Effect of CO₂ concentration on high-temperature carbon fixation and pozzolanic activity of high-calcium fly ash

To simultaneously capture CO₂ emitted from coal-fired power plants and enhance the pozzolanic activity of high-calcium fly ash, this study investigates the carbon fixation behavior of high-calcium fly ash through dry carbonation under various atmospheric conditions. The influence of carbon fixation on the pozzolanic activity of the fly ash is also evaluated. The atmospheres used for dry carbonation include air, simulated flue gas, a mixture of O₁C₄(V_O2∶V_CO2=1∶4), and pure CO₂. The physical and chemical characteristics of high-calcium fly ash before and after carbonation were analyzed using XRD, FTIR, N₂ adsorption, and SEM–EDS. Carbon fixation performance was assessed through thermogravimetric analysis (TG). The pozzolanic activity was evaluated by measuring the compressive strength of cement-hardened pastes incorporating carbonated fly ash at different curing ages. The results show that the carbon fixation rates under flue gas, the O₁C₄ mixture, and pure CO₂ atmospheres were 7.63%, 8.25%, and 11.03%, respectively. The corresponding contributions of unit CO₂ to carbon fixation (i.e., the ratio of fixation rate to CO₂ content in the atmosphere) were 0.44, 0.10, and 0.11, indicating that high-calcium fly ash exhibits high dry carbonation efficiency, with optimal performance under flue gas conditions. XRD results revealed stronger CaCO₃ diffraction peaks in the carbonated S_Fg, S_O1C4, and S_C samples, while SEM images showed irregular block-shaped calcite particles adhered to the ash surface. The carbonated fly ash participated in cement hydration, contributing to improved volumetric stability. Following carbonation in flue gas, O₁C₄, and pure CO₂ atmospheres, the 7-day compressive strength of fly ash-cement hardened pastes increased by 6.4%, 48.5%, and 72.5%, respectively, compared to uncarbonated samples. Similarly, 28-d compressive strength increased by 7.6%, 13.8%, and 44.7%, respectively. These results demonstrate that carbonation significantly enhances the pozzolanic activity of high-calcium fly ash, likely due to the formation of monocarbonate hydrates from the reaction of CaCO₃ with hydrated calcium aluminate. This stabilizes ettringite and results in a denser pore structure, reducing porosity and improving compressive strength. Based on carbon fixation efficiency and compressive strength development, carbonation under a pure CO₂ atmosphere yielded the most favorable results. It is therefore recommended to increase CO₂ content during carbonation to enhance the pozzolanic activity of high-calcium fly ash, supporting its use as a cementitious material capable of capturing CO₂ from flue gas while delivering high pozzolanic activity.