Structure and properties of lime alumino-borate glasses

The aim of our study is to investigate the structural effects of the substitution of aluminum by boron in lime alumino-borate glasses. In this case a competition between two network formers can be observed. It was proven that aluminum can be in four, five and six-fold coordination in calcium aluminate and aluminosilicate glasses, whereas boron can be in three and four-fold coordination in borate and borosilicate glasses. To understand what happens when boron replaces aluminum, different glasses were made starting with Ca-aluminate glass, C12Al14O33, i.e. C12A7 in cement nomenclature, and then progressively substituting aluminum by boron until B fully replaces Al. Raman, B-11 NMR and Al-27 NMR spectroscopies were used to investigate the structure of these glasses at medium and short range order respectively. Density, DSC measurements were also done in order to characterize properties at a macroscopic scale. NMR and Raman spectra show that aluminum in Ca-aluminate glass, C12A7, is in four-fold coordination, more precisely in T-3 and T-4 species (T-n, indicates aluminum in tetrahedral sites with n Bridging Oxygens). Whereas, when boron replaces aluminum, BO3 are the main species formed in good agreement with the two spectroscopic methods. These triangular units break the tetrahedral network already formed by AlO4- leading to an important drop of T-g. As the concentration of boron increases, NBOs are formed to create BO4 tetrahedral units, with the presence of calcium acting as a charge compensator. These tetrahedral structures start to increase the polymerization of the network. Glass transition temperature, Tg, continues to decrease but with a slower decline. In this study, we will present all these results and discuss the influence of the structure of the network on the macroscopic parameters. (C) 2016 Elsevier B.V. All rights reserved.