# Physical and Mechanical Properties and Damage Mechanism of Sandstone at High Temperatures
## 6 Conclusions  
In order to study the macroscopic and microscopic properties of sandstone at high temperatures, uniaxial compression tests were carried out on sandstone at different temperatures as well as at different rates of warming to study the changes in the physical and mechanical properties of the sandstone and to analyze the mechanism of its damage evolution by acoustic emission technology and scanning electron microscopy, and the following conclusions were mainly drawn:  
(1) As the temperature rises, sandstone physical properties appear in the phenomenon of mass reduction, volume expansion, density reduction, and wave velocity reduction, with the most apparent changes at 500-600 \({}^{\circ}\)C. The peak stress and modulus of elasticity of sandstone showed a tendency of increasing and then decreasing with temperature, with 500 \({}^{\circ}\)C as the threshold temperature, which was 69.48% and 30.43% higher than that of room temperature, and the peak strain showed a tendency of increasing with temperature, and slightly decreased at 200 \({}^{\circ}\)C and 1200 \({}^{\circ}\)C. The modulus of elasticity and peak stress show a decreasing trend with an increasing rate of warming, and the peak strain does not change much.  
(2) The pattern of maximum ringing counts of sandstone is the same as that of its strength, and the maximum ringing counts increase and then decrease with the increase in temperature and increase with the increase in warming rate. The aggravation of thermal damage makes the ringing counts more frequent, and the cumulative ringing counts show an upward trend with the increase in both temperature and heating rate. The cracking threshold and damage threshold stresses and their ratios are highly correlated with the peak strength, and both of them show a specific decreasing trend with the increase in temperature, which indicates that the high-temperature treatment will cause the sandstone to crack prematurely.  
(3) In the macro and microscopic damage pattern, before 500 \({}^{\circ}\)C, the thermal expansion within the sandstone internal crystals gradually closes the sandstone's pores and cracks, improving the sandstone's densification and mechanical properties. The macroscopic damage mode is dominated by shear damage, and the fracture morphology is mainly characterized by brittle morphology. After 600 \({}^{\circ}\)C, the large amount of development of cracks makes the structural integrity of sandstone deteriorate continuously, and the  
Figure 17: Micromorphology changes at different heating rates.  
mechanical properties gradually decrease. The macroscopic damage mode is dominated by cleavage damage, and the fracture surface is characterized by ductile morphology. By analyzing the changes in micromorphology under different heating rates at 1200 \({}^{\circ}\)C, the crack length and pore size will grow with the increase in the heating rate under thermal shock.  
Conceptualization, L.Z.; methodology, Y.Z.; validation, X.G. and P.W.; formal analysis, M.L.; data curation, F.Z.; writing--original draft preparation, L.Z. and Y.Z.; writing--review and editing, L.Z. and Y.Z.; visualization, P.W.; funding acquisition, L.Z. and M.L. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Natural Science Foundation of China (52074240, 52274140, and 51974296), the State Key Laboratory for GeoMechanics and Deep Underground Engineering, China University of Mining and Technology (SKLGDUEK2212), the Excellent Science and Technology Innovation Team of Jiangsu Province (Engineering Structural Safety under Complex Environmental), the sixth phase of Jiangsu Province \"333 talents\" training support special--card neck technology research.  
Not applicable.  
The data presented in this study are available on request from the corresponding author.  
The authors are grateful for the equipment support provided by Xuzhou University of Technology.  
The authors declare no conflict of interest.