## 2 Sample Preparation and Test Procedures  
Sandstone with clear horizontal bedding was taken as the research object in the test. All samples were taken from the same sandstone mined by a stone factory in Qingdao, Shandong Province, China. The samples appeared purple-grey and grey-white in color, and the weathering degree was low. The mineral composition is mainly feldspar, quartz, clay minerals (such as kaolinite, illite, etc.), and iron-bearing minerals. The average density was 2.63 g/cm\({}^{3}\), and the average longitudinal wave velocity approximately 4598 m/s. Different directions of this sandstone were drilled to obtain rock samples with different bedding dip angles of 0\({}^{\circ}\), 30\({}^{\circ}\), 45\({}^{\circ}\), 60\({}^{\circ}\), and 90\({}^{\circ}\) (Figure 1) (\(\beta\) is the angle between the bedding plane and horizontal direction). The test samples were processed into 360 standard cylinder samples with a diameter of 50 mm and a height of 100 mm (ISRM 2007).  
Considering the general temperature in high-temperature geological environments such as underground coal gasification (800-1100 \({}^{\circ}\)C), burnt rock in coal field geology (up to 2000 \({}^{\circ}\)C), geothermal resource development, underground rock engineering encountering fires, and existing laboratory testing equipment and site conditions, a heating temperature range from 25 \({}^{\circ}\)C (original state) to 1000 \({}^{\circ}\)C was selected. The heating gradient was 200 \({}^{\circ}\)C, with a total of five gradients, including six temperature settings: 25 \({}^{\circ}\)C, 200 \({}^{\circ}\)C, 400 \({}^{\circ}\)C, 600 \({}^{\circ}\)C, 800 \({}^{\circ}\)C, and 1000 \({}^{\circ}\)C. Based on the common burial depth of sandstone in rock engineering, 15 MPa was set as the upper limit of the confining pressure, and uniaxial compression tests and triaxial compression tests were conducted with confining pressures of 5 MPa, 10 MPa, and 15 MPa.  
The instruments and equipment used in this test included an electronic balance (precision 0.001 g), Vernier calliper (precision 0.02 mm), color analyzer (TES-135A comes from Beijing, China), ultrasonic detector (Pundit-PL200 PE comes from Beijing, China, 250 kHz), high-temperature furnace (SG-XL1200 comes from Beijing, China, maximum temperature 1200 \({}^{\circ}\)C, precision \(\pm\) 3 \({}^{\circ}\)C), and mechanical testing machine (HSW-1000B comes from Beijing, China).  
The test process was as follows: the rock sample was placed at room temperature under ventilation for 2 weeks to eliminate the influence of natural moisture content. The rock samples were put into the high-temperature furnace for heat treatment according to the designated temperature groups. The heating rate was 3 \({}^{\circ}\)C/min, and the temperature was kept constant for 2 h after being heated to the target temperature. After the power  
Figure 1: Sampling diagram of sandstone samples with different bedding angles.  
was turned off, the rock samples were naturally cooled to room temperature in the furnace; chromaticity tests, wave velocity tests, and uniaxial and triaxial compression tests were conducted on the rock samples after high-temperature treatment, and photos of the damaged sample morphology were taken.