Mineral Carbonation of Red Mud Using CO2 Microbubbles under Ambient Conditions

This study investigated the mineral carbonation and neutralization behavior of red mud (RM) using CO<sub>2</sub> microbubbles (CO<sub>2</sub> MBs) under ambient temperature and pressure conditions, and further assessed the feasibility of utilizing carbonated RM as a cement substitute. Batch experiments were conducted at various solid-to-liquid ratios (S/L=0.001–1.0), monitoring pH, electrical conductivity (EC), and aqueous carbonic acid (H<sub>2</sub>CO<sub>3</sub>(<sub>aq</sub>)) concentrations. In the RM–CO<sub>2</sub> MBs system, pH initially dropped sharply and then recovered to the buffering zone (pH 7–8.5), while EC exhibited a rapid rise followed by gradual decline, indicating sequential ion release and carbonate precipitation. The H<sub>2</sub>CO<sub>3</sub>(<sub>aq</sub>) concentration decreased over time due to both carbonation consumption and pH-induced speciation shift. In continuous experiments (reactor dimensions: D=14.6cm, H=34 cm, S/L=0.025), both powdered (RM-P) and sludge-type (RM-S) samples achieved neutralization (pH=7) within 4 minutes, accompanied by a characteristic EC decrease–rebound pattern. The total inorganic carbon (TIC)-based CO<sub>2</sub> uptake of RM-S reached 8.87g-CO<sub>2</sub>/kg-RM, corresponding to approximately 84% of the theoretical maximum carbonation potential (TMCP). Mortar specimens incorporating carbonated RM as a partial cement replacement (0–15 wt%) exhibited decreasing compressive strength with increasing substitution ratio, yet 5 wt% replacement maintained adequate strength for non-structural construction materials. These results demonstrate that CO<sub>2</sub> MBs enable rapid (≤2 min), high-efficiency carbonation and neutralization of RM under ambient conditions without pressurized systems. The proposed process provides a low-energy, environmentally friendly pathway for simultaneous CO<sub>2</sub> sequestration and red mud valorization, contributing to sustainable carbon-neutral technology.