## 3 Results
### Mechanical Properties  
#### 3.2.1 Uniaxial Compressive Strength  
Uniaxial compressive strength is the peak stress that the sample can withstand during loading in the uniaxial compression test. The uniaxial compressive strength of the original rock (limestone specimen without thermal shock treatment) was 91.45 MPa (\(\sigma\) = 4.456), and the results of the uniaxial compressive strength of heated limestone specimens affected by thermal shock are shown in Figure 5.  
It can be seen that the uniaxial compressive strength of the rock sample increases and then decreases with the increase in heating temperature T at a given cooling temperature from Figure 7A, and the inflection point temperature is 200 \({}^{\circ}\)C. When the cooling temperature is 20 \({}^{\circ}\)C, the average uniaxial compressive strength of limestone increases from 73.63 MPa (\(\sigma\) = 3.83) at 100 \({}^{\circ}\)C to 81.13 MPa (\(\sigma\) = 1.89) at 200 \({}^{\circ}\)C, an increase of 9.8%, and then decreases to 44.47 MPa (\(\sigma\) = 7.37) at 600 \({}^{\circ}\)C, a decrease of 45.2%. It is worth noting that the strength values are very strongly scattered for the temperatures 100, 200, and 300 \({}^{\circ}\)C. This is because the mechanical strength of limestone is more influenced by the primary tectonics in the lower temperature section. Limestone of different formations can have large differences in the thermal-mechanical response. Therefore, the experimental results reflect more scattered numerical points.  
This is because thermal shock causes thermal expansion and deformation of the limestone sample below 200 \({}^{\circ}\)C, which is believed to close the primary voids, thus leading to an increase in the uniaxial compressive strength of the specimen. With the increase in heating temperature, the internal voids of the specimen re-open and generate new pores and fissures, forming a complex network of fissures leading to the decrease in the uniaxial compressive strength of the specimen. By analyzing the test results, it was found that the uniaxial compressive strength of limestone correlated well with the heating temperature at a given cooling temperature. Therefore, the two parameters were fitted in the range of 200 \({}^{\circ}\)C to 600 \({}^{\circ}\)C and were found to have an exponential relationship:  
\[\sigma_{\text{c20 }^{\circ}\text{C}}=114.8e^{\circ}(-0.002\text{T}),\text{ (R}^{\text{\text{\text{\text{\text{\text{\text{ \text{ \text{ \text{ \text{ \text  
where \(\sigma_{\rm c}\) is the uniaxial compressive strength at different cooling temperatures (MPa); T is the heating temperature, and its range is from 200 \({}^{\circ}\)C to 600 \({}^{\circ}\)C; R^2 is the correlation coefficient.  
From Figure 7B, it can be seen that the uniaxial compressive strength of high-temperature limestone samples increases with the increase in cooling temperature. As an example, the limestone sample was cooled at different temperatures at 600 \({}^{\circ}\)C: the average uniaxial compressive strength decreased from 51.46 MPa (\(\sigma\) = 5.97) at 100 \({}^{\circ}\)C to 44.47 MPa (\(\sigma\) = 7.37) at 20 \({}^{\circ}\)C, an increase of 13.6%.  
This is because the greater the temperature difference per unit distance and per unit time, the higher the temperature gradient, the greater the thermal stress, and the more serious the damage caused to the rock sample.