Coupled Effects of Elevated Water Pressure and Limestone Powder on Thaumasite Sulfate Attack in Cement Mortar

Thaumasite sulfate attack (TSA) under elevated water pressure has important implications for the durability of deep underground concrete structures, yet the deterioration process and the coupled effect of water pressure and carbonate supply remain insufficiently understood. In this study, laboratory pressurized sulfate exposure tests were conducted to investigate the evolution of macroscopic performance and microstructure of cement mortars with different limestone powder contents (0%, 15%, and 30%) under water pressures of 0, 2.5, and 5.0 MPa. The results show that elevated water pressure promotes sulfate ingress into the mortar and accelerates later-stage strength loss; this interpretation is supported by the depth-dependent distribution of soluble SO42− measured in mortars without limestone powder. Two-way ANOVA indicates that both water pressure and limestone powder content have significant effects on compressive strength, and their interaction becomes statistically significant at 120 d. XRD, FT-IR, and SEM/EDS results show that, under elevated water pressure and high limestone powder content, the corrosion products gradually evolve from gypsum-related products to ettringite- and thaumasite-related products, with a certain spatial differentiation. Specifically, the gray–white, mud-like surface products are consistent with thaumasite-rich assemblages, whereas the needle- and column-like crystals in the interior are consistent with ettringite-rich assemblages. Overall, elevated water pressure mainly promotes sulfate transport, while limestone powder mainly increases carbonate availability. These two factors may jointly intensify TSA deterioration in mortar through a pathway involving transport enhancement, carbonate supply, corrosion product evolution, and aggravated macroscopic damage. This study provides a reference for understanding the sulfate deterioration mechanism of limestone powder-containing cement-based materials in deep underground environments under elevated water pressure.