Calcium alumino-silicates hydrates (C-A-S-H) carbonation kinetics

Proceeding page 3612 de "Conference Proceedings of the 15th ICCC, Prague, 16-20 September 2019"International audienceIn this study, we intended to characterize the impact of carbonation on the main cement hydrate (C A S H) in terms of chemistry and kinetics of degradation as well as to provide a better understanding on the carbonation products’ properties. We synthetized C-A-S-H with increasing calcium content, i.e. Ca/Si ratios ranging from 0.80 to 1.40 and Al/ Si ratios of 0.05 and 0.10. Based on thermogravimetric, 27Al and 29Si magic angle spinnining nuclear magnetic resonance and x-ray diffraction preliminary results, it was observed that C-A-S-H and C-S-H generated, after carbonation, the same major and minor calcium carbonates polymorphs (vaterite and aragonite, respectively) and the same amorphous product (silica gel). For the C-S-H, the silica gel embedded the fraction of calcium not transformed in calcium carbonate and, it was also the case for C-A-S-H silica gel which incorporates also aluminium. It was also found that coupling a high aluminium content with a high calcium content, i.e. Ca/Si higher than 0.95, calcium aluminates hydrates of pentahedral and octahedral coordination (third aluminate hydrate, TAH) are produced. The pentahedral product is located in the C-A-S-H interlayers and the TAH on its surfaces. The presence of those species correlated with a lower kinetic of degradation for the C A S Hs. A proper understanding of the mechanisms involved requires further studies but from these preliminary results, the hypothesis of a CO2 access to the C-A-S-H limited by the calcium aluminates with Al in pentahedral environment and TAH seems acceptable