# The Regulation of Thermodynamic Behavior and Structure of Aluminosilicate Glasses via the Mixed Alkaline Earth Effect

## TL;DR

This study explores how changing the ratio of CaO and MgO affects the structure and properties of aluminosilicate glasses.

## Contribution

The study reveals the non-monotonic structural changes and thermodynamic behavior due to the mixed alkaline earth effect in aluminosilicate glasses.

## Key findings

- Replacing MgO with CaO increases non-bridging oxygen, leading to glass depolymerization.
- Q4 units in the [SiO4] tetrahedron show a minimum at R = 0.7.
- Thermal stability and mechanical properties deviate from linearity with changing R.

## Abstract

This work systematically altered the molar ratio of CaO and MgO (R = [CaO]/[(CaO + MgO)], mol%) to elucidate the underlying mechanisms driving the observed changes in macroscopic properties. The results indicated that as CaO increasingly replaced MgO, the rise in the content of non-bridging oxygen led to the depolymerization of the glass structure. A quantitative analysis of Qn units in the [SiO4] tetrahedron using 29Si MAS NMR revealed that a non-monotonic variation appeared when the Q4 unit reached a minimum at R = 0.7. Meanwhile, the chemical environment of aluminum also varies with the R, and the presence of high-coordinated aluminum species is observed when Ca2+ and Mg2+ ions coexist. In terms of overall performance, both density and molar volume exhibited a linear trend. However, thermal stability, viscosity, characteristic temperatures (including melting temperature, Littleton softening temperature, working point temperature, and glass transition temperature), and mechanical properties showed deviations from linearity. Additionally, four non-isothermal thermodynamics was employed to quantitatively assess the thermal stability of samples C-0.7 and C-1. The insights gained from this study will aid in the development of advanced glass materials with tailored properties for industrial applications.