Residual Mechanical Performance and Mass Loss of Heat-Cured NaOH-Activated Fly Ash/Slag and Portland Cement Mortars After Exposure to Elevated Temperatures

This study compares the residual mechanical performance and mass loss of NaOH-activated Fly Ash (FA), Ground Granulated Blast Furnace Slag (GGBFS) and FA/GGBFS mortars with those of CEM I 42.5R and CEM I 52.5R mortars after exposure to 300, 600 and 900 °C. The alkali-activated mortars were heat-cured at 100 °C for 3 days, whereas the Ordinary Portland Cement (OPC) mortars were water-cured for 28 days, to compare the residual performance of the materials produced under their respective curing regimes. After 900 °C, the GGBFS mortar retained 43.4 MPa in compression (57.5%) and 2.88 MPa in flexure (44.9%), against 6.4–9.1 MPa (11.7–14.6%) and 1.29–1.40 MPa (12.8–16.3%) for the OPC mortars, both differences being statistically significant for each OPC mortar. The FA/GGBFS mortar (33.9 MPa, 50.9% in compression) exceeded the OPC mortars only in mean value; the differences were not statistically significant. GGBFS showed the highest mass loss (7.83% at 900 °C). In a correlation analysis, mass loss mirrored strength loss in the OPC mortars (R2 = 0.92–0.98) but not in the alkali-activated ones (R2 ≤ 0.11), showing that mass loss alone was not a consistent indicator of thermal damage across binder systems. The results indicate that the GGBFS-based NaOH-activated mortar is a promising alternative to OPC for precast building elements that must retain residual load-bearing capacity after high-temperature exposure.