## 4 Discussions
### Mudstone Hydration  
According to the mudstone water immersion test, it is known that the clay mineral content of the sampled mudstone is significant, mainly composed of illite and kaolinite, with solid hydrophilicity and apparent expansion and disintegration in water, so the damage caused by hydration in the natural mudstone sample needs to start from the change of the microstructure of the mudstone after being exposed to water.  
Clay minerals are a kind of aluminum-silicate crystal composed of two typical wafers superimposed on each other. One is a silica-oxygen wafer, with six silica-oxygen tetrahedra constituting a silicon sheet and adjacent tetrahedra sharing an oxygen atom; the other is an aluminum-hydroxide wafer, with four aluminum-hydroxide octahedra comprising an aluminum sheet. Silicon-oxygen wafers and aluminum-hydroxide wafers constitute two kinds of cell: the first is a 1:1 cell, the cell structure for a layer of silicon and a layer of aluminum together; the second is a 2:1 cell, the cell structure for the top and bottom of the two layers of silicon sandwiched between a layer of aluminum. The two types of cells can be arbitrarily combined, and different clay minerals are macroscopic manifestations of the various combinations of the two cell types. Montmorillonite, illite, and kaolinite are the three most common major clay minerals [37].  
Figure 18 illustrates the individual crystal cell structure of montmorillonite, which comprises two layers of silica-oxygen tetrahedra held between a layer of aluminum-hydroxide octahedra. The O\({}^{2-}\) to O\({}^{2-}\) linkage of the two crystal layers is weak, making the crystal layer easily infiltrated by water. This results in a macroscopic manifestation of solid hydrophilicity. Ilmenite, on the other hand, is the weathered material of mica in an alkaline environment, and its single crystal cell junction consists of three layers. A large number of potassium ions between the two crystal layers strengthens the linkage, making it stronger than montmorillonite but weaker than kaolinite due to structural issues. Kaolinite is formed by the superposition of a layer of silica-oxygen tetrahedra and a layer of aluminum-hydroxide octahedra. Large particles and poor hydrophilicity mainly characterize it.  
Figure 17: \(T_{2}\) spectral area of natural mudstone under different freeze–thaw cycles.  
When natural mudstone samples come into contact with water, the microstructure of the mudstone changes. Initially, the water molecules penetrate the surface pores and cracks of the mud. This results in the formation of more pores on the surface. As the freezing and expansion force takes effect, the pore space starts expanding and connecting, resulting in an increase in volume. Gradually, water penetrates the interior of the mudstone, triggering the formation of secondary pores, which leads to a rise in the number of pores and damages the sample. There are two specific reasons for this: Firstly, under the action of water, some of the potassium feldspar in the mudstone transforms into kaolinite, creating secondary porosity due to the differences in structure, composition, and density between the two. The samples used in this paper contain potassium feldspar. Secondly, the clay mineral particles adsorb molecules that combine with water, resulting in local expansion and more significant tensile stress, which leads to the generation of secondary porosity.