## 3 Results  
### Variation Laws of Surface Micro-Pore Structure Under Different Corrosion Mechanisms  
Nuclear magnetic resonance (NMR) was used to analyze the pore structure of sandstone before and after exposure to different dissolution conditions, complemented by scanning electron microscopy (SEM) to observe microscopic morphological changes. In NMR analysis, the relaxation time (T\({}_{2}\)) and signal intensity of the T\({}_{2}\) relaxation spectrum correspond to pore size and quantity, respectively. Specifically, relaxation times of [0.01, 3], [3, 50], and [50, 10,000] ms correspond to small, medium, and large pores in sandstone [17]. Using NMR technology, the evolution of the pore structure under static and dynamic dissolution conditions was systematically characterized. The T\({}_{2}\) relaxation spectra, as shown in Figure 6, provide a detailed visualization of pore size distribution and connectivity changes, offering critical insights into sandstone degradation mechanisms. This combined NMR and SEM approach enables a comprehensive understanding of microstructural alterations induced by acidic dissolution, essential for developing effective preservation strategies for sandstone-based cultural heritage. As shown in Figure 6, the T\({}_{2}\) relaxation spectra display a bimodal distribution. In untreated samples, the significant signal amplitude difference between the two peaks indicates a clear boundary between small pores and medium-to-large pores, suggesting poor pore connectivity [18]. With increasing dynamic cyclic dissolution time, the amplitude difference between the two peaks gradually decreases, and the boundary between small and medium-to-large pores becomes smoother, indicating improved pore connectiv  
Figure 5: Experimental flowchart. Figure 6: NMR tests under different dissolution conditions. (**a**) dynamic cyclic dissolution, (**b**) static immersion dissolution. static immersion dissolution, the amplitude difference between the two peaks increases over time, and the boundary between small and medium-to-large pores remains distinct, suggesting that pore connectivity remains relatively poor. These observations highlight the distinct effects of dynamic and static dissolution on the sandstone pore structure: dynamic dissolution enhances pore connectivity by promoting the merging of small and large pores whereas static dissolution primarily enlarges existing pores without significantly improving connectivity. These findings offer valuable insights into the microstructural changes induced by different dissolution processes, critical for understanding sandstone degradation mechanisms and developing targeted conservation strategies. Based on nuclear magnetic resonance (NMR) test results, Figure 7 illustrates the changes in porosity under different dissolution conditions. As shown in the figure, sandstone porosity progressively increases with dissolution time under both dynamic cyclic dissolution and static immersion conditions. For dynamic cyclic dissolution, porosity shows a strong linear relationship with dissolution time during Stage I (for example, 0-48 h as shown in Figure 7a), followed by an exponential relationship in Stage II (for example, 48-192 h as shown in Figure 7a). In contrast, static immersion dissolution leads to an exponential increase in porosity over time. During Stage I (for example, 0-240 h as shown in Figure 7b) of static immersion, porosity rapidly increases to 1.33 times the initial value, while, in Stage II (for example, 240-720 h as shown in Figure 7b), the rate of porosity increase slows, reaching 1.48 times the initial value after 720 h of dissolution. These trends highlight the distinct mechanisms of porosity evolution under dynamic and static dissolution: dynamic dissolution accelerates porosity growth through enhanced pore connectivity whereas static immersion results in a more gradual increase, primarily driven by the enlargement of existing pores.