Multi-scale quantitative analysis of grotto sandstone degradation considering inorganic salt phase transitions

The salt deterioration of the surface and host rock mass of grottoes poses a serious challenge to the protection of grottoes. The existing indoor experimental research on salt weathering mainly focuses on characterizing the deterioration process. It is challenging to realize the multi-scale quantitative analysis of the complex salt weathering process, especially the lack of systematic research on time factors and inorganic salt phase transition. This study selected the fresh sandstone on the south bank of Baoding Mountain in Dazu and performed the dry-wet cycle test of mixed salt solution by a multi-scale experimental method. The experimental variables included concentration and cycle times. Combined with acoustic emission monitoring technology, qualitatively and quantitatively studied the time-dependent deterioration of grotto sandstone under salt weathering, and the water-salt system revealed the key mechanism of time-dependent deterioration of Big Buddha Bay rock mass. The low concentration mixed salt has little effect on sandstone samples due to the limited number of cycles. The image comparison analysis shows that the deterioration degree of the sample at 1.5 mol/L (referred to as 1.5 M) concentration is similar to that of the existing grottoes, which can effectively simulate the salt weathering in the actual environment. When the concentration increases to 2.0 M, the salt crystal penetrates the sample, and the erosion depth can reach 4.5 mm. The acoustic emission analysis shows that the proportion of tensile cracks is always higher than that of shear cracks. Under the action of dry-wet cycles, as the number of cycles increases, the failure mode of the sample gradually changes from shear-dominated to tension-dominated. The phase change of inorganic salt on grotto sandstone shows that sodium sulfate caused significant volume expansion and crystallization stress due to different hydrate phase changes, and the damage was the most significant.