Evolution of macroporosity in fired mudstone clay

Samples of naturally occurring mudstone clay fired at temperatures higher than 1150°C exhibited unusual swelling, with localised densification and glassy shell and apparent bulk density of about 700 kg m^-3. The swelling was a result of the development of a foam type macroporous structure during sintering. Thermal analysis indicated that the development of the foam structure started at around 1150°C, and that it was not accompanied by any detectable mass change. A combination of thermal analysis and X-ray diffraction data suggest that macroporosity developed as a result of volumetric expansion accompanying thermally induced vitrification of the starting crystalline phases. It is believed that the transition to amorphous phase leads to release of structurally locked water molecules, which at high temperature behave as supercritical steam and act as a porogenic agent.