## 5 Conclusions  
This study utilized the response surface method to optimize and verify the influencing factors of MICP-solidified uranium tailings. The mechanical strength of solidified uranium tailings under the optimized parameters was determined by an unconfined compression test, and the composition and microstructure of the tailings and mineralization products were analyzed by XRD, SEM and CT scanning. The main conclusions are as follows:  
1. Using response surface methodology (RSM), experiments were designed to investigate the effects of three controllable factors--the bacteria - cementation ratio, cementing solution concentration, and grouting amount--on the unconfined compressive strength \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Characteristic Parameters**} & \multicolumn{5}{c}{**Number of Grouting Cycles**} \\ \cline{2-6}  & **0** & **1** & **3** & **5** & **7** \\ \hline Number of pores & 365,181 & 297,308 & 274,916 & 234,699 & 209,808 \\ Pore volume V/mm\({}^{3}\) & \(2.41\times 10^{4}\) & \(2.30\times 10^{4}\) & \(1.96\times 10^{4}\) & \(1.89\times 10^{4}\) & \(1.84\times 10^{4}\) \\ Porosity & 33.24\% & 32.78\% & 27.48\% & 27.12\% & 26.25\% \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Pore structure parameters of uranium tailings with different grouting rounds. Figure 10: Pore parameters and peak stress variation curves with different grouting times. (UCS) of MICP-treated uranium tailings. The results indicate that the mathematical model constructed through RSM is statistically significant, with cementing solution concentration and grouting amount identified as the two primary factors influencing the UCS of the solidified uranium tailings. The findings convincingly demonstrate that conducting injection operations in a native bacterial environment is feasible, resulting in a significant improvement in the strength of the uranium tailings. 2. The optimized injection parameters derived from RSM were applied to perform seven injection cycles on the uranium tailings. The UCS of the treated samples increased by 27 times compared to the control group that received only water, and by 6 times compared to the group that received only the cementing solution. Moreover, the measured values closely aligned with the predicted values. This confirms the effectiveness of RSM in optimizing injection parameters, providing a viable approach for similar microbial engineering injection projects. 3. Microstructural analysis was conducted using techniques such as XRD, SEM, and CT scanning to examine the composition and microstructure of the tailings and mineralization products. The results revealed distinct calcite characteristic peaks in the MICP-treated tailings, indicating a strong adhesion of the formed calcium carbonate crystals to the uranium tailings, with interlocking structures. This suggests that the enhancement in strength is closely related to the cementation effect of calcium carbonate within the particle pores. The generation of calcium carbonate contributes to the structural integrity of the initially unstructured soil, thereby increasing its resistance to disturbance. As the number of injection cycles increased, both the number and volume of pores exhibited a decreasing trend, indicating that the accumulation of calcium carbonate in the pores progressively filled the voids, transforming the originally loose sand into a denser state. The cementation and filling effects of calcium carbonate significantly improved the strength and stability of the uranium tailings. The results of this study indicate that effective reinforcement of uranium tailings can be achieved even in a native bacterial environment, demonstrating the environmental adaptability of MICP injection technology. Furthermore, RSM effectively addresses the challenges of parameter determination in engineering injections, allowing for the derivation of optimal injection parameters through a limited number of experiments.