Carbonation Enhancement of C3S2 by Carbonic Anhydrase-Producing Bacteria: Performance, Microstructure, and Mechanism.

CO2 mineralization of low-calcium silicate minerals is a promising approach for reducing carbon emissions from construction materials, but the carbonation of rankinite (3CaO·2SiO2, C3S2) is still restricted by slow reaction kinetics and the additional energy demand of conventional accelerated carbonation. In this study, carbonic anhydrase (CA)-producing bacteria were introduced to enhance the carbonation curing of C3S2, and their effects on mechanical performance, CO2 sequestration capacity, phase evolution, pore structure, and microstructure were systematically investigated. The results showed that CA-producing bacteria markedly promoted early carbonation and improved the overall performance of C3S2 specimens. At a dosage of 2.0%, which was the highest investigated in this study, the 7-day compressive strength reached 51.7 MPa, and the CO2 sequestration efficiency increased to 19.68 wt.%, which were 2.5 times and 40.7% higher than those of the blank group, respectively. XRD and FT-IR results showed that bacterial incorporation changed the polymorph distribution of CaCO3, with increased relative contents of aragonite and vaterite. X-CT, MIP, nanoindentation, and SEM observations further revealed a greater carbonation depth, a refined pore structure, and more uniform CaCO3 precipitation within the matrix. These improvements are mainly associated with CA-catalyzed CO2 hydration, together with the nucleation and pore-filling effects of bacterial cells and associated solid carriers. This work provides a bio-assisted strategy for improving the carbonation activation of C3S2 and developing low-carbon cementitious materials.