Low-carbon and highly efficient sand solidification based on the combined bio-carbonation of reactive magnesia and enzymatically induced carbonate precipitation

There is an increasing demand for innovative low-carbon alternatives to effectively improve soil properties to promote sustainability and achieve carbon neutrality. However, both the bio-carbonation of reactive magnesia cement (RMC) and enzymatically induced carbonate precipitation (EICP) had limitations, including inadequate strength and solidification inhomogeneity despite demonstrated effective for sand solidification. Therefore, the combination bio-carbonation of RMC and EICP was proposed to address their respective drawbacks. In addition to the combined treatment, other treatment methods (e.g., pure RMC hydration, bio-carbonation, and EICP) were also utilized to compare treatment effects under different treatment conditions (e.g., varying RMC contents, urea concentrations, and treatment cycles). Results showed that the combined treatment could effectively address the issue of insufficient precipitation resulting from low RMC concentrations or excessive CO₂ levels, thereby both reducing the permeability of treated sand and enhancing its strength to improve the overall treatment efficacy. With one treatment cycle, the combined treated sample with 20 % RMC and 3 M urea concentration exhibited a higher strength, while the sample with 15 % RMC had better solidification effects after two treatment cycles. Compared to the bio-carbonation treatment, the combined treatment resulted in higher proportions of artinite, while obtaining lower proportions of nesquehonite, demonstrating an influence of calcium addition on the mineralogy of magnesium precipitates. The combined treatment can achieve both strength enhancement and homogenization of solidification as a low-carbon and highly efficient solidification method, showcasing significant application potential in geotechnical engineering and material engineering fields.