## 3 Results and Discussion
### Microstructure Analysis  
In order to explore the microstructure of fractured sandstone grout-reinforced specimens under an acidic environment and freeze-thaw cycles, the specimens without treatment, soaking in acidic solution, freezing and thawing in an acidic environment for 5 times, 10 times, 20 times, and 30 times were selected as the scanning specimens, and the scanning electron microscope scanning test was carried out. Six different magnification images of 200, 500, 1000, 3000, 5000, and 10,000 times were obtained, respectively, and the action mechanism of an acidic environment and a freeze-thaw cycle on sandstone and grout-reinforced specimens was revealed. In the microstructure of the original specimens and under the observation of a 200 times electron microscope, the grouting material and sandstone are well cemented, the grouting material and sandstone are closely structured, a small number of cracks are found on the surface, the pores are small, and there are a small number of disordered mineral particles in the rock mass. After being magnified 5000 times, the surface of sandstone mineral crystal is relatively smooth, and the surface cracks of grouting material are disorderly distributed. In the microstructure of grout-reinforced specimens after 7 days of acid solution immersion and under the observation of a 200 times electron microscope, there are different degrees of corrosion on the surface of the grouting material and the sandstone, and compared with the original specimens, the development of micro cracks increases, but the development of large cracks decreases, the surface of main mineral crystals is smooth, and there is a large area of smooth area. After being magnified 5000 times, it can be seen that the acid corrosion of grouting material and sandstone is more significant, and the crack of grouting material extends and expands. The above conclusions show that under the influence of acid solution corrosion, grouting and solid grouting materials and sandstone react with the acid solution. Free mineral particles are generated, and free mineral particles are distributed around pores and cracks. The microstructure of grout-reinforced specimens after 30 acid environment freeze-thaw cycles is shown in Figure 12c. By comparing images with the different number of freeze-thaw cycles under the observation of a 200 times electron microscope, it can be seen that the grout-reinforced specimens after different freeze-thaw cycles have different degrees of damage. After five freeze-thaw cycles, there is a small range of particle shedding between grout-reinforced specimens grouting material and sandstone surface, and there are obvious free mineral particles on the surface of sandstone due to freeze-thaw cycles. Figure 11: The peak strength of full-through and semi-through grout-reinforced specimens changes with the number of freeze–thaw cycles: (**a**) full-through grout-reinforced specimens; (**b**) semi-through grout-reinforced specimens. Amplifying the electron microscope to 5000 times, it can be seen that the surface of the grouting material has obvious crack development and expansion, and the surface of the sandstone has an obvious \"honeycomb\" phenomenon. Due to the coupling effect of acid solution corrosion and a freeze-thaw cycle, the cementation of grouting materials and minerals in sandstone is obviously weakened. After ten freeze-thaw cycles, the grouting and solid grouting material is more obvious than the sandstone surface shedding after five freeze-thaw cycles, the micro-crack development on the surface of the grouting material increases, and the \"honeycomb\" area on the sandstone surface is more obvious. When the number of freeze-thaw cycles reaches 30 times, the loss of grouting and solid particles leads to a rougher surface, and the number of surface holes and cracks increases significantly. The surface roughness changes significantly compared with the original specimens of grouting and solid.