Andalusite: A promising material for manufacturing high-quality refractories

The process of mullitization of andalusite is reviewed to show that this phase transformation is based on the mechanisms of a pseudomorphism that fixes the grain shape and the chemistry of solid-phase processes. Mullite is oriented in correspondence with the generating crystal of andalusite along the C axis. A single crystal of andalusite transforms into a compound consisting of a mullite single crystal with capillary pores filled with a glass phase saturated with amorphous silica. The capillaries form a network of well-connected tubular pores with a micron cross section, and they are positioned along the C axis common for the newly formed mullite and the original andalusite. About 80% of the glass phase remains in pores of capillaries of the formed mullite. The results of the study were tested in reactions between andalusites of various grain compositions and powder alumina and are now used for fabricating unshaped refractory mixtures with an ultralow content of cement and cement-free mixtures with finely milled andalusite and alumina.