## 4 Microstructure Analysis
### Pore Scanning (CT)  
The previous test results demonstrated that the MICP grouting reinforcement technology can effectively enhance the overall mechanical properties of uranium tailings. The reinforcement effect is particularly important when reinforcing uranium tailings through MICP grouting. A CT scanning system was used to scan the grouting process in stages to study the pore evolution process of the MICP grouting body. Image reconstruction technology was used to visualize the internal pore structure space of the grouting sand column. In addition, the structural characteristic parameters, such as the number and volume of pores, were quantitatively characterized. The instrument used in the CT scan test was a high-resolution 3D X-ray microscope (XRM) from Carl Zeiss (Germany). The measurement accuracy was set to 39.8708 \(\upmu\)m based on the lateral dimensions of the samples. The reconstruction area was selected in the CT scan results using Avizo 2020 software, and the image was subjected to logarithmic filtering, geometry, and beam-hardening correction to achieve the best image processing effect. Subsequently, the reconstructed sections were processed to obtain the internal pore distributions and structural parameters of the grouting body. Figure 8: SEM after solidification of uranium tailings; subfigure (**a–c**) demonstrated the crystal morphology of calcium carbonate, subfigure (**d–f**) illustrated the different cementation forms between calcium carbonate and tailings particles. Based on CT slice reconstruction, Avizo 2020 software was used to reconstruct and optimize the 3D image of the sand column to understand the morphology and process of the dynamic change of pore structure of the uranium tailing sand column under different grouting stages more intuitively. The 3D pore reconstruction model of each grouting stage was then constructed. (Figure 9a). Considering that the porosity of the sand pillars was very small and greater than 10%, the three-dimensional reconstruction drawings of the five groups of sand columns were very similar, which was not conducive to observation. Consequently, the pore network model was used for analysis, as presented in Figure 9b-f. It can be noted that the pores with relatively large volumes are mostly distributed in the upper part of the sand column, and the number decreases with an increase in grouting rounds. A single large pore appeared near the edge of the lower part of the sand column during the first grouting process (Figure 9b), which may have been associated with the high local grout pressure during the first grouting round, inducing the formation of individual large pores at the edge. Under the action of MICP, the ratio of the pore volume to the surface area of the uranium tailing sand column at different time periods can quantitatively describe the degree of pore occupancy and elongation of its internal structure. These parameters are key indicators for evaluating the evolution of the internal pore structure and the overall degree of cementation of the sand column. By identifying and analyzing the porosity of the CT images, the pore structure characteristic parameters under different processing stages can be calculated, as shown in Table 6.  
An unconfined uniaxial compression test was conducted on each group of tailing samples, and the variation trend of the peak stress of the number of pores and volume under different grouting rounds of the uranium tailings was plotted, as shown in Figure 10. It can be concluded that with an increase in the number of grouting rounds, the change trends of the pore number and volume were opposite to those of the stress peak. From the  
Figure 9: Three-dimensional pore reconstruction and connected pore network model of uranium tailings under different grouting times; subfigure (**a**) 3D reconstruction images of porosity from CT scanning, subfigures (**b–f**) illustrated the pore network models under different grouting frequencies.