# Microbial Precipitation of Calcium Carbonate for Crack Healing and Stabilization of Sandy Soils
## 3 Results
### Stabilization of Sandy Soils Using MMT and MICP  
In general, the more silt and clay in a soil, the stronger it is. For this reason, it was expected that the addition of small MMT particles would improve the strength of the sandy soil samples in the present study during the MICP process by filling the pores of the sand and acting as a support. It was observed that the surface pores of the sandy soil samples containing MMT (10-30%) were filled with the precipitate formed via MICP, with the MMT levels affecting the efficiency of sand cementation. In particular, the resulting soil cake  
Figure 3: Ratio of crack to (**A**) sandy soil surface area and (**B**) crack reduction rate according to the spray treatment cycles of different solutions. Figure 2: Changes in the pattern of surface cracks of sandy soil according to the spray treatment cycle for each solution: (W) water, (CS) cementation solution, (B) bacterial culture, (BM) bacterial metabolites. had heights of 4.5 cm, 5.0 cm, and 6.0 cm for MMT mixing ratios of 10%, 20%, and 30%, respectively (Figure 4). Without MMT, the surface of the sand appeared to be firmly bonded after the MICP treatment, but the shape of the cake collapsed when removed from the cylindrical container, indicating that the sand grains were not cemented evenly. The hardness of the cemented sandy soil samples was measured using a pocket cone penetrometer, with the compressive strength of the soil cake found to be about 0.2 MPa with 10% of MMT and about 3.3 (\(\pm\)0.4) MPa at 20% and 30% of MMT. The strength of the sand was also improved with the addition of only MMT and CS (i.e., no microbes), but not to the same extent as with MICP. With 10% of MMT, the strength of the sand cake was 0.05 MPa, which was 20.4% of that observed with MICP. Similarly, with 20% and 30% of MMT, the strength rose to 0.15 MPa and 0.68 MPa, but these values only represent 4.6% and 19.3% increases, respectively, of the MICP-induced strength (Figure 5). The amount of CaCO\({}_{3}\) precipitated in the presence of MMT increased by approximately 1.8-fold compared to sand alone (Table 1). In particular, 0.014 g of CaCO\({}_{3}\) was found in 1 g  
Figure 4: Soil cases formed through MICP treatment on sand (MMT 0%) and sand with various levels of MMT (10–30%): (**A**,**B**) MMT 0%, (**C**,**D) MMT 10%, (**E**,**F) MMT 20%, and (**G**,**H) MMT 30%. of the 100% sand sample, compared with 0.018 g, 0.022 g, and 0.027 g in the 10%, 20%, and 30% MMT soil samples, respectively. The increase in CaCO\({}_{3}\) precipitation with a higher MMT content is important for the enhancement of the MICP, in which two mechanisms may be involved. In the first, the microbial activity and urea degradation rate may have been enhanced by MMT. Previous studies have compared the effects of various soil properties on MICP, finding that the urea degradation of microorganisms is promoted by the presence of MMT [30]. In addition, in the second mechanism, the Ca ions present between the MMT layers may have leached out during the reaction process, contributing to the precipitation of CaCO\({}_{3}\). Ca leaching from 2 g of MMT in 40 mL of pure water, the microbial growth medium, or the growth medium containing bacteria was thus investigated. It was found that 36.6 mg/L of Ca was leached from distilled water after 7 d, in addition to large amounts of Na. In the growth medium, the amount of leached Ca was 214 mg/L, compared to only 4.78 mg/L in the growth medium with urea and microorganisms. MMT can also fill the spaces between sand grains and become cemented during MICP treatment. Because the water-soluble organic matter on the microbial cell membrane and that on the surface of the MMT particles are negatively charged, the formation of CaCO\({}_{3}\) may have been promoted via the chelation of Ca\({}^{2+}\)[1; 32]. In a cross section of the sand cake observed under a stereoscopic microscope, it was found that the spaces between the