## 3 Mesostructural Evolution of Sandstone Under Dry-Wet Cycles
### Variation of Mineral Composition of the Sandstone  
In addition to changing the pore structure, the dry-wet alternation conditions also led to the change of the mineral composition of the rock specimens. To further analyze the change of mineral composition during the dry-wet alternation process, an X-ray diffraction spectrum test on sandstone powder was conducted as the dry-wet alternation numbers changed. Furthermore, the precipitates in the water solution after the dry-wet cycles were dried. Sandstone powder samples with different dry-wet alternation numbers are shown in Figure 10a. The mineral contents of rock specimens with different dry-wet alternation numbers are plotted in Figure 11.  
Figure 11 shows the contents of feldspar (potassium feldspar and albite) and calcite in the test rock samples gradually decreased with the increase of the dry-wet alternation number. In nature, the proportions of feldspar and calcite are 36% and 1.8%, respectively, while the proportions of feldspar and calcite decrease to 25.5% and 0.5% when \(n=20\). TCCM also decreases from 17.2% to 10.6% as the dry-wet alternation goes on. Due to the ion hydrolysis reaction, exchange reaction and protonation reaction of feldspar after wetting, the content of chlorite, montmorillonite and illite gradually decreases. In particular, the precipitation of chlorite is more obvious than that of illite and kaolinite, while the variations of kaolinite content is not obvious with the increase of dry-wet cycles [47; 48; 49]. As a result,  
Figure 10: Experimental sample powders: (**a**) sandstone powders from different dry–wet cycles; and (**b**) the final precipitate. Figure 9: Variation rule of each grade of pore throat with the number of dry–wet alternation cycles. the volume of pores and pore throats in the sandstone increases gradually [50]. Besides, the total content of clay minerals decreases with the increase of the dry-wet alternation number. The experimental results were similar to the results obtained by [PERSON] et al. [51]. However, quartz and anhydrite increased gradually due to the reduction of water-soluble mineral content in the rock samples. The decreases in the clay minerals weakened the strength of the grain skeletons in the rock specimens, leading to the damage and degradation of the rock. After the tests, the mineral composition of the sediment in the solution was measured, which reflected a series of physicochemical reactions between the rock samples and the solution. Table 1 shows the solution comprised quartz (42.3%), feldspar (26.0%), calcite (7.9%), dolomite (4.5%), anhydrite (2.7%) and TCCM (16.6%), while the rank order of the mineral contents was the following: quartz > TCCM > plagioclase > calcite > potassium feldspar > anhydrite. The quartz in the sediment mainly came from quartz particles on the surface of the rock samples, which peeled off during the dry-wet cycle. The process was relatively intense during the testing, so the quartz content in the sediment was larger than that of other minerals. The content of feldspar, calcite and clay minerals in the sediment was relatively large because these minerals were dissolved in water or reacted with water during the dry-wet cycle, resulting in the continuous increase of pore volume in the rock samples. The decrease of feldspar, calcite, clay minerals and the continuous development of pores and pore throats eventually led to the rock strength weakening. The results were consistent with the variation of total porosity and pore throat values in Section 3, which explains the essential reason for the increase of porosity of sandstone specimens with an increasing dry-wet alternation number from the microscopic perspective. Meanwhile, in the experiments, it was found that the mass of the gypsum accounted for about 2.7% of the sediment after 20 dry-wet cycles. This shows that the filler gypsum of the samples also dissolved in water during the dry-wet alternation action. Therefore, the water's effect on the filler gypsum was also a factor.