## 4 Discussion  
The observed changes in microstructure and macroscopic mechanical properties can be explained by the following mechanisms:  
(1) Composition, cementation mode, and pore structure characteristics of samples: Based on the relevant literature and standards, clay minerals in sandstone are primarily concentrated in intergranular pores or adhered to particle surfaces. They exist in dispersed, linear, bridge-like, and film-like forms. Integrating these results with the Scanning Electron Microscopy results in this study, a simplified diagram (Figure 12) was designed to demonstrate the distribution and variations in cementation, pore structure, water content, and crack formation in sandstone samples over different soaking durations. In Figure 12, it can be observed that with increasing soaking time, the number and size of microscopic cracks gradually increase, as indicated by the black lines. Water initially diffuses to the cementation points between the quartz framework and clay minerals. Therefore, cracks first form at these points. As soaking time progresses and water continues to diffuse, some clay minerals absorb water and expand, weakening the cohesion between their particles. These cracks then begin to extend between the clay minerals, increasing in both number and size. After 60 days of soaking, water has fully infiltrated the clay minerals, and the microcracks between the clay minerals gradually connect, leading to a loose connection between the clay minerals, accompanied by the peeling and detachment of the clay minerals. The reasons for these changes are primarily explained in Sections 2 and 3. Figure 12: Simplified diagram of microstructural features at different soaking times: (**a**) dry state; (**b**) submerged for 14 days; (**c**) submerged for 30 days; (**d**) submerged for 60 days. (2) Forms of water: Water can exist in rocks in different forms, including gaseous, solid, and liquid states. Based on its proximity to rock particles, liquid water can be categorized into strongly bound water (less than 0.5 \(\upmu\)m) closest to the particle surface, weakly bound water (5-10 \(\upmu\)m) as the binding force decreases, capillary water due to surface tension, and free water [40]. These differences in water forms also lead to different reactions of feldspar sandstone components with water, as explained in (3). (3) Mechanisms of hydration in feldspar sandstone components:  
a. A schematic diagram of the sandstone components in a dry state is shown in Figure 13a. During the initial stage of rock soaking, the pore water is mainly composed of bound water. During the water absorption process, molecular forces are predominant, making it easier for water to enter larger pores between quartz particles [7]. As water molecules infiltrate the spaces between quartz particles, a bound water film forms at the cementation points between quartz particles and clay minerals in the sandstone. Under external load, the pore space is either compressed or stretched. When primary pores close and the pore walls come into contact, the bound water film acts as a lubricant, reducing friction between particles [41], as shown in Figure 13b. b. As the soaking time reaches a certain level, in rocks containing clay minerals, some water adsorbs onto the surface of clay minerals, while some may interact with the interlayers of clay minerals. Instead of causing significant interlayer expansion, as seen in highly expansive clays, the primary effect is a thickening of the bound water film at the particle surfaces. This increases the intergranular spacing and leads to slight intergranular expansion, particularly in minerals like chlorite [42; 43]. This mechanism results in a weakening of the cementation ability within the clay minerals, as the water influences the interactions between the particles. c. Prolonged soaking leads to a gradual increase in water absorption by clay minerals. The XRD results indicate that the clay minerals in sandstone are predominantly kaolinite.