Low energy synthesis of cement compounds in molten salt

Abstract

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 β-dicalcium silicate (β-Ca₂SiO₄, β-C₂S) and tricalcium silicate (Ca₃SiO₅, C₃S) has been attempted in fused sodium chloride (NaCl). The synthesis of β-Ca₂SiO₄ was carried out by the reaction of CaCO₃ with SiO₂ in molten NaCl (CaCO₃-SiO₂-NaCl mole ratios 2∶1∶19·2 at 908°C, 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 electron microscopy. For the synthesis of Ca₃SiO₅, reactants with a molar ratio of 3∶1 were used (CaCO₃–SiO₂–NaCl 3∶1∶19·8 at 908 and 1000°C, 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 Ca₃SiO₅. These observations, coupled with previous studies on the solid state synthesis of Ca₃SiO₅, indicate that β-Ca₂SiO₄ is an intermediate compound, requiring in excess of 1140°C to react with CaO in order to form tricalcium silicate.