Applied Mineralogy of Cement & Concrete 9781501508356, 9780939950881

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REVIEWS IN MINERALOGY AND GEOCHEMISTRY Volume 74

2012

Applied Mineralogy of Cement & Concrete EDITORS Maarten A.T.M. Broekmans Geological Survey of Norway Trondheim, Norway

(NGU)

Herbert Pöllmann Martin Luther Universität Halle (MLU) HALLE (Saale), Germany

ON THE COVER: Background image: polymict aggregate in concrete, containing several types of sand-/siltstone in various colors, black lydite, chert/flint in various shades with and without cortex, white vein quartz and reddish quartzite, etc. Several lithologies behave alkali-reactive as revealed by internal cracking, presence of dark rims, etc. © Maarten ATM Broekmans. Front cover, top: Panorama of the Three Gorges Dam in China facing south, one of the largest concrete structures in the world, still under construction in October 2004. The dam with the Chinese characters on the right was temporary only, and was removed by blasting when the main structure was finished in 2009. © Maarten ATM Broekmans. Front cover, bottom: Rosette of layered doublehydrate (LDHt) phase in CAC. © Herbert Pôllmann.

Series Editor: Jodi J. Rosso MINERALOGICAL SOCIETY OF AMERICA GEOCHEMICAL SOCIETY

Reviews in Mineralogy and Geochemistry, Volume 74 Applied Mineralogy of Cement & Concrete ISSN ISBN

1529-6466

978-0-939950-88-1

COPYRIGHT 2 0 1 2 THE M I N E R A L O G I C A L

S O C I E T Y OF A M E R I C A

3 6 3 5 CONCORDE PARKWAY, SUITE 5 0 0 CHANTILLY, VIRGINIA, 2 0 1 5 1 - 1 1 2 5 , U . S . A . WWW.MINSOCAM.ORG The appearance of the code at the bottom of the first page of each chapter in this volume indicates the copyright owner's consent that copies of the article can be made for personal use or internal use or for the personal use or internal use of specific clients, provided the original publication is cited. The consent is given on the condition, however, that the copier pay the stated per-copy fee through the Copyright Clearance Center, Inc. for copying beyond that permitted by Sections 107 or 108 of the U.S. Copyright Law. This consent does not extend to other types of copying for general distribution, for advertising or promotional purposes, for creating new collective works, or for resale. For permission to reprint entire articles in these cases and the like, consult the Administrator of the Mineralogical Society of America as to the royalty due to the Society.

Applied Mineralogy of Cement & Concrete 74

Reviews in Mineralogy and Geochemistry

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FROM THE SERIES EDITOR Several years in the making, Applied Mineralogy of Cement & Concrete was finally brought to completion in 2012 by the persistent efforts of the volume editors, Maarten Broekmans and Herbert Pollmann. Their efforts are greatly appreciated. Any supplemental materials associated with this volume can be found at the MSA website, www.minsocam.org/MSA/RIM. Errata will also be posted there. The reader will also be able to find links to paper and electronic copies of this and other RiMG volumes. Todi 3". Posso, Series Editor West Richland, Washington April 2012

PREFACE Since its inception in 1974, the "Reviews in Mineralogy" ("-and Geochemistry" from 2000) series has published over seventy volumes covering a diverse range of topics from theoretical to applied, and f r o m very specific to generic. The idea for this R i M G volume was initially conceived in 2006, and the revised proposal was approved by the M S A Council in 2009. 'Building materials' as a generic term encompasses steel, aluminum, copper and a range of metal alloys, glass and glaze, particulate materials like sand, gravel, or crushed rock, and natural stone of sedimentary, igneous or metamorphic origin. Each of these materials sees a wide range of applications, from structural/bearing via functional to merely ornamental and decorative. The wide range of 'building materials' application is achieved through an equally wide range of processing, f r o m use 'as is' (e.g., stacking boulders to make a retaining wall), through simple re-dimensioning and fitting (e.g., splitting and sizing of roofing slate) to purification and complex treatment in multi-stage processing (e.g., glass, Portland cement clinker, concreting). The use of building materials, their applications and processing has changed considerably with the development of civilization and technology. Consequently, comprehensive coverage of building materials, applications, processing and history would require multiple volumes. The present R i M G volume contains a selection of papers on the applied mineralogy of cement and concrete, by far the most popular modern building material by volume, with an annual production exceeding 9 billion cubic meters, and steadily growing. Not even all 'concrete' topics can be covered by a single volume, but an interesting assortment was finally obtained. The seven chapters deal with mineralogy and chemistry of (alumina) clinker production and hydration (Pollmann), alternative raw clinkering materials to reduce C 0 2 1529-6466/12/0074-0000S05.00

DOI: 10.2138/rmg.2012.74.0

Applied Mineralogy

of Cement

& Concrete

- Preface

emission (Justnes), assessment of clinker constituents by optical and electron microscopy (Stutzman), industrial assessment of raw materials, cement and concrete using X-ray methods in different applications (Meier et al.), in situ investigation of clinker and cement hydration based on quantitative crystallographic phase analysis (Aranda et al.), characterization and properties of supplementary cementitious materials (SCMs) to improve cement and concrete properties (Snellings et al.), and deleterious alkali-aggregate reaction (AAR) in concrete (Broekmans). Finding reliable volume contributors is never an easy task, and we are immensely grateful to all authors for their submissions and for keeping deadlines. We are also greatly indebted to RiMG Series Editor Jodi J. Rosso for editorial assistance and her responsiveness to quickly elucidate emerging copyright matters. Finally, we are thankful to our numerous 'concrete colleagues' in the field, for having inspired us unknowingly during the many meetings and gatherings we have had with them through the years.

Trondheim / Halle (Saale) late April 2012 Maarten A.T.M. Broekmans Geological Survey of Norway (NGU) Herbert Pöllmann Martin Luther Universität Halle (MLU)

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Applied Mineralogy of Cement & Concrete 74

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TABLE OF CONTENTS 1

Calcium Alumínate Cements Raw Materials, Differences, Hydration and Properties Herbert Pôllmann

INTRODUCTION Raw materials MANUFACTURE OF CAC CHEMICAL AND MODAL MINERAL COMPOSITION CALCIUM ALUMINATE CEMENT - PHASE DIAGRAMS Mineralogical variability CHEMICAL AND MINERALOGICAL PHASE COMPOSITION Calcium aluminates Calcium silicates Calcium aluminum silicates and aluminum silicates Calcium aluminum ferrites Compounds containing magnesium and other species HYDRATION OF CALCIUM ALUMINATE CEMENTS Description of crystalline hydration products of CAC Descriptions of amorphous, pseudocrystalline and crystalline aluminum hydroxides and sulfur-containing phases Hydration mechanisms and setting of CAC Hydration of CAC under the influence of different admixtures CRYO-SEM INVESTIGATIONS OF CAC HYDRATION: MICROSTRUCTURE DEVELOPMENT Effects of Li 2 C0 3 accelerator on CAC hydration Effect of Fe impurities on CAC hydration Hydration mixtures with different other materials CAC - application and other cementitious mixtures ACKNOWLEDGMENTS LITERATURE

v

1 1 3 5 11 13 17 17 20 23 27 30 36 39 43 51 54 56 59 60 61 65 65 65

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Alternative Low-C0 2 "Green" Clinkering Processes Harald Justnes

ABSTRACT INTRODUCTION CEMENT CHEMISTRY BACKGROUND Cement chemist's short hand notation Clinker production for Portland cement High belite cement clinker Calcium sulfoaluminate cement (CSA) Calcium aluminate cement (CAC) MINERALS AS ALTERNATIVE TO LIMESTONE General Gypsum Wollastonite Larnite, bredigite and calcio-olivine Spurrite and associated minerals Hydrogrossular Anorthite and anorthosite ( ON( I 1SIONS REFERENCES

3

83 83 84 84 84 89 89 91 93 93 93 94 95 95 96 96 96 97

Microscopy of Clinker and Hydraulic Cements Paul E. Stutzman

ABSTRACT INTRODUCTION PORTLAND CEMENT PHASE COMPOSITION CLINKER MICROSCOPY SPECIMEN PREPARATION FOR MICROSCOPY Materials for sample preparation Preparation of clinker Polished powder mounts of Portland cementitious materials Cutting and grinding Polishing Etching for light microscopy SRM clinker Point count analysis SCANNING ELECTRON MICROSCOPY ANALYSIS SEM imaging of microstructure Image processing Direct methods for development of standard reference materials Phase estimates by microscopy and quantitative XRD Certified values by consensus means Application to cements vi

101 101 102 104 105 106 106 107 107 108 113 113 114 119 120 131 133 137 137 137

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SEM imaging of fly ash SUMMARY REFERENCES

4

139 141 143

Industrial X-ray Diffraction Analysis of Building Materials Roger Meier, Jennifer Anderson, Sabine Verryn

ABSTRACT INTRODUCTION METHODOLOGY Phase identification with XRD Phase quantification by using X-ray diffraction data Full pattern cluster analysis Computed tomography APPLICATIONS Raw materials/quarry Preheater/calciner Clinker/kiln Cement Hydrated cement Concrete SAMl'l i : PREPARATION CONCLUSION REFERENCES

5

147 147 149 149 149 152 152 154 154 156 156 158 159 163 164 166 166

Rietveld Quantitative Phase Analysis of OPC Clinkers, Cements and Hydration Products Miguel A. G. Aranda, Angeles G. De la Torre, Laura Ledn-Reirta

BRIEF INTRODUCTION I III! RIETVELD METHOD General issues Structural description of the phases present in OPC materials Whole-pattern quantitative phase analysis approaches SAMPLE PREPARATION AND DATA COLLECTION SELECTED EXAMPLES OF RIETVELD QUANTITATIVE PHASE ANALYSIS Clinkers Cements Hydration products Durability studies Selective dissolution vii

169 17(1 172 173 177 179 180 181 185 187 190 191

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INTERCOMPARISON AND COMPARISON WITH OTHER METHODS Bogue and reverse Bogue calculation Optical and scanning electron microscopies Thermodynamic modeling Thermal measurements Calorimetric data Nuclear Magnetic Resonance (NMR) spectroscopy GUIDELINES FOR RIETVELD QUANTITATIVE PHASE ANALYSES Crystal structures Sample preparation and data collection Data analysis Final check FINAL REMARKS AND OUTLOOK ACKNOWLEDGMENTS REFERENCES

6

192 193 193 193 194 194 194 195 195 196 197 198 198 201 201

Supplementary Cementitious Materials Ruben Snellings, Gilles Mertens, Jan Elsen

INTRODUCTION DEFINITION AND CLASSIFICATION OF SUPPLEMENTARY CEMENTITIOUS MATERIALS Definition Classification MINERALOGY AND CHEMISTRY OF SCMS Natural SCMs Thermally activated SCMs By-product SCMs THE POZZOLANIC REACTION The pozzolanic reaction mechanism Pozzolanic activity Hydration mechanism and kinetics of blended cements REACTION PRODUCTS Product assemblages Hydration thermodynamics PROPERTIES OF MORTAR AND CONCRETE CONTAINING SUPPLEMENTARY CEMENTITIOUS MATERIALS Properties of uncured mortar and concrete containing SCMs Properties of hardened mortar and concrete containing SCMs Durability of mortar and concrete containing SCMs CONCLUSIONS ACKNOWLEDGMENTS REFERENCES

viii

211 214 214 214 216 216 224 231 241 242 248 252 253 254 256 260 260 261 264 266 267 268

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Deleterious Reactions of Aggregate With Alkalis in Concrete Maarten A.T.M. Broekmans

INTRODUCTION Concrete in the built environment What is deleterious A A R ? W h y is A A R important? HISTORY A N D B A C K G R O U N D OF A S R First recognition Global and local acceptance of A A R Remediation and prevention ORIGIN O F ALKALIS IN C O N C R E T E The Na 2 0-equivalent Alkali from raw materials for Portland clinkering Infiltrated alkali f r o m seawater and deicers Alkali released from aggregate ALKALI-REACTIVITY POTENTIAL OF 'SILICA' Quartz properties and its solubility under A S R conditions Moganite, chalcedony and opal Solubility of silica under ASR conditions ALKALI-SILICA REACTION P R O D U C T S Dissolution of silica under ASR conditions Appearance and chemical composition of ASR gel Gel crystalline structure ALTERNATIVE ALKALI-REACTIVE M I N E R A L SPECIES Natural and industrial glass Common rock-forming silicate minerals Alkali-reactivity of carbonate rocks and minerals Other alkali-reactive species not being minerals LABORATORY A S S E S S M E N T OF ASR C O N C R E T E Sample acquisition and handling Impregnation-fluorescence petrography Quantification of ASR damage and development over time Characterization of the aggregate in ASR concrete In situ chemical analysis of ASR gel by SEM-EDS or E P M A THE ( RYSTAI U N I T Y INDEX OF QUARTZ SELECTED TOPICS F O R F U T U R E R E S E A R C H Reliable identification of quartz/silica properties governing alkali-reactivity Extraction of alkali-reactive aggregate from field concrete Dissolution of quartz/silica under ASR conditions Nano-structure of ASR gel Effect of lithium on ASR SUMMARY AND CONCLUSIONS ACKNOWLEDGMENTS REFERENCES

ix

279 279 280 281 282 282 282 284 285 285 285 287 291 295 295 300 301 303 303 305 308 312 312 313 317 321 321 321 324 326 331 335 339 340 340 341 341 341 342 342 343 343

1

Reviews in Mineralogy & Geochemistry Vol. 74 pp. 1-82, 2012 Copyright © Mineralogical Society of America

Calcium Alumínate Cements Raw Materials, Differences, Hydration and Properties Herbert Pollmann University of Halle-Wittenberg Von Seckendorffplatz 3 D-06120 Halle (Saale) Germany e-mail:

[email protected]

INTRODUCTION High alumina cement was used widely in the UK after World War I, expressing its higher content of aluminum oxide in comparison to Portland cement. Several descriptions of investigations on calcium aluminate cements appeared, starting around 1850, with a first patent field in 1888 (Scrivener and Capmas, in Hewlett 1998). More widely known is the work of Bied (1909, 1926) filing a patent in 1909 for the fabrication of cement using bauxite or some similar aluminum or iron-rich material, with low Si0 2 -contents and limestone. In 1918, the trade name Ciment Lafarge Fondue was used for the first time. Meanwhile in the USA, Spackman (1908, 1910a,b) developed cementitious material marketed under the name of Alca natural cements. Several patents were applied and granted (Bates 1921). A description of non-Portland cements was given by Muzhen et al. (1992). The reason for looking into alternative cement materials was to develop cements with improved stability against sulfate corrosion. Nowadays, calcium aluminate cements are used specifically for their distinct properties (Brown and Cassel 1977), some of which are presented in Table 1. Calcium aluminate cements do have special applications and are therefore widely used despite the fact that worldwide fabrication is by no means comparable to OPCs (Hohl et al. 1936; Garcés et al. 1997; George 1976, 1980a,b, 1983, 1990, 1997; George and Montgomery 1992; George and Racher 1996; Gartner et al. 2002). Scrivener and Taylor (1990) and Scrivener et al. (1997a,b) described calcium aluminate cements and their use and microstructural developments. The use for experimental purposes was described by Auer et al. (1995). Thermal analyses for thermogravimetry of CAC-fraction and formation was discussed by Chudak et al. (1982,1987). The CAC concretes and reactions were studied by Dunster et al. (1997,2000) and Deloye et al. (1996) studied the so-called "Portland Fondu." Raw materials CACs are mainly produced out of limestone and bauxite (Bolger 1997). Sometimes hydrated lime, laterite, bauxite (Valeton 1986; Sehnke 1995), or alumina is also used as raw material. Reduced qualities are mainly obtained by increased content of silica (

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Calcium Alumínate

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33

(1956). A structure description is given by Hanic et al. (1980). Pleochroite earns its name from its extremely strong pleochroism in optical microscopy even in extra thin sections, resembling strongly colored tourmaline in standard sections of 30 |im thickness (Ingham 2010). A structural model of pleochroite is given in Figure 43 and a SEM micrograph shows pleochroite in Figure 44. The hydration was studied by Kapralic et al. (1989). The composition in CAC clinker was described by Midgley (1979). Podworny et al. (2007) gave microstructural data for the same. Ca3Mg(Si04)2 - merwinite. Moore and Araki (1972) and Yamaguchi and Suzuki (1967) described the atomic arrangement of merwinite. A natural occurrence of merwinite in Crestmore/USA was described by Larsen and Foshag (1921). The structural model of merwinite is shown in Figure 45 and a SEM micrograph of merwinite is shown in Figure 46. CaMg(Si04) - monticellite. Monticellite and its possible solid solutions with olivine are described by Yang (1973), it occurs mainly in slags. Initially, monticellite was called "shannonite" based on an incorrect chemical analysis omitting Mg. This was corrected shortly after renewed analysis revealed its true composition (see footnote 3 on page 79 in Tilley 1929). Currently, the mineral name shannonite refers to a rare lead carbonate; its use for a CaMgsilicate is thus obsolete and should be avoided. A structural model is displayed in Figure 47.

34

Pôllmann

Ca2MgSi207- âkermanite. Solid solutions within the melilite-series were investigated by Swainson et al. (1992). The system diopside-âkermanite-nepheline was studied by Onuma and Yagi (1967), and Omar et al. (1986) discussed some of its phase relations in petrological systems. A SEM-micrograph is presented in Figure 48. Options for cation substitution are many in âkermanite and a characteristic disilicate structure is shown in Figure 49.

Figure 45. Structural model of merwinitephase.

Figure 46. SEM micrograph of merwinite.

MERW13 20KV

X3.000

Figure 47. Structural model of monticellitephase.

KONTRON

Calcium Alumínate

35

Cement

Figure 48. SEM micrograph of ákermanite. Copyright from Calcium and Calcium Alumínate Cements, edited by R.J.Mangabhai and F.P.Glasser, London, IOM Communications, 2001

AKERM2

20KV

raw IRON

Figure 49. Structural model of ákermanitephase.

Ca3Fe2TiOs - tricalcium ferro-titanate. Depending on available iron and titanium contents, formation of C 3 FT is possible in some CACs. A structural model was provided by Rodriguez-Carvalhal et al. (1989) (Fig. 50). A natural mineral equivalent has not yet been identified. CaTiOj - perovskite. Perovskite and related/derived phases play an important role in technology, notably in the electro ceramic field as semiconductors, piezoelectric materials and in superconducting materials (Hazen 1988), but they can also be found in CAC. Manganese containing perovskites were described by Galasso (1969), Buttner and Malsen (1992), Pôllmann et al. (2001), and Stôber et al. (2009a,b, 2010a,b). Silicates with a perovskite structure were described by Goldschmidt and Rait (1943). Kay and Bailey (1957) described structure and properties. Perovskites do not contribute to hydration with formation of new phases. Compositions of perovskite-like phases C^Fe^Ti^O^,.! were described by Causa et al. (1992). Gloter et al. (2000) showed TEM evidence of perovskite-brownmillerite coexistence. Koppmans et al. (1983) performed some Neutron powder diffraction studies. Octahedral tilting among perovskite relatives is described Woodward (1997). A structural model is indicated in Figure 51.

36

Póllmann

Titanomagnetite structure and composition are described in detail by Wechsler et al. (1984). Table 13 gives a summary of different magnesium containing phases in CAC.

HYDRATION OF CALCIUM ALUMINATE CEMENTS The hydration of CAC is strongly dependent of temperature applied and therefore different hydration products are formed (Edmonds and Majumdar 1988a,b, 1989a,b). The main reactions of calcium aluminates are described in the following schemata at different temperatures and are mainly crystalline CAH-phases, but include also different AH-phases, which can be amorphous or crystalline. Early hydration was investigated by Gotz-Neunhoffer (2003a,b, 2005), Gotz-Neunhoffer and Neubauer (2002, 2003, 2004, 2005), and Pollmann (2007, 2008). At elevated temperatures, stratlingite C 2 ASH s or ferrate hydrates may also be identified in the complex hydration microstructures. Some less reactive compounds may get involved in the hydration, because the hydration of more reactive compounds produces

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Figure 62. Transformation reactions of aluminum hydroxides at different temperatures. (Hauschild 1963; Torkaretal. 1961; Weis 1966).

The layered crystalline structure of gibbsite is given in Figure 63. Table 17 summarizes some different hydrated aluminum phases. Sulfur-containing phases. Sulfur-containing phases can occur in some CACs due to impurities. Sulfide occurring phases were especially known from CAC of Rolandshiitte/ Liibeck-Germany. Some other sulfate containing minerals are summarized in Table 18. Specifically, calcium sulfate can be formed in CAC in minor qualities or some hydrates like bassanite or gypsum by slight hydration conditions, but these phases are mainly known in the so called ternal mixtures of CAC with OPC and a sulfate source. Sulfoaluminate cement contains as a basic mineral the yeelimite compound.

48

Pöllmann

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y transformation there is a substantial change in lattice constants, recognizable but the fact that the density of the y-modification is 2.94 g/ cm 3 or about 10% lower than the P-modification with 3.20 g/cm 3 . As a result of this change in modification an originally compact burning product cracks and rapidly disintegrates into dust (so called "dusting") as soon as the temperature during cooling falls below 500 °C. Furthermore, the P-modification is hydraulically active while the y-modification is NOT. This is the second good reason to rapidly cool the clinker leaving the kiln. Dicalcium silicate modifications can take up more foreign ions in their crystal lattices than tricalcium silicate. Individual modifications can be stabilized in this way so that the modification which is stable at high temperatures is retained on cooling to normal temperatures. The impure P-modification of dicalcium silicate is called belite. Microprobe examinations of

Alternative

Low-C02

"Green" Clinkering

Processes

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industrial clinkers show that belite always contains A1 as a foreign ion. Na, Mg, P, S, K, Ti, V, Cr, Mn and Fe may also be present (Barnes et al. 1978). The high temperature modifications of C 2 S can be stabilized by certain foreign constituents. Depending on the nature and the concentration of the foreign substances or foreign substances it is possible to produce certain modifications selectively. Examples of proven stabilizers are Ca 3 (P0 4 ) 2 , Na4P 2 0 7 , V 2 0 5 , B 2 0 3 , SrO, BaO, K z O and combinations of Na z O and Fe 2 0 3 (Regourd and Guinier 1974; Ghosh 1985; Suzuki et al. 1986). Tricalcium aluminate, C 3 A, is the component in the C-A binary system richest in C It melts incongruently at 1542 °C dissociating into solid C and a melt accordingly poorer in C. Pure C 3 A occurs only in one cubic crystal modification (Mondal and Jeffrey 1975). The crystal lattice of C 3 A can take up various foreign ions in solid solution, especially Fe 3+ , Mg 2+ , Si4+, Na + and K + , as well as Cr3+, Ni 2+ and Zn 2+ (Stephan et al. 1999). The alkalis play a special role as their incorporation into the lattice changes its symmetry from cubic through orthorhombic to monoclinic. Stephan and Plank (2007) studied C 3 A doped with Fe 2 0 3 , Si0 2 and N a 2 0 in terms of changes to the crystal lattice as well as influence on the hydration. The change in reactivity by doping with foreign oxides was not found to be directly linked with the intensity of changes in the lattice parameters, but more a function of the kind and concentration of doped foreign oxide. Na 2 0 was found to retard the reactivity of the C 3 A in short and medium terms, even when the dosage was kept below the concentration where the crystal structure changes from cubic to orthorhombic (< 2.4%). The retardation was, however, compensated when the hydration proceeded. The retarding effect of N a 2 0 on C 3 A was amplified by the combined doping with Fe 2 0 3 . The Fe 2 0 3 compound in the cement clinker was originally assigned a composition corresponding to the formula Ca 4 Al 2 Fe 2 O 10 or C 4 AF in short hand notation (Hansen et al. 1928). It is therefore known as tetracalcium aluminoferrite or in natural rocks as brownmillerite after its discoverer (Hentschel 1964). C 4 AF is a phase from the incomplete solid solution series between dicalcium ferrite, C 2 F, which is stable under normal conditions and dicalcium aluminate, C 2 A, which only can be produced under high pressure (250 MPa at 1250 °C). This means that the Fe 3+ and Al 3+ ions are interchangeable in this compound within certain limits. The mixed crystals of this series should therefore have the formula C 2 (A,F). The end member of this solid solution series richest in A consists of 70% C 2 A and 30% C 2 F (Majumdar 1965). In the ternary phase diagram C-A-F by Newkirk and Thwaite (1958), it is shown that the calcium aluminoferrite crystal that is in stable equilibrium with C and C 3 A has a composition corresponding to 48 mol% C 2 A and 52% C 2 F and therefore very close to the composition C 4 AF. The calcium aluminoferrite mixed crystal can take up foreign ions in the crystal lattice (Hansen et al. 1928). The incorporation of Mg 2+ is of particular industrial significance, but it can also take up manganese, Mn 2+ (Guttmann and Gille 1929a,b,c; Goffin and MuPgnug 1933a,b; Parker 1952; Akatsu and Maeda 1967; Kondo et al. 1978), titanium, Ti4+ (Marinho and Glasser 1984) and chromium, Cr6"1" (Sakurai and Sato 1968). Silicon, Si4+, is apparently not incorporated in the crystal lattice (Ono et al. 1985), but is present in the form of C 2 S inclusions (Neubauer et al. 1996). A SEM back-scattered electron image of cement grains cast in epoxy and plane polished to jim fineness is shown in Figure 1. The angular crystals of C 3 S and rounded crystals of C 2 S can clearly be seen embedded in a white mass of C 4 AF and C 3 A. The mix of C 3 A and C 4 AF is often referred to as the interstitials since they are surrounding C 3 S and C 2 S crystals as they were the melt from where the silicates where grown. When such a clinker mass is ground, each resulting grain may be composed of all 4 phases. Thus, one cannot simulate cement by blending the 4 pure phases together in their respective amounts.

88

Justnes

Figure 1. S E M image of a large cement grain from ground clinker showing that in consists of several phases; edgy C 3 S to the left and rounded C 2 S (striped) in the right part, both grown from the frozen melt of C 3 A and C 4 AF surrounding them (called "interstitials").

Minor elements are often added deliberately to the raw meal as mineralizers. They may also be present as contaminations in the raw meal or in alternative fuels and raw materials (AFR), but they will all affect the clinkerization as if they were added as mineralizers. Thus, the effect of mineralizers is of importance to alternative raw materials. Mineralizers are compounds (mostly inorganic) which influence the process of reactions in the solid, liquid and solid-liquid interface face during burning of cement clinker. The possible effects can be summarized in changes of the chemical, mineralogical, structural, textural, mechanical and physical properties. The effects of the mineralizers can often be caused by specific elements in the compound added to the cement raw mix. Small additions of selected elements, often referred to as foreign ions, can alter the properties of the melt extensively. The modified properties can easily be seen in terms of reactivity and burnability of the raw mix. Mineralizers can be active at several stages in the burning process. The raw meal largely consists of ground limestone, clay and quartz. Some corrective (auxiliary) materials as iron oxide and bauxite (not shown) are also interground. The main reactions/mechanisms as the temperature increase are: calcinations (decarbonation) of the limestone at 700-900 °C: C a C 0 3

CaO + C 0 2

•

transformation of quartz from low- to high-temperature modification

•

formation of calcium aluminates (C 12 A 7 ), dicalcium silicate (belite = C 2 S) and ferrite phase [C 2 (A,F)] above 700 °C

•

formation of molten phase at 1300 °C and tricalcium silicate (Alite = C 3 S) is formed by the reaction: C 2 S + CaO C3S

Mineralizers are reported to affect decarbonation, the formation of alite and properties of the melt and stabilize the hydraulic belite polymorphs (a- and P-forms). Substances that are affecting the melt properties are also called fluxes. Reaction temperatures are often decreased when applying mineralizer(s) which means increased burnability of the mix. The effects of adding a mineralizer to the cement raw mix can be the following:

Alternative Low-C02 "Green" Clinkering Processes

89

•

decreased burning temperature due to changes in the reactivity and burnability

•

acceleration of the clinkerization reactions (higher activity of the clinker minerals) at lower burning temperatures, e.g., fluorosilicates

•

altered surface tension (e.g., S0 4 2 - ) and viscosity of the melt

•

formation of new intermediate phases as well as more stable phases when approaching firing temperatures

•

controlled polymorphism of the clinker minerals

•

changed properties of the produced cement (e.g., hydraulic activity and strength development)

An extensive literature review on the effect of minor elements was carried out by Moir and Glasser (1992). They explored the effects of minor components in view of the periodic table of elements and divided them into the following groups; alkalis, transition metals, halogens and p-block elements. High belite cement clinker High belite cement is essentially a Portland cement where the content belite, C 2 S, is much higher (45-60%) than that of alite, C 3 S (20-30%), or quite the opposite of an ordinary Portland cement (OPC). The lower calcium level should then give less C 0 2 emission providing the source is limestone. In addition, this cement requires about 100 °C lower kiln temperature than OPC, requiring less fuel and hence somewhat lower C 0 2 emission also for this reason. High belite cement is harder to grind than OPC and will require some extra energy in this respect. Gartner (2004) made an estimate of C 0 2 savings changing from a modern OPC with 65% C 3 S to a high belite cement with little or no C 3 S and found that the overall reduction in limestone consumption in total would not be more than 8%. Even allowing for the ensuing reductions in burning temperature, he pointed out that the likely maximum total C 0 2 emission savings only would be in the order of 10%. He also said that this has to be balanced against the fact that high belite clinkers are very hard to grind and thus require more energy. Very low rates of strength development are also considered unsatisfactory by most costumers. Chatterjee (2003) reviewed the status of high-belite cement and concluded that the interest in it has grown over the three last decades due to its anticipated multidimensional benefits like lower energy consumption, raw materials conservation and constructional durability of the resultant concrete. However, the product and its manufacturing technology are yet to be of extensive commercial significance as there are still no viable technologies to substantially enhance the intrinsic low reactivity of the belite phase and to generate large surface area for the cement at a reasonable energy input to achieve a higher degree of hydration in concrete (also see Popescu et al. 2002). It seems that high belite cement in practice is produced in Japan, India and China (Sui and Yao 2003) and that the application first and foremost is as low heat cement in massive structures like dams. However, low heat can also be obtained by for instance using large content of supplementary cementing materials as blast-furnace slag and fly ash in the concrete (also see Janotka and Krajci 1999). For the sake of cement with less C 0 2 emission it does not seem like high belite cement is worth pursuing with its low savings potential (about 10%), in particular when bearing in mind the low strength development rate that would have hampered building productivity. Calcium sulfoaluminate cement (CSA) Calcium sulfoaluminate cement (CSA) has recently been promoted as the cement for sustainable development (Alaoui et al. 2007) as a typical cement composition is 53% C 4 A 3 S,

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18% C 2 S, 12% C and 15% C 4 AF (2% residual). According to their comparison with Ordinary Portland cement reproduced in Table 3, the C 0 2 emission is in theory not only less for calcium sulfoaluminate cement (-43%), but also the specific heat consumption during clinkering ( - 1 4 % ) due to lower temperature, as well as lower crushing energy ( - 4 0 % ) since the minerals are more friable. Table 3. Comparing C 0 2 emission (from raw materials) and energy of making ordinary Portland cement vs. sulfoaluminate cement (Alaoui et al. 2007).

Parameter C 0 2 emitted per tonne clinker specific heat consumption during clinkering 1 energy for crushing 3

Ordinary Portland Clinker

Sulfo -aluminate Clinker

535 kg/t

305 kg/t

3.845 GJ/t 2

3.305 GJ/t

45 - 50 kWh/t

20 - 30 kWh/t

'Popescu et al. 2002, 2 BAT is 3.1 GJ/t, 3 Janotka and Krajci 1999

Gartner (2004), however, pointed out the many practical problems with cementing systems based on ettringite (C 6 AS 3 H 3 2 ) as binder, such as sulfoaluminate cements, especially the problem of controlling the expansion associated with the reaction. Calcium sulfoaluminatebased cements are increasingly being used in special applications where high early strengths and self-stressing or shrinkage compensation are required (e.g., self-leveling screeds), but the more general application to concrete is limited to China, where a wide range of C 4 A 3 S-based cements have been developed and normalized under the name of the "Third Cement Series" with the acronym "TCS" (Zhang et al. 1999). In the Chinese literature, it is stated that TCS, which are based on clinkers containing C 4 A 3 S, belite and ferrite in various proportions as their major phases, can be used in a wide variety of applications depending on phase composition and on the amount of gypsum or anhydrite interground to make the final cement. The TCS technology practiced in China lately seems to mainly be based on clinkers with rather high C 4 A 3 S contents (60-70%), utilized in the pre-stressed concrete sector in which the rapid strength development at moderate curing temperatures, plus self-stressing, are economic advantages. TCS is usually manufactured using bauxite as a principal raw material, making them relatively expensive compared to Portland cements. The TCS approach has been reinvestigated in some eastern European countries (Palou et al. 2003) to make energy-efficient sulfoaluminate-belite cements with lower C 4 A 3 S contents and higher belite contents than TCS produced in China. However, available strength results are disappointing, probably due to the same problem of low belite reactivity in high belite Portland cements, so TCS cements may not offer very significant global C 0 2 savings if the usually construction productivity is to be maintained. Li et al. (2007) acknowledged the great potential of calcium sulfoaluminate cement in reducing C 0 2 emission by at least 20-30% compared to an OPC of equal performance providing the clinker is produced in a modern rotary kiln. They also added that a great deal further careful study will be required to fully understand the hydration of these interesting and novel cements in order to better optimize their compositions and thereby further decrease manufacturing C 0 2 emissions for equal concrete performance. Valenti et al. (2007) also pointed out a few other environmentally friendly aspects of calcium sulfoaluminate cements. Firstly, industrial wastes and by-products difficult to reuse and dispose can be used as raw materials for its clinker production, such as fluidized bed combustion waste, red mud, low-quality pulverized coal fly ash and chemical gypsum. Secondly, the intergrinding

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of large amounts of gypsum with the clinker enables reduced clinker content and enhanced chemical gypsum utilization, in particular flue gas desulfurization gypsum generated worldwide in increasing amounts. Pera and Ambroise (2004) listed the following advantageous applications of sulfoaluminate cements: •

development of concrete with high early strength: 40 MPa in 6 h and > 55 MPa at 24 h

•

design of self-leveling screed with limited curling when unbounded to its support

•

design of self-leveling topping mortar presenting the following properties: time of workability > 30 min, set within 75 min and low drying shrinkage (< 250 )im/m)

•

glass fiber reinforced cement (GFRC) composites that can be demolded 4 h after casting and present high ductility and durability after aging in different weathering conditions

Quillin (2001) acknowledged a very good sulfate resistance of sulfo-aluminate-belite cement, but the chloride diffusion was higher than Portland cement, and especially the carbonation rate. However, he admitted that the durability may have been improved using a suitable plasticizer to achieve a lower w/c (used w/c = 0.56). Glasser and Zhang (2001) evaluated the durability of 14 year old reinforced concrete pipes (w/c = 0.25) exposed to the tidal zone in China, and found that the mild steel mesh reinforcement was without corrosion. This may have been due to a dense matrix and rapid self-desiccation that is difficult to re-saturate. The aspects concerning durability has made applications of calcium sulfoaluminate cement outside China limited to for instance rapid repair mortars and self-leveling screeds. In China with its > 106 t/year production it is also used in construction, but apparently in low performance structures as in in-door housing etc. However, this makes quite a bit of the total concrete market in a society, so this cement may be worthwhile looking further into due to its large saving potential in C 0 2 emission and considering the fast strength development enabling faster building processes. Calcium alumínate cement (CAC) Calcium aluminate cements (CACs) (Hewlett 1998) are cements consisting predominantly of hydraulic calcium aluminates. Alternative names are "aluminous cement," "high-alumina cement," and "Ciment Fondu" in French. They are used in a number of small-scale, specialized applications. An extensive review on calcium aluminate cements is given in Pollmann (2012, this volume). The method of making CACs from limestone and low-silica bauxite was patented in France in 1908 by Bied, from the Pavin de Lafarge Company. The initial development was as a result of the search for cement offering sulfate resistance. Subsequently, its other special properties were discovered, and these guaranteed its future in niche applications. The cement is made by fusing together a mixture of a calcium-bearing material (normally limestone) and an aluminum-bearing material (normally bauxite for general purposes, or refined alumina for white and refractory cements). The liquefied mixture cools to a basalt-like clinker which is ground alone to produce the finished product. Because complete melting usually takes place, raw materials in lump-form can be used. A typical kiln arrangement comprises a reverberatory furnace provided with a shaft preheater in which the hot exhaust gases pass upward as the lump raw material mix passes downward. The preheater recuperates most of the heat in the combustion gases, dehydrates and de-hydroxylates the bauxite and de-carbonates the limestone. The calcined material drops into the cool end of the melt bath. The melt overflows the hot end of the furnace into molds in which it cools and solidifies. The system is fired with

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pulverized coal or oil. The cooled clinker ingots are crushed and ground in a ball-mill. In the case of high-alumina refractory cements, where the mix only sinters, a rotary kiln can be used. The main active constituent of calcium aluminate cements is monocalcium aluminate (CaAl 2 0 4 or CA in the cement chemist short-hand notation). It usually contains other calcium aluminates as well as a number of less reactive phases deriving from impurities in the raw materials. Rather a wide range of compositions is encountered, depending on the application and the purity of aluminum source used (Taylor 1997). Constituents of some typical formulations are given in Table 4. The mineral phases all take the form of solid solutions with somewhat variable compositions. Because of their relatively high cost, calcium aluminate cements are used in a number of restricted applications: 1) in construction concretes, rapid strength development is achieved, even at low temperatures, 2) in construction concretes, high chemical resistance is possible, 3) in refractory concretes, strength is maintained at high temperatures and 4) as a component in blended cement formulations, various properties such as ultra-rapid strength development and controlled expansion can be obtained. Incorrect use of calcium aluminate cements has led to widespread construction problems, especially during the third quarter of the 20 th century when this type of cement was used because of its faster hardening properties. After several years some of the buildings and structures collapsed due to degradation of the cement and many had to be torn down or reinforced. Heat and humidity accelerate the degradation process known as "conversion;" the roof of a swimming pool was one of the first structures to collapse in the UK (http://webs. demasiado.com/forjados/patologia/aluminoso/index.htm). In Madrid, Spain, a large housing block nicknamed Korea (because it was used to house Americans during the Korean war), built ~1951-54 was affected and had to be torn down in 2006. Also in Madrid, the Vicente Calderón soccer stadium was affected and had to be partially rebuilt and reinforced (www.elmundo.es/ papel/2007/02/07/madrid/2082060.html). Table 4. Oxide and mineral composition of some calcium aluminate cements (CAC).

Oxide/Mineral Si0 2 (S) A1203 (A) Fe 2 0 3 (F) CaO (C) MgO (M) Na 2 0 (N) K 2 0 (K) Ti0 2 (T) monocalcium aluminate (CA) dodecacalcium hepta-aluminate (C12A7) monocalcium dialuminate (CA2) belite (C2S) gehlenite (C2AS) ferrite (C4AF) pleochroite ( - C ^ M S ) wiistite (F) corundum (A)

General Purpose

Buff

White

Refractory

4.0 39.4 16.4 38.4 1.0 0.1 0.2 1.9 46 10 0 7 4 24 1 7 0

5.0 53.0 2.0 38.0 0.1 0.1 0 1.8 70 5 0 5 14 5 1 0 0

2.7 62.4 0.4 34.0 0.1 0 0 0.4 70 0 17 0 11 2 1 0 0

0.4 79.6 0 19.8 0 0 0 0.1 35 0 30 0 1 0 0 0 33

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Calcium alumínate cements (CACs) are like calcium sulfoaluminate cements (TCS) usually manufactured using bauxite as a principal raw material, which is the main reason why they are relatively expensive compared to Portland cements. All CACs has monocalcium aluminate (CA) as the main cementing mineral (see Table 4) and thus they have a fairly low raw material derived C 0 2 emission. However, in addition to its higher raw materials cost, the general purpose clinker ("Ciment Fondu") is made by a melt process, which is not very energy efficient compared with the Chinese TCS approach (which makes use of conventional rotary kilns).

MINERALS AS ALTERNATIVE TO LIMESTONE General Calcium sources other than limestone can, in theory, be used as raw materials, but according to Gartner (2004) there are not really any other sufficiently widespread and sufficiently concentrated sources of calcium available. However, it will also help to replace parts of the calcium with other minerals than limestone, not necessarily all of it. On the other hand, local deposits may play a role for new plants to be built as limestone may become scarce some places. Important developing countries like India, for instance, only have limestone sources available that can serve cement plants to be built until 2012 for their expected production time thereafter, according to Kulkarni (2011). It should be reinstated once more that the easiest short-time measure to reduce C 0 2 emission from Portland cement production, is to replace part of the clinker with supplementary cementing minerals (e.g., fly ash, blast furnace slag, calcined clay, impure limestone or calcined marl) as outlined by Justnes (2007a,b 2010). However, that is the topic of another contribution to this MSA Short Course (Snellings et al. 2012, this volume). Generally speaking, one should look for natural calcium silicates that also may contain hydroxyl groups or hydrate water without any problem. Minor content of carbonate may also be acceptable. The aluminate and ferrite content cannot be too large if it should significantly replace calcium and silicon in Portland cement clinker, since the maximum content in this clinker typical is 7% A1 2 0 3 (A) and 3% Fe 2 0 3 (F) as pointed out in Table 1. High A (« 39%) and F 16%) is found in CACs (see Table 4), but then the silica (Si0 2 or S) content is low 4%). The CSA is also high in alumina (30% or 36% A depending on type), but also here silica is low (6-8% S) in addition to low ferrite ( 28 d) ages. Belite occurs in clinker as the second framework grain, typically appearing as rounded grains with a lameller structure (appears cross-hatched) that is brought out by chemical etching. Tricalcium aluminate (Ca 3 Al 2 0 6 ) comprises from 1% to 18% by mass of a clinker, and may be referred to as C 3 A for the pure phase or aluminate for the industrial phase. It can occur in one or more of three crystallographic forms dependent upon the amount of chemical substitution. The aluminates react rapidly, but contribute little to the cement strength. Tetracalcium aluminoferrite (Ca 2 AlFe0 5 ), or C 4 AF is found in concentrations between 5% to 15% by mass of clinker. The ferrite phase is a solid solution with variable Al 2 0 3 /Fe 2 0 3 ratios and does incorporate substituted Mg +2 , Si44, Na +1 , and K +1 ions (Hofmanner 1975). The solid solution phases are not distinguishable using light microscopy, and the ferrite phase is highly reflective and unaffected by common etching reagents. The aluminate and ferrite phases are part of a liquid phase in the clinkering process, forming a matrix linking the silicate framework grains. Their texture is influenced by the cooling rate, and may be coarse-grained for slowly cooled and fine-grained for rapidly cooled clinker. The fine-grained varieties can be difficult to distinguish using a light microscope and the terms "undifferentiated" and "differentiated" matrix is a commonly used descriptor. White cement is manufactured from clinker characteristically devoid of C4AF. The iron and minor amounts of manganese in the ferrite phase in Portland cement result in a strong coloration, imparting the typical gray color to the clinker nodules and to the cement. Periclase (MgO) is found in clinker as a result of MgO in the limestone feed material. Clinker with MgO contents in excess of 2% will generally exhibit some periclase. It is dispersed

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throughout the clinker, within and between silicate and matrix phases as equant crystals, but may also assume a dendritic morphology. Periclase is relatively hard and resistant to etching reagents, making it stand out in topographic relief, and readily identified in low to mediummagnifications as pink grains when rocking the focus above and below the optimum. Periclase exhibits slow reactivity, forming brucite (Mg(OH) 2 ) with a substantial volume increase. In a hardened concrete, this volume increase may not be accommodated and the strain may result in cracking. ASTM C 150 specification limits the total MgO content of cement to 6% in an effort to reduce the propensity for unsoundness. Some microscopy studies have questioned this limit, placing more emphasis on the size and distribution of periclase (Taylor 1997). Free lime (CaO) may occur in clinker where the C a 0 : S i 0 2 ratio of the raw feed material is too high, the raw feed material is not homogeneous, or from over-sized limestone fragments. Free lime, like periclase, may hydrate after the cement paste has hardened, with an increased potential for cracking. Free lime readily hydrates upon exposure to humid air, converting to Portlandite over a short time. The microscope and a qualitative scan of the clinker should yield clues to the source of free lime, which is an important part of monitoring the efficiency and effectiveness of the clinker production process (Hofmánner 1975). The alkali sulfates occur as a number of different mineral forms, the most common being arcanite (K 2 S0 4 ) and aphthitalite ((K,Na) 3 Na(S0 4 ) 2 ). Less common are calcium langbeinite (K 2 Ca 2 (S0 4 ) 3 ), thenardite ((Na,K)S0 4 ), and anhydrite (CaS0 4 ) (Taylor 1997). In light microscopy, these phases are grouped as "alkali sulfates" and identifiable (as a group) with appropriate etching reagents. In SEM imaging, some distinctions may be made based upon X-ray microanalysis, especially if complementary X-ray powder diffraction data are available. Gypsum (CaS0 4 -2H 2 0) is added to the clinker during the cement grinding process. Although the term gypsum is used, either through impurities in the raw material or due to dehydration in the grinding process, the actual mineral composition of the gypsum may include bassanite (CaS0 4 -0.5H 2 0, often called hemihydrate or plaster), and anhydrite (CaS0 4 ). The availability of sulfate ions directs the hydration reactions involving the aluminate phases to form ettringite preferentially to the AFm phases, which are responsible for the stiffening phenomena called flash set where, as the name indicates, the concrete mixture stiffens rapidly and becomes unworkable during placement and finishing. Another phenomenon, called false set, involves the hydration of bassanite to gypsum with a concomitant increase in the rheological properties. In contrast to flash set, this problem may be overcome with time and additional mixing. The clinker pore system may also be of interest, especially if an assessment of clinker grindability is being made. The pore system is a network of inter-connected and occluded voids that may vary significantly across a clinker nodule. Ideally, the embedding epoxy fills the voids to enhance their contrast with the other clinker phases. The discipline of clinker petrography has evolved to better understand the clinkering process, to evaluate process-related problems in clinker production, and to better relate clinker compositional and textural attributes to concrete performance.

CLINKER MICROSCOPY Microscopic examination is a direct means of analysis, in contrast to the indirect Bogue calculations that transform a bulk chemical analysis into phase estimates. While quantitative microscopy is an ideal technique for clinker analysis in cement manufacturing, it is not widely used today due to the time involved in a proper analysis. It remains very useful as a rapid screening tool to evaluate clinker production through semi-quantitative assessment of microstructural features and their association with potential processing problems. In addition,

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the microscope is used to evaluate the raw mix that is fed into the cement kiln as part of a quality control program (Fundal 1980; Miller 1981; Campbell 1999). Finally, the microscope has been invaluable in efforts to develop Standard Reference Materials for cement clinker, and to capture the compositional and textural aspects of hydraulic cements, fly ash, and slag for use in developing three dimensional microstructure development models of the hydration process. The microscope is a relatively rapid means to assess the clinker fabric for the cement manufacturer. Petrographic analyses of clinker utilize descriptive terminology for phase assemblages, texture and fabric, similar to that in igneous and metamorphic petrology. As described by Hofmanner (1975), clinker fabric is comprised of three features: phase association, texture, and structure. Phase association refers to the types and amounts of the constituent phases in the clinker. Texture stands for the size, shape, and distribution of the phases, and structure refers to potential grain orientation and the pore system. Textural terms commonly used are euhedral (or idiomorphic) for well-formed crystals, subhedral for moderately well formed crystals, and anhedral (xenomorphic) for poorly formed crystals, along with descriptive terms like dendritic, lath-shaped, and amoeboid. In addition to the individual phase descriptions, the phase distribution is an important reflection of the grinding, homogenization, and mixing of the source materials throughout the clinkering process. Phases may occur as clusters with distinct boundaries, in clusters with a ragged boundary, and as clusters forming streaks across the clinker nodule. These modes of occurrence may vary widely across a nodule so care must be taken to examine large areas in the examination of clinker. If the phase abundance is the primary goal for the examination, crushing the clinker should provide a more uniform specimen for examination. For cements, this is less of a problem as the grinding provides a degree of homogenization. Much effort has been directed toward relating clinker fabric with the manufacturing process, specifically anomalies in the processes of raw feed production and homogenization, and the clinkering processes Campbell (1999). Hofmanner (1975) provides an excellent chart that relates textural and structural aspects of the clinker constituents to the raw mix fineness, homogeneity, and chemical composition, the kiln burning conditions, and cooling conditions. Furthermore, according to Hofmanner (1975), the ideal clinker has a uniform distribution of all phases, euhedral to subhedral alite crystals, rounded belites, a fine-grained, differentiated matrix, and only minor, well-dispersed crystals of free lime. Semi-quantitative descriptions of clinker microstructure are useful in trying to evaluate the clinkering process. For example, if a clinker exhibits an abundance of belite, little alite, and no free lime, the raw mix may have had a low Ca0:Si0 2 ratio. Conversely, a clinker with abundant alite, little belite, and little free lime will have had a high Ca0:Si0 2 ratio of the raw mix. As the raw mix deviates from the ideal particle size distributions and homogeneity, a more heterogeneous clinker will result. Similarly, if the proportions of lime to silica or of aluminum to iron deviate from the ideal, changes in the phase proportions and microstructure will occur. The chart provided in Hofmanner (1975) on interpretation of clinker texture should be useful in unraveling the effects of confounding processes in clinker microstructure. Changes in cement manufacture toward greater energy efficiency, recycling of kiln dust, and use of waste materials as raw materials and fuels have added new variables in the clinkering process, and an update of this useful chart would be beneficial to the industry.

SPECIMEN PREPARATION FOR MICROSCOPY Obtaining a representative sample can be a challenge in a cement plant given the throughput of a few hundred tons per hour, or more. Often the objective of the study is to specifically analyze materials that are unusual, like fragments of material dissimilar to the clinker. For routine analyses, Hofmanner (1975) provides an example plan where three 2 kg

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samples are takes at five-minute intervals from a selected sampling location. These samples are blended and split using a riffle splitter or by cone-and-quartering to a 500 g sample. The remaining 500 g sample is crushed to a 5 mm particle size, blended, and then further split to the desired sample size to produce two specimens. Proper specimen preparation methods facilitate examination and interpretation of microstructural features. Improper preparation methods, however, may obscure features, and even create artifacts that may be easily misinterpreted. Reflected light microscopy and scanning electron microscopy using backscattered electron and X-ray imaging require a highly polished surface. The polished surface has two distinct advantages to a fracture surface: 1) clear definition of the constituent phases, and 2) a planar surface amenable for quantitative analysis. Using a representative sample, specimens are potted in an epoxy resin to permeate the material's pore system and to encapsulate the particles. The specimens are then cut or ground to expose a fresh cross section of particles, lapped to smooth the surface, and then polished using a series of successively finer grades of diamond paste. This polishing stage may be subdivided into a coarse polish where the grinding damage is removed to expose a blemish-free cross section, and a fine-polishing stage that removes the fine scratches that obscure the details of the microstructure. Each of these steps will be illustrated subsequently. Epoxy impregnation of the pore system serves three purposes: A) it fills the voids, B) it encapsulates the particles, creating a solid that is better able to resist plucking and spalling during the polishing process, and C) it enhances contrast between the pores and cementitious material. With relatively high permeability materials or powders such as clinker or Portland cement, an epoxy of low viscosity is necessary while for the less permeable cement pastes and concretes an ultra-low viscosity epoxy aids in rapid infiltration of the pore structure. The selection of epoxy depends upon the materials and the means for analysis. For polished sections, the ideal epoxy will exhibit good capillarity, wet and bond to pore walls and edges, fill voids, and will not leave any residue on the specimen surface that will adversely affect etching (for light microscopy) or backscattered electron contrast, will not soften (promoting particle plucking) from exposure to cleaning agents such as ethanol or acetone, and, particularly for SEM analyses, is relatively beam-stable. For clinker, a medium-viscosity epoxy 1 , 2 is used to promote rapid infiltration of the accessible void spaces. Vacuum assist pulls the air out of the less accessible void spaces and forces the epoxy into these voids. While complete permeation may not be possible in many cases as a result of occluded voids, an additional step of back filling the open voids and a second epoxy cure after sectioning may prove useful. A higherviscosity epoxy is used for cement, fly ash, and slag powders. This epoxy cures harder and is a better match to the material when polishing. Materials for sample preparation A list of equipment and materials necessary for preparation of polished specimens is given in Table 1. For some items, substitution may be possible if comparable supplies are available in the laboratory. The list is presented in order of use of the equipment or supplies. Preparation of clinker Clinker nodules or crushed fragments are placed in a mold container and surrounded by epoxy leaving a top surface exposed to the laboratory air, allowing the epoxy to be drawn 1 Certain commercial materials and equipment are identified in order to adequately specify experimental procedures. In no case does such identification imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the items identified are necessarily the best available for the purpose. 2 Suitable materials include L.R. White, hard grade for ultra-low viscosity, and Epotek 301 for medium viscosity, and Epotek 353ND for powders.

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Table 1. Equipment and supplies for preparation of polished sections.

Item

Purpose

Potting epoxies (medium and low viscosity)

for powders and hardened pastes

Mold cups

potting specimens

Mold release

facilitates removal of specimen / epoxy

Metal trays to hold specimens

contains any leaking epoxy

Vacuum chamber and pump

vacuum impregnation

Drying / curing oven

capable of at least 65 °C

Diamond blade wafering saw

cutting after curing

Propylene glycol

diamond saw cutting lubricant

Alcohol: 200 proof ethanol, or isopropyl

alternate cutting lubricant, cleaning aid

Acetone

final cleaning aid

Ultrasonic bath

specimen cleaning

Lapidary wheel (minimum 200 mm)

grinding and polishing

Diamond pen

label engraving

Abrasive papers (silicon carbide)

coarse to fine grinding, 240 to 1200 grit

Polishing cloths (low-relief)

6

Diamond paste for polishing

6, 3, 1, 0.25

Lint-free cloths

specimen handling and cleaning

Compressed air

specimen cleaning and drying

Vacuum dessiccators

specimen storage

and finer polishing in non-aqueous suspension

into the microstructure by capillary suction. To speed the infiltration, the specimen may be completely immersed in epoxy, and a vacuum drawn to remove remaining air. When the bubbling stops the vacuum may be released, forcing the epoxy into the pore system. The epoxy is cured, and then is ready for the cutting and polishing. Figure 1 shows an example of a potted, polished set of clinker, cement, and fly ash specimens. Polished powder mounts of Portland cementitious materials Powder mounts are prepared by suspending cement powder in epoxy, curing the epoxy, cutting and polishing a surface of the powder/epoxy composite (Fig. 2). To save preparation time, multiple specimens of powders may be mounted simultaneously by preparing sample disks that are drilled for each specimen; a 4 mm hole being suitable for obtaining a representative sampling of most powder specimens. A reference mark is then cut into the disk so it may be oriented in the microscope; a dry cut using the diamond wafering saw is suitable for this operation. The cement powder is mixed with a few drops of epoxy, adding powder to form a cohesive ball. The cement/epoxy mixture is placed in a drill cavity and pressed to fill the base of the cavity. The mixture is then consolidated in the sample mold by sharply tapping it on the laboratory bench top, and the epoxy is cured according to the manufacturer's guidelines. After curing, the specimen is removed from the mold, labeled and a fresh surface is exposed using a wafering saw or by grinding. Cutting and grinding The purpose of the cutting, grinding and polishing steps, which are common to all preparations, is to expose a fresh, flat surface. Diamond blade slab or wafering saws, lubricated using propylene glycol, are suitable for exposing a fresh surface. This surface needs to be smoothed by grinding. Abrasive papers of 400 and 600 grit (silicon carbide paper) used dry are also suitable for rapid removal of material by grinding. Coarser abrasive may be used if

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Figure 1. Polished sections of clinker (upper-left), while cement and fly ash powder samples contain multiple samples that have been indexed for identification (mm scale at base).

necessary. Using successively finer grades of abrasive paper removes damage produced by the earlier grit. After the 600 grit grind, the surface is smooth enough for polishing with the diamond pastes. Visual examination of the specimen allows one to identify when the abrasive has cut the entire surface. Grinding striations on the specimen surface indicate that grit has completely removed a layer of material. By alternating grinding directions by 90 degrees one can insure that the entire surface has been ground. These operations damage the specimen surface necessitating a polishing step that is described next. Polishing Polishing removes the damage imparted by the grinding operations. Using a sequence of successively finer polishing pastes composed of fine diamond particles (ranging from 15 )im

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Figure 2. Specimen mounts prepared using the same epoxy used to pot the powders. Cured disks (1) are pre-drilled (2) to accommodate multiple powder mixtures (3), are cured, cut and polished (4), and ready for the microscope.

down to 0.25 )im) and a lap wheel covered with a low-relief polishing cloth, an undisturbed microstructure is exposed and the fine scratches of the polishing operation are removed. The coarse polishing stage eliminates the grinding damage zone, exposing a relatively undamaged cross section, and the final polishing removes residual scratches from the coarse polishing stage, ideally leaving a damaged zone that is essentially invisible for the imaging method applied. Figures 3 and 4 illustrate the increased clarity of a clinker microstructure as the grinding damage is progressively removed with initial polishing stages using a 1200 grit silicon carbide paper or 9 |im diamond paste. These images are from a reflected light microscope from unetched specimens. Figure 3 (upper image) after sawing using a diamond blade wafering saw shows only the outline of the fragment, with no internal details discernable. In Figure 3 (lower image), the 6 )im polish is beginning to cut the topographically high portions off the specimen to reveal cross sections of the crystals of the specimen. This stage is perhaps the most important step, as incomplete exposure of undisturbed microstructure will result in the polishing of only a portion of the specimen, leaving an inordinately large apparent void system. A reflected light microscope is helpful to examine the clinker fragments, or a large cement grain to confirm that no grinding damage remains before moving on to the polishing steps. Figure 4 (upper image) illustrates a specimen with a minor amount of grinding damage, evidenced by the elongated, irregular-edged dark voids within the clinker crystals. Additional time spent coarse polishing almost completely removes the grinding damage, leaving a haze of fine scratches (Fig. 4, lower image). This specimen is now ready to move on to the final polishing stages. Subsequent polishing stages of 3 |im, 1 |im, and 0.25 )im pastes remove fine scratches from the 6 jim polish, further improving constituent definition.

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Figure 3. Reflected light microscope images of a clinker surface after saw-cutting and grinding using 600grit silicon carbide (upper image) exhibits no discernible microstructure due to the rough surface. Increased polishing time (bottom image) using 6 |jm diamond paste progressively removes grinding and cutting damage pits and begins to reveal the underlying microstructural features.

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Cements

Mi '

d 1*&

Ait

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r

Jf

is'-

V

. y.

t*m

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j f

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r

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rr

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Field Width = 250 (jm I 7 / " £I

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Field Width = 250 Mm Figure 4. Reflected light microscope images showing the effects of grinding and polishing. The upper image shows grinding pits due to incomplete grinding. The lower image shows a specimen where the coarse polishing has removed all the grinding pits, and the specimen is now ready for the 3 |jm and finer polishing steps to remove any fine scratches.

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The final polish must meet a number of criteria to ensure one achieves the best possible surface for analysis: a) few scratches on the specimen surface, b) sharp particle and pore perimeter edges (over-polishing will tend to round corners), c) well-defined phases and crystal boundaries, d) minimal surface relief, e) no etching due to the polishing process, f) epoxy completely fills all voids, g) no polishing media residue trapped within voids or on the surface, and h) the surface is cleaned using ethanol or isopropyl, followed by an acetone rinse. A thorough cleaning is necessary to provide a surface that will allow a uniform etch or carbon coating (for SEM examination). An example of a well-polished clinker fragment is shown in Figure 5. In this example, some of the pores appear unfilled and show reflection from the pore walls. The ferrite phase is the most distinct with its high reflectivity, but the remaining phases appear similar. The use of chemical etchants was devised as a means to enhance distinction between phases through enhanced coloration of reaction products and enhanced grain boundaries, facilitating examination by light microscopy.

Figure 5. SRM 2688 after polishing displays a surface suitable for etching for light microscopy or ready for SEM examination. Voids to the right are not filled with epoxy, resulting in some light reflection from the void walls.

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Etching for light microscopy Polished sections of clinker in the light microscope have little contrast aside from the more highly reflective ferrite phase. The use of etchants provides contrast between the phases due to selective reaction with the reagents and resulting precipitation of the reaction products, imparting color to each phase. Relief polishing produces enhanced grain boundaries, facilitating phase identification. Directions for making etch solutions and their applications are taken from Campbell (1999). Figure 6 shows the effect of a 0.1 mol/L aqueous solution of KOH applied to a polished clinker surface for 30 s, rinsed with isopropyl and dried. This etchant results in the blue coloration of the alumínate phase, darkening the alkali sulfates, and imparting coloration to free lime. This is typically followed by a nital (1.5 mL of nitric acid in 100 mL of ethanol) etch for about 10 s, then rinsed with isopropyl that results in a blue coloration for alite and brown for belite. The vapor of hydrofluoric acid makes an ideal etchant, leaving a uniform coloration to the silicates, slightly affecting the aluminates, darkening the alkali sulfates, and with little affect on periclase and the ferrite phase. The hazards in handling this acid, however, make it less popular.

aluminate

Field Width = 250 |jm Figure 6. SRM 2686 KOH etch provides some phase contrast by imparting a blue coloration on the alumínate matrix phase, the silicates alite and belite are uniformly affected, and the ferrite phase, unaffected, remains bright.

SRM clinker The Standard Reference Material (SRM) clinkers are used for developing and testing methods of quantitative phase analysis (Stutzman et al. 2008). These clinkers were selected as representative of the range of North American clinker production with respect to phase abundance, crystal size, and crystal distribution. The reference values represent consensus means and un-

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certainties based upon three independent analytical methods: (1) quantitative XRD, (2) light microscope point counts, and (3) image analysis of scanning electron microscope image sets. Clinker 2686a is intermediate in crystal size relative to the other SRM clinkers, and exhibits heterogeneous phase distribution and little free lime (Fig. 7). Alite occurs as subhedral to anhedral crystals approximately 30 |im in size. Belite occurs in large clusters with an approximate crystal size of 20 |im. The matrix is differentiated with a medium- to fine-grained lath-like ferrite, and fine-grained aluminate filling the inter-lath voids. Equant periclase crystals up to 15 |im are common throughout the microstructure and the alkali sulfate phases aphthitolite and arcanite are disseminated throughout the microstructure. According to the Hofmanner (1975) diagnostic chart, the belite clusters may represent heterogeneity or over-sized silica grains in the raw mix. The fine-grained matrix indicates rapid cooling from 1300 °C, and the slightly elevated belite content may reflect a slightly low CaO content. SRM 2687 is a fine-grained clinker with abundant subhedral alite with a grain size of about 40 jim, few belite clusters with rounded grains scattered throughout the clinker, and some diffuse free lime clusters (Fig. 8). This may be interpreted as a raw feed having a high CaO content or perhaps heterogeneous blending. The matrix is abundant, but poorly differentiated, indicating a rapid cooling. Alkali sulfates are common and dispersed throughout the clinker along grain boundaries. SRM 2688 is a coarse-grained, homogeneous clinker with euhedral to subhedral alite up to about 150 |im in size, and well-dispersed, rounded belite of size up to about 40 |im (Fig. 9). The matrix exhibits two distinct types, both well differentiated where ferrite occurs as either a coarse lath-like or a medium-grained dendritic form, aluminate occurs as fine-grained with the dendritic ferrite, and more lath-like with the coarse ferrite (Fig. 10). Periclase is a minor component and is fine-grained and dispersed within the matrix. Alkali sulfates are uncommon and reside within the matrix. The coarse-grained nature of the clinker silicates may reflect a long residence time in the kiln above 1300 °C, or a high maximum temperature. The coarse matrix may reflect a slow cooling. Clinker nodules may experience different thermal histories from the outer-portion relative to the core, especially if they are large. Examination of a wide cross section of particles may be necessary to be able to make definitive conclusions on a clinker. Point count analysis Quantitative microscopy is based upon the relationship between area fraction and volume fraction. For composites consisting of randomly distributed, randomly oriented phases, the planar area fraction is an unbiased estimator of the volume fraction (Chayes 1956; Campbell and Galehouse 1991). The Glagolev-Chayes method, referred to as the point count method, is perhaps the most widely used technique in quantitative mineralogical analysis when using a microscope, and is the basis for ASTM C 1356, Standard Test Method for Quantitative Determination of Phases in Portland Cement Clinker by Microscopical Point-Count Procedure. This procedure utilizes a grid of points to sample the clinker polished section (Fig. 11). Selecting a combination of eyepiece reticule and magnification in conjunction with the clinker crystal size to achieve spacing so as adjacent points ideally do not fall on the same crystal is a general rule and sampling in a regular pattern across the sample. About 3000 to 4000 points on clinker phases are necessary to have reasonable counting statistics for the measurements and a duplicate sample is recommended Hofmanner (1973). Calculate mass percentages by multiplying the volume fractions by the density of the corresponding clinker phase and normalizing the totals to 100%. A number of sources of systematic and random error are present in any quantitative analyses, influenced by sample preparation, specimen polishing and etching, and operator experience in point counting and phase identification. A statement of uncertainty should accompany any measurement. Uncertainty analysis for each phase fraction estimate is based on counting statistics. Assume a representative sample

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Figure 7. SRM 2686a exhibits a heterogeneous silicate distribution and a fine-grained, differentiated matrix. Hydrofluoric acid vapor etching imparts color to the silicates, slightly darkens the aluminates and periclase, while the ferrite phase appears unaffected.

Figure 8. SRM 2687 with a hydrofluoric acid vapor etch shows the fine-grained character, the predominance of alite and the undifferentiated matrix.

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Figure 9. SRM 2688 with a hydrofluoric acid vapor etch shows uniform etching of the hexagonal alite rounded belite showing the lamellar structure, and a matrix of lath-like ferrite (bright white) and alumínate (off-white). Some belite dots occur in the matrix and some ferrite dots occur within the alite crystals.

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Figure 10. SRM 2688, hydrofluoric acid vapor etch with the dendritic texture of ferrite (upper) and lathlike texture of ferrite (lower), very fine-grained belite within the matrix, fine-grained ferrite inclusions in alite, and decomposition to belite along alite grain boundaries.

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Figure. 11. Single sampling field for SRM 2688 clinker with a hydrofluoric acid vapor etch and 9-point grid where six points fall on alite, and one each on belite, alumínate, and ferrite.

was collected and that the sample size was reduced using proper practice. The measurement uncertainty of the phase fraction is related to the number of points falling on the phase and the total number of points counted, as described in Hofmanner (1973), Neilson and Brockman (1977), and Howarth (1998). The latter two references provide a more recent look at the sampling and measurement uncertainty associated with point counting, and also provide recommendations on sampling and methods to calculate confidence bounds on estimates from point count data. Using the clinker data provided in Table 2 on four specimens counted to about 3100 points each where N is the total number of counts, n the counts per phase, a n d / i s the inverse of the F probability distribution for (1 - a , 2(N - n + 1), In), available from the F table or calculated through a spreadsheet function. The upper, p(n)", and lower p(n)', 95% confidence bounds are calculated as: p(n)" = 100 / [l + (N - n) / {(/? +1)/}] p{n)' = 100 / [l + {(# - n +1)/} / n]

SCANNING ELECTRON MICROSCOPY ANALYSIS The scanning electron microscope provides sets of images that are suitable for processing and analysis. The uniformity of the backscattered electron and X-ray images makes it possible to perform image processing (feature extraction) and analysis (measurements) for quantitative microscopy. SEM analysis is perhaps the only microscopic means to characterize fine-grained,

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Table 2. Example point count data on four replicate S R M 2686 specimens, mean area fraction and 95% confidence bounds using a light microscope and point count analysis (Kanare 1987).

Counts (sample)

Phase

Area Fraction

Density (g/cm3)

alite

1892

1809

1936

1865

3.18

61.0

58.4

62.5

60.2

beli te

708

781

637

730

3.30

22.8

25.2

20.5

23.5

aluminate

39

38

34

42

3.06

I.3

1.2

1.1

1.4

ferrite

354

357

387

384

3.77

II.4

11.5

12.5

12.4

periclase

87

107

105

63

3.58

2.8

3.5

3.4

2.0

alkali suif,

12

3

6 10

2.67

0.4

0.2

3.34

0.3

0.1 0.1

0.0

3

0 1

0.0

0.3

3098

3100

3100

free lime Totals

Phase

3100

Area Fraction Mean

95 % Confidence Bounds Upper

Lower

/

alite

60.5

61.2

59.8

1.0307

beli te

23.0

23.7

22.4

1.0355

aluminate

1.2

1.4

1.1

1.1372

ferrite

12.0

12.4

11.5

1.0461

periclase

2.9

3.2

2.7

1.3664

alkali suif,

0.2

0.2

1.0893

free lime

0.2

0.3

0.1 0.1

1.3751

multi-phase particles like cement, fly ash, and slag. The cement grinding operation reduces the clinker nodules to a size distribution that spans from about 45 |im to down to about 1 |im, destroying the clinker crystal arrangements that are so useful for phase identification. Light microscopy is of limited use because of the fine particle size and the potential for any etching operation to partially or completely dissolve the finer-sized particles. The multiple SEM phase and chemical imaging modes help overcome these limitations for qualitative and quantitative microscopy (Scrivener 1987; Stutzman 1994, 2007; Bentz and Stutzman 1994; Stutzman et al. 2008; Bullard et al. 2011). While the traditional point count analysis is readily accomplished using a SEM, image processing and analysis using the full image field is common and is amenable to automation in both data collection and analysis. Interestingly, this type of analysis essentially replicates that performed in the early days of microscopy when sketches were made of the fields of view to quantify phase fractions (Insley and Frechette 1955). SEM imaging of microstructure The combined information from backscattered electron (BE) and X-ray (XR) imaging from the same field of view is used to quantify phase abundance. These two kinds of images are captured simultaneously and registered (same field of view), and present different perspectives (phase and chemistry) that will be used in conjunction with fabric characteristics (framework, matrix, or dispersed phase) to identify and quantify the constituent phases. In the BE compositional image, local brightness is proportional to the individual phase average atomic number ( Z ) . The backscatter coefficient r| is a measure of the backscattered electron fraction and, following Goldstein et al. (2003), is estimated using the mass fractions and r| values for each constituent. Table 3 lists phases that are found in clinker, cement, and

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Table 3. C o m m o n phases in ordinary Portland c e m e n t clinker, blast f u r n a c e slag, and fly ash, ordered per material with decreasing brightness in S E M - B E imaging, with average atomic n u m b e r Z and backscatterd electron coefficient r|.

Phase ferrite free lime alite belite arcanite aluminate-cubic aluminate-orth. aphthitalite anhydrite bassanite gypsum thenardite periclase

Z

Tl

16.65 16.58 15.06 14.56 14.41 14.34 13.87 13.69 13.42 13.03 12.12 10.77 10.41

0.186 0.188 0.172 0.166 0.165 0.164 0.159 0.159 0.154 0.149 0.138 0.125 0.121

Phase

Z

Tl

Slag merwinite average slag gehlenite melilite akermanite

13.71 13.36 13.11 12.80 12.25

0.157 0.153 0.150 0.147 0.105

Fly Ash quartz mullite hematite magnetite

10.80 10.69 20.59 21.02

0.125 0.124 0.223 0.227

pozzolanic mineral additions in descending order of their backscattered coefficient and gray intensity. The contrast between alite ( Z = 15.06) and belite ( Z = 14.56)js relatively strong and their distinction is clear, while that between belite and cubic aluminate ( Z = 14.34) is generally too weak to distinguish these constituents. These values are estimates and the actual backscattered expression of a phase will depend upon any chemical substitution and the image collection dwell time. Longer image collection times may improve the distinction between phases. Typical SEM operating conditions for clinkers and cements are 10 kV accelerating voltage, about 3 nA probe current, which is adjusted to maximize count rates while keeping an X-ray dead time below 40% when collecting X-ray images, and 5 min per frame scan rate (1024 x 768 pixels) to minimize image noise. The magnification is adjusted to retain a 0.75 pm/pixel spatial resolution. Changes in the accelerating voltage affect both the size of the interaction volume, and as such, the spatial resolution, and it affects which peaks will appear, and the relative peak heights, in the X-ray spectral response. Practically, compromises are necessary and 10 kV represents a value that provides a reasonable spatial resolution, improves X-ray response for the lighter elements (Na, Mg, Al), and yet is sufficient to excite the heavier elements (Fe). A 10 keV beam (in BE mode) striking a calcium silicate has a signal interaction volume and an X-ray volume resolution that is about 0.75 pm (J. Davis, personal communication). X-rays are generated as a result of the interaction between the high-energy electron beam and the specimen. The X-ray spectrum consists of the characteristic lines for each element present, represented by peaks on the spectrum, and a background of white radiation. X-ray microanalysis may be used to identify and quantify chemical composition of phases that can be used to generate images of element spatial distribution. The latter capability is particularly useful for image processing as it enables a set of element spatial distribution images to be included in the image processing. XR imaging is necessary for distinguishing between phases that have the same BE coefficient yet are compositionally distinct and for identification of phases that are not easily detected in the BE image; periclase and some alkali sulfates will appear dark, like voids. Combining the XR and BE images allows a degree of phase discrimination that is not available from any single image. An example of the backscattered electron image and X-ray spectrum for aluminate from SRM 2686a is provided in Figure 12. While the SEM images lack the color information of the

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Alumínale

C4KA1

Energy (KeV) Figure 12. SRM 2686a BE "compositional image" (upper) with 1) free lime, 2) ferrite, 3) alite, 4) belite, 5) alumínate, 6) periclase, 7) void, and 8) alkali sulfate and (lower) X-ray spectrum for alumínate indicates the presence of its constituent elements by their characteristic X-ray lines.

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light microscope images, the view is not unfamiliar to the microscopist, and is perhaps easier to view and understand due to the uniformity of appearance for each phase, the flexibility with the wide range of magnification, and the ancillary information provided by X-ray microanalysis. As with the light microscope, identification is made by a combination of phase morphology, association with other constituents (framework grain or matrix phase), BE brightness, and chemistry. For this clinker field of view, there is abundant alite, minor belite, a differentiated matrix with abundant ferrite and little aluminate, widely-dispersed periclase, a cluster of free lime, and some alkali sulfate along void and fracture walls. Figures 13 through 19 illustrate the microstructure of the three NIST SRM clinkers by SEM BE imaging from low to high magnifications. Descriptions of these clinkers were provided earlier with the light microscopy. SRM 2686a (Figs. 13-14) is a medium-grained, heterogeneous clinker with a fine-grained, differentiated matrix, abundant well-dispersed periclase as equant and occasionally dendritic crystals, and common alkali sulfates along pore walls and inter-grain boundaries. SRM 2687 (Figs. 15-17) is a fine-grained, moderately heterogeneous clinker with common belite nests, a fine-grained, poorly differentiated matrix, moderate alkali sulfate, and occasional free lime. The BE imaging mode shows a fine-scale differentiation of the aluminate and ferrite matrix phases in Figures 16 and 17, where the ferrite appears with a web-like texture within the aluminate. This feature was not observable in the light microscope. SRM 2688 (Figs. 18-19) is the coarsest-grained clinker of the three. It has a homogeneous distribution of phases and a well-differentiated matrix of ferrite and aluminate that ranges from very coarse, lath-like crystals to a finer-grained dendritic ferrite with fine-grained aluminate. A BE and X-ray image set from SRM clinker 2686a is presented in Figure 20, with the backscattered electron image labeled BE, and the X-ray images labeled by their respective elements. A common means of image collection stores the spectrum accumulated at each pixel into a file called a data cube. From this file the total spectrum may be calculated for any region and images of element spatial distribution (X-ray images, or maps) may be generated for subsequent analysis. The images displayed here were considered the most useful for phase identification. More element images may be collected or extracted from the data cube if thought necessary. Image analysis will use a subset of these images to reduce redundancies that can confound the mineral phase identification (segmentation) process. An example of redundancy is between the BE and Fe image. Ferrite is the second brightest (Table 3) phase and exhibits a unique grey level. The Fe image duplicates this with an image with inherently greater noise. The use of the Fe image for cements will not add any new information, and may confound the analysis due to the additional noise. Typically for clinker and cements, calcium, silicon, aluminum, magnesium, iron, potassium, sodium, and oxygen are the principal images selected from an initial screening of the data. A visual assessment is usually all that is needed to identify significant images necessary to extract the set of constituent phases. Fly ash and slag are inherently more complex and will require additional images. An example may be seen with aluminate and belite having distinct chemical compositions, yet exhibit a similar backscattered electron coefficient. Figure 20 illustrates this with relatively large, rounded belite grains appearing at the same grey level as the matrix-phase aluminate. However, belite contains appreciable silica while aluminate contains aluminum, so use of one or the other of the X-ray images will serve to distinguish these phases. Similarly, the calcium sulfate addition (for cement) and the alkali sulfates are usually difficult to see given the high brightness and contrast of the BE image. The X-ray images Ca, S, K, and Na may be used to aid in their distinction. The mineral phase identification process may be visualized through a process whereby the BE and XR images are merged into a red-green-blue composite image. A useful combination for cement color composites is BE = red, Mg = green and Al = blue to see the belite-aluminate

HV WD 10.0 k v 10.39 mm

-100

Mm-

Figure 13. SRM 2686a at low magnifications illustrates distribution of phases and porosity. Abundant alite, clustering of belite, a differentiated matrix comprised mostly of ferrite and some fine-grained alumínate, and uniform distribution of periclase is typical for this clinker.

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Figure 14. SRM 2686a at higher magnification with belite inclusions in alite crystals, a differentiated matrix dominated by the ferrite phase, and both equant and dendritic (lower image) periclase.

126

Figure 15. SRM 2687 is a fine-grained clinker with abundant alite, some small nests of belite, variable porosity, and abundant, undifferentiated matrix.

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Figure 16. For SRM 2687, higher magnification images begin to reveal details of the matrix, which consists of both a medium and fine-grained ferrite.

HV WD 10,0 kV 12.13 mm

1 o fj rn

Figure 17. SRM 2687 matrix appears to be a mixture of medium-grained ferrite, massive aluminate and a web-like textured ferrite intermixed with the aluminate.

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Figure 18. SRM 2688 is a coarse-grained clinker with uniform phase distribution, no free lime, a mediumto coarse-grained well differentiated matrix, and occasional presence of periclase and alkali sulfates within the matrix and along grain boundaries.

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Figure 19. SRM 2688 at higher magnifications showing the belite and ferrite inclusions within alite, and a medium- to coarse-grained, well-differentiated matrix.

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Figure 20. Backscattered electron (BE) and X-ray image set for SRM 2686a that will be used to segment the sample into the constituent phases.

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distinction, limestone, and the contribution of Mg in identification of periclase. In this case, the addition of the aluminum image makes the aluminate appears purple compared to the dark red of the belite, allowing their distinction. Another useful combination is BE= red, K=green and S=blue to highlight the locations of the gypsum addition to the cement, and alkali sulfates within and along cement grains. Processing the image set serves to enhance the distinction between the constituent phases by reducing noise and unwanted signals. This may be accomplished, as needed, by use of a median filter to reduce noise, yet retain edge definition, followed by clipping to establish a lower threshold to eliminate the background signal introduced by the continuous background of the X-ray spectrum. If difficulties are encountered in subsequent processing, the median filtered image may be useful in achieving a more successful segmentation. All these operations may be interactively applied using most image analysis codes, and are illustrated here using ImageJ (this processing and analysis code is available from the National Institutes of Health, http://rsbweb.nih.gov/ij/index.html). Image processing Traditional image processing methods use Boolean logic and mathematical operators to threshold phases individually using one or more images from a set. These operations create binary images for each phase that are subsequently merged into a composite image indexed by phase. For example, ferrite is typically a bright phase and may be segmented using only the BE image on that basis. This may also be accomplished using thresholding tools where the upper and lower grey level bounds are interactively set prior to creating the binary image. A more difficult example may be found in the aluminate phase in Figure 21 where in the BE image, aluminate exhibits a gray level similar to that of belite, making these phases indistinguishable from the BE image alone. However, aluminate contains appreciable amounts of aluminum so we can make the distinction using a combination of the BE and aluminum images. This example makes use of a thresholding tool to set upper and lower gray level bounds for the BE and Al image to create the respective binary images, BE-t and Al-t. The Al-t is subtracted from BE-t to create a binary of belite distribution and the belite binary is subtracted from the BE-t to create a binary of aluminate distribution. Some general criteria for making phase distinctions for clinker and cement are presented below. Criteria for identification and segmentation of cement and clinker phases: • • • • • • • • • • •

Free lime - strong calcium, rounded grains, brightest BE Ferrite - high iron, prismatic matrix phase, bright BE Aluminate - matrix phase, high aluminum, low magnesium, intermediate BE Belite - rounded grains, low aluminum, intermediate BE Alite - medium-high BE, principal phase, hexagonal shape Periclase - equant to dendritic habit, may occur anywhere, high magnesium, low BE Alkali sulfate - along grain boundaries or voids, high sulfur, high K, medium Na Gypsum - high sulfur, calcium, low potassium, low BE Quartz - high silicon, intermediate BE Kaolin - high aluminum, high silicon Slag - sharp, angular grains, high silicon, magnesium, and aluminum, low BE

While this approach works well, it is tedious since constituents are segmented individually and then merged into a final image. Difficulties in reconciling the composite image are encountered with this process as areas of overlapping phase assignments and holes from incomplete segmentations will need to be resolved. In certain circumstances, however, it can be a rapid and useful means to isolate specific features for quantitative analysis.

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1. 2. 3.

Threshold BE and Al X-ray image Subtract Al-t from BE-t to get belite Subtract belite binary from BE-t to get alumínate

Figure 21. Segmentation of belite and alumínate, which have similar BE grey levels, requires the additional Al image, thresholding the BE (BE-t) and Al (Al-t) images and finally image subtraction to generate binary images of belite and alumínate distribution. Field width = 400 |jm.

Multi-spectral processing developed for analyzing hyperspectral remote sensing data provides an alternative method of image processing. Van Niekerk (2003) and Lydon (2005) used one such code, Multispec© (Landgrebe 2003; Landgrebe and Biehl 2011) for geological studies of rocks and the success of their efforts prompted application of this type of code to processing SEM image sets. Combinations of BE and XR images of cementitious materials are displayed to identify the constituent phases, to establish a user-defined training set of image regions typical of each phase (including voids), and to classify each pixel into the phase group to which it most likely belongs. This is in contrast to the sequential creation of binary images and eventual reconciling of image overlaps and dropouts. The suitability of the classifier and the operator-designated training set in segmentation may be assessed by a visual

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evaluation of the resulting segmented image and by evaluation of the accuracy in which the training set was properly classified. Multi-spectral processing begins with reading the subset of B E and X R images into a suitable image processing software application. Different combinations are displayed for phase identification and establishing the user-defined training set (Fig. 22). By merging individual images into a red-green-blue channel pseudo-colored image, the constituent phases may be highlighted and identified. Regions of each phase (including void space) are defined with a training set, with at least 100 pixels each to define each phase. The next step is to select a classifier and group like pixels and review the performance of that classifier on the training set. This may be accomplished graphically using an X - Y plot, demonstrated in Figure 23 with the B E image on the X-axis and aluminum channel on the Y-axis. The ferrite phase, being the brightest, is plotted high along the X-axis, reflecting it's large B E signature. The distinction between belite (Class 2) and aluminate (Class 3), while having similar B E signatures, becomes clear with the addition of the aluminum image data. The classifier extrapolates the training set characteristics for each phase for the entire image to complete the classification. Reviewing the performance of the classifier on the designated training field pixels provides an initial sense of the success of the segmentation (Table 4). A successful classification will have high reference accuracies for each phase. The miss-classifications in the matrix to the right in Table 4 provide some clues to the nature of any miss-classifications. Often, the addition of a larger number of training pixels will reduce the classification error through an improved definition of the range of class attributes. The best assessment of the segmentation will be a comparison of the segmented image to the original B E image. Figure 24 shows the original B E and thresholded binary images for each phase in the field of view. Examination of the phase assignments provides a visual assessment on the success of the segmentation. The resulting image is typically displayed as a single image where each phase is uniquely identified by a color and an index value. These are important for visualizing the results and for subsequent measurements of phase abundance and surface area. Since clinker is a heterogeneous product, some measure of the uncertainty is required (Table 5). The results of a clinker SRM development by image analysis and X-ray powder diffraction are examined next. Direct methods for development of standard reference materials The data set of SEM and X R D analyses for the development of SRM 2686a provides an opportunity to examine the results from two unique, direct methods for phase analysis. These data were used to establish consensus means and consensus uncertainty values for phase abundance for this SRM clinker. SRM certification requires at least two independent methods of analysis when no single method can provide the necessary level of accuracy and/or when there is no single method whose sources of uncertainty are well-understood and quantified. A common goal in the analysis of such data is to compute a consensus mean value and consensus uncertainty to that value (Stutzman and Leigh 2002). Error in microscopy may be due to incorrect classification as a result of operator misidentification of a phase, or due to irresolvable finely divided interstitial phases, or edge effects, which may preclude their proper classification. X R D data may be biased due to improper sample preparation, incomplete identification of phases as a result of difficulties in resolving weak diffraction peaks, the non-suitability of the structure models, effects of microabsorption (if significant), and the inability to identify and control correlations between variables. References on specimen preparation for X R D analysis include Bish and Reynolds (1989) and Bish and Plotze (2010). References specific to Rietveld analysis of X R D data include Young (1995) and McCusker et al. (1999). About 300 kg of clinker was obtained from a cement plant as sieved material in a size interval from 4 mm to 13 mm, representing a sampling of nodules that underwent a common

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Figure 22. Selection of typical regions for each phase establishes the training set for classification. Field width = 250 |jm. Bi Plot of Channels Z vs 1

Channel2ZBSp

8

23

46

69

92

115 138 161 Channel 1

184

207

230

253

276

Figure 23. The addition of the aluminum image (Y-axis) to the BE image (X-axis) shows the clustering facilitating the belite (2) and alumínate (3) distinction in the training set. Additional phases shown include alite (1), and ferrite (4).

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Table 4. Output from the classification provides a means to assess the re-classification of the training set pixels, the types of miss-classifications, and area fractions. TRAINING CLASS PERFORMANCE (Resubstitution Method) Number of Samples in Class Project Class Name

Class Number

Reference Number Accuracy Samples (%)

1

2

3

4

5

6.

7

8

alite

1

100.0

1362

1362

0

0

0

0

0

0

0

belite alumínate ferrite periclase

2 3 4 5

100.0 98.0 96.1 100.0

746 252 233 644

0 0 0 0

746 2 0 0

0 247 0 0

0 1 224 0

0 0 0 644

0 0 0 0

0 0 9 0

0 2 0 0

alkali sulfate free lime void

6 7 8

99.9 100.0 100.0

1536 1450 848

0 0 0

1 0 0

0 0 0

0 0 0

0 0 0

1534 0 0

0 848 0

1 0 1450

7071

1362

749

247

225

644

1534

857

1453

100

99.6

100

99.6

100

100

98.9

99.8

TOTAL Reliability Accuracy (%)

CLASS DISTRIBUTION FOR SELECTED AREA Class

Number Samples

Percent

1 alite 2 belite 3 alumínate

116,731 12,084 4,294

60.8 6.3 2.2

4 5 6 7

11,639 6,670 6,181 7,492

6.1 3.5 3.2

ferrite periclase alkali sulfate free lime

8 void TOTAL

26,909

3.9 14.0

192,000

100.0

Table 5. Phase abundance results from SEM / image analyses on n = 20 specimens, expressed as mass fractions. alite

belite

aluminate

ferrite

periclase

. sulfate

Average

64.2

la

0.5

19.7

2.2

10.1

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0.7

0.7

0.1

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0.1

0.1

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0.3

1.6

2.7

0.8

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Phase Alite Belite Aljminate Ferrite Perielase Alkali Sulfate Free Lime

Index A B C D E F G

Pixels 124977 7568 3057 12523 6865 5759 1473

Density 3 18 3.31 3.03 3.73 3.58 2.66 3.35

Mass % 76.0 4.8

1,8 8.9 4.7 2.9 0.9

Figure 24. SEM BE image, individual phase binary images, and calculated mass fractions for this image field. This is typically rendered as a single, indexed image with pseudo-color to show phase assignments.

thermal history. This sample was not intended to be representative of the bulk clinker production, but was sampled and processed with the intent of creating a homogeneous lot of clinker fragments. The nodules were stage-crushed using a jaw crusher and sieved to capture the size fraction between 3 mm and 4 mm. The fragments were homogenized using a V-blender and packaged in containers each containing about 7 g of clinker. Sampling followed a random-stratified scheme, taking 16 of the containers for XRD analysis and 10 samples for SEM analyses. In preparation for the XRD analysis, each container was split into duplicates using a cone-and-quarter method, and the splits were

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ground individually to a particle size of less than 10 jim. The splits were analyzed in triplicate. Microscopy samples were potted and polished as described earlier. Between 7 and 12 fields of view were collected for each clinker specimen in the SEM and the images were processed as described above. Two individuals performed these analyses independently. The box plot provides a graphical comparison of the XRD and microscopy results through assessment of the alignment or misalignment of median values and differences in interquartile ranges. The phase abundance analysis characterized by important features of the box plot: 1.

The width of each box is proportional to sample size,

2.

The median value is used for its resistance to outliers, and is identified by the "X" inside the box,

3.

The interquartile range ("middle half') of the data are represented by the vertical extent of the box,

4.

The extremes (minimum and maximum) are represented by the ends of the vertical lines projecting out of the box, and

5.

Circles outside the extremes of each box represent outliers.

Phase estimates by microscopy and quantitative XRD The XRD data for alite (Fig. 25) and belite (Fig. 26) exhibit greater precision than the microscopy data and, while the boxes overlap, both phase median estimates by XRD are lower than those by microscopy. The heterogeneity of the phase distribution is responsible for the greater variability in the microscopy. Preferred orientation corrections were not made for alite, a phase that cleaves in a manner that is subject to orientation effects. Preliminary tests with orientation corrections where intensity and orientation were simultaneously refined indicated that they tend to increase bias, probably as a result of correlations between refined variables. Given that all the phase abundance values are correlated, any orientation corrections will influence all phase estimates, so they were not used. The XRD values for aluminate (Fig. 27) and ferrite (Fig. 28) are similar to these from microscopy but are consistently lower. Periclase estimates (Fig. 29) exhibit reasonably close agreement between XRD and microscopy. The alkali sulfate data (Fig. 30) represent a sum of arcanite and aphthitalite since the distinction was not made by SEM imaging. Certified values by consensus means Certified values (Table 6) are unweighted averages of diffraction and microscopy mean values. The Type B on bias (BOB) approach (ISO GUM) to consensus means was used to establish the certified phase values (Levenson et al. 2000). The method is designed to handle cases where the number of analytical methods is small (2-5), and the ordinary sample standard deviation is an inadequate estimate of the uncertainty of the systematic effects (Levenson et al. 2000). Application to cements The SEM provides an opportunity to quantitatively describe fine-grained multi-phase powders in a way not possible using light microscopy. Cement imaging poses a more complicated problem with material preparation, image interpretation and analysis. The use of a harder epoxy has proven useful in the polishing of an encapsulated powder by reducing particle plucking and having an epoxy with polishing characteristics closer to that of the powder. Data interpretation is complicated due to the loss of many phase associations due to the grinding. An advantage to powdered materials though is that the particle size reduction tends to produce a more homogeneous sampling compared to clinker. There are numerous multi-phase particles to assist in initial phase identification and the images are typically uniform such that criteria for identification made in one region carries through across the entire image field. Additional phases, such as cal-

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Table 6. Certified values for SRM 2686a; mass fractions (%) with Mean and k = 2 (95%) expanded uncertainty (2Uc) (Stutzman et al. 2008). SRM 2686a

Alite

Belite

Aluminate

Ferrite

Periclase

Alkali Sulfate

Mean 2Uc

63.53 1.04

18.80 1.10

2.46 0.39

10.80 0.84

3.40 0.23

0.86 0.17

cium sulfates, and pozzolanic additions, such as limestone, fly ash, or slag, must be considered. Previewing the image set generally provides for identification of these constituents (Table 3). Figure 31 shows the original BE image of a polished cement section and the resulting segmented image. From the segmented, indexed image, measurements of area fraction, surface perimeter fraction, and spatial distribution may be made for the constituent phases (Bentz et al. 1999; Bullard et al. 2011). Taking these data and combining them with X-ray computed tomography images of real cement particles has enabled the generation of virtual cement particles (Fig. 32) with the phase and textural characteristics of actual industrial cements, which has been invaluable in the development of virtual cement hydration models (Bullard et al. 2011). S E M imaging of fly ash Since the 1950's, supplementary mineral admixtures (SCM) have been used in conjunction with cements. These are typically waste products from other industrial processes, yet have the potential of adding value to concrete products because of reduced cost and increased durability. SCMs fall into two general classes, pozzolanic and hydraulic. Pozzolans react with the calcium hydroxide in Portland cements upon hydration to form additional hydration products, such as calcium-silicate-hydrate, one of the principal phases in hardened cement paste. Pozzolans such as fly ash and slag are generally high in S i 0 2 and A1 2 0 3 . Limestone is a special case as it too will react with the pore solution of hydrating cement, accelerating the reactions of some of the cement phases and the formation of monocarbonate (Taylor 1997). Hydraulic mineral admixtures are capable of reacting with water and setting without the addition of Portland cement. The particle size of these materials may be as fine as, or finer, than that of cements and may also affect the system as filler particles between the hydrating cement grains and as nucleation sites for hydration products (Taylor 1997). Fly ash is a waste byproduct from coal-burning power plants (Fig. 33). The mineral fraction of the coal, principally clays, calcite, quartz, and pyrite, becomes partially molten during the combustion process, cools post-combustion in the flue gasses, and is collected in precipitators designed to remove the ash. The ash is comprised of some crystalline phases, but is predominantly amorphous and contains some residual carbon. The mineral, glass, and carbon portions of the ash reflect a combination of the organic and mineral composition of the source coal, the burning and precipitation processes at the power plant, and any subsequent processing that may serve to beneficiate the ash. The glassy portion is thought to be the reactive portion of the ash and the approximately spherically shaped particles are a result of its formation in the flue gasses. Blast furnace slag (Fig. 34) is a waste product from the production of iron Snellings et al. (2012, this volume). The slag is skimmed from the molten iron and is quenched. The process produces an angular, glassy, uniform calcium alumino-silicate product that is crushed and graded based upon particle size, sulfur and sulfate content, and pozzolanic activity. The characteristic shapes of these materials make them relatively easy to identify in cements when examining powder mounts with a light microscope, and in polished section in the scanning electron microscope.

140

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e o O ItLassesbackground I Alitfl I Be lite Alurmnate I Ferrite |Periclase Arc Q rite ICalcite Gypsum I Void

Figure 31. Segmentation of a polished section of hydraulic cement, following the same techniques used for clinkers, resulting in a segmented, indexed image. Field width = 500 |im.

Fly ash and slag phase characterization are difficult because of the lack of crystalline phases and limited knowledge of their glassy components (Chancey et al. 2010). The mineral constituents of these two latter materials can exert significant influence on their performance in a concrete, yet are not generally characterized. The glass phase(s) of these materials is not quantified and is typically considered as a single, homogeneous phase, which may not be the case. Spot X-ray analysis shows that slag is often comprised of calcium, silicon, and magnesium, with smaller amounts of aluminum, sulfur, and potassium. While the homogeneity and angularity of the slag is initially apparent in the backscattered electron image, local regions high in silicon and in aluminum indicate a heterogeneity resulting from very finely divided discrete phases.

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Figure 32. Virtual cement particle created by combining the results from analysis of SEM image data and X-ray computed tomography. Reconstruction of an actual cement particle is used in three-dimensional cement hydration modeling.

The SCMs are a case where the mixture of crystalline and glassy phases is ideally addressed through multiple analyses. Combined quantitative XRD and SEM with image analysis provide a means to explore the complicated microstructures of these materials to better understand their phase composition and textures and perhaps to explore potentially new methods for classification. The analytical methods are applied in the same manner as used for cements; a powder for XRD using an internal standard, and polished powder mounts for SEM.

SUMMARY Microscopy has played a significant role in developing our understanding of cementitious materials compositions and their effects on cement and concrete performance. It continues to play an important role in the evaluation of cement clinker kiln operations and is a relatively straightforward quantitative tool for assessing phase compositions. Point-counting for quantitative phase abundance is a mature method, yet is one of the few direct methods to determine clinker phase compositions. The application of the scanning electron microscope with X-ray microanalysis complements light microscopy by not only providing analyses of clinker, but also of the more difficult fine-grained powders of Portland cement and pozzolans like fly ash, and slag. These images may also be point-counted, but image processing and analysis provides a means for full-field quantitative measurements on area fraction and surface perimeter fraction, and spatial distribution for the constituent phases. These data, coupled with X-ray computed tomography, is providing the means to generate three-dimensional particles that capture the characteristics of phase abundance and texture and are invaluable in the development of three-dimensional computer simulation models of cement hydration.

142

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••

Figure 33. Fly ash BE and XR image set illustrate the chemical and compositional complexity of fly ash.

Microscopy of Clinker and Hydraulic Cements

143

Figure 34. Slag BE and XR images show the uniform, angular nature. Subtle differences in particle chemistry allow division of this slag into two distinct subgroups.

REFERENCES ASTM C150 / C150M-11 (2011) Standard Specification for Portland Cement. American Society for Testing and Materials West Conshocken, PA, Annual Book of ASTM Standards, (4.01): Cement ASTM C595 / C595M-11 (2011) Standard Specification for Blended Hydraulic Cements. American Society for Testing and Materials West Conshocken, PA, Annual Book of ASTM Standards, (4.01): Cement ASTM CI 157 / C1157M-11 (2011) Standard Performance Specification for Hydraulic Cement. American Society for Testing and Materials West Conshocken, PA, Annual Book of ASTM Standards, (4.01): Cement

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ASTM C1356 (2011) Standard Test Method for Quantitative Determination of Phases in Portland Cernent Clinker by Microscopical Point-Count Procedure American Society for Testing and Materials West Conshocken, PA, Annual Book of ASTM Standards, (4.01): Cement Bates PH, Klein AA (1917) Properties of the calcium silicates and calcium aluminates occurring in normal Portland cement. Technological Papers of the Bureau of Standards 78:1-38 Bentz DP, Stutzman PE (1994) SEM analysis and computer modeling of hydration of Portland cement particles. In: Petrography of Cementitious Materials. American Society for Testing and Materials, ASTM S TP 1215:60-73 Bentz, DP, Stutzman PE, Haecker CJ, Remond S (1999) SEM/X-ray imaging of cement-based materials. Proceedings of the 7 th Euroseminar on Microscopy Applied to Building Materials (EMABM), 457-466 Bhatty JI, Miller FM, Kosmatka S (eds) (2004) Innovations in Portland Cement Manufacturing. Portland Cement Association, Skokie/IL Bish DL, Plötze M (2010) X-ray powder diffraction with emphasis on qualitiative and quantitative analysis in industrial mineralogy. Advances in the characterization of industrial minerals. EMU Notes Mineral 9:35-76 Bish DL, Reynolds RC Jr (1989) Sample preparation for X-ray diffraction. Rev Mineral 20:73-99 Bogue RH (1955) The Chemistry of Portland Cement. 2nd ed, Reinhold, New York Bogue RH (1961) Origin of the special chemical symbols used by cement chemists. J PCA Res Develop Lab 3:20-21 Brown LS (1948) Microscopical study of clinkers in long-time study of cement performance in concrete. PCA Bull 26:877-933 Bullard JW, Lothenbach B, Stutzman PE, Snyder KA (2011) Coupling thermodynamics and digital image models to simulate hydration and microstructure development of Portland cement pastes. J Mater Res 26:609-626 Campbell DH (1999) Microscopical Examination and Interpretation of Portland Cement and Clinker. 2ni Edition. Portland Cement Association, Skokie/IL Campbell DH, Galehouse JS (1991) Quantitative clinker microscopy with the light microscope. Cem Concr Aggr 13(2):94-96 Chancey RT, Stutzman P, Juenger MCG, Fowler DW (2010) Comprehensive phase characterization of a class F fly ash. Cem Concr Res 40:146-156 Chayes F (1956) Pétrographie Modal Analysis. An Elementary Statistical Appraisal. Wiley, New York EN197-1 (2011) Cement - part 1: Composition, specifications and conformity criteria for common cements. English version. CEN, Brussels: pp 29 Fundal E (1980) Microscopy of cement raw mix and clinker. F. L. S. Review 25:1-15 Goldstein J, Newbury DE, Joy DC, Lyman CE, Echlin P, Lifshin E, Sawyer L, Michael JR (2003) Scanning Electron Microscopy and X-Ray Microanalysis. A Text for Biologists, Materials Scientists, and Geologists. 3rd edition. Springer Verlag, Berlin Hewlett PC (ed) (1977) Lea's Chemistry of Cement and Concrete. 4th Edition. John Wiley, New York Hofmänner F (1975) Microstructure of Portland Cement Clinker. Holderbank Management and Consulting, Ltd., Holderbank/CH Howarth RJ (1998) Improved estimators of uncertainty in proportions, point-counting and pass-fail test results. Am J Sei 298:594-607 Insley H, Frechette V (1955) Microscopy of Ceramics and Cements. Chapter 5, Special techniques. Academic Press, New York, p 177-207 Joint Committee for Guides in Metrology (2008) Evaluation of measurement data. Guide to the expression of uncertainty in measurement (GUM). 1st edition, corrected version 2010. BIPM, Sèvres, France. JCGM 100:1-134 Kanare HM (1987) Production of Portland cement clinker phase abundance standard reference materials. Final Report, CTL Project No. CRA012-840, Construction Technology Laboratories, Skokie/IL Kosmatka SH, Kerkhoff B, Panarese WC (2002) Design and Control of Concrete Mixtures, PCA Engineering Bulletin 001. Portland Cement Association, Skokie/IL Landgrebe D (2003) Signal Theory Methods in Multispectral Remote Sensing. Wiley Interscience, New York Landgrebe D, Biehl L (2011) An Introduction to Multispec©. http://dynamo.ecn.purdue.edu/~biehl/MultiSpec, retrieved 04 January 2012 Le Chatelier H (1905) Experimental Researches on the Constitution of Hydraulic Mortars. English translation by JL Mack, McGraw, New York Levenson MS, Banks DL, Eberhardt KR, Gill LM, Guthrie WF, Liu HK, Vangel MG, Yen JH, Zhang NF (2000) An approach to combining results from multiple methods motivated by ISO GUM. J Res NIST 105(4):571-579 Lydon JW (2005) The measurement of the modal mineralogy of rocks from SEM imagery: the use of Multispec© and ImageJ freeware. Geol Survey Canada Open File 4941:1-37

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McCusker LB, Von Dreele RB, Cox DE, Louer D, Scardi P (1999) Rietveld refinement guidelines. J Appl Crystallogr 32:36-50 Miller (1981) Microscopy as an aid in evaluation of mix burn ability and clinker formation. Proc. 3rd Int Conf Cem Micro, Duncanville, TX, 181-192 Neilson MJ, Brockman GF (1977) The error associated with point-counting. Am Mineral 62:1238-1244 Rankin GA (1915) The constituents of Portland cement clinker. J Ind Eng Chem 7:466-474 Rankin GA, Wright EE (1915) The ternary system Ca0-Al 2 0 3 -Si0 2 ; with optical study. Am J Sci 39:1-79 Scrivener KL (1987) The microstructure of anhydrous cement and its effect on hydration. Mater Res Soc Symp Proc 85:39-46 Snellings R, Mertens G, Elsen J (2012) Supplementary cementitious materials. Rev Mineral Geochem 74:211278 Stutzman PE (1994) Scanning electron microscopy imaging of hydraulic cement microstructure. Cem Concr Comp 26/8:957-966 Stutzman PE (2007) Multi-spectral SEM imaging of cementitious materials. Proc. 29th Int Conf Cement Micr, Québec City, Canada Stutzman PE, Bentz DP (1993) Imaging of cement and image-based simulation of hardened cement microstructure. Proc 15th Int Conf Cem Micr, Duncanville Texas, 312-323 Stutzman PE, Leigh S (2002) Phase composition analysis of the NIST reference clinkers by optical microscopy and X-ray powder diffraction. NIST Technical Note 1441:1-44 Stutzman PE, Lespinasse G, Leigh S (2008) Compositional analysis and certification of NIST reference material 2686a. NIST Tech Note 1602:1-49 Taylor HFW (1997) Cement Chemistry. 2ni Edition. Thomas Telford, New York Van Niekerk D (2003) Modal analysis and phase identification in meteorite thin sections using freeware for PC. Annual Lunar and Planetary Sci Conf, League City, TX http://www.lpi.usra.edu/meetingsApsc2003/ pdfZ2015.pdf Young RA (ed) (1995) The Rietveld Method. Int Union of Cryst Monographs on Crystallography 5:1-308

4

Reviews in Mineralogy & Geochemistry Vol. 74 pp. 147-165,2012 Copyright © Mineralogical Society of America

Industrial X-ray Diffraction Analysis of Building Materials Roger Meier PANalytical Lelyweg 1, PO Box 13 7600 AA Almelo, The Netherlands e-mail:

[email protected]

Jennifer Anderson PANalytical 117 Flanders Road Westborough, Massachusetts 01581, U.S.A.

Sabine Verryn PANalytical (Pty) Ltd 363 Oak Avenue Ferndale 2194, South Africa

XRD Analytical and Consulting cc 75 Kafue Street Lynnwood Glen 0081, South Africa

e-mail: sabine. verryn@xrd. co.za

ABSTRACT X-ray analysis of polycrystalline powder samples has grown beyond its roots in the world of laboratory research and is regarded as one of the most powerful industrial process-control tools in the field of building materials and minerals. This is the key to characterize the element and the phase composition of the material. This is largely due to the development of industrial X-ray analytical systems, which have transformed these advanced analytical techniques born in the laboratory into a robust, workmanlike and easy-to-use tool for today's heavy industries. X-ray diffraction (phase analysis) opens enormous possibilities for process and quality control. Moreover, the recent development of ultra-high-speed X-ray detectors allows for "on the fly" quantitative X-ray diffraction analysis and truly interactive process control. Hydration of cements can be studied relative ease. Additionally Computed X-ray Tomography (CT) can yield valuable information in the study of mortars and concrete.

INTRODUCTION Heavy duty environments like those found in the building materials and minerals industry are notoriously difficult for sensitive test and process-control equipment. The need to often work in dusty conditions, high humidity and extreme temperatures places special demands on such equipment, which must combine ruggedness, ease of use and reliability with the precision and sensitivity of a laboratory instrument. This transition from laboratory instrument to industrial system is not always simple but is nevertheless essential if the industry is to benefit from the latest analytical tools. One of the most recent analytical tools to make this transition is X-ray diffraction (XRD). Although the power of XRD as the only analytical technique capable of dis1529-6466/12/0074-0004505.00

DOI: 10.2138/rmg.2012.74.4

Meier, Anderson,

148

Verryn

tinguishing crystalline phases is well recognized, it has long been considered a tool best suited to the laboratory, because of its low measurement speed and need for specialist knowledge. All this had changed, however, with the introduction of the latest generation of industrial X-ray equipment. These instruments are not only able to withstand the rigors imposed by an industrial regime; they are also designed to be exceptionally easy to use even by non-specialist operators. Moreover, even speed is no longer an issue with the introduction of ultra-fast linear and two dimensional XRD detectors. Figure 1 shows the differences between a point detector and linear detector setup. The linear detector can be regarded as a multiple detector array that collect the data in parallel and gain therefore in the overall measurement speed. The introduction of this silicon-based technology gives X-ray detectors up to 150-fold increase in data acquisition speed. This means that a scan formerly requiring three hours of data collection time is now recorded in less than two minutes with, moreover, no compromise on resolution. Various sample chambers can be used to study hydration in different environments and under different conditions. Modern X-ray diffraction equipment allows for fast exchange of sample stages as well as optics. This paper describes the industrial applications, rather than the detailed description of methods such as Rietveld analysis, which is covered in great detail by Aranda et al. (2012, this volume). Different systems and software packages are available and in use. A volume of Reviews in Mineralogy, dedicated to Modern Powder Diffraction (Bish and Post 1989), comprised a series of key articles on the basics of powder diffraction, sample preparation and synchrotron and neutron powder diffraction. In that volume, quantitative phase analysis was discussed in detail by Snyder and Bish (1989). There the Reference Intensity Ratio

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approach (Davis 1990) (also known as Chung method - see Chung 1974a,b, 1975), the method of standard additions (also known as spiking method) and the full pattern-fitting approach using both the Rietveld method (Rietveld 1969; Young 1993) and the observed patterns method (Smith et al. 1987) are presented and are therefore not discussed much further here. Although XRF analysis is a valuable tool and often quoted here, in this paper the authors want to show that quantitative XRD and statistical evaluation can be taken from the laboratory environment into the industrial environment for plant control. Often XRF results are used to calculate the Bogue values on the basis of equilibrium conditions. In the past the results obtained by the Bogue method were mostly close to the real phase composition, because the process followed conditions close to the modeled equilibrium by Bogue. Nowadays, due to the use of alternative fuels, which introduce many "new" elements into the process and can change the burning conditions the trend towards a higher material throughput, where the materials have not enough time to reach the equilibrium condition, the results obtained by Bogue may significantly different compared with the "true" results by microscopy and X-ray diffraction. METHODOLOGY Phase identification with XRD In the production of minerals and building materials, accurate information about the phase composition is essential for determining the quality of the semi-finished and finished products and allowing manufacturers to optimize the efficiency of their process. Although many nondestructive analytical techniques exist to measure the concentrations of elements within a sample, in almost all cases the properties of the material are not determined by the relative amounts of elements alone, but more importantly by the crystalline phases present. Qualitative phase analysis is one goal of an X-ray diffraction experiment. Establishing which phases are present in a sample is usually the first step of a whole series of analyses and form the basis of investigations on how much of each phase is present (quantitative phase analysis). All crystalline materials have their own unique, characteristic X-ray finger print (or stick pattern), based on their crystal structure. When diffraction data for a particular sample is compared against a database of known materials, the crystalline phases within the sample can be identified (Fig. 2). This classic X-ray diffraction analysis approach takes into account the peak positions and net profile data of every peak in a single diffraction pattern. The identification of mineral phases in a sample is a powerful tool for characterization of materials. Modern XRD software provides search-match routines to aid in phase identification. Phase quantification by using X-ray diffraction data Possible phase composition of Portland cement in cement plants is traditionally estimated using optical microscopy by either point counting or image analysis, which is highly operator dependent. The preparation and analysis time limits its usefulness for process control. Alternatively, it is derived from chemical analysis, usually by X-Ray Fluorescence employing the Bogue method (Bogue 1929; Taylor 1989). However, these methods have sources of error, such as sampling difficulties for microscopic investigations and the use of inappropriate phase compositions using the Bogue method (Aranda et al. 2012, this volume). Quantitative XRD gives a direct measurement of the phase content of building materials, and, as shown in the introduction, fast XRD systems for quantitative XRD analysis of building materials are becoming more widespread. XRD can even be used for plant and kiln control. The classical calibration methods. Quantitative phase analysis using diffraction data can be done with a number of methods. These methods are described in various classical works

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Verryn Characteristic Qifractotjairo

Quartz (SiO,) +

X

1/

Salt (NaCI) Superposition of the peaks

Si0 2 + NaCI

\ 1/

Figure 2. Principle of the phase identification of multiphase diffraction diagrams.

(Nuffield 1966; Klug and Alexander 1974; Cullity 1978; Snyder and Bish 1989; Zevin and Kimmel 1995; Jenkins and Snyder 1996). The calibration method assumes that the phase composition of the sample is known. The intensity information of one peak (or a group of peaks), belonging to the phase to be analyzed, is used to quantify the abundance of that phase in the sample. The measured peak area or peak intensity is compared against a calibration curve built from samples with known concentrations of the phase (or phases) in question. There are several drawbacks to these methods for the building materials industry. Standards used for this method are often expensive, unstable or otherwise difficult to obtain. Additionally, the method cannot be extended to include all phases relevant to the cement industry. There are extensive overlapping peaks in the diffraction patterns of cementitious materials and several phases exhibit preferred orientation making the calibration of many phases impossible. Only very few phases can be calibrated for, these include free lime in clinker and in some cases calcite in cement. Aldridge (1982) also states that these XRD methods were generally unsatisfactory. Rietveld analysis. Nowadays, quantitative XRD analysis from powder diffraction data is mainly based on the Rietveld method (Rietveld 1969; Hill and Howard 1987; Bish and Howard 1988; Bish and Post 1993; Madsen and Scarlett 2000, 2008). The Rietveld method was first described in 1966 by Hugo M. Rietveld (Rietveld 1966, 1969) to refine crystal structures from powder data measured on neutron diffractometers. The method was first reported at the seventh Congress of the IUCr in Moscow in 1966 (Rietveld 1966). Aranda et al. (2012, this volume) address in detail the description of the Rietveld method, therefore here only a brief summary, as this paper deals with the actual application in industry. The Rietveld method is based on the idea that the overall scale of a phase is proportional to its abundance in a phase mixture. The intensity relationship between peaks in a diffraction pattern is governed by the crystal structure of each of the phase present in the mixture. A calculated model of the diffraction data is optimized to match that of the observed data and to determine the quantities of the phases present. Sophisticated Rietveld analysis software packages perform a comparison of measured and calculated profiles and minimize the differences between the profiles by a least-squares fit of many parameters. This is a profile fitting method that takes into account the whole diffraction pattern. Figure 3 shows a graphical representation of a Rietveld refinement of an industrial Portland cement clinker.

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Counts 10000 - C:\Program Files\PANalyt¡calWPeit HighScore P • : C 3 S - A l i t e , Nishi et al 7 3 . 2 % I É C 2 S - beta - Belite ( M u m m e ) 10 5 % Calcium oxide - Lime 0.8 % Brownmillerite (2/1.52/0.48/5)8.5 % C 3 A - A l u m í n a t e cubic 5.7 % |

C 3 A - N a - A l u m í n a t e ortho, N I S T 0 . 3 % M a g n é s i u m oxide - Periclase 0.0 %

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Figure 3. Graphical output of a Rietveld refinement of a Portland cement clinker: lower graphics difference between calculated and observed pattern.

Rietveld analysis can be performed automatically with robotic software modules. Specifically designed for unattended, industrial Rietveld analysis, this application also contains the option to create tailor-made reporting, putting Rietveld analysis into the reach of the nonspecialist while providing extended possibilities for more advanced investigations. The traditional Rietveld method uses a refinement strategy, in which the full diffraction profile of high quality is fitted according to a model with known crystal structures. Within the constraints of the application, the traditional Rietveld method is the only method for analysis of complex mixtures of several crystalline phases that show peak overlaps. That yields precise, relatively correct results—enough to verify any changes and to maintain the status quo. It is recommended to compare the precise Rietveld quantitative data to other results obtained by established techniques like optical microscopy. This referenced Rietveld method enables an absolutely correct XRD quantification, accurate enough to push the limits of process control. For example a high quality Portland cement clinker allows you to use more additions in place of valuable clinker. It also allows for a tighter control of the different sulfate phases in OPC (ordinary Portland cement), resulting in more constant hydraulic properties, which facilitates the use of the final product in building projects. For more details on using the Rietveld analysis of cements and the optimization of the method for cement analysis the reader is referred to Aranda et al. (2012, this volume). Apart from the described methodologies in analysis of Building materials, it must be mentioned that it is also possible to quantify amorphous components present in building materials. This is often achieved by the addition of a standard as described by Walenta and Fiillmann (2004) in their papers relating to slag and fly ash, which are used as alternative raw materials for cement and concrete production, as well as hydrated cement analysis.

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Full pattern cluster analysis Modern X-ray diffraction equipment as described above allows the rapid collection of hundreds of scans in a short time. This can be useful in application areas such as process and quality control in the building materials industries. For such large amounts of data, it is very time consuming and often not practical to analyze every individual measurement. This implies that a data reduction tool is required. Cluster analysis starts with a set of scans that are compared and sorted into classes based on their similarity. The similarity comparison is based on the peak and intensity information, but in this case the statistical contribution counts as well. The theory of this method is presented in various literature citations (Lance and Williams 1966; Mardia et al. 1979; Rousseew 1987; Kelley et al. 1996; Lohninger 1999). In summary, cluster analysis is a method that uses statistical methods to simplify the analysis of large amounts of data by: •

Automatically sorting all scans of one or more experiments into classes of closely related scans

•

Identifying the most representative scan of each class

•

Identifying the two most different scans of each class

•

Identifying outliers not fitting into any class (non-members).

This drastically reduces the amount of data that has to be processed, because only representative scans, outliers and sometimes the most different scans are analyzed in more detail. Further cluster analysis can be used to discover hidden features/structures in the data as it is sensitive to small, otherwise unnoticeable, changes in the obtained information. Comparisons of the full peak and profile of every powder diffraction pattern in a set of n patterns with every other pattern can be presented as a correlation matrix. The correlation matrix is used as input to a hierarchical agglomerative cluster analysis, which puts the patterns into classes defined by their similarity. This method starts with each data-set representing a distinct cluster. At each step of the analysis, two clusters with the highest degree of similarity are merged into a single cluster. The process stops with the final step, when only one cluster containing all data-sets remains. The result of this analysis step is usually displayed as a dendrogram (Fig. 4). A well-known and in principle unsolved problem is to find the "right" number of clusters (Kelley et al. 1996). This means cutting the dendrogram at a given dissimilarity and retaining a meaningful set of clusters, where the scans inside a cluster are closely related while the different clusters are different enough to keep them apart. Principal Components Analysis (PCA) can be carried out as an independent method to visualize the quality of the clustering. The correlation matrix is used as input. The method can handle enormous amounts of data and additionally can also deliver process relevant information in the form of a "yes" or "no" result. The cluster approach uses all relevant information in a measurement and included in the analysis is subtle information which is covered in the noise. The method is not based on crystallographic principals and therefore the analysis is not directly related to the Bragg peak information. It is a non-biased method and no mistake can be made based on wrong assumptions. While the cluster analysis can be performed automatically by modern software, some parameters can be changed by the user to optimize the result. Computed tomography X-ray CT (computed tomography) is a non-destructive technique for visualizing features in the interior of solid objects, and for obtaining digital information on their 3D geometries and properties. Although this is a technique more suitable for a central research laboratory, it gives valuable information and is therefore included here (Desrues et al. 2006).

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Figure 4. The dendrogram is a graphical display of the result of an agglomerative hierarchical cluster analysis (actual cut-off indicated by a stippled line).

The fundamental principle behind computed tomography is to acquire multiple views of an object over a range of angular orientations. In this way, additional dimensional data are obtained in comparison to conventional X-radiography, in which there is only one view. In our experiments we use the so-called volume CT method, where a cone beam or highly-collimated, thick, parallel beam is used in combination with a 2D (area) detector. The radiation transmitted through the object at each angle is measured and the detector data is stored as 2D X-ray images. The series of 2D X-ray projections, used to generate 3D images, is a collection of images acquired while progressively rotating the sample step-by-step through a full 360-degree rotation within the field of view at increments of less than 1 degree per step. These projections, effectively X-ray attenuation data, represent a measure of the reduction in X-ray intensity that result from absorption and scattering by the sample and contain information on the position and density of absorbing object features within the sample. The accumulated 2D projections data is then used for the numerical reconstruction of the volumetric data (volume rendering). This volume data is compiled as a visualization of the reconstructed layers in a 3D view by CT reconstruction software, which provides these 3D volume results using a Filtered Back-Projection algorithm, the co-called "Feldkamp" algorithm (Feldkamp et al. 1989). The 3D CT data are rendered as voxels (volume element) with threedimensional resolution depending on the X-ray detector pixel size. In general, any sample that fits entirely within the field of view and is completely penetrated through all directions perpendicular to the rotational axis can be imaged in this way. The energy spectrum of the X-ray source defines the penetrative ability of the X-rays, as well as their expected relative attenuation as they pass through materials of different density. Higher-energy X-rays penetrate more effectively than lower-energy ones, but are less sensitive to changes in material density and composition. The X-ray intensity directly affects the signalto-noise ratio and thus image clarity. More detailed description can be found in Bowen and Tanner (1998) and Stock (2008).

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The usefulness of this method in building materials investigations has been described in various papers (Masad et al. 2002; Gopalakrishnan et al. 2007; Schmidt et al. 2010). APPLICATIONS The use of X-ray techniques to characterize the materials involved in the production process of cementitious materials results in most of the cases in benefits for the process and/or the product. The X-ray diffraction methodologies shown here range from phase identification to full pattern phase quantification or even statistical interpretation of the collected data and can be applied during the different process steps. The degree of utilization of the obtained results varies in terms of the process details, the available equipment and the process management. The examples mentioned below should provide a rough idea of the working principle and corresponding achievable benefits. The sequence of the applications is presented to follow the standard workflow of the cement manufacturing process (Fig. 5).

Quarry

Raw Meal Preparation Kiln

Dispatch

Figure 5. The three main steps during the cement manufacturing process are the preparation of the raw mixture, the production of clinker (thermal process) and the grinding/blending of the cement.

Raw materials/quarry The overall goal of the raw mixture preparation is to achieve a homogeneous mixture of limestone, clay and other needed compounds, such as bauxite, iron ore and sand, to produce a proper kiln feed with the correct chemical composition. The corresponding main application is the determination of the chemical composition, where X-ray fluorescence in the laboratory and neutron activation based methods on the belt are mandatory on a routine bases to run the process. In order to make cement it is essential to know the raw materials. An oxide analysis of the raw materials is the first step, which provides input for a wide variety of ratios and moduli that relate oxide compositions to one another. These include: LSF (lime saturation factor), SM (silica modulus or ratio), AR (alumina-to-iron ratio), and other lesser-used formulas like the hydraulic modulus. The silica ratio represents the burnability of a raw mix. The burnability impacts how much energy is put into the system. As the ratio of silica to alumina plus iron increas-

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es, it becomes harder to "burn" and more difficult to combine the raw materials into the phases needed. As the ratio decreases, the tendency for fluxing (the ability of the solid materials to become liquid) increases, and the combining reactions become easier. Another consideration is that silica present as quartz is generally more difficult to combine than silica present as silicates. The alumina-to-iron ratio is important because it controls the potential C 3 A/C 4 AF ratio in the finished cement, which is important because of sulfate resistance, heat generation, and admixture compatibility issues. The lime saturation factor controls the potential C 3 S to C 2 S ratio in the finished cement. C 3 S governs the early age strength development while C 2 S hardens slowly and contributes largely to strength increases at ages beyond 7 days (Moore 1982; Taylor 1990). In addition, X-ray diffraction can contribute in many cases to improve the overall performance. Especially important is the detection of crystalline phases which contribute in a negative way to the process. The detection of coarse quartz in the limestone or high amounts of chlorine-containing clay varieties can be applied during the exploration phase or at different steps of the raw mixture preparation. The location of the sampling as well as the timing has to be adapted to the process to maximize the resulting benefits. Quartz in the raw mix can influence the clinkerization process in a negative way. Coarse quartz grains need more energy (time and temperature) to fully react with the other involved minerals of the raw mix to form C3S and C2S. Furthermore, it is most likely that coarse quartz pieces will increase the wear of the raw mill and therefore reduce the mill life. The presence of quartz can be monitored by automatic Rietveld analysis (Fig. 6). Certain elements, like chlorine, will cause the formation and precipitation of 'unwanted' minerals during the calcinations and/or sintering process. These "sticky" minerals, for example paraspurrite, can cause clogging in the pre-heater/calciner or even in the cement kiln. A continuous monitoring of the incoming materials or the corresponding XRF analysis during the exploration phase can prevent the clogging events by a proper raw material selection and/ or additional blending steps. Using XRD (and XRF) quality control of gypsum is important as natural or artificial gypsum are often mixtures of gypsum, hemihydrate and anhydrite and other phases, which do have different reaction behavior, but can be easily distinguished by XRD. Purity of limestone

Position

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Figure 6. Rietveld refinement of a raw mix.

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and other raw materials such as blast furnace slag and fly ash can be assessed. Early detection of detrimental phases, such as pyrite, is possible. Preheater/calciner The introduction of pre-heaters around 1930 and pre-calciners in 1970 (Peray 1998) increased the efficiency of the clinker sintering significantly. However, next to the intended advantages, any additional step in the production process carries the risk of possible problems. Hence, one of the most critical process steps during Portland Cement Clinker production is the operation of the preheater system. The major risk in case of the preheater/calciner is the blockage by clogging as described before, which increases the risk of blockages and downtimes. The origin of this potential problem is the formation of detrimental phases, which influence the flow of the hotmeal and lead to the build-up of cloggings. A complete phase analysis of the material before and after the preheater/calciner can provide data to recognize the imminence of problems. The detrimental phase formations can be monitored directly by X-ray diffraction using Rietveld phase quantification in real time. This not only helps to completely understand the process, but also allows proactive strategies, when clogging precursor phases occur, for reducing and solving blockage problems. The monitoring of the material flow in the preheater/calciner is also gaining more and more importance due to the fact that alternative fuels bring critical elements, such as chlorine, into the process The range of fuels is extremely wide. Traditional kiln fuels are gas, oil or coal. Materials like waste oils, plastics, auto shredded residues, waste tyres and sewage sludge are often proposed as alternative fuels for the cement industry. Also all kinds of slaughterhouse residues are offered as fuel nowadays (Káántee et al. 2000). In many cases, spot checks of the fuels alone are not sufficient, due to the inconsistency of the alternative fuel composition, which shows huge variations and low sampling frequency is by far not sufficient to handle the possible variations. The regular analysis using automated Rietveld procedures of the involved material streams delivers the key information for a smooth process. The tracing of the efficiency of the heating/calcinations process presents also valuable information for process improvement. The commonly used parameter is the degree of calcinations, which can be directly determined by measuring the ratio of free lime vs. carbonate+free lime (Fig. 7). Clinker/kiln The chemical reactions that occur in the kiln are described in detail by Hewlett (1998). The temperature is increased when going from the meal feed to the rotary kiln. The most important oxides that participate in the reactions are CaC0 3 , Si0 2 , A1 2 0 3 and Fe 2 0 3 . Up to about 700 °C water is removed from the meal. In the preheating section (700-900 °C ) calcination as well as an initial combination of alumina, ferric oxide and silica with lime takes place. Between 900 °C and 1200 °C belite, C2S (= 2Ca0-Si0 2 ), forms. Above 1250 °C a liquid phase appears and this promotes the reaction between belite and free lime to form alite, C3S (= 3Ca0-Si0 2 ). During the cooling stage the molten phase forms C3A, tri calcium aluminate, (= 3Ca0-Al 2 0 3 ) and if the cooling is slow alite may dissolve back into the liquid phase and appear as secondary belite. Calculations of clinkering reactions are described by Barry and Glasser (2000). Usually the production of clinker is done so that one type of clinker allows the plant to manufacture several well-defined types of cement that comply with the physical demands as specified by cement standards. Figure 8 shows the schematic view of clinker formation reactions. Already for a long time the clinker material is tested on its free lime content. The usual target values for the free lime concentration are around 1%. Significant lower values for the free lime concentration indicate a too high temperature during the sintering process. On the one hand this causes wastage of fuel and on the other hand it can influence the clinker properties, especially the grindability, in a negative way. Higher contents of free lime appear if the temperature during the sinter process has been too low and/or the raw meal has been too coarse

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and/or the reaction time has been too short. Free lime concentrations above 2% can generate problems in durability of the concrete. The free lime will react with water and form the phase portlandite. The portlandite phase requires more space than the free lime and causes, if present in higher amounts, cracks in the concrete. High amounts of sodium and potassium lead to the formation of orthorhombic instead of cubic C 3 A and both modification show differences in reactivity and hydration. The ratio C 3 A-

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cubic/C 3 A-orthorhombic influences the water consumption of the cement, as there is higher water consumption by orthorhombic C 3 A. In the case that Na and K are fixed in alkali-sulfates, these elements are not available for the incorporation in C 3 A and by changing the degree of sulfatization the ratio of C 3 A-cubic/C 3 A-orthorhombic can be controlled. The increasing use of alternative fuels introduces more variations in the clinker composition and in the burning conditions. The subsequent potential implications on the product properties require a full quantitative phase analysis of the clinker to be able to produce a constant product with the desired properties. Table 1 gives an overview of the main phases—property relations in clinker and cement. A more complete list of phases can be obtained from Aranda et al. (2012, this volume). Bahtty (1995) describes the role of minor elements in cement manufacture in more detail. Pollmann (2002) describes the composition of cement phases as well as influences of different phases and elements. Another related opportunity to improve the consistency of the product and to influence the cement manufacturing process in apositive way is the monitoring of the bypass dust composition. The composition of bypass dust is based on the elements, which cannot be incorporated in the clinker minerals. The elemental as well as the phase composition of the bypass dust provides important information of the process. The statistical data analysis by using cluster analysis, as described above, can be applied as well to monitor effects of the raw feed changes and fuel composition on the process. The subsequent interaction on the process is based on the obtained cross correlations of historical data. The described cluster analysis can be also applied for the clinker to get a visually easy readable presentation of the data. Often the principal component analysis plot is used to display the results of the cluster analysis. Differences in the phase composition and therefore also in the material properties are seen as distances of the displayed positions (Fig. 9). Cement The full crystalline phase quantification of the cement can contribute to the prediction of the product properties. The usual strength tests take a relatively long time before the results become available. By that time, the corresponding material is often already used and is it very difficult and expensive to correct possible discrepancies afterwards. The results of the X-ray analysis are available much faster, allowing corrective actions to be taken much earlier in the process.

Table 1. Phase property relations in clinker and cement.

Cement Phase

Trivial Name

Relevance

CaO Ca(OH)2 C3S C2S

Free Lime Calcium hydroxide/ Portlandite Alite Belite

Kiln Temperature Control Kiln Temperature - Cooling Conditions early strength, hardness later strength, hardness

C3A C4AF MgO CaS0 4 -2H 2 0

Alumínate Ferrite Periclase Gypsum

Setting Time Color expansion hydration control

CaS0 4 -0.5H 2 0 CaS0 4 CaC0 3

Hemihydrate Anhydrite Calcite

hydration control hydration control

Si0 2

Quartz Glass

insufficient grinding temperature

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ASTM C 150, Standard Specification for Portland Cement, recognize eight basic types of Portland cement concrete (see Table 2). There are also many other types of blended and proprietary cements that are not mentioned here. The permissible additions of slag, fly ash and other extenders are shown in Bye (1999). Examples of Rietveld quantifications of different cement types are shown in Figures 10-12. The quantitative phase analysis also provides information about various process steps. For example are there phase changes between the different sulfate phases (gypsum, hemihydrates and anhydrite) which would be an indicator for the milling conditions and can be used in many cases to increase the mill efficiency. At the same time, the quantitative ratio of the sulfate phases together with the abundance of the calciumaluminate polymorphs (cubic and orthorhombic C 3 A) is important for the workability/setting time of the concrete. Another important benefit of the X-ray analysis is the quantitative compound determination of cement additions (Bye 1999). Nowadays, the Rietveld quantification includes not only the crystalline phases, but the Rietveld analysis can also provide information about non-crystalline compounds of the cement, like blast furnace slags, fly ashes and other pozzolanic materials (Walenta and Ftillmann 2004; Westphal et al. 2009, 2010). See also Figures 11 and 12. The statistical cluster data analysis gives a user-friendly presentation of process trends and Figure 13 shows the principle component analysis plot of some different cement types as listed in Table 2 and again the material properties are seen as distances of the displayed position. Hydrated cement X-ray diffraction analysis has also been widely used to determine the hydration of hardened cement paste (Scrivener et al. 2004; Hoshino et al. 2006; Hesse et al. 2009). In earlier days, the results were mainly qualitative, but using modem equipment together with Rietveld analysis

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Table 2. A S T M types of P o r t l a n d c e m e n t .

Type

Name

I

Purpose

Normal

General-purpose cement suitable for most purposes.

IA

Normal - Air-Entraining

An air-entraining modification of Type I.

II

Moderate Sulfate Resistance

Used as a precaution against moderate sulfate attack. It will usually generate less heat at a slower rate than Type I cement.

IIA

Moderate Sulfate Resistance Air-Entraining

An air-entraining modification of Type II.

III

High Early Strength

Used when high early strength is needed. It is has more C 3 S than Type I cement and has been ground finer to provide a higher surface-to-volume ratio, both of which speed hydration. Strength gain is double that of Type I cement in the first 24 hours.

High Early Strength Air-Entraining

An air-entraining modification of Type III.

IV

Low Heat of Hydration

Used when hydration heat must be minimized in large volume applications such as gravity dams. Contains about half the C 3 S and C,A and double the C 2 S of Type I cement.

V

High Sulfate Resistance

Used as a precaution against severe sulfate action principally where soils or groundwaters have high sulfate content. It gains strength at a slower rate than Type I cement. High sulfate resistance is attributable to low C 3 A content.

IIIA

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an accurate analysis of hydration products can be achieved. The methods are, again, described in more detail by Aranda et al. (2012, this volume). With the reduced measurement times as described above, the quantification can be done "online" and the hydration process does not have to be stopped as the sample can be analyzed repeatedly at short intervals. Cluster analysis as described above can then be employed to evaluate the large data sets produced (Fig. 14). After identification of phases present, subsequent data sets can be evaluated using automated Rietveld analysis and scans can be visualized using 3D functions supplied by most modern XRD software (Fig. 15).

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Principi! Campa nent 1 Figure 13. Principal Component Analysis Plot of the Cluster analysis of different cement type samples black: CEM I 32.5, dark grey: CEM I 42.5, light grey: other).

I

Figure 14. Principal Component Analysis Plot of the Cluster analysis of a cement hydration study.

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< M t i o o o o t» t» t» t» ¿•1 uci1 u u u ^eî 70 wt% remains one of the fulfillment criteria for pozzolans in ASTM C618. In case of a one phase material the chemical composition can be considered as a meaningful parameter. However most natural and artificial SCMs consist of a heterogeneous mixture of phases and then a direct relationship between overall chemical composition and pozzolanic activity becomes less obvious. The fact that all correlations were reported for long term pozzolanic activity and/or performance indicates that the characteristics and the distribution of the reaction products should relate SCM chemistry and long term performance. The eventual, long term reaction product assemblage is controlled by the overall chemistry of the active phases (Massazza 2001). Addition of blast furnace slag or metakaolin has been observed to change the Ca/Si ratio, the silicate polymerization and the morphology of the main C-S-H reaction products and can thus alter the permeability of the reaction product barrier layer and the eventual performance of the binder (Richardson 1999, 2004). As the main reaction products are calcium-silicate-hydrates (with some Al incorporation) and calcium-aluminate-hydrates (containing additional Si and Fe), the total Si0 2 + A1203 + Fe 2 0 3 content of the active phases may be considered as an indication of the Ca(OH) 2 binding potential of an SCM. In practice, it is very difficult to separate the contributions to the SCM activity of physical particle characteristics and mineralogical properties. This is considered one of the primary reasons of contradictory findings in literature concerning the relative activities of SCM phases. Furthermore, because SCMs of comparable origin often show broad similarities in physical particle properties if not in mineralogical and chemical composition, the widespread adoption of the genetic classification scheme of SCMs can be considered to remain sensible. External factors. The rate of the pozzolanic reaction also depends on the mix design, larger water/binder ratios will result in increased pozzolanic activity but will inevitably decrease the performance of the binder due to the increased overall porosity. The ratio of SCM over Ca(OH)2, or equivalently the ratio of SCM over Portland cement obviously affects the pozzolanic activity in increasing or decreasing the frequency of fulfillment of the reaction configuration. The Ca(OH)2:pozzolan ratio for optimal performance and activity is depending on the overall content, composition and activity of the constituent phases of the SCM, but is usually situated in between 1:1 (Murat 1983; Bakolas et al. 2006) and 2:1 (Takemoto and Uchikawa 1980; Costa and Massazza 1974). Pozzolans rich in A1 2 0 3 generally need higher Ca(OH) 2 :SCM ratios for optimal reactivity, and SCMs displaying hydraulic activity usually need much less Ca(OH) 2 to activate the hydration reactions (Lang 2002). In terms of Portland cement over SCM ratio optimal replacement percentages are often defined based on the desired properties of the hardened cement. In general, the optimal replacement ratio depends on the water demand, i.e., surface roughness and specific surface, and the activity of the SCM. The higher the water demand and activity, the lower the optimal replacement ratio is, typically 10-15 wt% for silica fume or metakaolin. Optimal replacement ratios defined for durability properties tend to be somewhat higher than ratios for optimal strength performance. At excessive replacement levels the pozzolanic activity is lowered because of the premature depletion of the solution alkalinity by the reacting SCM. A significant drop in solution pH below 10 may not only effectuate a decrease in pozzolanic reaction rate, but can also lead to the destabilization of AFm and AFt reaction products in the blended cement (Lothenbach et al. 2011). To increase the pozzolanic reaction rate, curing at elevated temperatures can be applied. The temperature dependence of the pozzolanic reaction on the short term can be described by the Arrhenius equation, implying an exponential dependence of the reaction rate on temperature (Snellings et al. 2009). The curing temperature should however not exceed the stability field of the C-S-H phase and result in the precipitation of more crystalline phases (typically above

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80-100 °C; Taylor 1990). The acceleration induced by elevated curing temperatures is much more pronounced for the pozzolanic reaction compared to the hydration of Portland cement constituents due to the higher activation energy of the pozzolanic reaction (Shi and Day 1993). Hydration mechanism and kinetics of blended cements In blended Portland cements the hydration reactions of the clinker phases are complemented by the pozzolanic or hydraulic reactions of the added SCM. Although the hydration processes of the clinker phases follow different mechanisms and rates than the pozzolanic or hydraulic reactions of the SCM, the clinker hydration in blended cements is influenced by the presence of SCMs. Reaction kinetics, products and the properties of fresh and hardened pastes can be manipulated by the replacement of a fraction of the Portland cement by SCMs. Both the properties of the SCM as the mix design are determining factors with the potential to affect all stages of the hydration and pozzolanic reactions in the blended cement. Influence of SCMs on the hydration of clinker phases. To eliminate the interference of simultaneously occurring reactions in a blended cement, the effect of SCM addition on the hydration kinetics of single clinker compounds has been investigated by numerous researchers. The hydration mechanisms of the individual clinker components have been recently reviewed by Gartner et al. (2002) and Bullard et al. (2011). Similar to the pozzolanic reaction mechanism, the hydration of C 3 A in the presence of gypsum and the hydration of C3S experience both a brief initial phase of high reactivity followed by a dormant period and an eventual main reaction stage. C3S. The main component of Portland cement is C 3 S, constituting 60-70 wt% of the cement. In general, the addition of an SCM has an accelerating effect on the hydration of C 3 S (Ogawa et al. 1980). The heat evolution rate during the main reaction period and the cumulative amount of heat released over the complete reaction are increased, especially when recalculated to the C3S content in the samples (Massazza 2001). The effect of the SCM on the early reaction is mainly governed by its fineness, as illustrated in Figure 29. The initial dissolution period is 12 C3S alone 20 % Silica Fume (SF) SF Surface area 50 m2/g

10-

130 m2/g 200 irv' (j 300 m2/g pH 13.5). However, such deicers represent concentrated solutions of weak acids, with consequences for solution behavior and their interaction with concrete (see below). Alkali contents of the solutions as applied are high, with ~23 wt% K for potassium formate, a little lower (~20 wt%) for an acetate solution. Commercial solutions usually also contain a dye to help uniform application additives to prevent steel reinforcement corrosion, and others. Sodium from ordinary salt dissolved in sea water or applied as deicer is the dead-painted external source of alkali potentially contributing to deleterious AAR, and has been subject of many studies until recent (e.g., Chatterji et al. 1987; Nixon et al. 1987; Sibbick and Page 1987; Kawamura et al. 1996; Duchesne and Berube 1996; Shayan 1998; Berube and Dorion 2000; Katayama 2004b; Shayan et al. 2010), including both field structures and concrete/mortar prepared for laboratory testing. Infiltration of NaCl—irrespective its origin—is the primary cause of chloride-induced reinforcement corrosion, a mechanism that is essentially confined from the concrete surface to the outermost steel reinforcement mesh, i.e., the cover. Removal of the concrete cover by demolition until behind the reinforcement and restoring to original profile with fresh concrete (often sprayed) is dusty, noisy, laborious and hence costly, and therefore often undesirable or impractical. Alternatively, electro-chemical extraction of infiltrated chloride is possible by embedding a temporary external Ti-wire mesh in an alkaline waste-paper or wood saw-dust pulp to the structure surface, and applying an electrical current (DC). The electrochemical repair process produces additional alkalinity as OH~ around the internal reinforcement by electrolysis of the concrete pore fluid, whereas charged species migrate in the electrical field created between the internal reinforcement and temporary external mesh, direction depending on charge sign. Thus, negatively charged chloride Cl~ is driven towards the electrically positive outer mesh, as is the newly created OH~ (re-) saturating the concrete cover. In contrast, positively charged species like the alkalis Na + and K + are drawn towards the electrically negative inner reinforcement. Electrochemical chloride-extraction is often less successful than anticipated (or hoped for) as chloride tends to get immobilized in the paste by alumino-ferrite present in the paste, through precipitation in compositions near ~hydrocalumite Ca 2 Al[0H] 6 [Cl 1 _ x 0HJ-3H 2 0. These represent complex calcium-aluminate hydrate compounds closely related to "Friedel's salt" (e.g., Fischer et al. 1980), but still rather poorly characterized in the cement paste (e.g., BirninYauri and Glasser 1998). Arya and Xu (1995) noticed different extent of chloride bonding by cement pastes of different composition, whereas Suryavanshi et al. (1995) demonstrate the complexity of chloride binding by the paste, which mechanism also involves sodium. Whereas infiltration of aggressive solutions is mostly limited to the exposed concrete surface, penetration to greater depths is strongly facilitated by an existing crack fabric. Broekmans (2002) studied complete top-bottom cross sections of the decks in two ASRdamaged viaducts scheduled for demolition and replacement. The fabric and spatial distribution of cracks in the decks' cross sections were determined on fluorescence-impregnated plane

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sections cut lengthwise from 0100 mm cores cf. Danish Standard 423.39 (2002). Results were presented as a "damage rating index" (DRI) with arbitrary units 0-5, ranging from 'no visible cracks' to 'incoherent disintegrated concrete.' Attributed DRI values were interpreted in terms of 'system openness' rather than 'amount of ASR damage,' as genuine-ASR cracks might have been enhanced by mechanical load during service. Whole-rock geochemistry on bulk concrete and total chloride by ISE after dissolution in excess nitric acid were determined on separate cores 0170 mm drilled in closest possible conjunction with the cores for petrography. Each 0170 mm core was divided in sixteen equally high sections, and each section was analyzed separately (Broekmans 2002; also see Broekmans 2006). Figure 3 above shows cores BB243/4/5 from one viaduct, cores BB246/7 from another. Values equal to or below LLD (0.005 wt%) are plotted as 1/10 that value and occur in all cores, implying a 'chloride-free' initial background. Both structures had been exposed to NaCl deicer during service life, but not to marine water. The coincidence between chloride content and DRI is striking, with the exception of the central portion in core BB246, in which the cracks in sections 6, 8-9, and 11 represent artifacts from previous strength testing in the laboratory (see Den Uijl et al. 2000), thus were never exposed to chloride in the field. Figure 3 shows that the interior of a 600-1000 mm thick concrete deck (tapering) is open to communicate with the outside world through cracks present. Apparently, these provide a convenient pathway for the infiltration of fluids carrying dissolved species like chloride, as well as for leaching of other species. The concentration of chloride (or of any other infiltrated

chloride concentrations in bulk concrete by ISE [wt%] BB243

BB244

BB245

BB246

BB247

Figure 3. Distribution of infiltrated chloride (solid lines) vs. cracking DRI (dashed lines) across viaduct decks. Adapted from Broekmans (2002).

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species, for that matter) in the crack itself and its direct vicinity will be (substantially) higher than the plotted values, as it is in fact averaged out over the entire core section by the wholerock analytical procedure. In situ assessment of chloride contents by EPMA in thin sections across the crack fabric will give more realistic concentrations. While above excursion to chloride illustrates the openness of cracked concrete for hydrous solutions, the situation may be different for the infiltration of Na and/or K along the same crack fabric. Diamond (1997) reports that drying and rewetting of concrete cores tends to reduce the alkali content of expressed pore fluid, and argues this is due to consolidation in the paste. Many if not most field concretes damaged by deleterious AAR have been exposed to drying and rewetting cycles during their lifetime, though conditions may range widely from arctic to tropical, with wetting from seasonal to permanently humid. Hong and Glasser (1999, 2002) did laboratory experiments with synthetically prepared amorphous CSH and CASH 'gels,' and exposed these to Na and/or K solutions at water:solid ratios much higher than in field concrete. Combining observations from these papers seems to suggest that alkalis are partly absorbed to the paste minerals' surface, partly consolidated in the paste through exchange and more difficult to remobilize upon rewetting: Diamond (1997) found that four months of continuous moisture saturation only remobilized 10-20% of the fixed alkali. Thus, exsiccation could be effective to halt the (further) development of deleterious AAR. However, any cracks present would facilitate infiltration of fresh alkalis gradually over time increasing the total amount of alkali present, or replenishing the amount fixed with unfixed alkalis from the concrete interior. Without cracks, redistribution of alkalis would be predominantly diffusion controlled, several orders of magnitude slower than via cracks serving as convenient fluid pathways. An excellent investigation of field concrete damaged by infiltrated alkalis combining multiple analyses is Katayama et al. (2004), and includes a calculated estimate of the alkali-balance. Another type of AAR damage from alkali infiltration has gained increasing attention recently. Since about 1990, K-acetate solutions have been applied as non-corrosive deicers on (mainly military) airfields in the United States of America as well as on the German Autobahn. Damage patterns are typical for infiltration-controlled mechanisms, with extensive peripheral cracking, and much less farther away from exposed surfaces or edges (Giebson et al. 2010). Even when the alkalinity of solutions as applied is around pH 11 and hence 'safe for concrete,' practice proves different. Diamond et al. (2006) added portlandite Ca[OH] 2 (an abundant paste constituent in common OPC concrete) to laboratory prepared KAc solutions, and measured their pH to jump to >15! Laboratory concretes produced with low-alkali cement and several types of aggregate did expand rapidly upon exposure to this kind of solutions cf. ASTM C1293-08b (2008), whereas the same mixes behave non-expansive when exposed to 1 N NaOH in the same procedure. In the same experiments, the pH of K-acetate deicer increased from initially ~pH 11 for virgin solutions until >pH 14 after exposure to concrete, coinciding with earlier observations. In concentrated solutions, however, activities of the dissolved species have to be taken into account, so that the actual increase of [OH~] in this situation is less than 10 5 = 100,000-fold than suggested by the increase in pH. Extensive research by Giebson et al. (2010) on synthetic and real pore solutions, model calculations and performance tests on concrete and mortar specimen, concludes that two mechanisms seem to enhance alkali reactivity of the aggregate: 1) the additional supply of alkalis, and 2) the dissolution of portlandite Ca[OH] 2 into the pore solution increasing [OH~] and rising pH. In summary, infiltration of alkalis through solution ingress is able to induce deleterious AAR in concrete, especially in structures with a large exposure area and a small 'body volume,' for instance pavements, airfield runways, viaducts, and the like. Depending on exposure conditions, and the nature of the infiltrating alkali solutions, the degree of potential enhancement of deleterious AAR may vary from 'negligible' to 'most effectively.'

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Alkali released from aggregate Release of alkalis Na and/or K is a matter of intensive dispute, in concrete structures suffering from deleterious ASR as well as in testing and assessment of aggregate materials for use in concrete. Alkali release from rock forming minerals under natural conditions has been studied extensively. The Reviews in Mineralogy volumes by Hochella and White (1990) and White and Brantley (1995) present excellent reviews and a wealth of literature references. Alkali release from common rock forming minerals exposed to 'concrete conditions' (either laboratory simulated, or in real life) has been investigated by Diamond et al. (1964), Van Aardt and Visser (1977a,b, 1978), Way and Cole (1982), Berube et al. (1988,1990,2002), Choquette et al. (1987, 1991), Kawamura et al. (1989), Goguel (1995), Berube and Duchesne (1996), Constantiner and Diamond (2003), Berube and Fournier (2004), Leemann and Holzer (2005), Lu et al. (2006), Wang et al. (2008), Locati et al. (2010), and others, in both sialic and mafic concrete aggregate lithologies. In practice, K-feldspar and sodic plagioclase appear to be the most susceptible for alkali release especially with increasing degree of alteration to kaolinite or saussurite, together with micas and other alkali-phyllosilicates due to their perfect cleavage and loosely bonded alkalis. In existing concrete, a number of aggregate lithologies have been indicated to release poorly bonded alkalis from certain minerals, in particular K-feldspar partly altered to sericite or kaolinite, but also muscovite, illite, and others. Supplementary cementitious materials (SCMs; also see Snellings et al. 2012, this volume) like e.g., blast furnace slag, fly ash, microsilica or others that are added to OPC to incorporate and immobilize alkali in the paste during their hydration and thus inhibit deleterious AAR, are feared ineffective against internally released alkalis, for at least two main reasons. First, there is timing: alkalis in the paste are released instantly with the addition of mix water, and most of it is again consolidated in the paste during setting and hardening, essentially a few months in traditional OPC concrete (though hydration continues as long as unhydrated clinker is available and has access to moisture). The internal pore volume in individual particles is generally low for most natural rock types used as concrete aggregate (typical range 1.0-0.1 vol%, but often lower). In addition, interstitial pores between mineral grains are generally small (large voids are undesirable as they reduce compressive strength) and with low permeability due to small and/or necked interconnections. Porosity (and permeability) of aggregate compares as 'low,' relative to bulk traditional concrete with 15-20 vol%. Thus, concrete aggregate stored and stockpiled outside is moisture saturated, so that infiltration of dissolves species through the existing pore system into the interior of individual particles (whether inhibiting or enhancing deleterious AAR) will be predominantly diffusion-controlled, and hence slow. Conversely, 'exfiltration' of soluble species through leaching via the aggregate's pore system will be slow as well. Therefore, whereas paste alkalis are immobilized at an early age, alkalis released by aggregate become available only after some delay, the extent of which depends on existing porosity and permeability of the actual aggregate lithology, as well as particle size, grain size and fabric parameters together determining the effective length of the diffusion pathway. By the time the alkali concentration in the releasing particle's interior has reached a critical level, the immobilizing action of the SCMs added may not be adequate to prevent deleterious AAR. Second, there is the issue of location: alkalis released in the interior of an aggregate particle cannot be immobilized by SCMs outside that particle, unless both reservoirs are able to communicate chemically (see Fig. 4). Again, this will be diffusion-dominated, and communication would only be effective if 'alkalis-to-be-immobilized' were indeed present, which only happens delayed.

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Figure 4. Optical micrograph showing ASR gel coating an air void, in a slag cement concrete with characteristic blue-green paste. As paste alkalis had been immobilized by the slag, the gel was produced with alkalis released in the particle's interior (top right corner, micaceous siltstone). While a curiosity in Dutch (slag) concrete, what would happen if a given aggregate contains a substantial proportion of such particles? The circle marked D refers to a detail shown in Figure 8.

Alkalis Na and/or K are present in the most common rock forming minerals, e.g., acidic plagioclase (albite-oligoclase, Nai_j:CarAlSi3_.rAlr08, with x = 0-0.3), K-feldspar (KAlSi 3 O s ), muscovite (KAl2Si3AlO10[OH]2), illite (-Ko^AloSisssAlaesOjofOHJo), and other micas or phyllosilicates, alkali amphiboles, etc., all rather common and occasionally abundant in typical concrete aggregate lithologies, except limestone. The weak bonding of Na + (and to a somewhat lesser extent of K + ) is largely due to its monovalent nature, combined with a small radius of ~100 pm (K+ ~140 pm) in an oxide or silicate matrix. This renders Na (and K) prone to removal from the crystal structure relative to other species in the mineral structure, for instance by dissolution in aqueous solution, by evaporation upon exposure to an electron probe beam (see below), or otherwise. The structure of micas and a number of clay minerals can be described in a simplified way as consisting of a single layer of A10 4 [0H] 2 -0ctahedra sharing edges (representing gibbsite), flanked on either side by layers of Si0 4 -tetrahedra arranged in six-rings, with all tetrahedraapices sharing O with the gibbsite layer. This Si-Al-Si-oxyhydroxide sandwich layer is often abbreviated as "TOT layer," for tetrahedral-octahedral-tetrahedral. In muscovite, TOT layer stacks are kept together by K + ions positioned above the centers of two opposite silica sixrings, defining a TOT-TOT interlayer. Muscovite easily parts into flexible and elastic thin flakes, defining the perfect basal cleavage characteristic for micas. Instead of K + , the interlayer may contain other ionic species to create other minerals, e.g., by Na + (paragonite), Ca 2+ (margarite, a brittle mica), whereas partial filling with K + , vacancies and/or hydronium H 3 0 + gives illite. Part of the Si4+ in the sandwich layer can be replaced by additional Al 3+ to maintain charge balance. Instead of Al3+, the octahedral layer in the sandwich may also be constructed of Mg 2+ and/or Fe 2+ in variable ratios (together with a range of other species usually in minor quantities), thus making common micas like e.g., phlogopite (K-Mg with subordinate Fe), biotite (K-Fe

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with subordinate Mg), celadonite, glauconite, and many more (also see Hazen and Wones 1972, 1973). The presence of less common chemical species replacing Al3+ in the tetrahedraloctahedral-tetrahedral sandwich combined with substantial consequently makes less common mica minerals that are rare to exotic in concrete aggregate. The efficacy of alkali release into solution is in principle related straight-forwardly to available surface area, or inversely to mineral grain size. Thus, a reduction of mineral grain size due to tectonic milling (here under subgraining) or otherwise facilitates alkali release, as does the presence of cleavage trails (notably in feldspars and micas) or microcracks by providing access for the pore solution to the grain's interior. In addition, alteration through retrograde metamorphism or weathering may render the alkali-bearing mineral more susceptible to alkali release. Known examples include e.g., sericitized or kaolinitized K-feldspar, saussuritized plagioclase, muscovite altered to illite. In effect, both latter features (i.e., Assuring, alteration) represent net grain size reduction as well. Alkali release from other minerals than mica and phyllosilicates is similar along broad lines. Planes of weakness (as e.g., cleavage planes at atomic scale, grain/twin/exsolution boundaries at crystal lattice scale, interstitial pore space at particle interior scale, total accessible surface area at particle exterior scale) facilitate fluid communication in and out. Common rock forming minerals containing alkalis and with pronounced cleavage include micas (e.g., muscovite, biotite, phlogopite, celadonite, glauconite, phengite) and intermediates towards clays, and to a lesser extent also alkali-amphiboles. Though (acidic) plagioclase and K-feldspar have three independent cleavages, their higher mechanical tenacity would render them less susceptible to leaching. On the other hand, altered plagioclase and K-feldspar may be more prone to alkali release due to the fine-grained nature of the alteration products (saussurite and sericite/kaolinite, respectively), often poorly crystalline. Grattan-Bellew and Beaudoin (1980) describe the release of K + from natural phlogopite branded as "Suzorite" (composition reported as K2Mg432Fei16Alo.35[Si5 75Al2.2502o][OH]4) added to OPC matrices with different initial Na 2 0-equivalents. In all combinations investigated, concrete with phlogopite expanded more than the equivalent mixes without, though some remained under the expansion limit. Phlogopite exposed to lime water (saturated Ca[OH] 2 solution) with or without various amounts of added NaOH (representing simulated concrete pore fluid) was found to release K + into solution, whereas the exposed phlogopite was demonstrated by SEM-EDS to have a lower Mg/K ratio, however exchange of Mg for Ca could not be confirmed. Also see Figure 5, showing alkali release from alkali-reactive sand-/siltstone in Dutch concrete from the mid-1960's. It is interesting to take a look at the observations of Boles and Johnson (1983). They added 1 g of chopped biotite and/or muscovite to solutions initially alkalized with 0.1 M NaOH to pH 11.5, then acidified by titration with 0.1 M HC1 to pH 4. They found that biotite acts as a base raising pH by absorbing H+, throughout the entire pH range investigated. In contrast, below ~pH 5.5 muscovite acts as an acid shedding H+. The net capacity to increase pH under alkaline conditions is attributed to differences in mineral-structural chemistry, muscovite being di-trigonal, biotite tri-trigonal, interlayer species present (K+, Na+, hydronium) and the resulting surface charge and charge density. Tentative calculations suggest that pH increase may be up to several orders at close range to the mineral surface. Above experiments seem confirmed by observations on biotite in certain oil-bearing sandstones, in which calcite or prehnite exclusively occur intercalated within biotite cleavage, which is attributed to local stabilizing through H-capture by the biotite host. Similar observations have been reported by Claeys and Mount (1991, and references therein), Nijland et al. (1994) for hydrogarnet lenses in biotite, and Nijland (pers. comm. 1997) who reported portlandite precipitated within and around biotite flakes in OPC concrete.

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Figure 5. Element maps by E P M A showing distribution of K (left) and Na (right) in an alkali- reactive sandstone, extruding alkali-silica gel into the surrounding paste through a crack. Detrital grains of elongate muscovite and more rounded K-feldspar and albite are easily recognized in the particle's interior.

Along the same line of thought, Bj0rkum (1996) identified detrital muscovite flakes penetrating into quartz grains, without bending or buckling. A simple calculation using elasticity properties of muscovite showed that the flakes are mechanically too weak to penetrate. Furthermore, SEM-cathodoluminescence unequivocally demonstrated the detrital origin of the penetrated quartz grains, as opposed to precipitated quartz of neogenic origin. In summary, the observed penetration is interpreted as a result of quartz dissolution enhanced by muscovite through an increase of the local pore fluid pH. This mechanism has been successfully applied to describe the compaction of North Sea sandstones, and is supported by petrographic observations (Oelkers et al. 1996). Whether mica-enhanced quartz dissolution is genuinely catalytic, or alternatively involves the interaction of K + at some stage, requires more research. Some alkali-reactive sandstone in Dutch concrete reveal droplet-shaped volumes with increased interstitial porosity between quartz grains at the ends of detrital muscovite flakes (Broekmans and Jansen 1998). Element maps by EPMA reveal that the alkali-reactive sandstone particles are substantially richer in K (and to a lesser extent also in Na) than the embedding cement, and that alkali-rich gel is extruded into the surrounding paste through AAR-induced cracks (see e.g., Fig. 6). Lithologies with exceptionally high alkali contents include peralkaline rock types like syenite and albitite which are often near monomineralic. Thus, a hypothetical syenite rock consisting of 100 vol% K-feldspar KAlSi 3 O s would contain up to 16.9 wt% K 2 0 ( = 1 1 . 1 wt% Na 2 O eq ), and an albitite with 100 vol% albite NaAlSi 3 O s in the same manner 11.1 wt% N a 2 0 (of course = 11.1 wt% Na 2 O eq ). Yet higher alkali contents are certainly possible (e.g., kalsilite K A l S i 0 4 with 29.8 wt% K 2 0 ) , but would be highly unusual for concrete aggregate, and descriptions of such cases seem absent from the literature (e.g., Diamond 1992). Gillott and Rogers (1994, 2003) describe alkali release from the uncommon carbonate mineral dawsonite (NaAl[C0 3 ][0H] 2 ) in concrete made with silico-carbonatite aggregate (see e.g., Stevenson and Stevenson 1965). Dawsonite occurs widespread in a broad range of lithologies as an authigenous cement mineral in carboniferous sandstones and/or alkaline shales (e.g., Anthony et al. 1990-2003, and references therein). The stability of dawsonite s.s. and ion-exchanged relatives (NH 4 + , K + ) at pH 2-14 and ambient P and T in various solutes has been

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Figure 6. Optical micrographs showing detrital muscovite in a similar sandstone as in Figure 5, in crossed polarized light (XPL), and in fluorescence (FL). The increased fluorescence at the tip ends of the mica flake may indicate catalytic action. Intergranular cracks visible in FL are connected to a larger network of cracks ultimately extruding ASR gel into the paste.

investigated by Stoica and Perez-Ramirez (2010). At pH 14, they found Na-dawsonite and its potassium and ammonium relatives to dissolve completely, whereas at lower pH, dawsonites decompose into a number of phases including alumina hydrates, depending on dissolved species present. Moreover, SEM observations revealed extensive recrystallization of dawsonite in carbonate solutions of ~pH 12, demonstrating easy cation exchange. Both dissolution and cation exchange may have contributed to the deleterious ASR described by Gillot and Rogers (1994, 2003). ALKALI-REACTIVITY POTENTIAL OF 'SILICA' Quartz properties and its solubility under ASR conditions By far the most abundant mineral in concrete aggregate worldwide is quartz. The name quartz seems derived from Old German Querklufterz (Liischen 1979), describing reject material unfit for further processing as queer vein ore (i.e., "gangue"; also compare querulant and quarrel). The abundance of quartz in chert as well as in most other crustal rocks justifies an elaborate description of its ideal and real structures and chemical compositions. Structure and composition determine the mechanical, physical and chemical properties of quartz, essential for understanding behavior in real life applications, including its use for concrete aggregate and its undesirable dissolution in structures under ASR conditions (Dove and Rimstidt 1994). Eventually, this may provide essential clues for optimizing test and assessment methods for aggregates and constituents, as well as criteria for how to distinguish the good from the bad and ugly. In the following, we will use "quartz " as a synonym for a-quartz. Further specification will be supplied as appropriate. Crystal structure of quartz, silica polymorphism and phase transitions. Ideally, the chemical composition of quartz is Si0 2 , silicon dioxide. The crystal structure can be described as composed of Si0 4 tetrahedra, with O located on each apex and Si in its center of gravity. Each oxygen atom is shared with an adjacent tetrahedron, reducing the contribution of each O to any tetrahedron to only half, and thereby reducing the composition of bulk quartz to Si0 2 . The structure of the a-quartz polymorph stable at temperatures below 573 °C is best explained as a distorted version with lower symmetry of P-quartz that is stable above that

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temperature (Heaney 1994). A view down the c-axis (i.e., viewing a high-quartz crystal on its pyramid top down its length axis) reveals silica tetrahedra arranged in six rings. Each ring is composed of two intertwined helical chains // c-axis of silica tetrahedra with six-fold symmetry, each chain extending over three 'ring storeys' (and that pattern being repeated indefinite). All chains have the same chirality, i.e., the same sense of rotation + translation = screw direction, resulting in left- or right-handed lattice. The chains are linked by independent silica tetrahedral not belonging to either of the helices. Thus, the whole quartz structure in fact represents one single polymer molecule, with six-sided di-trigonal channels parallel to its c-axis. Upon cooling through the P-a transition temperature at 573 °C, the Si-O bond length changes only marginally. In contrast, bond angles change substantially. Due to rotation of silica tetrahedra around the a-axis (oriented perpendicular to the c-axis), the helical chains buckle and contract along the c-axis. The tetrahedral rotation occurring at 573 °C reduces the initial hexagonal symmetry of the six rings in P-quartz with all equal angles, into six rings with ditrigonal symmetry with three wider angles alternated by three tighter angles. This a-quartz structure is stable from ambient temperatures (and far below) up to 573 °C. Looking at thermal behavior of a-quartz, linear expansion is 'normal' but highly anisotropic: quartz expands approximately only half as much along its c-axis compared to perpendicular equatorial directions (Rosenholtz and Smith 1941). At 573 °C, the c-axis shows a stepwise length increase induced by the a - P transition, after which further thermal expansion continues as 'normal' and again linear. The large thermal expansion anisotropy of a-quartz results in substantial internal stress even over relatively small temperature intervals, making it an important yet often underestimated factor in rock weathering and alteration (Siegesmund et al. 2003). The a - P transition of quartz at 573 °C merely involves change of bond angles by rotation of Si0 4 tetrahedra and is hence called displacive. Thus, the transition is instantaneous and reversible. Upon further heating, further transitions may occur, from P-quartz to cristobalite at 870 °C, at 1470 °C from cristobalite to tridymite, and melting at 1705 °C (e.g., Heaney 1994). However, in practice, heating of dry a-quartz at atmospheric pressure results in transition to P-quartz at 573 °C, and a transition directly to tridymite at ~1470 °C, entirely skipping the intermediate phase cristobalite. Upon cooling, (high-) cristobalite transforms displacively to low-cristobalite at 230 °C, whereas (high-) tridymite transforms to low-tridymite in the same fashion (Putnis and McConnell 1980). Depending on heating-cooling history of the silica, heating-cooling trajectories may see alternative transitions to different polymorphs (Heaney 1994). All these transitions are very slow and require exceedingly long dwell times near the transition temperature (i.e., months rather than weeks) in pure silica, some even to the extent that effectively, the transition is undetectable. Transition speeds are greatly increased by a large factor by the addition of fluxing agents or mineralizers (e.g., Na-tungstate) to pure silica, so that heating-cooling experiments can be conducted over the course of hours, and no phases are being skipped (e.g., Kiihnel et al. 1987). The main reason for skipping intermediate phases when transitioning from one polymorph to the other, as well as the sluggish reaction rates, is that all these phase transitions require a lot of energy to break Si-O bonds and rearrange all silica tetrahedra into the new structure. Therefore, this type of phase transition is called 'reconstructive:' complete reconstruction is required as the new structure cannot be formed through mere displacement (e.g., rotation, translation) of tetrahedra in the precursor polymorph. Note that both cristobalite and tridymite also have low- and high-modifications that can transform through displacive transformation into the other, instantaneously and reversibly. As a result, polymorphs may persist and/or can even be formed meta-stable outside their formal stability region. An example is volcanic glass, a common matrix component in basalt rock, or making its own rock type known as obsidian. Upon aging (no laboratory has the luxury

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of geological time) under ambient conditions, the unstable random glass structure crystallizes to form cristobalite (e.g., Smith et al. 2001). That variety of obsidian is known as snowflake obsidian, indeed with cristobalite snowflakes. The cristobalite has a denser tetrahedra packing than the glass precursor, thus creating pore space. The process is known as "devitrification" and has also been identified in medieval stained glass windows and artifacts (e.g., Garcia-Vallès et al. 2003). The newly created pore space provides access channels for further fluid infiltration (e.g., Çopuroglu et al. 2009). A similar devitrification process in opaline silica ages amorphous silica (opal-A, opal-AN) into cristobalite (opal-C, opal-CT). Meta-stability of silica is a delicate and potentially dangerous state in an AAR-context. Transformation to the stable polymorph can be triggered by only a minor event, but may then occur catastrophically. The catastrophic transformation of meta-stable olivine to the spinel structure under earth mantle conditions has been suggested as a cause for deep-seated earthquakes (Putnis and McConnell 1980, p 192). Similarly, one could imagine some meta-stable silica polymorph to go into solution very quickly when exposed to highly alkaline ASR conditions. Summarizing the above, quartz has a di-trigonal symmetry consisting of interconnected Si0 4 tetrahedra sharing apical O. The tetrahedra are arranged in two intertwined helical chains linked by independent tetrahedra. The resulting crystal structure has six-sided di-trigonal channels running along the c-axis and parallel to the helical chains. As all tetrahedra are interconnected, a single quartz crystal in fact represents one single silica polymer molecule. The a-(3 transition in quartz at 573 °C occurs instantly and is reversible due to its displacive nature, and similar displacive low-high transformations in cristobalite and tridymite are known to occur. In contrast, phase transitions between different polymorphs (i.e., quartz, cristobalite, tridymite, glass) are reconstructive, and hence sluggish. This latter quality provides the main reason for the meta-stable persistence of silica polymorphs well outside their stability region, prone to catastrophic transformation. Quartz crystal chemistry and substitutions. The idealized chemical composition of quartz is Si0 2 , with linked Si0 4 tetrahedra forming a crystal structure with di-trigonal channels running parallel to the c-axis. Though certain qualities of quartz are among the purest minerals found in nature (i.e., ultrapure quartz known as lascas), rock quartz does normally contain a number of chemical impurities. These can be incorporated in the crystal structure in a number of ways. Si4+ can be conveniently substituted by Ti4+ or Ge4+, which bears little to negligible effect on the crystal structure of the quartz host. Alternatively, Si4+ can be substituted by Al3+ or Fe 3+ that both fit well in a tetrahedral position provided by the surrounding four oxygens. However, to maintain charge balance, trivalent replicants must be supplied with additional monovalent species, for instance Na+, K + or Li+, which all can be conveniently hosted in the di-trigonal channels. The Al3+ + Li+ coupled substitution is occasionally dubbed "spodumene substitution," and indeed, the spodumene (LiAlSi2Oe) pyroxene structure is a distorted derivative of the quartz structure through complete substitution of Si4+ with Al3+ + Li+ in Si 4+ Si 2 0 6 , just an alternative notation of ordinary quartz (Palmer 1994). Another option for charge compensation involving Al3+ is coupled substitution of yet another Si4+ with P5+. Thus, ordinary quartz noted alternatively as Si 4+ Si 4+ 0 4 then becomes Al 3+ P 5+ 0 4 . The mineral A1P04 is isostructural with quartz, and is known from nature as berlinite with (optical and other) properties virtually identical to quartz (e.g., Motchany and Chvanski 2001), and the 2Si —» Al+P replacement is therefore known as "berlinite substitution." Upon exposure to natural irradiation, Al-substituted quartz turns brownish to smoky quartz, whereas Fe-substituted quartz turns purple into amethyst. Without irradiation, Al- and/or Fesubstituted quartz remains colorless (Nassau 1983). Other colors may be caused by mineral

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impurities, for instance avanturine green is colored by green fuchsite mica flakes, chrysoprase by green pimelite flakes, and some blue or rose-colored quartzes by micro-fine fibers of dumortierite (Applin and Hicks 1987). Another common substitution in quartz is the replacement of Si4+ by 4H+, changing one Si0 4 tetrahedron into (OH)4, which arrangement in quartz is known as a silanol group. Especially quartz formed under hydrothermal conditions (natural or laboratory) contains a considerable amount of silanol, and is hence described as "wet quartz," as opposed to "dry quartz" with little or no silanol. Both varieties behave markedly different upon stress, wet quartz being softer by a factor ~10 (Kekulawala et al. 1978). Remarkably, amethyst (i.e., Fe-doped quartz) behaves about as soft as wet quartz, both being about half as hard as heat-treated dry quartz. Obviously, the presence of water as silanol in the quartz structure has a very significant effect on lattice rigidity. Many more element substitutions have been described in quartz (Miiller et al. 2003; Gotze et al. 2004), but the above are the most common and widespread. Which are present in quartz from a given lithology depends on geological conditions under which the quartz was deposited, e.g., P, T, stress and strain, fluid present (amount and speciation), and species dissolved in the fluid including mineralizers and solvents. Thus, the composition of quartz reflects its geological background and history to a large extent. Some substitutions have been calibrated and can now be used as an instrument, for instance Ti in quartz as a very accurate geothermometer (Wark and Watson 2004). Apart from bonded impurities as the lattice substitutions described above, the channelized quartz structure is able to accommodate virtually the entire periodic system (Kats 1962a,b). Using an electrical field, chemical species can be introduced from an aqueous solution into quartz in its structural channels (e.g., Kronenberg and Kirby 1987). This is by no means a novel technique: the phenomenon was already described by Curie (1889), the brother of Mme Curie who discovered radioactivity. Changing polarity will again remove most of the introduced chemical species from the structure. Impurities are very common in natural quartz, which is why only the cleanest qualities of highest purity are suitable for production of Si-metal for electronics and solar cells. While chemical impurities incorporated in the quartz lattice are very common, their concentrations are generally low, rather on the order of a few hundreds to a few thousands of ppm [mg/kg] altogether, ranking quartz among the purest minerals occurring in nature. On the other hand, even such low impurity levels do have a marked and measurable effect on its properties, for instance compressive strength (Kekulawala et al. 1978), as well as solubility (Dove and Rimstidt 1994). Microstructure of quartz• The Si-O bond character in quartz is predominantly covalent, with only ~25% ionicity (Stewart et al. 1980; compare halite NaCl with 63%), explaining the generally poor solubility of quartz in aqueous solutions. Obviously, bonds surrounding chemical substitutions (i.e., foreign species) differ from bulk quartz structure, due to valence differences between the original Si4+ and the substituting species (e.g., Al3+, Fe3+, 4H+, Ti4+, Ge4+, P5+), atomic/ionic radius and resulting field strength, as well as the presence of balancing charges (e.g., Li+, Na+, K + ) at positions not normally occupied. For instance, 4H+ substituting lSi 4+ to form a silanol group replaces a single Si bonding its four surrounding O's with four mutually repelling -OH hydroxyl groups. Consequently, the crystal structure around impurities is distorted relative to the pure ideal material. Thus, impure quartz is under constant mechanical stress, which increases its susceptibility to stress corrosion (e.g., Hadizadeh and Law 1991). The above discusses the effects of various impurities on the mechanical and chemical stability or solubility of quartz at the atomic scale, assuming an otherwise contiguous structure. At a scale of observation of a few hundred up to several thousand unit cell axis lengths (i.e.,

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a few tens of nanometers up to several microns, the structure of natural quartz can usually be regarded as truly contiguous. Larger contiguous volumes ('domains') are known to exist but are not common in natural quartz; they are more typical of synthetic quartz grown under carefully controlled conditions not exposed to mechanical deformation. Domain size is in its own right important in quartz dissolution as inversely related to surface area, in the same fashion as grain and particle sizes at even larger scales. The definition of a domain is a net sum of the various definitions of its borders, representing some type of discontinuity terminating a contiguous crystal structure. Discontinuities include low- or highangle dislocation walls, twin interfaces, or grain boundaries. In general, a crystal structure is distorted by any foreign species present, as discussed extensively above. In addition, the structure can also be distorted by errors that render repetition of crystal-structural build units less than perfect. Such defects in the structure may for instance be an additional atom or ion at a position that normally is unoccupied, or a vacancy at a location that normally would be occupied. Stacking faults result from dislocated atomic bonds latching on to where they wouldn't in the ideal structure. Imagine a growing crystal plane on which 'fundamental crystal building blocks' are being deposited, not unlike Lego bricks of uniform size and shape being stacked in a completely regular pattern. Accidentally, a bond is formed between the low edge of one Lego brick just getting attached, with the top edge of an adjacent brick so that the landing brick's position is tilted and a step is created. In reality, the misfit of the brick is minimized by redistributing the tilted bond angles over a larger number of adjacent bricks (real-life crystal building blocks are much more flexible than the macroscopic Lego bricks in above rationalization). The result is the formation of a so-called screw dislocation, where the amount of dislocation is largest at the core (one single step) and evens out with increasing distance away. Many more types of dislocations do exist, for which the reader is referred to Putnis and McConnell (1980) and particularly Poirier (1985). Depending on conditions, dislocations can promote crystal growth and/or dissolution (rates), due to interatomic bonds being under stress at the dislocation core. In minerals under natural/geological conditions, dislocations occur together in so-called "dislocation walls," forming planar arrangements not predicted by laws of crystal symmetry, often curved and/or highly irregular. The dislocation angle is variable, i.e., the degree of misorientation between the individual lattices on either side of the dislocation wall, and a distinction is made between low- and high-angle boundaries. Lattice misorientations 13) in decreasing order as: opal-A, opal-C, opal-CT (Gutteridge and Hobbs 1980), moganite, chalcedony, fine grained quartz (i.e., chert and flint, but also siltstone), and finally, coarse grained quartz. Probably, as data on the solubility of opal-AN and moganite under ASR conditions in concrete are scarce. However, moganite is absent in rocks older than 100-150 Ma, whereas opaline silica is absent in younger rocks of only 60 Ma, which seems to confirm the greater stability of moganite relative to opal. By comparison, quartz is present in the very oldest rocks currently known, with ages up to 4.3 Ga, even including some chert beds. To the best of the author's knowledge, alkalireactivity of moganite has never been specifically reported, though it must represent a common constituent in Danish, Dutch, German, Belgian, French and UK concrete aggregate containing potentially alkali-reactive chert from the Cretaceous (Heaney and Post 1992). The solubility of silica is furthermore affected by presence of other dissolved species in the concrete pore solution, which may go either way. This effect is known as "salting-in" or "salting-out," respectively, depending whether solubility increases or decreases (e.g., Marshall and Warakomski 1980; Chen and Marshall 1982; Marshall and Chen 1982; Curtil et al. 1992; Dove and Rimstidt 1994; Broekmans 2002, 2004a). Diamond (1989) reported that solubility of opaline silica in alkaline NaOH solution decreased substantially in the presence of Ca(OH)2, confirming salting-out behavior (also see Curtil et al. 1992). Opaline silica. Alkali-reactivity of opaline silica has been extensively investigated, e.g., Vivian (1950), Tang and Xue (1962), Diamond and Thaulow (1974), Diamond (1976), Ludwig and Bauer (1976), Midgley (1976), Poole (1976), Diamond (1978), Baker and Poole (1980), Gutteridge and Hobbs (1980), Figg (1981), Kawamura et al. (1983), Giovambattista et al. (1986), Curtil et al. (1992), Scrivener and Monteiro (1994), Tajing et al. (1995), Rodrigues et al. (2001), Mitchell et al. (2004), Ponce and Batic (2006), Bulteel et al. (2010), and as part of the general review on ASR and performance testing by Lindgard et al. (2012). The violent alkali-reactivity potential of opal s.l. poses problems with reliable testing of aggregate materials. Around the turning of the millennium, some Danish laboratories were experimenting with further acceleration of mortar bar tests to decrease duration, as desired by building contractors. Thus, the experiment temperature of a given procedure was increased

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from 38 °C to 60 °C. Despite using a well-known highly alkali-reactive opaline limestone of local origin, initially no expansion was found. Thus, the experiment was repeated after double checking all materials, equipment, instrumentation and settings, to find the same result: no expansion at all, whereas structures containing the same limestone are known to develop damage only short time after. Post-mortem thin section petrography on the exposed test specimen revealed a spongy fabric of 'empty' cement paste, the voids containing only scarce relics of dissolved and disintegrated opaline limestone. Such outcome is false and potentially poses a great danger: if expansion were the only criterion used to accept or reject a material, then poorly performing aggregate might be falsely approved for use in concrete. Some countries have included post-mortem thin section petrography as a yet unofficial addition to the standard test procedure, to verify the nature of the expansion data, irrespective whether or not local accept/reject criteria based on expansion behavior have been fulfilled. The opposite situation has been encountered as well, where aggregate performing well seems to have been incorrectly labeled as potentially alkali-reactive. Some sea dredged Cretaceous cherts reveal internal porosity in thin section petrography and therefore classified potentially alkali-reactive and hence unsuitable for use as concrete aggregate, conform Dutch guideline CUR-Recommendation 89 (2008). However, expansion testing has not been able to confirm the alleged alkali-reactivity potential: the material behaves essentially non-expansive and stays within defined limits. The material has resided at the seabed for hundreds to possibly thousands of years, during which soluble silica species (i.e., opal, moganite) may have been dissolved and removed by the saline seawater. However, to the knowledge of the authors, this hypothesis has never been verified by mineralogical or geochemical analysis. Opaline silica often occurs in the interstitial pore space between detrital grains associated with neogenic clay minerals. This is difficult or even impossible to detect in thin section petrography, but may render a given lithology potentially alkali-reactive. Moganite and chalcedony. The presence of moganite and/or opaline silica in silt- and sandstones may well explain the reactivity observed in some varieties. Moganite has been demonstrated identical with lutecite in Parisian Basin sandstones, and there seems no reason why moganite could not also be present in silt-/sandstone from other localities. However, its problematic detection (and reluctant official acknowledgement as a mineral) may have prevented its identification as a common constituent in lithologies frequently used for concrete aggregate, including chert, flint, siltstone and sandstone. To the best knowledge of the present authors, the alkali-reactivity potential of moganite in terms of ASR-susceptibility has never been specifically investigated. Chalcedony has been suggested a fibrous silica composite with a-quartz and moganite in variable proportions. Depending on moganite content and internal porosity, the solubility of chalcedony may differ over a wide range as reported in the existing literature. Comparatively, the solubility of chalcedony is intermediate between that of moganite and pure a-quartz. The variable solubility under natural geological conditions might possibly coincide with the different behavior of (lithologies containing) chalcedony as observed in concrete: some varieties are alkali-reactive, others behave innocuous. Uniform and fully reliable petrographic criteria to distinguish reactive from innocuous chalcedony are currently lacking, though internal porosity is most typically taken as it is perhaps the easiest property to assess. Fine and coarse grained a-quartz. Dissolution behavior of a-quartz is also determined by grain size. Depending on the material's nature, grain size may either represent "domain size" or "particle size," both of which may apply to the silica polymorphs occurring in chert and flint. There are fundamental differences between convex (i.e., spherical) and concave surfaces (i.e., hollow, enclosed). Individual a-quartz grains smaller than ~0.1 mm have a measurably /«creased solubility relative to bulk quartz (Dove 1995), which can be attributed to a

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shifting ratio between surface vs. bulk thermodynamic free energies towards smaller grain/ domain/particle sizes. Meanwhile, concave surfaces of equally small radius like for instance fractures from brittle deformation/compaction, interstitial pores between fine sedimentary detritus, nanopores in sponge spiculae, contact points between adjacent quartz grains, have a markedly decreased solubility, for the same thermodynamic background (Brantley 1992). As a consequence, a-quartz particles 2H 2 0: its solid solutions and their role in chloride binding. Cem Concr Res 28(12):1713-1723 Bish DL, Reynolds RC Jr (1989) Sample preparation for X-ray diffraction. Rev Mineral 20:73-99 Bish DL, Plötze M (2010) X-ray powder diffraction with emphasis on qualitative and quantitative analysis in industrial mineralogy. Christidis GE (eds), EMU Notes Mineral 9:35-76 Bj0rkum PA (1996) How important is pressure in causing dissolution of quartz in sandstones? J Sediment Res 66:147-154 Bluhm H, Frey W, Giese H, Hoppé P, Schultheiß C, Sträßner R (2000) Application of pulsed HV discharges to material fragmentation and recycling. IEEE Trans Dielectr Electr Insul 7:625-636 Blum AE, Yund RE, Lasaga AC (1990) The effect of dislocation density on the dissolution rate of quartz. Geochim Cosmochim Acta 54:283-297 Bogue RH (1929) Calculation of the compounds in Portland cement. Ind Eng Chem Anal 1(4):192-197 Boles JR, Johnson KS (1983) Influence of mica surfaces on pore-water pH. Chem Geol 43:303-317 Bönen D, Diamond S (1994) Interpretation of compositional patterns found by quantitative energy dispersive X-ray analysis for cement paste constituents. J Am Ceram Soc 77:1875-1882 Borbély G, Beyer HK, Karge HG, Schwieger W, Brandt AH, Bergk KH (1991) Chemical characterization, structural features, and thermal behavior of sodium and hydrogen octosilicate. Clays Clay Miner 39(5):490-497 Bowden FP, Hughes TP (1937) Physical properties of surfaces IV - Polishing, surface flow and the formation of the Beilby layer. Proc Roy Soc 160A:575-587 Brantley SL (1992) The effect of fluid chemistry on quartz microcrack lifetimes. Earth Planet Sei Lett 113:145156

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Brindley GW (1969) Unit cell of magadiite in air, in vacuo, and under other conditions. Am Mineral 54:15831591 Brisard S, Chae RS, Bihannic I, Michot L, Guttmann P, Thieme J, Schneider G, Monteiro PJM, Levitz P (2012) Morphological quantification of hierarchical geomaterials by X-ray nano-CT bridges the gap from nano to micro length scales. Am Mineral 97:480-483 Broekmans MATM, Jansen JBH (1998) Silica dissolution in impure sandstone: application to concrete. J Geochem Explor 62:311-318 Broekmans MATM, Nijland TG, Jansen JBH (2001) Origin and mobility of alkalies in two Dutch ASRconcretes II: microscale element distribution around sandstone and chert. Implications for the mechanism of ASR. In: Procs 8th EMABM, Stamatakis M, Georgali B, Fragoulis D, Toumbakari EE (eds), Athens, Greece, p 85-92 Broekmans MATM (2002) The alkali-silica reaction: mineralogical and geochemical aspects of some Dutch concretes and Norwegian mylonites. PhD-thesis Utrecht University, The Netherlands. Geologica Ultraiectina 217:1-144 Broekmans MATM (2004a) Structural properties of quartz and their potential role for ASR. Mater Charact 53(2-4):129-140 Broekmans MATM (2004b) Qualities of quartz and the alkali-silica reaction in concrete. In: Procs 8th Intl Cong Appi Min, Pecchio M, Andrade FRD, D'Agostino LZ, Kahn H, Sant'Agostino LM, Tassinari MMML (eds), Aguas de Lindoia, Brazil, p 661-664 Broekmans MATM (2004c) The crystallinity index of quartz by XRD, its susceptibility for ASR, and a brief methodological review. In: Procs 12th ICAAR, Tang M, Deng M (eds), Beijing, China, p 60-68 Broekmans MATM (2006) Sample representativity: effects of size and preparation on geochemical analysis. In: Marc-André Bérubé Symposium on Alkali-Aggregate Reactivity in Concrete. Fournier B (ed), Montréal, Canada p 1-19 Broekmans MATM, Wigum B J (eds) (2008) Procs 13"1 ICAAR, Trondheim, Norway: ppl336 + xvii. Broekmans MATM (2009) Petrography as an essential complementary method in forensic assessment of concrete deterioration: two case studies. Mater Charact 60(7):644-655 Broekmans MATM, Fernandes I, Nixon P (2009) A global pétrographie atlas of alkali-silica reactive rock types: a brief review. In: Procs 12th EMABM. Middendorf B, Just A, Klein D, Glaubitt A, Simon J (eds) Dortmund, Germany, p 39-50 Brouxel M (1993) The alkali-aggregate reaction rim: Na 2 0, Si0 2 , K 2 0 and CaO chemical distribution. Cem Concr Res 23:309-320 BS 7943-99 (1999) Guide to the interpretation of petrographical examinations for alkali-silica reactions. British Standard BSI 07. British Standard Institution, London, United Kingdom 20 p Buhrke VE, Jenkins R, Smith DK (eds) (2001) A Practical Guide for the Preparation of Specimens for X-ray Fluorescence and X-ray Diffraction Analysis. Wiley-VCH, New York Bulteel D, Garcia-Diaz E, Dégrugilliers P (2010) Influence of lithium hydroxide on alkali-silica reaction. Cem Concr Res 40(4):526-530 BurzoE (2009) 8.1.5.15 Kanemite and ekanite groups and related silicates. In: The Landolt-Bòrnstein Database, Wijn HPJ (ed), Springer Verlag, Berlin/Heidelberg: p 57 Cabri LJ, Rudashevsky NS, Rudashevsky VN, Oberthiir T (2008) Electric-pulse disaggregation (EPD), hydroseparation (HS) and their use in combination for mineral processing and advanced characterization of ores. Canadian Mineral Processors 40th Annual Meeting, Proceedings 14:211-235 Carroll D, Starkey HC (1971) Reactivity of clay minerals with acids and alkalies. Clays Clay Miner 19:321-333 Casey WH, Bunker B (1990) Leaching of mineral and glass surfaces during dissolution. Rev Mineral 23:397426 Casey WH, Westrich HR, Banfi eld JF, Ferruzzi G, Arnold GW (1993) Leaching and reconstruction at the surfaces of dissolving chain-silicate minerals. Nature 366:253-256 Cembureau (2012) Construction in Europe Main activities 2010. Billion euro: 1312. www.cembureau.be/sites/ default/files/Construction2010.pdf, retrieved dd. 26 January 2012 Cerny P, Burt DM (1984) Paragenesis, crystallochemical characteristics, and geochemical evolution of micas in granitic pegmatites. Rev Mineral 13:257-297 Chapman DL (1913) A contribution to the theory of electro capillarity. Philos Mag 25:475-481 Charlwood RG, Solymar ZV (1995) Long-term management of AAR affected structures: an international perspective. In: Procs 2ui Intl Conf on AAR in Hydroelectric Plants and Dams, Chattanooga, Tennessee. US Committee on Large Dams (USCOLD), Denver, Colorado, p 19-55 Chatterji S, Jensen AD, Thaulow N, Christensen P (1986) Studies of alkali-silica reaction. Part 3. Mechanisms by which NaCl and Ca(OH)2 affect the reaction. Cem Concr Res 16:246-254 Chatterji S, Thaulow N, Jensen AD (1987a) Studies of alkali-silica reaction. Part 4. Effect of different alkali salt solutions on expansion. Cem Concr Res 17:777-783

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