Hydration of Portland cement (Type I) pastes as a function of particle size (< 70 mu m, < 90 mu m) was studied by stray-field magnetic resonance imaging (STRAFI-MRI) and hydrogen maps, from different types of water (capillary, gel or chemically bound water), enabled to follow the corresponding spatially-resolved kinetics. A Portland cement with a particle size < 45 mu m, but containing additives, was also studied for comparison.