# Physical and Mechanical Properties and Damage Mechanism of Sandstone at High Temperatures
## 3 Test Result  
### Physical Property Change  
The physical properties of sandstone, such as color, mass, volume, density, and elastic wave velocity, change after experiencing high temperatures. Figure 1d shows the color change in sandstone after experiencing different temperatures. At room temperature, the sandstone is off-white, and at 100-200 \({}^{\circ}\)C, the color is similar to room temperature, and the change is not apparent. A significant darkening of the color was observed between 300 and 600 \({}^{\circ}\)C due to the combustion of organic matter. When the temperature increased to 700 \({}^{\circ}\)C, the color changed to reddish brown, presumably because the sandstone contains hematite reduced to iron at high temperatures. However, according to the XRD results in Figure 1c, no hematite material was found at room temperature, probably because the small content of diffraction peaks is not obvious. The mass, volume, density, and elastic wave velocity of sandstone before and after high temperature were measured one by one, and four change rate indexes were defined: mass loss rate, volume expansion rate, density reduction rate, and elastic wave velocity reduction rate. These indexes are specifically defined as follows:  
\[\eta_{m}=\frac{m_{0}-m_{1}}{m_{0}}\]  
\[\eta_{\circ}=\frac{v_{1}-v_{0}}{v_{1}}\]  
\[\eta_{\rho}=\frac{\rho_{0}-\rho_{1}}{\rho_{0}}\]  
\[\eta_{p}=\frac{p_{0}-p_{1}}{p_{0}}\]  
In the equations, \(\eta_{m}\), \(\eta_{\circ}\), \(\eta_{\rho}\), \(\eta_{p}\) are the mass loss rate, volume expansion rate, density reduction rate, and wave speed reduction rate of sandstone before and after high temperature, respectively; \(m_{0}\), \(v_{0}\), \(\rho_{0}\), \(p_{0}\) are the mass, volume, density, and elastic wave speed of sandstone before high temperature; \(m_{1}\), \(v_{1}\), \(\rho_{1}\), \(p_{1}\) are the mass, volume, density, and elastic wave speed of sandstone after high temperature. Figure 3 shows the variation in the physical properties of sandstone at different temperatures. Figure 3a reflects the variation in the mass loss rate with temperature, which increases with increasing temperature. Before 500 \({}^{\circ}\)C, the mass loss rate grows steadily, and the average value of the mass loss rate is between 0.68% and 1.65%. The mass loss rate increases rapidly at 500-600 \({}^{\circ}\)C, from 1.65% to 4.24%, an increase of 156.6%. After the heat treatment temperature of 600 \({}^{\circ}\)C, the mass loss rate lies between 4.24 and 4.99%, and the growth rate slows down. Figure 3b reflects the change in volume expansion rate with temperature, and the change in the volume expansion rate is not obvious before 500 \({}^{\circ}\)C. The volume of sandstone expanded rapidly from 500 to 600 \({}^{\circ}\)C, from 0.25% to 1.21%, and the volume expansion rate increased rapidly after 700 \({}^{\circ}\)C, and the volume expansion rate under heat treatment at 1200 \({}^{\circ}\)C reached 3.71%. Figure 3c shows the variation in the density reduction rate of sandstone with temperature, which is similar to the trend shown in Figure 3a. Figure 3d shows the variation in the wave velocity reduction rate with temperature, the wave velocity reduction rate increases from 4.34% to 13.18% before 500 \({}^{\circ}\)C, and the wave velocity reduction rate between 500 and 600 \({}^{\circ}\)C is 39.05% at 600 \({}^{\circ}\)C, which is 196.28% compared with that at 500 \({}^{\circ}\)C, and the wave velocity reduction rate increases to 62.21% at 1200 \({}^{\circ}\)C.  
It is summarized that the law of change in physical properties of sandstone with temperature is found to be mainly divided into three stages:  
Stage I (100-500 \({}^{\circ}\)C): In this stage, the mass is steadily reduced due to the escape of water inside the sandstone [26]; the internal primary pores are contracted, the volume expansion is not apparent, and the density is reduced; the elastic wave velocity is slowly reduced. Stage II (500-600 \({}^{\circ}\)C): This stage is the mutation stage.