Hydration products of tricalcium silicate (C3S) are the calcium silicate hydrates (C-S-H) and portlandite. Silica fume, added to anhydrous cement in industrial formulations, reacts with portlandite and leads to C-S-H slightly different from the previous one (this reaction is called the pozzolanic reaction). C3S hydration at 120 degrees C With and without silica fume has been studied by two H-1 nuclear magnetic resonance techniques : combined rotation and multipulse spectroscopy (CRAMPS) and magic angle spinning (MAS). The static spectra are broadened by the proton-proton dipolar interaction. The H-1 MAS technique does not allow us to remove the effects of this interaction completely and cannot be in this case a quantitative method. Therefore the CRAMPS technique, which can remove the broadening of the interaction because of the use of a multipulse sequence associated with the sample rotation, was used. It is shown that the CRAMPS NMR spectra allow us to describe the action of silica fume on the hydrates and to reveal the competition between portlandite formation around the C3S grains and portlandite dissolution near the silica grains until the complete dissolution of portlandite.