In the present paper a new accelerating additive for cement mixtures, based on transition metal silicate hydrates,is described.The new additive is highly effective in promoting the nucleation and growth of silicate hydrates of portland cement.The additive contributes to developing high early strength and improving the microstructure of hydrated cement paste and represents a new development in the field of concrete admixtures.The structure and characteristics of the new additive were investigated by XRPD and 29Si-NMR.Hydration kinetics of cement pastes containing different dosages of the new admixture were studied by XRPD and compared to equivalent admixture-free cement pastes.The data confirmed that the new admixture significantly increases the C3S dissolution and the C-S-H formation rates and reduces the induction period.Concrete tests confirmed a beneficial effect of the admixture on early strength development.The precipitation of C-S-H induced by the copper silicate hydrates in the capillary porosity refines the microstructure of the cement paste, as demonstrated by SEM micrographs, and reduces the water permeability and the durability of concrete structures.