Tricalcium silicate (C3S, C: CaO, S: SiO2), the main constituent of cement, reacts with water to form calcium silicate hydrates (C-S-H). Despite its practical importance, the kinetics of this reaction is not yet fully understood. Different mechanisms have been proposed, and NMR has proven to be successful in elucidating the structure of hydrates when the reaction is stopped or completed. However, a precise picture of the dynamics of silicate hydrates formation in-situ is still lacking and impedes a definite choice between the proposed models. In this presentation, we show that the carefully synthesized 29Si-enriched C3S allows following in-situ and quantitatively its hydration process in presence of aluminate ions by a combination 29Si NMR, 27Al NMR and calorimetry. First, in pure water, we obtain the transient local molecular composition of the hydrates at different stages of hydration. In particular, during the deceleration period the hydrate precipitation rate decreases faster than the amount of hydroxylated C3S surface, suggesting that the C3S is partially covered by C-S-H and that the surface area available for silicate dissolution decreases. Then we investigate the effect of aluminate ions on the hydration of C3S. This is an issue of increasing importance in low CO2 cements where clinker is replaced by supplementary cementitious materials. We show that a pH sensitive retardation of C3S hydration by aluminate ions occurs initially but that the amount of the hydrates formed increases later. Our experimental results can be interpreted assuming that aluminates hinder C3S dissolution. This view is supported by molecular dynamics simulations establishing that aluminates can adsorb on hydroxylated C3S through ionic interactions between aluminate and calcium ions, as well as through hydrogen bonding with silicate surface groups. This interaction is pH dependent and, consequently, the retardation effect of aluminates varies during the advancement of hydration. These results are of interest to various fields beyond cement science such as geochemistry and environmental sciences.

Tricalcium silicate hydration in absence and presence of aluminate ions: A step towards a molecular understanding of how the hydration rate of cement depends on its formulation

253rd National Meeting (2017)

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