Effect of Calcium Carbonate Deposition Induced by Microorganisms and Plant Urease on Sand Reinforcement

Abstract

Soil solidification based on microbial mineralization is an environmentally friendly and sustainable technology. This study utilized microbial induced calcium carbonate precipitation (MICP) and urease-induced calcium carbonate precipitation (EICP) to solidify standard sand and silty sand. The physical and mechanical properties of the soil samples before and after solidification were tested, and the mechanisms of Sporosarcina pasteurii and urease-induced calcium carbonate solidification were analyzed. The results showed that the compressive strength of standard sand after MICP and EICP treatment was higher than that of silty sand. MICP treatment resulted in significantly higher compressive strength compared to EICP treatment. MICP formed a "skeleton" with calcium carbonate, enhancing shear strength and compressive strength but reducing permeability. EICP sealed the pores with calcium carbonate crystals, improving impermeability. The mechanical properties of solidified silty sand were worse due to particle shape and size, but it had better impermeability. During solidification, Sporosarcina pasteurii mainly stayed at the contact points between sand particles, with extracellular polymeric substances (EPS) containing negative ion groups. This enabled stronger adsorption capacity for calcium ions and facilitated the formation of "nucleation sites". Larger-sized, higher-strength calcium carbonate crystals were produced by MICP, aggregating at particle contact points. MICP treatment resulted in a sand microstructure resembling a "skeleton", enhancing shear strength, compressive strength, and permeability. In contrast, EICP directly used smaller-sized urease enzymes, which were more likely to be free in the pores. This caused the catalytically precipitated calcium carbonate to deposit between the pores, closing some of them and improving permeability. However, EICP often produced calcium carbonate in a disordered aggregate form with smaller size and brittle texture. The solidified samples were more brittle and prone to brittle failure. The research findings have certain guiding significance for sand soil solidification engineering.