## 3 Experimental Results  
### Macroscopic Weathering Indicators  
#### 3.1.1 Apparent Morphology  
The pores on the rock surface become filled with sulfate solution, which subsequently leads to the formation of crystals due to fluctuations in temperature and humidity. This process initiates a complex sequence of physical and chemical reactions, ultimately resulting in discernible changes in the rock's overall appearance. With each successive cycle, the degree of rock weathering gradually intensifies, accompanied by a deepening of surface damage. This phenomenon closely mirrors the actual weathering process experienced by rock relics. The apparent morphology of the sandstone samples at different cycling times is shown in Figure 4. It can be observed that the damage range of the surface layer of the sandstone samples gradually expands, and the thickness of the weathering layer continues to deepen. During cycles 1 to 10, the upper region of the sandstone exhibits characteristics such as edge defects, dissolution holes, and dissolution pits, while other areas remain smooth. During cycles 11 to 15, the corners change from \"right angles\" to \"rounded corners\", and the number of pore fractures continues to increase. The surface roughness is distinctly enhanced the microstructure becomes loose, and the weathering thickness reaches up to 3 mm. At this stage, the damage range is still concentrated in the upper-middle region. During cycles 21 to 25, the surface layer of the rock sample completely peels off, and visible dissolution holes and pits are scattered around, indicating a high degree of softening. At the commencement of the experiment, the rock samples were 50 mm square cubes. Following 25 cycles of salt weathering, the length, width, and height of the rock samples were reduced to approximately 14 mm, 10.5 mm, and 9 mm, respectively. Consequently, the total volume of the rock samples witnessed a reduction of approximately 10%. The deterioration of the apparent morphology of limestone is depicted in Figure 5. It is noteworthy that the edges of the limestone samples, resembling the key carving areas of rock statues, exhibit a subtle gravel-like texture and varying degrees of structural damage when compared to other regions. These differences can be attributed to factors such as cutting, akin to the base or corners of cave statues. Two primary characteristics of the damage can be observed, as follows. The first is the accumulation of white material, wherein white material accumulates in the surface layer of the limestone sample, and as the number of cycles increases, the amount of white material also increases. Consequently, the range of damage expands. The second is edge damage and particle detachment, wherein the edges of the limestone exhibit slight damage and particle detachment while the remaining areas remain smooth. In general, the deterioration of the apparent morphology of limestone is minimal, with the damage primarily concentrated at the edges of the sample. This indicates an evolutionary trend of expansion from the periphery towards the center. Figure 4: Apparent morphological characteristics of the sandstone samples. (**a**) After 10 cycles; (**b**) after 15 cycles; (**c**) after 20 cycles; (**d**) after 25 cycles. Based on the aforementioned analysis, it can be inferred that sandstone is more susceptible to surface damage under sulfate erosion than limestone. Both types of rocks exhibit a consistent deterioration pattern, characterized by a top-to-bottom and outer-to-inner progression (refer to Figure 6). Despite sharing common trends in damage under similar weathering conditions, the apparent morphological characteristics of weathering distinctly differ between the two rock types. During the initial stage of salt weathering, the test samples undergo continuous salt absorption until they reach a saturation point.