## 3 Results and Discussion
### Microstructure and Mineral Composition  
Figure 7 shows the SEM images of the samples subjected to different drying\(-\)wetting cycles in the acidic solution with pH = 3. For the untreated sample (\(n=0\)), the surface was smooth and had no obvious fractures and pores. The microstructure was compact, and the mineral particles were well-cemented to each other. After 10 drying\(-\)wetting cycles, the sample surface still maintained an appreciable homogeneity with a small number of pores. The majority of pores were fine and irregular and had almost no connectivity with each other. As the number of drying\(-\)wetting cycles increased, the pores on the surface increased gradually, and the cementation between the mineral particles became weak, accompanied by a change from granular particles to play particles. Moreover, some isolated pores gradually expanded and converged into large pores or fractures. These led to a loose microstructure. When the number of drying\(-\)wetting cycles reached the maximum, the microstructure of the sample had essentially changed in contrast to the untreated sample. The changes mainly indicated that the intact and dense structure became a loose structure with the stacking of fragmented particles, and the pores and fissures were well-developed. Figure 7: SEM images of samples subjected to different drying\(-\)wetting cycles in the acidic solution with pH = 3: (a) \(n=0\); (b) \(n=10\); (c) \(n=20\); (d) \(n=30\); (e) \(n=40\). In order to investigate the effect of solution pH values on the microstructure of red sandstone, the SEM images of the samples before and after cyclic drying\(-\)wetting treatment in the solutions with pH = 7 and 5 were supplemented, as shown in Figure 8. Before cyclic drying\(-\)wetting treatment, the microstructures of the samples soaked in different solutions were compact and smooth with a high cementation degree, and they had few significant differences. This indicated that the effect of the solution pH values on the microstructure of the untreated samples was not evident. As expected, the effect reached a maximum after 40 drying\(-\)wetting cycles. At pH = 3, the microstructure deterioration was the most serious. The microstructure changed from densification to looseness, accompanied by abundant pores and fissures. The mineral particles became platy and were stacked together in a disorderly manner. The deterioration of microstructures in the solution with pH = 7 was the mildest, and the deterioration in the solution with pH = 5 fell in between that of the other conditions. For the sample subjected to 40 drying\(-\)wetting cycles in the neutral solution, the microstructure deteriorated, but some large particles still existed and were not completely fragmented. In short, the microstructure became increasingly worse as the drying\(-\)wetting cycles and acid concentration increased. Figure 9 shows the XRD patterns and relative mineral contents of the samples under different drying\(-\)wetting cycles and pH solutions. It can be observed that quartz was the most abundant component in the samples, followed by feldspar. The other components included calcite and a small number of clay minerals. Due to the chemical reactions with hydrogen ions (H\({}^{+}\)), the contents of the minerals in the acidic solutions were generally less than those in the neutral solution, while the content of quartz showed the opposite trend. At pH = 7, the contents of the minerals decreased slightly with increasing drying\(-\)wetting cycles, except quartz. This was the result of the hydrolysis reaction [14]. At pH = 5, the contents of feldspar and calcite decreased significantly with increasing drying\(-\)wetting cycles, while the content of quartz increased. The content of clay minerals showed an irregular change. At pH = 3, the maximal variation in the mineral contents is observed.