Quantitative Studies on the Microstructures of Ternary CaO–Al₂O₃–SiO₂ Glasses, Melts, and Correlation with Their High-Temperature Viscosities

Microstructure of CaO–SiO₂-based glassy samples with various Al₂O₃ contents was examined quantitatively by Raman spectroscopy and ²⁷Al MAS NMR. Sequence of multiple cluster models of aluminosilicate system modified with Ca²⁺ and Na⁺ cations has been designed, and Raman spectra simulation was carried out after geometric optimization by quantum chemistry (QC) ab initio calculation. The functional relationship between Raman scattering cross section (RSCS) and stress index of silicon-oxygen tetrahedron (SIT) for aluminosilicates was established, which was applied to the calibration of experimental Raman spectra. Some five-fold coordinated aluminum (Al^V, around 5%) and less than 2% six-fold coordinated aluminum (Al^VI) were detected by ²⁷Al MAS NMR, while most of aluminum remained in tetrahedral sites (Al^IV). The hyperfine quantitative results of Raman spectroscopy and NMR showed a gradual production of Al^IV with the addition of Al₂O₃, along with the significant adjustment of Q_i species distribution, in which Q₁, Q₂ reduced and fully polymerized Q₄ increased, while Q₃ showed a non-monotonic variation and obtained the maximum at Al₂O₃ = 18 mol%. Furthermore, the effects of aluminum to bridging oxygen bond types (T–O_b, T = Si, Al) and the degree of polymerization were also discussed in detail. The evolution of microstructure and its correlation with the viscosity of CaO–SiO₂-based melts, incorporating various Al₂O₃ additives, have been investigated by employing in situ high-temperature Raman spectroscopy at 1823 K and viscosity model. These structural features related to composition are essential theoretic foundation to understand their properties. The average Q_i evolution culminates in an overall enhancement of the degree of polymerization. Viscosity was determined utilizing a rigorously selected viscosity model, elucidating a consistent upward trajectory as Al₂O₃ content is incrementally added. Furthermore, a quantitative analysis of the relationship between viscosity and structure was conducted based on the average number of non-bridging oxygen per network-forming tetrahedron (NBO/T). It provides valuable insights for examining and predicting viscosity behavior of aluminosilicate systems.