## 4 Discussion and Analysis  
### Exploration of the Correlation between Pore Characteristics and Macroscopic Indicators  
Based on the above analysis, assessment of the negative impacts of sulfate erosion heavily relies on variations in pore structure, a term encompassing pore quantity, pore size, distribution, and connectivity. Furthermore, this variation is inherently connected to macroscopic indicators [32; 33]. In this study, the pore damage factor (Equation (5)) was introduced as an independent variable, while macroscopic indicators such as surface hardness (reflecting the microstructure of the rock surface), wave velocity (reflecting the cracks in the rock), and strength parameters (representing the macroscopic manifestation of rock internal and external fissure) are treated as dependent variables. The relationship between pore damage and the extent of changes in macroscopic indicators is investigated based on the aforementioned curves [34]. \[D_{\varphi}=\frac{\varphi_{n}-\varphi_{o}}{1-\varphi_{o}} \tag{7}\]  
In the equation, \(D_{\varphi}\) represents the pore damage variable of the rock sample after \(n\) cycles, \(\varphi_{o}\) represents the initial porosity of the fresh rock sample, and \(\varphi_{n}\) represents the porosity of the rock sample after \(n\) cycles. Figure 14 shows relationship curves between the pore damage factor and macroscopic indicators for sandstone and limestone; moreover, the horizontal coordinates of each point  
Figure 13: Pore size distribution curves of limestone and sandstone under sulfate erosion. (**a**) Sandstone pore size distribution characteristics; (**b**) limestone pore size distribution characteristics. in the figure correspond to the pore damage factors of rock samples after 0 to 25 cycles (from left to right). When comparing the horizontal coordinates of each point in Figure 14a,b, one can observe that with same cycle times and after the same number of cycles, the pore damage factor of sandstone is approximately 2-4 times higher than that of limestone, indicating a more remarkable change in the pore structure of sandstone. Changes in the macro index values of wave velocity, surface hardness, and compressive strength, alongside the pore damage factor, can be fitted with exponential, exponential, and linear functions, respectively. However, the magnitude and rate of change differ distinctly between the two rock types. In sandstone, the wave velocity exhibits a slow-to-fast change trend, with an increasing pore damage factor; meanwhile, limestone shows an opposite trend, with a larger magnitude of change. Notably, when the pore damage factor ranges from 0.06 to 0.08, the outer layer of the sandstone sample experiences severe microstructural damage, resulting in a notable decrease in hardness. Furthermore, there is a strong linear correlation between the pore damage factors and strength parameters for both rock types, with fitting coefficients exceeding 0.98. These findings suggest that non-destructive testing methods, such as infrared and ultrasonic techniques, can be employed during the investigation of rock cultural heritage sites. The impact of infrared photography is contingent upon the interaction mode of the object with light. Objects that possess a high degree of infrared absorption appear darker in the image, while those with lower absorption exhibit brighter colors. These methods can help characterize the pore structure of heritage sites, determine the weathering conditions of different parts of the heritage sites, and predict the structural safety and stability of key areas in stone caves [35]. Analysis of the Differential Sensitivity of Macroscopic Indicators to Sulfate Erosion in Sandstone and Limestone  
Due to the variations in initial lithology, including primary structure, pore characteristics, and material composition, the physical property indicators of sandstone and limestone exhibit different degrees of deterioration under sulfate erosion.