Formation and transition of sodium calcium silicate compounds in the presence of alumina based on solid-state reaction

Abstract

In order to synthesize sodium calcium silicate compounds in the low calcium system, the thermodynamics, kinetics, and transition mechanism of sodium calcium silicate compounds in the presence of Al2O3 based on the solid-state reaction were systemically studied by XRD, Raman, TG-DSC, and SEM methods using analytical reagents of Na2CO3, CaCO3, SiO2, and Al2O3. The results show that the thermodynamic stability of Na2CaSiO4 and Na2Ca2Si2O7 is better than other sodium calcium silicate compounds in the Na2O-CaO-Al2O3-SiO2 quaternary system when the temperature is above 860 °C. Na2Ca2Si2O7 and NaAlO2 form at the beginning via the direct reactions between the initial reactants, and then Na2Ca2Si2O7 reacts with NaAlO2 and Na2SiO3 to convert into Na2CaSiO4 and Na1.75Al1.75Si0.25O4. The sintering reactions reach an equilibrium when the temperature is above 1050 °C, and the stable phases mainly include Na2CaSiO4, NaAlO2, and Na1.75Al1.75Si0.25O4. Furthermore, the direct phase formation reactions occur in the temperature range of 730–903 °C, while the phase transition reactions occur above 903 °C. The formation kinetics based on the Na2O-CaO-Al2O3-SiO2 system during the sintering process corresponds to the F2 model and A1 model in the temperature ranges of 730–842 °C and 842–903 °C, respectively.