Carbonation curing for wollastonite-Portland cementitious materials: CO₂ sequestration potential and feasibility assessment

Carbonation curing to produce building products could provide a promising route to energy-efficient cementation process and economic CO2 sequestration. The potential benefit from material selection and optimization should be explored to optimize the environmental performance of CO2-cured materials. In this study, the accelerated mineral carbonation of wollastonite-Portland cement (WPC), a low carbon binder, is investigated by combining a detailed study on its CO2 mineralization capacity and on the physicochemical evolution in microstructure. Up to 25 wt.% of natural wollastonite is employed to replace the CO2-intensive ordinary Portland cement. During the carbonation curing process, WPC pastes exhibited CO2 uptakes up to 20 wt.% under moderate pressures (≤2.5 MPa). Also, a dense structure with substantially high polymerization degree and fine pores was clearly discerned in cured WPC paste. The pore-creating effect from evaporated pore water and the porosity-filling effect from carbonated calcium silicates were found to dominate the microstructure in the early-stage and mid-late stage reactions, respectively. Results also revealed the positive impacts of adding the wollastonite mineral: (i) The diluted effect of wollastonite enhanced the pore-creating effect in the early-stage; (ii) The consumption of wollastonite and formation of Ca-modified silica gel primarily happened in the mid-late stage, which helped increasing the degree of polymerization. Associated with the structural evolution, the cured WPC pastes exhibited superior compressive strength (over 80 MPa) after the carbonation curing: the maximum increment could exceed 350%. These results demonstrate the feasibility of CO2 mineralization of WPC to produce green building material without compromising mechanical performance.