Using in situ X-ray diffraction in conjunction with the Rietveld method, we have studied the dynamics of phase formation in the early stages of hardening of mechanically activated blended binders based on Portland cement and mineral admixtures: granulated magnesia-ferriferous slag and nepheline concentrate. The results demonstrate accelerated clinker hydration in the composition of the blended cements compared to admixture-free Portland cement, which correlates with the compressive strength of the samples.