*4.1. Compressive Strength*

The compressive strength of a geopolymer is contingent on several factors. These factors include the strength of the gel phase, the ratio of gel phase to undissolved Al-Si particles, the distribution and hardness of the undissolved Al-Si particle size, the amorphous nature of the geopolymer or the degree of crystallinity, and surface reactions between the gel phase and undissolved Al-Si particles [76,103–105]. For instance, curing at elevated temperatures for more than two hours appears to promote the development of compressive strength. Nevertheless, curing at 70 ◦C appears to increase compressive strength significantly more than curing at 30 ◦C during the same period. Table 6 displays the compressive strength and influencing parameters from prior studies.


**Table 6.** Compressive strength and factors affecting.

Figure 6 shows the compressive strength using kaolin from prior studies summarized from Table 1. The best compressive strength by using kaolin as raw materials, 68 MPa was obtained at 28 days of curing by using high density water glass about 1.5 g·cm−3. Choosing a suitable water glass density and type of alkali activator helps to ensure that the geopolymer mortar used as ceramic has good compressive strength. Generally, the ratio of each binder either solid or liquid will result in different compressive strength due to phase and bond formation in geopolymer system.

**Figure 6.** Reported compressive strength using kaolin.

#### *4.2. Chemical and Mineralogical Composition*

The chemical composition of raw materials as determined by X-ray fluorescence impacts the development of geopolymerization and the kind and quantity of zeolite [12,22,63,107]. The primary chemical component of kaolin, kaolinite, is dehydroxylated at temperatures reaching 550 ◦C, hence converting its long-range organised microstructure to an amorphous state. Consequently, kaolinite has been converted to metakaolin [48,50,74]. In geopolymer synthesis, the kind and temperature of thermal treatment affect the reactivity of metakaolin. Due to its relatively well-defined chemical structure, chemical composition, and properties, which increase its reactivity [50,57], kaolin is also considered one of the most essential precursors for geopolymer synthesis [22,49,50,56,68,108]. According to several studies, the use of kaolin as a raw material for the synthesis of geopolymer is environmentally friendly because it generates less carbon dioxide than the production of Portland cement. Table 7 displays the chemical composition of kaolin geopolymer.

**Table 7.** Chemical composition of kaolin geopolymer.


Alumina (Al2O3) and silica (SiO2) content, in general, have the greatest impact on the geopolymerization. Other mineral compositions also play a part, including magnesium oxide, MgO (speeds up the hydration reaction and may cause low porosity and high bulk

density due to the large volume of the hydrate) [49], iron(III) oxide, Fe2O3 (able to exhibit adsorptive, ion-exchange, and catalytic properties similar to those of zeolitic aluminosilicate molecular sieves) [110], and calcium oxide, CaO (acts as to harden at room temperature without affecting the mechanical properties of the final product) [22].
