## 1 Introduction  
Rock-cut caves and stone carvings represent a significant portion of ancient stone architectural heritage, as their surface layers predominantly preserve critical cultural and historical information. Over time, the surface materials of these stone structures are continuously exposed to environmental factors, including water and moisture, resulting in a gradual decline in their physical and mechanical properties. This deterioration leads to various surface defects, significantly compromising the long-term durability and stability of these ancient structures. Therefore, a comprehensive investigation into the changes in the physical and mechanical properties of stone surfaces under natural environmental conditions is crucial for guiding preventive conservation and restoration efforts. The climate in Southwest China (Figure 1) is characterized by year-round high temperatures, high humidity, and abundant rainfall, rendering it one of the regions most severely impacted by acid rain in the country. Sandstone, a natural material, is widely used in constructing rock-cut caves and stone carvings due to its fine-grained and soft texture, enabling intricate carving. However, prolonged exposure of sandstone to dynamic acid rain cycles and static immersion in residual rainwater has caused severe damage to the surface layers of sandstone-based rock-cut caves and stone carvings in this region. Understanding the mechanisms underlying this deterioration is crucial for developing effective preservation strategies to protect these invaluable cultural heritage sites. Sandstone, composed of calcium, argillaceous cement, and feldspar minerals, is highly susceptible to acidic dissolution, which gradually alters its pore structure [1]. Numerous studies have investigated this phenomenon. For example, [PERSON] et al. [2] used CT imaging to show that acid immersion significantly increases sandstone porosity, accompanied by a notable rise in the number of interconnected pores, pore throats, and their equivalent radii. Similarly, [PERSON] et al. [3] conducted experiments on feldspathic sandstone immersed in acidic solutions (pH 2 and 5), showing that acid exposure increases porosity, reduces the proportion of small pores, and increases the proportion of medium and large pores. Other studies have also demonstrated that immersion in acidic solutions increases sandstone porosity, accompanied by a decline in mechanical parameters, including compressive strength, elastic modulus, peak strain, P-wave velocity, shear strength, internal friction angle, cohesion, and creep failure strength [4; 5; 6]. Furthermore, changes in the pore structure significantly affect the spontaneous moisture absorption process of sandstone surfaces. Moisture absorption acts as a critical medium for transporting erosive substances into the  
Figure 1: Schematic diagram of the study area. internal structure of building materials [8; 9]. [PERSON] et al. [10] analyzed the correlation between porosity characteristics and adsorbed moisture content in conglomerate samples, finding that vapor adsorption positively correlates with porosity (a pore diameter \(>\) 0.2 \(\upmu\)m significantly affects vapor adsorption). These findings highlight the complex interplay between acidic dissolution, pore structure evolution, and moisture absorption in sandstone, emphasizing the need for further research to guide conservation strategies for sandstone-based heritage structures. Previous research indicates that dissolution processes primarily alter the pore structure of rocks, resulting in two significant outcomes: reduced surface strength and changes in the hygroscopic properties of the rock surface [11; 12; 13]. Based on methodologies used in prior studies, most researchers have focused on static immersion experiments with standard sandstone samples to investigate changes in physical and mechanical properties after acid-induced dissolution. These studies have offered valuable insights into the effects of static immersion on sandstone degradation.