3. Results and discussion
3.1. Compressive strength of the geopolymer
The compressive strength of the geopolymer as a function of calcination temperature and dosage of KAc are presented in Fig. 2. It denotes that the compressive strength of GT-CSK is noted as 0 MPa between 300 °C and 400 °C because samples do not harden, which proposes that these calcination temperatures are not sufficient for CSK to form metakaolin, and it cannot form the geopolymer by alkaline activation. Meanwhile, no samples harden under the calcination temperature of 300 °C. However, when the calcination temperature increases above 350 °C, the compressive strength of GT-CSK-K3 is the highest, around 31 MPa. The results demonstrate that the optimum dosage of KAc is 30% by mass of CSK and the lowest calcination temperature is 350 °C which is sufficient for the formation of metakaolin that can form the geopolymer by alkaline activation.