Geopolymers based on natural and synthetic metakaolin - A critical review

This critical review is the follow up of a feature article titled Environmental implications of Geopolymers, online on 29 June 2015. It has been written in compliance with a decision of Elsevier and Geopolymer Institute to join forces, distill and distribute the best research publications contained in their combined archives, through a series of Elsevier-Geopolymer Institute Virtual Special Issues on Geopolymer Science, online 29 June 2015.

The author was assisted in this review by Xuemin Cui and Dechang Jia.

Much of the original research into geopolymers was conducted on calcined kaolinitic clay precursors known under the generic term of metakaolin. Although metakaolin reacts in alkaline as well as in acidic medium, the present issue focusses exclusively on the alkaline route. The acidic phosphate-based technology will be presented in another issue, in the next future.

Forty years ago, in October 1975, in our CORDI laboratory (later Cordi-Géopolymère) in Saint-Quentin, France, we were testing a new French metakaolin brand named Argical®. It was manufactured with an advanced technology in a flash calciner instead of being roasted in a rotary kiln or a vertical multiple-hearths oven. We discovered that this metakaolin was reacting very well with soluble alkali silicates. I recognized the potential of this discovery and presented an Enveloppe Soleau for registration at the French Patent Office. It was the first mineral resin ever manufactured. Chapter 1 of the book Geopolymer Chemistry and Applications describes this major milestone. The title of the patent, Mineral polymer, was self-evident (Davidovits, 1979).

In 1983, at the Central laboratory of Lone Star Industries, Houston, USA, we started the development of advanced cementicious materials. This research yielded the discovery of the first metakaolin-based geopolymer cement (the cement PYRAMENT). However, we had to test at least 10 different metakaolin brands in order to find the right product, which would react as a geopolymeric precursor, in alkaline medium. Indeed, at that time, the bulk of the various metakaolins was used essentially as fillers in the paper making and plastic industry. Its specific chemical reactivity towards alkalis remained confined in the production of very special products, namely synthetic zeolites, especially the type Zeolite A. In addition, it was striking to discover that the metakaolin sources for zeolite manufacture were according to Breck (1974) of two types, one calcined at 550°C (low temperature metakaolin) and the second at 925°C (high temperature metakaolin). Both metakaolins reacted weakly compared to the metakaolin we had been working with in France. We recognized that we had had luck when starting the geopolymer research, in Saint-Quentin. We had tested the right source of metakaolin, from the beginning. And we became aware of one major parameter in geopolymer science, namely the calcining temperature of the geological kaolinitic clays.

The chemical formula for kaolinite is Si₂O₅Al₂(OH)₄. From a geopolymer standpoint we may write ºSi-O-Al-(OH)₂ with the covalent aluminumhydroxyl - Al-(OH)₂ side groups of the poly(siloxo) hexagonal macromolecule [Si₂O₅]_n. This new structural approach has profound consequences with regard to a better understanding of geopolymerization mechanisms. In particular, according to the reaction:

Si₂O₅Al₂(OH)₄ - > Si₂O₅Al₂O₂ + 2H₂O

metakaolin results from the dehydroxylation of the OH groups in kaolinite. The reactive molecule is an alumino-silicate oxide Si₂O₅Al₂O₂, or  º Si-O-Al=O. This suggests strong chemical reactivity for this aluminium oxide, as opposed to the traditional way of writing 2SiO₂·Al₂O₃. Metakaolin is not alumina! See for details in Chapter 8 of the book Geopolymer Chemistry and Applications.

According to MacKenzie (1985), calcined kaolinite still contains 10% of untouched –OH groups. We coined it KANDOXI, acronym for KAolinite, Nacrite, Dickite OXIde. Other researchers who continued to use the general term metakaolin did not adopt this label. Therefore, after several discussions, we decided to change the terminology in introducing MK-750, meaning metakaolinite calcined at 750°C. It will be used all throughout this review.

However, in addition to temperature control, it is the kiln technology, which determines the feasibility and production of the alumino-silicate oxide MK-750. In calcination carried out in a rigid vertical multiple-hearths calciner, a sufficiently low water vapour pressure is maintained during the entire roasting process, providing the desired chemical reactivity (Al in 5-fold coordination). Same for products manufactured in a flash calciner. This is not the case for metakaolins obtained in a rotary kiln, commercialized as Portland cement additives. Unfortunately, this later product is more and more used in geopolymer research because it is easily available. This raises new concerns in terms of reactivity and reproducibility of the results obtained with this raw material essentially tailored for Portland cement applications, not for geopolymer technologies.

The present special issue, acknowledging the abundance of publications on metakaolin-based geopolymers, tries to collect the best of the scientific production for the understanding of geopolymer resins, binders and cement properties. Our review will also pinpoint those papers where the authors overlooked the important parameters mentioned above.

The geopolymer chemistry was invented 40 years ago because we had the luck to get the right geological raw material and the appropriate calcination process. Today, there exist new methods based on the synthesis of alumino-silicates. In short, we have natural metakaolin MK-750 and artificial, or synthetic metakaolin SMK-750. Therefore, this review is split into two subthemes, namely:

- Geopolymers based on natural metakaolins MK-750;

- Geopolymers based on synthetic metakaolins SMK-750;

1. Geopolymers based on natural metakaolin

Our first contact with MK-750 in 1975 was very exciting. We had prepared a mixture of 1 kg powder with 0.3 kg NaOH solution 12M and let it mature in a plastic bag for a while. After 1 hour, we were surprised by the high amount of water vapour and condensation seeping outside of the bag, with a temperature exceeding 100°C, and a polyethylene bag totally destroyed. We had discovered one of the major properties of MK-750, namely its powerful exothermicity and reactivity in alkaline medium. This characteristic was mentioned in all our earlier published papers, for example Davidovits (1991, 1994, 1999). It helped us to develop a standard method for quality control and selection of reactive raw materials. This technique was successfully used during our selection of the US metakaolin brands mentioned in the introduction. Despite this highly propagated property, the amount of research carried out and the number of published papers on this topic is surprisingly low. The paper by Xiao Yao et al. (2009) is worthwhile of mentioning. It confirms the various parameters involved in the development of this exothermicity, namely: curing temperature, alkalinity ratio SiO₂:Na₂O.

a) Reactivity and calcination methods:

The reactivity and exothermicity are closely related to the calcination method of the raw kaolinitic clay source. The standard parameters set at the Geopolymer Institute and industry partners laboratories are described in Chapter 8 of the reference book Geopolymer Chemistry and Applications. They involve the grinding of the clay, and calcining in an electric oven, in air, at 750°C for 3 hours; the heating time from 20°C up to 750°C is 1h 30 min., the cooling time 1h 30 min, totalizing 6 hours in the furnace. What we find in the literature is totally different, so that it is hard to compare any results deriving from these researches. The following non-exhaustive list is representative of what we came across during this critical review of publications; it gives the calcination temperature and the time (yet we do not know if it includes the heating and cooling times).

- Zibouche et al., (2009), 800°C, 2 h.

- Zhang Yunshen et al, (2009), 700°C, 12 h.

- Rowles et al., (2009), 750°C, 24 h.

- Yao et al., (2009), 900°C, 6 h.

- Medri et al. (2011), 750°C, 15 h.

Others are simply using commercial brands:

- Zhang et al., (2009), commercial MetaMax® from Engelhardt (now BASF).

- Favier et al., (2013, 2015), commercial Argical® M1000 from Imerys.

- Medri et al., (2010), commercial Argical® M1000 and M1200 S from Imerys.

Medri et al. (2010) tested two metakaolins manufactured industrially by the company Imerys with two different kiln technologies. One called M1000 is calcined in a rotary kiln and characterized by rounded massive aggregates of lamellar particles. The second, called M1200S, calcined in a flash kiln, is made up of fine lamellar particles with lower agglomeration. Granulometric distributions are broad and multimodal for both metakaolins. M1000 particles are coarser with a mean grain size d₅₀=6.5 μm, while M1200S has a d₅₀=1.7 micron. Scientists prefer the brand M1000 because of its low water demand. M1000 like the brand MetaMax and others are essentially commercialized as additive to Portland cement. The calcination cycle lasts 4 hours at 700–750°C in the rotary kiln (production 10 tonnes/h). On the opposite, the product M1200 is processed in a flash calciner with an air flow at 950°C during 1 second time (production 1 tonne/h). We mentioned in the introduction the beneficial influence of this flash calcined metakaolin in the discovery of geopolymer science. Some years later, in the 1980s on, the product was no longer manufactured and replaced by another brand, also very popular in geopolymer research, MetaStar 501 (same as Polestar 501) from ECC Int. UK, now Imerys. Highly refined kaolin clay is calcined in a vertically oriented multiple hearth furnace. The material is moved by mechanical rakes across each hearth and then drops to the next hearth below. Each hearth has separate temperature controls and, unlike with rotary kilns, the time that the material is inside the furnace and the temperature gradient that the material is exposed to is precisely controlled ensuring the consistent production of high- purity highly reactive metakaolin. It is our reference material (coined Kandoxi and later MK-750) in our publications (see Chapter 8 of the reference book Geopolymer Chemistry and Applications).

b) MAS-NMR spectroscopy:

In addition to exothermicity, the best investigation tool to determine the geopolymeric reactivity of metakaolin is the ²⁷Al MAS NMR spectroscopy. According to Medri et al., (2010), the simulation of the NMR spectra for the commercial brands showed that the relative concentration of the aluminium species Al(6), Al(5) and Al(4) is:

- M1000: 35% Al(6), 50% Al(5), 15% Al(4), with Al(5) + Al(4) = 65%.

- M1200S: 25% Al(6), 55% Al(5), 20% Al(4), with Al(5) + Al(4) = 75%.  

In comparison,

- MK-750 (MetaStar 501): 24% Al(6), 49% Al(5), 27% Al(4) with Al(5) + Al(4) = 76%.

The geopolymeric reactivity of M1200S is equivalent to the one of MK-750, and far better than M1000.

The introduction of MAS-NMR spectroscopy in the study of silicates by Lipmaa et al. (1980), Mäller et al. (1981), transformed our view and our knowledge on silicate structures. The most important contribution for our study was made in 1985 by MacKenzie’s team from New Zealand. The first paper by Meinhold, MacKenzie and Brown (1985), reads: “Metakaolinite contains 11 to 12% of the original kaolinitic water, associated with the 8% of aluminium atoms which remain in undistorted sites. Approximately one-half of these well-defined aluminium sites are octahedral, one-quarter are tetrahedral, and the remainder are either tetrahedral, or some other regular site with chemical shift intermediate between tetrahedral and octahedral.” This chemical shift intermediate will be later assigned to pentahedral aluminium sites Al(5). But, in their second paper, MacKenzie et al. (1985), the authors could not claim of having discovered this new Al(5). It was against the view of the main stream of scientists at that time.

Three years later, the paper by Sanz et al. (1988) confirmed the presence of Al(4) and Al(5), in addition to Al(6). They applied MAS-NMR spectra to follow the kaolinite-mullite transformation with temperature from 20°C to 1200°C. The ²⁷Al MAS-NMR spectrum for raw kaolinite (20°C) contains one resonance at 0 ppm characteristic of Al(6). When the sample is heated, the intensity of this line decreases up to 980°C, then increases again at 1055°C (transformation to mullite). In the medium temperatures above 400°C, that is during the dehydroxylation phases of kaolinite, two new lines appear at 55–60 ppm assigned to Al(4) and at 25–30 ppm assigned to Al(5). The intensity of the Al(5) line increases with temperature from 450°C to 850°C, then it decreases and disappears at 980°C. The maximum intensity is spread 700–850°C. The Al(4) line remains with the same intensity.

Nonetheless, studies on the calcination time at definite temperatures are seldom. None of the papers cited above are mentioning the reason why the calcination was run at a particular temperature and during a given time. For example, why the authors in Zibouche et al. (2009) calcined their kaolinitic clays at 800°C for 2 hours, and not at 750°C, 24 hours long like in Rowles et al., (2009) ? Or why Xiao Yao et al., (2009), fired at 900°C for 6 hours, when we already knew, thanks to the research published by Sanz et al., (1988) that the intensity line of the most reactive Al specie, Al(5), diminishes above 850°C. From unpublished, internal source, Wang Meirong (2011), we learn, that when calcination time at 900°C is less than 4h, the characteristic NMR resonances for Al(4) and Al(5) are very well defined. However, when calcination time is prolonged, these reactive species, especially (Al)5, decrease in intensity, due to the metakaolinite-spinel-mullite transformation.

c) Reaction mechanism:

Another important issue in metakaolin-based geopolymerization relates to its reaction mechanism. At the beginning of geopolymer research (Davidovits, 1976) and afterwards for at least 25 years, it was assumed that the geochemical syntheses occurred through hypothetical oligomers (dimer, trimer). Further polycondensation of these hypothetical building units provided the actual structures of the three-dimensional macromolecular edifice. Review papers published at the First Geopolymer Conference in 1988, and at the second, 11 years later, in 1999, could not present scientific details describing the actual reaction mechanism (Davidovits, 1988, 1991, 1999) (see the page Science at the Geopolymer Institute).

The most important contribution to this issue is the paper by North and Swaddle (2000). Using ²⁹Si and ²⁷Al NMR spectroscopy, they suspected the presence of solute species with Si-O-Al sialate linkages in concentrated solutions. One major improvement in their research was that their study was carried out at low temperature, at 5°C and below. Indeed, it was discovered that the polymerization of oligo-sialates was taking place on a time scale of around 100 milliseconds, i.e. 100 to 1000 times faster than the polymerization of ortho-silicate. At room temperature, or higher, the reaction is so fast that it cannot be detected with conventional equipment. They chose KOH over NaOH used in their previous study because concentrated KOH alumino-silicate solutions resist gelation longer than their NaOH analogues. Due to the very weak signal of ²⁹Si, the NMR experiments had to be run up to 3 days long to get significant detailed spectra. They successfully detected five solute species, two linear molecules and three cycles:

- one ortho-sialate  (OH)₃-Si-O-Al(OH)₃ for Si:Al=1;

- one linear ortho(sialate-siloxo) (OH)₃-Si-O-Si(OH)₂-O-Al(OH)₃, one cycle ortho(sialate-siloxo), for Si:Al=2;

- two cycles ortho(sialate-disiloxo), for Si:Al=3.

The hypothetical oligomers set forth in geopolymer synthesis were no longer virtual molecules. As a matter of fact, they exist in soluble forms and are stable in concentrated solutions at high pH. Swaddle’s study confirmed the polymerization mechanisms tentatively reported earlier by Davidovits (1976) with linear oligo-sialate, oligo(sialate-disiloxo) and rings or cycles, as starting geopolymer building units.

As Zhang Yunshen et al. (2009) stressed out, most of the researchers describe the formation process of metakaolin-based geopolymers (in his case geopolymer cement) by means of a traditional hydration mechanism as for Portland cement or alkali slag cement. For these cement scientists, the geopolymerization reaction involves a 3-step process, namely dissolution–reorientation– polycondensation. However, the setting and hardening of geopolymer cement is so rapid that this 3-step process almost takes place at the same time. Therefore, it is impossible to isolate the three steps by experimental study, yielding no better understanding of the details of each step until now.

Nevertheless, for the main stream of scientists, the first step involves the dissolution of metakaolinite releasing monomeric Si(OH)₄ and Al(OH)₄, even when we know from Swaddle’s work that the reaction in strong alkaline medium between the monomeric species is so fast that it becomes undetectable. In fact, the suggested dissolution mechanism of metakaolinite should be similar to the one set fourth in Duxson and Provis (2008) paper for alumino-silicate glass. The breakdown of the alumino-silicate glass molecular structure in high pH leads up to a well-defined molecule, namely the otho-sialate (OH)₃-Si-O-Al(OH)₃ before the supposed final release of monomeric Si and Al. But, we already know that the monomers immediately polymerize into this same ortho-sialate in solution, according to Swaddle. In other words, there is no isolated monomeric Al and Si. The breakdown of metakaolinite in strong pH, which is always the case in geopolymerization, does not follow the mechanism set forth for acidic and low-medium alkaline mediums with its preferential solubilisation of monomeric Al(OH)₄ followed by Si(OH)₄.

Therefore, the first step of the reaction mechanism is the formation of this otho-sialate (OH)₃-Si-O-Al(OH)₃ molecule and results from the breakdown of the Si-O-Si chain (see the chemical mechanism displayed at the page Science of Geopolymer Institute internet site http://www.geopolymer.org/science/about-geopolymerization/ and in Chapters 7–8 of Geopolymer Chemistry and Applications).

Supporting information to this claim is provided by another important paper by Bauer and Berger (1998). They followed the entire reaction of kaolinite in KOH solutions and examined the dissolution rate of kaolinite in high pH KOH solutions (0.1 to 4M KOH) at temperatures of 35°C and 80°C in a batch reactor. They found that the aqueous concentrations of Si and Al increased linearly with log (t) whatever the temperature and the KOH concentration was. More, the amounts of Si and Al are identical with time, i.e. one Si is released together with one Al, simultaneously. This result is in favour of the presence in solution of the stable otho-sialate (OH)₃-Si-O-Al(OH)₃ molecule. Since the 1970s, in all papers, the concentrations of Al in solutions are analysed colorimetrically with an UV-visible spectrophotometer, using the Catechol violet method. Then, separately, Si concentrations are measured with the Molybdate blue method. Applied to the oligo-sialate molecule, this method gives 1 Al for 1 Si in solution, even when there is no monomeric Si and Al.

Despite the strong evidence towards the formation of the oligo-sialate molecule, several authors like Buchwald et al. (2011) continue to propagate a model system based on the interaction of monomeric Na-silicate and Na-aluminate. Like many others, they do not take into account the macrocospical structure of the raw material metakaolin, per se. The calcination of kaolinite does not destroy the lamellar structure of the tabular shaped metakaolinite pellets. Electron microscope studies by German scientist Eitel showed, as early as 1939, that kaolinite particles retain their hexagonal outline far above the dehydration temperature, up to 750°C. The alkaline attack starts on the outer faces of the metakaolinite particle. It continues, layer by layer, from the edges to the inside. This is a very important feature, which induces two different geopolymerization mechanisms.

We discovered the implications of this structural parameter by chance. Our PhD student M. Zoulgami (1997-2000) prepared a MK-750-based Na–poly(sialate-siloxo) geopolymer (with Si:Al=2) dedicated to bio-material applications. This implied a drastic diminution of the pH from pH 10.5–11 to a neutral value in the range of 7.5, by heat-treatment at 750°C. In the paper by Zoulgami et al. (2002), the X-ray diffraction of the dried geopolymer showed two major crystalline phases: nepheline (NaAlSiO₄), Si:Al=1, and albite (NaAlSi₃O₈), Si:Al=3. These two phases are coexisting in the ratio 50/50 and the bulk reactional mix corresponds to the formula

Na_4.5Al_4.5Si_9.04O_27.11, i.e. NaAlSi₂O₆ or Si:Al=2

The EDS analyses performed on this geopolymer treated at 750°C show that the bulk chemical composition is equal to NaAlSi₂O₆. This means that both geopolymer phases, namely nepheline and albite detected by X-ray diffraction, are in solid solution, on the nano scale. Duxson et al. (2005) also noticed for temperature above 500°C the formation of nepheline with Na and of a not well-defined component similar to albite. One geopolymerization generates the formation of a Si:Al=1 geopolymer, the second a Si:Al=3 geopolymer, with an overall value Si:Al=2.

As recognized by MacKenzie’s team in Bo Zhang et al. (2009) paper, MK-750-based geopolymer binders and cements are different from the simple alkali activation with NaOH, published by several authors. This NaOH simple alkali-activation of metakaolin generates products of the type Zeolite A. For Bo Zhang et al. (2009), a geopolymer cement or a geopolymer binder implies the reaction of MK-750 with soluble (Na, K) alkali silicates. The authors suggest that the various silicate units (Q₀, Q₁, Q₂, Q₃) present in the alkali-silicate solution have a templating function during geopolymerization. 

The relatively recent papers by Favier et al. (2013, 2015) are the most important papers so far, dealing with this important and evident structural parameter. Their elastic modulus study on MK-750-based geopolymer suspensions confirms the 2 step reaction mechanism mentioned above. At the very early stage of the reaction (fewer than 15 min after the beginning of mixing), aluminosilicate molecules  with Si:Al ratio in the range of 2 to 3 are formed at the grain boundaries, while the rest of the solution is still mainly composed of silicate/siloxonate oligomers of type Q₀, Q₁, Q₂ and also Q₃. In the second phase, the geopolymer hardens with a second rapid increase of the elastic modulus. This fast increase indicates that a new chemistry is taking over, with Si:Al=1, which does not involve any siloxonate molecules, only NaOH. As the geopolymerization progresses, at the scale of dozens to hundreds of nanometres, different sialate species coexist as a distribution of solid solutions between Si:Al=1 to Si:Al=4, more specifically poly(sialate) of the nepheline type, Si:Al=1, together with poly(sialate-disiloxo) of the albite type, Si:Al=3, and others.

Accordingly, the geopolymerization of MK-750 with (Na,K)-silicates results from two reaction mechanisms taking place in the following order:

- Phase 1: outer faces/edges reaction involving Na+, K+, OH− and Q₀, Q₁, Q₂ and Q₃ siloxonates; resulting geopolymer with atomic ratio Si:Al between 1 and 4;

- Phase 2: inner particulate reaction with only Na+, K+, OH−; resulting geopolymer with atomic ratio Si:Al=1.

d) Applications

MK-750-based geopolymers have a variety of applications. They are new binders and resins for coatings and adhesives, fibre composites, waste encapsulation and new cement for concrete. The wide variety of applications includes: fire-resistant materials, decorative stone artefacts, thermal insulation, low-tech building materials, low energy ceramic tiles, refractory items, thermal shock refractories, bio-technologies (materials for medicinal applications), foundry industry, cements and concretes, composites for infrastructures repair and strengthening, high-tech composites for aircraft interior and automobile, high-tech resin systems, radioactive and toxic waste containment, arts and decoration, cultural heritage, archaeology and history of sciences.

Some of these applications were already presented in the issue Environmental Implications http://www.materialstoday.com/polymers-soft-materials/features/environmental-implications-of-geopolymers/; it discusses essentially those dealing with fire and heat resistance, on the one hand, toxic- and radioactive waste management and cement applications, on the other hand. In particular, several published scientific LCA papers claim that, in terms of CO₂ emission, geopolymer cement was not better than Portland cement, and worse for other parameters. These statements are based on methodological errors and false calculations of the CO₂ emission values for geopolymer cement/concrete. The problem is that these false values are taken for granted by other scientists without any further consideration, as for example in a recent paper by Provis et al. (2015).

The route to high-temperature ceramics via MK-750 geopolymerization was pioneered by W. Kriven and her team at the University of Illinois, USA. They converted fully condensed MK-750-K–poly(sialate-siloxo) into high valuable leucite ceramic KAlSi₂O₆. This demonstrated an alternative route in the production of these highly valuable refractory ceramics, without the necessity of expensive equipment, long processing times, and costly precursors (Kriven et al., 2003, 2006). One paper by Kriven et al. (2005) was presented at the Geopolymer 2005 Conference. Dechang Jia and his team at Harbin Institute of Technology, China, also focused their research on this geopolymer route. When compared to hydrothermal methods, geopolymer technology is advantageous in low cost and short fabricating time; with the proper processing procedure, geopolymer can be directly converted into the final structural leucite, kalsilite or pollucite ceramic. See the video of the Keynote paper, High-temperature geopolymer carbon fiber reinforced composites and their derived CMC, by Dechang Jia at Geopolymer Camp 2011 at http://www.geopolymer.org/conference/gpcamp/gpcamp-2011/.

He et al. (2010) stressed how the geopolymer route is of great interest. They discovered that the addition of cesium stabilized the geopolymeric leucite. The cesium substitution for potassium was effective in stabilizing the cubic polymorph of leucite to room temperature. In the paper by He et al. (2011), they used MK-750 containing a certain amount of silica (quartz). They obtained the transformation into leucite at a temperature as low as 800–900°C instead of 1050–1100°C with pure kaolin and the ceramic firing.

2. Geopolymers based on synthetic metakaolin

The conclusion of the pioneer paper by Davidovits (1976) states: “… we still use natural polymeric minerals for transformation into other mineral polymers. The works made recently on the low- temperature synthesis of clay minerals pioneered especially on kaolinite suggest that in the near future it will be possible to perform the synthesis of man-made rocks from monomeric elements, for example from silico-aluminate monomers or oligomers.”

The idea would be to manufacture kaolinite-like products and to calcine them like for MK-750. But, these first synthesis from year 1976 and older were still complicated to carry out. A progress towards a simpler synthesis of kaolinite-like minerals, Si₂O₅Al₂(OH)₄, may be traced back to De Kimpe et al. (1981). They used a sol-gel method involving the simultaneous hydrolysis of TEOS tetraethylorthosilicate (Si(OC₂H₅)₄) as Si source and aluminium isopropoxide (Al(OCH(CH₃)₂)₃ for the Al source.

a) Different synthesis methods

A real breakthrough towards synthetic metakaolin SMK-750 occurred in 2005 with the research carried out by W. Kriven and her team at University of Illinois at Urbana Champaign, USA. Since 1995, W. Kriven used a sol-gel technique known as the “steric entrapment method” to create a variety of ceramic oxides. Her team, Gordon et al. (2005), explains how during the organic steric entrapment method, water solutions of aluminium nitrate nonahydrate Al(NO₃)₃ 9H₂O and sol-SiO₂, are added to a soluble polymer, typically polyvinyl alcohol PVA, dissolved into solution together and mixed. After mixing, the solute (water or alcohol) is removed by drying at 110°C and the resulting powder is calcined. During calcination, the anionic components of the salts are sublimed and the cation components oxidize. The resulting powder is chemically homogenous with a Al₂O₃,1.3SiO₂ composition and after calcination at 700-800°C, the SMK-750 synthetic metakaolin has a surface area of 166m²/g and a d₅₀ particle size of 1.62 μm.

We have presently 3 published methods for synthetic metakaolin SMK-750, namely:

- Steric entrapment method, according to Kriven’s team, Gordon et al. (2005), involving aluminium nitrate nonahydrate Al(NO₃)₃ 9H₂O, sol-SiO₂ and polyvinyl alcohol PVA.

- Sol-gel system according to Xuemin Cui and his team at Guangxi University, Nanning, China. Cui et al. (2008), include aluminium nitrate nonahydrate Al(NO₃)₃ 9H₂O and TEOS tetraethylorthosilicate (Si(OC₂H₅)₄).

- Sol-gel procedure according to De Kimpe et al. (1981), used by Chan and his team at National Taiwan University, Taipei, Taiwan. Tsai et al. (2010) combine aluminium isopropoxide (Al(OCH(CH₃)₂)₃ and TEOS tetraethylorthosilicate (Si(OC₂H₅)₄).

Actually, the three methods are based on sol-gel, which yields SMK-750-types with high purity and homogeneous chemical composition. However, there are also differences between them. Kriven’s and Cui’s methods use aluminium nitrate nonahydrate as the Al source, but different Si sources are selected, i.e. sol-Silica or TEOS. Sol-silica is cheaper in price than TEOS and this could have an incidence on the choice of the technique. There are also differences in properties.

In the sol-gel SMK-750 preparation method after Cui et al. (2010), the starting materials and solvents are mixed in the following molar ratios: Al₂O₃:SiO₂ is 1:2 and SiO₂:H₂O to ethanol EtOH is 1:18:12. In a typical synthesis, two solutions are prepared while stirring;

- Solution A, tetraethylorthosilicate TEOS is dissolved in EtOH;

- Solution B, aluminium nitrate nonahydrate ANN is dissolved in a mixture of EtOH and distilled water.

- Solution B is then added to the solution A slowly while stirring, and the resulting mixture is maintained at 70 °C until a gel formed.

- The gel is then dried at 105 °C.

- Finally, the dried gel powder is calcined in air at temperatures 700 °C to 800 °C, for 2 h to form the SMK-750 synthetic metakaolin.

The SMK-750 synthetic metakaolin has a surface area in the ranges 200-600 m²/g and a d₅₀ particle size of 250 nm (0.25 mm).

As for the sol-gel procedure according to De Kimpe et al. (1981), the preparation is described in Tsai et al. (2010) paper :

- Aluminium iso-propoxide (Al(ⁱPO)₃) and tetraethyl orthosilicate (TEOS) are added into deionized (DI) water at regular time intervals.

- The solution is further agitated for 24 h to complete the precipitation process.

- The mixture is dried at 60°C for 36 h and ground.

- The powder is then mixed with 10 mL of 0.1 M KOH solution. The mixture is hydrothermally treated at 240°C in an autoclave with a duration varying from 48 h to one week.

- After the heat treatment, the product is collected by suction filtration and dried at 60°C for 24 h. It is calcined at 700°C for 10h to form the SMK-750 synthetic metakaolin.

This third preparation introduces a hydrothermal treatment in autoclave at 240°C for several days, to be compared with the two other methods that require a simple drying at 105-110°C, before calcination. We must remember that De Kempe’s target was the synthesis of kaolinite. Indeed, according to the XRD analysis, the product obtained after the autoclave treatment is crystalline with the same XRD patterns as natural kaolinite. In comparison, the kaolinite-like powders obtained with the other methods have an amorphous hump at 25° 2theta.

To sum up: the autoclave treatment is not necessary in the preparation of SMK-750. The choice between the three methods will depend on the availability and costs of the chemical reactants and solvents.

The noticeable difference is the particle size. The SMK-750 prepared with Cui’s method is nano-scaled, with an average particle diameter of about 0.25 μm, while the SMK-750 obtained according to Kriven’s method is of micron scale, with an average particle diameter of about 1.6 μm or higher. This higher particle size is probably due to aggregation or densification. The authors (Gordon et al., 2005) noticed that “.. it had been found previously that trace alcohol on the surface of SMK prevents the reaction with alkali-silicate solutions.” As a consequence, the preparation includes a second heat treatment at 800°C to remove all organics from the surface of the SMK-750. This additional calcination generates densification and agglomeration of the particles. In Medri et al. (2010) paper, natural MK-750, brand M1200S, calcined in a flash kiln, has a d₅₀=1.7 μm. This size is similar to Kriven's SMK-750 powder. Yet, its surface area is low, only 23 m²/g, to be compared with 166 m²/g and higher, up to 600 m²/g for the synthetic metakaolins.

b) MAS-NMR spectroscopy

At the first glance, ²⁷Al and ²⁹Si MAS-NMR spectra of SMK-750 synthetic metakaolins are similar to those obtained for natural MK-750. They display Al resonances assigned to Al(4), Al(5) and Al(6), but the intensities are different.  The relative concentrations of the aluminium species Al(6), Al(5) and Al(4) of our reference MK-750 (MetaStar 501) and SMK-750 obtain with the method by Cui according to Zheng et al. (2013) are :

- SMK-750: 44.4% Al(6), 49.6% Al(5), 6.1% Al(4), with Al(5) + Al(4) = 55.7%.

-   MK-750: 24.0% Al(6), 49.0% Al(5), 27%  Al(4), with Al(5) + Al(4) = 76%.

The amount in Al(4) is surprisingly low in the sol-gel product and the geopolymeric reactivity of SMK-750 is less than the one of MK-750. This is due to the high amount of Al(6) species, twice the amount in synthetic MK than in natural MK-750, probably attributed to remains of precipitated nano-size Al₂O₃ particles.

It should be noted that according to Cui et al. (2010), the resonances related to Al(4) and Al(5) come into sight in the spectra of the calcined powders at temperatures as low as 200°C-300°C. Their intensities increase up to 800°C, essentially for Al(5). Similar resonances usually take shape in kaolinite calcination at temperatures over 450 °C; see the paper by Sanz et al. (1988) on the transition of natural kaolinite to metakaolinite.  

There are also several ²⁹Si resonances in SMK mainly centred at -101.9 and -105.2 ppm, corresponding to Q₄(1Al) and Q₄(0Al), respectively. The ²⁹Si spectra for SMK-750 show a high concentration in SiO₂, which is not found in natural MK-750 when quartz SiO₂ is absent. Like for aluminium, the high concentration of SiO₂ is probably a result of precipitated nano-size colloidal SiO₂.

c) Reactivity

Forty years ago, we discovered the reactivity of natural metakaolin partly because of its exothermicity. We also complained about the lack of research carried out on the study of this parameter, during geopolymerization. The contact with synthetic metakaolin SMK-750 is even more dramatic. In any papers referenced in this critical review, we find a sentence similar to this one: "... the reaction rate is very fast and the preparation process is highly exothermic. So, the reaction device should be put in ice water bath to lower the reaction rate and to prevent the rapid setting of the geopolymer ."

The high reaction rate may be due to the very high surface area of SMK-750, namely 166 m²/g and higher, up to 600 m²/g. This is to be compared with the values for MK-750 that are in the range of 15 to 23 m²/g. In addition, the morphology of SMK-750 is quite different. Although SMK-750 shows sheet-like morphologies similar to natural MK-750, the layered structure of SMK is clearly not the same. Cui et al (2008) observed the morphology after acidic leaching. The synthetic metakaolin has an irregular layered structure with individual layers likely composed of many particle clusters.

TEM morphology of the SMK-750-based geopolymer is similar to that of natural metakaolin. But the resulting nano-particulates (we called them geopolymer micelles) are smaller, in the 4 nm in diameter range, to be compared with 10-20 nm for MK-750-based geopolymer. It has a denser and more homogeneous microstructure.

The mechanical properties depend on the calcination temperature. According to Cui et al. (2010), the compressive strength is 4.8 MPa for SMK-750 treated at 300, 400 and 500°C, 14.2 at 600°C, reaches 28 MPa for 700-800°C and drops to nil when calcined at 900°C.

d) Applications

It is obvious that the SMK-750-based geopolymer technology is not targeted towards mass applications. So far, the purpose of the scientists involved in these developments was to implement a system providing high purity products. It is very difficult to employ the conventional approach with natural or waste products to prepare geopolymers which can meet the stringent requirements for medical applications. Therefore, it is of great interest to develop a synthetic protocol to prepare geopolymers with well-defined chemical composition. This was the target of Chan and his team from Taiwan as well as for Cui and his team at Nanning.

This is also the case in the development of high temperature ceramics, by following the geopolymer route already mentioned for natural MK-750-based geopolymer.  Jia’s team at Harbin Institute of Technology, China, employs the steric entrapment method for the manufacture of pollucite ceramic. In the paper by He and Jia, (2013), the results show that the whole thermal shrinkage (ΔL/L₀) of the Cs-based geopolymer from 25 to 1400°C was 0.17. Thermal shrinkage before 800°C was caused by capillary shrinkage, which contributed to 32.1% of the whole shrinkage. From 800 to 1200°C, thermal shrinkage was caused by viscous sintering, corresponding to 67.9% of the whole shrinkage. The onset and ending sintering temperatures of the Cs-based geopolymer using synthetic metakaolin were much lower than those of the one using natural metakaolin reported by Bell et al. (2009). The average coefficient of thermal expansion of the pollucite ceramic was 2.8×10^-6/°C.

---------------------------------

The present selection of papers dedicated to natural metakaolinite MK-750 in geopolymer synthesis begins with the previously cited Davidovits (1975) document presented at the French Patent Office in Paris. The hand-written text of the Enveloppe Soleau was filed on 29/12/1975, number 70528, at Institut National de la Propriété Industrielle, INPI, Paris. It describes how metakaolin was reacting very well with soluble alkali silicates. It is reproduced with the English translation from French in Chapter 1 of the reference book by J. Davidovits, Geopolymer Chemistry and Applications, since the 3rd edition, 2011 onwards. The link to the free download PDF file of this Chapter 1 is located at http://www.geopolymer.org/shop/product/geopolymer-chemistry-applications/.

We mentioned that the strong exothermicity of the reaction remains a domain where few research has been carried out. The Chapter 8 of the reference book Geopolymer Chemistry and Applications provides several examples. We only found one paper worth to be cited in this review. Published in 2009 by Yao et al., it is titled Geopolymerization process of alkali–metakaolinite characterized by isothermal calorimetry. The authors measured the exothermicity in relation with curing temperature from 20°C to 80°C and other parameters. But, like others, they explain the formation of intermediary ortho-sialate (OH)₃-Si-O-Al(OH)₃ by interaction of monomeric Si and Al, followed by polycondensation into what they call "small catenulate gels". Another point to criticize in this work is the structural representation of MK-750 in their Figure 2. First, in the Al-O-Al layer, the Al atom is represented being tetravalent (AlO₄), in the same way as the Si-O-Si network with its tetravalent Si⁴⁺ configuration. The oxygen atom is also tri-valent O^3-, which is a nonsense. This is a major error, also found in numerous scientific papers, namely the confusion between chemical valence and physical coordination. In metakaolin, the Al atom is tri-valent Al³⁺ but Al is tetracoordinated, Al(4), or pentacoordinated, Al(5) to oxygens. Their false structural representation is copy-pasted in numerous publications dealing with metakaolin-based geopolymer.

The list of papers selected for their data on calcination temperature or provenience, starts with Zibouche et al. (2009) titled Geopolymers from Algerian metakaolin. Influence of secondary minerals. The choice of 750°C, 15 hours is not explained and the purpose of the paper is to demonstrate that any kaolinitic clays are suitable for geopolymerization.

Zhang Yunshen et al, (2009), worked at 700°C for 12 h. Their research contradicts the ideas presented by the mainstream of cement-scientists on the geopolymerization mechanism: Study of ion cluster reorientation process of geopolymerisation reaction using semi-empirical AM1 calculations. Their structural representation of the geopolymerization mechanism is interesting. They state: " By far the important conclusions from these studies are that the major structures and energetics of silicates or alumino-silicates can be accounted for by short-range directional forces or covalent bonding in a traditional sense".

Rowles and O'Connor (2009), calcined at 750°C for 24 h in their publication Chemical and Structural Microanalysis of Aluminosilicate Geopolymers Synthesized by Sodium Silicate Activation of Metakaolinite. Their SEM imaging reveals the presence of a two-phase microstructure; the matrix phase being the fully formed geopolymer with Si:Al=3, while the grain phase is reminiscent of, but chemically dissimilar to, the MK precursor with Si:Al=1. The problem is that the authors did not recognize that these 2 phases are due to the initial layered arrangement of the tabular shaped metakaolin particulates. This feature was explained above in the section Reaction mechanism. It is strange to discover that this unique structural property of metakaolinite is ignored or not taken into consideration by the majority of scientists who try to explain the geopolymerization of MK-750-based geopolymers.

The paper by Medri et al. (2010) is titled Role of the morphology and the dehydroxylation of metakaolins on geopolymerization. It provides MAS-NMR spectra for two commercialized metakaolins, Argical M1000 and Argical M1200S. They are manufactured with two different technologies: rotary kiln for M1000 and flash calciner for M1200S. We stressed out above their differences in terms of Al(4), Al(5), Al(6) concentration in the section MAS-NMR spectroscopy. This paper is very interesting from this point of view. However, geopolymerization was carried out in a weak alkali medium, with a Na-silicate solution with the molar ratio SiO₂:Na₂O=2. We know that geopolymerization should be performed in the range SiO₂:Na₂O=1.25 to 1.80.

The most important study is the one performed by Favier et al. (2015) titled A multinuclear static NMR study of geopolymerization. It is the follow up of Mechanical properties and compositional heterogeneities of fresh geopolymer pastes published in 2013. This study provides strong evidence for a heterogeneous formation and a two-step mechanism. This was already analyzed in the section Reaction mechanism. In this study, they focussed on the solid/liquid heterogeneous suspension. Using static liquid-phase NMR they could specifically monitor the chemical speciation of the mobile species in the suspending liquid (ions, oligomers or small gel particles). It is also one of the rare research, which takes into account the exact molecular composition of the alkali-silicate solution, namely the simultaneous presence of highly depolymerised species, isolated ortho-silicate Q₀, as well as polymerised silicon oligomers, with silicon speciation Q₁, Q₂, Q₃.

Another paper by Zhang B. et al. (2009), Crystalline phase formation in metakaolinite geopolymers activated with NaOH and sodium silicate, stresses the same importance for the molecular structure of the various silicate units (Q₀, Q₁, Q₂, Q₃) present in the alkali-silicate solution. They suggest that they have a templating function during geopolymerization. 

We already mentioned the importance of the research carried out in 1985 by the team around MacKenzie, Meinholds et al. (1985) and MacKenzie et al. (1985). Their discovery of the Al(5) resonance, even if they did not precisely claim it, together with the study by Sanz et al. (1988) changed our view on the molecular structure of metakaolin. It is the first milestone that had a profound impact on our personal development in geopolymer technology.

The second important milestone occurred 15 years later. Titled Kinetics of Silicate Exchange in Alkaline Aluminosilicate Solutions the paper by North and Swadle (2000), clarified our comprehension of the chemical mechanism. It was presented above. As a matter of fact, it is practically unknown by the main stream of scientists. This lack of knowledge explains their incorrect interpretation of the actual chemical mechanisms. The molecular structure of the soluble sialate molecules are displayed at two places in the Geopolymer Institute internet pages: http://www.geopolymer.org/science/scientific-means-of-investigation/

or the video at

http://www.geopolymer.org/conference/webinar/webinar-spring-2014-geopolymer-web-workshop-apr-8-9/, see  Video Talk 1 / Part 2, What is a geopolymer, at time 8:00.

Although their paper is dealing with kaolinite, not metakaolinite, the study performed by Bauer and Berger (1998) is the third important milestone. Titled Kaolinite and smectite dissolution rate in high molar KOH solutions at 35° and 80°C, it demonstrates that in concentrated KOH solution, there is a simultaneous presence of measured Si(OH)₄ and Al(OH)₄ species. This is in opposition to the traditional view of preferential dissolution of monomeric Al, long before Si comes in solution. It supports the formation of the ortho-sialate molecule (OH)₃-Si-O-Al(OH)₃. The chemical mechanism is described at the internet page of the Geopolymer Institute, About Geopolymerization: http://www.geopolymer.org/science/about-geopolymerization/

Buchwald et al., (2011), in their paper Condensation of aluminosilicate gels—model system for geopolymer binders, continue to propagate a model based on monomeric Al and Si.

The fourth milestone is the research carried out in M. Zoulgami PhD thesis. Her paper, Zoulgami et al. (2002), Synthesis and physico-chemical characterization of a polysialate-hydroxyapatite composite for potential biomedical applications, shows the presence of two geopolymers in solid solution, one with Si:Al=1, the second with Si:Al=3, providing the average composition of Si:Al=2.

Even if it is a general description of alkali-activated materials, the comments made in the recent review by Provis et al (2015), Advances in understanding alkali-activated materials, have a negative impact on the good name of geopolymer cements. We already explained above how they are propagating false CO₂ emissions values for geopolymer cement.

We already mentioned how the route to high-temperature ceramics via MK-750 geopolymerization was pioneered by W. Kriven and her team at the University of Illinois, USA. The publications Kriven et al., (2003), Microstructure and Microchemistry of Fully-Reacted Geopolymers and Geopolymer Matrix Composites, as well as Kriven et al., (2006), Microstructure and nanoporosity of as-set geopolymers, are focussing on the method of preparation of leucite type KAlSi₂O₆ ceramics.

The geopolymer route with MK-750-based geopolymer is also of great interest for the preparation of high-temperature stable ceramics developed by Jia and his team from Harbin Institute of Technology, China. According to He et al. (2010), Effect of cesium substitution on the thermal evolution and ceramics formation of potassium-based geopolymer, the properties of leucite type ceramics are enhanced. Yet, in another paper by the same group, He et al., (2011), Thermal evolution and crystallization kinetics of potassium-based geopolymer, the authors discuss the preparation and crystallization of leucite type ceramics. Contrary to Kriven's paper and fully-reacted geopolymer, here, the method is not 100% satisfactory. Indeed, due to the existence of unreacted metakaolin, K-geopolymer was inhomogeneous at the molecular level. Thus a higher energy was required for the structural rearrangement during the change from amorphous to crystallized phase.

------------------

The selection of papers dedicated to synthetic metakaolinite SMK-750 in geopolymer synthesis begins with the previously cited paper by De Kimpe et al., (1981), Hydrothermal Formation of a Kaolinite-like Product from Noncrystalline Aluminosilicate Gels. It does not deal with the synthesis of geopolymer but presents the sol-gel route to the preparation of synthetic kaolinite.

The contribution of Gordon et al., (2005), Comparison of naturally and Synthetically-Derived, Potassium-Based Geopolymers, stresses out the development carried out by W. Kriven and her team. It has been discussed above and represent the first successful preparation of SMK-750-based geopolymer.

The second most important contribution comes from Cui and his team from the Guangxi University, Nanning, China, who provided the first MAS-NMR spectra of synthetic SMK-750 and geopolymers: Cui et al., (2010), Characterization of chemosynthetic Al2O3-2(SiO2) geopolymers. The presence of Al in Al(5) configuration is related to geopolymer reactivity. The very low content in Al(4), and the extremely high exothermicity of the geopolymeric reaction further demonstrates its major role.

A more recent paper from Cui and his team, Zheng et al. (2015), Alkali-activation reactivity of chemosynthetic Al2O₃–2SiO₂ powders and their ²⁷Al and ²⁹Si magic-angle spinning nuclear magnetic resonance spectra, provides quantitative data for the MAS-NMR investigation.

We already analysed the characteristics of the publication by Tsai et al., (2010), Solid-state NMR study of geopolymer prepared by sol–gel chemistry. The MAS-NMR spectra are slightly different and are closer to natural metakaolinite. However, the obtained geopolymer seems to be similar as those synthetized according to Kriven and Cui. Their goal was to develop a method that could offer a good opportunity for the medical applications of geopolymer.

The Kriven's method was also used by Jia's team at Harbin Institute of Technology. In He and Jia, (2013), Low-temperature sintered pollucite ceramic from geopolymer precursor using synthetic metakaolin, they claim that the synthetic metakaolin route is providing a better sintering and therefore higher quality pollucite ceramic.  

The editors hope this selection will inspire additional, and much-needed, research and standardisation on the preparation, manufacture, of natural and synthetic metakaolin-based geopolymers, bearing in mind that industrialization and commercialization already started with the production of several applications, worldwide.

Geopolymer Institute references:

Davidovits J., (1976), Solid phase synthesis of a mineral blockpolymer by low temperature polycondensation of aluminosilicate polymers, IUPAC International Symposium on Macromolecules Stockholm; Sept. 1976; Topic III, New Polymers of high stability. PDF file at: http://www.geopolymer.org/library/technical-papers/20-milestone-paper-iupac-76/

Davidovits J., (1979), Polymère Minéral, French Patent Applications FR 79.22041 (FR 2,464,227) and FR 80.18970 (FR 2,489,290); US Patent 4,349,386, (1982) Mineral polymers and methods of making them.

Davidovits J., (1991), Geopolymers: Inorganic Polymeric New Materials, J. Thermal Analysis, 37, 1633–1656. PDF file at: http://www.geopolymer.org/library/technical-papers/12-geopolymers-inorganic-polymeric-new-materials/

Davidovits J., (1994), Geopolymers: Man-Made Rock Geosynthesis and the Resulting Development of Very Early High Strength Cement, J. Materials Education, Vol.16 (2&3), 91–139. PDF file at: http://www.geopolymer.org/library/technical-papers/3-geopolymers-inorganic-polymeric-new-materials/

Reference book: Joseph Davidovits, Geopolymer Chemistry and Applications, 2nd ed. 2008, 3rd ed. 2011, 4th ed. 2015, Geopolymer Institute, ISBN 4th ed. 9782951482098.

Elsevier's references:

Bauer A. and Berger G., (1998), Kaolinite and smectite dissolution rate in high molar KOH solutions at 35° and 80°C, Appl. Geochem., Vol. 13, No. 7, 905– 916.

Buchwald A., Zellmann H.-D., Kaps Ch., (2011), Condensation of aluminosilicate gels—model system for geopolymer binders, Journal of Non-Crystalline Solids, 357, 1376–1382.

Cui X-M, Zheng G-J, Han Y-C, Feng S., Ji Z., (2008), A study on electrical conductivity of chemosynthetic Al2O3–2SiO2 geopolymer materials, Journal of Power Sources 184, 652–656.

Cui X-M., Liu L-P, Zheng G-J, Wang R-P, Lu J-P, (2010), Characterization of chemosynthetic Al2O3-2SiO(2) geopolymers, Journal of Non-Crystalline Solids, 356, 72-76.

Favier A, Habert G., d'Espinose de Lacaillerie J.B., Roussel N., (2013), Mechanical properties and compositional heterogeneities of fresh geopolymer pastes, Cement and Concrete Res., 48, 9–16.

Favier A, Habert G., Roussel N., d'Espinose de Lacaillerie J.B., (2015), A multinuclear static NMR study of geopolymerization, Cement and Concrete Research 75, 104–109.

He P., Jia D., Wang M. and Zhou Y., (2010), Effect of cesium substitution on the thermal evolution and ceramics formation of potassium-based geopolymer, Ceramics International, 36, 2395–2400.

 He P., Jia D., Wang M. and Zhou Y., (2011), Thermal evolution and crystallization kinetics of potassium-based geopolymer, Ceramics International, 37, 59–63.

Medri V., Fabbri S., Dedecek J., Sobalik Z., Tvaruzkova Z., Vaccari A., (2010), Role of the morphology and the dehydroxylation of metakaolins on geopolymerization, Applied Clay Science, 50,538–545.

Medri V., Fabbri S., Ruffini A., Dedecek J., Vaccari A., (2011), SiC-based refractory paints prepared with alkali aluminosilicate binders, Journal of the European Ceramic Society, 31, 2155–2165.

Provis J. L., Palomo A., Shi C., (2015), Advances in understanding alkali-activated materials, Cement and Concrete Research, 78A, 110-125.

Tsai Y-L, Hanna J. V., Lee Y-L, Smith M. E., Chan J. C. C., (2010), Solid-state NMR study of geopolymer prepared by sol–gel chemistry, Journal of Solid State Chemistry, 183, 3017–3022.

Yao X., Zhang Z., Zhu H., Chen Y., (2009), Geopolymerization process of alkali–metakaolinite characterized by isothermal calorimetry, Thermochimica Acta, 493, 49–54.

Zhang Yunsheng, Jia Y., Sun W., Li Z., (2009), Study of ion cluster reorientation process of geopolymerisation reaction using semi-empirical AM1 calculations, Cement and Concrete Research, 39, 1174–1179.

Zheng G., Cui X., Huang D., Pang J., Mo G., Yu S., Tong Z., (2015), Alkali-activation reactivity of chemosynthetic Al2O3–2SiO2 powders and their 27Al and 29Si magic-angle spinning nuclear magnetic resonance spectra, Particuology, 22, 151–156.

Zibouche F., Kerdjoudj H., d'Espinose de Lacaillerie J-B., Van DammeH., (2009), Geopolymers from Algerian metakaolin. Influence of secondary minerals, Applied Clay Science, 43, 453–458.

Other Readings

Breck D.W., (1974), Zeolite Molecular Sieves, Structure, Chemistry and Use, John Wiley & Sons, New York, 771 pp.

De Kimpe C.R., Kodame H., Rivard R., (1981), Hydrothermal Formation of a Kaolinite-like Product from Noncrystalline Aluminosilicate Gels, Clays and Clay Minerals, Vol. 29, No. 6, 446-450.

Duxson P., Lukey G.C. and van Deventer J.S.V., (2005), The effect of alkali metal type and silicate concentration on the thermal stability of geopolymers, Geopolymer 2005 Proceedings, 189–193.

Duxson P. and Provis J. L., (2008), Designing Precursors for Geopolymer Cements, J. Am. Ceram. Soc., 91 [12] 3864–3869.

Gordon M., Bell J., Kriven W.M., (2005), Comparison of naturally and Synthetically-Derived, Potassium-Based Geopolymers, Advances in Ceramic Matrix Composites, Ceramic Transactions, 165, 95-165.

He P. and Jia D., (2013), Low-temperature sintered pollucite ceramic from geopolymer precursor using synthetic metakaolin, J Mater Sci., 48, 1812–1818.

Kriven W.M., Bell, J., Gordon, M., (2003), Microstructure and Microchemistry of Fully-Reacted Geopolymers and Geopolymer Matrix Composites, Ceramic Transactions, 153, 227–250.

Kriven W.M., Bell J., Gordon M. and Wen G., (2005), Geopolymers, more than just a cement, Geopolymer 2005 Proceedings, 179–183.

Kriven W.M., Bell J., Gordon M. (2006), Microstructure and nanoporosity of as-set geopolymers, Ceramic Engineering and Science Proceedings, 27 (2), pp. 491–503.

Lippmaa E., Mägi M., Samoson A., Engelhardt G. and Grimmer A.-R., (1980), Structural Studies of Silicates by Solid-State High-Resolution 29Si NMR, J. Am. Chem. Soc., 102, 4889–4893.

North M.R. and Swaddle T.W., (2000). Kinetics of Silicate Exchange in Alkaline Aluminosilicate Solutions, Inorg. Chem., 39, 2661–2665.

Mackenzie K.J.D., Brown I.W.M, Meinhold R.H. and Bowden M.E., (1985), Outstanding Problems in the Kaolinite-Mullite Reaction; Sequence Investigated by 29Si and 27Al Solid-State Nuclear Magnetic Resonance: I, Metakaolinite, J. Am. Ceram. Soc., 68 [6], 293–297.

Meinhold R.H., MacKenzie K.J.D. and Brown I. W. M.., (1985), Thermal reactions of kaolinite studied solid state 27-AI and 29-Si NMR, Journal of Materials Science Letters 4, 163–166.

Rowles M. R. and O'Connor B. H., (2009), Chemical and Structural Microanalysis of Aluminosilicate Geopolymers Synthesized by Sodium Silicate Activation of Metakaolinite, J. Am. Ceram. Soc., 92 [10], 2354–2361.

Sanz J., Madani A., Serratosa J.M., Moya J.S. and Aza S., (1988), Aluminum-27 and Silicon-29 Magic-Angle Spinning Nuclear Magnetic Resonance Study of Kaolinite-Mullite Transformation, J. Am. Ceram. Soc., 71 [10] C-418-C-421.

Wang Meirong (2011), Geopolymerization mechanism of aluminosilicate geopolymer and microstructure and properties of fly ash cenosphere/geopolymer composite, (PhD thesis), Harbin: Harbin Institute of Technology, 2011.,

Zhang B., MacKenzie K. J. D, Brown I. W. M., (2009), Crystalline phase formation in metakaolinite geopolymers activated with NaOH and sodium silicate, J. Mater. Sci., 44, 4668–4676.

Zoulgami M., Lucas-Girot A., Michaud V., Briard P., Gaudé J. and Oudadesse H., (2002), Synthesis and physico-chemical characterization of a polysialate-hydroxyapatite composite for potential biomedical application, Eur. Phys. J. AP, 19, 173–179.