## 4 Discussion  
### Analysis of Sandstone Corrosion Mechanism Under Acid Rain Exposure  
The primary mineral constituents of the rock samples are quartz, plagioclase, and calcite. Among these, the main chemical compositions of plagioclase and calcite are CaAl\({}_{2}\)Si\({}_{2}\)O\({}_{8}\), NaAl\({}_{2}\)Si\({}_{3}\)O\({}_{8}\), and CaCO\({}_{3}\), respectively. During the experimental process, the following chemical reactions predominantly occur [23; 24]:  
\[ \begin{array}{c}\mathrm{CaCO_{3}+2H^{+}\rightarrow Ca^{2+}+H_{2}O+CO_{2}}\\ \mathrm{CaAl_{2}Si_{2}O_{8}+2H^{+}+H_{2}O\rightarrow Al_{2}SiO_{5}(OH)_{4}+ Ca^{2+}}\\ 2\mathrm{NaAl_{2}Si_{3}O_{8}+2H^{+}+H_{2}O\rightarrow Al_{2}SiO_{5}(OH)_{4}+ 4 SiO_{2}+2 Na^{+}}\end{array} \]  
Due to the rapid flow of water after dynamic dissolution, the dissolution solution does not reach saturation, enabling the dissolution process to continue. Consequently, soluble minerals in the rock are continuously lost, increasing the number and size of pores within the rock. This is manifested as increased porosity with dynamic dissolution duration. Simultaneously, under prolonged rainwater immersion, sandstone minerals are gradually dissolved, and small pores and micro-fractures progressively develop, resulting in an exponential increase in porosity, with the rate of increase gradually slowing down. The mechanism underlying the staged changes in porosity with increasing dissolution time is analyzed as follows:  
(1) Under dynamic cyclic dissolution, erosion by the dynamic solution carries reaction products away with the water, driving the sandstone's chemical reactions forward. This results in increased secondary minerals, some of which are carried away while others remain on the mineral surface. Overall, this leads to pore enlargement and an improved sandstone microstructure. (2) In the initial stages of static immersion dissolution, the sandstone microstructure is severely disrupted, leading to a significant increase in porosity. As the immersion time progresses, secondary minerals generated by the reaction remain near the reaction sites, accumulating and precipitating, gradually blocking the pores. The deposition of secondary  
Figure 18: Mass loss rate under different dissolution conditions. (**a**) Static immersion dissolution, (**b**) dynamic cyclic dissolution. minerals closes the pores, reducing the contact area for subsequent reactions and slowing the rate of porosity increase. (3) Therefore, dynamic dissolution significantly alters the sandstone microstructure, causing a substantial increase in porosity. In contrast, static dissolution impacts the sandstone microstructure more gradually, with the rate of porosity increase slowing over time. Mechanistic Analysis of Sandstone Deterioration in Physical and Mechanical Properties Under Acid Rain Exposure  
Physicochemical reactions lead to a continuous decline in particle bonding strength, gradual disappearance of soluble particles, increasing mass loss, and cumulative porosity increase, resulting in the progressive deterioration of overall physical and mechanical parameters. Due to significant changes in the pore structure, the number of pores increases and their size gradually enlarges. Macroscopically, this is reflected in the gradual rise in the moisture absorption curve with increasing porosity and the decline in mechanical properties as porosity increases. Previous studies indicate that the rapid water absorption phase is primarily controlled by capillary action (rapid water penetration along capillary pores) while the stable phase is mainly governed by water diffusion through interlayer pores in sandstone [25; 26]. The following mechanistic analysis explains the deterioration of surface physical and mechanical properties during static and dynamic dissolution processes:  
(1) Under dynamic cyclic dissolution, capillary pore numbers gradually increase and both surface hardness and surface longitudinal wave velocity decrease as porosity increases.