International Summit on Cement Hydration Kinetics

Jeffrey Thomas

Northwestern University

It is well established that the early hydration of cement and C₃S proceeds by nucleation and growth, but application of the widely used Avrami model to kinetic data is problematic. The boundary nucleation and growth (BNG) model provides a better physical depiction of hydration, because it accounts for the fact that nucleation is restricted to the surface of the cement particles. Application of the BNG model to kinetic data for C₃S hydrated with and without CaCl₂ acceleration provides valuable insights.  While the growth rate of hydration product is similar in the two cases, the nucleation rate is greatly increased by CaCl₂. CaCl₂ also significantly increases the total amount of early hydration.  A concept that will be introduced and discussed is that of a reaction volume, defined as the total volume available for early nucleation and growth of hydration product. A review of the effects of CaCl₂, w/c, and powder surface area on the amount of early hydration suggests that the reaction volume extends a fixed distance away from the particle surfaces. The reaction volume can be effectively increased by adding pure C-S-H to the paste at the time of mixing to seed the hydration process, providing nuclei in the pore space away from the particles. This results in a paste with much less capillary porosity. A simple digital model of the hydration process is able to reproduce the effects of the C-S-H seed with surprising accuracy.