The properties of cementitious materials are related to the microstructure of their binder matrix, which forms by a sequence of dissolution-precipitation reactions converting the clinker phases into hydrates, among which the main hydration product is calcium silicate hydrate (C-S-H).Here, we monitor non-invasively the evolution of the cement paste microstructure during hydration by "pencil beam" synchrotron X-ray diffraction-enhanced computed micro-tomography (XRD-CT).This innovative technique yields phase-resolved images of the hydrating cement paste at different stages, which are combined to map the sites where dissolution and precipitation processes occur.The results indicate that the nucleation mechanism of C-S-H changes when cement hydrates in the presence of poly-carboxylate ether (PCE) superplasticizers, which are commonly used to control the paste rheology.The observed change is essential to understand the development of the cement microstructure and to provide a direct link between the reaction kinetics and the physico-mechanical properties of the system.