Geopolymers and Other Alkali-Activated Materials

16

Geopolymers and Other Alkali-Activated Materials John L. Provis and Jannie S.J. van Deventer

16.1 ALKALI ACTIVATION: INTRODUCTION AND BACKGROUND This chapter addresses the class of materials which are generated through combination of an alkali source and an aluminosilicate component, to form a binder which is based on alkali–aluminosilicate chemistry. These binders, termed alkaliactivated materials (AAMs), have been developed and commercialised for various reasons,1,2 but are currently primarily receiving interest due to the desire of the construction sector to reduce its environmental footprint through the use of waste-derived materials and alternative processing routes. The most commonly used alkali sources are the hydroxides or silicates of sodium or potassium, which are mostly provided in aqueous form and combined with a solid (powdered) aluminosilicate precursor to produce the AAM binder. Commonly used aluminosilicate powders include blastfurnace slag, coal fly ash, and calcined clays, although many others can also be used as will be discussed later in this chapter. The savings in CO2 emissions which can be generated through the use of alkali-activated binders in place of Portland cement are made available mainly through the avoidance of a high-temperature calcination step in the production of AAMs from industrial by-products, although the alkali-activator component does itself incur some significant environmental cost.3 The carbon dioxide emissions profiles which have been calculated in academic studies describing the potential replacement of Portland cement by an AAM range from a saving of 97% up to a possible small increase in emissions under varying circumstances, but in most cases it appears that a CO2 emissions saving of 50%–80% is possible.1,3 There are also some significant performance drivers which favour the use of AAMs in a range of applications in civil infrastructure and other aspects of the construction industry, but the key driver for the use of these materials at present is related to environmental performance. The first reported use of sodium or potassium salts as an ‘alkaline activator’, that is, to generate an elevated pH in the presence of a powdered silicate inducing its reaction to form a hydraulic binder, dates to the late 1800s and early 1900s, when uhl5 each patented binder mixes based on mixtures of ground slag and alkaline components. Purdon in 19406 Whiting4 and K€ then performed a detailed laboratory study on cements consisting of slag and various sodium salts, with and without lime, which led to commercial production in Belgium of an alkali-activated slag cement known as ‘Purdocement’ during the 1950s, using a combination of Na2SO4 with either lime or a small amount of Portland cement clinker as activator.7 However, there was only limited activity in this area in the succeeding decades, as the focus of research and development remained in the production and optimisation of Portland cements and the concretes synthesised from them. In the mid-1950s, Glukhovsky began to develop binders from clays in combination with solutions containing alkali metals.8 He called the binders ‘soil cements’ and the corresponding concretes ‘soil silicates’; these materials have since been used in many infrastructure projects in the former Soviet nations, and are widely standardised in that region in a number of strength grades. In the late 1970s, the French chemical engineer Davidovits reignited interest in this area with the development of alkaliactivated binders based on metakaolin, and coined the term ‘geopolymer’ to describe his products.9 These materials were initially targeted for fire resistance applications, but moved into more general application as a high early strength construction material,10 including hybrid alkali–clinker–aluminosilicate systems with various admixtures used to regulate setting and hardening. The first description of the formation of a strong and durable binder by alkali activation of fly ash was published by Wastiels et al.,11,12 and this has become a particularly fruitful area of academic investigation since this time due to the low cost, and wide diversity, of fly ashes available worldwide. Specific technical areas in which the process of alkali activation has been proposed to offer benefits include1,13–15:
Low-CO2 construction through the use, or valorisation, of recovered or waste materials as precursors; this can also bring low cost if the activator can be sourced cost-competitively
High mechanical performance (compressive, flexural and abrasion resistance) in the hardened state, including rapid strength gain in mixes formulated and cured appropriately
Good performance under fire conditions, including low thermal conductivity
Potentially very high resistance to acid and chemical attack Lea’s Chemistry of Cement and Concrete. https://doi.org/10.1016/B978-0-08-100773-0.00016-2 © 2019 Elsevier Ltd. All rights reserved.

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Chemistry and mineralogy which can be tailored for selective immobilisation or encapsulation of problematic wastes Adhesion to many substrates for use as a repair material, and strong bond to metallic reinforcing High internal pH enabling passivation of reinforcing steel Low permeability when formulated at low water content

Each of these advantageous characteristics can be achieved by the correct formulation, placement, curing and use of alkaliactivated binders. However, it is essential to stress that it is unlikely that any single formulation will achieve all of these characteristics simultaneously. The alkali content of almost all alkali-activated binders exceeds the allowable alkali limit specified for Portland cement in regulatory standards worldwide (e.g. 0.60% Na2Oeq in cement in ASTM C150,16 or values ranging from 2.0 to 5.0 kg/m3 concrete depending on the aggregate reactivity, in Appendix D of BS 8500-217). This means that performance-based testing of these materials will certainly be required to justify their resistance to alkali–aggregate reaction processes; indications to date are that the high reactive aluminium content of alkali-activated binders is very helpful in this regard.18 It should also be mentioned that alkali-activated binders are not explicitly covered by any of the prescriptive cement categories in the major international standards regimes under which cements and concretes are specified. Pure performancebased standards such as ASTM C115719 offer scope for the use of these materials, and work is now ongoing in several jurisdictions to develop specifications and standards which would enable the specification of AAMs directly under prescriptive or semi-prescriptive frameworks.

16.2 CLASSIFICATION OF ALKALI-ACTIVATED BINDERS The simplest way to classify alkali-activated binder structures is according to the calcium content of the binder, which plays an essential role in defining the phase assemblage comprising the hardened material. Low-calcium alkali-activated binders (for which metakaolin and fly ash are the primary aluminosilicate precursors) form an alkali aluminosilicate gel with a highly cross-linked tetrahedral framework structure, resembling a zeolite but lacking long-range chemical order, whereas higher calcium binders (such as those derived from blastfurnace slag) are dominated by a hydrous chain silicate which much more closely resembles the hydration products of Portland cement.1,20 The intermediate region, as depicted in Fig. 16.1, can feature gel coexistence if the binder chemistry is designed to stabilise both types of gel. These cases will be discussed in turn in the following sections; firstly the low-calcium case, then the high-calcium and finally the intermediate region including discussion of hybrid alkali–Portland cement–aluminosilicate binder systems. Fig. 16.2 shows a more detailed schematic of the process and products of alkali activation, as a function of precursor chemistry and reaction time. The reaction process, whether the calcium content of the precursor is high or low, will start with a series of dissolution steps releasing Al, Si and Ca (if present) into the solution, which may already be rich in Si if a silicate activator is used. These species react and reorient themselves into an assemblage of binder products, the exact details of which will depend on the binder chemistry and curing regime imposed. The reaction products then continue to evolve towards a mature microstructure, refining and/or filling the pores within the material, and yielding a hardened product which can display highly desirable properties and performance, as discussed throughout the remainder of this chapter.

16.3 LOW-CALCIUM ALKALI-ACTIVATED SYSTEMS 16.3.1 Alkali Activation of Aluminosilicates Alkali hydroxides are the simplest activator which can be used in an alkali-activated binder, and are usually dissolved to form an alkaline solution which is then mixed with an aluminosilicate powder to form a solid binder (Eq. 16.1). It is also possible to calcine solid aluminosilicates together with alkali hydroxides to form a precursor for ‘just add water’ binder synthesis,21,22 Eq. (16.2), but this process is much more technically challenging to scale beyond small scale laboratory research.

Tobermorite-type (‘C-A-S-H’) structure Zeolite-type (‘N-A-S-H’) structure Low

Ca content

High

FIG. 16.1 Gel types present in alkali-activated binder systems as a function of calcium content.

Geopolymers and Other Alkali-Activated Materials

Dissolution of solid aluminosilicate source Highly alkaline environment drives dissolution of Al and Si; calcium availability decreases at high pH as portlandite saturation is reached

781

Silicate species supplied by activating solution Smaller species prevail at high pH; oligomeric species (up to 8 Si units) in systems richer in Si

Rearrangement and exchange among dissolved species

Gel nucleation

Nucleation sites influenced by Si content of activator; can be close to or far from particle surfaces

High Ca (medium-high Mg)

Low Ca Layered double hydroxides

C-A-S-H gel

(low Mg)

N-A-S-H gel

Crystalline zeolites

Primary binder gels in bold, secondary products in plain text

Development and evolution of hardened material microstructure and properties FIG. 16.2 Schematic showing the process and reaction products of alkaline activation under different conditions and at different compositions.

ASx + NH ! N-A-S-H ðx  2 for metakaolin, up to  8 for fly ashÞ

(16.1)

Ny ASz + H ! N-A-S-H ðy  1 and usually 2  z  6Þ

(16.2)

During the alkali activation of fly ash in a hydroxide environment, the glass component of a fly ash particle is etched by the hydroxide activator, releasing silicate and aluminate species which can then reorient in solution and polymerise to form aluminosilicate gel units. This leads to the formation of reaction products both outside and inside the particle,23 giving a final microstructure which contains embedded fly ash particles with varying degrees of reaction, Fig. 16.3.

50 µm FIG. 16.3 Backscattered electron image of an alkali-activated fly ash, showing partially-reacted spherical fly ash particles, some of which are hollow, and also a broken hollow-shell morphology containing gel reaction products as well as smaller fly ash spheres. (Sources: Modified from Ismail I, Bernal SA, Provis JL, San Nicolas R, Hamdan S, van Deventer JSJ. Modification of phase evolution in alkali-activated blast furnace slag by the incorporation of fly ash. Cem Concr Compos 2014;45:125–35; Courtesy I. Ismail and S. Bernal.)

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Si layer Al layer

FIG. 16.4 Structural model representing metakaolin, as calculated by density functional theory simulation in White et al.25, showing the high degree of structural disruption in Al layers induced by dehydroxylation of kaolinite.

The situation for a metakaolin particle is broadly similar, with the hydroxide activator attacking the layered structure of the precursor particle to release the silicate and aluminate species, probably with some selective release of Al from the crystallographically strained sites in the dehydroxylated clay structure,24 Fig. 16.4. The final microstructure of the metakaolinbased binder is intrinsically mesoporous,26 and will usually involve a significant fraction of embedded unreacted precursor particles.27 The location at which gel nucleation takes place—either on, or away from, the particle surfaces—is important in determining the final extent of reaction achieved. This can be controlled to some extent by the addition of soluble silica to the activator,28,29 or by seeding with high-surface area nanoparticles,30 although it is likely that the seeding approach is more useful in the absence of added soluble silica due to masking of the seeds by sorbed silicates when present. The rheology of alkali-activated fly ash binders is in general described rather well by the Bingham model,31 while the behaviour of alkali-activated metakaolins is rather more complex due to the presence of plate-shaped metakaolin particles, and (particularly in the presence of a silicate activator) the rapid generation of ‘bridges’ of early-age reaction products which form between the high-surface area precursor particles.32,33 The water demand of a fly ash-based binder is in general comparable to that of Portland cement, with good workability often achievable at water/solids ratios below 0.5 in fly ash-based concrete mixes,34 while even in paste format, mixes based on rotary-kiln calcined metakaolin often require water/solids ratios closer to 1.0 to enable mixing and moulding. The viscosity of a concentrated alkali silicate solution can also be challenging in terms of workability in the fresh state, particularly when a high silica concentration is used.20

16.3.2 Binder Structure The binders formed through alkali hydroxide activation of low-calcium precursors are dominated by an alkali aluminosilicate gel, abbreviated here as N-A-S-H without excluding the possibility of replacement of Na by K or other alkalis. Aluminium and silicon atoms with tetrahedral coordination to oxygen are bonded to form a highly cross-linked structure, an aluminosilicate ‘framework’, with alkali metal cations (‘non-framework’ ions) charge balancing the tetrahedral Al(III) sites,9,20,35–37 Fig. 16.5. The gel binder structure is able to be represented as a disordered analogue of a zeolite structure, with similar ˚ ngstr€oms.35,38,39 chemical structural features but lacking long-range order beyond a length scale of several A Detectably crystalline alkali aluminosilicate phases are also developed over time within the binder, with higher temperatures and higher water contents generally favouring higher crystallinity.35,40–43 This is significant because thermal and/or steam curing are often applied to hydroxide-activated aluminosilicate binders, which would otherwise show slow strength development at room temperature. If there is some calcium present, it appears to be able to replace a small fraction of the alkalis in a charge balancing role in the N-A-S-H gel, analogous to its role in crystalline aluminosilicate zeolites, although this is to some extent defined by kinetic considerations. If the calcium is released very rapidly into a silica-free activating solution, it can form discrete nanoscale particulates (which are presumably hydroxides, although this has not been fully confirmed) instead of becoming more homogeneously incorporated into the gel as is the case when more silica is present in the activating solution.44 The exact nature of the crystallites which form in NaOH-activated binders depends to a significant extent on the Si/Al ratio of the reactive component of the aluminosilicate precursor. When activating metakaolin, with an Si/Al ratio close to 1.0, these crystallites are predominantly members of the sodalite family of feldspathoids, particularly hydrosodalite or

Geopolymers and Other Alkali-Activated Materials

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FIG. 16.5 Sketch of a fragment of the N-A-S-H type framework structure. Purple, Si; red, Al; grey, O; green, Na. Note that the Na+ charge-balancing ions are adjacent to oxygen atoms that are linked to tetrahedral Al sites.

hydroxysodalite (Na6+ x[SiAlO4]6(OH)xnH2O) depending on the alkalinity of the system. Some zeolitic phases such as zeolite Na-A (NaAlSiO4nH2O) and/or low-silica forms of faujasite (NaAlSi1+ xO4+2xnH2O) are observed, either transiently or as stable reaction products42,45,46; this parallels the reaction sequences and crystal products observed when metakaolin and NaOH are allowed to react under hydrothermal conditions at higher liquid/solid ratios.47,48 Because fly ashes generally have a reactive Si/Al ratio greater than 1, the products of their alkali activation with NaOH will also have a higher Si/Al ratio than their metakaolin-based counterparts. This leads to formation of chabazite-Na (also known as herschelite, NaAlSi2O6nH2O) and/or faujasite, both zeolites, in addition to sodalite-group feldspathoids as in metakaolin systems.49–51 Binders activated by KOH tend to show a much lower extent of crystallinity than comparable NaOH-activated materials, across the range of relevant Si/Al ratios. Nuclear magnetic resonance (NMR) spectroscopy has proven to be a key tool in the analysis of alkali-activated binder structures, due to its ability to directly probe the bonding environments of atoms in non-crystalline phases. Most studies have focused on the analysis of the bonding environments of Si and Al37,52–56 which can be defined in terms of Qn(mAl) notation (Fig. 16.6), although important information has also been obtained from the study of non-framework species such as 23Na, 1H and 39K.57–60 NMR analysis of the silicon environments within hydroxide-activated aluminosilicate binders has shown that Q4(mAl) sites predominate, with a distribution of values of m between 1 and 4 but mainly at the higher end of this range.54,61,62 The concentration of bound hydroxyl groups (and thus the presence of silicon in Q3 environments) is low in hardened lowcalcium alkali-activated binders.58 Larger cations are preferentially bound by the gel, demonstrated by the fact that in a binder mix with an overall Na:K ratio of 1:1, the Na/K ratio of mobile species in the pore solution is much higher than the Na/K ratio of cations bound to the gel.60 This is potentially of relevance in the use of AAMs to immobilise nuclear waste, as 137Cs is both a large alkali cation and a radionuclide of some importance, and is preferentially bound into the aluminosilicate gel in the presence of smaller alkali cations according to leaching tests.63 The binder gel structure formed by combining an alkali silicate activator with a low-Ca aluminosilicate raw material is largely similar to the structure formed through hydroxide activation of the same precursor. The main chemical differences lie in the Si/Al ratios of the respective gel products, which are higher when additional Si is supplied through the activator, and this also reduces the tendency towards crystallisation, as the rearrangement of the initially formed gel is less favoured. This also carries over onto a microstructural length scale, as silicate-activated binders tend to show a more homogeneous structure OR4

Si R 3O

OR1 OR2

FIG. 16.6 Illustration of Qn(mAl) nomenclature. The value of n (0  n  4) indicates the number of R groups that are either tetrahedral Al or Si, where m (m  n) is the subset of these that are tetrahedral Al.

784 Lea’s Chemistry of Cement and Concrete

on a length scale of microns or more, whereas hydroxide-activated binders display a much more evidently particulate structure, with larger pores23,27(Fig. 16.7). This is attributed to the greater chemical and structural stability of the gels formed at higher Si content, whereas microstructural evolution of low-silica gels takes place more readily via processes related to gel syneresis (densification and expulsion of water)27 and crystallisation.64 This is also linked to the loss of strength that is sometimes observed at extended curing times in low-calcium alkali-activated binders; when the water content is high, and thus there exists sufficient mobility and space for structural rearrangement, microstructural densification can lead to a reduction in mechanical strength at later age, whereas systems produced at lower water content and/or with more silica present are much less subject to this process.64 Silicate activation of low-calcium binders is usually achieved most effectively by the addition of dissolved alkali silicates to the solid precursor; solid silicate sources have been trialled in fly ash-based binders,65,66 but tend to give slow strength development. There are various ways of manipulating the composition of a sodium silicate solution, and each has a somewhat different effect on the binder structure. Moving from a hydroxide to a silicate solution tends to lead to a higher strength, lower porosity binder,27,50,67–70 up to a solution modulus (molar ratio SiO2/M2O, where M is Na or K) of between 1 and 2, where the exact value of this optimum depends on the nature of the aluminosilicate precursor.71 This is also seen in the microstructural results in Fig. 16.7, where the sample depicted in Fig. 16.7D was synthesised with an activator modulus of 1.5, and shows both the highest strength and the most uniform microstructure among the samples analysed in that study.27 This is related to the reactivity and reaction kinetics of the silicate species provided by the activator, where excessively fast or excessively slow

Si/Al: 1.15

Si/Al: 1.40

Si/Al: 1.65

Si/Al: 1.90

Si/Al: 2.15

FIG. 16.7 SEM images showing the progression in gel structure from highly particulate to highly uniform with addition of Si to the activator; all samples depicted were synthesised from a metakaolin with Si/Al ¼ 1.15, with any additional silica above this ratio supplied by the activator. (Source: Reproduced with permission from Duxson P, Provis JL, Lukey GC, Mallicoat SW, Kriven WM, van Deventer JSJ. Understanding the relationship between geopolymer composition, microstructure and mechanical properties. Colloids Surf A Physicochem Eng Asp 2005;269(1-3):47–58; copyright Elsevier B.V.)

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reaction of these initially dissolved silicates can reduce the final strength achieved.72 An optimal Na/Al ratio in the binder appears to be around 1.035,46; this will vary for less reactive precursors where additional alkalis may be required to accelerate setting and hardening. The pore solution in an alkali-activated fly ash or metakaolin binder is essentially a concentrated alkali hydroxide solution,73 with the concentrations of silicate and aluminate species probably matching their respective equilibrium solubilities once the binder has hardened and matured. However, there are few data available to enable these solutions to be fully characterised. This high alkali concentration is important in understanding the carbonation mechanism of alkali-activated binders, as it controls the carbonate–bicarbonate ratio which defines pH through a buffering mechanism,74,75 but can also be detrimental if excess alkalis are mobile and cause efflorescence.76

16.3.3 Alternative Activators for Low-Ca Systems There are also some possible alternative activators which can be used to replace alkali silicates or hydroxides in production of low-Ca binders, as these are the components which introduce the greatest environmental cost to these materials as a whole. Of the various alternatives that have been tested, probably the most promising is a concentrated sodium aluminate solution, which can be sourced as a by-product of extractive metallurgy, and will react with a suitable fly ash to give a strong hardened binder77,78; this pathway is probably less appropriate for metakaolin as a precursor due to its already high Al content. Alkali carbonate and sulfate activators tend to give inconveniently slow strength development in the absence of high concentrations of Ca, and so these are more amenable to use in combination with higher calcium precursors or with some percentage of Portland cement clinker to produce a hybrid binder system, as will be discussed in detail below.

16.3.4 Fly Ash Chemistry in Alkali-Activated Binders As discussed in Chapter 10, fly ashes are broadly classified in both the ASTM and EN standardisation environments according to the ratio of network-forming (particularly Si and Al) to network-modifying (particularly Ca) oxides. The alkali activation of ashes which are classified as Class F under ASTM C618, or approximately equivalently as ‘siliceous’ (V) under EN 197-1, generally yields a product which is mainly aluminosilicate in nature and follows the structural descriptions presented in the preceding section. However, the factors which control the reactivity of a fly ash in alkali activation are not necessarily identical to those which control its pozzolanic reactivity, as the chemistry of the alkali activation process is different in several essential aspects compared to the pozzolanic process. Probably the most fundamental of these is the need for an ash used in alkali activation to react rapidly within the first 24 h to yield a material with a sufficiently short setting time for engineering applications; this is less important in Portland cement blends where the extent of reaction of the fly ash while the material remains in the fluid state is minimal, and where its contribution to the chemistry of the materials takes place mostly at later age.79 Additionally, the alkali activation reaction as depicted in Eq. (16.1) fundamentally involves the attack of an alkali metal-rich solution on the fly ash glass, while the pozzolanic reaction in a Portland cement-based system involves a pore solution saturated with respect to portlandite and thus with calcium playing a much more significant role in the dissolution of the fly ash particles. To this end, the amount of aluminium available in the reacting system (and particularly at early age) is crucial in a fly ashbased AAM, as in other AAMs,80 as this is the component which induces the cross-linking, and thus resistance to hydrolysis, of the aluminosilicate gel. If the reactive Al content is too low, the reaction product may be soft and lack strength, or may simply desiccate (i.e. lose its free water) to form a gel which shows acceptable mechanical strength, but which is not stable upon exposure to moisture. It has been identified that the calcium content of fly ash also plays a significant role in defining its reactivity, which is consistent with the known role of network modifying cations in defining the susceptibility of fly ash glasses to dissolution.81 To address these issues in parallel, a pseudo-ternary classification for fly ashes was developed82 based on the overall quantities in the ash of silica, alumina and a scaled sum of alkali and alkaline earth metal (i.e. glass network modifier) content, shown schematically in Fig. 16.8 and exemplified by the data set summarised in Table 16.1, which are taken from reference.34 The presence of a sufficient content of Al and of network-modifying species is a necessary, but not sufficient, condition for strength development, as the combination of these components leads to the formation of a reactive aluminosilicate glass within the fly ash. The data in Table 16.1, which are sorted by potential network modifier content (i.e. the sum of CaO + MgO + Na2O + K2O, ordered from low to high in Table 16.1) show that the ashes with an intermediate value of this parameter generally produce alkali-activated concretes with the best mechanical performance when activated under the specific conditions of the study reported.34 This set of ashes do not have a wide variation in aluminium content, so it is not possible to use these data to assess

786 Lea’s Chemistry of Cement and Concrete SiO2

Strength

Low

High

Sum of alkali and alkaliearth oxides, scaled by cation charges

Al2O3

FIG. 16.8 Schematic depiction of the correspondence between fly ash composition and binder strength. To reduce fly ash composition onto this pseudoternary diagram, compositions are renormalised to omit all components not shown. (Source: Data from Duxson P, Provis JL. Designing precursors for geopolymer cements. J Am Ceram Soc 2008;91(12):3864–9.)

TABLE 16.1 Summary of Selected Data for Alkali-Activated Fly Ash Concretes CaO + MgO + Na2O + K2O (%)

SiO2 + Al2O3 + Fe2O3 (%)

41.62 38.00 36.08 35.87 34.80 30.02 29.06 24.56 17.32 16.95 14.59 13.91 11.51 10.28 9.83 9.44 9.39 7.38 6.06

50.80 54.71 57.49 57.98 59.59 65.33 64.23 72.23 79.79 80.00 83.29 83.33 86.26 87.70 88.07 88.59 81.30 88.56 89.90

CaO (%)

Passing 45 mm (%)

Concrete Density (kg/m3)

Compressive Strength (MPa)a

Flexural Strength (MPa)b

Setting Time (min)

Activator/ Fly Ash Mass Ratio

33.39 28.47 28.53 28.07 26.19 23.53 22.45 18.72 11.66 12.93 9.23 10.60 6.90 5.64 5.48 5.01 4.64 5.00 1.97

80.9 81.2 85.9 83.8 85.7 84.3 83.0 74.2 67.3 68.8 63.2 63.1 63.0 63.8 66.2 61.7 87.5 63.5 71.3

2323 2339 2323 2323 2323 2307 2323 2339 2371 2291 2291 2355 2291 2307 2291 2307 2371 2307 2243

36.5 42.8 57.2 52.3 39.2 61.4 59.5 80.4 53.7 55.9 43.4 62.2 46.1 46.7 47.6 46.8 46.6 40.4 47.4

3.58 5.18 5.27 4.72 4.19 6.23 4.48 5.27 4.43 4.30 4.24 4.83 4.71 5.30 5.58 4.61 6.31 4.14 5.12

4 3 2 2 6 8 1.5 20 285 25 45 55 400 320 350 240 480 180 480

0.40 0.50 0.40 0.45 0.40 0.45 0.45 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.50

Determined according to ASTM C39, using cylindrical specimens, 150 mm diameter  300 mm height. Determined according to ASTM C78, via three-point flexural loading of 100  100  400 mm prismatic concrete specimens. All mixes were formulated with 494 kg fly ash per m3 of concrete, activated by a 1:1 mass ratio mixture of 14 M NaOH and a commercial sodium silicate of modulus 2.0, dosed at activator/fly ash mass ratios between 0.40 and 0.50 for each sample as shown. Each row corresponds to a different source of fly ash. Ashes with excessive loss on ignition are excluded from the analysis; all ashes shown have loss on ignition <5%. Source: Data from Diaz-Loya EI, Allouche EN, Vaidya S. Mechanical properties of fly-ash-based geopolymer concrete. ACI Mater J 2011;108(3):300–6. a

b

the influence of that parameter, although its importance in the production of an alkali-activated binder gel with high mechanical strength is well known.83 The ashes shown in Table 16.1 with higher calcium content, which did not in general lead to high concrete strengths under the activation conditions studied in34 according to the data in Table 16.1, would be expected to react well under a less alkaline environment, similar to what has been observed for alkali activation of blastfurnace slag (with a CaO content of 42%), where the combination of this precursor with a sodium silicate activator of higher modulus was observed to yield mortars with a significantly higher compressive strength.84 It is possible that a similar trend may hold for very high-calcium fly ashes—some of which are actually self-cementing, and harden adequately for use in concretes without addition of any alkalis85—although further systematic, parametric work is obviously required in this area.

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Additionally, such analysis can only offer an indication of potential reactivity of an ash, as factors such as the particle size and the unburnt carbon content of a fly ash can never be captured by such a simplified analysis, and are known to be important in defining performance during alkali activation. The particle size of a fly ash is important because the initial reaction processes take place at the particle surfaces, and so a coarse fly ash will react slowly. Mechanochemical activation86 is effective in enhancing the performance of some fly ashes, but the change from spherical to angular particles induced by grinding can have a negative influence on water demand. A high unburnt carbon content is undesirable in alkali activation because it can lead to consumption of the activator by sorption onto the high-surface area carbon particles.87 This is a key aspect in which tighter quality control of fly ashes will be able to bring much greater reproducibility in AAM production, as the unburnt carbon content of fly ashes can vary significantly between batches from a single source, often much more so than the bulk composition or particle size distribution. The fly ashes classified as Class C according to ASTM C618, and analogous ‘calcareous’ (W) ashes in EN 197-1, often contain more than 20 wt.% CaO (as much as 40 wt.% in some cases) and are less widely studied for AAM synthesis than are their lower calcium counterparts. Although ‘Class C’ and ‘high-Ca’ are often used as synonyms in the academic literature, it is important to note that the ASTM C618 specification actually defines the Class C grouping in terms of the SiO2 + Al2O3 + Fe2O3 content being between 50% and 70%, in addition to various other requirements related to particle size, sulfate and unburnt carbon content. The EN 197-1 ‘calcareous’ classification explicitly requires the ash to contain more than 10% reactive CaO, and any ash with more than 15% CaO is required to display hydraulic properties to meet this specification. Hydraulic ashes do offer good potential for use in alkali activation, but control of the heat release and possible dimensional changes caused by hydration of free lime are key considerations. As noted above, high-Ca fly ashes do not always produce AAM binders with higher strengths than their low-Ca counterparts,49,88 and the higher Ca ashes tend to offer much less robustness in mix design with regard to the range of activator compositions which can offer acceptable workability and strength development rates. This discussion highlights one of the main drawbacks associated with the use of high-calcium fly ashes: the variability between the different ashes available within this classification (whether or not complying with the Class C requirements of ASTM C618) is much greater than in the case of lower calcium ashes, and so much more careful mix design work is required for each source of ash, leading to a system which is less robust than is the case for low-Ca fly ash-based AAMs.

16.3.5 Natural Mineral Resources as Precursors Metakaolin, which is produced by calcination of kaolinite clay (Al2Si2O5(OH)4) at temperatures of around 550°C–750°C as discussed in Chapter 10, is the main natural mineral resource which is used in AAM synthesis. Calcination of kaolinite at these intermediate temperatures (below the temperature at which kaolinite recrystallises to form mullite, approximately Al6Si2O13) results in a disruption of the layered structure of the clay through dehydroxylation,89,90 producing lattice strain in the aluminate layers in particular and rendering the material highly reactive. This material is classified under Class N in ASTM C618, and group Q in EN 197-1. The use of metakaolin in production of alkali-activated binders mainly yields materials which are useful in ‘ceramic-like’ (including refractory) applications rather than as a Portland cement alternative, as its high water demand leads to difficulties in concrete production.91 Flash calcination of kaolinite at a higher temperature (exceeding 1000°C) for a very short time leads to the formation of metakaolin with a more spherical particle morphology,92,93 which appears to offer significant benefits regarding water demand, and is now being deployed at commercial scale. Blended alkali-activated binders using metakaolin in combination with a fly ash or slag as primary aluminosilicate source, and hybrid binders involving Portland cement clinker and an alkali source, will be discussed in detail below. Non-kaolinitic clays are also of interest in alkali-activation processes; a thermally treated muscovite-rich mining waste has been valorised in this way94 in the development of AAMs for various applications including concrete repair. Illitesmectite clays can be thermally treated to produce a precursor for alkali activation,95 while mechanochemical treatment of pyrophyllite (Al2Si4O10(OH)2) was more effective than thermal treatment in rendering it amenable to AAM production,96 and effective pretreatment of halloysite, a polymorph of kaolinite, can be achieved by both thermal and mechanical processes.97 In some instances, the Fe content of the clay has also been identified as being important in determining reactivity during alkali activation and the nature of the reaction products formed.98 Natural pozzolanic materials sourced from various locations worldwide99–101 have shown good performance in alkali activation. The reactive phases—both glassy and crystalline—present in volcanic ash, particularly where there is also a high amorphous silica content, mean that the natural pozzolans which are well suited to use in blends with Portland cement are also likely to be amenable to alkali activation. The content of reactive Al in many natural pozzolans is relatively low, but blending with supplementary sources such as metakaolin or calcium aluminate clinker can aid in overcoming this, and also reduces the prevalence of efflorescence.100,102

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16.4 HIGH-CALCIUM ALKALI-ACTIVATED MATERIALS The development and assessment of alkali-activated binders based on calcium-rich precursors such as blastfurnace slag (BFS) and other Ca-rich industrial by-products have been conducted for over a century.5,6 The chemistry and basic engineering aspects of alkali-activated BFS-based binders have been discussed in a number of reviews in the scientific literature, including by Krivenko and by Wang et al.,103–106 and this is the most technologically mature area of development and deployment of alkali activation technology in most parts of the world.13,107 This advanced stage of development is reflected in the much more advanced status of standardisation efforts related to high-calcium alkali-activated binders compared to other classes of AAMs. A full suite of prescriptive standards for alkaliactivated slag-based cements and concretes of different grades and purposes exist in Ukraine and neighbouring nations,108 and these have been utilised throughout that region to produce large volumes of concrete for commercial application.109 Activation of blastfurnace slag by a chemical activator is described in the Swiss national document SIA 2049,110 where the nature of the activator is not strictly specified as long as sufficient performance is achieved.

16.4.1 Activators for BFS Systems Over a period of several years, BFS will hydrate in water, even in the absence of an external alkali source, to form a hardened binder consisting mainly of aluminium-substituted calcium silicate hydrate (C-A-S-H), with secondary phases including hydrotalcite–group phases forming in parallel, depending on the details of the chemistry of the slag. The cement literature usually describes these layered Mg–Al phases simply as ‘hydrotalcite’, but in mineralogical nomenclature111 the observed compositions can include quintinite, Mg4Al2(OH)12CO33H2O and/or meixnerite Mg6Al2(OH)184H2O as well as true hydrotalcite, Mg6Al2(OH)16CO34H2O, depending on the availability of Mg and carbonate. In an alkali activation process, the activator accelerates this reaction to enable setting and hardening within an acceptable time frame, which is achieved by the generation of an elevated pH. Alkali silicates and hydroxides bring this high pH directly, while alkali carbonates generate hydroxide for the activation process through reactions involving calcium released as the BFS dissolves, and/or from added lime,6 via precipitation of insoluble calcium carbonate.112,113 Various other weakly alkaline salts can take on a similar role, most notably sodium sulfate,6,107,114 but tend to require the addition of a secondary Ca source and/or a significant quantity of Portland cement clinker as will be discussed in Section 16.5. The combination of BFS with an alkali carbonate activator and an inorganic carbonatebinding agent (e.g. calcined technical-grade hydrotalcite) has also been shown to accelerate the setting and hardening compared to a carbonate-activated binder without the mineral additive,115 forming additional hydrotalcite-group minerals in the hardened binder which can then contribute to generation of desirable durability properties.116 The silicate activators used in the production of BFS-based binders are usually supplied as a commercial sodium silicate (‘waterglass’) solution, or as a spray-dried powder. If a powdered activator is selected (which is often convenient for transport), it is then usually dissolved in the mix water prior to combination with the BFS powder to avoid complications related to the sometimes slow dissolution of alkali silicate powders. An alkali silicate powder can also be blended or interground with the BFS, which reduces occupational health and safety complications related to the handling of large volumes of alkaline solutions, but can result in greater variability in early strength.65,117 Nonetheless, the use of powdered sodium silicate in conjunction with a lime slurry to activate BFS has been demonstrated at concrete scale, providing high early strength and good slump retention.118,119 Various alternative sources of silicate for use in the activator may also provide advantages in terms of price and/or environmental footprint in some circumstances. Silica fume or nanosilica,120,121 rice husk ash,122 and siliceous glass waste123 have been assessed as substitute silica sources introduced into the mix by dissolution or leaching in NaOH, with some success. The inclusion of soluble silicate in the activator tends to be very beneficial to the properties of the final binder compared to the use of hydroxide alone, as the silicate present in solution is able to rapidly complex with the calcium as it is released from the BFS particles, which enhances the progress of the reaction and enables a higher final extent of BFS dissolution to be achieved. The complex rheology of silicate-activated binders does present a disadvantage; these mixes can show significant thixotropy124 and have additional shear rate dependencies in rheological properties which mean that they cannot be described straightforwardly via the Bingham yield stress fluid model.125,126 Sodium carbonate activation of BFS has formed the basis of many of the AAM concretes which have been used commercially for the past 50 years in eastern and central Europe,103,107,127 as it is a more readily available and lower cost component than hydroxide or silicate activators, although at the cost of a slower reaction process due to its relatively moderate pH. Finely ground limestone has been demonstrated to add useful properties to Na2CO3-activated binders, in terms of both performance and cost,128,129 where binders with 3-day strengths exceeding 40 MPa and a claimed 97% reduction in CO2equivalent emissions compared to Portland cement have been described in the scientific literature.

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16.4.2 Binder Structure in High-calcium AAMs As mentioned above, when BFS is activated by an alkaline solution, the main reaction product which gives strength and coherency to the binder is a C-A-S-H type gel with a disordered tobermorite-type structure.104,130–134 Roy et al.135 who activated BFS with a range of different alkali and alkali-earth metal hydroxide solutions at a fixed pH, identified similar reaction products in each of these systems. A wide range of secondary reaction products will also form, depending on the chemistry of the slag and the activator,20,136 particularly calcite (CaCO3) in alkali carbonate-activated binders,128 hydrotalcite-group (Mg, Al)-layered double hydroxides when sufficient MgO is present,137–139 and sometimes also calcium-containing layered double hydroxides and/or highly cross-linked aluminosilicate products of low crystallinity. Distinct zeolitic reaction products are also sometimes observed as activation products when the BFS has a very low Mg content, commonly in the gismondine type structure family (Ca1–xNa2xAl2Si2O8nH2O with varying degrees of Na/Ca substitution),140 while thomsonite (NaCa2Al5Si5O206H2O) has also been observed in aged activated BFS pastes.141 Additional phases which have been observed in silicate-activated BFS include siliceous hydrogarnets such as katoite (Ca3Al2(SiO4)3–x(OH)4x with 1.5 < x < 3),55 and sometimes also distinct crystalline AFm phases including str€atlingite (Ca2Al[AlSiO2(OH)10]2.5H2O).142,143 After several years of curing of an alkali-activated BFS concrete, veins of a phase that appears to be a disordered calcium hydroxide product have also been observed, consistent with ongoing calcium release from remnant slag particles and potentially offering some degree of crack closure within the mature material.144 The C-A-S-H is often partially cross-linked (containing Q3 site environments, Fig. 16.9), which means that accurate calculation of the mean chain length, Al/Si ratio and degree of cross-linking in the C-A-S-H gel requires deconvolution of 29Si magic angle spinning (MAS) NMR spectra, followed by the use of these results in a calculation model which describes the different structural constraints of both cross-linked and non-cross-linked tobermorite units.132 The degree of cross-linking tends to be lower in the C-A-S-H gels formed in NaOH-activated BFS than when a silicate activator is used,130,145 while the Ca/(Si + Al) ratio of the hydroxide-activated system will be higher because no extra Si is supplied by the activator. However, independent of the activator used, the C-A-S-H type gel formed through the activation of BFS has a lower Ca content than a hydrated Portland cement system. Tetrahedral aluminium in C-A-S-H gels is mostly present in bridging sites within the chains,132,146–149 and this is the main site at which cross-linking between the silicate chains takes place. Alkali substitution into charge-balancing sites is also very possible in these gels, in competition with calcium and hydrogen cations in the interlayer region.134,146 This information regarding the thermodynamic preferences for substitution into different sites within the gel structure has been used, along with an extensive body of literature data describing solubility in the C-N-A-S-H quinary system, to develop an ideal solid solution model for the alkali-substituted C-A-S-H gel,146 enabling more accurate description of the phase assemblage in alkali-activated BFS systems through geochemical-type calculations.150 The pore structure of alkali-activated binders has been described in the scientific literature as falling anywhere in the range from highly impermeable to extremely permeable. This obviously depends on the mix design (particularly water content), and the methods of sample preparation and measurement, as the preconditioning of samples via a harsh drying regime will induce cracking of the samples prior to analysis.151–154 An increasing activator dose tends to give refinement of the pore structure of BFS-based binders,155 similar to the case for fly ash-based materials.70

16.4.3 Pore Solution Chemistry Considering the very high pH of most alkali-activated binder systems in the fresh state, it may appear that it should be fairly straightforward to retain a sufficiently high pH to passivate steel reinforcing. However, the fact that the pH is not buffered by

FIG. 16.9 Schematic representations of cross-linked and non-cross-linked tobermorite structures which represent the generalised structure of the C-(N)-A-S-H type gel. Sites occupied by Al are shown in red, and silicate sites are colour coded as follows: Grey, Q3; Dark blue, Q3(1Al); Green, Q2; Light blue, Q2(1Al); Yellow, Q1. (Source: Modified from Myers RJ, Bernal SA, Provis JL, Gehman JD, van Deventer JSJ. The role of Al in cross-linking of alkali-activated slag cements. J Am Ceram Soc 2015;98(3):996–1004.)

790 Lea’s Chemistry of Cement and Concrete

dissolution of a solid hydroxide phase (the role filled by portlandite in hardened Portland cement) means that if high alkalinity is to be retained in service, durability of a reinforced AAM concrete element depends even more strongly on the transport properties of the binder itself,13 particularly in preventing alkalinity loss through leaching or carbonation.73,74 Similar to the case for low-calcium systems as mentioned above, the pore solution chemistry of BFS-based alkaliactivated materials is fundamentally controlled by a high concentration of alkali hydroxides, with pH generally between 13–14, and the concentration of dissolved silicates falling to sub-millimolar levels (limited by the solution reaching saturation with respect to the C-A-S-H gel146) in a mature binder, as summarised in Table 16.2. Dissolved Al concentrations are also approximately in the millimolar range, while the collected data for calcium concentrations are rather variable but also generally around the millimolar range (Table 16.2). Reports of dissolved sulfur concentration and speciation are relatively scarce, but this element (present in either oxidised or reduced form, and supplied by the BFS156,160) can be a very significant contributor to the pore solution chemistry at concentrations exceeding 100 mM, which is particularly important in calculations of parameters such as activity coefficients which depend on ionic strength (sum of squared charges) rather than simply concentration as many of the sulfur-containing species are divalent. The sulfur concentration and speciation within the pore solution of BFS-based binders also control the redox environment within the material, where BFS inherently generates a reducing environment, with Eh (redox potential) values as negative as 400 mV.161 This is important in defining the relationship between the pore solution and any embedded steel reinforcing elements in an AAM.162

16.4.4 Effects of BFS Characteristics Blastfurnace slags from different continents can show significant diversity in chemistry depending on the nature of the ore and fluxes being processed in the blastfurnace, the construction and operating conditions of the furnace and various other parameters such as the rate of quenching (see Chapters 2 and 11). The standard specifications for slags to be used in Portland cement based blends are generally based on performance tests with some limited consideration of chemistry according to pass/fail criteria on a single calculated compositional ratio, The ratios specified in different parts of the world vary greatly, and have been designed from the basis of many different chemical theories regarding the precise nature of the glassy phase in slags107; for example, EN 197-1 requires that the mass ratio (CaO + MgO)/SiO2 in the slag must exceed 1.0 to ensure that the slag is sufficiently basic for use in blends with Portland cement. However, greater subtlety in understanding of the chemistry of the slag itself is essential in enabling the optimisation of AAM mix design and performance, because there is no clinker component to moderate differences in hydration chemistry between slags in the case of an AAM.117 The slags used in alkali activation are overwhelmingly amorphous, and the minor (<5%) crystalline content of some commercial slags does not appear to interfere significantly with the reaction process because there is generally sufficient glass to enable the reaction to proceed. A finer slag also brings more rapid reaction, but this can come at the cost of a higher water demand and/or reduced workability if the particle size is too small.106,117,163,164 An increased content of MgO in alkali silicate-activated BFS tends to give higher compressive strength137,165 as well as improved resistance to carbonation.139 This has been associated with the formation of larger quantities of hydrotalcite-group phases as the final destination of the Mg released by the BFS as it dissolves, which then reduces the Al content of the C-A-S-H gel and leads to a structure more similar to that of the C-S-H formed by Portland cement hydration.139 However, this was not the case for NaOH-activated BFS binders, where only slight variations in strength were observed as a function of MgO content.137 In blastfurnace slags with different Al2O3 contents, an increase in Al2O3 reduces the extent of reaction at early times of curing, and consequently decreases the compressive strength of activated BFS binders, but shows little influence on later age properties other than the formation of a small amount of str€atlingite.166 It has been identified that the extent of slag hydration is the key factor determining mechanical properties across a range of slag and activator compositions.167 The selection of activator type and composition is dependent on the specific slag chemistry in order to give optimal strength development,107,168,169 for example, it is known that a more basic slag requires a higher modulus waterglass for optimum strength development.117

16.4.5 Non-Blastfurnace Slag Precursors Steel production, from iron and/or recycled scrap, is a multi-stage process which results in the production of a variety of slags. Several of these slags contain hydraulic or latent hydraulic compounds, and can display very good binding properties under the action of a proper alkali-activator to enhance early strength development and final binder performance,170–172 although the significant free lime content of many of these slags causes challenges in appropriate matching of activator and precursor to achieve a dimensionally stable binder.

TABLE 16.2

Pore Solution Chemistry of Alkali-Activated BFS as Reported in the Literature 156

157

Activator

Sodium silicate

Concentration (mM) Si Ca Al Na Mg K Sa OH

1 day

7 days

28 days

180 days

Sodium silicate 7 days

7 days

11 0.3 7 1500

8.5 0.4 4.8 1300

9.6 0.4 4.3 1400

5.9 0.5 3.2 1300

75 0.3 3.7 1200 0.1

7.1 0.2 4.8 1800 0.01

29 150 340

38 403 270

45 319 340

48 420 340

NaOH

73

158

Sodium silicate 90 days

1 M NaOH

0.5 M NaOH

0.1 M NaOH

NaOH

28 days

56 days

28 days

56 days

28 days

56 days

28 days

Sodium silicate 28 days

0.7 0.5 0.3

0.7 0.5 0.3

0.5 0.8 0.2

0.4 1 0.2

0.1 8 0.1

0.2 9.4 0.09

0.5 0.1 <0.01

0.18 <0.01

263

263

31

26

13

11

<1 <1 3617

159

77 278

a Gruskovnjak et al.156 also reported sulfur speciation: S(VI) remained approximately constant at 41 6 mM from 1 to 180 days, while S(IV) increased from 16 to 24 mM, and S(-II) increased from 87 to 350 mM during this time.

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Reference

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792 Lea’s Chemistry of Cement and Concrete

There are numerous other pyrometallurgical processes operated worldwide producing non-ferrous metals which result in the production of various volumes of silicate-based slags. Many studies of the alkali-activation of non-ferrous slags have been published, with the focus usually on parametric mix design using a particular slag available in a specific region. Such work has been published for slags resulting from the production of phosphorus,173 nickel,174 ferrochrome,175 ferronickel176 and various other metallurgical processes.177,178 Care is required to avoid problems related to the release of toxic components, particularly hexavalent chromium, and radioisotopes from some of these slags, and the available volumes of material tend to be low and localised. Nonetheless, even materials containing problematic contaminants can find application for niche purposes such as in immobilisation or encapsulation of other hazardous or radioactive wastes, in landfill capping, and in other scenarios in which interaction with the biosphere is restricted, subject to sufficient quality control in the raw materials supply and binder production.107

16.5 INTERMEDIATE CALCIUM SYSTEMS The ability to gain attractive synergies, both in technical performance and in environmental footprint, through the combination of high-calcium and low-calcium precursors in an alkali-activated binder system, has generated much interest in this area in recent years. These binders make use of the stable coexistence of tobermorite-like C-A-S-H gels and alkali aluminosilicate N-A-S-H gels (sometimes also named ‘geopolymeric gel’ in this context) to produce a binder which can offer some of the advantages of each type of system, for example, the excellent chemical resistance of the N-A-S-H type gel and the space-filling and pore-blocking capabilities of the C-A-S-H gel.13,179,180 The use of less reactive aluminosilicate components (e.g. a low-grade fly ash) in combination with alkalis plus a more reactive calcium-rich precursor also offers scope for the valorisation of otherwise low-utility materials.

16.5.1 Gel Coexistence in Blended Binders The main target in the design of a blended alkali-activated binder is usually to achieve the stable coexistence of C-A-S-H and N-A-S-H gels. This requires manipulation of reaction kinetics of both the Ca-rich and Ca-poor solid precursors; if calcium is released too rapidly into an alkaline solution without sufficient silica it can precipitate as portlandite which restricts the formation and structural development of C-A-S-H phases, while if the early reaction of the Ca-rich component consumes too much of the alkali from the activator, a less reactive aluminosilicate may remain largely unreacted throughout the process of setting and hardening.181–185 Given the difficulties associated with directly synthesising and analysing intermixed C-A-S-H and N-A-S-H gels to generate a detailed scientific understanding of phase relationships in blended binder systems, the understanding of gel coexistence has been enhanced by the study of synthetic mixtures in which gels are synthesised independently, and then either modified through addition of calcium, alkalis or aluminium, or the two gels combined together to observe compatibility.186,187 The simultaneous incorporation of aluminium and alkalis into an initially pure C-S-H gel leads to cross-linking of the tobermorite chains and the crystallisation of str€atlingite as the alumina content saturates the C-A-S-H bridging sites, but not the formation of an identifiable discrete N-A-S-H gel.186 Upon mixing of synthetic C-S-H and N-A-S-H gels in water,187 the two gels appeared to remain stable and segregated into discrete regions on a length scale of tens of nanometres, rather than showing full chemical intermixing, indicating that gel coexistence may be thermodynamically driven. This is consistent with results which identified a secondary N-A-S-H phase in an Al-rich alkali-activated slag binder system,132 as well as data for blended fly ash-slag alkali-activated binders where the two discrete types of gel chemistry were evident in spatially resolved elemental analysis at late age, rather than forming a homogeneously intermixed gel product (Fig. 16.10).188 Sodium aluminate activation of a high-calcium fly ash,189 blending of metakaolin with BFS and waterglass140,182,190 and activation of synthetic powder precursors185 have also been reported to result in coexistence of N-A-S-H and tobermorite-like C-A-S-H products. The other important factor to consider in a binder system with two coexisting gel phases is the difference in the role of the alkali cation between the two gel types. The alkali cations associated with the N-A-S-H gel structure are an essential and intrinsic component of the gel, occupying non-framework sites and charge balancing the tetrahedral aluminium. These cations are to a significant extent exchangeable when the material is synthesised at high water content,191 but are not as freely removed under conditions such as high-pressure pore solution extraction or leaching in pure water, where there is no incoming cation to replace the alkali in its charge balancing role. Conversely, the alkalis in the C-A-S-H gel occupy a role in the interlayer which can more favourably be filled by calcium; this is made particularly evident by the fact that the addition of more BFS to a blended binder tends also to increase the Na+ concentration of the pore solution which is extracted by leaching tests,73 showing that the predominance of the C-A-S-H gel within the binder reduces the overall degree of binding

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793

SiO2

Unreacted slag 100 slag 75 slag/25 fly ash 50 slag/50 fly ash 25 slag/75 fly ash 100 fly ash Unreacted fly ash

CaO

Al2O3

FIG. 16.10 Pseudo-ternary phase diagram developed from SEM-EDX data for slag-fly ash blended binders, produced using sodium metasilicate (Na2SiO3) as an activator. (Sources: Data from Ismail I, Bernal SA, Provis JL, San Nicolas R, Hamdan S, van Deventer JSJ. Modification of phase evolution in alkali-activated blast furnace slag by the incorporation of fly ash. Cem Concr Compos 2014;45:125–35; Diagram courtesy of S. Bernal.)

of the alkalis. This observation raises questions related to why the addition of a calcium source has been seen to be effective in reducing the effects of efflorescence in AAMs,102 but it seems that the partial closure of the pore structure which is induced by the formation of the C-A-S-H gel is sufficient to counteract the effects of additional alkalis present in the pore volume that does remain.

16.5.2 Activators for Intermediate Ca Systems The optimal selection of activators for an intermediate-calcium AAM binder depends very strongly on the nature of the precursor blend used to produce the material, as it is imperative that the compatibility between all components leads to desirable workability and setting behaviour as well as high final strength. An alkali silicate activator with modulus between 1 and 2 is probably the most versatile option in terms of giving good strength development across a wide range of binder compositions, but is also relatively expensive and can sometimes give mixes with problematic workability. Alkali hydroxide activation can give very good results for low-calcium binder systems, while carbonate or sulfate activation in the presence of a rapidly reacting secondary calcium source (effectively generating alkali hydroxides in situ) can also give good performance if the mix composition can be balanced appropriately. The following sections will address various popular combinations of precursors which are used in the design of such mixes.

16.5.2.1 Aluminosilicate + Ca(OH)2 + Alkali Source The alkali–lime (or portlandite)–aluminosilicate binding systems can be viewed in many cases as a modification of classical lime-pozzolan cements, with the addition of an alkali source to accelerate reactions and enhance reaction of the aluminosilicate component.107 When a very high activator concentration is used in a blend of portlandite and an aluminosilicate (fly ash, metakaolin or natural pozzolan), the portlandite does not contribute significantly to the reaction process as its solubility is suppressed at high pH, and the main reaction product is a N-A-S-H based binder.183 At a lower activator concentration in hydroxide, silicate or carbonate-activated binder systems, C-A-S-H gel is formed in parallel with the N-A-S-H product, with their relative predominances (and thus influence on the properties of the binder) determined by the ratio of the Ca source to the aluminosilicate source.183,184,192 The use of a sulfate activator in such systems results in the formation of AFt and/or AFm phases as secondary products, which incorporate much of the available Al in the reacting mixtures and provide a valuable space-filling effect,193–196 although the hardening and strength development of such systems can be rather gradual.107 The activation of low-Ca fly ash with Ca(OH)2 and NaOH yields a binder based on C-A-S-H and katoite.197

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16.5.2.2 Calcined Clay + BFS + Alkali Source When the calcium source in a blended binder is BFS, careful consideration of its dissolution kinetics is required in optimising the performance of the material as a whole. The first solid component to react is usually the BFS, particularly the fine particles present.198 At very high alkalinity, the availability of Ca from the BFS particles is restricted by the blockage of the BFS particle surfaces by formation of early-age reaction products, and the low solubility of Ca2+ at high pH.182 When the initial activator pH is more moderate—either through the use of a silicate solution with a modulus exceeding 2, or by use of a carbonate activator—the release of calcium from the BFS particles is promoted by the formation of low-solubility calciumcontaining phases such as CaCO3 in carbonate-containing systems, or C-A-S-H gels in the presence of dissolved silica. These reactions lead to an increase in pH through the release of hydroxide ions, which activates the aluminosilicate precursor and generates the N-A-S-H gel component of the coexisting binder phase assemblage. The mechanical strength development of the binder is often highest in the case where the coexistence of C-A-S-H and N-A-S-H gels is reached,190,199 although this is not universally true.84 Because the early reaction is controlled by the BFS, the activators used in these systems often follow the formulations used for BFS alone, and in particular the use of sodium silicate as an activator is popular in such binder systems. Within this compositional regime, binders richer in metakaolin can benefit from the use of a higher activator modulus (molar ratio SiO2/ Na2O) to supply additional Si to form stable reaction products with the high levels of Al made available by the metakaolin; correspondingly, the BFS/metakaolin ratio required for optimal compressive strength development tends to increase with activator modulus.84,200–202 16.5.2.3 Fly Ash + BFS + Alkali Source The possibility to combine fly ash, BFS and a source of alkalis to produce an AAM binder has been the subject of interest since 1977,203 which pre-dates the published research into solely fly ash-based systems. These materials can show 28-day strengths approaching 100 MPa, which has attracted significant interest from industry. However, due to the chemical and mineralogical complexity of these materials, they are only recently becoming better understood and more deeply analysed in the scientific literature. The addition of blastfurnace slag is known particularly to improve the early-age strength development of a mostly fly ashbased binder, much more so than highly reactive calcium sources such as lime, because the progressive release of calcium can induce the formation of C-A-S-H gel binding phases in the presence of a silicate activator, and/or silica released by the slag and fly ash, whereas simply adding the same quantity of calcium in a rapidly soluble form would lead to its precipitation as a hydroxide. For example, addition of 4% BFS was sufficient to improve the 14-day compressive strength of a 90% fly ash/10% metakaolin blend by more than 40%.204 The binder phase assemblage in an alkali-activated fly ash–BFS blend is strongly dependent on the blending ratio of the two solid precursors,188,205,206 as illustrated in Figs 16.2 and 16.10, and the mechanical strength,205 pore structure,70,180 chloride migration and diffusion rates153 and resistance to acid attack207 are therefore also influenced by this parameter. The main reaction products are coexisting C-A-S-H and N-A-S-H gels,188,205,206 and this coexistence becomes particularly evident under accelerated carbonation conditions where the chain structure of the C-A-S-H gel is significantly damaged by the conversion of its calcium to CaCO3, while the highly coordinated network structure of the N-A-S-H component is much less altered on a nanostructural level under these conditions.55 The detailed understanding of the gel structures in blended fly ash-BFS systems, and in complex blended AAM systems in general, is now beginning to benefit from advances in highresolution analytical instrumentation which can be applied to the analysis of these materials, but there is still a large volume of work remaining to be conducted in this area.

16.5.3 Hybrid PC–Alkali–Aluminosilicate Binders Arguably the most complex among the various blended AAM binder systems available are those which contain some level of Portland cement clinker; nonetheless, these materials were successfully commercialised in North America in the 1980s under the trade name Pyrament,208,209 meeting with acceptance and support from both military and government specifiers.208,210,211 From a practical perspective, an attractive feature of such materials is that the alkaline component can often be provided as a solid, with the clinker providing the driving force to initiate the reaction process and generate desirable early-age properties, while the alkali source and the aluminosilicate component react at later age with each other, and with the portlandite made available through clinker hydration. More recent interest in low-CO2 cement and concrete production has further driven interest in hybrid binders.179 Particular interest has been generated by systems which use sodium sulfate as the alkaline component, in combination with

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50%–70% fly ash or metakaolin, and the remainder Portland cement (clinker with or without gypsum),212,213 sometimes with the addition of finely ground limestone as a secondary calcium source.214 Binders with Portland cement as the primary reactive component, a relatively high volume of fly ash, and a small addition of NaOH as an accelerator, have also been described in the scientific literature, with particular emphasis on the development of rheology-control admixtures which are stable and effective in the highly alkaline pore solution present in such a binder.215 In addition to Portland cement, the other type of clinker which has been used as a secondary calcium source in alkaliactivated hybrid binder systems is calcium aluminate cement, which can be blended with an alkali activator along with aluminosilicates such as calcined clays216 or natural pozzolans.102 The calcium aluminate cement acts as a highly reactive source of both calcium and aluminium, and so is suited for use in combination with a main aluminosilicate precursor which is lacking in one or both of these elements. When used in low fractions, that is, essentially as a mineral admixture rather than as a major binder component, calcium aluminate cement contributes Al to the growth of N-A-S-H phases, and Ca as a minor charge balancer in those phases, rather than showing the formation of its usual crystalline hydration products.102,216

16.6 ADMIXTURES IN ALKALI-ACTIVATED BINDERS As described in Chapter 14, there exists a suite of highly developed and optimised organic admixtures which are effective in manipulating the properties of Portland cement-based materials in both the fresh and hardened state. However, the influence of the majority of these components on the properties of AAMs has not been studied systematically in the academic literature. The available studies have generally shown that admixtures behave differently in controlling the properties of AAMs compared with Portland cement-based binders. Because of the complex rheology of many alkali-activated materials in the fresh state, there have been several attempts to develop or apply organic admixtures to improve early age workability. Probably the first such detailed study was related to the Finnish ‘F-Concrete’ in the 1980s217,218 where various ‘F-activators’ were used; in one notable case, an admixture containing lignosulfonate plus small quantities of sodium gluconate and tributyl phosphate was used to enhance flow, control setting and reduce air entrainment of a slag-based binder activated by a mixture of NaOH and Na2CO3.218 Some of this complexity is illustrated in Fig. 16.11, where it is seen that even the Herschel–Bulkley model cannot fully describe the flow curves (shear stress–shear rate relationships) of three pastes based on BFS, with the addition of PCE in the absence and presence of an alkali activator. The PCE is seen to plasticise this particular system, and the activator then gives a further reduction in yield stress. It is evident that the application of the Bingham model to these data could not give physically

FIG. 16.11 Flow curves for pastes of ground granulated blastfurnace slag (BFS) blended with water alone, with water plus a polycarboxylate ether (PCE), and with an alkali-activator added. Model fits (solid lines) are calculated using the Herschel-Bulkley model, with exponents of 0.91 (BFS + water), 1.38 (BFS + PCE) and 1.17 (BFS + PCE + activator) for the three systems depicted. (Source: Data courtesy of V. Anderson and A. Wilson, Schlumberger Gould Research, in conjunction with P.C. Hewlett and M. Liska, David Ball Group plc.)

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meaningful predictions of the yield stress; in fact, Bingham model fits to data for BFS-based binders in the academic literature have occasionally resulted in negative calculated yield stresses due to the curvature in the flow curve at low shear rates. A negative yield stress is clearly an unphysical result and indicates that the model is unsuitable. However, the effectiveness of flow control admixtures appears to be much more limited in alkali-activated pastes containing sodium silicate as activator, across various different types of molecular architecture and functionality which comprise the available toolbox of admixtures for Portland cement-based materials.165,219 The sodium silicate itself shows significant plasticising performance in BFS-based pastes,126 but only limited gains beyond this point have been demonstrated to date. Various studies of laboratory-synthesised AAM concretes have appeared to show some flow-enhancement effect from the admixtures, but it is not clear whether this is a true plasticising effect, or more related to air entrainment and/or set retardation. Among the available options, commercial polycarboxylates have shown some potential value in enhancing paste rheology,219,220 and tailoring of the molecular architecture of the polycarboxylates to suit the particular surface chemistry of specific alkali-activated binder systems has led to the development of molecules which are relatively effective in systems including a silicate-activated slag221 and a hybrid NaOH-activated Portland clinker–fly ash binder.215 The surface chemistry of the solid aluminosilicate component,126,215,222,223 as well as the high pH of the activator solution which can cause hydrolytic degradation of the polymer molecules,224 have been shown to be key factors influencing the performance of these admixtures. In particular, the surface charge of the particles (both magnitude and sign) has been identified to differ in alkali-activated slag systems as a function of the level of alkali silicate or hydroxide addition,126 as well as the magnitude varying by as much as a factor of three between different sources of fly ash.225 The zeta potential also varies between each different face of a kaolinite crystal, particularly between the silica face, the alumina face, and the edge surfaces226; although data do not currently seem to be available for the different surfaces of a metakaolin particle after dehydroxylation, it is likely that the situation would be similar in this instance also. The diversity (and sensitivity) of zeta potential values across the range of useful precursors to alkali-activated materials opens significant scope for important future developments in this area. The control of setting time in AAMs can be achieved through the use of a wide range of organic and inorganic additives, depending on the chemistry of the binding system. For calcium-rich systems, retardation can be achieved through the addition of components which bind the calcium and prevent C-A-S-H formation, such as phosphates and borates,227–229 or in most AAM systems by reducing the activator dose or concentration. Acceleration is usually achieved through an increase in activator dosage or by the addition of a rapidly dissolving calcium source (particularly lime). An increase in temperature is also generally effective in accelerating the setting and hardening process, although sometimes at the cost of a slight reduction in final strength.

16.7 PERFORMANCE, DURABILITY AND OPEN QUESTIONS The deep understanding of binder chemistry developed during more than 100 years of work in alkali activation in academia and industry has provided strong confidence in the performance and durability of these materials in service. For applications with demanding mechanical load requirements, the strong bond of the AAM paste to steel reinforcing230–232 and the high flexural strength that can be achieved by alkali-activated concretes at a given compressive strength grade233 may also offer attractive possibilities. The excellent resistance of AAMs to aggressive conditions such as acid, sulfate and thermal load has been demonstrated18,234,235; in each of these cases, the relatively low content of calcium of an AAM binder when compared with Portland cement is seen as being advantageous, as the mechanisms of degradation which prevail under each of these sets of conditions are specifically related to the breakdown or alteration of calcium-rich binder products which are either not present, or not dominant, in AAMs.236 A particular example of this was described by Ismail et al.237 who observed that immersion in Na2SO4 solution induced almost no chemical or microstructural alteration in AAMs based on a 1:1 fly ash–BFS blend. The relatively low-calcium nature of the binding phases, and specifically the absence of AFm phases from the AAM phase assemblage, meant that the expansive processes observed upon exposure of hydrated Portland cement to Na2SO4 solution did not take place, and the material remained undamaged. It should be noted that low calcium content cannot be considered a panacea for durability problems; Lloyd et al.207 observed an improvement in the resistance of AAMs to attack by acids when calcium was supplied by addition of BFS, compared to binders based on fly ash alone, linked to the reduced permeability of the BFS-containing binders. In the general case of material design for high performance and durability, there will be a compromise between the development of a binder with intrinsically high resistance to the degradation processes which are specific to calcium-rich phases (particularly structural damage through decalcification), and the generation of a refined pore structure with space-filling hydrate compounds, which generally requires the presence of sufficient calcium. Each different mode of degradation will require a different balance between chemistry and microstructure to provide optimal performance—and in fact this is also true when

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comparing different test methods designed to probe each mode of degradation. Factors such as the question of whether a test is accelerated or extended, static or dynamic, and continuous or cyclic, will lead to different results and recommendations regarding ‘optimal’ binder chemistry in terms of durability performance.15 This means that it is essential to consider the correspondence (or lack thereof) between test and service conditions when selecting a binder composition for a specific service environment.236 This is true for other classes of cements as well as AAMs, but becomes particularly critical for AAMs due to the strong relationships between chemistry and microstructure in these materials. However, there are still some remaining issues in the area which require further scientific attention; these include:
It is essential to control curing conditions for AAMs to mitigate drying shrinkage, which can otherwise be a significant issue.155,238,239 Propylene glycol-based shrinkage-reducing admixtures240 and internal curing agents241 have shown valuable characteristics in this regard. The interactions between shrinkage and cracking, and the role of concrete mix design in controlling both of these characteristics, also require further attention.242,243 It is also noted that chemical processes leading to microstructural evolution in the binders can also manifest as autogenous shrinkage at relatively early age; this may or may not induce cracking,244,245 depending on the creep properties of the material.246
As indicated in the discussion above, the selection and optimisation of testing methodologies for durability analysis of AAMs requires careful attention; these materials are often tested according to the same protocols used for Portland cement-based materials, including parameters such as curing conditions, sample preconditioning methods and age at testing. These conditions will not always have the same influence on structural evolution (including issues such as microcracking under drying environments during preconditioning154) when the binder chemistry is changed away from a Portland cement-based system; this issue is not limited to AAM binders, but has become particularly evident in the case of AAMs as these materials have become a prominent alternative to Portland cement in infrastructure applications requiring detailed service life predictions. The question of carbonation in alkali-activated concretes appears to require particular care in the design of testing methods, as acceleration of this degradation process in a truly representative way (i.e. retaining the correct mechanisms and pore solution chemistry) is extremely difficult to achieve.74 Many accelerated tests appear to show very poor performance of AAMs under carbonation attack, whereas this is not necessarily the case in actual service conditions, as the highly alkaline pore solution interacts specifically with the high concentrations of CO2 used in accelerated testing to produce reaction products and mechanisms which do not accurately represent performance in service.
The development of admixtures which are specific to the chemistry of AAMs is still in its infancy, but is essential if these materials are to find broader application in cast-in-place concreting, and also for the production of very high-performance concretes at low water content.
The corrosion chemistry of steel reinforcing elements within alkali-activated concretes requires further analysis to enable reliable service life predictions.
The standardisation of alkali-activated binders and concretes, ideally via performance-based specifications, appears essential in enabling their broader uptake in most jurisdictions.247 Related to this, the characterisation and quality control of waste-derived precursors is essential; the same is true for such materials when used as supplementary cementitious materials in Portland cement blends,248 but the situation for AAMs is more pressing due to the absence of clinker in these materials.
Accurate life cycle assessment of alkali-activated concretes requires further geographic specificity and precision in both inventory and process-related calculations. The emissions profile of alkali activation is highly region specific due to differences in the processes of production of activators, and in generation and/or transport of precursors,249 which raises the need for numerous localised case studies to give an accurate representation of the savings which are achievable through the uptake of this technology.

REFERENCES Provis JL, van Deventer JSJ, editors. Alkali-activated materials: state-of-the-art report, RILEM TC 224-AAM. Dordrecht: Springer/RILEM; 2014. J.L. Provis, Alkali-activated materials. Cem Concr Res, in press, https://doi.org/10.1016/j.cemconres.2017.02.009. Habert G, Ouellet-Plamondon C. Recent update on the environmental impact of geopolymers. RILEM Tech Lett 2016;1:17–23. Whiting J. Manufacture of cement. U.S. Patent 544,706; 1895. K€ uhl H. Slag cement and process of making the same. U.S. Patent 900,939; 1908. Purdon AO. The action of alkalis on blast-furnace slag. J Soc Chem Ind Trans Commun 1940;59:191–202. Buchwald A, Vanooteghem M, Gruyaert E, Hilbig H, de Belie N. Purdocement: application of alkali-activated slag cement in Belgium in the 1950s. Mater Struct 2015;48(1-2):501–11. 8. Glukhovsky VD. Gruntosilikaty (Soil silicates). Kiev: Gosstroyizdat; 1959. 154 pp.

1. 2. 3. 4. 5. 6. 7.

798 Lea’s Chemistry of Cement and Concrete 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.

19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42.

Davidovits J. Geopolymers—inorganic polymeric new materials. J Therm Anal 1991;37(8):1633–56. Bennett DFH. Innovations in concrete. London: Thomas Telford; 2002. Wastiels J, Wu X, Faignet S, Patfoort G. Mineral polymer based on fly ash. J Resour Manage Technol 1994;22(3):135–41. Patfoort G, Wastiels J, Bruggeman P, Stuyck L. Mineral polymer matrix composites, In: Proceedings of brittle matrix composites 2 (BMC 2). Cedzyna, Poland: Elsevier; 1989. van Deventer JSJ, Provis JL, Duxson P. Technical and commercial progress in the adoption of geopolymer cement. Miner Eng 2012;29:89–104. Duxson P, Provis JL, Lukey GC, van Deventer JSJ. The role of inorganic polymer technology in the development of ‘Green concrete’. Cem Concr Res 2007;37(12):1590–7. Provis JL, Palomo A, Shi C. Advances in understanding alkali-activated materials. Cem Concr Res 2015;78A:110–25. ASTM International. Standard specification for portland cement, ASTM C150/C150M-18. West Conshohocken, PA; 2018. British Standards Institution. Concrete—complementary British Standard to BS EN 206-1—part 2: specification for constituent materials and concrete, BS 8500-2:2015 +A1:2016. London; 2016. Abora K, Belen˜a I, Bernal SA, Dunster A, Nixon PA, Provis JL, Tagnit-Hamou A, Winnefeld F. Durability and testing—chemical matrix degradation processes. In: Provis JL, van Deventer JSJ, editors. Alkali-activated materials: state-of-the-art report, RILEM TC 224-AAM. Dordrecht: Springer/ RILEM; 2014. p. 177–221. ASTM International. Standard performance specification for hydraulic cement, ASTM C1157/C1157M-17. West Conshohocken, PA; 2017. Provis JL, Bernal SA. Geopolymers and related alkali-activated materials. Ann Rev Mater Res 2014;44(1):299–327. Kolousˇek D, Brus J, Urbanova M, Andertova J, Hulinsky V, Vorel J. Preparation, structure and hydrothermal stability of alternative (sodium silicatefree) geopolymers. J Mater Sci 2007;42(22):9267–75. Feng D, Provis JL, van Deventer JSJ. Thermal activation of albite for the synthesis of one-part mix geopolymers. J Am Ceram Soc 2012;95(2):565–72. Ferna´ndez-Jimenez A, Palomo A, Criado M. Microstructure development of alkali-activated fly ash cement: a descriptive model. Cem Concr Res 2005;35(6):1204–9. White CE, Provis JL, Proffen T, van Deventer JSJ. Molecular mechanisms responsible for the structural changes occurring during geopolymerization: multiscale simulation. AIChE J 2012;58(7):2241–53. White CE, Provis JL, Proffen T, Riley DP, van Deventer JSJ. Density functional modeling of the local structure of kaolinite subjected to thermal dehydroxylation. J Phys Chem A 2010;114(14):4988–96. Benavent V, Frizon F, Poulesquen A. Effect of composition and aging on the porous structure of metakaolin-based geopolymers. J Appl Crystallogr 2016;49(6):2116–28. Duxson P, Provis JL, Lukey GC, Mallicoat SW, Kriven WM, van Deventer JSJ. Understanding the relationship between geopolymer composition, microstructure and mechanical properties. Colloids Surf A Physicochem Eng Asp 2005;269(1-3):47–58. Lloyd RR, Provis JL, van Deventer JSJ. Microscopy and microanalysis of inorganic polymer cements. 1: remnant fly ash particles. J Mater Sci 2009;44 (2):608–19. Lloyd RR, Provis JL, van Deventer JSJ. Microscopy and microanalysis of inorganic polymer cements. 2: the gel binder. J Mater Sci 2009;44(2):620–31. Rees CA, Provis JL, Lukey GC, van Deventer JSJ. The mechanism of geopolymer gel formation investigated through seeded nucleation. Colloids Surf A Physicochem Eng Asp 2008;318(1-3):97–105. Palomo A, Banfill PFG, Fernandez-Jimenez A, Swift DS. Properties of alkali-activated fly ashes determined from rheological measurements. Adv Cem Res 2005;17(4):143–51. Favier A, Habert G, d’Espinose de Lacaillerie JB, Roussel N. Mechanical properties and compositional heterogeneities of fresh geopolymer pastes. Cem Concr Res 2013;48:9–16. Steins P, Poulesquen A, Diat O, Frizon F. Structural evolution during geopolymerization from an early age to consolidated material. Langmuir 2012;28 (22):8502–10. Diaz-Loya EI, Allouche EN, Vaidya S. Mechanical properties of fly-ash-based geopolymer concrete. ACI Mater J 2011;108(3):300–6. Provis JL, Lukey GC, van Deventer JSJ. Do geopolymers actually contain nanocrystalline zeolites?—a reexamination of existing results. Chem Mater 2005;17(12):3075–85. Melar J, Renaudin G, Leroux F, Hardy-Dessources A, Nedelec J-M, Taviot-Gueho C, Petit E, Steins P, Poulesquen A, Frizon F. The porous network and its interface inside geopolymers as a function of alkali cation and aging. J Phys Chem C 2015;119(31):17619–32. Walkley B, San Nicolas R, Sani M-A, Gehman JD, van Deventer JSJ, Provis JL. Phase evolution of Na2O–Al2O3–SiO2–H2O gels in synthetic aluminosilicate binders. Dalton Trans 2016;45(13):5521–35. Bell JL, Sarin P, Provis JL, Haggerty RP, Driemeyer PE, Chupas PJ, van Deventer JSJ, Kriven WM. Atomic structure of a cesium aluminosilicate geopolymer: a pair distribution function study. Chem Mater 2008;20(14):4768–76. White CE, Provis JL, Llobet A, Proffen T, van Deventer JSJ. Evolution of local structure in geopolymer gels: an in-situ neutron pair distribution function analysis. J Am Ceram Soc 2011;94(10):3532–9. Ofer-Rozovsky E, Arbel Haddad M, Bar Nes G, Katz A. The formation of crystalline phases in metakaolin-based geopolymers in the presence of sodium nitrate. J Mater Sci 2016;51(10):4795–814. Ma X, Zhang Z, Wang A. The transition of fly ash-based geopolymer gels into ordered structures and the effect on the compressive strength. Constr Build Mater 2016;104:25–33. Zhang B, MacKenzie KJD, Brown IWM. Crystalline phase formation in metakaolinite geopolymers activated with NaOH and sodium silicate. J Mater Sci 2009;44(17):4668–76.

Geopolymers and Other Alkali-Activated Materials

799

43. De Silva P, Sagoe-Crentsil K. Medium-term phase stability of Na2O–Al2O3–SiO2–H2O geopolymer systems. Cem Concr Res 2008;38(6):870–6. 44. Provis JL, Rose V, Bernal SA, van Deventer JSJ. High resolution nanoprobe X-ray fluorescence characterization of heterogeneous calcium and heavy metal distributions in alkali activated fly ash. Langmuir 2009;25(19):11897–904. 45. Provis JL, Yong SL, Duxson P. Nanostructure/microstructure of metakaolin geopolymers. In: Provis JL, van Deventer JSJ, editors. Geopolymers: structure, processing, properties and industrial applications. Cambridge, UK: Woodhead; 2009. p. 72–88. 46. Rowles M, O’Connor B. Chemical optimisation of the compressive strength of aluminosilicate geopolymers synthesised by sodium silicate activation of metakaolinite. J Mater Chem 2003;13(5):1161–5. 47. Rocha J, Klinowski J, Adams JM. Synthesis of zeolite Na-A from metakaolinite revisited. J Chem Soc Faraday Trans 1991;87(18):3091–7. 48. Barrer RM, Mainwaring DE. Chemistry of soil minerals. Part XIII. Reactions of metakaolinite with single and mixed bases. J Chem Soc Dalton Trans 1972;(22):2534–46. 49. Oh JE, Monteiro PJM, Jun SS, Choi S, Clark SM. The evolution of strength and crystalline phases for alkali-activated ground blast furnace slag and fly ash-based geopolymers. Cem Concr Res 2010;40(2):189–96. 50. Criado M, Ferna´ndez-Jimenez A, de la Torre AG, Aranda MAG, Palomo A. An XRD study of the effect of the SiO2/Na2O ratio on the alkali activation of fly ash. Cem Concr Res 2007;37(5):671–9. 51. Rees CA, Provis JL, Lukey GC, van Deventer JSJ. Attenuated total reflectance Fourier transform infrared analysis of fly ash geopolymer gel aging. Langmuir 2007;23(15):8170–9. 52. Criado M, Ferna´ndez-Jimenez A, Palomo A, Sobrados I, Sanz J. Alkali activation of fly ash. Effect of the SiO2/Na2O ratio. Part II: 29Si MAS-NMR survey. Microporous Mesoporous Mater 2008;109(1-3):525–34. 53. Singh PS, Trigg M, Burgar I, Bastow T. Geopolymer formation processes at room temperature studied by 29Si and 27Al MAS-NMR. Mater Sci Eng A Struct Mater Properties Microstruct Process 2005;396(1-2):392–402. 54. Duxson P, Provis JL, Lukey GC, Separovic F, van Deventer JSJ. 29Si NMR study of structural ordering in aluminosilicate geopolymer gels. Langmuir 2005;21(7):3028–36. 55. Bernal SA, Provis JL, Walkley B, San Nicolas R, Gehman JD, Brice DG, Kilcullen AR, Duxson P, van Deventer JSJ. Gel nanostructure in alkaliactivated binders based on slag and fly ash, and effects of accelerated carbonation. Cem Concr Res 2013;53:127–44. 56. Brus J, Abbrent S, Kobera L, Urbanova M, Cuba P. Advances in 27Al MAS NMR studies of geopolymers. Annu Rep NMR Spectrosc 2016;88:79–147. 57. Rowles MR, Hanna JV, Pike KJ, Smith ME, O’Connor BH. 29Si, 27Al, 1H and 23Na MAS NMR study of the bonding character in aluminosilicate inorganic polymers. Appl Magn Reson 2007;32:663–89. 58. Duxson P, Lukey GC, Separovic F, van Deventer JSJ. The effect of alkali cations on aluminum incorporation in geopolymeric gels. Ind Eng Chem Res 2005;44(4):832–9. 59. Barbosa VFF, MacKenzie KJD. Synthesis and thermal behaviour of potassium sialate geopolymers. Mater Lett 2003;57(9-10):1477–82. 60. Duxson P, Provis JL, Lukey GC, van Deventer JSJ, Separovic F, Gan ZH. 39K NMR of free potassium in geopolymers. Ind Eng Chem Res 2006;45 (26):9208–10. 61. Palomo A, Alonso S, Ferna´ndez-Jimenez A, Sobrados I, Sanz J. Alkaline activation of fly ashes: NMR study of the reaction products. J Am Ceram Soc 2004;87(6):1141–5. 62. Provis JL, Duxson P, Lukey GC, van Deventer JSJ. Statistical thermodynamic model for Si/Al ordering in amorphous aluminosilicates. Chem Mater 2005;17(11):2976–86. 63. Perera DS, Vance ER, Aly Z, Davis J, Nicholson CL. Immobilization of Cs and Sr in geopolymers with Si/Al  2. Ceram Trans 2006;176:91–6. 64. Lloyd RR. Accelerated ageing of geopolymers. In: Provis JL, van Deventer JSJ, editors. Geopolymers: structure, processing, properties and industrial applications. Cambridge, UK: Woodhead; 2009. p. 139–66. 65. Yang K-H, Song J-K, Ashour AF, Lee E-T. Properties of cementless mortars activated by sodium silicate. Constr Build Mater 2008;22(9):1981–9. 66. Yang KH, Song JK. Workability loss and compressive strength development of cementless mortars activated by combination of sodium silicate and sodium hydroxide. J Mater Civil Eng 2009;21(3):119–27. 67. Criado M, Ferna´ndez-Jimenez A, Palomo A. Alkali activation of fly ash. Effect of the SiO2/Na2O ratio. Part I: FTIR study. Microporous Mesoporous Mater 2007;106(1-3):180–91. 68. Steveson M, Sagoe-Crentsil K. Relationships between composition, structure and strength of inorganic polymers. Part I. Metakaolin-derived inorganic polymers. J Mater Sci 2005;40(8):2023–36. 69. Steveson M, Sagoe-Crentsil K. Relationships between composition, structure, and strength of inorganic polymers. Part 2. Fly ash-derived inorganic polymers. J Mater Sci 2005;40(16):4247–59. 70. Lloyd RR, Provis JL, Smeaton KJ, van Deventer JSJ. Spatial distribution of pores in fly ash-based inorganic polymer gels visualised by Wood’s metal intrusion. Microporous Mesoporous Mater 2009;126(1-2):32–9. 71. Provis JL, Harrex RM, Bernal SA, Duxson P, van Deventer JSJ. Dilatometry of geopolymers as a means of selecting desirable fly ash sources. J NonCryst Solids 2012;358(16):1930–7. 72. Provis JL, van Deventer JSJ. Geopolymerisation kinetics. 2. Reaction kinetic modelling. Chem Eng Sci 2007;62(9):2318–29. 73. Lloyd RR, Provis JL, van Deventer JSJ. Pore solution composition and alkali diffusion in inorganic polymer cement. Cem Concr Res 2010;40 (9):1386–92. 74. Bernal SA, Provis JL, Brice DG, Kilcullen A, Duxson P, van Deventer JSJ. Accelerated carbonation testing of alkali-activated binders significantly underestimates service life: the role of pore solution chemistry. Cem Concr Res 2012;42(10):1317–26. 75. Pouhet R, Cyr M. Carbonation in the pore solution of metakaolin-based geopolymer. Cem Concr Res 2016;88:227–35.

800 Lea’s Chemistry of Cement and Concrete 76. Zhang Z, Provis JL, Reid A, Wang H. Fly ash-based geopolymers: the relationship between composition, pore structure and efflorescence. Cem Concr Res 2014;64:30–41. 77. Phair JW, van Deventer JSJ. Characterization of fly-ash-based geopolymeric binders activated with sodium aluminate. Ind Eng Chem Res 2002;41 (17):4242–51. 78. Nugteren H, Ogundiran MB, Witkamp G-J, Kreuzer MT. Coal fly ash activated by waste sodium aluminate as an immobilizer for hazardous waste, In: 2011 World of coal ash conference. Denver, CO: ACAA/CAER; 2011. 79. Durdzi nski PT, Ben Haha M, Bernal SA, De Belie N, Gruyaert E, Lothenbach B, Menendez Mendez E, Provis JL, Sch€oler A, Stabler C, Tan Z, Villagra´n Zaccardi Y, Vollpracht A, Winnefeld F, Zaja˛c M, Scrivener KL. Outcomes of the RILEM round robin on degree of reaction of slag and fly ash in blended cements. Mater Struct 2017;50(2):#135. 80. Benavent V, Steins P, Sobrados I, Sanz J, Lambertin D, Frizon F, Rossignol S, Poulesquen A. Impact of aluminum on the structure of geopolymers from the early stages to consolidated material. Cem Concr Res 2016;90:27–35. 81. Durdzi nski PT, Snellings R, Dunant CF, Haha MB, Scrivener KL. Fly ash as an assemblage of model Ca–Mg–Na-aluminosilicate glasses. Cem Concr Res 2015;78(B):263–72. 82. Duxson P, Provis JL. Designing precursors for geopolymer cements. J Am Ceram Soc 2008;91(12):3864–9. 83. Ferna´ndez-Jimenez A, Palomo A, Sobrados I, Sanz J. The role played by the reactive alumina content in the alkaline activation of fly ashes. Microporous Mesoporous Mater 2006;91(1-3):111–9. 84. Bernal SA, Mejı´a de Gutierrez R, Rose V, Provis JL. Effect of silicate modulus and metakaolin incorporation on the carbonation of alkali silicateactivated slags. Cem Concr Res 2010;40(6):898–907. 85. Berry M, Stephens J, Cross D. Performance of 100% fly ash concrete with recycled glass aggregate. ACI Mater J 2011;108(4):378–84. 86. Kumar S, Kumar R, Alex TC, Bandopadhyay A, Mehrotra SP. Influence of reactivity of fly ash on geopolymerisation. Adv Appl Ceram 2007; 106(3):120–7. 87. van Riessen A, Chen-Tan N. Beneficiation of Collie fly ash for synthesis of geopolymer: part 1—beneficiation. Fuel 2013;106:569–75. 88. Winnefeld F, Leemann A, Lucuk M, Svoboda P, Neuroth M. Assessment of phase formation in alkali activated low and high calcium fly ashes in building materials. Constr Build Mater 2010;24(6):1086–93. 89. MacKenzie KJD, Brown IWM, Meinhold RH, Bowden ME. Outstanding problems in the kaolinite-mullite reaction sequence investigated by 29Si and 27 Al solid-state nuclear magnetic resonance: I. Metakaolinite. J Am Ceram Soc 1985;68(6):293–7. 90. White CE, Provis JL, Proffen T, Riley DP, van Deventer JSJ. Combining density functional theory (DFT) and pair distribution function (PDF) analysis to solve the structure of metastable materials: the case of metakaolin. Phys Chem Chem Phys 2010;12(13):3239–45. 91. Provis JL, Duxson P, van Deventer JSJ. The role of particle technology in developing sustainable construction materials. Adv Powder Technol 2010; 21(1):2–7. 92. San Nicolas R, Cyr M, Escadeillas G. Characteristics and applications of flash metakaolins. Appl Clay Sci 2013;83-84:253–62. 93. San Nicolas R, Cyr M, Escadeillas G. Performance-based approach to durability of concrete containing flash-calcined metakaolin as cement replacement. Constr Build Mater 2014;55:313–22. 94. Pacheco-Torgal F, Castro-Gomes J, Jalali S. Investigations about the effect of aggregates on strength and microstructure of geopolymeric mine waste mud binders. Cem Concr Res 2007;37(6):933–41. 95. Buchwald A, Hohmann M, Posern K, Brendler E. The suitability of thermally activated illite/smectite clay as raw material for geopolymer binders. Appl Clay Sci 2009;46(3):300–4. 96. MacKenzie KJD, Komphanchai S, Vagana R. Formation of inorganic polymers (geopolymers) from 2:1 layer lattice aluminosilicates. J Eur Ceram Soc 2008;28(1):177–81. 97. MacKenzie KJD, Brew DRM, Fletcher RA, Vagana R. Formation of aluminosilicate geopolymers from 1:1 layer-lattice minerals pre-treated by various methods: a comparative study. J Mater Sci 2007;42(12):4667–74. 98. Gomes KC, Lima GST, Torres SM, De Barros S, Vasconcelos IF, Barbosa NP. Iron distribution in geopolymer with ferromagnetic rich precursor. Mater Sci Forum 2010;643:131–8. 99. Kamseu E, Leonelli C, Perera DS, Melo UC, Lemougna PN. Investigation of volcanic ash based geopolymers as potential building materials. Interceram 2009;58(2-3):136–40. 100. Bondar D, Lynsdale CJ, Milestone NB, Hassani N, Ramezanianpour AA. Effect of adding mineral additives to alkali-activated natural pozzolan paste. Constr Build Mater 2011;25(6):2906–10. 101. Najafi Kani E, Allahverdi A. Effect of chemical composition on basic engineering properties of inorganic polymeric binder based on natural pozzolan. Ceram-Silikaty 2009;53(3):195–204. 102. Najafi Kani E, Allahverdi A, Provis JL. Efflorescence control in geopolymer binders based on natural pozzolan. Cem Concr Compos 2012;34(1):25–33. 103. Krivenko PV. Alkaline cements, In: Proceedings of the first international conference on alkaline cements and concretes; 1994. Kiev, Ukraine: VIPOL Stock Company. 104. Wang S-D, Pu X-C, Scrivener KL, Pratt PL. Alkali-activated slag cement and concrete: a review of properties and problems. Adv Cem Res 1995;7 (27):93–102. 105. Shi C, Qian J. High performance cementing materials from industrial slags—a review. Resour Conserv Recycl 2000;29:195–207. 106. Puertas F. Cementos de escoria activados alcalinamente: situacio´n actual y perspectivas de futuro. Mater Constr 1995;45(239):53–64. 107. Shi C, Krivenko PV, Roy DM. Alkali-activated cements and concretes. Abingdon, UK: Taylor & Francis; 2006. 376 pp. 108. Ko LS-C, Belen˜a I, Duxson P, Kavalerova E, Krivenko PV, Ordon˜ez L-M, Tagnit-Hamou A, Winnefeld F. AAM concretes: standards for mix design/ formulation and early-age properties. In: Provis JL, van Deventer JSJ, editors. Alkali-activated materials: state-of-the-art report, RILEM TC 224-AAM. Dordrecht: Springer/RILEM; 2014. p. 157–76.

Geopolymers and Other Alkali-Activated Materials

801

109. Provis JL, Brice DG, Buchwald A, Duxson P, Kavalerova E, Krivenko PV, Shi C, van Deventer JSJ, Wiercx JALM. Demonstration projects and applications in building and civil infrastructure. In: Provis JL, van Deventer JSJ, editors. Alkali-activated materials: state-of-the-art report, RILEM TC 224AAM. Dordrecht: Springer/RILEM; 2014. p. 309–38. 110. Schweizerisches Ingenieur and Architektenverein (SIA). Anforderungen an neue Zemente (Merkblatt 2049). Z€urich: Switzerland; 2014. 111. Mills SJ, Christy AG, Genin J-MR, Kameda T, Colombo F. Nomenclature of the hydrotalcite supergroup: natural layered double hydroxides. Mineral Mag 2012;76(5):1289–336. 112. Bernal SA. Advances in near-neutral salts activation of blast furnace slags. RILEM Tech Lett 2016;1:39–44. 113. Bernal SA, Provis JL, Myers RJ, San Nicolas R, Van Deventer JSJ. Role of carbonates in the chemical evolution of sodium carbonate-activated slag binders. Mater Struct 2015;48(3):517–29. 114. Mobasher N, Bernal SA, Provis JL. Structural evolution of a sodium sulfate activated slag cement. J Nucl Mater 2016;468:97–104. 115. Ke X, Bernal SA, Provis JL. Controlling the reaction kinetics of sodium carbonate-activated slag cements using calcined layered double hydroxides. Cem Concr Res 2016;81:24–37. 116. Ke X, Bernal SA, Provis JL. Uptake of chloride and carbonate by Mg-Al and Ca-Al layered double hydroxides in simulated pore solutions of alkaliactivated slag cement. Cem Concr Res 2017;100:1–13. 117. Wang SD, Scrivener KL, Pratt PL. Factors affecting the strength of alkali-activated slag. Cem Concr Res 1994;24(6):1033–43. 118. Collins FG, Sanjayan JG. Workability and mechanical properties of alkali activated slag concrete. Cem Concr Res 1999;29(3):455–8. 119. Collins F, Sanjayan JG. Strength and shrinkage properties of alkali-activated slag concrete placed into a large column. Cem Concr Res 1999;29 (5):659–66. 120. Rousekova´ I, Bajza A, Zˇivica V. Silica fume-basic blast furnace slag systems activated by an alkali silica fume activator. Cem Concr Res 1997;27 (12):1825–8. 121. Rodrı´guez ED, Bernal SA, Provis JL, Paya J, Monzo JM, Borrachero MV. Effect of nanosilica-based activators on the performance of an alkaliactivated fly ash binder. Cem Concr Compos 2013;35(1):1–11. 122. Bernal SA, Rodrı´guez ED, Mejı´a de Gutierrez R, Provis JL, Delvasto S. Activation of metakaolin/slag blends using alkaline solutions based on chemically modified silica fume and rice husk ash. Waste Biomass Valorization 2012;3(1):99–108. 123. Puertas F, Torres-Carrasco M. Use of glass waste as an activator in the preparation of alkali-activated slag. Mechanical strength and paste characterisation. Cem Concr Res 2014;57:95–104. 124. Cheng Q-H, Sarkar SL. A study of rheological and mechanical properties of mixed alkali activated slag pastes. Adv Cem Based Mater 1994;1 (4):178–84. 125. Palacios M, Banfill PFG, Puertas F. Rheology and setting of alkali-activated slag pastes and mortars: effect of organic admixture. ACI Mater J 2008;105 (2):140–8. 126. Kashani A, Provis JL, Qiao GG, van Deventer JSJ. The interrelationship between surface chemistry and rheology in alkali activated slag paste. Constr Build Mater 2014;65:583–91. 127. Xu H, Provis JL, van Deventer JSJ, Krivenko PV. Characterization of aged slag concretes. ACI Mater J 2008;105(2):131–9. 128. Sakulich AR, Miller S, Barsoum MW. Chemical and microstructural characterization of 20-month-old alkali-activated slag cements. J Am Ceram Soc 2010;93(6):1741–8. 129. Moseson AJ, Moseson DE, Barsoum MW. High volume limestone alkali-activated cement developed by design of experiment. Cem Concr Compos 2012;34(3):328–36. 130. Ferna´ndez-Jimenez A, Puertas F, Sobrados I, Sanz J. Structure of calcium silicate hydrates formed in alkaline-activated slag: influence of the type of alkaline activator. J Am Ceram Soc 2003;86(8):1389–94. 131. Richardson IG, Groves GW. Microstructure and microanalysis of hardened cement pastes involving ground granulated blast-furnace slag. J Mater Sci 1992;27(22):6204–12. 132. Myers RJ, Bernal SA, San Nicolas R, Provis JL. Generalized structural description of calcium-sodium aluminosilicate hydrate gels: the crosslinked substituted tobermorite model. Langmuir 2013;29(17):5294–306. 133. Puertas F, Palacios M, Manzano H, Dolado JS, Rico A, Rodrı´guez J. A model for the C-A-S-H gel formed in alkali-activated slag cements. J Eur Ceram Soc 2011;31(12):2043–56. € ¸elik VO, White CE. Nanoscale charge-balancing mechanism in alkali-substituted calcium–silicate–hydrate gels. J Phys Chem Lett 2016;7 134. Ozc (24):5266–72. 135. Roy A, Schilling PJ, Eaton HC, Malone PG, Brabston WN, Wakeley LD. Activation of ground blast-furnace slag by alkali-metal and alkaline-earth hydroxides. J Am Ceram Soc 1992;75(12):3233–40. 136. Myers RJ, Bernal SA, Provis JL. Phase diagrams for alkali-activated slag binders. Cem Concr Res 2017;95:30–8. 137. Ben Haha M, Lothenbach B, Le Saout G, Winnefeld F. Influence of slag chemistry on the hydration of alkali-activated blast-furnace slag. Part I: effect of MgO. Cem Concr Res 2011;41(9):955–63. 138. Ferna´ndez-Jimenez A, Puertas F. Effect of activator mix on the hydration and strength behaviour of alkali-activated slag cements. Adv Cem Res 2003;15 (3):129–36. 139. Bernal SA, San Nicolas R, Myers RJ, Mejı´a de Gutierrez R, Puertas F, van Deventer JSJ, Provis JL. MgO content of slag controls phase evolution and structural changes induced by accelerated carbonation in alkali-activated binders. Cem Concr Res 2014;57:33–43. 140. Bernal SA, Provis JL, Mejı´a de Gutierrez R, Rose V. Evolution of binder structure in sodium silicate-activated slag-metakaolin blends. Cem Concr Compos 2011;33(1):46–54. 141. Bernal SA, Provis JL, Brice DG, Kilcullen A, Duxson P, van Deventer JSJ. Accelerated carbonation testing of alkali-activated binders significantly underestimate the real service life: the role of the pore solution. Cem Concr Res 2012;42(10):1317–26.

802 Lea’s Chemistry of Cement and Concrete 142. Chen W, Brouwers HJH. The hydration of slag, part 1: reaction models for alkali-activated slag. J Mater Sci 2007;42(2):428–43. 143. Lothenbach B, Gruskovnjak A. Hydration of alkali-activated slag: thermodynamic modelling. Adv Cem Res 2007;19(2):81–92. 144. San Nicolas R, Bernal SA, Mejı´a de Gutierrez R, van Deventer JSJ, Provis JL. Distinctive microstructural features of aged sodium silicate activated slag concretes. Cem Concr Res 2014;65:41–51. 145. Schneider J, Cincotto MA, Panepucci H. 29Si and 27Al high-resolution NMR characterization of calcium silicate hydrate phases in activated blastfurnace slag pastes. Cem Concr Res 2001;31(7):993–1001. 146. Myers RJ, Bernal SA, Provis JL. A thermodynamic model for C-(N-)A-S-H gel: CNASH_ss. Derivation and validation. Cem Concr Res 2014;66:27–47. 147. Churakov SV, Labbez C. Thermodynamics and molecular mechanism of Al incorporation in calcium silicate hydrates. J Phys Chem C 2017;121 (8):4412–9. 148. Haas J, Nonat A. From C–S–H to C–A–S–H: experimental study and thermodynamic modelling. Cem Concr Res 2015;68:124–38. 149. Richardson IG. Tobermorite/jennite- and tobermorite/calcium hydroxide-based models for the structure of C-S-H: applicability to hardened pastes of tricalcium silicate, b-dicalcium silicate, Portland cement, and blends of Portland cement with blast-furnace slag, metakaolin, or silica fume. Cem Concr Res 2004;34(9):1733–77. 150. Myers RJ, Lothenbach B, Bernal SA, Provis JL. Thermodynamic modelling of alkali-activated slag cements. Appl Geochem 2015;61:233–47. 151. Kuenzel C, Vandeperre LJ, Donatello S, Boccaccini AR, Cheeseman C. Ambient temperature drying shrinkage and cracking in metakaolin-based geopolymers. J Am Ceram Soc 2012;95(10):3270–7. 152. Ismail I, Bernal SA, Provis JL, Hamdan S, van Deventer JSJ. Drying-induced changes in the structure of alkali-activated pastes. J Mater Sci 2013;48 (9):3566–77. 153. Ismail I, Bernal SA, Provis JL, San Nicolas R, Brice DG, Kilcullen AR, Hamdan S, van Deventer JSJ. Influence of fly ash on the water and chloride permeability of alkali-activated slag mortars and concretes. Constr Build Mater 2013;48:1187–201. 154. Yang K, Yang C, Magee B, Nanukuttan S, Ye J. Establishment of a preconditioning regime for air permeability and sorptivity of alkali-activated slag concrete. Cem Concr Compos 2016;73:19–28. 155. Melo Neto AA, Cincotto MA, Repette W. Drying and autogenous shrinkage of pastes and mortars with activated slag cement. Cem Concr Res 2008;38:565–74. 156. Gruskovnjak A, Lothenbach B, Holzer L, Figi R, Winnefeld F. Hydration of alkali-activated slag: comparison with ordinary Portland cement. Adv Cem Res 2006;18(3):119–28. 157. Puertas F, Ferna´ndez-Jimenez A, Blanco-Varela MT. Pore solution in alkali-activated slag cement pastes. Relation to the composition and structure of calcium silicate hydrate. Cem Concr Res 2004;34(1):139–48. 158. Song S, Jennings HM. Pore solution chemistry of alkali-activated ground granulated blast-furnace slag. Cem Concr Res 1999;29:159–70. 159. Kim JC, Hong SY. Liquid concentration changes during slag cement hydration by alkali activation. Cem Concr Res 2001;31(2):283–5. 160. Chaouche M, Gao XX, Cyr M, Cotte M, Frouin L. On the origin of the blue/green color of blast-furnace slag-based materials: sulfur K-edge XANES investigation. J Am Ceram Soc 2017;100(4):1707–16. 161. Glasser FP. Mineralogical aspects of cement in radioactive waste disposal. Mineral Mag 2001;65(5):621–33. 162. Mundra S, Bernal SA, Criado M, Hlava´cˇek P, Ebell G, Reinemann S, Gluth GJG, Provis JL. Steel corrosion in reinforced alkali-activated materials. RILEM Tech Lett 2017;2:33–9. 163. Brough AR, Atkinson A. Sodium silicate-based, alkali-activated slag mortars. Part I: strength, hydration and microstructure. Cem Concr Res 2002;32 (6):865–79. 164. Talling B, Brandstetr, J. Present state and future of alkali-activated slag concretes, In: Third international conference on fly ash, silica fume, slag and natural pozzolans in concrete, ACI SP114. Trondheim, Norway: American Concrete Institute; 1989. 165. Douglas E, Brandstetr J. A preliminary study on the alkali activation of ground granulated blast-furnace slag. Cem Concr Res 1990;20(5):746–56. 166. Ben Haha M, Lothenbach B, Le Saout G, Winnefeld F. Influence of slag chemistry on the hydration of alkali-activated blast-furnace slag. Part II: effect of Al2O3. Cem Concr Res 2012;42(1):74–83. 167. Winnefeld F, Ben Haha M, Le Saout G, Costoya M, Ko S-C, Lothenbach B. Influence of slag composition on the hydration of alkali-activated slags. J Sustain Cem-Based Mater 2015;4(2):85–100. 168. Shi C, Day RL. Selectivity of alkaline activators for the activation of slags. Cem Concr Aggregates 1996;18(1):8–14. 169. T€anzer R, Buchwald A, Stephan D. Effect of slag chemistry on the hydration of alkali-activated blast-furnace slag. Mater Struct 2015;48(3):629–41. 170. Shi C, Wu X, Tang M. Research on alkali-activated cementitious systems in China. Adv Cem Res 1993;5(17):1–7. 171. Natali Murri A, Rickard WDA, Bignozzi MC, van Riessen A. High temperature behaviour of ambient cured alkali-activated materials based on ladle slag. Cem Concr Res 2013;43:51–61. 172. Bernal SA, Rodrı´guez ED, Kirchheim AP, Provis JL. Management and valorisation of wastes through use in producing alkali-activated cement materials. J Chem Technol Biotechnol 2016;91(9):2365–88. 173. Shi C. Study on alkali activated phosphorus slag cement. J Nanjing Inst Chem Technol 1988;10(2):110–6. 174. Bin X, Yuan X. Research of alkali-activated nickel slag cement, In: Proceedings of the second international conference on alkaline cements and concretes, Kiev. Ukraine: ORANTA; 1999. 175. Narang KC, Chopra SK. Studies on alkaline activation of BF, steel and alloy slags. Silic Ind 1983;9:175–82. 176. Komnitsas K, Zaharaki D, Perdikatsis V. Geopolymerisation of low calcium ferronickel slags. J Mater Sci 2007;42(9):3073–82. 177. Zosin AP, Priimak TI, Avsaragov KB. Geopolymer materials based on magnesia-iron slags for normalization and storage of radioactive wastes. Atomic Energy 1998;85(1):510–4.

Geopolymers and Other Alkali-Activated Materials

803

178. Kalinkin AM, Kumar S, Gurevich BI, Alex TC, Kalinkina EV, Tyukavkina VV, Kalinnikov VT, Kumar R. Geopolymerization behavior of Cu–Ni slag mechanically activated in air and in CO2 atmosphere. Int J Miner Process 2012;112–113:101–6. 179. Shi C, Ferna´ndez-Jimenez A, Palomo A. New cements for the 21st century: the pursuit of an alternative to Portland cement. Cem Concr Res 2011;41 (7):750–63. 180. Provis JL, Myers RJ, White CE, Rose V, van Deventer JSJ. X-ray microtomography shows pore structure and tortuosity in alkali-activated binders. Cem Concr Res 2012;42(6):855–64. 181. Yip CK, Lukey GC, Provis JL, van Deventer JSJ. Effect of calcium silicate sources on geopolymerisation. Cem Concr Res 2008;38(4):554–64. 182. Yip CK, van Deventer JSJ. Microanalysis of calcium silicate hydrate gel formed within a geopolymeric binder. J Mater Sci 2003;38(18):3851–60. 183. Alonso S, Palomo A. Calorimetric study of alkaline activation of calcium hydroxide-metakaolin solid mixtures. Cem Concr Res 2001;31(1):25–30. 184. Dombrowski K, Buchwald A, Weil M. The influence of calcium content on the structure and thermal performance of fly ash based geopolymers. J Mater Sci 2007;42(9):3033–43. 185. Walkley B, San Nicolas R, Sani M-A, Rees GJ, Hanna JV, van Deventer JSJ, Provis JL. Phase evolution of C-(N)-A-S-H/N-A-S-H gel blends investigated via alkali-activation of synthetic calcium aluminosilicate precursors. Cem Concr Res 2016;89:120–35. 186. Garcı´a Lodeiro I, Ferna´ndez-Jimenez A, Palomo A, Macphee DE. Effect on fresh C-S-H gels of the simultaneous addition of alkali and aluminium. Cem Concr Res 2010;40(1):27–32. 187. Garcı´a-Lodeiro I, Palomo A, Ferna´ndez-Jimenez A, Macphee DE. Compatibility studies between N-A-S-H and C-A-S-H gels. Study in the ternary diagram Na2O–CaO–Al2O3–SiO2–H2O. Cem Concr Res 2011;41(9):923–31. 188. Ismail I, Bernal SA, Provis JL, San Nicolas R, Hamdan S, van Deventer JSJ. Modification of phase evolution in alkali-activated blast furnace slag by the incorporation of fly ash. Cem Concr Compos 2014;45:125–35. 189. Oh JE, Moon J, Oh S-G, Clark SM, Monteiro PJM. Microstructural and compositional change of NaOH-activated high calcium fly ash by incorporating Na-aluminate and co-existence of geopolymeric gel and C–S–H(I). Cem Concr Res 2012;42(5):673–85. 190. Buchwald A, Hilbig H, Kaps C. Alkali-activated metakaolin-slag blends—performance and structure in dependence on their composition. J Mater Sci 2007;42(9):3024–32. 191. Bortnovsky O, Deˇdecˇek J, Tvaru˚zˇkova´ Z, Sobalı´k Z, Sˇubrt J. Metal ions as probes for characterization of geopolymer materials. J Am Ceram Soc 2008;91(9):3052–7. 192. Granizo ML, Alonso S, Blanco-Varela MT, Palomo A. Alkaline activation of metakaolin: effect of calcium hydroxide in the products of reaction. J Am Ceram Soc 2002;85(1):225–31. 193. Shi C. Early microstructure development of activated lime-fly ash pastes. Cem Concr Res 1996;26(9):1351–9. 194. Shi C, Day RL. Chemical activation of blended cements made with lime and natural pozzolans. Cem Concr Res 1993;23(6):1389–96. 195. Shi C, Day RL. Pozzolanic reaction in the presence of chemical activators. Part II: reaction products and mechanism. Cem Concr Res 2000;30 (4):607–13. 196. Shi C, Day RL. Pozzolanic reaction in the presence of chemical activators. Part I: reaction kinetics. Cem Concr Res 2000;30(1):51–8. 197. Williams PJ, Biernacki JJ, Walker LR, Meyer HM, Rawn CJ, Bai J. Microanalysis of alkali-activated fly ash–CH pastes. Cem Concr Res 2002;32 (6):963–72. 198. Lecomte I, Liegeois M, Rulmont A, Cloots R, Maseri F. Synthesis and characterization of new inorganic polymeric composites based on kaolin or white clay and on ground-granulated blast furnace slag. J Mater Res 2003;18(11):2571–9. 199. Yip CK, Lukey GC, van Deventer JSJ. The coexistence of geopolymeric gel and calcium silicate hydrate at the early stage of alkaline activation. Cem Concr Res 2005;35(9):1688–97. 200. Burciaga-Dı´az O, Escalante-Garcı´a JI, Arellano-Aguilar R, Gorokhovsky A. Statistical analysis of strength development as a function of various parameters on activated metakaolin/slag cements. J Am Ceram Soc 2010;93(2):541–7. 201. Bernal SA, Mejı´a de Gutierrez R, Provis JL. Engineering and durability properties of concretes based on alkali-activated granulated blast furnace slag/ metakaolin blends. Constr Build Materi 2012;33:99–108. 202. Bernal SA, Provis JL, Rose V, Mejı´a de Gutierrez R. High-resolution X-ray diffraction and fluorescence microscopy characterization of alkali-activated slag-metakaolin binders. J Am Ceram Soc 2013;96(6):1951–7. 203. Smith MA, Osborne GJ. Slag/fly ash cements. World Cem Technol 1977;1(6):223–33. 204. Li Z, Liu S. Influence of slag as additive on compressive strength of fly ash-based geopolymer. J Mater Civil Eng 2007;19(6):470–4. 205. Puertas F, Martı´nez-Ramı´rez S, Alonso S, Va´zquez E. Alkali-activated fly ash/slag cement. Strength behaviour and hydration products. Cem Concr Res 2000;30:1625–32. 206. Puertas F, Ferna´ndez-Jimenez A. Mineralogical and microstructural characterisation of alkali-activated fly ash/slag pastes. Cem Concr Compos 2003;25(3):287–92. 207. Lloyd RR, Provis JL, van Deventer JSJ. Acid resistance of inorganic polymer binders. 1. Corrosion rate. Mater Struct 2012;45(1-2):1–14. 208. Husbands TB, Malone PG, Wakeley LD. Performance of concretes proportioned with Pyrament blended cement. Construction Productivity Advancement Research Program, report CPAR-SL-94-2. Vicksburg, MS: U.S. Army Corps of Engineers. 1994; 106 pp. 209. Wheat HG. Corrosion behavior of steel in concrete made with Pyrament® blended cement. Cem Concr Res 1992;22:103–11. 210. Ozyildirim C. A field investigation of concrete patches containing Pyrament blended cement. Richmond, VA: Virginia Department of Transportation; 1994. 11 pp. 211. Zia P, Leming ML, Ahmad SH, Schemmel JJ, Elliott, RP. Mechanical behavior of high performance concretes, vol. 2: production of High performance concrete, SHRP-C-362. Washington, DC: Strategic Highway Research Program, National Research Council; 1993.

804 Lea’s Chemistry of Cement and Concrete 212. Garcia-Lodeiro I, Fernandez-Jimenez A, Palomo A. Hydration kinetics in hybrid binders: early reaction stages. Cem Concr Compos 2013;39:82–92. 213. Donatello S, Ferna´ndez-Jimenez A, Palomo A. Very high volume fly ash cements. Early age hydration study using Na2SO4 as an activator. J Am Ceram Soc 2013;96(3):900–6. 214. Bernal SA, Skibsted J, Herfort D. Hybrid binders based on alkali sulfate-activated Portland clinker and metakaolin, In: XIII international congress on the chemistry of cement, Madrid; 2011. 215. Marchon D, Sulser U, Eberhardt A, Flatt RJ. Molecular design of comb-shaped polycarboxylate dispersants for environmentally friendly concrete. Soft Matter 2013;9:10719–28. 216. Ferna´ndez-Jimenez A, Palomo A, Vazquez T, Vallepu R, Terai T, Ikeda K. Alkaline activation of blends of metakaolin and calcium aluminate cement. Part I: strength and microstructural development. J Am Ceram Soc 2008;91(4):1231–6. 217. Kukko H, Mannonen R. Chemical and mechanical properties of alkali-activated blast furnace slag (F-concrete). Nordic Concr Res 1982;1:16.1–16.16. 218. Byfors K, Klingstedt G, Lehtonen HP, Romben L. Durability of concrete made with alkali-activated slag, In: Third international conference on fly ash, silica fume, slag and natural pozzolans in concrete, ACI SP114. Trondheim, Norway: American Concrete Institute; 1989. 219. Puertas F, Palomo A, Ferna´ndez-Jimenez A, Izquierdo JD, Granizo ML. Effect of superplasticisers on the behaviour and properties of alkaline cements. Adv Cem Res 2003;15(1):23–8. 220. Criado M, Palomo A, Ferna´ndez-Jimenez A, Banfill PFG. Alkali activated fly ash: effect of admixtures on paste rheology. Rheol Acta 2009;48:447–55. 221. Kashani A, Provis JL, Xu J, Kilcullen AR, Qiao GG, van Deventer JSJ. Effect of molecular architecture of polycarboxylate ethers on plasticizing performance in alkali activated slag paste. J Mater Sci 2014;49(7):2761–72. 222. Palacios M, Houst YF, Bowen P, Puertas F. Adsorption of superplasticizer admixtures on alkali-activated slag pastes. Cem Concr Res 2009;39:670–7. 223. Palacios M, Bowen P, Kappl M, Butt HJ, Stuer M, Pecharroma´n C, Aschauer U, Puertas F. Fuerzas de repulsio´n de aditivos superplastificantes en sistemas de escoria granulada de horno alto en medios alcalinos, desde medidas de AFM a propiedades reolo´gicas (Repulsion forces of superplasticizers on ground granulated blast furnace slag in alkaline media, from AFM measurements to rheological properties). Mater Constr 2012;62(308):489–513. 224. Palacios M, Puertas F. Stability of superplasticizer and shrinkage-reducing admixtures in high basic media. Mater Constr 2004;54(276):65–86. 225. Li H, Zhang J, Zhao Y, Wu C-Y, Zheng C. Wettability of fly ashes from four coal-fired power plants in China. Ind Eng Chem Res 2011;50(13):7763–71. 226. Gupta V, Hampton MA, Stokes JR, Nguyen AV, Miller JD. Particle interactions in kaolinite suspensions and corresponding aggregate structures. J Colloid Interface Sci 2011;359(1):95–103. 227. Gong C, Yang N. Effect of phosphate on the hydration of alkali-activated red mud-slag cementitious material. Cem Concr Res 2000;30(7):1013–6. 228. Lee WKW, van Deventer JSJ. Effects of anions on the formation of aluminosilicate gel in geopolymers. Ind Eng Chem Res 2002;41(18):4550–8. 229. Nicholson CL, Murray BJ, Fletcher RA, Brew DRM, MacKenzie KJD, Schm€ucker M. Novel geopolymer materials containing borate structural units, In: World congress geopolymer 2005. Saint-Quentin, France: Geopolymer Institute; 2005. 230. Bernal S, de Gutierrez R, Delvasto S, Rodriguez E. Performance of an alkali-activated slag concrete reinforced with steel fibers. Constr Build Mater 2010;24(2):208–14. 231. Moser RD, Allison PG, Williams BA, Weiss Jr CA, Diaz AJ, Gore ER, Malone PG, Improvement in the geopolymer-to-steel bond using a reactive vitreous enamel coating. Constr Build Mater 2013;49:62–9. 232. Sofi M, van Deventer JSJ, Mendis PA, Lukey GC. Bond performance of reinforcing bars in inorganic polymer concrete (IPC). J Mater Sci 2007;42 (9):3007–16. 233. Provis JL, Bı´lek V, Buchwald A, Dombrowski-Daube K, Varela B. Durability and testing—physical processes. In: Provis JL, van Deventer JSJ, editors. Alkali-activated materials: state-of-the-art report, RILEM TC 224-AAM. Dordrecht: Springer/RILEM; 2014. p. 277–307. 234. Bernal SA, Bı´lek V, Criado M, Ferna´ndez-Jimenez A, Kavalerova E, Krivenko PV, Palacios M, Palomo A, Provis JL, Puertas F, San Nicolas R, Shi C, Winnefeld F. Durability and testing—degradation via mass transport. In: Provis JL, van Deventer JSJ, editors. Alkali-activated materials: state-of-theart report, RILEM TC 224-AAM. Dordrecht: Springer/RILEM; 2014. p. 223–76. 235. Bernal SA, Krivenko PV, Provis JL, Puertas F, Rickard WDA, Shi C, van Riessen A. Other potential applications for alkali-activated materials. In: Provis JL, van Deventer JSJ, editors. Alkali-activated materials: state-of-the-art report, RILEM TC 224-AAM. Dordrecht: Springer/RILEM; 2014. p. 339–79. 236. Bernal SA, Provis JL. Durability of alkali-activated materials: progress and perspectives. J Am Ceram Soc 2014;97(4):997–1008. 237. Ismail I, Bernal SA, Provis JL, Hamdan S, van Deventer JSJ. Microstructural changes in alkali activated fly ash/slag geopolymers with sulfate exposure. Mater Struct 2013;46(3):361–73. € Karahan O. Influence of activator on the strength and drying shrinkage of alkali-activated slag mortar. Constr Build 238. Duran Atiş C, Bilim C, C ¸ elik O, Mater 2009;23(1):548–55. 239. Sagoe-Crentsil K, Brown T, Taylor A. Drying shrinkage and creep performance of geopolymer concrete. J Sustain Cem-Based Mater 2013;2(1):35–42. 240. Palacios M, Puertas F. Effect of superplasticizer and shrinkage-reducing admixtures on alkali-activated slag pastes and mortars. Cem Concr Res 2005;35(7):1358–67. 241. Sakulich AR, Bentz DP. Mitigation of autogenous shrinkage in alkali activated slag mortars by internal curing. Mater Struct 2013;46(8):1355–67. 242. Collins F, Sanjayan JG. Cracking tendency of alkali-activated slag concrete subjected to restrained shrinkage. Cem Concr Res 2000;30(5):791–8. 243. Bernal SA, Mejı´a de Gutierrez R, Pedraza AL, Provis JL, Rodrı´guez ED, Delvasto S. Effect of binder content on the performance of alkali-activated slag concretes. Cem Concr Res 2011;41(1):1–8. 244. H€akkinen T. The influence of slag content on the microstructure, permeability and mechanical properties of concrete. Part 1: microstructural studies and basic mechanical properties. Cem Concr Res 1993;23(2):407–21.

Geopolymers and Other Alkali-Activated Materials

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245. H€akkinen T. The influence of slag content on the microstructure, permeability and mechanical properties of concrete. Part 2: technical properties and theoretical examinations. Cem Concr Res 1993;23(3):518–30. 246. Ma Y, Ye G. The shrinkage of alkali activated fly ash. Cem Concr Res 2015;68:75–82. 247. Hooton RD. Current developments and future needs in standards for cementitious materials. Cem Concr Res 2015;78(A):165–77. 248. Juenger M, Provis JL, Elsen J, Matthes W, Hooton RD, Duchesne J, Courard L, He H, Michel F, Snellings R, de Belie N. Supplementary cementitious materials for concrete: characterization needs. Mater Res Soc Symp Proc 2012;1488. https://doi.org/10.1557/opl.2012.1536. 249. McLellan BC, Williams RP, Lay J, van Riessen A, Corder GD. Costs and carbon emissions for geopolymer pastes in comparison to Ordinary Portland Cement. J Clean Prod 2011;19(9-10):1080–90.

FURTHER READING 250. Juenger MCG, Winnefeld F, Provis JL, Ideker J. Advances in alternative cementitious binders. Cem Concr Res 2011;41(12):1232–43. 251. Myers RJ, Bernal SA, Provis JL, Gehman JD, van Deventer JSJ. The role of Al in cross-linking of alkali-activated slag cements. J Am Ceram Soc 2015;98(3):996–1004. 252. Zhang J, Shi C, Zhang Z, Ou Z. Durability of alkali-activated materials in aggressive environments: A review on recent studies. Constr Build Mater 2017;152:598–613. 253. Ye H, Radlin´ska A. Shrinkage mitigation strategies in alkali-activated slag. Cem Concr Res 2017;101:131–43. 254. White CE, Olds DP, Hartl M, Hjelm RP, Page K. Evolution of the pore structure during the early stages of the alkali-activation reaction: an in situ smallangle neutron scattering investigation. J Appl Cryst 2017;50(1):61–75. 255. Krivenko P. Why alkaline activation – 60 years of the theory and practice of alkali-activated materials. J Ceram Sci Technol 2017;8(3):323–34. 256. Simon S, Gluth GJG, Peys A, Onisei S, Banerjee D, Pontikes Y. The fate of iron during the alkali-activation of synthetic (CaO-)FeOx-SiO2 slags: An Fe K-edge XANES study. J Am Ceram Soc 2018;101(5):2107–18. 257. Richardson IG, Li S. Composition and structure of an 18-year-old 5M KOH-activated ground granulated blast-furnace slag paste. Constr Build Mater 2018;168:404–11. 258. Luukkonen T, Abdollahnejad Z, Yliniemi J, Kinnunen P, Illikainen M. One-part alkali-activated materials: A review. Cem Concr Res 2018;103:21–34. 259. Ke X, Criado M, Provis JL, Bernal SA. Slag-based cements that resist damage induced by carbon dioxide. ACS Sust Chem Eng 2018;6(4):5067–75.