Alternative batch compositions in the glass-forming region of the Na2O–CaO–SiO2 system

An alternative new method of pre-reacted batches, without free silica, is proposed in the glass-forming region of the system Na2O–CaO–SiO2. Compositions were prepared by solid state reaction, as pellets and as a loose powders. The batches were treated with successive heating in the 400–900 °C interval, just before the liquid phase is obtained. The evolution of the phase formation was followed by X-ray diffraction (XRD) and the liquid phase was observed by polarized light optical microscopy. It has been found that the use of pellets favors the presence of Na2Ca(CO3)2, which enhances the formation of several ternary phases and avoids the formation of free calcium oxide. Three mixtures were selected on the basis that they showed the lowest free silica content, therefore being considered the best candidates to be prepared using raw materials supplied from the glass industry. The best compositions were localized in the low silica content, since the liquid phase appears above 1100 °C.