Durability Properties of PVA-Strengthened Waste-Based Foam Lightweight Soil Under Freeze–Thaw Cycles and Solution Immersion Conditions

Traditional cement-based foamed lightweight soils suffer from high construction costs, poor durability, and low solid waste utilization efficiency, which severely restrict their engineering application. A novel polyvinyl alcohol (PVA)-reinforced solid waste-based foamed lightweight soil is fabricated using Bayer red mud, mineral powder, and fly ash. To clarify the durability evolution mechanisms, systematic freeze–thaw cycling, long-term water immersion, and sodium sulfate erosion tests were conducted on PVA-reinforced solid waste-based, unreinforced solid waste-based, and pure cement-based specimens. The results demonstrate that the PVA-reinforced specimen achieves optimal freeze–thaw resistance with only 17.10% strength loss after 50 cycles, owing to the internal three-dimensional fiber network that restrains crack propagation and enhances matrix toughness. It also exhibits excellent long-term water immersion stability, with a mild strength increment of 4.04–10.33% after 120 days. In contrast, the CN exhibited a strength increase of 43.62%, attributed to its lower initial strength caused by incomplete hydration; however, its final strength remained between those of the other two groups. In sulfate environments, unreinforced solid waste-based specimens present superior corrosion resistance, while PVA fiber-induced interconnected pores slightly weaken sulfate erosion resistance. Microscopic analysis confirms that the generation of alunite and gypsum hydration products fundamentally causes performance discrepancies among different specimens. Different from previous studies focusing on single fiber modification or single solid waste partial replacement of cement, this study innovatively adopts a composite modification strategy of “multi-solid waste alkali-activated matrix + PVA fiber toughening”, and systematically reveals the durability evolution mechanism under multiple harsh environments.