## 3 Experimental Results
### Macroscopic Weathering Indicators
#### 3.1.5 Uniaxial Compressive Strength  
1. Characteristics of Uniaxial Compression Failure  
The characteristics of uniaxial compression failure for the two types of rocks undergoing sulfate erosion are depicted in Figure 10. Let us first analyze the failure characteristics of sandstone. 1. For the rock samples subjected to cycles 1 to 15, the predominant failure modes are brittle and tensile failure. The axial force compresses the fractures within the rock sample and expands its volume, resulting in a relatively intact middle portion, while the remaining parts fracture into multiple small pieces. The overall integrity of the rock sample is poor. 2. At cycles 15 and 20, the bonding between particles weakens, leading to the emergence of multiple vertical cracks that penetrate the rock sample. 3. By cycle 25, the surface layer of the rock has completely softened, and a large number of cracks appear within the rock sample. Applying minimal pressure causes vertical cracks to form at the sides, resulting in sample failure. Despite this, the rock sample still maintains a relatively intact overall appearance, reflecting the softening and plasticity of sandstone after prolonged salt weathering. In general, the initial state of the limestone samples is good, the particles are tightly arranged, and the damage caused by sulfate weathering is slight. The differences in the characteristics of uniaxial compression failure after different numbers of cycles are similar. As shown in Figure 10g,h, in the examples of failure after 25 days, the weathered limestone notably and primarily fails through axial splitting, displaying brittle damage characteristics. Multiple vertical cracks penetrate the rock sample at the sides, indicating a high degree of fragmentation and poor integrity. 1. Uniaxial Compressive Strength  
Figure 11 presents the variation curves of unconfined compressive strength (UCS) and its loss in samples of sandstone and limestone subjected to sulfate erosion. The curves clearly illustrate that as the number of cycles increases, the strength values of both rock types consistently decrease. However, there are notable disparities in the rate of degradation between the two. Specifically, the strength values of sandstone demonstrate an initial rapid decline, followed by a slower decrease. This trend can be accurately described and modeled using an exponential function:  
\[\text{UCS}=a-b\times c^{n} \tag{5}\]  
Figure 10: Uniaxial compression failure characteristics of sandstone and limestone under sulfate erosion. (**a**) 0 cycles; (**b**) 5 cycles; (**c**) 10 cycles; (**d**) 15 cycles; (**e**) 20 cycles; (**f**) 25 cycles; (**g**) 0 cycles; (**h**) 25 cycles. where UCS represents the uniaxial compressive strength, \(n\) represents the number of cycles, and \(a\), \(b\), and \(c\) are constants. The strength values of limestone exhibit a trend of slow degradation followed by rapid decline, and also follow an exponential function trend:  
\[\text{UCS}=ae^{-n/b}+c \tag{6}\]  
In this equation, UCS represents the uniaxial compressive strength, \(n\) represents the number of cycles, and \(a\), \(b\), and \(c\) are constants. Based on the magnitude of the strength variation, the sandstone sample experienced a decrease in strength from 77.81 MPa to 28.37 MPa after 25 cycles, while the limestone sample underwent a change from 139.26 MPa to 92.91 MPa. Both rock types exhibit a decrease of approximately 50 MPa. After 10 cycles, the surface structure of the sandstone sample was markedly different, having been damaged and softened, while the salt absorption remained stable. The integrity of the rock gradually improved, and the strength gradually decreased until it reached the limit value of the test sample. Due to the favorable initial condition of the limestone, the strength of the fresh limestone reaches an impressive 140 MPa. Moreover, the impact of salt crystallization on the limestone during the cycle is relatively limited, resulting in only minor damage.