Effect of Mixed Alkali on Structure and Properties of R₂O–CaO–Al₂O₃–SiO₂ Glass

This study was to investigate the influence of mixed alkali effect on the structure and properties of a R₂O–CaO–Al₂O₃–SiO₂ glass system, to provide theoretical support and practical guidance for optimizing the formulation of high–level radioactive liquid waste (HLW) glass vitrification technology. The safe disposal of HLW is a critical aspect for the sustainable development of the nuclear industry. The Joule-heated melter vitrification as an internationally engineered and implemented method could immobilize radioactive waste via melting HLW with base glass frit in an electric furnace to form a stable vitrified product for long-term containment.

Glass samples with varying Z-values (i.e., Z = n(Na₂O)/(n(Na₂O) + n(Li₂O)) were prepared by a high-temperature melting method. Their structure and properties were systematically analyzed by infrared spectroscopy, Raman spectroscopy, and high-temperature resistivity analysis.

The structural characterization revealed that the silicate tetrahedral units Q² and Q³ are the primary structural units influencing glass properties. The structure exhibits regular changes as Z-value increases. At Z=0.2, the proportions of Q³ and Q⁴ reach the maximum values, while Q¹ and Q² reach the minimum values. At Z=0.6, Q¹, Q³, and Q⁴ proportions reach the maximum values, while Q² reaches the minimum value, indicating the most compact glass structure and the most significant mixed-alkali effect.

The results of performance tests demonstrate that the physical and thermal properties both are governed due to the mixed–alkali effect The density and thermal expansion coefficient both increase with increasing Z-value. The density reaches the maximum value as Z=0.6, while the thermal expansion coefficient has the minimum value. The glass transition temperature (T_g) initially decreases and then increases, reaching its minimum as Z=0.2. The high-temperature viscosity has the maximum value as Z=0.4 and the minimum value as Z=0.6.

For the electrical properties in single-alkali systems, the resistivity at the melting temperature (T₂) decreases in an order of K₂O > Na₂O > Li₂O, while it at the forming temperature (T₄) decreases in an order of K₂O > Li₂O > Na₂O. In the mixed-alkali system, the resistivity reaches the maximum value at Z of 0.6–0.8, being correlated to the "blocking effect" of alkali ions. The activation energy for electrical conduction changes nonlinearly with Z-value. In single-alkali systems, it increases in the order of Na₂O < Li₂O < K₂O.

This study clarified the mixed-alkali effect on the R₂O–CaO–Al₂O₃–SiO₂ glasses, particularly highlighting the low viscosity as Z = 0.6 and the high resistivity characteristics as Z = 0.6–0.8. These findings could provide a clear guidance for glass formulation design.