CO₂ micro-nanobubbles-enhanced carbonation with reactive magnesia for rapid and sustainable stabilization of soft soils

Traditional methods for soft soil stabilization are commonly challenged by high-carbon emissions and slow early-strength development. Although the carbonation between reactive magnesia (MgO) and CO₂ offers a low-carbon alternative, its gaseous reaction pathway suffers from slow CO₂ diffusion, easy escape, and non-uniform stabilization. Inspired by the excellent mass-transfer efficiency and buffering capacity of micro-nanobubbles (MNBs), this study proposes a liquid-phase carbonation method using CO₂-MNBs together with reactive MgO, with the objective of evaluating its effectiveness for soil stabilization. The physicochemical properties of CO₂-MNBs were first characterized and compared with conventional CO₂ bubbles (CO₂-Bs). Unconfined compressive strength tests were then performed on stabilized soils treated with these solutions at different gas–liquid ratios. The results show that CO₂-MNBs significantly promotes early-strength development in carbonated stabilized soil. Within less than 3 days of curing, the CO₂-MNBs stabilized soil at a gas–liquid ratio of 0.4 achieves the 28-day design strength of cement-stabilized soil. This early-age strength is 7.2 times greater than that achieved with conventional CO₂-Bs over the same age. The strength continued to increase with curing time, showing more significant gains at higher gas–liquid ratios. Microstructural analysis revealed that the rapid densification within 3 days is attributed to the formation of hydration products such as brucite and carbonation products including hydrated carbonates, which effectively bonded soil particles and filled pores. The method presented here offers an efficient, low-carbon, and sustainable alternative to traditional cement-based soil stabilization, demonstrating significant potential for large-scale application in carbon-conscious geotechnical engineering.