Densification behaviour of a red firing Brazilian kaolinitic clay

This study focuses on the complexity of the densification of a Brazilian kaolinitic clay sintered in air at various temperatures. The used kaolinitic clay comes from a large structural ceramics production region in South-Eastern Brazil (Campos-RJ). The samples were dry pressed in a cylindrical die and sintered at temperatures ranging from 750 °C to 1150 °C. XRD and SEM were used to identify the present phases and the densification level. The densification was measured by five parameters: specific surface, linear shrinkage, water absorption, apparent density, and compressive strength. The results revealed that sintering is governed by different mechanisms according to the temperature. A series of phase transformations takes place during sintering, which play an important role on the densification. Below 950 °C, sintering is governed by solid state diffusion mechanisms. Above 950 °C, sintering is governed by viscous flow mechanism with high influence on the densification of the green body. In addition, the red clay presented a refractory behaviour during firing due the high amount of kaolinite clay mineral, high loss on ignition, and the presence of gibbsite, which impairs sintering, resulting in greater porosity of the specimens.