Evaluation of the thermal stability of metakaolin-based geopolymers according to Si/Al ratio and sodium activator

The thermal stability of geopolymer pastes with different Si/Al ratios (1.5–3.0) and sodium activators (Na₂SiO₃ and/or NaOH) were evaluated in this study. The inclusion of Na₂SiO₃ induced an active geopolymer reaction, enhancing pre-heating compressive strength, but gradually reduced from 300 °C due to vapor pressure and microstructural deterioration caused by entrapped physically bound water. Conversely, the strength of geopolymers containing only NaOH remained stable or increased up to 600 °C, owing to additional geopolymerization from unreacted raw materials and compact matrix induced by thermal shrinkage. The aluminosilicate networks of all geopolymers remained relatively stable until crystalline nepheline formed above 800 °C; larger crystalline particles were formed in geopolymers with Na₂SiO₃ owing to its denser geopolymer matrix, whereas smaller crystalline particles were formed in geopolymers with smaller Si/Al and only with NaOH, resulting in fewer voids and stable mechanical strength. Swelling of silica was notable in Na₂SiO₃-activated geopolymer, resulting in greater strength loss upon heating, indicating that Na₂SiO₃ hinders the thermal stability of geopolymers at extreme temperatures.