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.

This study proposed an improved bio-carbonation of reactive magnesia cement (RMC) method for dredged sludge stabilization using the urea pre-hydrolysis strategy. Based on unconfined compression strength (UCS), pickling-drainage, and scanning electron microscopy (SEM) tests, the effects of prehydrolysis duration (T), urease activity (UA) and curing age (CA) on the mechanical properties and microstructural characteristics of bio-carbonized samples were systematically investigated and analyzed. The results demonstrated that the proposed method could significantly enhance urea hydrolysis and RMC bio-carbonation to achieve efficient stabilization of dredged sludge with 80% high water content. A significant strength increment of up to about 1063.36 kPa was obtained for the bio-carbonized samples after just 7 d of curing, which was 2.64 times higher than that of the 28-day cured ordinary Portland cement-reinforced samples. Both elevated T and UA could notably increase urea utilization ratio and carbonate ion yield, but the resulting surge in supersaturation also affected the precipitation patterns of hydrated magnesia carbonates (HMCs), which weakened the cementation effect of HMCs on soil particles and further inhibited strength enhancement of bio-carbonized samples. The optimum formula was determined to be the case of T = 24 h and UA = 10 U/mL for dredged sludge stabilization. A 7-day CA was enough for bio-carbonized samples to obtain stable strength, albeit slightly affected by UA. The benefits of high efficiency and water stability presented the potential of this method in achieving dredged sludge stabilization and resource utilization. This investigation provides informative ideas and valuable insights on implementing advanced bio-geotechnical techniques to achieve efficient stabilization of soft soil, such as dredged sludge. (c) 2024 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).