## 4 Discussion  
Effect of Steel Slag Substitution Rate on the Reaction Mechanism and Microstructure of Barrier Materials  
As can be seen in Figure 3, with the increase in steel slag substitution rate (0-50%), the unconfined compressive strength showed a linear decrease, which was closely related to the change in material microstructure by the steel slag substitution rate. Figure 9 shows the SEM images of soil samples of the same age (28d), same bentonite admixture (54%), and different steel slag substitution rates (0, 12.5%, 25%, 37.5%, and 50%) magnified by a factor of 5000 after the infiltration tests were completed. When there is no steel slag incorporation, a small amount of flocculated structure can be observed, and the material is more porous and well connected internally. With the increase in the steel slag substitution rate, needle-like tricalcium aluminate crystals began to appear, and hydrated calcium silicate gel gradually decreased. Tricalcium aluminate crystals fill the pores, forming a dense skeletal structure. Because the hydration products of steel slag-cement-bentonite material mainly from cement and steel slag is a gelling material, the hydration of cement can provide an alkaline environment for the hydration of steel slag, and with the increase in steel slag, the hydration of cement is weakened, resulting in the generation of less and less Ca(OH)\({}_{2}\), which is not conducive to stimulating the activity of steel slag. Steel slag and cement both contain tricalcium aluminate; the hydration product of tricalcium aluminate is calcium aluminate. The content of calcium aluminate will gradually increase with the reaction process. Due to the expansion of calcium aluminate, the gradual increase in the content of calcium aluminate will reduce the pore space of the soil samples, thus reducing the permeability of the soil samples; at the same time, excessive calcium aluminate will reduce the strength properties of the soil samples. This microstructural evolution directly explains the changes in the macroscopic mechanical properties: the swelling of tricalcium aluminate to fill the pores reduces the permeability (e.g., permeability coefficient decreases with the increase in steel slag substitution rate in Figure 7), but an excess of tricalcium aluminate inhibits the generation of the C-S-H gel, leading to weaker cementation and, thus, a reduction in the compressive strength (e.g., the strength decreases with the increase in steel slag substitution rate in Figure 3 decreases). Figure 8: Comparison of 28d permeability coefficient of different materials with bentonite dosage in the literature [24; 25; 26]. The mechanism of microstructure influence on macroscopic properties is clarified by referencing SEM images (Figure 9) with mechanical and permeability data (Figures 3 and 7). For example, at 50% steel slag substitution (Figure 9e), a dense tricalcium aluminate skeleton fills the pores, consistent with a low permeability coefficient of 2.30 \(\times\) 10\({}^{-6}\) cm/s (Figure 7c); however, a reduction in the gelling products results in a compressive strength of only 58.18 kPa (Figure 3a). Figure 9: SEM image of steel slag–cement–bentonite material with 54% bentonite dosage 28 days after completion of penetration test. Analysis of the Correlation Between Cohesion and Unconfined Compressive Strength of Barrier Materials  
In the process of barrier wall design for special sites on slopes, it is difficult to ensure stability by simply relying on the unconfined compressive strength alone, and the shear strength index can only be obtained through indoor tests, which is a process that consumes energy and material resources. This subsection links the unconfined compressive strength and cohesive strength and studies the relationship between the two changes through the unconfined compressive strength of the material to predict the cohesive strength of the material, which can be convenient and effective for ensuring that the design value meets the engineering needs.