The Influence of Sand Ratio on the Freeze–Thaw Performance of Full Solid Waste Geopolymer Concrete

To clarify the effect of sand ratio on the freeze–thaw performance of full solid waste geopolymer concrete (FSWGC) and establish a constitutive model for its post-freeze–thaw mechanical behavior, FSWGC was prepared via alkali activation—using fly ash, slag, silica fume as cementitious materials, and cold-bonded geopolymer lightweight aggregates (CBGLAs) and recycled sand as aggregates. With sand ratios (0.45, 0.55, 0.65) as the core variable, rapid freeze–thaw tests were conducted to measure mass loss, relative dynamic elastic modulus, mechanical properties, and axial compressive stress–strain characteristics of FSWGC. Results show that higher sand ratios significantly aggravate freeze–thaw damage: after 100 cycles, the 0.65 sand ratio specimen has a mass loss rate of 4.61% and a relative dynamic elastic modulus retaining only 34.4% of its initial value, with accelerated strength degradation. This is due to yjr weakened wrapping of recycled sand by cementitious materials, forming a weak interfacial transition zone. The modified Guo constitutive model for FSWGC, and the further established model considering freeze–thaw cycles, accurately describe the stress–strain curve of FSWGC before and after freeze–thaw. This study provides theoretical and experimental support for FSWGC mix optimization, durability design, and mechanical response calculation in cold regions.