*3.2. Si and Al Composition*

Geopolymer materials composed of Si and Al have more potential as ceramics. As one of the most important clay minerals, kaolin has a high porosity, strong mechanical stability, and low thermal conductivity in geopolymers [22,50,71]. During the firing of kaolin, the type and quantity of secondary phases can have a significant impact on the thermal properties of the raw materials [48,68]. Iron oxide is very significant [49,72]; Fe2O3 can exist in raw materials as either mineral complexes or silicate structures. The addition of Fe2O3 in kaolin not only increases the quantity of mullite phase at lower temperatures (1050 ◦C), but also improves the crystallisation of mullite at higher temperatures [19,73,74]. Table 3 lists the Si and Al content of numerous kaolin types successfully employed in geopolymer production.


**Table 3.** Si and Al content of kaolin for geopolymer synthesis.

The percentage content range of Si and Al was from 32.04% to 70.32% and 11.60% to 44.2%, respectively, while the lowest particle size was 1.3 μm and lowest surface area is 1.57 m2/g. According to previous research, mechanical activation altered the particle size and specific surface as well as the kaolin's reactivity with respect to the geopolymerization reaction, hence increasing the compressive strength of the geopolymers [69,81–83]. This increase was attributed to the smaller particle size and altered shape, which allowed for a faster dissolution of the particles in the activating solution [82,84]. The initial crystalline structure of clay minerals that already exist is broken during dehydroxylation, making the substance reactive; obviously, the higher the level of dehydroxylation, such as amorphousness, the more reactive the material [22,49,50,76]. Kaolin as a geopolymer material which has high Si Al content is highly suitable in ceramic application

There is a critical alkaline concentration that can achieve the maximum compressive strength, and a higher concentration does not favour the formation of geopolymers, according to prior research that used kaolin as a single source material to synthesise geopolymer products and investigate the effect of different alkaline activator concentrations on the

compressive strength [22,56,85,86]. Important criteria that influence the degree of the geopolymerization process and the reaction duration with the alkali activator, respectively, are the Si-to-Al ratio and the ageing time [26,63,87]. Table 4 summarises the utilisation of alkali activator in past studies on kaolin.


**Table 4.** Previous research on kaolin and alkali activator.

The reaction of alkaline activators has been observed; NaOH and Na2SiO3 are commonly used at a range of 6 to 10 molar NaOH concentration. In the synthesis of kaolin geopolymers, the higher molar concentration of KOH, 16 Molar, is employed. In addition, high concentrations of KOH are utilised to enhance the solubility of Al3+ and Si4+ ions [26,88].

In addition to the selection of raw materials and manufacturing conditions, geopolymers can exhibit a vast array of qualities and characteristics. In general, the properties of geopolymers are highly reliant on the composition of the reactants, particularly the Si/Al ratio, process, and method, mixing design, and alkali activator type. In addition, the most important component in determining the application sectors of geopolymers is the sintering process, which is the most crucial step in the fabrication of ceramic geopolymers.
