Properties of MgO–MA bricks can be further improved by using fused
aggregates to partially or completely replace sintered aggregates. An effective
additive for improvement of properties of MgO–MA bricks is ZrO2, which
enhances hot strength slag corrosion resistance. Addition of ZrO2 to MgO–
MA bricks alleviates some problems attributable to spinel. Lime, from impurities
in the magnesia-spinel bricks or from other sources such as slags, extracts the
alumina within the spinel to form low-melting calcium aluminates. As a result,
the hot strength of the MgO–MA will be decreased. However, when ZrO2 is
added, it reacts with lime to form high-melting (25508C) because CaZrO3 reaction
between ZrO2 and CaO is more thermodynamically favorable than reaction
between Al2O3 and CaO, so avoiding formation of low-melting calcium aluminates.
ZrO2 can be incorporated in the form of single-phase ZrO2 or in the
form of Al2O3–ZrO2 coexisting grain, but the latter appears more effective in
improving the properties, because of formation of both CaZrO3 and in-situ spinel.
Gruver (23) showed that the modulus of rupture (MOR) of MgO–MA bricks at