A comparative ²⁹Si solid state NMR study of kinetics of the hydration reactions in cement pastes based on rapid-hardening ordinary Portland cement (SH) and on the mechanochemically modified cement (MSH) is presented. The mechanical activation of a cement/silica fume blend in a vibrating mill accelerates the hydration reactions by 15-20%, especially during the initial period of hardening. Variations in relative intensities of ²⁹Si resonances assigned to the hydration products in SH/MSH blends suggest different structures of hydrated SH nets. This can be correlated with a pronounced increase of the MSH-cement binding capacity reported earlier.