High-resolution solid-state Si-29 nuclear magnetic resonance (NMR) characterization of hydrated C3S provides a means of selectively observing the hydrogen-containing phases. In particular, by applying the cross-polarization (CP) technique, we demonstrated that a contrast can be obtained, not only for hydrated against anhydrous phases, but also for hydrogen-hydrated against deuterium-hydrated phases. Selectively deuterated samples were thus prepared by a H2O/D2O alternated hydration procedure. The progress of the polycondensation of orthosilicates as derived by H2O was followed, demonstrating that a separate microphase of anhydrous C3S prevailingly reacted at any step. An induction period, showing a low reactivity and mainly formation of Q0 hydrated was observed during the first hydration steps of selectively deuterated samples. The efficiency of the masking effect of D2O was also indirectly proven by comparison with calorimetric measurements.