A novel approach for improving aqueous carbonation kinetics with CO2 micro- and nano-bubbles

The kinetics of carbonation change dramatically depending on the mode of gas–liquid–solid interactions. While prior studies have highlighted the significance of improving liquid–solid interaction by transferring carbonation from a gaseous environment to an aqueous environment, addressing the gas–liquid mass transfer limitation is becoming vital for further accelerating carbonation. Therefore, in this study, micro- and nano-bubbles (MNBs) were used as a novel accelerator for recycled cement paste (RCP) carbonation. The rate of calcium carbonate (Cc) precipitation, the CO 2 utilization efficiency, the composition and morphology of the carbonated RCP , and the changes in solution chemistry were examined and compared to carbonation using normal bubbles. The results indicated that the high surface-to-volume ratio and long lifetime of MNBs contributed to a rapid mass transfer of gaseous CO 2 to aqueous carbonate species. It created stronger attractions between the negatively charged MNBs and the calcium ions, which remarkably enhanced RCP dissolution and Cc precipitation. Meanwhile, the huge amount of MNBs and the significant inter-particle collision changed Cc nucleation behaviour and resulted in RCP breakage. These small changes in CO 2 -water-RCP interaction induced complete differences in the carbonation kinetics and RCP microstructures, where it was found that (i) carbonation efficiency was improved by ∼ 5 times, (ii) the CO 2 utilization efficiency was improved by ∼ 3.5 times, reaching 82.2 %, (iii) carbonation products had a more open microstructure with 72.2 % higher porosity, and an ultra-fine RCP with the specific surface area of 135 m 2 /g was produced. Generally, the use of CO 2 MNBs breaks through the rate limitation of aqueous carbonation. It contributes to the fundamental understanding of the control over carbonation kinetics and carbonation products.