Heat Resistance of MKG

This chapter investigated the degradation process of metakaolin-based geopolymers under high temperatures of up to 1000 °C. The study focused on examining changes in the microstructure, phase assemblage, and mechanical properties of the samples. The results indicated that the type of activator—Na2SiO3 + NaOH (Na/Na) and Na2SiO3 + KOH (Na/K) solution—influenced the degradation mechanisms of geopolymer mortars at high temperatures. Geopolymers based on (Na, K) exhibited superior thermal resistance above 200 °C, revealing higher compressive strength, lower porosity, and reduced cracking tendency compared to those based on (Na, Na). Mortars based on (Na, Na) experienced more mass loss, leading to significant drying shrinkage, and further crack development at 200 °C. At temperatures exceeding 200 °C, crack development and material property degradation resulted in the decline of the mechanical properties of geopolymers. In contrast to their (Na-Na) counterparts, (Na–K)-based geopolymers showed improved chemical stability and did not develop new crystalline phases over 1000 °C. Higher temperatures (1000 °C) caused geopolymers to sinter significantly, resulting in a thick and homogenous matrix and bettering the specimens’ mechanical characteristics. Overall, the results point to the critical role that Na+ and K+’s mutual encouraged effects play in the development of cracks, sintering, and new crystallization in geopolymers at high temperatures.