## 5 Conclusions  
Static dissolution refers to the chemical weathering process in which sandstone is submerged in stagnant acidic solutions for extended durations. Dynamic dissolution involves the combined chemical-mechanical erosion process that takes place when sandstone is exposed to flowing acidic aqueous solutions. This study compares the effects of static immersion and dynamic cyclic dissolution on the sandstone surface pore structure, mechanical strength distribution, and moisture absorption properties, revealing the mechanisms and differences between the two dissolution methods. The main conclusions are as follows:  
(1) Under different dissolution methods, sandstone porosity shows a gradual increasing trend, but the effects of dynamic and static dissolution on pore structure modification differ significantly. Under dynamic cyclic dissolution, porosity increase can be divided into two phases: Phase I is characterized by linear growth, with the proportion of large pores gradually increasing; Phase II shows exponential growth in the proportion of large pores, leading to an exponential increase in porosity. In contrast, under static immersion dissolution, porosity increases logarithmically, with the growth rate gradually slowing, and the proportion of large pores initially rises rapidly before gradually declining. Overall, dynamic cyclic dissolution has a significantly stronger effect on pore structure modification than static dissolution. (2) Under both dissolution methods, surface hardness and longitudinal wave velocity decrease gradually with increasing dissolution time, but their distributions are affected differently. For dynamic cyclic dissolution, surface hardness decreases and shifts toward lower values, with a decline in distribution uniformity; longitudinal wave velocity also shifts toward lower values, with significant fluctuations in uniformity. For static immersion dissolution, surface hardness decreases and shifts toward lower values, with little change in distribution uniformity; longitudinal wave velocity decreases and shifts toward lower values, with an expanding distribution range and gradually declining uniformity (Figures 12 and 15). Dynamic cyclic dissolution has a more intense and uneven impact on sandstone mechanical properties, while static immersion dissolution has a slower and more uniform effect. (3) Moisture absorption curves under different dissolution methods rise gradually with increasing porosity. They can be divided into two phases (Figures 16 and 17): Phase I (0-20 min) shows an increasing rate of moisture absorption with a longer dissolution time and higher porosity; Phase II (20-90 min) exhibits a decreasing rate of moisture absorption with a longer dissolution time and higher porosity, indicating that dissolution accelerates the attainment of equilibrium in surface moisture content. Additionally, experiments demonstrate that dynamic cyclic dissolution results in greater moisture absorption and a faster attainment of equilibrium. (4) This study provides critical insights into the degradation mechanisms of sandstone under dynamic and static dissolution conditions, which are prevalent in high-temperature, humid, and acidic environments like Southwest China. The findings offer a scientific basis for developing targeted preservation strategies for ancient sandstone structures, such as rock-cut caves and stone carvings, which are integral to cultural heritage. By understanding the differential effects of dynamic (acid rain) and static (water immersion) dissolution on pore structure, mechanical properties, and hygroscopic behavior, conservation efforts can be tailored to mitigate specific degradation patterns. This research is particularly relevant for heritage sites exposed to acid rain and prolonged moisture, enabling the design of preventive measures to enhance the long-term durability and stability of these invaluable structures. **Author Contributions:** Conceptualization, Q.L. and C.L. ; methodology, C.L. and D.H.; software, D.H. and C.W. ; validation, Q.L. and C.W.