Freeze–Thaw Resistance of Class C/Class F Fly Ash Geopolymer Mortars with Different Ash-to-Sand Ratios

This paper investigates the freeze–thaw durability of geopolymer mortars synthesized from class C and class F fly ash, with varying ash-to-sand ratios ranging from 1:1 to 1:1.8. Optimizing freeze–thaw resistance is critical for promoting the practical application of geopolymer materials in cold regions, where cyclic freezing and thawing significantly threaten long-term durability. The performance of the mortars was evaluated through laboratory freeze–thaw cycling and natural environmental exposure. Freeze–thaw resistance was assessed by measuring mass loss and compressive strength after 60 laboratory cycles and 90 days of natural environmental exposure, while specimens cured under standard conditions were used as reference samples. The results demonstrate that the ash-to-sand ratio significantly influences durability performance. After 60 laboratory freeze–thaw cycles, specimens with a ratio of 1:1 exhibited a severe mass loss of 17.31%, whereas those with ratios between 1:1.4 and 1:1.8 maintained mass losses below 5%. Under natural environmental exposure, which reflects multiple coupled environmental factors such as moisture fluctuation, drying, and carbonation rather than freeze–thaw action alone, mass loss increased from approximately 2.26–3.64% at 15 days to 9.00–11.74% at 90 days. The geopolymer mortars with an ash-to-sand ratio of 1:1.4 exhibited superior freeze–thaw resistance, characterized by the lowest mass loss and the highest compressive strength. Microstructural and phase analyses indicated environment-dependent phase evolution and pore structure changes in the geopolymer matrix, which were associated with the observed durability performance. These findings contribute to the understanding of durability in geopolymer systems, offering insights into optimizing ash-to-sand ratios for enhanced freeze–thaw resilience.