The Al2O3–SiO2 phase diagram can be used to explain the “classes” of
alumina-silica brick in terms of generalized “refractoriness.” These classes are
given in Table 1 along with comments on the use of the products. It is easy to
see that as you progress across the chart to the right (increase the Al2O3 content
of the refractory), the refractoriness increases either because the mullite content
increases and/or because the solidus temperature jumps up above 60 mole %
Al2O3. The refractoriness increases because the glass content (quantity of vitrified
phase) decreases, leading the material to have higher resistance to shrinkage
on prolonged high-temperature exposure.