Abstract The paper investigates the hydration of ternary blends comprising Portland cement, blast-furnace slag and metakaolin. The isothermal calorimetry and “in situ” X-ray diffraction was used to evaluate the effect of metakaolin and blast-furnace slag on the early hydration of blends. XRD, DTA and SEM were used to analyze the hardened paste after 1month of hydration. Metakaolin influences significantly the kinetics, mechanism and products of hydration at initial period of hydration causing the rapid formation of ettringite. Hemicarbonate aluminate hydrate is formed as a product of ettringite conversion due to the presence of reactive calcite in Portland cement.