Effect of SO3 on formation and hydraulic reactivity of belite

SO₃ accelerates the formation of belite between 1100 and 1300°C. Its effeciency rests on the permanent generation of CaO in statu nascendi by the equilibria $\text{CaSO}{}_4\text{?}\text{CaO + SO}{}_3\text{and SO}{}_3\text{?}\text{SO}{}_2\text{+}\text{1}\text{2}\text{O}{}_2$. Via the gaseous phase this CaO is transported to SiO₂ and reacts there to belite. Only 0.5 – 1.5%SO ₃ are dissolved in belite stabilizing its β-modification, but without increasing the hydraulic reactivity markedly.