Let's Make a Mullite Matrix - Elkem

Hot M.O.R. (MPa)
30
25
20
15
10
5
0
1100 1200 1300 1400 1500 1600
Temperature (C)
6 wt% cement
0.5 wt% cement
No cement
Figure 7: Hot M.O.R. of fused alumina-based castables with 8 wt% microsilica as a
function of temperature. q=0.25, max. particle size 4mm, 13 vol% water for
casting (4.2wt%). From Ref. [7]
Ultralow cement castables:
In ultralow cement castables, the amount of peritectic liquid is significantly reduced, and the
fluxing effect is consequently much lower. The result is a castable that – provided the
aggregates are resistant- will be much more refractory than its low-cement counterpart. This
difference is clearly shown in Figure 7. In Figure 8, the dependence of hot-M.O.R. on
microsilica content is shown as a function of temperature. It is clear that with more
microsilica, more mullite precipitates and stronger castables are made. Theoretically the
peritectic should only account for 0.5% microsilica and mullite should be formed from the
excess. However, such high amounts are normally not detected 7 , which is probably connected
to kinetic hindrances.
The strong dependence of hot-M.O.R. on microsilica content at 1500°C as seen in Figure 8 is
again probably an effect of the melting of the peritectic phase. Although the melting point
theoretically is 1512°C, early investigations 6 revealed that it contains relatively significant
amounts of impurities, notably alkalis, which would lower melting temperature. A melting
around 1500°C is hence likely. Upon heating, this liquid attacks the mullite and it becomes
important to have massive precipitations in order to maintain strength to high temperatures.
Elkem Materials – page 11