Synchrotron energy-dispersive diffraction has been employed to monitor the hydration of CaAl2O4. Diffraction intensities of neat pastes, with a water-to-solid ratio (w/s) of 0.4, were employed to follow the hydration progress at temperatures ranging from 60 degrees C to 90 degrees C. The hydration rate of CaAl2O4 increased with temperature according to a three-dimensional diffusion-controlled model expressed by [1-(1-alpha)(1/3)](2) = kt, with reaction constants k ranging from 7.4 x 10(-1) min(-1) at 70 degrees C to 3.2 x 10(-2) min(-1) at 90 degrees C. The activation energy was 84 kJ mol(-1) K-1. The onset of Ca3Al2O6.6H(2)O diffraction peaks from CaAl2O4, at 90 degrees C, was within 3 min whereas at 60 degrees C it was after 8 min. The results show that Ca2Al2O5.8H(2)O is the intermediate in the hydration of CaAl2O4 to Ca3Al2O6.6H(2)O, in all cases. The hydration is explained in solid-state kinetics terms, Ca2Al2O5.8H(2)O playing a key role in CaAl2O4 hydration for triggering the hydration to form Ca3Al2O6.6H(2)O.