## 3 Results
### Variation Laws of Surface Micro-Pore Structure Under Different Corrosion Mechanisms
These findings offer critical insights into the time-dependent degradation of sandstone under different environmental conditions, providing a scientific basis for preserving sandstone-based cultural heritage. Furthermore, the proportions of small, medium, and large pores under dynamic and static dissolution conditions were analyzed as a function of dissolution time. As shown in Figure 8, the proportions of small, medium, and large pores exhibit distinct trends under dynamic and static dissolution conditions. Figure 7: Porosity variation under different dissolution conditions. (**a**) Dynamic cyclic dissolution, (**b**) static immersion dissolution. During Stage I of dynamic cyclic dissolution, the proportion of small pores decreases, while large pores increase rapidly, and medium pores show a slight decline. This stage is primarily characterized by large pore development. In Stage II, the proportion of large pores continues to increase exponentially, albeit at a slower rate than in Stage I, while small pores decrease linearly, and medium pores decline due to the continued expansion of large pores. In contrast, under static immersion dissolution, Stage I is marked by a rapid and significant decrease in small pores, accompanied by a sharp increase in large pores and a slight decline in medium pores. In Stage II, large pores gradually decrease while small pores begin to rise, and medium pores fluctuate. These trends highlight the distinct mechanisms of pore structure evolution under dynamic and static dissolution: dynamic dissolution promotes the merging of small pores into larger ones, enhancing pore connectivity, whereas static dissolution primarily enlarges existing pores without significantly improving connectivity. These findings offer critical insights into the time-dependent degradation of sandstone under different environmental conditions, providing a scientific basis for preserving sandstone-based cultural heritage. To investigate microscopic changes in the pore structure, scanning electron microscopy (SEM) was used to analyze the surface morphology of sandstone samples subjected to dynamic and static dissolution, e.g., the SEM images in Figure 9.  
Figure 8: Variation in small, medium, and large pore proportions under different dissolution conditions. (**a**) Dynamic cyclic dissolution, (**b**) dynamic cyclic dissolution. Figure 9: _Cont._  
Figure 9: Cont. Scanning electron microscopy (SEM) observations reveal distinct changes in the sandstone microstructure induced by dissolution processes. In untreated samples, the sandstone structure appears intact, with cementitious fragments filling spaces between mineral grains. Under dynamic cyclic dissolution, dissolution cavities and pits form within minerals and at mineral boundaries, indicating significant material loss and enhanced pore connectivity. In contrast, static immersion dissolution causes the gradual disappearance of cementitious materials at mineral edges, accompanied by widening and deepening boundary cracks. Additionally, as immersion time increases, gaps between minerals become progressively filled with secondary minerals. These microstructural changes highlight the distinct degradation mechanisms under dynamic and static dissolution: dynamic dissolution promotes interconnected pore network development through dissolution feature formation, whereas static dissolution results in localized damage and secondary mineral deposition, partially mitigating further degradation. These findings offer critical insights into the microscopic processes driving sandstone deterioration, providing a scientific basis for preserving sandstone-based cultural heritage. Figure 9: SEM analysis under different dissolution conditions.