The 3D phase distribution of cement pastes evolves during hydration and controls the rheology and mechanical properties of the paste. Synchrotron powder-diffraction micro-tomographic imaging is here employed to assess the cement phase spatial distribution in a totally non-invasive way. This technique can be used to produce distribution maps of the phases present in the hydrating cement paste. The method is applied to an ordinary Portland cement, hydrated in pure water or in the presence of nucleation seeds. The quantitative description of the phase spatial distribution by radial distribution functions allows the discrimination of different nucleation mechanisms.