## 4 Damage Mechanism of Mesostructure in Sandstone  
Based on the above results of NMR and SAXS testing on the meso-damage of jointed rock, the mesoscopic structural damage development law of sandstone during the dry-wet alternation action can be revealed. On the basis of the mesoscopic results, the meso-damage  
Figure 11: Mineral content of rock specimens with different dry–wet alternation numbers: (**a**) quartz, plagioclase, clay mineral; and (**b**) calcite, siderite, anhydrite. propagation law of jointed sandstone subjected to dry-wet alternation is illustrated in Figure 12, and the damage evolution process is as follows. 1. Pristine condition (Figure 12a). The sandstone was formed by mechanical, chemical and biological deposition of ore-forming materials. In nature, the mineral particles inside the sandstone are usually connected with cementitious materials in the form of porosity and contact; i.e., porous connection and contact connection. Besides, the gap between the mineral particles divided into pores and pore throats (Figures 6 and 8), and pore structure and throat features are all inner factors that affect and determine the mesoscopic damage evolution of rock samples under the dry-wet cycle. 2. Dry state (Figure 12b). In the dry state, due to the thermal expansion caused by changes in temperature, the compression and fragmentation of the inter-particles occurs in contact cementation, resulting in the formation of mineral particle debris. Meanwhile, the expansion of particles causes compression and fragmentation of the cementitious material between the elastic particles, inducing the generation of tiny amounts of cement debris. The combined effect of the two kinds of fragmentation mechanisms leads to microscopic structure changes in sandstone. Figure 12: Schematic diagram of meso-damage propagation law of jointed sandstone under dry–wet cycles. 3. Water-saturated state (Figure 12c). On the one hand, when the water permeates the rock specimens, the particle debris, cement debris and unstable mineral particles inside the rock (such as feldspar, calcite, etc.) are dissolved. The cohesion between the mineral particles will be weakened. This was demonstrated by the mineral analysis experiment in Section 3.3. On the other hand, the mineral particles will shrink slightly when the rock is cooled by water. The compression between particles will become tension, leading to microcracks in the contact cementation structure. Finally, the micropores continue to expand and new throats are produced, which further enhances the permeability of the rock sample. 4. The second dry state (Figure 12d). When the water-saturated rock sample is dried again, the contact and porous cementation are squeezed again due to thermal expansion. The force between elastic particles changes from tension to compression, resulting in the formation of mineral particle debris and cement debris. The mineral particles in the rock sample go through tension-compression transformation under alternating dry-wet cycles, leading to the continuous fragmentation and dissolution of soluble minerals and cements. Besides, some closed pores are transformed into open pores due to the formation of new cracks in the inter-particle structure [52]. Therefore, with an increasing dry-wet alternation number, the proportion of small pores and pore throats decreases, while the proportion of medium and large pores and pore throats increases (Figures 7 and 8). 5. Final state (Figure 12e). During the dry-wet alternation action, the size of pores and pore throats continues to expand due to the weakening of the cements between mineral particles. The proportion of mesopores and medium pore throats is significantly increased. Besides, the content of soluble minerals and cements in the pores of rock samples decreases continuously due to the erosion of water, so new pores and pore throats are gradually generated under the action of dry-wet alternation [44]. Meanwhile, the cementation effect on the inter-particles gradually decreases during the dry-wet cycle.