<p>A large group of industrial and municipal waste are glassy in nature. One of the main directions of their use is thermal treatment, which aims to melt them (solid waste) or sinter (powdered waste). Fine grained glass cullet as well as silica fly ash was the subject of our research. During heating of glassy powders, high sintering dynamics is observed along with the accompanying glass devitrification process. Our objective in this research was to partially or entirely decouple the simultaneous occurrence of glass crystallization reaction and the sintering process during the heat treatment of glassy wastes. We divided an experiment onto two parts, first we compared the effects of thermal treatment on glass in the form of: pressed powder and a bulk glass. In the second experiment the influence of the powder sample density on the progress of glass crystallization was analysed. Results of our study indicate that physical sintering of glassy powders can lower the devitrification temperature of this material. However, physical sintering is accompanied by shrinkage, which when high density is achieved can limit the expansive growth process of grains formed during devitrification.</p> Graphical abstract <p></p>