Mechanical behaviour and microstructure of granite residual bio-cemented soil by microbially induced calcite precipitation with different cementation-solution concentrations

Microbially induced calcite precipitation (MICP) stands as an environmentally friendly and promising technique for enhancing the performance of soil. Bacteria catalyze the hydrolysis of urea, prompting calcium ions to react with carbonate ions, ultimately forming calcium carbonate precipitation as a cement within soil grains. However, studies of using MICP to enhance granite residual soil (GRS) that is recognized as a problematic soil because of its wide grain size distribution are relatively rare. In this present study, bio-cemented GRS samples were prepared through grouting with Sporosarcina pasteurii as the colony and a mixture of urea and calcium chloride as the cementation solution. The effect of cementation-solution concentrations on the mechanical properties of the bio-cemented samples was analyzed through unconfined compression and triaxial shear tests. Furthermore, X-ray computerized tomography, scanning electron microscopy, and X-ray diffraction experiments were performed to reveal the mechanism of MICP from a microscopic perspective. The experimental results indicate that an optimal concentration of 2 mol/L achieved the highest level of cementation, resulting in an impressive 47.15% increase in the unconfined compressive strength of the GRS samples. The triaxial shear strength and stress paths of bio-cemented samples were affected by the cementation level. The variation of porosity indicated that CaCO3 precipitation improves soil densification by filling the macropores among the soil grains. The CaCO3 precipitates from the MICP treatment predominantly exist in the form of calcite crystals, serving to fill, wrap, and cement within the soil structure, thereby enhancing the cohesive and frictional forces exerted by the bio-cemented grains.