Carbonation induced phase evolution in alkali-activated slag/fly ash cements: The effect of silicate modulus of activators

In this study, the effects of silicate modulus of activators (Ms) on carbonation of alkali-activated slag/fly ash cements are studied and compared between natural (0.03–0.04% CO2) and accelerated (1% CO2) carbonation conditions, via XRD, FTIR and TG/DTG techniques. Carbonation results in the decalcification of the C-A-S-H gels and the formation of calcium carbonates and silica gels, while the N-A-S-H gels are essentially constant after carbonation. The main calcium carbonate phase after carbonation is calcite, and the increase of Ms leads to the generation of vaterite and aragonite. The carbonation behaviors of low-calcium system (20% slag) are different from the high-Ca systems (100% and 60% slag), while the carbonation rate and the amount of calcium carbonates generally decrease with the increase of Ms in all of the studied systems. This investigation provides an in-depth understanding of carbonation mechanisms of alkali-activated cements, and is essential for the establishment of carbonation models and prediction of long-term performances