STRUCTURE AND PROPERTIES OF CALCIUM ALUMINOSILICATE GLASSES

The structural role of aluminum in alkali silicate glasses has been a question of interest for some time. In spite of this interest, the question remains an unsettled one. The corresponding alkaline earth glasses have been much less studied, although it is probable that an understanding of the similarities and dissimilarities of these two families of glasses will provide some illumination of the structure and behavior of the alkali silicates. Here, a step is taken in this direction with a systematic study of the properties and spectra of a series of calcium aluminosilicates (Al2O3: 0-39 mol%; SiO2: 10-75 mol%; CaO: 13-65 mol%) as functions of composition. Results of measurements of density, refractive index, and microstructure are presented. No abrupt changes are observed in density or refractive index as functions of the molar ratio of Al2O3 to CaO. This may mean that aluminum ions are four-fold coordinated only in calcium aluminosilicate glasses. Minima in the molar volume of oxygen appear at lower silica content as the ratio of Al2O3 to CaO increases. TEM studies showed phase separation. FTIR spectra showed that the amount of non-bridging oxygens increased with the amount of excell lime. Again, no apparent evidence was found for the existence of six-fold coordinated aluminum ions.