## 4 Discussion  
### The Influence of Immersion Time in Mine Water on the Mechanical Properties of Coal Fracture  
Coal is composed primarily of organic compounds, along with a smaller proportion of inorganic minerals. Common inorganic minerals include clay minerals (such as kaolinite and chlorite), quartz, calcite, and pyrite. In the presence of water, coal is likely to undergo partial dissolution of its components. Mine water exhibits a complex composition of multivalent metal cations, which induce more sophisticated physicochemical interactions at the coal-water interface compared to distilled water systems. For instance, the dissolution of clay and carbonate minerals occurs; clay becomes soft or decomposes upon contact with water, while montmorillonite swells due to water absorption, resulting in the uncoordinated deformation of mineral particles. The presence of water molecules significantly reduces the intermolecular binding forces within coal matrices, consequently diminishing interparticle friction coefficients [45; 46; 47]. The experimental mine water exhibited weak alkalinity, wherein hydroxide ions (OH\({}^{-}\)) significantly decoupled molecular bonds within the coal matrix. This physicochemical interaction induced microstructural alterations in the coal specimens, ultimately compromising their mechanical integrity [46; 47; 48; 49]. These factors contribute to the formation of small pores and cracks in coal and lead to changes in the primary pore and crack structures, thereby affecting the coal's ability to resist fracture instability. The microscopic characteristics of coal under both non-immersion and immersion conditions with mine water were observed using scanning electron microscopy (SEM), as illustrated in Figure 10.  
From Figure 10a, it is evident that in the absence of mine water immersion, the coal exhibits certain primary pores and cracks, along with smaller cracks on the matrix; however, the overall structure remains intact. This integrity contributes to the relatively weak and brittle properties of the coal. Figure 10b illustrates coal that has been immersed for 30 days, revealing larger pores, elongated cracks, and an accumulation of pores and cracks that form  
Figure 10: Scanning electron microscopy (SEM) images of coal samples ((**a**) no immersion; (**b**) immersed for 30 days; (**c**) immersed for 60 days; (**d**) immersed for 120 days; (**e**) enlarged image of the fragmented structure; (**f**) pore and fracture connectivity in the coal layer). a relatively loose structure. This observation demonstrates that the corrosive effects of mine water immersion can lead to structural damage. The components present in mine water undergo physical and chemical reactions with coal, resulting not only in changes to the pore and fracture structures but also facilitating the entry of water molecules into the coal's interior through fracture channels. This process induces damage and alters the macroscopic mechanical response of the coal. According to Figure 10c, after immersion in mine water for 60 days, the pore diameters have increased and interconnected with the cracks, creating a fragmented structure. Figure 10d-f indicate that after 120 days of immersion, an increasing number of pores and cracks emerge, with the original pores and cracks further developing, leading to increased fragmentation. This may be attributed to the prolonged immersion in mine water, where the dissolution rate of coal components surpasses the adhesion rate of new substances to the matrix. The appearance of these fragmented structures on the coal surface significantly weakens its mechanical properties, facilitating the propagation of fracture cracks along the surface and natural weak planes. When coal samples are immersed in water, the clay minerals within them undergo softening and mudification [34]. As the immersion time increases, the internal structure of the coal gradually changes, resulting in the continuous generation of new micropores.