Thermodynamic description of the Na2O─CaO─SiO2 system and its application to solidification and heat treatment design

The Na₂O─CaO─SiO₂ ternary system, which is a very complex pseudo-ternary system, is a fundamental system widely used as glasses and glass–ceramics. Its thermodynamic description is crucial for understanding solidification and subsequent heat treatments. In this work, a thermodynamic assessment of the Na₂O─CaO─SiO₂ system was conducted by applying the CALPHAD method. The liquid phase in the Na₂O─CaO─SiO₂ system was successfully described by the two-sublattice partially ionic liquid model (Na⁺¹,Ca⁺²)_P(O⁻²,SiO₄⁻⁴,SiO₂)_Q and two new sublattice models of (Na⁺¹,Ca⁺²)₂(Na⁺¹,Va)₁(Si⁺⁴)₁(O⁻²)₄ and (Na⁺¹,Ca⁺²)₂^M1(Va,Na⁺¹)₃^M2(Ca⁺²,Na⁺¹)₃^M3(Ca⁺²)₁^M4(Si⁺⁴)₆(O⁻²)₁₈ with accurate cation distributions in their sublattices were developed to respectively describe the ternary solid solution phases βN1C1S1 and Combeite in this work. The other ternary compounds were treated as stoichiometric ones due to their limited solid solubilities. Based on the reliable experimental data, a set of self-consistent thermodynamic parameters was obtained. The calculated isothermal sections, vertical sections, liquidus projection, and thermodynamic properties can satisfactorily reproduce the experimental data within the experimental errors. In addition, the present thermodynamic description contributes to the reliable prediction for the melting temperatures and primary crystallization phases for the Na₂O─CaO─SiO₂ glasses, providing valuable insights for the design of glass and glass–ceramics compositions and heat treatment processes.