**5. Geopolymerization mechanism**

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 [41, 42]. 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

**Clay/Clay minerals Strength (MPa) Ref.**

Almandine 10.3c 8.5c [35] Grossular 16.7c 14.5c [35] Sillimanite 12.7c 6.5c [35] Andalusite 11.1c 8.8c [35] Kyanite 6.8c 6.3c [35] Pumpellyite 10.8c 8.8c [35] Spodumene 13.1c 5.0c [35] Augite 6.7c 5.0c [35] Lepidolite 4.3c 2.5c [35] Illite 7.1c 5.8c [35] Celsian 9.7c 8.7c [35] Sodalite 15.0c 10.3c [35] Stilbite 18.9c 14.2c [35] Heulandite 7.4c 5.6c [35] Anorthite 14.4c 6.0c [35] Kaolin - 2 – 10c [37] Clay residues 5.76 – 5.98f [38]

**KOH NaOH**

shown in **Figure 4**.

c

f

compressive strength;

**Table 2.** Strength result of clay/clay minerals geopolymers.

flexural strength

244 Cement Based Materials

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].

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

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.

**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].

$$\text{Al}-\text{Si material (s)} + \text{MOH (aq)} + \text{Na}\_2\text{SiO}\_3 \text{(s or aq)}\text{\textasciic} \tag{2}$$

Al–Si material (s)

**Figure 8.** Graphic model of alkali activation of geopolymers [54].

**Figure 7.** Schematic diagram of geopolymer formation [48].

[

<sup>M</sup>*a*((AlO2)*<sup>a</sup>* (SiO2)

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

*<sup>b</sup>*)*n*MOH · mH2 <sup>O</sup>] (4)

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$$\text{Al-Si material (s)} + \left[ \text{M}\_{\text{x}} \text{(AlO}\_{2}\text{)}\_{\text{x}} \left\{ \text{SiO}\_{2} \right\}\_{\text{y}} \cdot n\text{MOH} \cdot m\text{ H}\_{2}\text{O} \right] \text{gel} \Big|\,\tag{3}$$

Clay-Based Materials in Geopolymer Technology http://dx.doi.org/10.5772/intechopen.74438 247

$$\begin{aligned} \text{Zn (Si\$\div\$O\$, Al\$\div\$O\$)} + 2n\text{SiO}\_2 + 4n\text{H}\_2\text{O} &\longrightarrow \text{n (OH)}\\ &\text{(\$\div\$ \$)}\\ &\text{(OH)}\end{aligned} $$

$$\begin{array}{ccccc} \text{in (OH):} & \text{Si-O-Al}^{\text{(-}}\text{)-O-Si-(OH)}\text{jo} & \text{(-Na}^{+}\text{, K}^{+}\text{)-(-Si-O-Al}^{\text{(-}}\text{)-O-Si-O-}\text{)} + 4n\text{H}\_{2}\text{O} & \text{(-}\text{(-}\text{)}\text{)} + 2\text{O}^{-} & \text{(-}\text{)} + \text{(-}\text{)} \\ & \left| \begin{array}{c} \text{|} \qquad \text{|} \qquad \text{|} \qquad \text{|} \qquad \text{|} \qquad \text{|} \qquad \text{|} \qquad \text{|} \qquad \text{|} \qquad \text{|} \end{array} \right| \\ & \text{(OH):} \text{2} \qquad \text{(-}\text{)} + \text{(-}\text{)} + \text{(-}\text{)} \\ \end{array}$$

**Figure 7.** Schematic diagram of geopolymer formation [48].

**Figure 8.** Graphic model of alkali activation of geopolymers [54].

Al–Si material (s) + MOH (aq) + Na2 SiO3 (s or aq)↓ (2)

**Figure 6.** Formation of aluminum substituted silicate layer in metakaolin after attack by NaOH solution [47].

**Figure 5.** Reaction of kaolinite under alkaline medium (gray circle denotes the aluminum hydroxyl side groups) [46].

<sup>M</sup>*<sup>z</sup>* (AlO2)*<sup>x</sup>* (SiO2)*<sup>y</sup>* · *<sup>n</sup>*MOH · *<sup>m</sup>* H2 <sup>O</sup>] gel<sup>↓</sup> (3)

Al–Si material (s) + [

246 Cement Based Materials

$$\text{Al-Si material (s) } \left[ \text{M}\_{\text{s}} \text{(AlO}\_{2}\text{)}\_{\text{g}} \left( \text{SiO}\_{2}\right)\_{\text{b}} \right] \text{nMOH} \cdot \text{mH}\_{\text{g}} \text{O} \tag{4}$$

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 threedimensional geopolymer frameworks.

### **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.

fly ash-based geopolymers revealed smooth heterogeneous geopolymer matrix with remnant

**Figure 9.** ESEM micrographs of metakaolin geopolymers after (a) 10 minutes, (b) 3 hours, (c) 6 hours, and (d) 9 hours

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Differently, Wang etal. [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

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

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.

fly ash particles in the hollow cavities due to partially dissolution (**Figure 10**).

after the chemical reaction.

of mixing [57].

fine-grained microstructure [61].

**6.2. Mineralogy of clay-based geopolymers**
