Mullitization of Andalusite in Refractory Bricks

Mullite formation was studied in andalusite bricks fired in the range 1300-1600°C Mullitization of andalusite grains retains the grain shape and bulk chemistry. An andalusite monocrystal is transformed into a composite made of a mullite monocrystal (≃ 80 %)with a capillary network filled with a silica rich glass. The capillaries form interconnected tubes parallel to the c axis, common to the mullite crystal and the parent andalusite grain. Heating andalusite crystals reveals that only ≃10-15% of the glass is expelled out of the host grain. This observation is supported by volume balance calculations In spite of a high amount of glass, andalusite based refractories exhibit an excellent behaviour in industrial use resulting from the trapping of most of the glass in the mullite.