Durability and Microstructural Evolution of Class C/Class F Fly Ash Geopolymer Mortars with Different Ash-to-Sand Ratios Under Combined Freeze–Thaw and MgSO4

This study investigates the durability of Class C/Class F fly ash geopolymer mortars with varying ash-to-sand ratios under freeze–thaw cycles, MgSO4 exposure, and their combined action. The results showed that geopolymer specimens with ash-to-sand ratios of 1:1.4 and 1:1.6 exhibited excellent durability under individual freeze–thaw or MgSO4 attack. After 60 freeze–thaw cycles, the mass losses ranged from 1.44% to 3.19%, while the residual compressive strength remained between 103.7% and 107.0% of the initial value. After 90 days of MgSO4 exposure, the mass loss was limited to 0.28–1.40%, and the residual strength increased significantly to 147.5–159.8%. However, the combined effect of freeze–thaw cycles and MgSO4 corrosion was not simply additive. Under combined degradation, mass loss increased markedly to 7.29–9.28% after 60 cycles, while residual strength declined to 42.6–53.7%. The ash-to-sand ratio significantly influenced the pore size distributions. A direct relationship was observed among reaction products, microstructure, and freeze–thaw resistance. These findings provide insight into the durability mechanisms of fly ash geopolymer mortars and support their application in cold-region infrastructures.