# Microbial Precipitation of Calcium Carbonate for Crack Healing and Stabilization of Sandy Soils
## 3 Results
### Stabilization of Sandy Soils Using MMT and MICP
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**No.**} & \multirow{2}{*}{**Samples**} & \multicolumn{2}{c}{**Calcium Carbonate in**} & \multicolumn{2}{c}{**MMT (001)**} & \multicolumn{2}{c}{**Compressive Strength**} \\ \cline{3-6}  & & \multicolumn{2}{c}{**S**} & \multicolumn{2}{c}{**\%**} & \multicolumn{2}{c}{**2 Theta**} & \multicolumn{2}{c}{**d-Spacing (nm)**} & \multicolumn{2}{c}{**MPa**} \\ \hline
1 & Sand & 0.014 & 1.428 & - & - & - \\
2 & MMT 10\% & 0.018 & 1.774 & 6.81 & 1.296 & 0.24 \\
3 & MMT 20\% & 0.022 & 2.232 & 6.90 & 1.278 & 3.33 (\(\pm\)0.4) \\
4 & MMT 30\% & 0.027 & 2.691 & 6.95 & 1.270 & 3.53 (\(\pm\)0.2) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Mineralogical and physical properties of soil cakes formed using MICP treatment. Figure 5: Soil compressive strength of soil cakes formed through MICP treatment on sand (MMT 0%) and sand with various levels of MMT (10–30%)This indicated that the dissolved Ca was precipitated by MICP (Figure S1). In a growth medium with a high ionic strength, the Ca between MMT layers can leach out and be replaced with Na, while the growth of microorganisms can produce small organic molecules, such as EPS, which can be inserted between the MMT layers [31]. Therefore, the Ca-MMT present in the soil during the MICP treatment could act as a potential Ca source and contribute to the precipitation of CaCO\({}_{3}\). sand grains were filled with MMT particles, with the extent of this filling increasing with a higher MMT content (Figure 6). The MMT was supported by the sand grains, and the CaCO\({}_{3}\) that formed during the MICP process contributed to the stabilization of the sand by acting as a bridge between the particles. However, in the soil cake with 30% of MMT, the sand and MMT particles were compressed more densely than in the soil cake with 20% of MMT, leading to agglomeration and cracks between the clumps. Therefore, when the MMT content increases, the durability of the soil cake is weakened because shrinkage cracks occur when soils with high clay contents are dried. The width of the shrinkage cracks observed in the micrographs varied between 50 and 250 \(\upmu\)m (Figure 6F). Figure 6: Microscopic observation at low (10\(\times\)) and high (40\(\times\)) magnification of soil cake cross-sections formed through MICP treatment on sand (MMT 0%) and sand with various levels of MMT (10–30%): (**A,B**) MMT 10%, (**C,D**) MMT 20%, and (**E,F**) MMT 30%. Figure 7 shows the morphological characteristics of the particles in the soil cake observed using SEM. In the sample with 10% of MMT, the large particles (about 600 \(\upmu\)m in diameter) were mainly quartz (Si and O), with MMT covering their surfaces (Figure 7A). Under a high magnification, the MMT on the quartz surface consisted of flaky particles primarily containing Si, Al, and O with trace amounts of Fe and Ca. With 10% of MMT, it was observed that the quartz grains and the surface-covering MMT particles were separated (Figure 7B). However, in the sample with 30% of MMT, particle agglomeration with MMT was widely observed, and the quartz particles were covered entirely with MMT. Figure 7C presents the agglomerated MMT particles, while Figure 7D shows calcite crystals partially coated by MMT. An EDS analysis confirmed the presence of calcite based on the high Ca content. The precipitation of CaCO\({}_{3}\) in the soil samples was investigated using XRD patterns (Figure 8). The sandy soil was mainly composed of quartz, feldspar, and mica, and as the MMT addition increased from 10% to 30%, the peak intensity of MMT also increased. The position of the MMT diffraction peak shifted from a 20 of 5.9\({}^{\circ}\) to 6.9\({}^{\circ}\), and the d-spacing (001) decreased from 1.505 to 1.270 nm as the amount of MMT increased (Figure 8 and Table 1). The MMT d-spacing (001) remained at 1.505 nm in distilled water over 24 h but increased to 1.780 nm in the microbial growth medium containing organic matter such as glucose.