Carbonation behavior of calcium silicate hydrate (C-S-H): Its potential for CO2 capture

Calcium silicate hydrate (C-S-H), as the main component of cementitious materials, is susceptible to carbonation presenting a potential to capture CO2. Although some researches paid attentions to the carbonation behaviors of C-S-H given the durability of concrete structure, the kinetics of C-S-H carbonation as well as changes of C-S-H itself under carbonation have not been fully addressed. In this study, the carbonation kinetics of C-S-H with various calcium to silica ratio (Ca/Si) were investigated and the evolution of carbonation products in terms of compositions and micro-mechanical properties was quantitatively characterized by quantitative X-ray diffraction (QXRD), energy dispersive spectroscopy (EDS), and nuclear magnetic resonance (NMR). Three distinct periods are found in the course of C-S-H carbonation, encompassing dissolution period, diffusion period and slowly ongoing reaction period. A progressive decrease in Ca/Si and pH is observed in dissolution period with the formation of calcium carbonate, the reaction rate of which is strongly dependent on the initial Ca/Si of C-S-H. In the diffusion period, a small proportion of crosslinking occurs in C-S-H structure before producing a large amount of calcium modified silica gel, during when the Ca/Si and pH remain unchanged due to the incongruent dissolution of C-S-H as revealed by thermodynamic modellings. In the slowly ongoing reaction period, C-S-H decomposed completely and the mixture of microcrystalline calcium carbonate and calcium modified silica gel becomes the final product. The micro-mechanical property of C-S-H was significantly weakened by the change of phase assemblage. Our findings do not only advance our understandings on the deterioration of cementitious material under carbonation but also provides new insights into the CO2 capture using C-S-H widely existing in cementitious materials.