# Microbial Precipitation of Calcium Carbonate for Crack Healing and Stabilization of Sandy Soils
## 4 Discussion
### Combination of MMT and MICP for the Stabilization of Sandy Soil  
The cementation of sandy soil by MICP has been proven by many previous researchers, and Table 2 shows examples of recently reported biocementation studies mediated by microorganisms. [PERSON] et al. (2021) reported that high urea and calcium concentrations can improve the biocementation of sandy soil by increasing calcium carbonate precipitation [37]. On the other hand, this study showed that adding MMT to sandy soil was effective in stabilizing sandy soil by improving the MICP efficiency even at a low calcium concentration and simplified treatment. The effect of the combination of MICP and MMT is summarized in Figure 9. Because the bacterial cell walls, which are composed of peptidoglycan rich in  
Figure 8: XRD patterns of soil cakes formed through MICP treatment on sand (MMT 0%) and sand with various levels of MMT (10–30%).  
carboxylate groups, are negatively charged, Ca\({}^{2+}\) is attracted and serves as a nucleation site for CaCO\({}_{3}\)[38; 39]. MMT, which has excellent adsorption capacity, adsorbs these bacterial cells and promotes CaCO\({}_{3}\) nucleation and mineral crystal growth. This mechanism has been demonstrated in a previous experiment conducted by [PERSON] et al. (2021) [22]. In addition, the biofilm produced by the microorganisms interacted with MMT to form an organo-clay, leading to net contraction along the 001 lattice plane of MMT. It is known that the basal contraction of Ca-MMT can occur even when Na is substituted for Ca in a chemically Na-rich environment [18; 40]. However, in the present study, the 001 lattice plane of MMT tended to contract from 1.505 to 1.270 nm as the MMT content increased, even though the concentration of NaCl in the CS was the same. Therefore, it appears that the increase in the MMT content promoted the growth and activity of microorganisms and increased the production of biofilm and its substitution into MMT [1; 20; 21]. As an example of combining MICP and MMT for the treatment of contaminated soil, [PERSON] et al. (2017) cemented uranium tailings by adding different amounts of MMT to an MICP process, finding that MMT contributed to reducing the toxicity of uranium ions for microorganisms [1]. When the MMT content was 6%, the maximum strength of the cement body reached 2.18 MPa, representing a 47.66% increase compared to the case without MMT. These results indicate that adding MMT to the MICP process for soil stabilization can contribute to CaCO\({}_{3}\) precipitation and contaminant fixation by promoting microbial activity and mineral nucleation. Therefore, this combined process is likely to be effective not only for soils that require lower permeability and greater strength, but also for soils contaminated with pollutants.  
Nevertheless, the addition of MMT may inhibit the permeability of the solution for the biomineralization of calcium carbonate in soil. Therefore, the effects shown in these results can only be expected be positive when applied to sandy soil, where it is necessary to reduce the size of pores and lower permeability for stabilization. Additionally, efficiency can be increased if the solution for the biomineralization of calcium carbonate can be mixed with soil particles instead of pouring it during the process. However, if the MMT addition ratio is too high, internal cracks may occur due to the agglomeration of sand grains, so the addition amount needs to be adjusted according to the physical characteristics of the soil in the field.  
Figure 9: Schematic diagram of the effect of adding MMT to sandy soil on MICP and soil stabilization: acting as a support by filling the pores between sand grains and inducing calcium carbonate precipitation due to the attraction and adsorption of microbial cells and calcium ions.