Abstract The addition of CaCO3 and CaSO4 to Portland cement clinker influences the hydration and the strength development. An increase of the CaSO4 content accelerates alite reaction during the first days and results in the formation of more ettringite, thus in a higher early compressive strength. The late compressive strength is decreased in Portland cements containing higher quantities of CaSO4. The reduced late compressive strength seems to be related to an increase of the S/Si and Ca/Si content in the C–S–H. The presence of calcite leads to the formation of hemicarbonate and monocarbonate thus indirectly to more ettringite. Only a relatively small quantity of calcite reacts to form monocarbonate or hemicarbonate in Portland cement. Although hemicarbonate is thermodynamically less stable than monocarbonate, hemicarbonate formation is kinetically favored. Monocarbonate is present only after 1week and longer independent of the quantity of calcite available and the content of sulphate in the cement.