Influence of Silica Fume and Polycarboxylate Ether Dispersant on Hydration Mechanisms of Cement

Partial replacement of ordinary portland cement by silica fume (SF) accelerates its rate of hydration reactions. This acceleration is attributed to the enhanced heterogeneous nucleation of the main hydration product, i.e., calcium-silicate-hydrate (C-S-H), on the extra surfaces provided by SF. However, such enhancement of C-S-H nucleation is suppressed in the presence of polycarboxylate ether (PCE) dispersant, which is added to regulate the fluidity and rheological properties of fresh paste. A generalized phase boundary nucleation and growth model with time-dependent growth of C-S-H is used to fit the hydration rates of plain and binary (10% to 30% SF) cement pastes prepared with and without PCE. The results show that while SF accelerates cement hydration, increments in hydration rates are significantly smaller in relation to the extra surface area provided by SF. This is because of the agglomeration of SF particles which renders up to 96% of their surface area unavailable for C-S-H nucleation. Furthermore, it is shown that the hydration of cement, in both plain and binary pastes, is suppressed in relation to the PCE dosage. This is because of (a) adsorption of PCE molecules onto cement and SF surfaces resulting in inhibition of sites for product nucleation and (b) interaction of PCE with C-S-H, which suppresses growth of C-S-H throughout the hydration process. It is shown that the effects of nucleation site inhibition by PCE are more pronounced in SF as compared to cement. The outcomes of this study improve our understanding of the mechanisms that drive the hydration of cement in the presence of SF and PCEs.