Formation characteristics of sodium calcium silicate compounds based on the solid-state reaction

Abstract

The formation thermodynamics, phase transition and stability of sodium calcium silicate compounds under different calcination parameters in the Na2O–CaO–SiO2 system were studied using XRD, FTIR and SEM-EDS methods. As the Na2O/SiO2 ratio increases from 0.3 to 0.7 when the CaO/SiO2 ratio is 1.0, the formation sequence of sodium calcium silicate compounds is Na2Ca3Si2O8→Na6Ca3Si6O18→Na2Ca2Si2O7→Na2CaSiO4; as the CaO/SiO2 ratio increases from 0.3 to 1.2 when the Na2O/SiO2 ratio is 0.5, the formation sequence is Na6Ca3Si6O18→Na2Ca2Si2O7→Na2Ca3Si2O8. As the most stable sodium calcium silicate compound, Na6Ca3Si6O18 forms by the solid-state reaction of preformed Na2SiO3 with CaO and SiO2, while increasing the calcination temperature and holding time can promote its crystal stability. The decomposition of Na6Ca3Si6O18 in sodium aluminate solution follows the mixed control of the film diffusion and chemical reaction, and the corresponding activation energy is between 40 and 41 kJ/mol.