Performance optimization of silica fume–incorporated fly ash geopolymer mortars: mechanical, thermal, microstructural and environmental assessment

This study investigated the effects of silica fume substitution on the mechanical, microstructural, and environmental performance of fly ash-based geopolymer mortars cured at 60 °C and 90 °C. Silica fume replaced fly ash at levels between 2% and 10%, and the mixtures were evaluated for unit weight, water absorption, porosity, capillary water absorption, abrasion resistance, high-temperature resistance, thermogravimetric analysis (TGA), field emission scanning electron microscopy (FESEM), and life cycle assessment (LCA) analyses. The results indicated that incorporating 2–4% silica fume under 60 °C curing provided the highest compressive and flexural strengths while maintaining low porosity and water absorption. Higher replacement ratios led to matrix deterioration and increased porosity. Although curing at 90 °C accelerated early-age strength development, prolonged exposure caused moisture loss and microcrack formation, resulting in unstable strength behaviour. Mixtures containing 2% silica fume exhibited the best capillary water absorption and abrasion resistance. Under elevated temperatures, some mixtures showed strength gains at 300 °C, followed by reductions at 600 °C and partial recovery at 900 °C due to sintering effects. FESEM and TGA confirmed that lower silica contents promoted a denser gel structure. LCA results demonstrated that geopolymer mortars offer environmental benefits in terms of global warming potential. Overall, a 2% silica fume replacement provided the most balanced mechanical, microstructural, and environmental performance.