**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,

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

fly ash-based geopolymers revealed smooth heterogeneous geopolymer matrix with remnant fly ash particles in the hollow cavities due to partially dissolution (**Figure 10**).

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 after the chemical reaction.

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**

Gel 2 with high Si content as the reaction progresses and finally rearranged forming three-

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

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

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

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,

firstly mixed with liquid sodium silicate and the NaOH solution is added afterwards.

dimensional geopolymer frameworks.

248 Cement Based Materials

**5.1. Clay-based geopolymer formation**

crucial in order to attain high strength.

consistency without addition of excess water.

**6.1. Morphology**

**6. Characterization 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.

**Figure 10.** SEM micrographs of fly ash geopolymers cured at room temperature for 24 hours and at 80°C for another 24 hours [15].

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

**Figure 12.** XRD patterns of geopolymers from Algerian metakaolin, activated with alkaline sodium silicate solution and

molecules); 907 cm−1 (AlIV-OH vibrations); 799 cm−1 (SiO-symmetric stretch-

[25]. The FTIR bands associated with VI-fold coordinated Al vanished

ing) 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

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

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

molecules); 994 cm−1 (Si-O

Clay-Based Materials in Geopolymer Technology http://dx.doi.org/10.5772/intechopen.74438 251

geopolymer matrix. Similar trend was reported for fly ash geopolymers [70].

Kaolinite shows FTIR bands around 1113 cm−1 (Si-O bonds in SiO4

**7. Functional group identification**

cured at 50°C for 24 h with various Si/Al ratios [66].

bonds in SiO4

tetrahedral [73].

to the amorphous SiO2

**Figure 11.** SEM micrographs of metakaolin geopolymers [59].

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° [25, 48, 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

**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].

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].
