## 5 Discussion  
Wave velocity tests and NMR analysis can be used to evaluate the internal structure of unloaded rocks. Based on the above tests, the pore structure evolution of unloaded samples under dry-wet cycle was analyzed in detail through the mass, P-wave velocity, and NMR results. With the strengthening of dry-wet cycles, the P-wave velocity of the sample decreases continuously, indicating that the pore structure showed a gradual deterioration trend. The change of mass shows a three-stage change trend of rapid decline, uniform decline, and stability. T\({}_{2}\) spectrum distribution and spectrum area data show that the pore diameter increases with the increased cycling. It can be seen that the overall pore diameter and volume continue to increase with the strengthening of dry-wet cycles. This article involves multiple analysis angles such as mass, wave speed, and pores and realizes macro and micro-angle and multi-parameter change analysis of the same sample, thereby showing the evolution and damage of the pore structure of the sample with the dry-wet cycle from different angles. As the dry-wet cycle continues, various dissolved substances in the sample are transported, causing mass loss. As the cycles increase, the content of soluble substances inside the sample continues to decrease, which manifests as a gradual decrease and tends to be stable in the percentage of mass reduction in the middle and late stages of the cycle. The growth rate of the sample pore size and number gradually decreases, which is manifested in the gradual decrease of the wave speed reduction percentage, and the T\({}_{2}\) spectrum distribution and spectral area growth amplitude decreases. In summary, it can be seen from the above analysis that the dry-wet cycle affects the crack development time, thereby causing differential deterioration of the physical and mechanical properties of the sample. Combining the aforementioned quantitative reflections of mass, wave velocity, and NMR parameters on the pore structure and their evolution with cycles, the mechanism of the dry-wet cycle on unloaded samples can be divided into three periods: the early stage, the middle stage, and the late stage. The schematic diagram of the deterioration mechanism of the sample under the dry-wet cycle is drawn, as shown in Figure 11. Taking the longitudinal direction of the model as the analysis angle, the upper and lower ends of the model are the surface layer, and the middle of the model is the deep layer. The blue filling represents water molecules, the yellow filling represents cement, and the gray inclusions represent mineral particles. In the initial stage of the dry-wet cycle, due to the low interaction intensity, water molecules infiltrate into the surface layer along the pores and cracks of the sample, and the surface layer of the sample changes rapidly. Various types of unstable cements on the surface dissolved when exposed to water, and the cementation effect decreased, resulting in microscopic changes. Cracks and pores gradually develop, providing more reaction contact surfaces for the next stage of the dry-wet cycle. During the structural evolution process, part of the deep water is adsorbed on the surface of the particles, and part of the water is wrapped in closed pores, which is the main reason for the smaller difference in mass reduction at this stage compared with the middle stage. In the transition stage of the dry-wet cycle, the cumulative intensity is strengthened, and the crack development time gradually accumulates. On the one hand, the cement continues to dissolve, while on the other hand, on the basis of the development of surface pores and cracks in the early stage and the process of repeated dry-wet cycles, some cracks are generated at the contact of rock particles, closed pores are converted into open pores, and then water can enter deeper areas inside the rock, thus prompting the pores to continue to develop and expand from the surface to the inside.