Durability and Microstructure of Fly Ash/Silica Fume-Modified Geopolymer Concrete with Inorganic Aluminosilicate Polymer Gels Under Freeze–Thaw Cycles and Single-Side Salt Erosion

Geopolymer concrete contains inorganic aluminosilicate polymer gels formed through the activation of industrial solid wastes. This study investigated the effects of fly ash (FA) and silica fume (SF) on the durability and microstructure of geopolymer concrete exposed to freeze–thaw cycles and single-side salt erosion. Five mixtures were prepared using Baioheng geopolymer cement, with FA replacement levels of 15% and 25% and SF replacement levels of 3% and 5%. Mechanical tests, freeze–thaw tests, single-side salt-freezing tests, SEM-EDS, XRD, and CT analysis were conducted to evaluate the relationship between macroscopic performance and inorganic polymer gel structure. The results showed that 25% FA reduced compressive strength and freeze–thaw resistance, mainly due to insufficient reaction products and increased defect connectivity. In contrast, 3% SF improved the 56 d compressive strength by 13.24%, maintained the relative dynamic elastic modulus at 86.64% after 100 freeze–thaw cycles, and limited the mass loss to 0.72%. SEM-EDS and XRD results indicated that appropriate SF addition increased the Si/Al ratio and promoted the formation of C-(A)-S-H/N-A-S-H-related gel products, leading to a denser inorganic polymer matrix. However, excessive SF weakened the improvement effect, possibly due to local heterogeneity and dispersion difficulty. These results indicate that controlling the composition and spatial distribution of inorganic aluminosilicate polymer gels is essential for improving the salt-frost durability of geopolymer concrete.