Solid state sintering of red ceramics at lower temperatures

Red ceramic products based on clay minerals are normally fired above 900 °C. In practice, however, bricks have been produced at temperatures as low as 600 °C. In the present work the possibility of lower temperature solid state sintering of clay particles is theoretically explained based on geometrical aspects, vacancy diffusion and active surface bonding consolidation. A model of close-packed discs of a few micrometers in diameter and nanoscale thickness is proposed to explain a much more efficient sintering process for clay minerals than the classical spherical particle model. The proposed disc model presents specific characteristics of high contact surface area, lower porosity and nanoscale pores. The initial vacancy diffusion sintering rate is constant and several orders of magnitude greater than for the classical model. At 600 °C the completion of dehydroxylation in the clay crystals turns their flat surfaces into active sites for bonding consolidation.