Cross-Scale Quantitative and Qualitative Study of Grotto Sandstone Under Salt Weathering

Grottoes encounter significant challenges, such as weathering and water erosion, seriously threatening their preservation. Current laboratory research on grotto deterioration predominantly centers on characterizing the deterioration process, making it challenging to conduct cross-scale quantitative analysis of the intricate rock evolution process. This study examines sandstone from Baoding Mountain in Dazu and employs a cross-scale experimental approach to conduct dry–wet cycle tests on salt weathering. By integrating particle flow numerical simulation and inorganic salt deliquescence theory, the study elucidates the mechanism of aging degradation of sandstone caused by various salt solutions. The results indicate that the hierarchical order of the influence of salt solution types on rock degradation, ranked from highest to lowest, is as follows: sodium sulfate, magnesium sulfate, and sodium carbonate. The sandstone samples exhibited significant damage after 7 and 18 cycles of medium and high-concentration sodium sulfate solutions, respectively. In contrast, the strength of sandstone samples decreased by only 5.3% on average after 20 cycles of medium and low-concentration sodium carbonate solution. Subsequently, particle flow was analyzed numerically using the variable stiffness theory. The influence of salt solution on sandstone’s strength, deformation characteristics, internal pores, and fracture development can be quantitatively evaluated by examining five core mesoscopic parameters. Furthermore, the study highlights that the relative humidity required for the deliquescence of sodium sulfate and magnesium sulfate crystals exceeds 80%, making these crystals vulnerable to the repetitive dissolution–migration–precipitation processes in the climate conditions of Baoding Mountain, thereby accelerating the deterioration of rocks in the region.