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.