Comparative Study on the Deterioration of Surface Physical and Mechanical Properties of Sandstone Cultural Heritage Under Different Dissolution Conditions

In Southwest China’s high-temperature, humid, and rainy climate, ancient sandstone structures face significant deterioration due to acid rain and water accumulation, which cause dynamic and static dissolution. This degradation weakens the sandstone’s physical and mechanical properties, threatening the preservation of cultural heritage sites. Dynamic dissolution is the process of matter and energy exchange during fluid–rock or fluid–mineral interactions under dynamic conditions. Under dynamic conditions, continuously renewed fluids supply chemicals for dissolution and remove dissolved products, sustaining reactions similar to acid rain dissolution. Static dissolution is the dissolution–erosion process between fluids and rocks or minerals in a relatively stationary fluid environment. Unlike dynamic dissolution, which involves moving fluids, static dissolution occurs in nearly stagnant fluids, where rising product concentrations from acid–rock reactions may hinder further dissolution, akin to static immersion dissolution. This study systematically examined how different dissolution conditions affect sandstone’s pore structure, mechanical properties, and hygroscopic behavior. Nuclear magnetic resonance (NMR) and scanning electron microscopy (SEM) were used to analyze pore structure changes, while ultrasonic testing and Leeb hardness measurements assessed mechanical strength. Hygroscopicity was evaluated through non-destructive moisture testing in controlled environments. The results show that dynamic dissolution has a greater impact on sandstone than static dissolution. Both conditions increased porosity in two stages, but dynamic dissolution enhanced pore connectivity while static dissolution caused gradual porosity growth and localized cracks. Dynamic dissolution significantly reduced surface hardness and P-wave velocity, increasing hardness heterogeneity, whereas static dissolution had a milder effect. Additionally, dynamic dissolution notably increased sandstone’s hygroscopicity, with moisture absorption rising over time. This study highlights the distinct effects of dynamic and static dissolution on sandstone deterioration, offering insights for the preventive conservation of ancient stone structures. Tailored preservation strategies are essential for addressing these varying degradation mechanisms.