About the thermal transformations and sintering of a Ghassoul clay from Morocco

Abstract

A Ghassoul clay from Morocco was studied regarding the effect of (1) grinding and (2) selection of the particle size distribution (classification) on the thermal transformations during sintering. Ghassoul clay contains stevensite, dolomite and quartz as major minerals. Prior to the classification, the starting coarse aggregates of the raw clay were first dry-ground below 500 μm and the obtained particles of powder were then granulated in order to produce adequate pellets for shaping by uniaxial pressing. Classification was performed to obtain three grain size distributions, namely greater than 500 μm, between 500 and 315 μm and lower than 315 μm. As the grain size distribution became finer, the proportion of carbonate phases was increased. DTA–TG analyses were in agreement with this trend, and in addition, dilatometry analyses showed that the densification was shifted to lower temperatures when decreasing the size of granules. The major phases occurring after sintering at 1200 °C were diopside, enstatite and forsterite as expected from the equilibrium phase diagram. Moreover, the smaller clay fraction (typically lower than 40 μm) enhanced the formation of melilite and monticellite. Nevertheless, a swelling occurred during sintering that balanced the densification shrinkage. According to these results, the sintering of Ghassoul clay proceeds through several stages regardless of the particle size selection. With a mean size of granules lower than 315 μm, the densification can be achieved at 1100 °C, while for greater sizes, a similar densification state of the raw material requires a significantly higher temperature of sintering of 1200 °C. In addition, some dedensification drawbacks may occur when using granules whose size is greater than 315 μm and therefore could induce unexpectedly a significant damage to materials. The observed trends may be useful for further applications of such Ghassoul clay in the silicate ceramics field.