## 3 Experimental Results and Analysis
### Microanalysis Results for the MICP-Treated Soil
#### 3.5.2 XRD Analysis of the Soil Composition  
We conducted XRD experiments on MICP-treated soil and control group samples with ages of 7, 14, and 28 days using the Advance X-ray diffractometer (D8 advance; Bruker AXS, Madison, WI, USA). Figure 8 and Table 2 summarize the results. Dolomite and calcite are present in the soil in the form of calcium carbonate, and the sum of their contents is considered the total soil calcium carbonate content. The results obtained from XRD should be regarded as semi-quantitative references due to the inherent limitations in precision. Future research will incorporate chemical analysis to achieve more accurate and reliable assessments. Figure 7: SEM images of soil samples of different ages: (**a**) 7 days; (**b**) 28 days. The amount of precipitated calcium carbonate can reflect the effectiveness of MICP reinforcement. This paper uses the acid washing method to measure the amount of precipitated calcium carbonate. The specific steps are as follows:  
1. Take the sample that failed in the unconfined compressive strength test and dry it to a constant weight (recorded as M1);
2. Immerse and dissolve the sample using an excess of hydrochloric acid with a concentration of 1 mol/L. Stir until no bubbles escape from the soil. Then, rinse multiple times with an excess of deionized water and dry it in an oven;
3. Repeat the immersion step until no bubbles are produced, and dry it to a constant weight (recorded as M2). The calcium carbonate content in the sample is calculated using the following formula: \[C_{\text{CaCO}_{3}}=\frac{M_{1}-M_{2}}{M_{2}}\times 100\%\] (1)  
Based on the CaCl\({}_{2}\) concentration (1 mol/L), the Ca\({}^{2+}\) mass in the cementation liquid, corresponding to a per soil mass unit, is 5 g. In comparing the soil masses after different periods (Figure 9), after 14 days, the fixed Ca\({}^{2+}\) content in the soil was 6.2 g/kg, which was higher than the content in the cementation liquid. Thus, microbes continuously induced calcium ions to generate calcium carbonate or magnesium calcium carbonate, utilizing both Ca\({}^{2+}\) from the added cementation liquid and the original dissociative Ca\({}^{2+}\) in the soil. Consequently, the final generation amount was notably larger than the amount of Ca\({}^{2+}\) mixed in the cementation liquid. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Category** & **Chemical** & \multicolumn{2}{c}{**0 Days (Control Group)**} & \multicolumn{1}{c}{**7 Days**} & \multicolumn{1}{c}{**14 Days**} & \multicolumn{1}{c}{**28 Days**} \\ \hline Quartz & SiO\({}_{2}\) & 17.3 & 15.7 & 17.2 & 17.2 \\ Anorthite & Ca (AlSi\({}_{2}\)O\({}_{8}\)) & 0.4 & 0.5 & 0.7 & 0.6 \\ Microcline & K (AlSi\({}_{3}\)O\({}_{8}\)) & 29.6 & 33.5 & 29 & 29.1 \\ Pegmatite & KAl\({}_{2}\) (AlSi\({}_{3}\)O\({}_{10}\)) (OH)\({}_{2}\) & 47.3 & 43.8 & 45.6 & 46.1 \\ Dolomite & CaMg (CO\({}_{3}\))\({}_{2}\) & 1.6 & 1.9 & 2.3 & 1.8 \\ Calcite & Ca (CO\({}_{3}\)) & 1.7 & 2.8 & 2.9 & 3.3 \\ Others & / & 2.1 & 1.7 & 2.2 & 1.9 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Mass fractions for the composition of the MICP-treated soil. Figure 8: XRD patterns of the treated samples with different ages. The reinforcement effect is influenced not only by the concentration of CaCl\({}_{2}\) but also by the concentration of bacteria. [PERSON] et al. (2007) [32] found that there is a relationship between the concentration of bacteria and the morphology of calcium carbonate crystals. They found that at high bacterial concentrations, most calcium carbonate crystals were spherical, whereas at low bacterial concentrations, the crystals were predominantly regular rectangular parallelepipeds or agglomerates. Due to the limitations regarding this paper's scope, future research could explore this factor in greater detail. As shown in Figure 9, the experimental results indicate that the total calcium carbonate content in soil changes with the cultivation period.