# Microbial Precipitation of Calcium Carbonate for Crack Healing and Stabilization of Sandy Soils
## 5 Conclusions  
In this study, the effect of bacterial metabolites and montmorillonite on the surface crack healing and bioccmentation of sandy soils was evaluated to improve the efficiency of the MICP process. Crack healing efficiency rates of about 71% and 80% were observed via CaCO\({}_{3}\) precipitation promoted by living microorganisms (C5 + B) and microbial metabolites (CS + BM), respectively. The precipitation of CaCO\({}_{3}\) induced by microbes and their metabolites indicated that the healing of shrinkage cracks in sandy soils can occur in both microbial-growth-conducive and extreme environments. The addition of MMT to sandy soils for the MICP process also resulted in changes to the soil texture and increased its compressive strength. However, the effects of 20% and 30% of MMT on the strength of the sandy soil were similar, but at levels higher than 30% of MMT, desiccation cracking of clay occurred, indicating that the stability of the cementation effect may be weaker in the long term. The large surface area and adsorption properties of MMT were found to contribute to the improvement of MICP efficiency via the adsorption of microorganisms and Ca ions and the promotion of microbial activity and mineralization. Ca-rich MMT acted as a Ca source through the leaching of Ca ions from the mineral interlayers. In this study, the mixing ratio of MMT with sand was simplified to clarify its influence and characteristics, but for field applications, it will be necessary to adjust the mixing ratio of MMT more precisely based on the purpose of the MICP process and the site environment. In addition, further investigations on the roles of the constituents and organic molecules in the metabolites of _S. pasteurii_ in precipitating calcium carbonate will be required to clarify the mechanism. Overall, the addition of biological additives and/or montmorillonite to the MICP process can be effective for sandy soils that require the healing of desiccation cracks, reduced permeability, and greater strength, as well as for environments where the spread of contaminants is a concern. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **This Study** & **[PERSON] et al. (2021) [(37)]** & **[PERSON] et al. (2021) [(22)]** & **[PERSON] et al. (2017) [(1)]** \\ \hline \multirow{2}{*}{Used microbes} & _Sporosarcina pasteurii_ & _Strains of Sporosarcina_ & _Sporosarcina pasteurii_ & \multirow{2}{*}{_Bacillus pasteurii_} \\  & (KCTC 3558) & _genera_ & (ATCC11859) & \\ \hline \multirow{2}{*}{Urea} & 330 mM & 500 mM & 30.03 g & 0.05–0.15 mol/L \\ \hline \multirow{2}{*}{CaCl\({}_{2}\)} & 50 mM & 500 mM & 55.5 g & (Ca\({}^{2+}\)/urea) \\ \hline \multirow{2}{*}{Temperature} & 30 \({}^{\circ}\)C & 32 \(\pm\) 3 \({}^{\circ}\)C & 25 \({}^{\circ}\)C & 30 \({}^{\circ}\)C \\ \hline \multirow{2}{*}{Soil} & sand & Sand, loamy sand & Sand & Uranium tailings \\ \hline \multirow{2}{*}{Additives} & Montmorillonite & - & Na-montmorillonite & Montmorillonite \\ \hline \multirow{3}{*}{Metabolite} & Bacterial suspension, & & & \\  & soluble microbial & - & Bacterial suspension & \\  & products & & & \\ \hline \multirow{2}{*}{Treatment method} & Mixing & Spraying & - & Injection of \\  & & & & mineralizing liquid \\ \hline \multirow{2}{*}{Treatment cycles} & 1 & 1–3 & 1 & 1 \\ \hline \multirow{2}{*}{Minerals formed} & Calcite & Calcite & Calcite & Calcite, vaterite \\ \hline \multirow{2}{*}{Soil compressive strength} & \multirow{2}{*}{0.24–3.7 Mpa} & \multirow{2}{*}{1.67–5.3 Mpa} & \multirow{2}{*}{-} & 1.2–2.18 Mpa \\  & & & & \\ \end{tabular}
\end{table} 
Table 2: Bioccmentation studies of sandy soil mediated by microorganisms.