When exposed to carbonation, Na2CO3-activated slag (SCAS) concrete demonstrated 53% mass loss and 57% reduction in compressive strength due to the decalcification of C-(A)-S-H. Improvements in the carbonation resistance of SCAS mixes via the inclusion of hydromagnesite seeds (S) and reactive MgO (M) were reported in this study. The conversion of MgO into hydrated magnesium carbonates (HMCs) and the additional formation of hydrotalcite were observed in the presence of these additives. HMCs contributed to the binding network and limited the diffusion of CO2 into the sample. The decalcification of C-(A)-S-H was retarded via the absorption of CO2 in hydrotalcite, producing huntite. These changes in the reaction kinetics of samples involving M and S enabled the retention of hydration products and formation of additional carbonation products, leading to denser microstructures and ~40% increase in compressive strength after carbonation, which was ~10 times higher than the control sample (58 vs. 6 MPa).
- hydration product