The proton free induction decay of a portland cement paste in an advanced stage of hydration can be roughly divided into three main components: (1) a component with a very short spin‐spin relaxation time, T2, representing the protons of the solid OH groups and the water of crystallization, (2) a component with an intermediate T, representing the bonded water in the gel phase, and (3) a third component with a relatively long T2representing the water in the micropores and layers. The dependences of the intensities, T2's, and spin‐lattice relaxation times (T1's) of these three components on the cement hardening time have been determined. The proton spin‐lattice relaxation time of the “solid” component increases with hardening time whereas T1decreases for the other two components. The observed time dependence of the diffusion coefficient, D, of water in a tricalcium silicate paste supports the findings of the above correlat