Oil shale amended raw clay: Firing transformations and ceramic properties

Effects of oil shale additions (up to 20 wt.%) on the firing transformations and ceramic properties (water absorption, firing shrinkage and bending strength) of an illitic-chloritic non-calcareous raw clay were investigated in the range 700–1075 °C. For the first study, X-ray diffraction, thermal analyses, scanning electron microscopy and energy dispersive X-ray spectrometry were used. The results show that oil shale additions lead to the development of anorthite and diopside, and the vanishing of magnesian phases (Mg-spinel and olivine), particular for quantitative additions, as well as to a drastic decrease of the glassy phase. Anorthite developed from lime, supplied from calcite, and clay minerals breakdowns. The contribution of free silica to the latter reaction is plausible, especially at higher firing temperatures. For the development of diopside, higher calcium activities were needed. In such condition, Mg-phases vanished. As compared with the calcite effect, the organic matter associated with shale did not have an appreciable impact on the neoformation process. On the other hand, it is found that the measured ceramic properties experienced a marked change as a function of moderate oil shale additions (<12 wt.%) because of the glassy phase effect. In contrast, they remained practically unchanged for higher contents (>12 wt.%) because of calcium silicates formation.