Elucidating the Effect of Water-To-Cement Ratio on the Hydration Mechanisms of Cement.

Abstract

The hydration of cement is often modeled as a phase boundary nucleation and growth (pBNG) process. Classical pBNG models, based on the use of isotropic and constant growth rate of the main hydrate, that is, calcium–silicate–hydrate (C–S–H), are unable to explain the lack of any significant effect of the water-to-cement (w/c) ratio on the hydration kinetics of cement. This paper presents a modified form of the pBNG model, in which the anisotropic growth of C–S–H is allowed to vary in relation to the nonlinear evolution of its supersaturation in solution. Results show that once the supercritical C–S–H nuclei form, their growth remains confined within a region in proximity to the cement particles. This is hypothesized to be a manifestation of the sedimentation of cement particles, which imposes a space constraint for C–S–H growth. In pastes wherein the sedimentation of cement particles is disrupted, the hydration kinetics are no longer unresponsive to changes in w/c. Unlike C–S–H, the ions in solution are not confined, and hence, the supersaturation-dependent growth rate of C–S–H diminishes monotonically with increasing w/c. Overall, the outcomes of this work highlight important aspects that need to be considered in employing pBNG models for simulating hydration of cement-based systems.