# Physical and Mechanical Properties and Damage Mechanism of Sandstone at High Temperatures
## 5 Discussion  
The previous section analyzed the mechanical properties and damage characteristics of sandstone at high temperatures with different heating rates using the uniaxial compression test and acoustic emission test. In this section, the temperature effect and heating rate of the micro-morphological characteristics of sandstone fracture surfaces were analyzed by using a scanning electron microscope to ultimately reveal the damage and rupture mechanism of sandstone under the action of high temperatures. Figure 16 shows that pores and cavities already exist in the sandstone microstructure at room temperature. With the increase in temperature before 500 \({}^{\circ}\)C, the rock matrix and crystal particles are more tightly cemented, the thermal expansion of mineral particles leads to the closure of primary fissures, the degree of densification is increased, and the relative ratio of pores and cavities is significantly reduced compared with that at room temperature. At the same time, along with the evaporation of water, the brittleness of sandstone is enhanced, which also leads to the increase in peak stress and modulus of elasticity of sandstone with the increase in temperature, and the peak strain is not apparent with the change in temperature. From the fracture morphology, the fracture surface is mainly dominated by step-like patterns, river patterns, and trans crystalline cracks, which are typical brittle fracture patterns, and it can be concluded that the fracture surface of the sandstone at 500 \({}^{\circ}\)C shows significant brittle fracture. At 600 \({}^{\circ}\)C, thermal damage starts to occur in the sandstone structure, which is manifested by the sprouting of cracks and cavities in the crystal grains and matrix on the microscopic level and the gradual deterioration of the mechanical strength on the macroscopic level. At 1000 \({}^{\circ}\)C, the matrix of sandstone is no longer continuous and intact but is in a loose structure with a large number of microcracks, and there are a large number of thermal cracks on the crystal grains of sandstone, which results in the structure of the grain structure no longer being intact and generating a large number of fine particles and debris. The microcracks extend through each other, leading to a rapid decrease in the rock's load-bearing capacity, which also explains the increase in the decrease in peak stress and elastic modulus after 1000 \({}^{\circ}\)C in Figures 6a and 7a. Compared with the brittle fracture mode, more plastic deformation occurs on the fracture surface where ductile fracture occurs, and the fracture surface is not as flat and smooth as the brittle fracture, but a rougher fracture portion occurs. Comprehensive analysis of the fracture surface of sandstone after 600 \({}^{\circ}\)C ductile fracture morphology, mainly in the dimple, slip separation, and intergranular cracks, which is also a typical ductile fracture morphology, indicating that the fracture mode is a ductile fracture. From the fracture surface morphology, the brittle-ductile transition temperature of sandstone occurs between 500 and 600 \({}^{\circ}\)C, which is consistent with the previous conclusion. Figure 15: The variation law of stress threshold and proportion with heating rate at 1200 \({}^{\circ}\)C: (**a**) stress threshold; (**b**) proportion of stress threshold. Figure 16: Cont. From Figure 17, a large number of cracks and pores already existed in the sandstone at 1200 \({}^{\circ}\)C, but with the increase in heating rate, the thermal damage and thermal rupture were more serious, which was manifested by the increase in surface roughness of the samples, and the obvious increase in crack width, length, and pore size, which led to the decrease in mechanical properties of the sandstone with the increase in heating rate.