The thermal decomposition of sodium carbonate and calcium carbonate in molten sodium chloride and the molten salt synthesis (MSS) of sodium silicate and calcium silicate.

It is well-known that the global average surface temperature of the earth has risen by ~0.8°C during the 20th century. This can be attributed to an increase in quantities of greenhouse gasses (GHG's) in the atmosphere, arising from anthropogenic sources and changing land use. This temperature rise has been linked to major catastrophes, such as hurricanes, heat waves, floods, droughts, evaporation of lakes, rising sea levels and melting of ice glaciers. One of the main contributors to GHG emissions is energy-intensive industries (EIIs), such as the cement process. This process emits between 0.65-0.92 kg tonne-1 of carbon dioxide (CO2) of cement and accounts for 5% of global CO2 emissions annually. This is mainly due to the high temperatures required to achieve its process conditions (~1500°C), emitting CO2 directly (from limestone decomposition) or indirectly (through electricity usage). One method of reducing such emissions could be molten salt synthesis (MSS), which involves dissolving reactants in a molten salt and reacting in solution. MSS has proven to be an alternative route to many compounds; therefore in our project, we investigated the synthesis of the cementitious compounds; calcium metasilicate (Ca2SiO4) and sodium metasilicate (Na2SiO3) in sodium chloride (NaCl). Our results suggested a-Ca2SiO4 and a-Na2SiO3 could be produced at 830°C, however other compounds such as Ca3SiO5 required higher temperatures (>1100°C). The dissolution of the reactants; silicon dioxide (SiO2), calcium carbonate (CaCO3) and sodium carbonate (Na2CO3) in molten NaCl were also investigated at 830°C and our results suggested that Na2CO3 and CaCO3 decomposed to CO2, calcium oxide (CaO) and sodium oxide (Na2O), and SiO2 only dissolved with a limited solubility. This suggests that molten salts could be suitable media to reducing CO2 emissions from such processes and hence improve the overall energy requirement. These MSS reactions were also depicted on predominance diagrams, to illustrate how these compounds could be produced using electrolytic methods.