Let's Make a Mullite Matrix - Elkem

Figure 2: Hot modulus of rupture as a
function of time at 1400 and 1500°C for a
fused alumina based castable with 0.5wt%
hydraulic alumina in combination with
0.5wt% cement. 8wt% microsilica.
At 1400°C, the strengthening (which is caused by the mullite formation) continues for
perhaps more than a day, while at 1500°C the reaction is completed within a few hours at
temperature. At 1300°C, unpublished data have indicated that the strengthening by the
mullitization occurs at a very low speed.
Once the mullite bond is established, it is permanent. This means that if e.g. a castable like
that in Figure 2 is prefired for 5 hours at 1500°C and then tested at 1400°C, then the hot-
M.O.R. will be higher than for a sample which has been held at 1400°C for 5 hours. Around
25-30MPa would probably be obtained in such a test.
This example was for a castable with only minute amounts of cement (i.e. CaO) so that we
may correlate the strengthening with the predictions from the phase diagram, Figure 1, where
a liquid forms and mullite crystallizes from this liquid.
Castables normally contain several percent of cement to give green-strength. A certain
amount of calcia is thus introduced. Since this lime is localized in the bond phase, which
normally makes up around one third of a castable, the lime concentration becomes quite
significant and has to be accounted for.
The ternary phase diagram Al2O3 – SiO2 – CaO incorporates the cement:
It is generally accepted that cement in combination with microsilica and alumina gives lower
hot-strength with increased cement content. If you ask why this is so, the explanation is
normally some vague reference to “low melting liquids in the system”, which is an
explanation that may be good enough for some people, but has never satisfied me. During
many years of research in refractory castables, the following conviction has gradually
manifested:
Basis for all the explanations should be the phase diagram. Particularly for castables based on
relatively pure ingredients of silica, alumina and lime, it should be able to explain many
observations by interpretation of the phase diagram Al2O3 – SiO2 – CaO. One should however
always remember that the phase diagram supposes equilibrium, - which is normally not the
case in your castable.
Elkem Materials – page 3