The main crystal phase of mullite bricks is mullite (3Al₂O₃·2SiO₂). However, ordinary high alumina bricks only have alumina content that meets the standard, and their crystal phase is usually composed of a mixture of corundum, mullite and glass.
In actual use, mullite bricks have significant advantages in thermal shock resistance and creep resistance. They have better volume stability under high temperature environments and are less prone to deformation and cracking problems.