## 3 Results and Discussion
### CT Scanning of Layered Sandstone After High-Temperature Treatment  
CT was used to scan the sandstone core following treatment at different temperatures. The 2D images of sandstone core slices obtained via threshold segmentation are shown in Figure 9a, where the black areas represent pores, and the bright white bands represent horizontal bedding bands formed by the aggregation of magnetite grains. As shown in Figure 9a, the grayscale of the specimen slice at room temperature is low. As the temperature rises, the grayscale of the slice darkens significantly, indicating that the sandstone is dense and has less pores and cracks at room temperature. Following high-temperature treatment, the density of the pores in the slice increases significantly. The higher the temperature, the denser the pore distribution, and therefore, the darker the grayscale. Figure 9b depicts the 3D reconstructions of the pore distribution in the sandstone core following heat treatment at different temperatures. Table 1 lists the porosity data obtained from further quantitative calculations. As indicated by Figure 9b and Table 1, fine-grain sandstone containing bedding planes is dense with a porosity of only 3.0% at room temperature. After the sandstone core is heated to high temperatures, the number of internal pores significantly increases, thus increasing the pore distribution density. After the sandstone core is heated to 800 \({}^{\circ}\)C, its porosity increases to 19.4%. Figure 7: TG and DTG curves of layered sandstone. Figure 8: Microstructural characteristics of layered sandstone heat-treated at different temperatures. (**a,b**) Rock sample at room temperature; (**c**) Rock sample heated at 200 \({}^{\circ}\)C; (**d**) Rock sample heated at 400 \({}^{\circ}\)C; (**e**) Rock sample heated at 600 \({}^{\circ}\)C; (**f**) Rock sample heated at 800 \({}^{\circ}\)C. \begin{table}
\begin{tabular}{c c} \hline
**Temperature/\({}^{\circ}\)C** & **Porosity/\%** \\ \hline
25 & 3.0 \\
200 & 3.9 \\
400 & 7.5 \\
600 & 8.2 \\
800 & 19.4 \\ \hline \end{tabular}
\end{table} 
Table 1: Porosities of layered sandstone heat-treated at different temperatures. The pore length and aperture in the layered sandstone specimen, which was subjected to heat treatment at different temperatures, were calculated based on the 3D reconstructions. The minimum, maximum, and average values of pore length and aperture are listed in Table 2. As shown, the average values of pore length and aperture increase as the heating temperature increases. At 200 \({}^{\circ}\)C, the average values of pore length and aperture are similar to those at room temperature. At 400 \({}^{\circ}\)C, the average values of both pore length and aperture increase significantly, indicating a deterioration of internal failures in the sandstone core which results in larger pores. At 800 \({}^{\circ}\)C, the maximum value of pore length reached 729.42 \(\upmu\)m, while the maximum aperture value reached 371.10 \(\upmu\)m.  
The volume of the pores within the layered sandstone following high-temperature treatment were calculated; then, pores were categorized by volume as follows: small (1 \(\times\) 10\({}^{5}\)-1 \(\times\) 10\({}^{6}\)\(\upmu\)m\({}^{3}\)), medium (1 \(\times\) 10\({}^{6}\)-5 \(\times\) 10\({}^{6}\)\(\upmu\)m\({}^{3}\)), and large (5 \(\times\) 10\({}^{6}\)-1 \(\times\) 10\({}^{7}\)\(\upmu\)m\({}^{3}\)) pores. As shown in more detail in Table 3, at 200 \({}^{\circ}\)C, the numbers of small and medium pores in sandstone are similar to those at room temperature and do not undergo a significant increase compared to the numbers at 25 \({}^{\circ}\)C. At 400 \({}^{\circ}\)C, the numbers of small and medium pores grow significantly, indicating that primary cracks are widening and propagating while numerous new hot cracks are starting to form. At 600 \({}^{\circ}\)C, the numbers of small and medium pores grow slightly compared to the numbers at 400 \({}^{\circ}\)C but not to a significant degree.