## 3 Results and Discussion
### Hydraulic Erosion Condition of Scoured Slope  
Four modified slopes are shown in Figure 5. The loose phosphogypsum particles were solidified into a whole after treatment, and a thin layer of CaCO\({}_{3}\) was formed locally on S3 and S4. The rainfall scouring test lasted for one hour, and Figure 6 shows the morphology of the phosphogypsum slopes after scouring. The particles in S1 were connected by ordinary hydration, it can be seen that a hardened layer of a certain thickness was formed on the surface after hydration. In the early period of the test, only slight erosion occurred on the surface under the protection of the hardened layer. After 30 min, several cracks appeared on the hardened layer, and the non-hydrated loose particles were quickly washed away when the rainfall washed into the lower part of the hardened layer. More cracks were formed in the hardened layer under the scouring force of water and with the accumulation of shedding particles. The whole structure of the S1 slope was quickly destroyed. As for the microbially treated slopes S2, S3, and S4, the matrix was cemented tightly and uniformly without delamination, and while the structures of the slopes were not destroyed, the failure patterns were different. S2 was treated with OD\({}_{600}\) = 0.1, and the erosion type of slope is sheet erosion. The flakes were splashed, and the spalling area gradually expanded under rainfall scouring, meaning that the erosion surface was smooth. S3 was treated with OD\({}_{600}\)= 0.3, and it showed obvious massive spalling. Erosion pits appeared on the surface and were arranged along the runoff direction with the increasing erosion time, which finally lead to rills. S4 was treated with OD\({}_{600}\)= 0.5, and the solidification was inhomogeneous. The local surface had formed a compact overlay which was slightly damaged, while spallies appeared in the uncovered area, and the width of the spallies gradually widened with the continuous erosion, until the process finally became a form of gully erosion. Comparing the damage patterns and erosion details of four phosphogypsum slopes after scouring, the different curing effects of microbial treatment and ordinary hydration could be analyzed. The hardened layer of S1 was gypsum dihydrate formed by the hydration of gypsum hemihydrate. The thickness was 6 mm with a low density. The formation of the hardened layer hindered the seepage of water, so the phosphogypsum particles at the lower part of the hardened layer could not completely hydrate and were still in a loose state. When the rainfall scoured into the interior of the slope, an extensive area of loose particles eroded under the scouring, and the whole structure of S1 was quickly destroyed. The phosphogypsum particles in S2, S3, and S4 were modified by the microbial solution and solidified into a whole. The matrix under the scouring layer was tightly cemented. The surface did not appear to have any apparent cracks and overall slip. It indicated that the connection between the particles had changed after the microbial treatment. Under the condition of multiple cycles of different concentrations of bacterial solution, the properties and morphology of each modified slope are different. It could be seen that the particles on S2 were closely connected with a uniform texture, and only weak flake peeling occurred under the scour of water flow. However, both S3 and S4 were scoured out with erosion pits or gullies. A thin layer of calcium carbonate was formed on S3 and S4 due to the incomplete infiltration of the microbial solution. The eroded surface is rough and uneven with bulky particles. Therefore, the erosion resistance of the phosphogypsum slope had been improved by MICP. When the concentration of the cement solution is 0.5 M, the slope modification effect with a bacterial solution concentration of 0.1 is better than that of 0.3 and 0.5. The cumulative erosion weight and erosion velocity are shown in Figures 7 and 8.