Kinetics and Mechanism of Ternesite Formation from Dicalcium Silicate and Calcium Sulfate Dihydrate

The kinetics and mechanism of ternesite formation (calcium sulfosilicate, Ca₅(SiO₄)₂SO₄, C₅S₂$) were investigated by studying the reaction between beta-dicalcium silicate (β-C₂S) and calcium sulfate dihydrate (CaSO₄∙2H₂O). Mineralogical composition development was monitored using X-ray diffraction (XRD) and backscattered scanning electron microscopy (BSEM) coupled to energy-dispersive X-ray spectroscopy (EDS). Ternesite can form in the 1100 to 1200 °C range by the solid-phase reaction of β-C₂S and CaSO₄. The formation of ternesite is favored by increasing the sintering temperature or extending the sintering time. The solid-phase reaction is carried out by diffusion of CaSO₄ to β-C₂S. The kinetics equation of ternesite is consistent with three-dimensional diffusion models (3-D model, D3 model or Jander model). The equation of the D3 model is 1 - 2α/3 - (1 - α)^2/3 = kt. On the basis of the Arrhenius equation, the activation energy of ternesite is 239.8 kJ/mol.