Effects of D2O and Mixing on the Early Hydration Kinetics of Tricalcium Silicate

The hydration of tricalcium silicate powder with H2O and D2O was followed at three different temperatures using isothermal calorimetry. For both types of paste, the early kinetics could be accurately modeled, from the time of mixing to a point on the downslope of the main hydration peak, with a four-parameter nucleation and growth model. Pastes made with D2O followed the same reaction path as pastes made with H2O, with a reaction rate that was reduced by a factor of ∼2.7 at a given temperature. The activation energy for hydration with D2O is slightly higher than for H2O. It is proposed that the slower reaction kinetics and higher activation energy in the presence of D2O result from a kinetic isotope effect, whereby the increased mass of the hydrogen atom lowers the probability of the forward reaction at the atomic level.