We report on the novel application of quasi-elastic neutronspectroscopy to study thereaction mechanism between tricalcium silicate and water. The insitu data taken at aseries of fixed temperatures varying from 10 to 40 °C show that theamount of free water inthe system remains relatively constant for an initial period, rangingfrom 1 h at 40 °C to 16h at 10 °C. This is followed by an exponential decrease in theamount of free water thatcrosses over to a diffusion-limited behavior for later times. Fitsto an Avrami type behaviorallow us to determine an activation energy for the hydration process onthe order of 30kJ/mol.