allows for depositing certain, usually quite substantial, thickness of the spoil layer without fear of blurring it by sea currents or undulations, as this could create various obstacles in sea navigation. There is also no control and monitoring of such sludge storage sites (Boniecka et al. 2013). Pollutants of anthropogenic origin found in port sediments can result in secondary contamination during re-use (Dong et al. 2013, 2015). Numerous methodologies and approaches for their reuse have recently been developed worldwide (Hamer and Karius 2002). In general, the dredging sludge processing processes concern (Lirer et al. 2017): • • • •

drainage (mechanical or natural), particle separation (washing and sorting), biological, chemical and thermal treatment, immobilization of pollutants (thermal stabilization, chemical oxidation).

Detailed treatment of sludge is to improve their physicochemical and biological properties in order to degrade, remove or immobilize pollutants. The most common methods are: • physical treatment (flotation, vacuum extraction), • chemical treatment via chemical oxidant addition to trigger an elimination reaction, • solidification/stabilization by adding a binder in the form of lime, cement, clay or pozzolana. This process may be applied at low temperatures (110–150°C), • biological treatment methods (Rotamix™, Sesatec™, bioreactor, bioremediation, phytoremediation), which involve the use of enzymes, algae, fungi and bacteria to precipitate organic pollutants, including oils or polycyclic aromatic hydrocarbons (PAHs), • heat treatment (incineration, thermal desorption, oxidation, pyrolysis, vitrification) at high temperature from 600 to 1200°C, • flash calcination process used to treat the dredged sludge to activate certain sludge phases (including clay phases). All modifications caused by the treatment process are aimed at improving the sludge properties and producing an active material (Amar et al. 2020). However, they are costly, long-lasting and often not effective enough (SMOCS 2012).

6.7.1.2 Harbour sediment characteristics Chemical and physical characteristics Sediment characterization relates to the role of sediment as a contamination source. Sediments adsorb and trap pollutants deposited on the bottoms of seas and rivers, both from diffuse and point sources (Burton 2002). The sediments also retain nitrogen and phosphorus components. The sediments retain as well as transport heavy metals – e.g. Cr, Zn, Zn, Cd, Hg, As, Cu Pb, Ni and Cl (Baruzzo et al. 2005; Wei and Lin 2009; Huang et al. 2011; Lim et al. 2020). These metals originate – i.a. from underwater volcanic processes and weathered sedimentary rocks. They are characterized by high nutrient concentrations (P and N) and some organic pollutants (polychlorinated naphthalene and hexachlorobenzene that are subsequently discharged into reservoirs, rivers and lakes, which has a negative impact on aquatic ecosystems) (Peng et al. 2009; Chen et al. 2015a; Unyimadu et al. 2017).

6  •  Useful waste in the production of lightweight aggregates  203 They usually contain a high amount of water, organic matter as well as salts (He et al. 2020). The dredging material is a suspension consisting of solid grains (fine and coarse) in addition to a high amount of water, the chemical properties of which are dependent upon the dredging environment. For a long time, the sludge from dredging was considered waste and, as a consequence, was stored mainly as suspension. The grain size of the sediments varies. They are divided into fine particles up to 2 µm (clay), up to 16 µm (silt), from 63 mm to 64 mm (sand and gravel) as well as over 64 mm (rock). Sediment pollutants can be transported in various forms, either as dry gas, as dry solids or as wet deposition. Sediments are transported via ocean and wetland systems as well as tides (Nielsen 2009; Office of Naval Research 2008). The main chemical components of the harbour sediment that play a special role in the process of obtaining lightweight aggregates include SiO2, Al2O3, Fe2O3, CaO, Na2O, K2O and MgO, but their composition and quantity may significantly differ depending on the source of origin (Table 6.11). SiO2 and Al2O3 oxides contribute to the glassy phase formation of appropriate viscosity in the process of burning off the aggregates, while Fe2O3, K2O, Na2O, CaO and MgO as fluxes generate the gas necessary to create a porous structure. Low loss on ignition (LOI) values in sediments may indicate a higher density of lightweight aggregates (Lim et al. 2019). Analysis mineralogical and thermal Dredging sediments are a multiphase material, consisting of several crystalline minerals and a rare glass phase (Table 6.12). The mineral composition of all sediments was found to contain quartz. Plagioclases, the content of which often ranges from 10 to 40%, are represented by microcline and albite. In turn, the group of clay minerals, the amount of which can range from 25 to 55%, includes kaolinite (25–45%), illite (15–40%), chlorite (0–20%), smectite (< 10%) (Gilot et al. 2021). Usually, the carbonate minerals include calcite and dolomite, and the microcline and margarite groups are included in the microcline group. Occasionally, aragonite, rutile and hematite have been identified. Moreover, the content of the amorphous phase is low and is very rare in sediments. The differential thermal analysis of the sediments shows that the initial weight loss at ambient temperature to 150°C results from moisture evaporation. The release of water from minerals, decomposition of sulfur compounds and combustion of organic substances take place at temperatures in the range of 300–700°C (Qi et al. 2010). The decomposition of MgCO3, Mg(OH)2 and Ca(OH)2 occurs at a temperature of 350 to 500°C, whereas CaCO3 decomposes at 600–800°C (Garea et al. 2003). The evolution of SO2 results in a further weight loss at 900–1000°C, presumably due to the decomposition of alkali metal sulfates (Bethanis et al. 2002). Up to 1000°C, the formation of eutectic compounds may be favoured by the reaction of iron oxides (Anagnostopoulos and Stivanakis 2009). Low values of the loss on ignition LOI in the sediments may indicate a higher density (Lim et al. 2019). In turn, during the thermal analysis dredged harbour Bejaia port sediment, the weight loss at 30 to 200°C was 2.0% wt. as a result of physically absorbed water, indicated by a slight endothermic peak, the maximum of which is around 70°C. The weight loss at a temperature of 200 to 600°C is 6.0% by mass. Two thermal effects can be observed in this temperature range. The first exothermic peak was recorded at temperatures of 200–380°C, which is attributed to the combustion of the organic substance. The second endothermic peak, which characterizes the dehydroxylation of kaolinite and obtaining metakaolin, was recorded at 490°C. At 600–800°C, the greatest weight loss is 9.8% by mass and is the result of carbonate decomposition with CO2 release. Endothermic peak at 680°C has little effect on temperature. Finally, at 800–1100°C, a 1.2% weight loss occurred, attributed to the recrystallization and reorganization of metakaolin as well as the formation of aluminum-silicon spinel, as indicated by the low exothermic activity of the sludge (Slimanou et al. 2019).

204  Application of Waste Materials in Lightweight Aggregates

6.7.1.3  Harbour sediment application in civil engineering The storage of spoil at sea or on land leads to an irreversible loss of the obtained material as well as interference with the sea/land environment. The current pro-environmental tendency indicates the reuse of waste in practice, or its recycling, and not only its disposal. The best-known solutions for the use of spoil include: • • • • • • • • • •

artificial power supply for beaches, dunes, flood embankments, widening of land, drainage of wetlands, construction of artificial islands and reefs for fishing, expansion of land for wind fields, use in aquaculture and agriculture, filling mine areas, establishing parks, road construction as a load-bearing layer or substrate, substitute for cement and/or sand in concrete, use in ceramic materials such as brick and tiles (Siham et al. 2008; Belas et al. 2011; Oh et al. 2011; SMOCS 2012).

The reuse of sludge in the construction sector seems to be a promising solution because it decreases the depletion of natural resources as well as the emissions of carbon dioxide. Due to the high hydration of the sludge, up to 80%, it must be dried. In the case of contaminated sludge, it must additionally be cleaned or immobilized with hazardous substances, so that the requirements for a given application are met. This causes many difficulties, both economic and related to the need to respect environmental protection regulations. According to the regulations in force, such sediment cannot be dumped back into the sea. There is also a problem with its practical use because it is treated as hazardous waste. Component of cement, mortars, concrete In the framework of the sustainable development project, more and more attention is currently being drawn to the recycling and reuse of by-products and waste in the cement industry as well as the protection of natural resources. The application of sludge as supplementary cement materials (SCM) is one of the alternatives for reducing cement production and thus the CO2 emissions, the amount of which is 842 kg CO2/t of clinker (Snellings et al. 2017; Scrivener et al. 2018; Van Bunderen et al. 2019). Nevertheless, due to their heterogeneity and specific properties, appropriate treatment may be needed to improve some mechanical and physicochemical properties (Tribout et al. 2011). Cement production requires high temperatures, usually up to 1450°C, in order to degrade organic as well as inorganic pollutants. Heavy metals become stabilized, either by being trapped in cement kiln dust or in cement phases. The proposed technology has certain advantages – i.e. it employs existing technologies and facilities to develop significant amounts of dredged material, simultaneously reducing the need of cement producer for raw materials. A preferred alternative was the use of New York/New Jersey port dredging material at 1.49, 6.63 and 12.3% as a raw material to replace some of the other raw materials employed in the production of portland cement. The dredging material contained silica, oxide aluminum, iron and iron, essential for portland cement production. Depending on the quartz content of sediments, the working conditions of the furnace had to be modified. Thus, it is possible to replace 3–6% of the total charge material with dredging sludge after considering such factors as chloride scale deposition. High content of chloride in the output does not increase in the final product, although it remains a matter of practicality in production. A 6% substitution of the raw material in cement production would decrease the required fly ash amount by 100%,

TABLE 6.11 Oxide composition by weight (%) of harbour sediments in literature. AL2O3

FE2O3

CAO

MGO

NA2O

K2O

59.51

16.40

7.32

1.37

2.94

1.24

3.55

63.0

11.0

8.30

1.80

0.90

1.90

1.60

0.18

–

2.55

1.02

TIO2

SO3

P2O5

0.78

–

0.09–

–

–

–

–

0.28

LOSS ON IGNITION LOI 5.21 10.0

Derman and Schlieper (1999) Hamer and Karius (2002)

28.9

7.57

4.05

29.1

6.49

3.62

24.5

1.34

0.09

0.68

0.38

1.10

0.31

29.2

Komnitsas (2016)

37.1

4.97

2.63

21.3

1.55

1.16

1.41

0.34

0.62

0.09

27.34

Komnitsas (2016)

1.10

4.50

3.40

0.80

0.10

0.10

3.60

0.69

–

0.19

14.85

71.0

10.1

44.97 57.7 26.00 53.9

3.80

2.60

14.17

4.86

15.26

2.15

1.11

2.07

18.7

7.67

2.05

2.64

2.05

3.93

4.00

2.00

–

–

5.10

–

2.30

–

7.15

1.91

2.46

1.91

3.66

0.94

1.82

0.26

2.19

0.61

2.13

–

–

1.50

–

1.20

0.30

–

9.80 17.4

40.73

8.23

6.24

51.4

5.40

4.90

3.13 12.1

1.95

–

AUTHORS

6.60 – 6.80 21.1 –

7.67

Wei et al. (2008)

Mymrin et al. (2016) Slimanou et al. (2019) Lim et al. (2019) Chien et al. (2020) Lim et al. (2020) Wang et al. (2020a) Zentar et al. (2021)

6 • Useful waste in the production of lightweight aggregates

SIO2

205

206

COMPONENT

HAMER AND KARIUS (2002)

Silicon oxide

Q

Calcium carbonate

C

Iron oxide

H

WEI ET AL. (2008) Q

SIHAM ET AL. (2008) Q

Ha

Ha

Ky

I, Ph

P

SLIMANOU ET AL. (2019)

Other minerals

WANG ET AL. (2020c)

ZENTAR ET AL. (2021)

GILOT ET AL. (2021)

Q

Q

Q

Q

Q

Q

C

C, D

C, D

D

C

C, D

Ha

K, Mu

Ky, Al

K

I, K, S

Al

Al

Al, Mi

Al, Mi

Ma

Mu

Ha An

Mi

Micas Glass

LOUDINI ET AL. (2020)

H

Co

Feldspar

LIRER ET AL. (2017)

W

Sodium chloride I, Cl, K, S

Q

A, C

Aluminum oxide Silicates and aluminosilicates

MYMRIN ET AL. (2016)

Gl R

 

A (aragonite-CaCO3); Al (albite-NaAlSi3O8); An (analcime-NaAlSi2O6·H2O); C (calcite-CaCO3), Co (Corundum-Al2O3); Cl (chlorite-(Mg,Fe,Al)3(Si,Al)4O10(OH)2); D (dolomiteCaMg[CO3]2); Gl-amorphous components; H (hematite-α-Fe2O3); Ha (Halite-NaCl); I (illte-(K,H3O)(Al,Mg,Fe)2(Si,Al)4O10[(OH)2,(H2O)]; K (kaolinite-Al2(OH)4Si2O5); Ky (kyaniteAl2SiO5); Ma (magnetite-Fe3O4); M (mullite-K0.6–0.85Al2(Si,Al)4O10(OH)2; Mi (microcline-KAlSi3O8); Ma (margarite-(CaAl2(Si2Al2)O10(OH)2); Mu (muscovite-KAl2(Si3Al)O10(OH,F)2); O (orthoclase-KAlSi3O8); P (plagioclase-Na[AlSi3O8] to Ca[Al2Si2O8]); Ph (phillipsite-(Mg,Fe)3(Si,Al)4O10(OH)2·(Mg,Fe)3(OH)6); Q (quartz-SiO2); S (smectite- (Al,Mg)2[(OH)2/Si4O10] x Na0.33(H2O)4); Si (silimanite-Al2SiO5); R (rutile-TiO2); W (wustite-FeO).

Application of Waste Materials in Lightweight Aggregates

TABLE 6.12 Mineralogical composition of dredging sediments in literature.

6 • Useful waste in the production of lightweight aggregates

207

iron by 45% and bauxite by 8%. Numerous factors have to be taken into consideration to evaluate the feasibility and cost of this approach for the given cement manufacturing facility and dredging operation. Some of the significant costs to consider include transport to the cement producer, removal and disposal of rubble, sludge dewatering (if necessary), material delivery and storage on site, furnace modification to introduce the sludge into the hot part furnace, additional operating costs for cleaning the furnace. The reduction of these production costs may be affected by income from savings on raw materials and the lack of a dredged storage fee material. The potential of this technology can make a significant contribution to the sustainable management of dredging spoil (Dalton et al. 2004). Mortars and concretes can also be obtained with the dredged marine sediments. The sludge from the port of Dunkirk (France) was dried, ground and added to the cement in varying amounts of 10, 20 and 30%. Replacing the limestone with a 20% sediment improves the mechanical properties of the concrete (Zhao et al. 2018). The marine sediments from the port of Dunkirk were used without pretreatment for the production of concrete elements of the port shoreline. In order to maintain the good mechanical properties of concrete under the conditions of sulfate aggression and frost, the addition of 12.5% fine marine sediment was effective (Achour et al. 2019). The river sediment collected from Aire sur la Lys (North of France) was employed as a substitute for natural sand in an amount of 50% in resin mortars. Performance depends on such factors as the amount of resin and the amount of sludge used. The mechanical parameters and physicochemical properties of the products are satisfactory. Compared to cement-based mortars, the efficiency of polymer mortars with sediments is much higher. The polymer mortars show good thermal properties as well as good resistance to chemical action and thermal shock (Maherzi et al. 2020). Bricks Dredging sludge has a positive influence as an additive for brick production (Cappuyns et al. 2015). The port sediments of Dunkirk (France) as well as river dredged sediments originating from Usumacinta were employed in the production of adobe bricks. Bricks are made of 1, 2, 3, 4 and 5% of plant biomass – i.e. palm oil flower fibres and hemp shiv. The addition of natural fibres enhances the thermal and mechanical properties of adobe bricks. The tensile strength increases along with the number of fibres. The bricks containing Usumacinta sediments are characterized by the maximum strength with 4% addition of fibres, while the bricks with Dunkirk sediments with 1% addition of hemp shivs. Higher tensile strength of crude bricks from the Usumacinta sediment is derived from the sediment mineralogy and the morphology of palm oil flower fibres. The bricks have a tensile strength above 0.25 MPa, which is the value recommended by the New Zealand standards NZS 4298 (1998) (Hussain et al. 2022). Moreover, brick was obtained from harbour-dredged sediment – HDS from the port of Bejaia, which, in the amount of 5 10, 15, 20, 50 and 100% wt., mixed with the clay coming from local quarries in Rmila in Northeast Algeria. Although the content of iron oxide (Fe2O3) – i.e. red pigment – was high, the initially colourless bricks turned yellow. This probably results from the presence of carbonates, favouring the formation of calcium silicates and containing Fe3+, instead of hematite, in their structure (Singer and Singer 1963). Addition of the harbour-dredged sediment results in a slightly reduced bulk density (Slimanou et al. 2019). The urban river sediments obtained in the course of the Qinhuai River (China) dredging were employed as the basic raw material for manufacturing highly insulating bricks. The bricks, containing 50% of urban river sediments, fired at a temperature of 1050°C, show favourable properties such as compressive strength and thermal conductivity (Xu et al. 2014). The brick was also obtained from Yellow River sediments from Huayuankou (China) with the addition of inorganic cement material, polymeric binder and jute fibre. As a result of the tests, the

208

Application of Waste Materials in Lightweight Aggregates

sludge is pre-consolidated when the activated sludge/cement binder/sand mixture ratio is 65/25/10. The compressive strength as well as softening index of stabilized bricks is higher due to the addition of a polymeric binder and jute fibre (Liu et al. 2016a). Road construction Recycling of dredging spoil for road construction necessitates verifying certain geotechnical criteria, specified in building regulations. Tensile and proctor tests are the main methods of determining the suitability of a given material for use in one of the layers of a road structure. The Dunkirk sediments were used as road construction material; in particular, for the base and bearing layers of roads that require good mechanical properties. The problems of high salinity and water content of the sludge were solved by the sludge dewatering process and the addition of lime. The mixture containing only 6% portland cement may be successfully employed as a road layer material both in the base course and in the base layer. The mixture with the addition of sludge shows good workability. Moreover, it is highly competitive economically with the mixtures that are usually employed in road construction (Siham et al. 2008). An alternative method of sludge management is their valourization in road construction using the stabilization/solidification method. The sludge collected from the port of Dunkirk in northern France was mixed with a binder such as CEM I 52.5 R portland cement as well as calcium sulfo-aluminate cement (CSA) in amounts of 2, 4, 6%. Curing was performed for 3, 7 and 28 days. It turns out that, compared to the raw sludge, the thickening efficiency of the solidified sludge is much better, which reflects the improvement in the behaviour of the solidified sludge in a short curing time < 1 h. The I-CBR index increase is almost linear with the binder content increase. The binder addition to raw sludge effectively improves the mechanical properties such as modulus of elasticity, tensile strength and compressive strength. The mechanical parameters improve with the increased curing time and binder content (Zentar et al. 2021). Valourization of deepened sediments is an innovative solution to the shortage of building materials resources. Usage of sludge from Safi Harbor (Morocco) is a confirmation of the use of dredging sludge in road construction, especially as a base layer for pavements. They were taken from two port zones: the basin zone S1 and the channel zone S2 and modified with 7% portland cement. This dose is in line with the values usually used in road soil treatment, which is usually between 6 and 8%. Both sediments show different properties. Treatment of the sludge with cement improves the mechanical properties. The mixture with the addition of cement exhibits superior immediate bearing capacity compared to the mix not treated with cement. The immediate bearing capacity improves from 25 to 38% following processing; this value exceeded the one recommended for the soil employed in the base course layer (35%). This enhancement resulted from an increase in brevity and density (Loudini et al. 2020).

6.7.1.4 Harbour sediment application in lightweight aggregates The JCI/UPCYCLE Associates, LLC company has developed a concept for the management of dewatered material in the production of lightweight aggregate. The main factor in the implementation of research is the use of existing infrastructure, available equipment and fixed assets in conjunction with the use of proven unit operations. The compressive strength of aggregates indicates that the product is suitable for use in structural concrete. The aggregate water absorption is 12.8%; bulk specific gravity, 1.34 g/cm3; frost resistance, 0.7% and is resistant to weather conditions. The strength of the aggregate concrete produced from dredged materials far exceeds that of a control mix that uses commercially available aggregate (Derman and Schlieper 1999). To solve the problems related to the disposal of sludge, dredging sludge was used as a component of ceramsite, to which zeolite and bentonite were added. The dredging sludge plays the role of an aerating

6 • Useful waste in the production of lightweight aggregates

209

agent in the ceramsite production process. The SiO2 and Al2O3 contents of the sediments is lower than the requirements according to the three-component Riley diagram, while the amount of organic matter burned in the dredging sludge (21.1% wt.) is significantly greater than that of zeolite (0.92% wt.) or bentonite (3.45% wt.), capable of releasing substantial amounts of carbon in the course of the sintering process (Tozzi et al. 2020). At high temperatures, organic matter carbonates and reacts with iron oxide, releasing a great amount of gas, facilitating the production of interconnected pores (Toya et al. 2007). Nevertheless, an excessive share of dredging sludge caused an increased amount of residual carbon during the sintering stage, causing an increased amount of expanding gases and ultimately crushing of the granules and a decreased porosity (Wang et al. 2018). Moreover, the formation of a complex pore structure is directly related to the liquid phase amount and viscosity (Qin et al. 2016). Compared to the starting materials, in which the main mineral component is quartz (SiO2), its amount in the ceramsite significantly decreases. The observed decrease may stem from the consumption of this mineral in the course of the glassy phase formation (Ducman and Mirtic 2009). The glassy surface absorbs gases, resulting in the expansion and formation of the aggregate. The highest porosity was obtained with an optimal content of mass of sediment, zeolite and bentonite. The surface of the aggregate is rough; it has well-developed pore structure comprising interconnected elongated pores (Wang et al. 2020a). Appropriate conditions for the preparation of aggregates with dredging sediments and firing temperatures influence their quality. The value of the pressure used to form aggregate samples at the stage of their preparation has a significant impact on their bulk density. The preliminary heating of the aggregates is to remove most of the organic components and to avoid breaking the granules into smaller pieces. This was observed in the case of obtaining aggregates from the sludge from the local reservoir and the harbour sludge, which were mixed in five varying weight-ratios. The sediment mixtures were pressed at 3000 and 5000 psi. Preheating was carried out at the temperature of 500°C and sintering, at 1050, 1100 and 1150°C for 18 minutes. It turns out that the bulk density of lightweight aggregates always decreases with increasing firing temperature. The aggregates with the lowest bulk density of 0.49 and 0.69 g/cm3 were obtained in the sintering process at a temperature of 1150°C for pressed granules under the pressure of 5000 and 3000 psi, respectively. The particles that are closer to each other combine more easily and sinter at an elevated temperature. Therefore, the granules shaped under higher pressure require shorter time to create the sintered layer (i.e. the shell) which is much smoother and glassy. Following the shell formation, the gases produced following the expansion process increase the shell size. Then, the core area becomes porous, which reduces the density of the lightweight aggregate. The finer sludge is easier to sinter, creating a glassy surface. Additionally, the water absorption capacity increases along with the temperature of the process. A higher-temperature process environment causes the particles to move more actively, which facilitates sintering, resulting in greater gas production during the expansion process. The expansion process provided the gases with sufficient energy, whereas some of them escaped into the atmosphere via the sintered shell coating, leaving small pores on its surface. Aggregate forming pressure does not cause a significant change in water absorption (Wei et al. 2008). In some cases, pre-treatment has little effect on dry particle density. It was observed during the preparation of lightweight aggregate from harbour and basic sediments oxygen furnace (BOF) slag, which is the main waste product of the steelmaking process. Additionally, a powder made of glass waste was used as an additive improving the properties of aggregates. The mixture of ingredients was prepared in a different mass (port sludge:slag BOF:waste glass), pre-heated at 500°C for 10 minutes and sintered at various temperatures and times (1125, 1150, 1175, 1200°C for 5, 10 and 15 minutes). Pre-treated aggregates are characterized by lower water absorption. With or without preheating, the dry particle density does not evidently change, representing the range 1.83–1.89 g/cm3 under the influence of different preheating temperatures and the residence time of the pellets in the oven. This shows that the dry particle density is virtually unaffected by pre-treatment. Pyrolysis and dehydration are the main thermal processes before sintering (Qi et al. 2010; González-Corrochano et al. 2014). Absorbed and structured

210

Application of Waste Materials in Lightweight Aggregates

water is released, which prevents the lightweight aggregates from heating up suddenly at elevated temperatures. Organic substances as well as parts of minerals – e.g. CaSO4, MgCO3, CaCO3, Mg(OH)2 and Ca(OH)2 – found in the raw materials decompose during preheating, which causes the release of carbon monoxide, carbon dioxide, water vapour as well as sulfur monoxide. It is important to avoid the rapid release of large gas quantities if the surface of the granules is not completely vitrified during the sintering process, as this could cause the granules to break. Lo et al. (2016) noted that a suitable preheating process causes the unburned carbon and organic matter in the raw materials to oxidize, which provides an internal space for further aggregate expansion. When the preheating temperature is exceeds 500°C or the heating time is longer than 10 minutes, all parameter values are comparable. Hence, it should be noted that further increasing the temperature (up to 600°C) as well as time (up to 15 minutes) may not significantly enhance the properties of lightweight aggregates. The procedure for preheating described in previous works involved the range of 400–600°C for 10–20 minutes. It was adequate, providing enough gases to ensure porous structure formation and eliminate the negative effects caused by flameresistant gases in the course of the sintering process (González-Corrochano et al. 2014; Li et al. 2016; Qi et al. 2010). As opposed to apparent density and water absorption capacity, dry particle density increases with sintering temperature, which is very different from the lightweight aggregates made from high loss on ignition (LOI) waste – e.g. sewage sludge. Usually, the dry particle density has a strong relationship with mass loss – i.e. low particle density corresponds to high mass loss and vice versa (Cheeseman and Virdi 2005; Tuan et al. 2013). The weight loss characterizing all aggregates with reservoir additive sediments is variable at different temperatures of the sintering process. This can be attributed to the fact that the intensity of the melting reaction increases along with sintering temperature and leads to vitrification of the raw material particles, which causes shrinkage and reduction in volume of the sintered granules, thus increasing the particle density. In turn, extending the sintering time may improve the sintering reaction as well as promote the formation of a vitrified layer, which is less permeable, on the outer surface of the granules (Li et al. 2016), leading to a reduction in the water absorption capacity. A temperature in the range of 1150–1300°C is usually the optimal range for the sintering process. Liu et al. (2018) reported that the optimal sintering temperature for an aggregate produced from river sludge and sewage was 1100°C for 30 min (Liu et al. 2018). Liao and Huang (2011a) obtained a lightweight aggregate from reservoir sludge with high strength CaO, which were sintered at the temperature of 1200°C for 30 minutes. A lightweight aggregate characterized by high strength and low water absorption was obtained from raw materials such as port sludge, BOF slag and glass. After preheating at 500°C for 10 minutes, the granules were sintered at 1175°C for 15 minutes. Proper sintering procedure provides sufficient viscosity, enabling the formation of a glassy outer coating on the sintered granules, but in the meantime, internal gases are released, causing LWA to expand (Ayati et al. 2018; Dondi et al. 2016). The aggregates are also characterized by a very low leachability of water-soluble chlorides as well as heavy metals, which indicates the possibility of their use for further applications in civil engineering (Lim et al. 2019; Lim et al. 2020). The addition of two sodium salts of NaCl and Na2CO3 in the amount of 1.5% for industrial sludge and harbour-dredged sediment (marine clay) facilitates the process of obtaining ultralight aggregates. Depending on the type of salt, the size of the aggregates increases and the specific is lowered density. This applies to a mixture of industrial and marine sludge clay (sludge/clay) composed of 0/100, 20/80, 50/50, 80/20 and 100/0. The colour of the aggregates with NaCl is darker than the aggregates without salt. One hundred percent of dredged aggregate surface harbour-sediment fired at 1200°C melts away. The 20% sludge addition causes no significant volume increase. The aggregates containing 50/50 clay/ sludge have the same size as without the addition of NaCl. The addition of 1.5% Na2CO3 significantly increases the size of the aggregates compared to other additives; moreover, it acts as a blowing agent (Huang et al. 2016). At the temperature of 1100°C, 1 cm aggregates were obtained, while, at the temperature of 1200°C, the mixtures 0/100 and 20/80 produced significantly larger aggregates, whereas, at

6  •  Useful waste in the production of lightweight aggregates  211 a temperature of 1300°C, the surface of the granules melted. Specific density of lightweight aggregate (50/50) with the addition of NaCl, fired at 1200°C, is very low and amounts to 0.35 g/cm3. The aggregates with harbour-dredged sediment and with the addition of 1.5% Na2CO3, burned out at 1200°C, have true density densities equal to 0.9 g/cm3. It is lowered when sediments (20/80) are added to the raw material mixture. Then, an ultra-lightweight aggregate is formed. Since the true densities of all aggregates were higher than 2.8 g/cm3, the lightweight effect of formed aggregates resulted from the formation of large, closed, internal pores. All aggregates are therefore characterized by a low water adsorption coefficient, which allows for their practical use in construction. In the firing temperature range of 1000–1200°C, both salt additives melt rather than evaporate. Co-fusion of these two salts with different conjugated compounds can lower the melting points of mixtures. Therefore, the aggregates with the Na2CO3 admixture produced from a 20/80 mixture at a temperature below 1200°C can be used for the production of ultra-lightweight aggregates characterized by a low water adsorption coefficient (Chien et al. 2020). The influence of firing temperature on aggregate density was noticed when the raw material mixture comprising the harbour sediments as well as slag waste originating from steel industry was sintered. The sediments come from Taichun harbour with a constant build-up of ocean sediment resulting from its geographic location that receives ocean tide. Thus, for the burning of aggregates, much higher sludge content was used in relation to the slag/port sludge ratio (10:100 and 20:100). The granules were fired at a temperature of 950–1100°C for 18 minutes following pre-treatment at 500°C for 2 minutes. The pretreatment process is intended to prevent the bursting of the granules during rapid sintering. The colour of the aggregates is influenced by the presence of CaO. The granules fired at 1100° characterized by a slag to sludge ratio of 50/100 have a brighter colour compared to others, which results from the CaO content in the slag (21.75%). As expected, a higher sintering temperature will reduce the density of the particles. Thus, all granules sintered at 950–1050°C exhibit a density > 2.0 g/cm3, while, at 1100°C, they are below 2.0 g/cm3. The factors influencing the absorbability of aggregates include the degree of glass formation and its impact on the coverage of surface pores, the degree of connection of surface pores with internal pores. The aggregates fired at 1100°C show water sorption ranging from 7.11 to 11.48%; therefore, they meet the earlier-mentioned criterion. Increasing sintering temperature tends to decrease water sorption, mainly resulting from the formation of a glassy surface of the aggregates, which is characterized by a negligible number of surface pores, causing water sorption inhibition. The change of crystalline phases at high temperatures indicates sintering and glassy phase formation, which has an effect on gas entrapment and pellet expansion. This results in larger pores and a lower aggregate density (Wei et al. 2014). In order to reduce the energy consumption during the production of aggregates and to prevent the emission of harmful greenhouse gases, a method of obtaining them without sintering but by impregnation has been proposed. Dredging sludge (80% by weight) was mixed with cement, lime, phosphogypsum, fly ash and water glass (aggregates of untreated ULA). While forming the granules in the disc granulator, they were sprayed with a water glass solution (WLA aggregates). After granulation, the aggregates were hardened at ambient temperature. Then, some of the aggregates were impregnated with an organosilicon solution (10% wt.) by spraying. Wrap-shell lightweight aggregates (WSLA) were produced via a coating process in which a white glue emulsion (4% wt.) was used as the core layer in the raw mix. The main components of the dredging sludge are Al2O3 and SiO2, which are easily hydrated but do not react with water. When too much of the dredging sludge is mixed with lime, fly ash and portland cement, the degree of the pozzolanic reaction can be disturbed and, as a result, concrete with poor mechanical properties can be obtained. Therefore, the exact particle size and percentage of the dredging sediment must be specified. The resulting crust on the surface of the “wrap-shell” granules is a concrete layer that protects the aggregates against external forces, which is reflected in the excellent results of compressive strength or water absorption. Impregnation of unsintered aggregates effectively

212  Application of Waste Materials in Lightweight Aggregates prevents the penetration of water into their interior. Organosilicon impregnating agent, by reacting with water and the hydroxyl group, forms a silicon polymer playing the role of a hydrophobic membrane (Gong et al. 2006; Peng et al. 2017).

6.7.2 Reservoir sediments 6.7.2.1  Types of water reservoirs Reservoir sediment is an outdoor storage area in which an adequate amount of water is collected and maintained when there is a technological or agricultural reason. They are a key component of numerous water supply systems, worldwide. Reservoirs can be built in river valleys using dams, excavations in the ground or conventional construction techniques, such as bricklaying or pouring concrete. Their main uses include: • communal and drinking water supply, • cooling and industrial and water supply, • energy generation, • flood control and river regulation, • irrigation, recreational and commercial fishing, • sailing and other aesthetic, recreational applications, • sewers, • navigation, • waste disposal (under certain circumstances). These particular types of human activity are met by building reservoirs that are potentially subject to significant human control. The dam is a type of hydrotechnical structure, a barrier separating the valley rivers for damming water – usually, concrete, reinforced concrete or earth. A dam can be erected for various purposes: flood protection, electricity production, reservoir and water extraction, recreational value (Thornton et al. 1996). Impoundments and off-river reservoirs are two types of man-made water bodies that typically vary in morphology and size from small monofunctional reservoirs to huge and complex multi-functional reservoirs (Thornton et al. 1996). Depending on their purpose, there are three types of tanks: • valley-dammed reservoir – usually the largest type of reservoir, located in narrow valleys where huge amounts of water can be retained by the valley walls and the dam, • bank-side reservoir – formed when water is drawn from an existing stream or river and stored in a nearby reservoir, • service reservoir – built primarily to store water for later use, often in the form of water towers and other erected structures. The dam reservoirs are complex deposition systems that constitute a barrier to the natural transport of sediments along the riverbeds. Sedimentation in tanks causes serious problems with their operation. Due to the large variability in the shape of the bottom of reservoirs, total volumes of stored water, river discharges, cargo and sediment texture, it is difficult to develop universal models to achieve for dam reservoirs (Sedláček et al. 2022).

6  •  Useful waste in the production of lightweight aggregates  213

6.7.2.2 The reasons for the deposits-formation The water in the water reservoirs is very calm. For this reason, particles of sand, rock and other materials called sediments sink to the bottom, leaving the water completely clean. Over time, however, these deposits build up, significantly reducing the total amount of water in the tank. Sedimentation reduces the capacity of the tanks and the resulting benefits. Sustainable management of the body of water aims to contain sedimentation and reduce its adverse impact, and ultimately balance between sediment outflow and inflow that supports the storage capacity, while maximizing the benefits of the project. Effective sediment management can use a combination of strategies that may change over time as sedimentation becomes more advanced (Morris 2015). The changes in the weather make the natural flow of water in rivers and streams highly variable over time. The periods of overflow as well as valley flooding may be alternated with drought or low flow. The retention reservoirs collect water during higher-flow periods, which prevents floods; then, water is gradually released during the lower flow periods. Reservoirs are usually formed by damming rivers; however, off-canal reservoirs can be formed by diverting structures as well as pipelines or channels carrying river water into natural or artificial depressions of the site. When the flow of the stream is stopped in the reservoir, the flow velocity is reduced and sedimentation occurs. Therefore, the streams that carry a lot of suspended sediment are unfavourable places for reservoirs; siltation quickly reduces the capacity of the tank and can seriously reduce the lifespan of a small tank. Sedimentation is a common and serious problem in larger tanks as well. Since the removal of accumulated sediment from reservoirs is usually too expensive to be viable, the reservoirs in sediment-laden watercourses are usually planned to include a reserve of retention capacity to compensate for losses resulting from sedimentation. Even so, with the current sedimentation rate, most tanks are expected to have an expected lifetime of less than 100 years (www.britannica.com/technology/reservoir). The formation of a large amount of sediment in water reservoirs has a serious impact on the use of the reservoir, which makes it necessary to deepen it. Retention reservoirs retain over 100 billion m3 of sediment, which accounts for 26% of the global sediment supply to the ocean (Syvitski et al. 2005). More than 50% of the sediment stream may be trapped in regulated pools. Gradual sedimentation along with the slow pace of construction of new reservoirs has led to a decreasing reservoir capacity, and after taking into account the increase in population, the capacity per capita is rapidly declining (Annandale 2013). The effects of sediment retention are not limited to bodies of water but also extend downstream to the shoreline. Trapping of sediments in reservoirs, along with in-channel mining (Torres et al. 2017), have an adverse effect on the fluvial sediment balance and may even result in coastal erosion (Willis and Griggs 2003; Slagel and Griggs 2008). The balance between outflow and inflow of sludge will be restored once the tank is full and the advantages of storage are lost. This can be achieved while maintaining tank functions via sediment management activities. Most of the world’s tanks have been designed according to the “tank life” paradigm – “Life of reservoir”. The sediment inflow was calculated on the basis of the planning horizon of 50–100 years and a corresponding sludge-storage volume in the storage pool. The consequences of sedimentation were not taken into account. Designing and operating a reservoir with no long-term sediment management strategy is a nonsustainable approach and not a proper management strategy. We are currently dealing with aging reservoirs with ever-increasing sediment problems (Morris 2020). Additionally, water loss in a reservoir can occur through seepage through the dam foundations, seepage into the surrounding soil or rock as well as surface evaporation. The rate of evaporation is often reduced via chemical treatments, but they necessitate frequent re-use and do not reduce water losses to a significant extent. Biological methods, including windbreaks and floating plants, can greatly decrease evaporation but may only be employed in reservoirs with favourable conditions.

214  Application of Waste Materials in Lightweight Aggregates Given the importance of modern reservoirs for the sustenance of society as well as the inability to replace currently used dams with new designs, Morris and Fan (1998) noted that “While the focus of the 20th century on building new dams, the 21st century will need to focus on preventing sedimentation to extend the life of existing infrastructure. This task will be much easier if we start today” (Morris and Fan 1998). Maintaining long-term storage of water in the reservoir is a management decision. Suitable sites for new dams and reservoirs are limited by competing land-use patterns, hydrology, geology and topography. Currently, existing reservoirs occupy the optimal places as well as constitute a limited and unique resource (Annandale et al. 2016). This increase was compensated by the storage of sediment in the reservoirs compensated for, decreasing the sediment flow to the oceans by 1.4 billion tonnes per year. More than 100 billion tonnes of sludge is currently stored in tanks that have been built over the last 50 years (Wohl 2011). Other data analyses conducted by means of large-scale watershed models indicate that rivers around the world can transport around 15.5 billion tonnes of sediment into the sea per year (Syvitski and Milliman 2007). The data based on historical measurement data from thousands of rivers show that this figure may be higher, reaching 19.9 billion tonnes (Milliman and Fornworth 2010).

6.7.2.3 Sediments from water-reservoir characteristics Chemical and physical characteristics Reservoir and river sediments constitute deposited material comprising insoluble material – primarily soil and rocks – as well as organic matter and particles displaced from land areas to the ocean (Romero et al. 2009; Šlezingr et al. 2015). The deposits are rich in silicon oxides that regulate viscosity, directly affecting the structure of the inner pores. Therefore, they can be a raw material for lightweight aggregate production. The research shows that silicon dioxide is present in the amounts from 31.53 to 70.14%, Al2O3–7.67–25.2%. The presence of more SiO2 and Al2O3 in the raw materials improves the strength of the sintered materials (Wang et al. 2009b). Flux oxides such as Fe2O3 (4.32–10.9%), CaO (0.41–2.59%), MgO (1.64–5.46%), K2O (0.08–5.24%) and Na2O (0.01–1.50%) (Table 6.13). Inorganic oxides in the raw materials affect the glass phase viscosity, which, in turn, can impact the structure of the micropores. Raw materials for the production of lightweight aggregates should meet two requirements: (i) they should contain the substances that decompose or react chemically releasing gases at high temperature (ii) at the same time, a viscous, glassy phase must be produced that closes the gases released at high temperature which contribute to the formation of the internal structure of micropores. Analysis – mineralogical and thermal The mineral always present in the sediments is quartz (Table 6.14). Its presence is evidenced by the dominant amount of silicon dioxide in the sediments. The sediments also contain iron oxide in their chemical composition, which is sometimes difficult to identify by X-ray diffraction. It follows that Fe2+ ions replace the Mg2+ ion sites in the structure of clinochlore (Liao et al. 2013). In addition to clinochlore, clay minerals such as illite, plagioclase and muscovite are present in the sediments. Chlorite is a mineral, which constitutes a mixture of magnesium and iron aluminosilicates. Identification of thermal processes (DTA) taking place in sediments shows a strong endothermic peak at a temperature of 74°C, which proves the presence of physically adsorbed water as well as another endothermic peak at 536°C, which corresponds to the evaporation of crystalline water of illite and chlorite, finally indicating an exothermic peak at the temperature of 762°C due to the formation of mullite. Moreover, the current feldspar in sediments is liquefied at a temperature of 1000°C. The loss of mass when the temperature changes from 50 to 750°C is up to 7%, since the crystallisation water

SIO2

AL2O3

FE2O3

CAO

MGO

NA2O

K 2O

TIO2

ORGANIC SUBSTANCE

57.12

21.96

7.72

0.41

2.13

0.75

4.12

0.93

4.65

70.14

15.45

5.37

0.64

1.64

1.02

5.24

31.53

7.67

4.32

0.59

–

1.15

2.52

0.51

59.31

19.97

6.53

1.41

2.02

0.01

0.08

0.07

60.9

25.2

5.55

0.92

–

1.22

3.49

61.4

22.70

8.6

0.70

2.00

1.30

60.3

25.2

5.5

0.9

–

53.4

23.8

10.9

1.8

2.50

63.09

16.76

2.59

5.49

–

7.95

LOSS ON IGNITION

AUTHORS Hung and Hwang (2007)

–

Chiang et al. (2008)

4.18

Wei et al. (2008)

2.90

7.70

Tang et al. (2011)

–

–

17.65

Chiou and Chen (2013)

3.40

–

–

5.57

Liao et al. (2013)

–

3.50

–

–

1.50

5.10

–

1.00

2.90

2.37

1.04

–

–

Tuan et al. (2014) Chen et al. (2017) Junakova and Junak (2017)

6  •  Useful waste in the production of lightweight aggregates  215

TABLE 6.13  Oxide composition reservoir sediment by weight (%) in literature.

216  Application of Waste Materials in Lightweight Aggregates TABLE 6.14  Mineralogical composition of dredging sediments in literature.

COMPONENT Silicon oxide

HUNG AND WEI AND HWANG LIN (2007) (2009) Q

Iron oxide

Q

TANG ET AL. (2011)

CHEN ET AL. (2012)

Q

Q

CHEN ET AL. (2012) Q

LIAO JUNAKOVA ET AL. AND JUNAK (2013) (2017) Q

Q

Cn

W

Aluminum oxide

Co

Silicates and aluminosilicates

I, Cl

I, Cl

I, Cl,

Cn

Feldspar

P

P

P

Al

Micas

Mu

Mu

Mu

Al (albite-NaAlSi3O8); Cn (clinochlore-[(Mg, Fe)6(Si, Al)4O10(OH)8]; Co (Corundum-Al2O3); Cl (chlorite- (Mg,Fe,Al)3(Si,Al)4O10 (OH)2), I (illte-(K,H3O)(Al,Mg,Fe)2(Si,Al)4O10[(OH)2,(H2O); Mu (muscovite-KAl2(Si3Al)O10(OH,F)2; P (plagioclase-Na[AlSi3O8] to Ca[Al2Si2O8]); Q (quartz-SiO2); W (wustite-FeO).

in the mineral and the physically adsorbed water evaporate. This means that the raw pellets from the sludge should be pre-heated in order to prevent an explosion during firing in a kiln (Chen et al. 2012). It was noticed that the weight loss of the Hejiagou River in Harbin (China) sediments at temperatures up to 1200°C is also small and does not exceed 10%. This is because the sediments are mainly composed of inorganic compounds. At a temperature of about 133°C, endothermic processes take place as a result of the evaporation of absorbed water. The greatest loss of mass occurs at a temperature of 250 to 620°C and amounts to about 7% due to the removal of structural water made of the OH- ions present in the structure of clay minerals and some low-melting inorganic salts (Tay et al. 2003). As a result of oxidation and volatilization of most inorganic substances, the loss is 1.8% at temperatures from 620 to 1200°C (Liu et al. 2018).

6.7.2.4 Application of the water reservoirs in civil engineering Continuous availability of reservoir sediments and their specific chemical and physical properties make them a valuable raw material, in the construction industry (Mezencevova et al. 2012), for the concrete, ceramic tiles and bricks production (Torres et al. 2009; Tang et al. 2011). The subject of research of many scientists was the use of dredged materials as fine or coarse aggregate in the production of concrete (Millrath et al. 2001; Mensinger 2008; Said et al. 2015) or common aggregate in mortar (Kazi Aoual-Benslafa et al. 2015). In the sediments, the silt-clay fraction is predominant; thus, the quantity of material which can be used as filler to the concrete is limited due to the possible deterioration of the workability of the concrete mix and the mechanical properties of the concrete. The possibility of using the sludge extracted from the port in Oran (Algeria) as a partial replacement of cement in mortars was shown by Kazi Aoual-Benslafa et al. (2015). The authors, examining the weight loss and compressive strength of mortars supplemented with sediments in various proportions of 5, 10, 15 and 20% in relation to the cement weight, found that the sediments can partially replace the cement in the mortars. The 5% addition was found to be the most suitable (Kazi Aoual-Benslaf et al. 2015). The sediments extracted from the Ruzin reservoir, located at the inlet of the Hornad River and its main tributaries: Hnilca, Opatki and Beli in the N-E part of the Slovak Mountains, were used to partially substitute portland cement in concrete. The sediments were mechanically activated by dry grinding and chemical grinding with the addition of NaOH in order to enhance the technological properties

6  •  Useful waste in the production of lightweight aggregates  217 of concrete. The mixture, prepared with 40% of ground sludge, chemically activated, exhibited low compressive strength that amounted to 4.6 MPa following 28 days and 6.1 MPa following 90 days of hardening. The flexural strength following 28 days was consistent with that of the control. Following 90 days, the flexural strength had only reached a third of value characterizing the control mix. The decreasing tendency in strength can stem from the chemical composition of the sediment, with predominant lower bound calcium content and the incomplete transformation of structures in the newlyformed matrix. Therefore, the changes in the strength of samples over a longer period of time have to be monitored (Junakova and Junak 2017). The reservoir sediments retrieved from the drying beds at Shi-Men Dam in Tao-Yuan County, Taiwan, in combination with clay in an amount of 0–20%, which was fired at 1050–1150°C, were used to obtain ceramic brick. Higher sintering temperature reduces water absorption, but in turn, increases shrinkage, density and compressive strength. Densification of the sintered granules took place at the temperature of 1050°C. The shrinkage increased significantly with the sintering temperature; while improving the compressive strength, absorption decreased. All bricks met the criteria in accordance with the applicable standards (Chiang et al. 2008).

6.7.2.5 Application the water reservoir in lightweight aggregates The water reservoir sediments are an effective raw material for lightweight aggregate production. As a result of mixing with various wastes – e.g. fly ash from municipal waste incineration plants (MSWI), sewage sludge, waste glass, tile grinding sludge or NaOH and CaO – and subjecting them to heat treatment, a full-value building material is obtained that can be successfully used for construction concretes and high-flowing concrete (Tuan et al. 2014). Fine sediment was collected from the Shih-Men water reservoir in northern Taiwan, with 309 million m3 of water storage capacity. After 40 years, it accumulated in the amount of 73.5 million m3 that constitutes about 23.7% of the designed reservoir water capacity. This affects the use of the reservoir, necessitating its dredging. As a result of the high water-content, this fine sediment cannot be landfilled and should be temporarily stored in catchment basins downstream. An attempt was made to use the sludge for the production of lightweight aggregate, which was sintered at various temperatures at 1000, 1100, 1150, 1175, 1200, 1250 and 1300°C. The Shin-Men Reservoir soil comprises clay, silt as well as sand in the amount of 60%, 38% and 2%, respectively. It turns out that the aggregate density of gradually increases when the temperature in the kiln rises to 1000°C, and the sudden decrease in density occurs at a temperature range of 1000–1250°C and slightly over 1250°C. The processes that take place in this temperature range can be divided into three stages: (i) dehydration, in which the density of the granules increased from 2.1 to 2.2 g/cm3 as a result of water diffusion and evaporation out of the particle followed by its condensation; (ii) an expansion step where the density decreases from 2.2 to 1.35 g/cm3 as a result of gas formation inside the granules; and (iii) collapse stage as a result to particle melting as well as gas evolution. Apart from temperature, the sintering time is important as well. If it is too short, the aggregates will be porous due to gas evaporation, while, if it is longer, the surface will melt and gas bubbles will form inside, resulting in a porous aggregate. In this case, a firing time of 10–12 minutes is suggested. The aggregates that contain open or interconnected pores adsorb water similarly to a sponge, while a glassy coating on the aggregate surface or isolated pores causes them to absorb little water. Water is prevented from penetrating into the pores. The water absorption value is under 0.5% at temperatures above 1150°C (Hung and Hwang 2007). Fly ash from municipal waste incineration plants (MSWI) was added to the sludge from the scrubber ash and reservoir cyclone ash. The amount of ash was variable and amounted to 10, 30 and 50%. After the pelletization process, the granules were fired at the temperature of 1070, 1100, 1120, 1150°C. It turns out that the maximum fly ash content should not be more than 30%. The produced

218  Application of Waste Materials in Lightweight Aggregates aggregate has a specific density in the range of 0.88–1.69 g/cm3 as well as a crushing strength of 13.43 MPa. The water absorption decreases with increasing firing temperature. The water created on the surface of the glassy layer particles prevents water from penetrating into the pores, and the water absorption by the aggregate decreases. Water absorption ranges from 7.6 to 29%. The more reservoir sediment (90%) in the raw material mixture, the lower the aggregate water absorption is. Compared to normal aggregate, those with reservoir additive sediments contain more voids, so their water absorption coefficient and surface area are higher, whereas density is lower. The specific surface area decreases with increasing ash content, while it increases along with the sludge content. On the other hand, adding more MSWI ash – i.e. 30 and 50% at a temperature range of 1070–1120°C – increases the crushing strength. The failure point loading is increased, too, with sintering temperature and the amount of reservoir sediments in aggregate. The obtained coarse lightweight aggregate can be successfully used for the production of self-compacting lightweight concrete. Compressive strength is from 25 to 55 MPa. The electrical parameters – i.e. ultrasonic pulse velocity and resistivity of 3600 m/s and 8.5 kΩcm, respectively – meet the standard requirements and the concrete has good corrosion resistance (Hwang et al. 2012). The properties of the lightweight aggregates made of reservoir sediments largely depend on the firing conditions in the kiln – i.e. time – and the implementation of preheating and firing proper. The sludge from the Shihmen reservoir was used to obtain a lightweight aggregate. This reservoir, built in 1959, is located in the north of Taiwan and constitutes the third largest reservoir in the country. The deposits accumulating in it adversely affect the operation of the tank. In order for it to function, the sludge must be periodically removed. Thus, a fine clay precipitate with low or medium plasticity was used to produce a lightweight aggregate. During the initial preparation, the sludge was air-dried, crushed, screened, sorted and, as a result, sintered. Initial heating took place at a temperature of 500–700°C for 7.5 and 15 minutes, then expanding at 1100–1150°C after 4, 8 and 12 minutes. During this time, the minerals of the raw pellets soften, begin to melt and release gases. Ultimately, the sludge from the tank produces light pellets of various sizes, having a hard ceramic coating as well as porous core. After the sintering process, the pellets are cooled with cold air. It appears that, at both sintering temperatures, the particle density decreases after preheating is applied. The density of the granules annealed for 7.5 minutes is lesser than that of 15 minutes. The particle density decreases along with soaking time. Regardless of the pre-heating time (7.5 min or 15 min), the aggregate produced at the temperature of 1200°C has lower density than that manufactured at the temperature of 1175°C. Extending the firing time of the granules at the temperature of 1200°C from 4 to 12 minutes does not significantly affect the density of the granules. The relative densities of the aggregates produced in the range from 1.01 g/cm3 to 1.38 g/cm3 are much lower than normal aggregate, meeting the ASTM C 330 standard requirements. The strength of an aggregate increases along with bulk density and ranges from 7.2 to 13.4 MPa; therefore, the produced aggregates can be successfully used for construction concretes. Their 7-day strength is on average 82% of the 28-day strength – i.e. a typical value for this type of concrete. The flexural strength increases with the density of the aggregate; however, it decreases with the value of the w/c ratio (Tang et al. 2011; Chen et al. 2012). In order to create a glassy phase limiting the adsorption of water in aggregates, NaOH was added to water reservoir sediments as a source of sodium which reacts with the aluminosilicate compounds present in the sediments. Various concentrations of NaOH (26 M, 23 M and 20 M) were mixed with the wet and dry sludge. The granules were hardened at 120°C for 1 h, then calcined for 30 minutes at 1085, 1100, 1115, 1130 and 1145°C. Sodium hydroxide is highly alkaline and is the most commonly used active solution to dissolve silica and alumina. The bulk densities of dry sludge aggregates to which NaOH has been added are slightly lower than those of wet sludge aggregates. Their values slightly decrease with the temperature increase from 1.6 to 1 g/cm3. Greater NaOH solution concentration lowers the bulk density of the aggregates.

6 • Useful waste in the production of lightweight aggregates

219

This phenomenon was not found in the cases of the aggregates calcined at a temperature under 1105°C. This can result from the fact that the wet sediment with the addition of NaOH after thermal hardening forms a geopolymer. Geopolymers are materials that are formed in the course of the reaction between Al(OH)4- and Si(OH)4 after thermal curing. The strength of geopolymers is determined by temperature of curing, since it influences the condensation rate in the geopolymerization reaction (Rovnaník 2010). The wet sludge contains 30% water, facilitating the NaOH diffusion. Sodium hydroxide solution dissolves the silica as well as alumina in the sediment, so the properties of aggregates are comparable to those characterizing geopolymers. Above 1105°C, the strength of aggregates decreases due to their expansion. The downward trend in bulk density is comparable for the aggregates from dry sludge. The solution of NaOH comprises Na+ ions capable of reacting with the aluminosilicate compounds contained in the precipitate, forming a glassy phase. The high concentration of NaOH provides huge quantities of the vitreous phase to absorb gas, contributing to a greater number of pores. This decreases the bulk density and increases the water adsorption, which is still low – less than 5% – as a result of a low number of open pores. Water adsorption by the aggregate containing wet sludge increases along with calcination temperature. In contrast, the water adsorption of dry sludge aggregates does not improve with higher temperature. In the wet sludge, water is contained in the pores between the molecules. Following evaporation, the pores shrink, which causes the particles in the dried sludge to thicken. Moreover, this is the reason why the NaOH solution is easily diffused into the wet sludge. Water adsorption by aggregates increases with the NaOH concentration. High NaOH concentrations provide huge quantities of the vitreous phase to absorb gas, contributing to a greater number of pores. Thus, the bulk density is reduced and the water adsorption is increased. Previous studies (GonzálezCorrochano et al. 2010; Liao et al. 2011a) found that the glassy phase seals the pores, decreasing the occurrence of open pores. The addition of NaOH enhances pore closure in the wastewater reservoir and low water adsorption aggregate production (Liao et al. 2013). In other studies, it was proven that the properties of lightweight aggregates from sediments of a water reservoir with the addition of calcium oxide are influenced by the method of thermal treatment – i.e. the annealing speed (5 or 15°C/min) and temperature (1170 to 1230°C). The bulk density decreases with increasing firing temperature. According to Liao and Huang (2011a), a short but fast firing time should be used to lower the density value. The bulk density of the aggregates fired at the heating rate of 15°C/min for 15 at 1230°C is the highest because the heating time is the shortest, which leads to a slight expansion of the pellets. This result differs from that obtained by Chen et al. (2010), who found that a longer annealing time of the granules increases the bulk density. The reason for this is the use of a different method of heat treatment – i.e. preheating time and sintering (Chen et al. 2010). The aggregates produced with a long heating time (30 minutes) and a fast heating-rate of 15°C/min at a temperature of 1170°C meet the standard requirements and, produced using other heat treatment methods – i.e. soaked for 15 minutes at a speed of 5°C/min – meet the requirements standard but at temperatures above 1200°C. The aggregates with low bulk density show low compressive strength. The aggregates fired at 1230°C for 15 minutes and 15°C/min have the highest compressive strength (4.08 MPa). Water adsorption is less than 4%, which is advantageous for the aggregates for lightweight concrete. Water adsorption depends mainly on the presence of open pores in the aggregate. After reaching the right temperature, the glassy phase begins to flow and sealed the small pores, thus reducing the adsorption of water (Liao and Huang 2011a). To lower the firing temperature of the aggregates from reservoir sediments, glass wastes of varying degrees of fragmentation can be added to the raw material mixture. Fine glass waste powder smoothens the surface of the aggregates and reduces the pore size of the aggregate. The maximum failure point loading increased to 75.9%. The lightweight aggregates from reservoir sediments reach a specific weight of < 1.8 g/cm3 when the grinding degree of glass waste is >150 µm. On the other hand, when the grinding degree of glass waste was < 150 µm, waste glass is suitable for the aggregates of normal weight and specific gravity. The grain size of glass waste affects the lightness characteristics of the

220  Application of Waste Materials in Lightweight Aggregates aggregates. The addition of finer waste glass to the sludge beneficially affects the chemical-corrosion resistance and potential alkali-silica reactivity of lightweight aggregates (Chiou and Chen 2013). It is possible to obtain a synthetic lightweight aggregate with high functional parameters from a raw material mixture containing reservoir sediments and tile-grinding sludge. The Taguchi method involves an effective and simple methodology enabling to optimize the conditions of the aggregate production process and significantly reduces the number of tests. An orthogonal array matrix L16 (45) was assumed, which consists of five controlled, four-level factors (i.e. sediment content, preheating temperature, preheating time, sintering temperature and sintering time and a method of analysis of variance to study the influence of experimental factors on particle density, water absorption, expansion ratio). The Taguchi method can effectively reduce the number of tests required in a process design procedure (Roy 2001; Taguchi et al. 2005). The interactions between process parameters were omitted because the Taguchi method can detect their presence (Wahid and Nadir 2013; Suchorab et al. 2016). The factors affecting the engineering properties of sintered aggregates involve firing conditions and raw material composition (Chandra and Berntsson 2002; Tang et al. 2011; Chen et al. 2012). The cost and time of the experiment increase greatly along with design parameters or parameter levels; moreover, this also contributes to more complex experimental conditions. The Taguchi method only tests combination pairs, rather than all possible combinations, as is the case with factorial design. This is mainly because Taguchi orthogonal arrays are highly fractional orthogonal designs. Hence, these designs allow estimating the principal using a minimum number of experiments, saving resources and time (Taguchi 1987; Roy 1990). The firing of synthetic aggregates included preheating at the temperatures of 300, 500, 700, 900°C and the time of 7.5, 15, 22.5 and 30 minutes and sintering at the temperatures of 1125, 1150, 1750 and 1200°C, also at varying times: 10, 15, 20 and 25 minutes. The reservoir sediment content was 90, 80, 70, 60% wt. The temperature of sintering is the factor of greatest importance, which influences the physical parameters of aggregates. The maximum value of response was obtained at the highest sintering temperature. The sintering temperature is also the most significant factor affecting the overall density of the aggregate particles. The following parameters had the greatest share: sintering temperature, preheating temperature and preheating time, equal to 43.78%, 25.1% and 24.7%, respectively. The particle density of the sintered aggregate pellets vary in the range of 0.43–2 g/ cm 3 and the water absorption, 0.6–13.4%, while the expansion ratio varies in the range of 77.4%377.4%. The highest value of the expansion index was in the case of the aggregate with the lowest density and water absorption amounting to 6.8%. Again, the sintering temperature is the factor of key importance, influencing the expansion of the aggregates. The analysis shows that bloating is most influenced by time temperature sintering, preheat temperature, preheat time and sludge content, with percentages amounting to 44.23%, 25.19%, 24.73% and 4.5%, respectively. On the other hand, preheating has the greatest influence on roasting losses time, sludge content, sintering temperature and preheating temperature, reaching (49.84%), 21.84%, 13.89% and 8.70%, respectively. The aggregate density and water absorption are lower than those of Liapor™ and Leca™. These aggregates can be successfully used for thermal insulation. On the other hand, the aggregates with a low expansion ratio are characterized by a relatively larger particle density; therefore, they will not meet the requirements for lightweight aggregates (Chen et al. 2017). Generally, the literature proposes the influence of the role of iron compounds present in various raw materials, especially in clay, on the expansion effect of aggregates according to the following chemical reactions (6.1) to (6.3): 2 Fe2O3 → 4 FeO + O2↑(6.1) 6 Fe2O3 → 4 Fe3O4 + O2↑(6.2) 2 FeS + 3 O2 → 2FeO + 2 SO2↑(6.3)

6  •  Useful waste in the production of lightweight aggregates  221 As a result of Fe2O3 decomposition, O2 is released, which is responsible for the expansion mechanism. Therefore, for the reaction (6.3) to occur, it is necessary to have an appropriate amount of it and contact with FeS. As a result of the decomposition reaction (6.1), the trivalent iron is reduced to divalent iron and the average degree of oxidation is reduced from 3 to 2.67+ in reaction (6.2). The resulting Fe3O4 in reaction (6.2) is a complex salt formed by Fe2O3-FeO; therefore, reaction (6.2) may be treated as a partial Fe3O4 decomposition into FeO. The following reaction can be a clearer way of describing the process (Eq. 6.4): 6 Fe3O4 → 4 (Fe3O4-FeO) + O2↑ 

(6.4)

Contrary to reaction (6.1)–(6.2), reaction (6.3) does not cause changes in the Fe oxidation state. In turn, the simulation studies on synchrotron-based XAS technique shows that approximately 59% of the total iron is in the reservoir sediment is in the form of Fe2+; the majority of Fe is oxidized to Fe3+ in the core and shell of lightweight aggregate granules produced at 1050°C and in the core of the aggregate produced at 1150°C. Expansion occurred in the aggregate core produced at 1150°C, and it is mainly due to the FeSO4 decomposition to FeO with the simultaneous release of O2, SO2 and SO3. Generally accepted in the literature, the expansion mechanism involving Fe2O3, which undergoes chemical reduction to FeO, accompanied with O2 release to form a lightweight aggregate, is not always noted in all processes (Wei and Lin 2009).

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Subject index A

G

Aardelite, 2, 9, 144, 146, 170 alkali-carbonate reaction, 25 alkaline activators, 149, 151 alkali-silica reaction, 25, 26, 27, 37 analysis mineralogical, 116, 124, 203 apparent density, 19 application in civil engineering, 108, 117, 127, 140, 204 artificial aggregate, 1, 3, 4, 6, 8, 9, 38, 91

gas emissions, 58, 97, 99 geopolymers, 130, 143, 158, 180, 219 glass cullet, 101 characteristic, 155 production, 152 types, 154 granite-marble sludge, 102, 184 granulation, 9, 40, 61, 72, 146 agitation, 42 compacting, 46 double step, 47, 48 parameters, 74, 76 ground granular blast furnace slag, 148, 195

B binders, 69, 70, 72, 89, 95, 141, 168 black core, 21, 86, 121 bloating agents, 85 area, 69, 80, 81 index, 21, 80, 81 mechanism, 21, 86 bulk density, 19, 179

C calcium hydroxide (CH), 39, 59, 60, 72, 147 calcium silicate hydrate, 34, 39, 59, 60, 72, 146, 198 carbonation conditions, 59, 88, 89 cement production, 128, 140, 204 coal fly ash production, 132 compressive strength, 13, 20, 23, 32, 121 construction and demolition wastes, 10, 186, 189, 190, 192 cracking, 29 crushed brick, 192

D diagram of Cougny, 81, 82 diagram of Riley, 80, 81 drilling wastes, 179 dry particle density, 13, 19, 22

F flotation waste, 166, 173, 174 flux ratio, 79 fly ash, 6, 37, 38, 70, 75 characteristic, 136 classification, 134 definitions, 134 frost resistance, 13, 21, 90 furnace atmosphere, 82, 87, 88

H harbour sediment, 199 hard coal–mining wastes, 166 hardening technique accelerated carbonation, 58, 61, 63, 88, 90, 97 autoclaving, 55, 88, 146 cold bonding, 41, 69, 72, 90 microwave radiation, 40, 65, 67, 68, 150 sintering, 40, 48, 50, 68, 77

I individual crushing strength, 20, 94 industry of non-ferrous metals, 172 internal pores, 90, 121, 150, 211 iron ore mining, 175

L leachability, 96, 123, 130, 210 lignite mining, 171 liquid phase, 40, 48, 50, 52, 79, 86, 176 loose bulk density, 20, 92, 93 Lytag, 3, 119, 144, 145, 163

M macrostructure, 70, 90 mechanical properties, 9, 18, 69, 72, 74, 90, 93 mechanism of expansion, 27 microstructure, 90

273

274  Subject index N natural aggregate, 1, 4, 6, 9, 11 expanded clay, 2, 35, 77 expanded glass, 24, 32, 161 expanded perlite, 8, 35, 48, 70 expanded shale, 34, 51 microsphere, 36 oil palm shell, 25, 39 volcanic rock, 2, 5, 37

rotary kiln, 1, 2, 6, 35, 40, 48, 50, 82, 131, 163, 185, 198 rotation speed, 45, 61, 72, 76, 89

S

organic matter, 54, 77, 85, 97, 106, 114 oxide composition, 107, 115, 125, 138, 155, 205

sand replacement, 129 sand sludge, 102, 184 sewage sludge, 25, 54, 63, 82, 83, 96, 97, 100, 103, 105, 108, 110, 113, 210, 217 sewage sludge ash, 100, 113, 115, 117, 119 shape and texture, 21 sintered fly ash, 144, 146 sintering temperature, 19, 48, 51, 54, 77, 80, 82, 85, 90, 92, 96, 119, 121, 220 symptoms of alkaline reactivity, 29

P

T

particle density, 13, 19, 92 phosphate sludge, 182 physical properties, 13, 17, 18, 19, 73 pop-outs, 30 porosity, 4, 19, 93 pozzolanic reaction, 28, 36, 39, 70, 73, 90 pre-sintering, 53, 90 production parameters, 69, 71, 74

thermal analysis, 116, 117, 126, 203, 204

O

R raw material, 1, 3, 41, 70, 77, 89, 172, 180 recycled aggregate, 2, 4, 9, 12, 187, 189, 190, 191 recycled concrete aggregates, 59, 188, 189 reservoir sediments, 102, 212, 216, 217 recycled masonry aggregate, 190, 193, 194 rice husk ash, 69, 72, 73, 93, 101, 129, 130, 148

V viscosity, 32, 40, 52, 53, 78, 82, 85, 131, 155, 159, 162, 184, 203, 209, 214 vitreous phase, 69, 80, 85, 111, 162, 184, 219 void, 13, 19, 157

W waste rock, 166, 167, 172, 180, 192 water absorption, 13, 19, 22, 90, 93, 121 water treatment sludge, 97, 122, 124, 127, 131

Z zeolitic rocks, 184