Mechanical activation of cement blended with metallurgical wastes (blast furnace slag and silica fume) in a high energy vibrating mill was investigated. The results show that the mechanical activation of the solids led to a reduction of the ground product fineness and an effective dispersion/interaction of ultra-fine silica particles within cement particles, resulting in an increased rate of the hydration reactions of the slag cement. The hardened paste samples after activation possess higher bending and compressive strengths. Fourier transform infrared spectroscopy and nuclear magnetic resonance studies indicate the mechanism of the mechanical activation on the slag cement.