## 3 Results  
### Apparent Morphology  
The differences in rock samples under different freeze-thaw conditions can be seen from the macroscopic morphology during the tests, as shown in Figure 7. The damage to rock samples under immersion freeze-thaw is the most serious. Rock samples are damaged from the edges and corners, cracks continued to develop along the bedding, particles kept peeling off, and other parts of the rock suffered minor damage. Some rock samples are seriously damaged after 40 cycles. The rock sample as a whole is no longer intact and completely damaged after 80 cycles. The rock samples in the capillary freeze-thaw test showed salt precipitation gradually. After 10 cycles, layered destruction occurred about 2 mm from the bottom, indicating that salt still existed after desalting treatment. Cultural relics in the open air were generally still subjected to salt weathering, the bottom water content of the capillary rock was high, and the damaging effect of freeze-thaw was stronger. In the periodic saturated freeze-thaw test, the macroscopic morphology of rock samples did not change significantly within the testing cycle range, and the touch sensation became rough. A small number of particles peeled off at the edge of rock samples, and the edges and corners became uneven, with slight damage in local areas. \begin{table}
\begin{tabular}{p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt}} \hline \hline
**Parameter** & \begin{tabular}{c} **Wave** \\ **Velocity** \\ **v (km/s)** \\ \end{tabular} & \begin{tabular}{c} **Natural** \\ **Water** \\ **Absorption** \\ **W\({}_{a}\) (\%)** \\ \end{tabular} & \begin{tabular}{c} **Dry** \\ **Density** \\ **pd (g/cm)** \\ \end{tabular} & \
\begin{tabular}{c} **Open** \\ **Porosity** \\ **\(\varphi\) (\%)** \\ \end{tabular} &
\begin{tabular}{c} **Surface** \\ **Hardness \(r\)** \\ **(HL)** \\ \end{tabular} & **UCS (MPa)** \\ \hline \hline \multicolumn{2}{p{56.9 pt}}{The numerical} & 1.986 & 4.321 & 2.311 & 9.145 & 558 & 82.83 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Average value list of some physical indexes of yellow sandstone. Figure 6: Schematic diagram of the weathering index test. (**a**) Schematic diagram of wave velocity test, the arrows indicate the three test directions; (**b**) hardness test points, the circles indicate the different test areas. Figure 7: Variation characteristics of macroscopic morphology of rock samples under different freeze–thaw modes. (**a**) Immersion freeze–thaw 60 cycles; (**b**) immersion freeze–thaw 60 cycles; (**c**) immersion freeze–thaw 60 cycles. Figure 8 shows the current state of weathering on the facade of the North Grottoes in Qingyang, Gansu, China. It can be seen that, under the influence of freeze-thaw action and cave building environment, the bottom and interior of the cave are seriously damaged by seepage erosion, and the cave deteriorated obviously. There are many longitudinal and transverse pore cracks in the surface layer. In the severely weathered area, part of the surface layer has spalled off. The upper part of the cave, the middle part, and the Buddha in the cave were affected by capill, the particle loss is serious, and the surface has white crystal precipitation. The appearance of the cavity is similar to the test results, with obvious discoloration and longitudinal and transverse cracks [16]. In terms of the law of change of the micromorphology, the phenological deterioration rate of rock samples under different freeze-thaw modes is different, but the change law of rock samples under the three freeze-thaw conditions is similar. The sample with periodic saturated freeze-thaw cycles is taken as an example, and its micromorphology is shown in Figure 9. At the beginning of the test (Figure 9a), the particles were closely arranged, the pores and cracks were small, and the cement was filled between the particles. After 10 cycles (Figure 9b), the sample was in a state that was \"rapid metamorphic\".