Open access peer-reviewed chapter

Clay-Based Materials in Geopolymer Technology

Written By

Mohd Mustafa Al Bakri Abdullah, Liew Yun Ming, Heah Cheng Yong and Muhammad Faheem Mohd Tahir

Submitted: October 2nd, 2017 Reviewed: January 25th, 2018 Published: October 10th, 2018

DOI: 10.5772/intechopen.74438

From the Edited Volume

Cement Based Materials

Edited by Hosam El-Din M. Saleh and Rehab O. Abdel Rahman

Chapter metrics overview

3,136 Chapter Downloads

View Full Metrics

Abstract

The term “geopolymer” was introduced by Davidovits in the 1970s. The prefix “geo” was selected to symbolize the constitutive relationship of the binders to geological materials, natural stone and/or minerals. Geopolymer is mineral polymers of inorganic polymer glasses with structure resembling natural zeolitic materials. Previously, geopolymer formation used source materials such as clay (e.g. kaolin and calcined kaolin) or industrial by-product (e.g. slag and fly ash). The precursor material plays an important role in the formation of geopolymer. The source material provides silicon (Si) and aluminum (Al) for reaction by an alkali activator solution. The Si and Al contents in the source materials dissolve in the alkaline activator solution and then polymerize to form a polymeric Si-O-Al-O framework which becomes the binder. Geopolymeric materials are attractive because of their excellent mechanical properties; durability and thermal stability can also be achieved. Owing to their low calcium content, they are more resistant to acid attack than materials based on Portland cement. In addition, they are of great interest because of the reduced energy requirement for their manufacture and the higher sustainability. Recently the search for alternative low cost and easily available materials led among others to Clay. Clay generally consists of a mixture of different clay minerals and associated minerals, which are strongly affected by the nature of the parent rocks. These materials are extensively distributed over the surface of the world and may show certain reactivity after a thermal activation process shows a great potential to be utilized in geopolymer technology. This article presents the potential of different types of clay as the source materials for geopolymerization reaction in terms of morphological properties. Moreover, the mechanical and microstructural properties of geopolymer made with various kinds of clay and its potential application are also presented.

Keywords

  • geopolymer
  • inorganic polymer
  • clay

1. Introduction

In 1978, the word “geopolymer” was introduced by Davidovits [1]. In general, geopolymer is an inorganic polymeric material formed through the reaction between aluminosilicate sources and highly alkaline silicate solution, followed by curing at ambient or slightly higher temperature [2]. The formation process is termed as geopolymerization reaction.

Geopolymer has an empirical formula of:

MnSiO2zAlO2·wH2OE1

where M is cation such as K+, Na+ or Ca2+; nis the degree of polycondensation; zis 1, 2, 3 and wis the amount of binding water. It has three-dimensional Si-O-Al polymeric networks ranging from amorphous to semi-crystalline. Tetrahedral SiO4 and AlO4 are linked alternately by sharing oxygen atom as shown in Figure 1. As refer to Figure 1, the terminology of geopolymers can be categorized into three forms which are poly(sialate), poly (sialate-siloxo) and poly (sialate-disiloxo). The Al is in IV-fold coordination [3, 4]. This leaves a negative charge in the IV-fold coordinated Al that is charge-balanced by cations (Na+, K+, Li+, Ca2+, Ba2+, NH4+ and H3O+). The charge-balancing by cations is important in determining the structural integrity and fragility of geopolymers [5].

Figure 1.

Geopolymer systems based number of siloxo Si-O units [2].

The cations is usually contributed by alkaline silicate solution which is a mixture of alkali hydroxides (NaOH or/and KOH) and silicate solution (Na2SiO3 or/and K2SiO3) [6, 7, 8]. The alkali hydroxide is required for the dissolution of aluminosilicates while alkali silicate acts as binder, alkali activator and dispersant or plasticizer [9]. The alkali silicate solution contributes certain amount of SiO2 for the geopolymerization reaction [10].

Advertisement

2. Aluminosilicates

The aluminosilicate sources are materials rich in alumina and silica content (e.g. ashes [11, 12, 13, 14], clays [15, 16] or slag [17, 18]). Some other natural and artificial silicoaluminates such as zeolite [19] and magnesium-contained minerals [20] have also been used as an important source of Si4+ and Al4+ ions in the geopolymer binding system. Normally, the total composition of Al2O3 and SiO2 is more than 70%, preferable in reactive amorphous phase [3, 21]. In this book chapter, the utilization of clay or clay minerals in geopolymer formation is discussed.

Advertisement

3. Kaolin/kaolinite

Kaolinite is the most common clay mineral used in geopolymer synthesis. It has 1:1 uncharged dioctahedral layer structure (Figure 2a) whereby the layers are (Si2O5)n2− sheet and the Al(OH)3 (gibbsite) sheet linked by sharing oxygen atoms. The layers are held together by weak van der waals and hydrogen bonds leading to the layered structure (Figure 2b).

Figure 2.

Structure of kaolinite (above) and microstructure of kaolinite (below) [22].

Advertisement

4. Metakaolin

Thermal treatment of kaolinite leads to the transformation of crystalline phases into reactive amorphous phases [7], which is the active constituent that determines the final strength of geopolymers. The thermal treatment is usually carried out at temperature in the range of 550–800°C which accompanied by dehydroxylation of strongly bounded hydroxyl ions on the Al-constitutive layer. Thus, kaolinite is transformed into metakaolin.

Metakaolin also has layered structure as kaolinite even after the thermal treatment process. However, the layer structure appeared more open than kaolinite (Figure 3) [23, 24].

Figure 3.

SEM micrograph of metakaolin (800°C for 2 hours) [23].

Also, the thermal treatment destroys the hexagonal layer of kaolinite and causes atomic arrangement that converted the hexa-coordinated Al ions of kaolinite are converted into penta- and tetra-coordinated Al ions [25]. The amount of penta- and tetra-coordinated Al ions reflects the reactivity of metakaolin [24].

4.1. Clay-based geopolymers

Clays are frequently used as the source materials in geopolymer formation. They have a total composition of Al2O3 and SiO2 in the range between 70 and 90% (Table 1) wherein the composition of clay is dependent on the origin and the geology of the location. Initially, in the early stage of geopolymer development, kaolin/kaolinite is mostly used as the aluminosilicate sources [2, 6, 32, 33]. Later, the experimental work has expanded to calcined clays, ashes and slag. This is because kaolin/kaolinite shows low reactivity with alkaline silicate solution causing low strength products. It is deemed that the near zero charge between layers and the layered structure that does not permit the exchange of ions or other element. Hence, kaolin/kaolinite has low surface area for geopolymerization reaction. According to Heah et al. [34], the low surface area limits the dissolution of kaolin/kaolinite to provide Si4+ and Al4+ ions for further reaction. Comparatively, fly ash has greater surface area as they have spherical-shaped particles.

Clay/clay mineralSiO2Al2O3Fe2O3TiO2MgOP2O5Na2OCaOK2OMnOSO3LOI
Metakaolin [26]51.3544.240.980.900.480.450.160.130.080.010.72
Metakaolin [27]52.143.00.70.30.122.51.0
Metakaolin [28]59.734.10.90.20.10.11.2
Clay sediment from Occhito reservoir, Italy [29]47.515.66.72.40.310.21.915.4
Clay sediment from Sabetta reservoir, Italy [29]50.015.95.71.90.36.91.717.5
Kaolinite from Hiswa, Jordan [30]48.9225.167.520.860.210.160.210.681.40.012.9411.93
Kaolinite [31]49.3536.030.200.020.020.040.022.2911.94
Kaolinite [31]40.8639.870.390.460.120.010.120.1717.91
Kaolinite [31]42.6640.921.120.450.040.140.140.0914.13
Halloysite [31]48.1236.330.330.160.050.040.0314.8

Table 1.

Chemical composition of clays from different origins.

Summary of the compressive strength of geopolymers based on clay/clay minerals is tabulated in Table 2. The strength achieved by geopolymers based on clay/clay minerals is low. The addition of kaolinite as secondary source of aluminosilicate is necessary in order to achieve strength. Unfortunately, the use of kaolinite alone in geopolymer is not preferable as it will produce weak structure [35]. The statement is further supported by van Jaarsveld et al. [38] who concluded the strength of fly ash geopolymers degraded as the result of high kaolinite content (41%) addition. The main reason for the deterioration in strength is because not all kaolinite reacted in the reaction.

Clay/Clay mineralsStrength (MPa)Ref.
KOHNaOH
Almandine10.3c8.5c[35]
Grossular16.7c14.5c[35]
Sillimanite12.7c6.5c[35]
Andalusite11.1c8.8c[35]
Kyanite6.8c6.3c[35]
Pumpellyite10.8c8.8c[35]
Spodumene13.1c5.0c[35]
Augite6.7c5.0c[35]
Lepidolite4.3c2.5c[35]
Illite7.1c5.8c[35]
Celsian9.7c8.7c[35]
Sodalite15.0c10.3c[35]
Stilbite18.9c14.2c[35]
Heulandite7.4c5.6c[35]
Anorthite14.4c6.0c[35]
Kaolin-2 – 10c[37]
Clay residues5.76 – 5.98f[38]

Table 2.

Strength result of clay/clay minerals geopolymers.

compressive strength;


flexural strength


If the clays/clay minerals are heat-treated, the mechanical strength of the final products would increase [35, 39]. Pre-treatment is crucial to increase the reactivity of clays/clay minerals. The pre-treatment methods include mechanochemical, chemical and thermal treatments. MacKenzie et al. [40] reported that typical characteristic geopolymers are produced with heat-treated (200–1000°C for 2 hours) halloysite. Mechanochemical-treated (high-energy grinding for 20 hours at 400 rpm) halloysite showed less complete geopolymer formation. For acid-treated (0.1 M HCl) halloysite, the resulting geopolymers were poorly set while alkaline-treated (0.1 M NaOH) halloysite caused the formation of crystalline zeolites. Thermal treatment is the most used methods. Successfully calcined clays lead to highly pozzolanic amorphous phase. For instance, geopolymers from clay sediments treated at 750°C for 2 hours showed greater compressive strength (6–12 MPa) than those treated at 400°C (1–4 MPa) [29].

According to the author, thermally treated clay sediments exhibited improved surface area toward dissolution and geopolymerization reaction. Apart from the purely clay geopolymers, blended geopolymers are also produced with the addition of other materials such as calcium hydroxide, slag and ashes with the clay materials as the starting source material. When calcium hydroxide is added, the strength of the blended geopolymers does not degrade [4142]. Similarly, the addition of 30% slag in metakaolin geopolymers showed improvement in the mechanical strength. Slag acted as filler in the geopolymer structure and enhanced the mechanical properties. However, the slag addition is limited to below 50% as it will greatly deteriorate the strength at content beyond 50% [43]. The high calcium content in both calcium hydroxide and slag caused the formation of geopolymer matrix as the main phases and the calcium silicate hydrates (CSH) phases as the secondary phases [27, 43]. This can be clearly shown in Figure 4.

Figure 4.

SEM micrographs of (a) pure metakaolin; and (b) metakaolin-50% slag geopolymer (a—Geopolymer matrix and B—CSH phases) [43].

Advertisement

5. Geopolymerization mechanism

Geopolymer formation involves chemical reaction that transforms partially or totally amorphous aluminosilicates into three-dimensional polymeric networks. The geopolymerization reaction is exothermic. Under strong alkaline medium, the aluminosilicate sources dissolve into SiO4 and AlO4 tetrahedral units which later on participate in the polycondensation process [44, 45].

The chemical attack of kaolinite starts from the surface and edge and continues layer by layer inside the structure (Figure 5) [46]. The Al-substituted silicate layers formed and the structural deformed Al sites transformed into tetra-coordinated Al sites after attack by alkali hydroxide (Figure 6) [47].

Figure 5.

Reaction of kaolinite under alkaline medium (gray circle denotes the aluminum hydroxyl side groups) [46].

Figure 6.

Formation of aluminum substituted silicate layer in metakaolin after attack by NaOH solution [47].

Davidovits proposed the reaction mechanism as shown in Figure 7. The reaction aluminosilicates and alkali silicate solution produced geopolymers with Si-O-Al backbone.

Figure 7.

Schematic diagram of geopolymer formation [48].

In general, the geopolymerization mechanism is similar for all types of aluminosilicates. Most researchers agreed that the geopolymerization reaction involves dissolution, polycondensation and hardening process. The dissolution of aluminosilicates is initiated by presence of hydroxyl ions in the alkaline silicate solution which releases Si and Al species for further polycondensation reaction [9, 49]. The geopolymerization reaction is deemed occurs in multistep simultaneously [38, 42, 50] such as reorganization and diffusion of dissolved ions with formation of small coagulated structures, solid state transformation and hardening to form hard solid polycondensation to form aluminosilicate gel phases and dissolution of aluminosilicates in highly alkaline medium.

In addition, Xu and Van Deventer [51] suggested that geopolymer is formed through Eqs. (2)(4). Eq. (1) represents the mixing of aluminosilicates with alkali silicate solution. Geopolymer gel is formed in Eq. (3), while Eq. (4) shows the formation of geopolymer rigid solid.

AlSimaterials+MOHaq+Na2SiO3soraqE2
AlSimaterials+MzAlO2xSiO2y·nMOH·mH2OgelE3
AlSimaterialsMaAlO2aSiO2bnMOH·mH2OE4

According to Provis et al. [52, 53], the final geopolymer gel phase after extended curing process is different from the initial gel phase. The curing process allows continuous rearrangement of geopolymer gel phase toward more crosslinking and some zeolite crystals (more ordered phases) are formed in the geopolymer structure. Similar model is illustrated by Duxson et al. [54] in Figure 8. The intermediate product (Gel 1) having high Al contents transformed into Gel 2 with high Si content as the reaction progresses and finally rearranged forming three-dimensional geopolymer frameworks.

Figure 8.

Graphic model of alkali activation of geopolymers [54].

5.1. Clay-based geopolymer formation

Most usual method to form geopolymer is direct mixing of aluminosilicate with alkali silicate solution. After mixing, the geopolymer paste is compacted in molds and cured at room temperature or slightly higher temperature (20–80°C). To avoid extensive loss of moisture, the geopolymer paste is covered with a thin plastic film during the curing process. Besides, other mixing method has also been studied with different mixing sequences. The aluminosilicate is firstly mixed with liquid sodium silicate and the NaOH solution is added afterwards.

Based on Lecomte et al. [25], the normal mixing and separate mixing did not lower the degree of geopolymerization reaction of kaolin/white clay-slag blended geopolymers. However, separate mixing required additional water for mixing and hence detrimental to the mechanical strength. A contradict result is reported by Rattanasak & Chindaprasirt [55] based on fly ash geopolymer. The separate mixing permits more time for dissolution of aluminosilicates providing more dissolved species for the polycondensation process. This in turn leads to formation of stronger geopolymers. On top of that, the homogeneity of the geopolymer mixtures is crucial in order to attain high strength.

Regardless of the different mixing sequence, workability is an important criterion to be taken into consideration during geopolymer formation. Serious workability problem leads to compaction difficulty and produce weak geopolymer structure [23, 34]. For geopolymer based on clay, it usually requires excess water during the mixing process in order to achieve certain consistency. The addition of excess water will definitely decrease the mechanical strength of the final product. Comparison with fly ash geopolymers, the mixture of clay-based geopolymer is usually highly viscous and sticky [56]. The layer structure of clay induces greater inter-particle friction which limits the flowability of mixture. Unlike clay, fly ash has spherical-shaped particles. The imposed inter-particle friction is lesser and can acquire adequate consistency without addition of excess water.

Advertisement

6. Characterization of clay-based geopolymers

6.1. Morphology

As aforementioned, the kaolinite and metakaolin appears plate-like or layer-like structure. After the geopolymerization reaction, this layer-like structure changed. The morphology of clay-based geopolymer appeared sponge-like with globular units (Figure 9). The microstructure grows and develops over time starting from the precipitation of loosely-packed globular units on the metakaolin’s particle surface and densification of geopolymer matrix inside and outside voids [57, 58]. At the beginning the K/Al and Si/Al molar ratios are high due to leaching of Si from liquid sodium silicate. As time passed, more dissolved Al entered the geopolymer system and lowered the molar ratios [57]. Instead of globular units of geopolymer matrix, fly ash-based geopolymers revealed smooth heterogeneous geopolymer matrix with remnant fly ash particles in the hollow cavities due to partially dissolution (Figure 10).

Figure 9.

ESEM micrographs of metakaolin geopolymers after (a) 10 minutes, (b) 3 hours, (c) 6 hours, and (d) 9 hours of mixing [57].

Figure 10.

SEM micrographs of fly ash geopolymers cured at room temperature for 24 hours and at 80°C for another 24 hours [15].

Differently, Wang et al. [59] observed that the metakaolin geopolymers are not compact. The layer structure is remained in the geopolymer matrix after geopolymerization reaction (Figure 11). The remnant metakaolin geopolymer might have left and embedded in the geopolymer structure. Based on Rowles et al. [60], the residual raw particles in geopolymer structure may weaken the structure. This is because the residual particles act as stress concentration point that permit propagation of cracks and fractures. To our knowledge, complete geopolymerization reaction is not achieved. There must be come residual raw materials left in the structure after the chemical reaction.

Figure 11.

SEM micrographs of metakaolin geopolymers [59].

The mechanical strength of geopolymer is affected significantly by the density and porosity in the structure. High strength geopolymer is associated with low porosity, high dense and fine-grained microstructure [61].

6.2. Mineralogy of clay-based geopolymers

The X-ray diffraction (XRD) pattern of kaolinite consists mainly of crystalline phases [34]. The thermal treatment of kaolinite transformed the crystalline phases into amorphous phases. The metakaolin retains some long-range order as result of stacking of the hexagonal layers [62]. Therefore, metakaolin shows semi-crystalline to amorphous pattern with a halo at 2θ between 15 and 30° [56]. This diffuse halo represents the amorphous silica in metakaolin [63]. Marked shift in the scattering peak is observed after the geopolymer formation. The diffuse halo in metakaolin shifted to higher angle. Generally, geopolymers show completely amorphous X-ray diffraction (XRD) pattern with a diffuse halo peak at 2θ between 27 and 30° [2548, 57, 64, 65]. The primary binder phase in geopolymer matrix contributes to the amorphous characteristic and determines the strength of geopolymers. Also, in a study carried out by Wang et al. [59], the halo diffuse peak of metakaolin geopolymer fell at 2θ between 18 and 25°. Increasing Si/Al ratio reduces the angle of diffuse halo [65].

Crystalline phases, particularly zeolites, are usually grown in geopolymers in conjunction with the amorphous binder phases (Figure 12) [46, 66]. As zeolites have similar chemical composition with geopolymers, geopolymers are usually deemed as the zeolitic precursor. The main difference between them is that zeolite is crystalline while geopolymer is amorphous. The growth of crystalline phases is facilitated by the high water content, high curing temperature, aging and also extended curing period [4, 67]. Zeolites are highly porous and have poor mechanical properties. Some researches [68, 69] claimed that zeolite crystallites reinforce and improve strength of clay geopolymers. Yet, the long-term strength reduced. Even so, it is strongly believed that there is a tolerance limit on the crystalline phase’s content within the geopolymer matrix. Similar trend was reported for fly ash geopolymers [70].

Figure 12.

XRD patterns of geopolymers from Algerian metakaolin, activated with alkaline sodium silicate solution and cured at 50°C for 24 h with various Si/Al ratios [66].

Advertisement

7. Functional group identification

Kaolinite shows FTIR bands around 1113 cm−1 (Si-O bonds in SiO4 molecules); 994 cm−1 (Si-O bonds in SiO4 molecules); 907 cm−1 (AlIV-OH vibrations); 799 cm−1 (SiO-symmetric stretching) and 537 cm−1 (Si-O-AlVI) [24, 38, 71, 72]. On the other hand, as the kaolinite is thermally treated, the band at 1113 cm−1 shifted to lower wavenumber (~ 1031 cm−1) [23]. This is related to the amorphous SiO2 [25]. The FTIR bands associated with VI-fold coordinated Al vanished after calcination as a result of distortion of tetrahedral and octahedral sheets of kaolinite [24]. The band at around 781 cm−1 appears in metakaolin as the Al-O stretching vibration in AlO4 tetrahedral [73].

As the geopolymerization reaction progresses, shift of bands are observed. Clay-based geopolymer exhibits main FTIR absorption band at 990 cm−1 associated with the asymmetrical stretching of Si-O-Si and Si-O-Al bonds [42, 74]. This band is shifted from the band at 1031 cm−1 in metakaolin. In addition, this FTIR band becomes more intense as the reaction proceeds indicating more geopolymer networks are formed. The band is usually shifted to lower wavenumber from raw materials and further shifted to higher wavenumber as a consequence of curing process. This is because of the changes in the silicate network with more substitution of non-bridging oxygen and increasing substitution of Al in the silicate sites. This is proved by the model by Duxson et al. [54] who proposed the transformation from Gel 1 to Gel 2 over time, aforementioned. This band shift is also observed in fly ash geopolymers (Figure 13) [75].

Figure 13.

Shifts of FTIR bands from Gel 1 (G1) to Gel 2 (G2) [75].

Another bands at 720 cm−1 (Si-O-Si/ Si-O-Al stretching), 560 cm−1 (tetrahedral aluminum stretching bands) and 690-440 cm−1 (Si-O-Si/ Si-O-Al bending vibrations) are also present in clay-based geopolymers [41, 42, 56]. High Si content in geopolymer structure produces stronger geopolymers as the Si-O-Si bonds are stronger than Si-O-Al bonds [76].

Advertisement

8. Properties of clay-based geopolymer

Geopolymers exhibit excellent mechanical and physical properties, such as low density, good chemical, fire and thermal resistance, high mechanical strength, and so on. Therefore, they are widely applied in various fields as new materials with high tech application. Geopolymers harden rapidly. In general, metakaolin geopolymers set and harden within 24 h. Short set time of 4 h has been reported by De Silva et al. [77] cured at 40°C. Fpr fly ash based geopolymer paste, in another way, sets and hardens faster compared to metakaolin geopolymers. Accordance to Hardjito et al., fly ash geopolymer can be hardened up to 2 h when cured at 65 and 80°C [78]. However, setting time is significantly dependent on the curing temperature. The geopolymer will set faster when cured at higher temperature. At 50°C, geopolymerization process required 4 h. Furthermore, geopolymerization process needed 1.5 h and 0.5 h at 85 and 95°C, respectively [2]. If the geopolymer paste is cured at temperatures lower than ambient temperature, it might need more than 1 day to set. No degradation in the strength of geopolymers at 28 days even they set at a longer time, as reported by Rovnanik [74].

The bulk density of metakaolin geopolymers is reported in the range between 1.20 and 1.80 g/cm3. Thus, lightweight products can be made out of geopolymers. The bulk density reported is lower than ordinary Portland cement paste and almost or even lower than geopolymers based on slag and fly ash . For instance, ordinary Portland cement paste has density of more than 1.80 g/cm3 [35] while coal fly ash geopolymers have density in the range between 1.40 to 1.80 g/cm3 [79, 80]. Bulk density is mainly affected by the curing condition as well as other synthesis parameters, such as the nature of alkali metal silicate, the type of geopolymers and alkali concentration. Bulk density decreases with increasing curing temperature [74]. Compressive strength increses with the increases of bulk density. Almost similar bulk density values were recorded for K-based (1.39–1.82 g/cm3) and Na-based (1.25–1.72 g/cm3) metakaolin geopolymers. Na-based geopolymers are generally lighter than K-based geopolymers. This is due to K-based geopolymers are denser and contain fewer pores as aforementioned [65].

From the result obtained by De Silva et al. [77] in Figure 14, high SiO2/Al2O3 ratio in the initial composition shows longer setting and hardening times. Strength development of metakaolin geopolymers with SiO2/Al2O3 of 3.81 became high and stabilized at a later age, even though the setting time was longer. Setting time is short providing that there is high Al2O3 content; however, it will deteriorate strength due to low SiO2 content. Besides, the calcium content in the precursor materials would definitely affect the setting time. This is due to the fact that the Ca content provides extra nucleation sites for precipitation of dissolved species and hence leads to setting and hardening at a faster rate [55].

Figure 14.

Final setting times and compressive strength of metakaolin geopolymers with varying SiO2/Al2O3 molar ratios at constant H2O/Na2O molar ratio of 13.6 [81].

Geopolymers achieve compressive strength of 20 MPa after only 4 h at 20°C. The 28-day compressive strength of geopolymers could be as high as 70–100 MPa [1]. High strength means the easier or higher dissolution of source materials, generating more aluminosilicate species, which are the most important ingredients for geopolymerization process. The reaction extent of source materials can be measured directly by the compressive strengths of prepared geopolymers. The strength of geopolymers is dependent on the strength of gel phase, the amount of gel phase formed and amorphous nature of the reaction products [73].

On the other hand, geopolymers have excellent thermal stability with low shrinkage (2%). Geopolymers are stable up to 1000–1200°C [4, 58, 82, 83] and have ceramic-like structure [3]. Geopolymers are dimensionally stable in the working range between 250 and 800°C, accordance to Subaer and van Riessen [84]. In order to improve the thermal properties of geopolymers, filler (e.g. granite or quartz) and foaming agents (e.g. Al powder, hydrogen peroxide) have been added during geopolymer mixing. Addition of quartz or granite reduced shrinkage to 1% [85]. In addition, based on Rickard et al. [86], foamed geopolymers reinforced with polypropylene fibers achieved fire rating of at least 1 h (Figure 15).

Figure 15.

Cold side temperatures during the fire testing of four mixes of metakaolin geopolymers [86].

Foamed geopolymers have good potential for ambient application as thermal insulator while exhibiting low density and compressive strength. For fire resistance application, materials must have very low thermal conductivity and resistance to thermal damage as to achieve the similar fire rating. Contradict result was reported by Elimbi et al. [87], whereby metakaolin geopolymers decreased in strength when heated between 300 and 900°C. It was explained due to the progressive transformation of geopolymer matrix into crystalline phases. The metakaolin geopolymers were warped and glazed with cracks at 1000°C.

Geopolymers possess high perseverance in acidic and alkaline media [68, 88]. Comparatively, they are more stable under alkaline medium. No deterioration in mechanical properties when immersed in sea water (pH = 8) and sodium sulfate solution (5% Na2SO4) up to 360 days. On the other hand, geopolymers were severely attacked when immersed in HCl solution for long period. Compression strength decreased while mass loss of samples increased. This was probably due to the de-aluminum of geopolymer structure in highly acidic medium. De-aluminum leads to mass loss of geopolymer structure as the consequences of SiAOAAl bonds break that form more silicic acid ions in acid medium. The microstructure of the produced geopolymers became more porous (Figure 16) [30].

Figure 16.

SEM micrograph of kaolinite geopolymers subjected to acid attack test after 90 days [30].

Drying shrinkage is shrinkage of the geopolymer matrix as a result of the loss of unbounded water during the curing process. As aforementioned, the addition of filler minimizes shrinkage of geopolymer samples. In general, shrinkage occurs in greater tendency in materials with higher content of finer materials than those with high content of coarser materials [89]. For instance, for geopolymers with sand filler, the drying shrinkage recorded was 0.01% at 180 days. However, for geopolymers without sand filler, the drying shrinkage fluctuated between 0.03 and 0.04% [30].

Advertisement

9. Applications of geopolymers

Geopolymers have great potential for variety of applications. Some applications have been successfully commercialized and marketed such as PYRAMENT blended cement and GEOPOLYMITE binders. GEOPOLYMITE binders have been used in several fields such as molding, tooling, foundry work, building’s thermal insulation and furnace insulation while PYRAMENT blended cement has been adapted in civil engineering in the production of pre-stressed and precast concrete [90].

Besides, geopolymers have been used to produce high-quality brick and tiles. Previously, kaolinite geopolymers are formed through low-temperature geopolymeric setting (L.T.G.S.) followed by ultra-rapid fire at 1000–1200°C to form bricks and tiles [4]. Up to now, this similar method has still being investigated by several researchers [91]. The geopolymer ceramics are non-burning and fire resistant. Furthermore, a new development of ceramic materials is compressing geopolymer powder using powder metallurgy method followed by sintering at 1000–1200°C [92].

In 1994, fireproof geopolymer fiber-reinforced composites have been used for aviation applications as aircraft composites and cabin interiors (floor panels, sidewalls, ceiling and partitions) to eliminate cabin fire during the aircraft accidents. The idea was arised from the problem of the existing plastic materials that were combustible and emitted flammable gases when they burnt. Besides, geopolymers have been used by Formula One teams in car manufacturing due to its corrosive, fire and heat resistance [4].

Concern toward lightweight materials for easy transportation and less energy consumption has led to lightweight concrete materials from geopolymers in civil engineering [93]. Furthermore, the lightweight concrete facilitates structural loading bearing and acts as thermal insulator [94]. Studies on foamed geopolymers in thermal insulation materials for housing construction have also been studied [95]. Zhang et al. [96] made reflective and heat insulative coating from geopolymers. With the addition of pigments and fillers (such as hollow glass microspheres, talc powder and titanium dioxide), wetting agent, dispersing agent and water-retaining agent, the coating produced has 90% reflectivity and thermal insulation performance up to 24°C. Apart from thermal insulative properties, the synthesis of geopolymer for acoustic insulation has been reported by Hung et al. [97]. Geopolymers can adequately and potentially become sound insulating materials in construction and buildings. The density of geopolymer matrix affects the noise reduction coefficient.

In addition, according to Temuujin et al. [98], geopolymers are capable of anti-ultraviolet and anti-aging, which made them suitable as coating for exterior wall building to conserve energy. The studies on the thermal and fire performance of geopolymers have also been reported elsewhere [81, 99, 100, 101]. As mentioned earlier, geopolymers have molecular structures to resemble zeolitic materials. As such, they are able to immobilize toxic waste or heavy metals as they can absorb and solidify toxic chemical waste. This is beneficial to the immobilization technology [84, 102].

Porous geopolymers were prepared by Okada et al. [89] for use in cooling system. This idea was come about due to high water retention properties or slow water releasing properties of geopolymers. This makes geopolymers suitable for surface cooling by water evaporation that helps to curb the rising earth temperature due to human activities and country development. Potential use of geopolymer in infrastructure rehabilitation was suggested by Pacheco-Torgal et al. [103]. Geopolymer paste can function as sealer for structures and replaced epoxy adhesives in fiber-reinforced polymer retrofiting. Almost similar research was carried out by Geraldes et al. [104], whereby geopolymers are used as restoration materials for tiles. In order to further enhance the usage of geopolymers in civil engineering, researchers have investigated on one-part geopolymer system [105, 106, 107], whereby geopolymer mixture can be prepared by just adding water. The interest of this study is caused by the limitation of geopolymer technology for in-situ application which lowers its economical value.

n recent year, the study on geopolymers moves toward application as biomaterials. As proven by Pangdaeng et al. [28], geopolymer has good bioactivity and it is improved by the addition of white Portland cement. On the other hand, geopolymer as drug delivery system has also been studied by Jamstorp et al. [108] and Cai et al. [109]. Based on them, geopolymers possess variable pore-structure for the release of drug at target cell. This again extends the application of geopolymers in the medical fields.

References

  1. 1. Davidovits J. Geopolymers and geopolymeric new materials. Journal of Thermal Analysis. 1989;35(2):429-441
  2. 2. Davidovits J. 30 years of successes and failures in geopolymer applications. Market trends and potential breakthroughs, in Geopolymer 2002 Conference. 2002, Geopolymer institute, Saint-Quentin, France: Melbourne, Australia
  3. 3. Davidovits J. Geopolymers: Inorganic polymeric new materials. Journal of Thermal Analysis. 1991;37:1633-1656
  4. 4. Provis JL, Lukey GC, van Deventer JSJ. Do geopolymers actually contain nanocrystalline zeolites? A reexamination of existing results. Chemistry of Materials. 2005;17:3075-3085
  5. 5. Saidi N, Samet B, Baklouti S. Effect of composition on structure and mechanical properties of Metakaolin based PSS-Geopolymer. International Journal of Material Science. 2013;3(4):145-151
  6. 6. Barbosa VFF, MacKenzie KJD, Thaumaturgo C. Synthesis and characterisation of materials based on inorganic polymers of alumina and silica: Sodium polysialate polymers. International Journal of Inorganic Materials. 2000;2(4):309-317
  7. 7. Xu H, van Deventer JSG. Geopolymerization of multiple minerals. Minerals Engineering. 2002:15:1131
  8. 8. Davidovits J. Chemistry of geopolymeric systems. Terminology. In Second International Conference Geopolymere'99. 1999. Saint-Quentin
  9. 9. Komnitsas K, Zaharaki D. Geopolymerisation: A review and prospects for the minerals industry. Minerals Engineering. 2007;20:1261-1277
  10. 10. Singh PS et al. Geopolymer formation process at room temperature studied by 29Si and 27Al MAS-NMR. Materials Science and Engineering A. 2005;396:392-402
  11. 11. van Jaarsveld JGS. The physical and chemical characterisation of fly ash based geopolymers, in Department of Chemical Engineering. 2000, University of Melbourne: Victoria, Australia
  12. 12. Temuujin J, Riessen Av, MacKenzie KJD. Preparation and characterisation of fly ash based geopolymer mortars. Construction and Building Materials. 2010;24:1906-1910
  13. 13. Chindaprasirt P et al. Comparative study on the characteristics of fly ash and bottom ash geopolymers. Waste Management. 2009;29:539-543
  14. 14. Hardjito D, Shaw Shen F. Fly ash-based geopolymer mortar incorporating bottom ash. Modern Applied Science. 2012;4:44-52
  15. 15. Kong DLY, Sanjayan JG, Sagoe-Crentsil K. Comparative performance of geopolymers made with metakaolin and fly ash after exposure to elevated temperatures. Cement and Concrete Research. 2007;37:1583-1589
  16. 16. Yunsheng Z, Wei S, Zongjin L. Composition design and microstructural characterization of calcined kaolin-based geopolymer cement. Applied Clay Science. 2010;47(3-4):271-275
  17. 17. Wang SD, Scrivener KL, Pratt PL. Factors affecting the strength of alkali-activated slag. Cement & Concrete Research. 1994;24:1033-1043
  18. 18. Cheng TW, Chiu JP. Fire-resistant geopolymer produced by granulated blast furnace slag. Minerals Engineering. 2003;16:205-210
  19. 19. Villa C et al. Geopolymer synthesis using alkaline activation of natural zeolite. Construction and Building Materials. 2010;24(11):2084-2090
  20. 20. MacKenzie KJD et al. Magnesium analogues of aluminosilicate inorganic polymers (geopolymers) from magnesium minerals. Journal of Materials Science. 2013;48:1787-1793
  21. 21. Cioffi R, Maffucci L, Santoro L. Optimization of geopolymer synthesis by calcination and polycondensation of a kaolinitic residue. Resources Conservation and Recycling. 2003;40:27-38
  22. 22. Varga G. The structure of kaolinite and metakaolinite. Epitoanyag. 2007;59:6-9
  23. 23. Liew YM et al. Processing and characterization of calcined kaolin cement powder. Construction and Building Materials. 2012;30:794-802
  24. 24. Cristobal AGS et al. Zeolites prepared from calcined and mechanically modified kaolins: A comparative study. Applied Clay Science. 2010;49:239-246
  25. 25. Lecomte I et al. Synthesis and characterization of new inorganic polymeric composites based on kaolin or white clay and on ground-granulated blast furnace slag. Journal of Materials Research. 2003;18:2571-2579
  26. 26. Zhang HY et al. Development of metakaolin–fly ash based geopolymers for fire resistance applications. Construction and Building Materials. 2014;55:38-45
  27. 27. Buchwald A, Hilbig H, Kaps C. Alkali-activated metakaolin-slag blends—Performance and structure in dependence of their composition. Journal of Materials Science. 2007;42:3024-3032
  28. 28. Pangdaeng S et al. Apatite forming on calcined kaolin-white Portland cement geopolymer. Materials Science and Engineering C; 2015
  29. 29. Ferone C et al. Thermally treated clay sediments as geopolymer source material. Applied Clay Science. 2015;107:195-204
  30. 30. Slaty F et al. Durability of alkali activated cement produced from kaolinitic clay. Applied Clay Science. 2015;104:229-237
  31. 31. Takeda H et al. Characterization of zeolite in zeolite-geopolymer hybrid bulk materials derived from kaolinitic clays. Materials. 2013;6:1767-1778
  32. 32. Davidovits J, Sawyer JL. Early High-Strength Mineral Polymer. United States: Pyrament Inc, Houston, Tex; 1984
  33. 33. Davidovits J. Mineral Polymers and Methods of Making Them. United States; 1982
  34. 34. Heah CY et al. Study on solids-to-liquid and alkaline activator ratios on kaolin-based Geopolymers. Construction and Building Materials. 2012;35:912-992
  35. 35. Xu H, van Deventer JSJ. The geopolymerisation of alumino-silicate minerals. International Journal of Mineral Processing. 2000;59(3):247-266
  36. 36. Heah CY et al. Effect of curing profile on kaolin-based geopolymers. Physics Procedia. 2011;22:305-311
  37. 37. Kovarik T et al. A novel approach to polyaluminosialates curing process using electric boosting and temperature profile investigation by DSC. Journal of Thermal Analysis Calorimetry. 2015;121:517-524
  38. 38. van Jaarsveld JGS, van Deventer JGS, Lukey GC. The effect of composition and temperature on the properties of fly-ash and kaolinite-based geopolymers. Chemical Engineering Journal. 2002;89:63-73
  39. 39. Pacheco-Torgal F, Castro-Gomes J, Jalali S. Alkali-activated binders: A review. Part 2. About materials and binders manufacture. Journal of Construction and Building Materials. 2008;22:1315-1322
  40. 40. MacKenzie KJD et al. Formation of aluminosilicate geopolymers from 1:1 layer-lattice minerals pre-treated by various methods: A comparative study. Journal of Materials Science. 2007;42:4667-4674
  41. 41. Alonso S, Palomo A. Alkaline activation of metakaolin and calcium hydroxide mixtures: Influence of temperature, activator concentration and solids ratio. Materials Letters. 2001;47(1-2):55-62
  42. 42. Alonso S, Palomo A. Calorimetric study of alkaline activation of calcium hydroxide-metakaolin solid mixtures. Cement and Concrete Research. 2001;31(1):25-30
  43. 43. Yunsheng Z et al. Synthesis and heavy metal immobilization behaviours of slag based geopolymer. Journal of Hazardous Materials. 2007;143:206-213
  44. 44. Davidovits J. High-alkali cements for 21st century concretes. In Concrete Technology, Past, Present and Future. 1994: Metha PK, Farmington Hills: American concrete institute
  45. 45. Yip CK, Lukey GC, van Deventer JSJ. The coexistence of geopolymeric gel and calcium silicate hydrate at the early stage of alkaline activation. Cement and Concrete Research. 2005;35(9):1688-1697
  46. 46. Davidovits J. Geopolymer chemistry and applications. 2nd ed. 16 Rue Galilee, 02100 Saint Quentin, France: Institute Geopolymere; 2008
  47. 47. Singh PS, Bastow T, Trigg M. Structural studies of geopolymers by 29Si and 27Al MAS-NMR. Journal of Material Sciences. 2005;40:3951-3961
  48. 48. Davidovits J. Geopolymers: Inorganic polymeric new materials. Journal of material. Education. 1994;16:91-139
  49. 49. Davidovits J. Proceeding of the 1st International Conference on Geopolymer '88. 1988: Geopolymere, 16 Rue Galilee, 02100 Saint Quentin, France
  50. 50. Dimas D, Giannopoulou L, Panias D. Polymerization in sodium silicate solutions: A fundamental process in geopolymerization technology. Journal of Material Sciences. 2009;44:3719-3730
  51. 51. Xu H, van Deventer JSJ. The effect of alkali metals on the formation of geopolymeric gels from alkali-feldspars. Colloids and Surfaces A: Physicochemical and Engineering Aspects 2003;216:27
  52. 52. Provis JL et al. Modeling the formation of geopolymers, in Department of Chemical and Biomolecular Engineering. Victoria, Australia: The University of Melbourne; 2006
  53. 53. Provis JL, Deventer JSJv. Geopolymerisation kinetics. 2. Reaction kinetic modelling. Chemical Engineering Science. 2007;62:2318-2329
  54. 54. Duxson P et al. Geopolymer technology: The current state of the art. Journal of Material. Sciences. 2007;42:2917-2933
  55. 55. Rattanasak U, Chindaprasirt P. Influence of NaOH solution on the synthesis of fly ash geopolymer. Minerals Engineering. 2009;22(12):1073-1078
  56. 56. Liew YM et al. Optimization of solids-to-liquid and alkali activator ratios of calcined kaolin geopolymeric powder. Construction and Building Materials. 2012;37:440-451
  57. 57. Zhang YS, Sun W, Li ZJ. Hydration process of potassium polysialate (K-PSDS) geopolymer cement. Advances in Cement Research. 2005;17:23-28
  58. 58. Sun W et al. In situ monitoring of the hydration process of K-PS geopolymer cement with ESEM. Cement and Concrete Research. 2004;34:935-940
  59. 59. Wang H, Li H, Yan F. Synthesis and mechanical properties of metakaolinite-based geopolymer. Colloids and Surfaces A: Physicochemical and Engineering Aspects. 2005;268(1-3):1-6
  60. 60. Rowles MR et al. 29Si, 27Al, 1H and 23Na MAS NMR study of the bonding character in aluminosilicate inorganic polymers. Applied Magnetic Resonance. 2007;32:663-689
  61. 61. Steveson M, Sagoe-Crentsil K. Relationships between composition, structure and strength of inorganic polymers. Part 1: Metakaolin-derived inorganic polymers. Journal of Materials Science. 2005;40:2023-2036
  62. 62. Wong H et al. Characterization and thermal behaviour of kaolin. Journal of Thermal Analysis and Calorimetry. 2011;105:157-160
  63. 63. Belver C, Banares MA, Vicente MA. Preparation of porous silica by acid activation of metakaolins. In: 6th International Symposium on the Characterization of Porous Solids (COPS-VI). 2002. Alicante, Spain: Elsevier Science B.V
  64. 64. Davidovits J. Properties of geopolymer cements. In: Proceedings First International Conference. 1994. Kiev, Ukraine: Geopolymer Institute, Saint-Quentin, France
  65. 65. Lizcano M et al. Mechanical properties of sodium and potassium activated metakaolin-based geopolymers. Journal of Materials Science. 2012;47:2607-2616
  66. 66. Zibouche F et al. Geopolymers from Algerian metakaolin. Influence of secondary minerals. Applied Clay Science. 2009;43:453-458
  67. 67. Duxson P et al. The effect of alkali and Si/Al ratio on the development of mechanical properties of metakaolin-based geopolymers. Colloids and Surfaces A: Physicochem. Eng. Aspects. 2007;292:8-20
  68. 68. Palomo A et al. Chemical stability of cementitious materials based on metakaolin. Cement and Concrete Research. 1999;29:997-1004
  69. 69. Kolousek D et al. Preparation, structure and hydrothermal stability of alternative (sodium silicate-free) geopolymers. Journal of Materials Science. 2007;42:9267-9275
  70. 70. Criado M et al. An XRD study of the effect of the SiO2/Na2O ratio on the alkali activation of fly ash. Cement and Concrete Research. 2007;37:671-679
  71. 71. Granizo ML, Blanco-Varela MT, Martinez-Ramirez S. Alkali activation of metakaolins: Parameters affecting mechanical, structural and microstructural properties. Journal of Material Sciences. 2007;42:2934-2943
  72. 72. Galan E et al. Technical properties of compounded kaolin sample from Griva (Macedonia, Greece). Applied Clay Science. 1996;10:477-490
  73. 73. Ilic BR, Mitrovic AA, Milicic LR. Thermal treatment of kaolin clay to obtain metakaolin. Hemijska Industrija. 2010;64:351-356
  74. 74. Rovnanik P. Effect of curing temperature on the development of hard structure of metakaolin-based geopolymer. Construction and Building Materials. 2010;24:1176-1183
  75. 75. Criado M, Fernandez-Jimenez A, Palomo A. Alkali activation of fly ash: Effect of the SiO2/Na2O ratio. Part 1: FTIR study. Microporous and Mesoporous. 2007;106:180-191
  76. 76. Duxson P et al. Understanding the relationship between geopolymer composition, microstructure and mechanical properties. Colloids and Surfaces A: Physicochem. 2005;269:47-58
  77. 77. De Silva P, Sagoe-Crenstil K, Dirivivatnanon V. Kinetics of geopolymerization: role of Al2O3 and SiO2. Cement Concrete Research. 2007;37:512-518
  78. 78. Hardjito D, Cheak CC, Ing CHL. Strength and setting times of low calcium fly ash-based geopolymer mortar. Modern Applied Science. 2008;2:3-11
  79. 79. Andini S et al. Coal fly ash as raw material for the manufacture of geopolymer-based products. Waste Management. 2008;28:416-423
  80. 80. Swanepoel JC, Strydom CA. Utilisation of fly ash in a geopolymeric material. ApplGeochem. 2002;17(8):1143
  81. 81. Kamseu E et al. Insulating behavior of metakaolin-based geopolymer materials assess with heat flux meter and laser flash techniques. Journal of Thermal Analysis and Calorimetry. 2012;108:1189-1199
  82. 82. Davidovits J. Geopolymer chemistry and properties. In: Davidovits J, Orlinski J, editors. Proceedings of the 1st International Conference on geopolymer’88. Compiegne, France. p. 25-48
  83. 83. Schmucker M, MacKenzie KJD. Microstructure of sodium polysialatesiloxogeopolymer. Ceramics International. 2005;31:433-437
  84. 84. Chequer CD-L, Frizon F. Impact of sulfate and nitrate incorporation on potassium and sodium-based geopolymers: Geopolymerization and materials properties. Journal of Materials Science. 2011;46:5657-5664
  85. 85. Subaer v RA. Thermo-mechanical and microstructural characterisation of sodium-poly (sialate-siloxo) (Na-PSS) geopolymers. Journal of Materials Science. 2007;42:3117-3123
  86. 86. Rickard WDA, Vickers L, van Riessen A. Performance of fibre reinforced, low density metakaolin geopolymers under simulated fire conditions. Applied Clay Science 2013;73:71-77
  87. 87. Elimbi A et al. Thermal behavior and characteristics of fired geopolymers produced from local Cameroonian metakaolin. Ceramics International. 2014;40:4515-4520
  88. 88. Davidovits J. Recent progresses in concretes for nuclear waste and uranium waste containment. Concrete International. 1994;16(12):53-58
  89. 89. Okada K et al. Water retention properties of porous geopolymers for use in cooling applications. Journal of the European Ceramic Society. 2009;29:1917-1923
  90. 90. Davidovits J, Davidovics M. Geopolymer: Ultra-high temperature tooling material for the manufacture of advanced composites. SAMPE Symposium Exhibition. 1991;36:1939-1949
  91. 91. Kuenzel C et al. Production of nepheline/quartz ceramics from geopolymer mortars. Journal of the European Ceramic Society. 2013;33:251-258
  92. 92. Jaya NA, Abdullah MMAB, Ahmad R. Reviews on clay geopolymer ceramic using powder metallurgy method. Materials Science Forum. 2014;803:81-87
  93. 93. Pimraksa K et al. Lightweight geopolymer made of highly porous siliceous materials with various Na2O/Al2O3 and SiO2/Al2O3 ratios. Material Science Engineering, A. 2011;528:6616-6623
  94. 94. Aguilar RA, Diaz OB, Garcia JIE. Lightweight concretes of activated metakaolin-fly ash binders, with blast furnace slag aggregates. Construction and Building Materials. 2010;24(7):1166-1175
  95. 95. Prudhomme E et al. In situ inorganic foams prepared from various clays at low temperature. Applied Clay Science. 2011;51:15-22
  96. 96. Zhang Z et al. Preparation and characterization of a reflective and heat insulative coating based on geopolymers. Energy and Buildings. 2015;87:220-225
  97. 97. Hung T-C et al. Inorganic polymeric foam as a sound absorbing and insulating material. Construction and Building Materials. 2014;50:328-334
  98. 98. Temuujin J, Minjigmaa A, Rickard W. Preparation of metakaolin based geopolymer coatings on metal substrates as thermal barriers. Applied Clay Science. 2009;46(3):265-270
  99. 99. Henon J et al. Potassium geopolymer foams made with silica fume pore forming agent for thermal insulation. Journal of Porous Materials. 2013;20:37-46
  100. 100. Temuujin J et al. Thermal properties of spray-coated geopolymer-type compositions. Journal of Thermal Analysis and Calorimetry. 2012;107:287-292
  101. 101. Sukontasukkul P et al. Use of phase change material to improve thermal properties of lightweight geopolymer panel. Materials and Structures. 2016;49:4637-4645
  102. 102. Ponzoni C et al. Chromium liquid waste inertization in an inorganic alkali activated matrix: Leaching and NMR multinuclear approach. Journal of Hazardous Materials. 2015;286:474-483
  103. 103. Pacheco-Torgal F et al. An overview on the potential of geopolymers for concrete infrastructure rehabilitation. Construction and Building Materials. 2012;36:1053-1058
  104. 104. Geraldes CFM et al. Geopolymers as potential repair material in tiles conservation. Applied Physics A Material Science Process. 2016;122:197
  105. 105. Sturm P et al. Degree of reaction and phase content of silica-based one-part geopolymers investigated using chemical and NMR spectroscopic methods. Journal of Materials Science. 2015;50:6768-6778
  106. 106. Ke X et al. One-part geopolymers based on thermally treated red mud/NaOH blends. Journal of the American Ceramic Society. 2015;98:5-11
  107. 107. Peng MX et al. Synthesis, characterization and mechanisms of one-part geopolymeric cement by calcining low-quality kaolin with alkali. Materials and Structures. 2015;48:699-708
  108. 108. Jamstorp E et al. Mechanically strong geopolymers offer new possibilities in treatment of chronic pain. Journal of Controlled Release. 2010;146:370-377
  109. 109. Cai B, Engqvist H, Bredenberg S. Evaluation of the resistance of a geopolymer-based drug delivery system to tampering. International Journal of Pharmaceutics. 2014;465:169-174

Written By

Mohd Mustafa Al Bakri Abdullah, Liew Yun Ming, Heah Cheng Yong and Muhammad Faheem Mohd Tahir

Submitted: October 2nd, 2017 Reviewed: January 25th, 2018 Published: October 10th, 2018