Let's Make a Mullite Matrix - Elkem

65wt% cement. Or said in other words; a castable with less than 3.5wt% microsilica at
6.5wt% cement will start to dissolve mullite when it is heated.
One important corollary that may be found in the phase diagram is connected to the peritectic
at 1512°C, located close to the composition of Anorthite. Basic knowledge of phase diagrams
and crystallization paths tells us that if we have a liquid with a composition within the
compatibility triangle Corundum-Anorthite-Mullite (e.g. a molten castable) and cool it, then:
1) Corundum precipitates and the composition of the remaining liquid moves away from
corundum, until 2) mullite starts precipitating and, 3) the last liquid disappears at 1512°C at
the peritectic composition.
For the formation of mullite in a castable, the following reaction pattern has been suggested 6 :
Initially all microsilica and cement, possibly with some alumina, create a liquid
supersaturated in silica at temperatures from approximately 1300°C. The supersaturated liquid
crystallizes mullite until a stable composition is attained. This stable liquid has the peritectic
composition. One may add to this that upon heating, to 1512°C, the mullite is stable in this
environment, after which the mullite starts to dissolve in an opposite pattern as the
crystallization described above.
Such a peritectic liquid has been detected both directly 5,6 and indirectly 7 in castables with
mullite formation.
The existence of a liquid from which mullite forms is not only found in cement containing
systems, it is also a consequence of the meta-stable binary system shown in Figure 1. The
mullite formation is seen in Figure 2 as the strengthening of the castable with time.
Practical consequences:
Since the phase diagram indicates that we should expect a stable (more or less) liquid that
contains calcia, silica and alumina (15wt%CaO, 48wt%SiO2, 37wt%Al2O3), and that the
microsilica and cement react to form the origin of this liquid, we cannot expect that mullite
will be expelled unless there is an excess of microsilica. I.e. in a LCC with 6wt% CA cement
(70wt%A, 30wt%C), more than 5.7wt% microsilica is bound up in the peritectic liquid. If
6wt% microsilica was used in the castable, less than 1wt% mullite should be expected in the
final refractory, and total softening should commence below 1500°C on heating. More
microsilica gives proportionally more mullite and better strength should be expected.
It should be emphasized that it is not only the stability of mullite that matters. Since the
mullite is expelled from a liquid, the amount of residual liquid is important. To strengthen the
castable, each mullite crystal must connect two or more aggregate grains. With more residual
liquid more mullite is needed, and the stronger is the influence of increased temperature. A
powerful tool to overcome some of these problems is to reduce the level of cement, as will be
seen in the following section.
5 U. Schuhmacher: Untersuchungen an zementarmen und ultrazementarmen Korundfeuerbetonen. Dr. Ing.
thesis, Rheinish-Westfälishen Technishen Hochschule Aachen, Germany 1988.
6 B. Myhre, "Hot Strength and Bond-Phase Reactions in Low and Ultralow-cement castables" in
Proceedings of UNITECR´93, Oct. 31 - Nov. 3 1993 Sao Paulo, Brazil p. 583-94
7 B. Myhre, A.M. Hundere, H. Feldborg, C. Ødegård:” Correlation between mullite formation and
mechanical properties of refractory castables at elevated temperatures” Presented at VIII Int. Met. Conf.
Ustron, Poland. May 25-28, 1999
Elkem Materials – page 5