Low energy synthesis of cement compounds in molten salt

The world cement industry is responsible for .> 5% of the total anthropogenic carbon dioxide emissions blamed for causing global warming. The production of cement clinker minerals by precipitation from a molten salt solvent offers a potential route to energy reduction in cement manufacture. Molten salt synthesis of the major cement compounds b-dicalcium silicate (β-Ca₂SiO₄, β-C₂S) and tricalcium silicate (Ca₃SiO₅, C₃S) has been attempted in fused sodium chloride (NaCl). The synthesis of b-Ca₂SiO₄ was carried out by the reaction of CaCO₃ with SiO₂ in molten NaCl (CaCO₃-SiO₂-NaCl mole ratios 2 : 1 : 19?2 at 908uC, and 2 : 1 : 20?4, 2 : 1 : 13?5, 2 : 1 : 10?3 and 2 : 1 : 8 at 1140°C). The product was characterised by powder X-ray diffraction, Raman scattering and scanning electronmicroscopy. For the synthesis of Ca ₃SiO₅, reactants with a molar ratio of 3 : 1 were used (CaCO₃-SiO₂-NaCl 3 : 1 : 19?8 at 908 and 1000uC, and 3: 1 : 20, 3 : 1 :14, 3:1:9?9 and 3 : 1 : 8?1 at 1140°C). In all cases β-Ca₂SiO₄ was the principal product, with the CaO phase still present and (if any) only small quantities of Ca₃SiO ₅. These observations, coupled with previous studies on the solid state synthesis of Ca₃SiO₅, indicate that β-Ca ₂SiO₄ is an intermediate compound, requiring in excess of 1140°C to react with CaO in order to form tricalcium silicate.