Investigation of the sintering mechanisms of kaolin–muscovite

Kaolin Kga-1b and a muscovite from Bihar (India) and their mixtures were used to investigate the densifying mechanisms during the sintering. The kaolin–muscovite mixtures containing up to 25 mass% muscovite were studied by isothermal and non-isothermal methods such as the step-wise isothermal Dorn method and the constant heating rate developed by Bannister (1968) and Woolfrey and Bannister (1972).The densification of the reference proceeded through a viscous flow sintering due to the amorphous phases and diffusion at the grain boundaries at high temperatures (> 1200 °C). The activation energy was between 115 kJ/mol and 250 kJ/mol, with a peak value of 650 kJ/mol around 1250 °C due to the formation of mullite. The same trend was observed when 10 mass% muscovite were added. However, the densification started at 1000 °C and the related activation energy was 500 to 700 kJ/mol because of the formation of high-temperature phases (leucite and mullite).At muscovite contents > 10 mass%, the linear shrinkage was more sensitive to the muscovite dehydroxylaton. The densification was also controlled by grain boundary diffusion and viscous flow up to 1300 °C. Beyond this temperature, a dissolution-limited liquid sintering mechanism proceeded up to the end of sintering. The presence of crystallites, despite a larger amount of the viscous melt, led to a lower densification rate. The activation energy was below 250 kJ/mol.

Research Highlights
► The sintering of pure kaolin is governed by viscous flow and diffusion at grain boundaries.
► Related activation energies are increased from 250 to 600 kJ/mol around 1250 °C.
► Up to 10 mass%, muscovite enables a densification at lower temperature (1000 °C).
► Above 10 mass muscovite, a dissolution-limited liquid sintering prevails from 1300 °C.
► The formation of crystallised phases tends to slow down the densification rate.