Geopolymer technology, from fundamentals to advanced applications ...

Geopolymer technology, from fundamentals to advanced applications: a review Behzad Majidi Geopolymers are inorganic polymers with 3D framework structures having superior mechanical and physical properties. This new generation of cementitious materials has potential applications in fire resistant fibre composites, concretes for infrastructure applications, sealants and ceramics. Geopolymers are environmentally friendly substitutes for Portland cement: geopolymers in many applications not only reduce greenhouse gas emissions but also consume large volumes of industrial wastes such as fly ash, mine tailings and metallurgical slag. A brief review is given of geopolymer fundamentals and technology. Coverage includes structure and preparation, raw materials and the properties, particularly mechanical properties, of geopolymers. Current and potential applications of geopolymers and their advantages are introduced and discussed. The status of the geopolymer industry and the challenges that must be overcome to achieve wide acceptance and application of these novel cements is considered. Finally, future research topics are proposed. It is concluded that factors including the conservative view of new materials have limited application of geopolymers but progress is being made and geopolymers will be an important part of sustainable development in the twenty-first century. Keywords: Geopolymers, Geopolymerisation, Cementitious materials, Fibre reinforced composites, Carbon emissions, Calcining, Portland cement

Introduction Geopolymers emerged as a result of attempts to model the geological formation of zeolites. Zeolites are microporous crystalline solids with well defined structures. Generally they contain silicon, aluminium and oxygen in their framework and cations, water and/or other molecules within their pores. Because of their unique porous properties, zeolites are used in a variety of applications such as petrochemical cracking, water softening and purification, and in separation and removal of gases and solvents. Many zeolites occur naturally as minerals, and are extensively mined in many parts of the world. Others are synthetic, being made commercially for specific uses, or produced by scientists trying to understand more about their chemistry. Victor Glukhovsky1,2 is believed to be the first researcher to attempt to model the geological process of zeolite formation, in the 1950s. Zeolites were synthesised by alkali activation of alumino-silicates present in industrial materials or wastes. These novel binders were initially called ‘soil silicates’.1 Some authors3 believe that zeolitic compounds are the final, stable phase of a long-term conversion of primary phases to zeolites. This is in accord with investigations on ancient Roman cements that have indicated the presence of amorphous zeolitic compounds.4,5 The outstanding durability of ancient Roman cements and Glukhovsky’s work created interest in the potential to Shiz Concrete Industrial Complex, Iran, email [email protected]

ß 2009 W. S. Maney & Son Ltd. Received 17 March 2009; accepted 1 May 2009 DOI 10.1179/175355509X449355

produce new, high strength, durable cementitious materials. The most comprehensive research in this field was conducted by J Davidovits,6 who first applied the term ‘geopolymer’ to these alkali activated alumino-silicates. Geopolymers or ‘inorganic polymers’ are mineral polymer materials with a structure of 3D cross-linked polysialate chains.7 Geopolymers are produced by polymerisation of silicon, aluminium and oxygen species to form an amorphous three-dimensional framework structure.8,9 A geopolymer could be made by dissolving an alumino-silicate material such as kaolinite in highly alkaline environment such as NaOH or KOH solutions. Geopolymerisation is a process in which silicon, aluminium and oxygen atoms create a chain of SiO4 and AlO4 tetrahedra linked alternatively by shared oxygen atoms.10,11 The water to solid ratio in this process, if no aggregates are used, ranges12 from 0?3 to 0?4. The products are amorphous to semi-crystalline materials with superior mechanical behaviour.13–23 The reactants used to form conventional geopolymers are usually metakaolin as the Al–Si source and an activator solution containing reactive silicate anions and alkali cations.24 The focus of research in this field may be summarised as follows: (i) Al–Si source: identifying low cost, readily available materials suitable to participate in geopolymerisation. It has been shown that a wide range of natural materials and industrial wastes such as kaolin, fly ash, blast furnace slag, alkalifeldspars12 and tungsten mine waste13 can be used to make geopolymers

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Geopolymer technology, from fundamentals to advanced applications

1 Polysialate and polysialate-siloxo molecular structures and related frameworks26

(ii) alkali activation: analysing the effects of pH and alkali ions on process completion and the final properties of the product, e.g. it has been shown that K-feldspars show increased dissolution in NaOH solution compared with KOH solution, and thus confer higher compressive strength12 (iii) geopolymerisation: the mechanisms of the reaction have yet to be fully understood; the parameters affecting the process, microstructural reorganisation of the source materials and the reaction steps have been studied extensively by analytical methods. The unique properties of geopolymers – high early strength, extraordinary durability, resistance to chemical attack, ability to immobilise toxic atoms and environmental benefits such as low energy consumption and carbon dioxide emission in production – make geopolymers a strategic material for sustainable development and a serious alternative to Portland cement.

balance the negative charge. By dissolving an aluminosilicate powder in alkali solution such as NaOH, first 2 AlO 2 4 and SiO 4 tetrahedra are created and, according to the concentration of silicon in the solution, one of the above monomers is formed. The molecular arrangements in some geopolymer frameworks are shown in Fig. 1. A reaction mechanism for geopolymerisation proposed by Davidovits26 involves the chemical reaction of precursors such as alumino-silicate oxides (Al3z in IV-fold coordination) with alkali polysilicates, resulting in polymeric Si–O–Al bonds. To emphasise the IV-fold coordination of Al in these Al–Si minerals, these configurations are usually written as (Si2O5.Al2O3) rather than (2SiO2.Al2O3):

Chemical characteristics of geopolymers The following empirical formula has been postulated by Davidovits25 to describe geopolymers Mn ½{(Si{O2 )z {Al{On :wH2 O in which M is an alkali metal, z is 1, 2 or 3 and n is the degree of polymerisation. Based on the Si/Al ratio, three monomeric units may be defined: polysialate : SiO2 =Al2 O3 ~2, (Si{O{Al{O{) polysialatesiloxo, SiO2 =Al2 O3 ~4, (Si{O{Al{O{Si{O{) polysialatedisiloxo, SiO2 =Al2 O3 ~6, (Si{O{Al{O{Si{O{Si{O{): 2 These structures are composed of AlO 2 4 and SiO 4 tetrahedra. Cations of alkali or alkali earth metals (Naz, Kz, Ca2z) are required in the structure to

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The structure of geopolymers can be amorphous or semi-crystalline, depending on the condensation temperature. Amorphous polymers are obtained at 20– 90uC, whereas semi-crystalline polymers are obtained at 150–1200uC.27 In geopolymerisation, first alumino-silicate oxides dissolve in the alkali solution, then dissolved Al and Si complexes diffuse from the particle surfaces to the inter-particle spaces. Finally, a gel phase is formed from the polymerisation of added silicate solution and Al and Si complexes.

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2 Effect of calcination temperature and duration on release of silicate and aluminate monomers in alkali solution30

To produce geopolymer from kaolin or other clay materials three main steps are necessary: (i) thermal activation, the aim of which is to obtain a clay material with high chemical activity. In this process dehydroxylation of clay mineral leads to an unstable amorphous solid (ii) alkali activation: activated alumino-silicate material is dissolved in highly alkaline solution to produce silicate and aluminate monomers28 (iii) reactive setting or polycondensation, in which the silicate and aluminate monomers condense to a stable polymer network.29 To obtain a well structured geopolymer with acceptable mechanical properties, it is necessary to enhance the activity and solubility of Al–Si source materials in alkali solution. Thermal activation of the source material is one way to meet this condition and several investigations have examined the thermal activation process and its effects on final properties. Kaps and Buchwald30 used Fourier transform infrared spectroscopy to analyse the microstructural changes of kaolin during calcination. On calcining kaolin at 500uC, two distinct microstructural changes were observed: first the peaks corresponding to O–H bonding vibration (,3600 cm21) start to broaden, and completely disappear after 180 min. Simultaneously, aluminium coordination with oxygen changes and the peak corresponding to [Al–O]VI vibration disappears. X-ray diffraction analysis was also used to determine the results of calcination. Two distinct peaks could be usually identified in the XRD pattern of kaolinite-based clays: kaolinite and quartz. On calcining the material at 500–800uC, kaolinite crystals are completely broken down to an amorphous phase, metakaolin. Quartz crystals are also usually present in calcined clay, and the quartz peaks do not disappear from the XRD pattern of the calcined clay untile the calcining temperature is increased to 1400uC. However, in this case kaolinite, by losing SiO2, transforms to mullite instead of forming metakaolin, which decreases the

solubility of the material in alkali solution. The behaviour of crystalline SiO2 in geopolymerisation and the microstructural effect of free quartz particles on the mechanical behaviour of the final product are not fully understood and need to be investigated further. The duration and temperature of thermal activation directly affect the solubility of the clay in alkali solution.31,32 It has been shown30 that increasing the temperature of calcination increases the release of silicate and aluminate monomers in alkaline solution. As can be seen in Fig. 2, release of Al and Si monomers in NaOH solution for kaolin dehydroxylated at 750uC is greater than for that calcined at 500uC: the same solubility is observed for 500uC after 180 min as for 750uC after 60 min.

Kinetics of geopolymerisation Determining the key parameters in the kinetics of geopolymerisation is essential to better control setting time and microstructural development of geopolymeric gels. Geopolymerisation consists of dissolution and hydrolysis followed by a condensation step in an alkaline silicate plus alumino-silicate system. Experimental techniques such as calorimetry have been frequently used by researchers33–37 to investigate geopolymerisation kinetics. This technique is useful in determining the reactivity of calcined materials in alkali environments and so could help to optimise calcination. Rahier et al.38–40 used quasi-isothermal modulated differential scanning calorimetry to observe the changes in heat flow and heat capacity during the setting of geopolymeric gels. They showed that the reaction consists of at least two steps – dissolution and polymerisation – and that the second step is autocatalytic. It has been shown that the rate of condensation between silicate species is lower than that between aluminate and silicate species.41–43 The role of Al2O3 and SiO2 in geopolymerisation and its kinetics has been studied by De Silva et al.44 They concluded that the geopolymerisation kinetics and setting rate of geopolymeric gel are controlled principally by Al2O3, whereas


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the following ratios: 0:2vNa2 O=SiO2 v0:28 3:5vSiO2 =Al2 O3 v4:5 15vH2 O=Na2 Ov17:5

3 Proposed reaction sequence for geopolymerisation47

the Si content is responsible for later strength development of the product.44 Similar results have been reported by Provis et al.,45 who also showed that high silica systems react more slowly with a ‘pause’ in the latter stages of the reaction before further reaction occurs. The mechanism of Al speciation in accelerating the condensation step of geopolymer formation by means of calculation of the partial charge of aluminate and silicate species has been investigated by Weng et al.,46 who concluded that varying the particle size of metakaolin has a significant effect on the properties of hardened geopolymer. They reported that milled metakaolin powders with high specific surface area have shorter setting time, higher strength and a more homogeneous microstructure due to improved Al availability, as predicted by the partial charge model. Recently, Provis at al.47 have developed a model based on the work of Faimon48 to study the chemical reaction sequence and kinetics of geopolymerisation. They propose the reaction sequence for geopolymerisation indicated in Fig. 3. By postulating reactions for each step and corresponding kinetic expressions for each reaction, assuming that the stoichiometry of the reaction predicts the kinetics, they developed a comprehensive kinetic model for geopolymerisation. Applying the model to experimental data from the literature, they showed that the model could be used to determine the rate of geopolymerisation reaction and setting time for a wide range of Si/Al ratios in raw materials.

Mechanical properties of geopolymers Mechanical behaviour is a basic property in assessing an engineering material for a specific application. For geopolymers as novel cementitious materials, compressive strength is an important factor. Ever since their invention in the 1950s, the better compressive strength, setting time and durability of geopolymers over conventional cements have been perceived as advantages. However, the compressive behaviour of geopolymers varies according to the raw materials and processing method used. To obtain a geopolymer with high compressive strength, a high strength gel phase and high ratio of gel to non-polymeric phases are required. These factors relate directly to the type and molar ratios of oxides in the Al–Si source, type and pH of alkali solution and solubility of raw materials in the activator solution.49,50 Davidovits51 introduced three ‘key parameters’ for producing high strength geopolymers. Based on research on kaolinite-based geopolymers, he defined

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Zuhua et al.52 investigated the role of structural water in the compressive strength of kaolinite geopolymers. They showed that final strength of products increases on increasing the calcination temperature of kaolinite. This result seems to be due to higher activity of the clay calcined at higher temperature and also a lower structural water content (which has a negative effect on the strength of the product). In view of the importance of the dissolution of Al–Si species in alkali solution and of the polymerisation reaction, it is unsurprising that the characteristics of the alkali solution directly affect the microstructural reorganisation of the calcined clay and so the final mechanical properties of the product. It has been shown53 that flexural strength, compressive strength and apparent density of geopolymers increase as NaOH solution concentration increases from 4 to 12 mol L21 and the higher the concentration of NaOH, the higher the amorphous content of the products. Similar results have been reported previously.54 Although dissolution of Al–Si species increases on increasing the concentration of alkali solution, excessive amounts of NaOH or KOH in the aqueous phase decrease the SiO2/Na2O ratio and so inhibit polycondensation. Therefore, there is a limit for alkali hydroxide concentration in the activator solution to obtain a high strength gel phase (Fig. 4).55 The geopolymer properties reported in Fig. 4 were obtained on specimens aged for 7 days; for a simple comparison, a typical 7 day compressive strength of type-I Portland cement concrete is 19?0 MPa. It has been also shown55 that KOH provides more inorganic polymer precursors than NaOH since the larger Kz cation favours the formation of longer silicate oligomers, with which Al(OH) 2 4 prefers to bind; thus better setting and higher compressive strength is acquired (Fig. 4). Using alkali solution composed of alkali hydroxide and dissolved silicate has been found to be beneficial for compressive strength relative to alkali hydroxide alone. Dissolved silica not only balances the SiO2/Al2O3 and Na2O/SiO2 ratios in the mixture but also catalyses polycondensation by providing SiO 2 4 monomers and by and SiO 2 initiating polymerisation between AlO 2 4 4 tetrahedral units. Therefore, higher compressive strength may be obtained using an activator composed of soluble silicate and alkali hydroxide. Investigations30 on kaolinite based geopolymers have shown that addition of 25 wt-% (additional SiO2/solid material) sodium silicate solution increases 14 day compressive strength from 13 to 38 MPa. Similar results have been reported recently on ferronickel slag based geopolymers.55 It should be noted that, again, there is a limit for addition of silicates to the mixture (Si/Al51?90) and very high ratios of Si/Al are not advisable due to the negative effect on mechanical properties.56 High Si/Al ratios increase the porosity of the structure and also content of unreacted species, and these factors directly decrease compressive strength of the geopolymer.

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4 Effect of alkali hydroxide concentration on compressive strength of geopolymers aged for 7 days55

The presence of calcium compounds such as CaO and Ca(OH)2 has been shown57,58 to improve the compressive strength of geopolymers; here the effect of Ca(OH)2 is more pronounced. Precipitation of calcium silicate hydrate or calcium silicate aluminate phases and catalysis of the dissolution of Al–Si particles in alkali solution have been proposed as two reasons for the enhanced mechanical behaviour observed in geopolymers produced by addition of calcium compounds to raw materials.

Microstructure of geopolymers The mechanical properties discussed above result from changes in the geopolymer microstructure. Analytical techniques such as XRD, XRF, magic angle spinning nuclear magnetic resonance (MAS-NMR), SEM/EDS, TEM/EDS, Fourier transform infrared spectroscopy (FTIR) and thermogravimetry (TG) have been used to clarify the microstructural features of geopolymers. X-ray diffraction and TG tests are very useful to detect structural changes during calcination of raw materials. Characteristic peaks in the XRD pattern of kaolinite based clays correspond to kaolinite, quartz, mullite and illite. After calcination at 500–800uC, broadened XRD peaks are observed. Quartz crystals usually remain in calcined clay but kaolin crystals may be completely broken down to an amorphous aluminosilicate phase. Combining XRD analysis and TG enables the process to be optimised. Zuhua et al.52 reported from TG tests on calcined kaolin that the mass loss at 700, 800 and 900uC was 0?9, 1?9 and 2?3% respectively relative to the mass at 600uC. This mass loss is believed to derive from evaporation of structural water; since existing hydroxyl units in heated kaolin have a negative effect on its activity in the alkaline medium, it was concluded that an optimised calcination could be determined by from an amorphous XRD pattern and the maximum mass fall detected by TG tests. XRD cannot provide useful information for microstructural changes in the alkali activation step: usually there are no significant differences between XRD patterns of reacted and unreacted clays59,60 because most changes take place in amorphous phases of the material and there is no crystalline phase in the reaction products. However, FTIR and MAS-NMR analysis can provide very useful data on molecular changes during geopolymerisation. One significant molecular change

during alkali activation of alumino-silicates is the shift of the infrared band for Si–O–Si and Al–O–Si asymmetric stretching (950–1200 cm21) to lower wavenumbers. It has been shown61,62 that dissolving alumino-silicates in alkali solution increases the number of non-bridging oxygen atoms (NBOs) within the structure. This causes SiO42 and AlO42 units to become isolated and thus lower molecular vibration of Si–O and Al–O bands is observed. Here, Na cations balance the negative charges created by formation of Al–O–Si bands or removing NBOs. By means of 29Si and 27Al MASNMR, geopolymerisation mechanisms and the progress and changes in molecular arrangements can be followed63 (Fig. 5). As the reaction continues, the coordination of Al (IV,V,VI) in metakaolin changes almost completely to IV in the final product.63 The effect of temperature in accelerating the reaction can also be seen in Fig. 5.

Geopolymer composites In recent years advanced composite materials using geopolymer matrices have attracted significant attention. In 1996, Lyon et al.64 reported information on geopolymer carbon fibre reinforced composites, including their fire resistance. On the basis of ASTM E-162 tests, the flame spread index for geopolymer matrix composites reinforced with glass or carbon fibres was superior to that of composites with thermoset, phenolic or engineering plastics matrixes. As an engineering

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29 Si and 27Al MAS-NMR spectra for metakaolin based geopolymer cured at room temperature and 80uC63


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6 Residual flexural strength of cross-ply laminates after fire exposure64

material for infrastructure applications, the residual strength of composites after fire exposure is very important. Figure 6 indicates residual flexural strength of cross-ply laminated composites after fire exposure. It can be seen that, even after exposure to more severe thermal environment (800uC, 75 kW m22) than organic composites the geopolymer matrix composite has higher residual strength. Lin et al.65 used short carbon fibres with different lengths (2, 7 and 12 mm) to reinforce a geopolymer matrix. Regardless of fibre length, carbon reinforced geopolymer was found to exhibit pseudoplastic fracture, as opposed to the sudden fracture mode of pure matrix. Geopolymer matrix samples showed brittle failure mode in three-point bending tests, while composites deformed without complete fracture. The maximum flexural strength and fracture work was obtained by using 7 mm fibres. Addition of 7 mm fibres increased the flexural strength of the matrix from 16?8 to 91?3 MPa and work of fracture from 54?2 to 6435?3 J m22. Scanning electron micrographs of the tensile side and fracture surface of composite deformed in three point bending are shown in Fig. 7. Fibre pull-out appears to be the main toughening mechanism in these composites. The clean surface of the pulled-out fibres also indicates that matrix/fibre bonding was weak. More recently, similar results have been reported by Li and Xu,66 who investigated the impact loading response of basalt fibre reinforced geopolymers. The addition of basalt fibres was shown to enhance the deformation and energy absorption capacities of a geopolymer matrix significantly. Research67 on the application of a geopolymer matrix for repair and rehabilitation of reinforced concrete beams has indicated that the geopolymer performs better than organic polymers in terms of adhesion of carbon fibres to reinforced concrete beams. The tribological behaviour of metakaolinite based geopolymer composites has also been investigated. It was found that addition of PTFE powder to the geopolymer matrix changes the wear mode from severe to mild owing to a composite soft layer formed during the friction.68

Geopolymers and Portland cements Availability of raw materials and its ease of production and application have made Portland cement concrete the most popular and widely used building material. The application of concrete in infrastructure and transport has unquestionably improved the development of civilisation, economic progress and the quality of life.69

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7 Scanning electron micrographs a perpendicular and b parallel to fracture surface of carbon fibre/geopolymer composite65

However, some inherent disadvantages of Portland cement remain difficult to overcome. Perhaps the important of these is the high ‘carbon content’: 1 t CO2 per tonne ordinary Portland cement (OPC) is released to the atmosphere and y1?5 t per tonne of raw materials. This makes production of OPC extremely resource and energy intensive. Calculations70 indicate that the total energy required to produce Portland cement is y3630 MJ/t, compared with an energy consumption for geopolymer cement production of 990 MJ/t. The difference is mainly due to the lower calcination temperature for geopolymers (800 versus 1450uC). It has been estimated that total CO2 emission for production of geopolymer cements is only 0?184 t per tonne of cement, about one-sixth that of Portland cement.71 Considering global warming and international attempts to reduce greenhouse gas emissions, and government commitments under the Kyoto protocol, geopolymers as an alternative to Portland cements may potentially have a remarkable impact on CO2 emission reduction strategies. In addition, the poor resistance of OPC concrete to corrosion and chemical attacks is a concern for designers, since OPC can deteriorate when exposed to severe environments. In contrast, geopolymers have shown superior resistance against chemical attacks. Song et al.72 compared the durability of geopolymer and Portland cement concretes against sulphuric acid attack, concluding that the geopolymer concrete was highly resistant to sulphuric acid with a very low mass loss, ,3% (Fig. 8). Therefore, owing to their outstanding mechanical behaviour and environmentally friendly production, geopolymers have potential to substitute ordinary cements in construction applications or at least can decrease demand for Portland cement production. As discussed below, cost considerations also apply. Nevertheless, geopolymers are an ideal material for sustainable development, being characterised by:73 (i) abundant raw materials resources (ii) energy saving and environment protection (iii) simple preparation technique

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8 Mass change in 10% sulphuric acid in ASTM C267 test of geopolymeric and Portland cement concretes72

(iv) (v) (vi) (vii)

good volume stability short setting time ultra-high durability high fire resistance and low thermal conductivity (viii) ability to immobilise toxic atoms (ix) superior resistance to chemical attack. All these properties make geopolymers a good substitute for ordinary materials in fields of industry such as civil engineering, automotive and aerospace, non-ferrous foundries, waste management and art and decoration.

Geopolymer industry Portland cement is widely used around the world; with an annual consumption well in excess of 1000 Mt, water is the only material used in greater quantities by mankind. Over the 180 or so years it has been in widespread use, knowledge has built up about its processing and long-term behaviour, and limestone, from which it is derived, is one of the most abundant materials on the earth. Considering these factors, the market position of Portland cement is strong. For construction applications in particular, new materials must meet a set of highly prescriptive standards and validation procedures which is extremely expensive and time-consuming for manufacturers. Although the lack of knowledge, especially on the long term behaviour of geopolymers, cannot be neglected, an important reason for the slow development of markets for geopolymers is this conservative view of new materials. Consequently, it is not anticipated that geopolymers will supply a significant amount of the global need for cements in a near future. However, a geopolymer industry is forming and an increasing number of geopolymer supplier companies are becoming established based on research activities in universities and research institutes. Although there is no firm data on the market size, geopolymer concrete is now used in the transport sector in the USA and more recently in Australia. The short setting time of geopolymer cement makes it an ideal solution for repairing highways and airport runways. It is estimated that geopolymer products as pre-cast concretes are 10–15% more expensive than OPC concretes which, although the properties and service lives are not the same, limits demands for geopolymer concretes. However, when it comes to concretes for severe environmental conditions such as oil wells, geopolymers are very cost effective choices. Note, however, that the cost of geopolymer concretes varies according to raw materials. Selecting an Al–Si material allowing acceptable levels of mechanical properties to be obtained using low concentrations of alkali atoms and silicate units to is the key to produce


cheaper geopolymer concretes. The effect of carbon taxes on the cement and concrete industry will also help the geopolymer industry to enhance its competitiveness with respect to Portland cements. For example, the European Emissions Trading Scheme (ETS),74 which determines taxes on carbon dioxide release in the range of J23–38/t CO2,75,76 will ensure that the economic advantage of ordinary cements does not continue at its current level. The ability of geopolymer matrixes to immobilise toxic materials and form an isolating coating for radioactive materials has been widely tested and appears to be accepted. In 1998 a pilot-scale experiment was successfully carried out in the Wismut mine water treatment facility at Schlema-Alberoda, Germany.77 This study showed geopolymer matrixes to be a mature and cost-efficient solution to many problems where hazardous residues must be treated and stored under critical environmental requirements. In another successful application, a geopolymeric shelter has been applied to encapsulate high level waste from the failed Chernobyl reactor 4. Another promising application of geopolymers is advanced fire resistant geopolymer composites. Geopolymers reinforced with carbon or glass fibres exhibit extraordinary mechanical properties at elevated temperatures (Fig. 6) and are ideal materials for aerospace applications. In 1994, a geopolymer matrix composite was successfully used in a Formula 1 racing car, replacing titanium parts in exhaust system, and subsequently these composites have been widely adopted in racing cars where their thermal properties are effective.78 Carbon fibre reinforced geopolymer composites do not ignite, burn or release any smoke after exposure to severe heat flux, which makes them appropriate materials for aircraft cabin fire protection, substituting for ordinary polymer matrix composites. The US Air Force is now uses bombers equipped with geopolymer composites as fire resistant materials.

Future research Geopolymer technology is gaining interest because of the successful application of products in various fields, driven by the superior properties of geopolymers relative to currently used materials. In addition, the environmental impact of the production process of Portland cement will drive active consideration of alternatives, including geopolymers. However, the research community first needs to address existing gaps in knowledge in geopolymerisation and the properties of geopolymers. It is possible to outline some key areas for future research. First, improved characterisation of raw materials appropriate for geopolymerisation is necessary. Balancing mixture composition to meet ‘key parameters’, as suggested by Davidovits, usually leads to contradictory results. It seems that XRF analysis of a material and knowledge of the different oxide contents alone cannot be used to predict precisely the response of the material in alkali activation. The relationship between XRD pattern and behaviour of the aluminosilicate phases present in clays and wastes during geopolymerisation should be considered and studied. Second, the effects of the physical properties of the Al–Si source on dissolution of the material in the alkali solution and the kinetics of geopolymerisation deserve


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further investigation. For example, the effects of particle size, morphology and zeta-potential on setting time, microstructure and mechanical properties of geopolymers needs to be clarified. Third, little has been published on the effects of curing conditions on the mechanical behaviour of geopolymers. Optimised curing condition and the effects of atmosphere humidity and heating of the paste on the properties of the final geopolymer should be elucidated. Fourth, the behaviour of geopolymer concretes with different reinforcements is not fully understood. For example, the corrosion mechanisms of steel embedded in geopolymer matrixes would benefit from further research. Finally, the lack of knowledge on the long term behaviour of geopolymers appears to be an important barrier to further uptake. Data on the response of geopolymeric products to severe environments and under creep and fatigue loading will be beneficial.

Conclusion A brief history of geopolymers and highlights of research activities in this area have been presented. Geopolymer technology has a history of a little more than half a century. Desirable properties, abundant raw materials and successful applications of geopolymers promise considerable progress in large scale production of geopolymers in the near future. In the past two decades, considerable research has been carried out throughout the world, providing a large volume of useful data and important findings on geopolymerisation and the properties of geopolymers. Various types of natural alumino-silicates and industrial by-products have been examined as raw materials for geopolymers and appropriate activation processes have been proposed. However, a lack of information on some aspects of geopolymerisation has become apparent and the research community should focus on these gaps. Despite the current status and wide acceptance of Portland cement, the desirable properties of geopolymers, their environmental benefits and the strong academic and commercial R&D activity suggest that geopolymer technology is poised for significant progress in the near future.

References 1. V. D. Glukhovsky: ‘Soil silicates’; 1959, Kiev, Gosstroyizdat Ukrainy Publishing (in Russian). 2. V. D. Glukhovsky: ‘Oil silicates: their properties, technology and manufacturing and fields of application’, DTech.Sc. Thesis, Civil Engineering Institute, Kiev, Ukraine, 1965. 3. M. L. Granizo: ‘Activation alcalina de metacaolin: desarrolllo de nuevos materials cementantes’, PhD thesis, University Autonoma of Madrid, 1998 (in Spanish). 4. D. H. Campbell and R. L. Folk: ‘The ancient Egyptian pyramids – concrete or rock’, Concrete Int., 1991, 29–44. 5. D. M. Roy and C. A. Langton: ‘Studies of ancient concretes as analogs of cementituos sealing materials for repository in Tuff’, Report LA-11527-MS, Los Alamos National Laboratory, USA, 1989. 6. J. Davidovits: ‘Soft mineralurgy and geopolymers’, Proc. 1st Int. Conf. on Geopolymers, Compiegne, France, June 1988, Vol. 1, 19– 23. 7. A. Palomo, M. W. Grutzeck and M. T. Blanco: Cement Concrete Res., 1999, 29, 1323.

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