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.