Experimental investigation and thermodynamic assessment of the Na2O–Al2O3–CaO system

Thermodynamic investigation of the Na2O–Al2O3–CaO system plays an important role in the applications of HACs and refractory. However, the phase relationships in the Al2O3-rich corner of the isothermal section at 1473 K for this system are uncertain. Key experimental investigation was performed and an improved method was developed to prepare the samples. Four three-phase regions of NC3A8+CA2+β-Al2O3, CA2+CA6+β-Al2O3, CA6+β-Al2O3+Al2O3 and NC3A8+NA6+β-Al2O3 were confirmed by X-ray diffraction. The Na2O–Al2O3 system was reassessed by CALPHAD method and the Na2O–Al2O3–CaO system was optimized by using the ionic two-sublattice liquid model (Na+1,Al+3,Ca+2)P(AlO2−1,O−2)Q to describe the liquid phase and a sublattice compound energy formalism model (Na+1)2(Ca+2,Na+1)1(Al+3)4(O−2)8(Va,Na+1)1 to describe the ternary compound NCA2. A set of self-consistent thermodynamic parameters was finally obtained, satisfactorily reproducing the experimental data and predicting the liquidus projection and primary phase regions. The present thermodynamic investigation of the Na2O–Al2O3–CaO system contributed to compositional design in HACs and refractory.