Second-generation MgO–MA castables with similar MgO–Al2O3 clinkers
have been developed using hydratable-alumina as a binder. The optimum
size distribution of both MgO and Al2O3 is critical. When MgO and Al2O3
react to form in-situ spinel, as discussed in Section V.B, a volume expansion
of 5 8% occurs, affecting the volume stability. By proper choice of reactive
alumina, and control of the sintering process, it is possible to achieve a volume
stable MgO–MA castable with 20–30% Al2O3. Adjustments (e.g., changing the
particle size of added Al2O3 and adding preformed spinel) can be made to the
formula, which allows control of the volume change at almost any given
alumina content. The advantage of second generation MgO–MA castables is
higher hot mechanical strength due to elimination of low-melting phosphates
from the binder. Slag tests show that slag penetration resistance increases as
alumina content increases although corrosion resistance decreases. In secondgeneration
MgO–MA castables, either fused or sintered spinel clinkers with
different stoichiometry can be used. Rigaud et al. (33) compared the effect of
MgO-rich (MR66) and two Al2O3-rich spinels (AR78 and AR90) on the Modulus
of Rupture (MOR) and corrosion resistance of MgO–MA castables, and
found that castables using MgO-rich spinel had slightly higher MOR and better
corrosion resistance than those using AR78 and AR90. The better corrosion
resistance using MgO-rich spinel is attributed to the higher MgO content, as discussed
by Zhang and Lee (34), since the solubility of MgO in silicate slag is
lower than Al2O3. Bi et al. (35) verified that increasing Al2O3–MgO ratio in
MgO–MA castables generally leads to better slag penetration resistance but
poorer corrosion resistance.