Effect of coal gangue fine aggregate on the dual durability of alkali-activated coal gangue-slag mortar: Pore evolution and deterioration mechanisms under sulfate attack and freeze-thaw cycling

This study investigates the influence of coal gangue fine aggregate (CGFA) on the sulfate and freeze–thaw resistance of alkali-activated coal gangue–slag mortar (AACGSM). Mortars containing 0–30% CGFA as mass replacement of standard sand were subjected to sulfate wet–dry cycling, freeze–thaw cycling, compressive-strength testing, XRD, SEM, FTIR, and BET analysis. The durability response was dosage-dependent. After 30 sulfate wet–dry cycles, 20CGFA showed the highest residual compressive strength coefficient (85.11%), indicating the best measured sulfate resistance. After 75 freeze–thaw cycles, 0CGFA retained the highest residual strength, whereas 20CGFA performed best among CGFA-containing mixtures. BET results showed lower cumulative pore volume for 20CGFA, while excessive CGFA increased pore connectivity and deterioration. The results support potential use of CGFA-containing AACGSM in non-structural and semi-structural applications in sulfate-bearing or seasonally frozen environments, although long-term and field validation remain necessary.