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

The hydration of tricalcium silicate powder with H 2 O and D 2 O 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 D 2 O followed the same reaction path as pastes made with H 2 O, with a reaction rate that was reduced by a factor of ∼2.7 at a given temperature. The activation energy for hydration with D 2 O is slightly higher than for H 2 O. It is proposed that the slower reaction kinetics and higher activation energy in the presence of D 2 O 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.