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. 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 caused 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.