Development of low-carbon cementitious materials via carbonating Portland cement-fly ash-magnesia blends under various curing scenarios: a comparative study

This study investigates a potential approach to produce low-carbon cementitious materials via using fly ash and reactive MgO as cement replacements, exposed to a pressurized CO2 curing regime. Pressurized CO2 with a concentration of 99.9% and pressures ranging from 0.1 MPa to 1.0 MPa were employed to accelerate the carbonation of MgO-fly ash-PC (Portland cement) blends in which up to 60% of PC was replaced with reactive MgO and fly ash. The carbon footprint of each cementitious material produced under various curing scenarios is evaluated based on life cycle assessment. Results indicate that after exposure to pressurized CO2, the cement pastes exhibited fast CO2 uptake, rapid compressive strength gain and microstructure densification due to the formation of carbonate products in terms of CaCO3 and/or (Ca, Mg)CO3 due to the incorporation of Mg2+ at the presence of MgO. The addition of fly ash facilitated carbonation, as it produced a more permeable microstructure for the ingress of CO2. A more rapid CO2 uptake and compressive strength development of the cement pastes was obtained as higher-pressure CO2 was used. In comparison to moist curing, pressurized CO2 curing yielded less carbon emissions. The CO2 emissions of the cement pastes decreased with increasing fly ash content as cement replacement but increased with the increase of MgO content from 20 to 40% by mass.