Influence of thermal treatment on the water release and the glassy structure of perlite

The effect of slow and rapid thermal treatment on water release and the structure of perlite was investigated by employing complementary techniques including X-ray diffraction, infrared spectroscopy and scanning electron microscopy. The study of several perlite samples, with different grain size and origin, has shown that rapid heating has a more pronounced effect on the glassy structure and that this is the only process capable of leading to perlite grain expansion. This process was simulated in a laboratory furnace allowing the careful control of temperature and time of treatment, and, thus, the description of their influence on the expansion process. The results show that molecular water released between 250 and 550 °C affects mostly the expansion process. Infrared spectroscopy provides evidence for additional water release, through dehydroxylation of Si–OH bonds, that may contribute also to expansion with a simultaneous development of the silicate network. The grain morphology was found to correlate with the expansion ratio. The presence of crystallites in raw perlite was shown to affect also the expansion process.