## 3 Results and Discussion  
Among the two forms of salt weathering, MgSO\({}_{4}\) caused the greatest amount of damage and mass loss to tuff samples. Figure 4 demonstrates that as the time of erosion increased, the mass of tuff samples soaked in distilled water and sodium sulfate changed only slightly, whereas the mass loss of tuff samples soaked in magnesium sulfate reached \(\sim\)10 g. The rate of mass loss was 1.56%. The results suggest that the presence of magnesium sulfate has a notable impact on both the expansion and dissolution of the tuff sample. The primary factor contributing to this phenomenon can be attributed to the process of salt crystallization [29]. The mass of the samples in each group of tests decreased as the freeze-thaw cycles increased in the second series of tuff samples. It revealed that the freeze-thaw cycle caused significant damage to the tuff. After 60 freeze-thaw cycles, the mass loss of tuff in the distilled water, sodium sulfate, and magnesium sulfate groups were 18.5 g, 17.2 g, and 17.3 g, respectively. The mass loss rates of tuff in the solutions of distilled water, sodium sulfate, and magnesium sulfate were 3.82%, 3.52%, and 3.58%, respectively. The observed mass loss rates of specimens immersed in sodium sulfate and magnesium sulfate solutions were marginally lower compared to those immersed in distilled water. This discrepancy may be attributed to the presence of salt crystals adhering to the specimen's surface, potentially obstructing certain micro-pores, and consequently reducing moisture infiltration into the rock specimen [30; 31]. \begin{table}
\begin{tabular}{c c c c c c} \hline
**Rock Type** & **Sample Size (mm)** & **Experiment Content** & **Experiment Condition** & **Test Number** & **Initial Mass (g)** \\ \hline \multirow{4}{*}{Tuff} & \multirow{4}{*}{\(\varphi 50\times 100\)} & \multirow{4}{*}{
\begin{tabular}{c} Salt weathering (25 \({}^{\circ}\)C) \\ \end{tabular} } & Distilled water & N1 & 482.12 \\  & & & Sodium sulfate & N2 & 485.05 \\  & & & Magnesium sulfate & N3 & 484.13 \\  & & Coupled frost and salt & Distilled water and freeze–thaw cycles & N4 & 484.50 \\  & & weathering & Sodium sulfate and freeze–thaw cycles & N5 & 487.60 \\  & & (\(-\)20\({}^{\circ}\)C) & Magnesium sulfate and freeze–thaw cycles & N6 & 483.00 \\ \hline \end{tabular}
\end{table} 
Table 1: Initial experiment conditions. Figure 3: Schematic diagram of CT scanning and 3D reconstruction of tuff sample. The evolution of the cross-sectional morphology of tuff as a result of salt weathering is depicted in Figure 5. After 10 days, the outer edge of the cross-section does not change significantly. When the number of days of erosion reached 40, the outer margin of the cross-section appeared fragmented due to the strong chemical reaction between the magnesium sulfate solution and the mineral element in tuff, which caused partial dissolution of the outer edge. After 60 days, the cross-section of tuff that had been soaked in sodium sulfate solution revealed severe damage along its upper edge, and a fracture appeared in the lower right region (Figure 5f). Almost the whole edge of the cross-section of tuff that had been soaked in magnesium sulfate solution was damaged after 60 days of erosion (Figure 5c). To analyze the change and evolution in pore characteristics quantitatively, all layers of scanning images were used to reconstruct the 3D model of the central pore distribution of tuff specimen. In Figure 6, the spectrum of porosity from 0 to 0.3 is represented by the color bar, with red representing locations with high porosity and blue representing regions with low porosity. The pore distribution of the sample exhibits notable heterogeneity in Figure 6. The primary factor contributing to this phenomenon is the diverse composition of the tuff. With an increase in the number of days and freeze-thaw cycles, the areas of high porosity  
Figure 4: Mass change in tuff samples under frost and salt weathering (**a**) salt weathering (**b**) coupled salt and frost weathering.