Early age accelerated carbonation of cementitious materials surface in low CO2 concentration condition

Accelerated carbonation curing contributes to the carbon sequestration effect of cementitious materials and enhances surface density. However, in large-scale applications, traditional high CO2 concentration conditions (>5 % CO2) are constrained by economic costs. This study traces the early-stage accelerated carbonation process of cement mortar under low CO2 concentration (3 % CO2) condition from the perspectives of the evolution of CaCO3 crystal types, phase transformations, and microstructural changes, and compares the results with those under the 20 % CO2 condition. The results show that the carbonation depth of mortar specimens cured under the 3 % CO2 condition is shallower, but the cumulative concentration of CaCO3 within the surface 3 mm is not significantly lower than that under the 20 % CO2 condition; The carbonation zone generates more low-crystalline forms of CaCO3; The accelerated carbonation process does not affect the formation of CH and ettringite. Microscopic results indicate that under the 3 % CO2 condition, the synergistic effects of hydration and carbonation are better, and appropriately extending the curing time helps to refine the pore structure of the carbonation zone and improve the density of the substrate surface.