Magnesia carbonation can be adopted as a soil solidification technology for geotechnical engineering. Recent studies have shown that urea decomposition under the catalyzation of ureolytic bacteria can provide a carbon source for magnesia carbonation. Although many related studies have been reported, the mechanical behaviour of the magnesia solidified soil, especially its durability and long-term performance, still require further deep investigations. Besides, the use of plant urease instead of bacteria for magnesia carbonation is also of research interest and requires further studies. In this study, we used crude soybean urease for the catalyzation of urea decomposition in order to provide carbon source for magnesia carbonation (soybean urease intensified magnesia carbonation, SIMC). The mechanical behaviour and durability of SIMC solidified soil under drying-wetting and soaking conditions in acid rain solution were investigated. For SIMC samples, the addition of urea and urease as internal carbon sources led to a much higher strength compared with those without them. The optimum urea concentration was 2 mol/L, and higher concentrations could have negative impact on the strength. As for magnesia, the highest strengths were obtained when the addition was 8 %. During the drying-wetting cycles and soaking tests with acid rain water, there was a generally moderate decreasing trend in strength for the SIMC samples with more drying-wetting cycles or soaking durations. However, the strength reduction ratio, which was defined as the long-term strength in acid environment to that in neutral environment, was much higher compared to the PC samples, implying a much stronger resistance to acid rain water. The mineralogical analysis revealed that hydrated magnesium carbonates were the major effective cementing materials.

In this study, enzyme-induced carbonate precipitation (EICP) combined with chitosan curing technology was used to improve the mechanical properties of standard sand, and the curing effect of EICP combined with different chitosan contents was studied by macroscopic tests, such as the unconfined compressive strength test, direct shear test, and calcium carbonate content test, and microscopic tests, such as scanning electron microscope (SEM) and nuclear magnetic resonance (NMR). The results show that compared with the pure EICP treatment, the unconfined compressive strength, shear strength, and calcium carbonate content of the sand treated by EICP combined with chitosan were significantly improved, and increased first and then decreased with the increase of chitosan content, reaching the maximum value when the content is 1.5%. The calcium carbonate content is positively correlated with the strength, indicating that calcium carbonate crystals can effectively play a role in filling and cementation. After the incorporation of chitosan, the shape of calcium carbonate crystals is still mainly spherical, but the number and volume become larger. At the same time, the incorporation of chitosan can greatly reduce the proportion of large pores and medium pores, significantly increasing the proportion of small pores, which greatly improves the pore structure.