**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 (Si2 O5 ) *n* 2− 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].
