## 4 Discussion  
Relationship between the Anisotropy of Physical and Mechanical Properties of Sandstone and Changes in Mineralogy and Microstructure  
Based on previous studies [24; 25], the change in sandstone is divided into four stages according to temperature to discuss the relationship between the anisotropy of the physical and mechanical properties of sandstone and the change in mineralogy and microstructure after high-temperature heating. (1) Room temperature to 400 \({}^{\circ}\)C  
With increasing temperature, the free water and weakly bound water in the sandstone are lost, and the internal cracks in the sandstone develop and expand [26]. However, due to the high temperature, the mineral particles expand and squeeze each other, and the cracks become compressed or even disappear [17; 27]. The loss of water and the reduction in fractures will lead to a decrease in sandstone quality, an increase in sandstone volume, and an increase in UCS and internal friction angle. At the same time, the propagation speed of the wave velocity in air is lower than that in liquid. Therefore, with increasing temperature, the wave velocity of sandstone decreases. Due to the difference in mineral composition between the bedding plane and sandstone matrix, the number of cracks developed along the bedding plane is greater than in the rock matrix, which increases the influence of bedding angle on the physical and mechanical properties of sandstone, leading to an increase in wave velocity, elastic modulus, and strength anisotropy. (2) 400 \({}^{\circ}\)C to 600 \({}^{\circ}\)C  
At this temperature stage, the first phase transformation of quartz (quartz lattice structure change, volume expansion), thermal expansion of feldspar, dehydroxylation of -clay minerals, and oxidation of iron-bearing minerals in sandstone mainly occur. The phase transformation of quartz and the uneven expansion of minerals lead to the development and expansion of a large number of intergranular cracks. However, at this stage, clay minerals such as kaolinite and illite begin to decompose and remove structural water, the particle surface becomes rough, the particle structure is unstable, and the connection is weakened. Some of the decomposition products may slip and fall into the intergranular cracks during the process of sandstone transport vibration and axial loading, so the strength of the sandstone is not significantly reduced. However, due to the large increase in cracks, the internal friction angle of the sandstone increases, and the wave velocity, elastic modulus, and cohesion decrease rapidly. At the same time, under the action of temperature, the number of fractures developing along the bedding further increases, and the anisotropy of the physical and mechanical properties of sandstone rapidly increases. \(600\)\({}^{\circ}\)C to \(800\)\({}^{\circ}\)C  
A large number of oxidized particles containing Fe and Al peel out from the sandstone and sinter with feldspar, quartz, etc., to form eutectic crystal particles that fill into larger cracks. When the sandstone is loaded, these eutectic crystal particles play a role in increasing the contact area and friction of mineral particles. The compressive strength of sandstone is improved to a certain extent [28; 29; 30]. Combined with the change curve of the longitudinal wave velocity, peak deviatoric stress, and elastic modulus, the longitudinal wave velocity and elastic modulus of the sandstone decrease significantly after heating at \(800\)\({}^{\circ}\)C, but the peak deviatoric stress does not decrease significantly, indicating that thermal damage, such as the development of microfractures, the increase in pores and the damage of the particle skeleton in sandstone, are relatively serious but have little impact on the strength of the sandstone. At this temperature stage, the internal cracks of sandstone are interconnected to form a dense fracture network, which leads to a reduction in the anisotropy of the sandstone.