The early hydration of three cementing materials has been investigated in situ by Synchrotron X-ray Tomographic Microscopy thanks to an automatic sample exchanger recently integrated at the TOMCAT beamline at the Swiss Light Source in Villigen (CH). Hydration has been investigated by following the three dimensional evolution of pastes up to 12 h, with particular attention to the consumption of anhydrous phases, the formation of hydrates and to changes in microstructure. A lecture key for the interpretation of the tomographic images is proposed, based on the relationship between the linear attenuation coefficient μ-function of the density and elemental composition of each mineral phase- and the corresponding grey tone experimentally observed. Some examples of application of the technique are proposed, such as: a comparison among the reaction kinetics of the three investigated cements, the study of morphological evolution of mineral phases, the analyses of the three-dimensional evolution of the pore network.