Upon the absence of alkalies, increasing sulfate contents in belite-rich cement clinkers induce a significantly higher belite content which is associated with a decrease in the alite content and, consequently, a reduction in the compressive strengths after 2 days of the resulting cements. The compressive strengths of the cements after 28 and 90 days are markedly raised by the addition of sulfate and this effect is enhanced by rapid clinker cooling. In the belite-rich cement clinker the major portion of SO 3 is incorporated in the belite as main clinker phase and occurs in the form of sulfate phases, mainly CaSO 4. Stabilization of the high-temperature forms α and α′ of belite cannot be achieved by SO 3 contents up to 1.5% ★) ★) Unless otherwise stated, percentages are by mass in belite-rich cement clinker, corresponding to 0.6 to 0.8% SO 3 in the belite phase. On the contrary, a lattice-disturbed sulfate-bearing β-phase of improved hydraulic activity results. At the presence of alkalies (sulfatizing degree 80–100), alkali sulfates are formed with K 2SO 4 being considerably more stable than Na 2SO 4 both in portland cement clinker and in belite-rich cement clinker. Na 2O is preferably incorporated both in the belite phase which is associated with the stabilization of high-temperature forms α and α′ and in the aluminate phase resulting in the formation of the orthorhombic modification. The addition of alkali oxides up toa maximum of 1.0% with a sulfatizing degree between 80 and 100, has positive effects on the strength development of the resulting cements.