## 2 Materials and Methods
### Experimental Procedure  
To investigate the evolution of the pore structure, mechanical properties, and hygroscopic characteristics of sandstone under dynamic and static dissolution conditions, the following experimental procedure was designed:  
(1) Sample Preparation: Sandstone slabs were dried in an oven at 105 \({}^{\circ}\)C and then cooled to room temperature in a desiccator to ensure a consistent initial state.  
(2) Dissolution Experiments: Sandstone slabs were subjected to static immersion and dynamic cyclic dissolution using a 0.01 mol/L dilute sulfuric acid solution. To simulate natural conditions and accelerate deterioration, static immersion tests involved open immersion of slabs in a container for 0, 240, 480, and 720 h, while dynamic cyclic dissolution tests were conducted using a dynamic circulation device. To maintain consistent dynamic dissolution conditions, the liquid level in the reaction device was kept constant, and the flow rate was stabilized at 1.2 L/min. Each dynamic dissolution cycle lasted 24 h, with a cumulative duration of 192 h.  
(3) Post-Dissolution Treatment: After dissolution experiments, the slabs were rinsed with deionized water. The sample requiring rinsing was placed into an appropriate container and distilled water was slowly added. The distilled water volume is typically 5-10 times that of the sandstone sample. Each sandstone sample was rinsed for 5-10 min, dried at 105 \({}^{\circ}\)C for 24 h, and cooled to room temperature in a desiccator. Twenty-five grid points were marked on each slab surface for ultrasonic wave velocity and hardness measurements to analyze mechanical strength distribution.  
(4) Hygyroscopicity Testing: Sandstone slabs were placed in a YB-60 constant temperature and humidity chamber set at 25 \({}^{\circ}\)C and 80% relative humidity for free moisture absorption. The moisture content of the slabs was monitored dynamically over time using a Concrete Moisture Encounter X5 moisture meter, Shanghai Laurier Instruments & Equipment Corporation, Shanghai, China.  
(5) Microstructural Analysis: Finally, nuclear magnetic resonance (NMR) and Phenom desktop scanning electron microscopy (SEM) were used to analyze changes in the microscopic pore structure of sandstone after static and dynamic dissolution.  
The pore structure of sandstone was analyzed using Niumag nuclear magnetic resonance (NMR) equipment, Suzhou Niumag Analytical Instrument Corporation, Suzhou, China. Niumag NMR systems, as advanced tools, are widely used in materials science, geology, petroleum exploration, and biomedical research. This method allows for non-destructive pore structure analysis, ideal for valuable or delicate samples.  
Microstructural analysis was conducted with a Phenom Pro benchtop scanning electron microscope (SEM). The Phenom Pro, equipped with a high-brightness CeB6 filament, produces focused electron beams to scan sample surfaces, capturing backscattered (BSE) and secondary electron (SE) signals for high-resolution compositional and topographical imaging. This advanced, user-friendly benchtop SEM system supports a wide range of research applications.  
This systematic experimental approach ensures a comprehensive understanding of sandstone degradation mechanisms under different dissolution conditions, offering valuable insights for preserving sandstone-based cultural heritage. The experimental procedure is illustrated in Figure 5.