## 3 Results and Discussion
### Mechanical Properties  
Figure 4 shows the stress\(-\)strain curves of typical samples under different drying\(-\)wetting cycles and pH solutions. On the whole, the curves can be divided into four phases, including the fissure closure, elastic, yielding, and failure phases. At the beginning of the axial load, the primary fractures and pores inside the samples were compressed, which may have caused concave stress\(-\)strain curves. However, the phase was inconspicuous in the present study.  
During the elastic phase, the stress\(-\)strain curves were approximately straight, in accordance with Hooke's law. As the axial load increased, the stress\(-\)strain curves entered the yielding phase, in which new fractures initiated and propagated, and the axial stress experienced a deceleration in growth until the peak stress. Afterward, the axial stress decreased rapidly with increasing axial strain, and the samples were destroyed with the formation of macro-cracks.  
It can be seen that as the number of drying\(-\)wetting cycles increased, the stress\(-\)strain curve gradually moved to the lower right, namely, the slope of the elastic phase and the peak stress decreased, yet the peak strain kept increasing roughly. Moreover, the drying\(-\)wetting cycles caused a difference in the shape of post-peak curves. The post-peak curve became flatter with increasing cycles. The reason is probably that the action of drying\(-\)wetting cycles can soften the particle cementation and generate new fractures inside the rock, resulting in the weakening of brittleness and the strengthening of ductility [34]. The effect of the solution pH on the stress\(-\)strain relationship was similar to that of drying\(-\)wetting cycles. As the acid concentration increased, the slope of the elastic phase and the peak stress also decreased. The stress\(-\)strain relationship manifested a tendency to change from brittleness to slight ductility.  
Figure 3: Relationships between the physical parameters of samples and the number of drying\(-\)wetting cycles: (**a**) mass; (**b**) P-wave velocity; (**c**) water absorption.  
Table 1 lists the elastic modulus and uniaxial compressive strength of the selected samples under different drying\(-\)wetting cycles and pH solutions. Both the mechanical parameters decreased monotonically with increasing drying\(-\)wetting cycles. For the acidic solution with pH = 3, the elastic modulus decreased by 38.55%, 47.40%, 58.72%, and 66.30% as the cycles progressed; the uniaxial compressive strength decreased by 19.96%, 27.27%, 35.96%, and 41.43%, respectively. Furthermore, the greater the acid concentration was, the more obvious the deterioration of mechanical properties was. After 40 drying\(-\)wetting cycles, the elastic modulus of the samples soaked in pH = 7, 5, and 3 solutions decreased by 51.33%, 55.37%, and 66.30%, respectively; the uniaxial compressive strength decreased by 27.28%, 32.63%, and 41.43%, respectively. It can be concluded that the influence of drying\(-\)wetting cycles and solution pH on the deformation property of red sandstone is greater than that on the strength property. Some scholars have obtained similar experimental results [36; 37]. However, the differences among these results cannot be ignored, especially the deterioration pattern and deterioration degree of mechanical properties. They are mainly dominated by the mineral composition and structure of rocks and experimental conditions [38].  
Figure 4: Stress\(-\)strain curves of typical samples under different drying\(-\)wetting cycles and pH solutions: (**a**) pH = 7; (**b**) pH = 5; (**c**) pH = 3.