Experimental study on fly ash slag-based mortar performance under elevated temperatures

As one of the primary building components exposed to fire, mortar plays a crucial role in the initial thermal response of building envelopes; therefore, its thermal stability and residual mechanical performance under elevated temperatures are significant. This study investigates the performance of mortar incorporating supplementary cementitious materials (SCMs) under elevated temperature exposure, aiming to improve thermal resistance, promote sustainability, and reduce CO $$_2$$ emissions. Mortar mixtures were prepared with a constant water-to-binder ratio of 0.485 and a binder-to-sand ratio of 1:2.75. A control mix (C1) consisting of 100% Portland pozzolanic cement (PPC) was compared with fly ash (FA)-based mixtures (30–70% cement replacement) and FA–ground granulated blast furnace slag (GGBS) blended mixtures (10–50% FA with a constant 20% GGBS replacement). The specimens were cured for 7 and 28 days and evaluated for compressive strength before and after exposure to elevated temperatures of 400, 600, and 800 $$^\circ $$ C. The results reveal that increasing FA content reduces ambient compressive strength by up to $$\sim $$ 54%; however, it enhances high-temperature performance, resulting in a strength gain of up to $$\sim $$ 33.7% after exposure to 400 $$^\circ $$ C. The incorporation of 20% GGBS improves early-age strength development and mitigates strength deterioration at 400–600 $$^\circ $$ C. Nevertheless, all mixtures undergo severe degradation at 800 $$^\circ $$ C, with an average strength loss of $$\sim $$ 81.5%, attributed to extensive microcracking and the decomposition of major hydration products, as confirmed by microstructural analysis. The FA–GGBS blended mortars exhibit superior thermal stability and residual strength retention, demonstrating their potential as sustainable alternatives for fire resistant construction applications.