*3.2. Pore Structure Analysis*

The Brunauer–Emmett–Teller (BET) method was used to determine the surface area of the unsintered and sintered kaolin-based geopolymers. The specific surface area and pore volume of unsintered and sintered geopolymers are depicted in Figure 4. Smaller particles resulted in larger surface areas. This is because sintered kaolin-based geopolymer has a larger surface area due to the removal of volatiles and impurities from the sample's surface. Sutama et al. [16] stated that the formation of pores on the sample may lower the compressive strength.

The unsintered kaolin-based geopolymer had the lowest surface area (2.3 m2/g) and pore volume (0.01 cm3/g). After sintering at 900 ◦C, the kaolin-based geopolymer's surface area (up to 245 m2/g) and pore volume (up to 0.025 cm3/g) increased relative to those of the unsintered kaolin-based geopolymer. Then, after sintering at 1100 ◦C, the surface area increased to 270 m2/g and pore volume increased to 0.04 cm3/g. The kaolin-based geopolymer was assumed to consist of mesopores in a small quantity, resulting in a higher surface area after sintering at high temperatures.
