Decalcification and carbonation actions investigated during alkali activation-induced calcium (alumino) silicate hydrates C-(A-)S-H gel evolution

CO 2 capture through mineral carbonation extends the applicability of carbon sequestration by enabling storage in various cementitious materials derived from solid wastes. Calcium-containing cementitious materials receive extensive attention due to their potential for carbon capture, however, the underlying mechanisms both decalcification and carbonation during composition-structure evolution remain to be fully elucidated. Herein, a mineral carbonation strategy was reported to enhance carbon uptake performance, depending on the evolution stage of calcium (alumino) silicate hydrates (C-(A-)S-H) gel induced by alkali activation. Sequential dynamic monitoring and experimental characterizations confirmed that the decalcification and carbonation processes are closely linked to Ca-bonding states within the C-(A-)S-H structure. The results indicate that the short chains of alumino-silicate and the majority of the weakly bonded Ca phases are more prone to carbonation. Estimation of carbonation capacities also revealed that samples with 3h-activation exhibit the highest carbonation capacities (80.5 %), showing the higher carbonate potential with free Ca and weakly bound Ca (e.g. Ca(OH) 2 and interlay Ca in tobermorite-like structure) than samples with more intralayer Ca. Our work can provide new insights into the understanding of alkali activation induced decalcification and the improvement on carbonation capacities in early-formed C-(A-)S-H analogues.