Microstructure refinement of calcium-sulfate-aluminate and portland cement (CSA-PC) hybrids with accelerated CO2 curing (ACC)

Calcium-sulfate-aluminate and Portland cement (CSA-PC) hybrids with balanced engineering properties would be a preferable solution to mitigate the increasingly raised CO2 emissions by PC. Meanwhile, CSA hydrates are rather active to carbonation, therefore, it is urgent to clarify how accelerated CO2 curing (ACC) impacts the microstructure and engineering properties of CSA-PC hybrids. Herein, an ACC scheme with the CO2 gas pressure of 0.5 MPa and duration of 24 h was designed to treat hybrid CSA-PC pastes and mortars. Mechanical properties and capillary absorption of the ACC-treated CSA-PC hybrids were evaluated with profound analysis of microstructure and mineral characteristics by SEM, MIP, Nitrogen adsorption, XRD and TG/DTG. Results show that the ACC treatment can systematically increase compressive strength by up to 65.2 % and depress capillary absorption rate by up to 72.2 %, respectively, for the optimal CSA-PC mixes. Rapid carbonation of the cement clinkers and hydrates, e.g., calcium hydroxide, ettringite, and ye'elimite, results in precipitation of CaCO3 that refines the pore structure and improves the material compactness. 25 % replacement of PC with CSA shows the highest strength gain to CO2 uptake ratio (SGCUR) of 1.05. The findings deepen the mechanistic understandings in microstructure refinement of ACC-treated CSA-PC hybrids with lower CO2 emissions.