Combined effects of precursor fineness and thermally expansive additives on the fire response of fly ash geopolymers

Fly ash-based geopolymers exhibit excellent mechanical performance under ambient conditions; however, their dimensional stability and residual strength can deteriorate substantially at elevated temperatures. This study investigates the effects of fly ash particle size and heat-expansive additives, including densified silica fume, raw perlite, and andalusite, on the thermomechanical performance, volumetric stability, and residual compressive strength of potassium-activated fly ash geopolymers exposed to standard fire heating for 0.5 and 2 h, followed by either natural cooling or water spraying to induce thermal shock. Finer fly ash increased geopolymerisation, and ambient strength by 38.3%, but increased thermal shrinkage and reduced early thermomechanical performance due to restricted vapour escape. Silica fume increased porosity at room temperature by 15.7%, induced expansion at 0.5 h, and substantially reduced thermal contraction at 2 h. It also achieved a residual compressive strength of 157% relative to the reference at 2h, attributed to its high content of unreacted and partially reacted amorphous silica. Andalusite significantly improved thermomechanical performance at 0.5 h by 72.8%, owing to its high thermal stability. Thermal shock generally arrested shrinkage and improved strength retention, while causing greater expansion in silica-rich mixes due to gel rehydration. The findings revealed strong relationships between fly ash particle size, initial porosity, amorphous silica content, thermal shrinkage, residual mechanical performance, and the potential of thermal-expansive additives under fire exposure.