Kyanite and andalusite convert in situ to mullite during firing, producing a controlled volume expansion that compensates for shrinkage in refractory mixes — a property difficult to replicate with pre-formed synthetic mullite. This in-situ conversion also bonds the refractory matrix more intimately. Cost is another factor: natural aluminosilicate minerals are generally less expensive than fused or sintered synthetic mullite, making them preferred for large-volume castable and brick applications.

Chamotte is specifically calcined fireclay (a refractory clay fired to reduce plasticity and shrinkage), used as a grog aggregate in refractory brick and castable mixes. It differs from calcined kaolin (a whiteware and paper mineral) and from calcined bauxite (a higher-alumina refractory). The alumina content, firing temperature, and intended refractory application should be documented on the commercial invoice to support correct classification and avoid confusion with other calcined clay products.