let's make a mullite matrix!
TRANSCRIPT
Let´s Make
a Mullite Matrix!
Published in Refractories Applications and News, in press.
The fine art of
Elkem Materials – page 1
Let’s make a mullite matrix!
Bjørn Myhre, Elkem Materials
P.O. Box 8126 Vaagsbygd, 4675 Kristiansand, Norway
Introduction:
In the paper “Let’s make a castable!”1, we went through proportioning of refractory castables,
and the beneficial effect microsilica addition has on casting properties.
It is however, not only flow and packing that are influenced by microsilica additions, also
high-temperature properties like hot-strength are affected. In the following paper, arguments
are put forward why microsilica additions at relatively high levels bring out the best of the
alumina-silicate castable. Implicitly this also shows why substitution of microsilica by
reactive alumina may not be such a good idea after all.
To begin with, let us look at the picture below:
These crystals are mullite crystals that are found on the fractured surface of an ultralow
cement refractory castable based on white fused alumina and microsilica, with only 0.5wt%
cement after hot-MOR testing at 1500°C. If the microsilica content is too low or the cement
content too high, these mullite crystals will not form. Quite opposite, instead of this strong
and robust bond, a liquid forms which causes catastrophic lowering of hot-strength. What
happens and why?
Alumina-silicate refractories:
It is commonly accepted that a mullite bond-phase is beneficial for high temperature
properties such as hot-strength and thermal-shock resistance. In bricks, the bond is normally
in place when the refractory is installed. In castables on the other hand, the mineralogical
make-up of the bond-phase goes through several intermediates during heat-up and hopefully
ends up with the desired physical and mineralogical properties at service temperatures. If the
composition of the bond phase is incorrect, the result may on the other hand be disappointing,
1 More about the proportioning and making of castables can be found in the paper “Lets make a castable”,
downloadable from www.refractories.elkem.com, also published in
“Refractories Applications and News” vol 13 (3)May/June 2008 p. 16-25. www.ranews.info
Elkem Materials – page 2
leading to softening and refractory failure at a much lower temperature than anticipated. If
one succeeds in creating a bond system with mullite formation though, the softening may be
postponed by several hundred degrees, which generally should provide improved service life.
Relevant assumptions:
In the following paper, it is assumed that the scenario is a refractory castable based on
alumina-silicate aggregates with fine alumina and microsilica in the matrix together with
cement. The variables are microsilica and cement, and it is assumed that there is always
enough alumina available for the mullitization reaction. Typical sources for such alumina are:
calcined alumina, reactive alumina, milled fused alumina (e.g.<74micron), milled tabular
alumina (e.g.<45micron). Clay and aluminasilicates from the sillimanite group are not that
suited since they all contain excess silica and are thus not ideal reaction partners for the
microsilica. In the following, microsilica is considered the sole source of silica. If other
aluminasilicates are used, parts of their silica content may take part in the reactions. Precise
quantities are difficult to estimate however.
The binary phase diagram Silica-Alumina:
Figure 1: The phase diagram Al2O3-SiO2 (Risbud and Pask 1978) 2
In Figure 1, the binary phase diagram after Risbud and Pask is shown with the metastable
system superimposed (dotted). The phase diagram shows us that if we heat a mixture of silica
and alumina, first of all a liquid appears from approximately 1200-1300°C, and from this
liquid, mullite is crystallized. At the lower temperatures, this is a slow process due to the high
viscosity of the silicate glass and good mullite yield may first be attained after several hours.
Figure 2 shows this temperature dependence for a castable with 0.5wt% cement and 8wt%
microsilica3. The castable was based on white fused alumina and hot-M.O.R. was
consecutively measured after different soak times.
2 S. H. Risbud and J. A. Pask J. Am. Ceram. Soc., 60(9-10)418-424 (1977)
3 B. Myhre and K. Sunde, "Alumina based castables with very low contents of hydraulic compound. Part
II." Strength and High-Temperature Reactions of No-cement Castables with Hydraulic Alumina and
Microsilica”, in Proc. UNITECR´95, Kyoto, Japan, Nov. 19-22 1995, p. II/317-24
Elkem Materials – page 3
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 4
Figure 3: The phase diagram Al2O3-SiO2-CaO after Osborn and Muan.4
Figure 3 shows the ternary phase diagram Al2O3-SiO2-CaO with additions relevant to the
present topic.
The gross composition:
First of all, when dealing with a mullite bond, the mullite must be stable. This means that the
composition of the castable and bond phase must be in one of the two compatibility triangles
indicated in Figure 3, either i) corundum, anorthite and mullite; or ii) mullite, anorthite and
silica. If the composition is outside, then mullite will not be stable, and if it forms at all, it will
dissolve over time. Most alumina castables are fortunately in the corundum corner of the
phase diagram. The hatched area in the corner indicates possible compositions (corundum
castables) and relevant silica and cement contents are indicated.
Cement and microsilica reactions:
One may assume that the cement and the microsilica react during heating6. If the cement
composition is indicated on the calcia-corundum join (here a 71%CA cement) and a line is
drawn towards the silica corner, one may by the use of the lever rule determine the minimum
silica/cement ratio that is needed to enter the stability regions of mullite. Here as indicated,
the minimum amount of microsilica as compared to cement is around 35wt% microsilica and
4 A. Muan and E.F. Osborn :"Phase Equilibria Among Oxides in Steelmaking" Addison-Wesley Publ.
Comp., Inc., 1965
peritectic
Elkem Materials – page 5
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 suggested6:
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 directly5,6 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 6
Examples:
In this part of the paper, some examples of the mullitization in castables will be shown. One
system based on white fused alumina, which serves as a model system, and one based on
Chinese bauxite aggregates.
Methodology:
Hot- Modulus Of Rupture (H-M.O.R.)
This picture shows the testing of the hot
Modulus of Rupture of a castable sample
(25x25x150mm) at a relatively low
temperature, around 700-800°C. Hot
M.O.R. testing gives relevant data on
strength of castable at temperature, but in
many cases where standards dictate a too-
short soak, the “equilibrium” values are
never obtained. This applies in particular
for some of the mullite forming reactions
we deal with here, and we therefore advise
a 24 hour soak prior to testing in order to
advance some of the slower reactions.
Common standards on hot-M.O.R. testing
normally open for a wide variation in
heating schedules and soak times, and
these should always be indicated in reports
and figures covering the issue. In Elkem’s
laboratories, heating rate is normally
300K/h (5K/min) up to temperature with a
subsequent soak of some 30 min to
equilibrate, or a longer soak depending on
scope and thermal history.
Refractoriness Under Load (R.U.L):
Refractoriness under load is another thermo mechanical technique that is useful in the
investigation of refractory castables. Briefly, the test consists of a furnace equipped with a
sample holder that allows the measurement of the sample height as a function of temperature
at a given load.
The temperature is normally increasing by 300K/h, as in the Hot-M.O.R. testing. In the Elkem
laboratories a load of 0.2MPa is normally applied, although some standards advise lower
loads for unshaped refractory products. The sample is in the shape of a cylinder with a central
bore in which a thermocouple and a measuring rod are placed which transmit the distance
between the top and bottom. The readings must be corrected for the thermal expansion of the
measuring rods in order to get correct numbers.
Elkem Materials – page 7
In this picture, the experimental set-up is
shown. The sample is the cylinder placed
between the two lightly yellow alumina
discs on top of the set up. Above the
sample is the furnace, which is lowered
onto the sample and the load is regulated
by a set-up with counter weights. It is seen
that this sample has been tested to a
temperature with significant subsidence,
due to the slight drum shape the cylinder
attains during testing with deformation.
As is the case with hot-M.O.R.
measurements, the results can be vastly
different depending on thermal history, and
unless pre-firing conditions are given, the
interpretation of the test may in some cases
be very difficult or close to meaningless.
Castables based on white fused alumina:
In the following section, the effect of microsilica and cement content on mullite formation
will be presented for our model system based on white fused alumina. All aggregates are a
high grade, white fused alumina. Together with calcined/reactive alumina, cement and
microsilica, the castables were composed to have the same particle size distribution and water
addition. To keep the particle size distribution as constant as possible, microsilica and reactive
alumina with similar PSD were substituted for each other. More results, incl. recipes can be
found in earlier papers6, 8,
9 downloadable through www.refractories.elkem.com.
The compositions are given in the Appendix, Tables 1 and 2, and are based on data from Ref.
[8]. Some of the results (Figures 4, 7 and 8) are taken from earlier investigations (1997-99)
and some (Figures 5, 6, 9, and 10) are recent (2007) remakes of these mixes. As some of the
original ingredients are no longer available, these have been replaced by recent replacement
materials. As the chemistry and PSD is very similar, it is considered justified to compare the
new results (obtained in 2007) with the old data (from 1997-99).
8 B. Myhre and Aase M. Hundere: “Substitution of Reactive Alumina with Microsilica in Low
Cement and Ultra Low Cement Castables. Part I: Properties Related to Installation and Demoulding”
in Proc. UNITECR´97, New Orleans, USA, Nov. 4-7 1997, p. 43-52 9 Aa. M. Hundere and B. Myhre:” Substitution of Reactive Alumina with Microsilica in Low Cement and
Ultra Low Cement Castables, Part II: The Effect of Temperature on Hot Properties.” Proc.
UNITECR’97, New Orleans, USA, Nov. 4-7, 1997 p. 91-100
Elkem Materials – page 8
Low cement castables
0
5
10
15
20
25
30
1100 1200 1300 1400 1500 1600
Temperature (C)
Hot M.O.R. (M
Pa)
8wt% MS
6wt% MS
4wt% MS
2wt% MS
Figure 4: Hot-M.O.R. of low cement (6wt% cement), fused alumina-based castables as a
function of temperature. Castables with different amounts of microsilica. 24 hours
at temperature. q=0.25, max. particle size 4mm, 13 vol% water for casting (3.8-
4.2wt%) From Ref.[7]
In Figure 4, results from a low-cement castable with 6wt% cement are shown for several
different microsilica contents. We see that at 1400°C, strength increases as the microsilica
addition increases. Particularly, levels of 6 and 8wt% microsilica seem beneficial to strength.
It should be noted that mullite was not detected7 at 1400°C unless 6 or 8% microsilica was
used. With 6wt% microsilica only 1wt% mullite was found, which is in good correlation with
the reactions and consequences described earlier in this paper. The peritectic liquid, being
approximately 11wt% of the castable may, at 1400°C, be either highly viscous or even
partially crystalline, which explains the strengthening effect seen at 1400°C. At 1500°C, the
peritectic composition melts and starts attacking the mullite bond, with more or less total
strength loss as result.
In Figure 5, the effect of pre-firing of a low-cement castable on R.U.L results is seen. In this
case we see that prefiring at high temperatures lowers the onset of “final subsidence”. Based
on the findings of the sample prefired at 1000°C, one could be led to believe that the castable
would tolerate a temperature of 1600°C. This is however difficult to understand if the hot-
M.O.R. results shown in Figure 4 are taken into consideration, and even more if the phase
diagram is consulted. The reason for this decline in refractoriness by the pre-firing, can be
explained in the following manner: The mullite bond establishes so rapidly that the bond
phase does not reach equilibrium. Then as equilibrium slowly commences, the bond phase is
attacked and slowly dissolved by the lime-containing liquid; probably only partly, but at least
sufficiently to allow a rapid subsidence at 1500°C.
Elkem Materials – page 9
-4
-3
-2
-1
0
1
2
3
0 200 400 600 800 1000 1200 1400 1600 1800
Temperature [°C]
Expansion [%]
Prefired 1000°C
Prefired 1500°C
Figure 5: Refractoriness under load for a white fused alumina-based LCC with 6% cement
and 8% microsilica. Samples pre-fired for 24 hours at 1000 or 1500°C.
-2
-1
0
1
2
0 200 400 600 800 1000 1200 1400 1600 1800
Temperature [°C]
Expansion [%]
LCC 4%MS
LCC 8%MS
Figure 6: Refractoriness under load. LCC based on white fused alumina. Samples with 4 and
8% microsilica pre-fired at 1500°C prior to testing.
Figure 6 shows that there is practically no difference in the R.U.L behavior for the LLC with
6% cement if 4 or 8% microsilica is used in the mix and if the castable has been pre-fired to
1500°C. Even if no additional refractoriness (as per R.U.L.) is gained by use of 8%
Elkem Materials – page 10
microsilica, due to placing properties and hot strength (Figure 4), it may be a good idea,
though, to use 8% microsilica and not substitute parts of it with reactive alumina.
Reduced cement castables:
We did see in Figure 4 that meltdown (at 1500°C) of massive amounts (11wt%) of peritectic
liquid softened the castable severely. The amount of this liquid may be reduced in two ways,
either by lowering the cement content, or by lowering the silica content. If the latter choice is
sought, according to the phase diagram, no mullite bond will be established, and also by
entering other compatibility triangles (e.g. less than 3.5wt% microsilica for 6.5wt% cement)
other eutectic and peritectic phases take over the role of “our” peritectic. These new phases
have even lower melting points, one as low as 1380°C. The latter is attained with microsilica
contents between 0.6 and 3.2wt%microsilica at 6wt% cement. One may remove microsilica
entirely, but then one excludes the use of alumina-silicate aggregates and fines.
To be frank, taking the refractoriness into consideration, it is a better proposal to reduce the
cement content. This is of course if the castable is intended to be used at temperatures
approaching 1500°C or higher, since at lower temperatures, the cement bond has so many
positive attributes, including a high green-strength, etc.
Figure 7 shows the effect of lowering cement content on hot-M.O.R. These castables all
contain white fused alumina and 8wt% microsilica, and the variable is the cement content. All
castables exhibit mullite formation, which is seen as an increase in hot-M.O.R. from 1300 to
1400°C. Typically there is a minimum at 1300°C, and at this temperature, some plasticity
may also be found. The softening may (at least for the 0.5wt% cement and no-cement
castables) be connected to the metastable liquid formation of the binary system SiO2-Al2O3
(Figure 1). At 1500°C both the 0.5wt% cement and the cement-free compositions exhibit
superior strength as compared to the low cement (6%) castable. The slightly better
performance at 1500°C for the cement-free castables as compared to the 0.5wt% castable,
may be attributed to the tiny amounts (1wt%) of peritectic liquid in the latter, which start
attacking the mullite bond upon further heating. It should again be stressed that the mullite
formation is irreversible at sub-solidus temperatures, (<1512°C) and that the measured
strength at 1300°C will be higher for a castable that has been pre-fired at higher temperatures.
This is because the mullite formation at 1300°C is kinetically inhibited, -probably by the high
viscosity of the liquid.
Elkem Materials – page 11
0
5
10
15
20
25
30
1100 1200 1300 1400 1500 1600
Temperature (C)
Hot M.O.R. (M
Pa)
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 detected7, 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 investigations6 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 12
0
5
10
15
20
25
30
1100 1200 1300 1400 1500 1600
Temperature (C)
Hot M.O.R. (M
Pa)
8wt% MS
6wt% MS
4wt% MS
2wt% MS
Figure 8: Hot-M.O.R. of ultralow cement (0.5wt% cement), fused alumina-based castables as
a function of temperature. Castables with different amounts of microsilica. 24 hours
at temperature. q=0.25, max. particle size 4mm, 13 vol% water for casting (3.8-
4.2wt%). From Ref.[7]
The irreversible nature of the mullite formation must be remembered, and the fact that the
softening at 1300°C is only a metastable phenomenon related to the mullite formation, as is
envisaged in Figure 9.
Figure 9 shows a comparison of Refractoriness Under Load (5°K/min) for two parallels with
different pre-firing conditions. The sample pre-fired at 1000°C starts to subside at
approximately 1300°C, and then from approximately 1500°C, mullite strengthens the sample
sufficiently to regain strength. Final softening commences around 1700-1750°C.
With mullitization in place before testing , e.g. with pre-firing at 1500°C, the softening does
not appear until the castable starts its final subsidence at temperatures around 1650-1700°C.
All in all, a castable showing remarkably good properties, considering that it contains 8wt%
microsilica, and a big improvement as compared to the low-cement castable shown in Figures
5 and 6.
Elkem Materials – page 13
-0.5
0
0.5
1
1.5
2
0 200 400 600 800 1000 1200 1400 1600 1800 2000
Temperature[°C]
Expansion [%]
ULCC 1000°C/24h
ULCC 1500°C/24h
Figure 9: R.U.L. (0.2MPa) of ultralow-cement castable based on white fused alumina with
0.5wt% cement and 8wt% microsilica as a function of pre-firing conditions.
In Figure 8, hot-M.O.R. of ULCC’s with various microsilica contents were shown. Based on
these results, a series of R.U.L. measurements were made and Figure 10 shows the results.
-1.5
-1
-0.5
0
0.5
1
1.5
0 200 400 600 800 1000 1200 1400 1600 1800 2000
Temperature [°C]
Expansion [%]
8%
0%4%
2% 6%
Figure 10: R.U.L. of ULCC pre-fired 24 hours at 1000°C. Castables where parts of the
m microsilica has been replaced by reactive alumina. Microsilica content indicated
n next to the curves.
Elkem Materials – page 14
The characteristics of these castables are that, except for the microsilica/reactive alumina
makeup, everything was sought to be kept constant. Particle size distribution was constant (by
replacement of microsilica by reactive alumina), water addition (in vol%) was constant, raw
materials were constant, etc. What we can see from Figures 8 and 10 is that a minimum
amount of microsilica (4%?) seems to exist, which has to be exceeded in order to get visible
effects of mullite formation. This may not only be caused by the reduction of mullite which
forms by the lowering of the microsilica content, but may also be connected to kinetic
hindrances. Another explanation is that in order to bond adjacent alumina grains a minimum
amount of mullite is required. The next thing that Figure 10 tells us is that it may not be a
good idea to replace microsilica by reactive alumina, - at least not in this kind of castable
based upon white fused alumina. With alumina-silicate raw materials, the picture may not be
that clear-cut, but these things have to be further investigated.
Castables based on bauxite aggregates:
So far, our examples have been based on very pure raw materials like white fused alumina.
Since natural raw materials are much more commonly used, it may be interesting to see how
use of such “non-ideal” components influences the mullite formation and strength. Thus, a
series of bauxite-based mixes were prepared. The compositions are given in Table 3 in the
Appendix . The castables were based on bauxite aggregates, but with milled, fused alumina
and calcined alumina together with cement and microsilica in the bond phase. Calgon
(SHMP) was used as dispersant.
In Figure 11, the development of hot-M.O.R. as a function of time is shown for two of the
compositions at 1200°C and 1300°C. The compositions shown are one with expected mullite
formation: (8% microsilica + 2.5% cement) and one with doubtful or no mullite formation:
(6% microsilica + 6% cement). The castable with 6% cement shows little development in
strength with time, and shows a drop from 1200°C to 1300°C, while the one with 2.5%
cement and 8% microsilica gets stronger both at 1300°C and also with time. During the first
few hours the 2.5% cement castable is weaker than the 6% cement castable due to
establishment of a liquid phase prior to the mullite formation. This occurs at approximately
100°C lower temperatures than in the purer white fused alumina system described above, but
in principle the same reactions are occurring. If the castable fulfils the requirements for
mullite formation, a liquid establishes, and from this liquid the strengthening mullite
precipitates. Typically, many refractory castables are based on a cement/microsilica ratio
close to 1 (e.g. 6%cement + 6% microsilica) and as the standards for hot-M.O.R. testing in
some countries advise 3 or 5 hours firing prior to testing, one may easily be mislead to choose
the 6% cement + 6% microsilica over the 2.5% cement + 8% microsilica castable.
Elkem Materials – page 15
0
2
4
6
8
10
12
14
16
18
20
0 5 10 15 20 25 30
Time [h]
hot-M.O.R [MPa]
6%MS, 6% cem. 1200°C
6%MS, 6%cem, 1300°C
8%MS,2.5%cem, 1200°C
8%MS, 2.5%cem, 1300°C
Figure 11: Bauxite-based refractory castables. Hot-M.O.R. as a function of time.
After 24 hours pre-firing, much of the mullitization is considered completed. Figure 12 shows
the hot-M.O.R. of bauxite castables with 3, 6 and 9% microsilica at 6% cement and also one
with 8% microsilica and 2.5% cement as a function of temperature (composition is given in
Table 3 in the Appendix ). The choice of 6% cement in combination with 6% microsilica is
indeed not the best if the hot-strengths shown in Figure 12 are examined. Unless more than
6% microsilica is used, there are no signs of the characteristic strengthening by mullitization.
This is in accordance with the mechanism sketched for the white fused alumina-based
compositions described earlier in this presentation, but as mentioned earlier, some 100°C
lower than in a pure system. The best results are again obtained by a lowering of the cement
content, while maintaining a relatively high microsilica level. Here 2.5% cement together with
8% microsilica was used.
Elkem Materials – page 16
0.00
5.00
10.00
15.00
20.00
25.00
30.00
35.00
1000 1100 1200 1300 1400 1500
Temperature[C]
hot-MOR [MPa]
9%MS, 6%cem
6%MS,6%cem
3%MS,6%cem
8%MS,2.5%cem
Figure 12: Bauxite-based castables. Hot-M.O.R. as a function of temperature. Castables pre-
fired 24 hours at test temperature.
-4
-3
-2
-1
0
1
2
0 500 1000 1500 2000
Temperature[°C]
Expansion [%]
9%MS, 6%cem
6%MS, 6%cem
3%MS, 6%cem
8%MS, 2.5%cem
Figure 13: R.U.L. (0.2MPa) of bauxite-based castables as a function of microsilica and
cement content. Samples had been pre-fired at 1000°C for 24 hours.
Refractoriness Under Load (R.U.L.) testing is often used to assess refractoriness of a material.
Together with strength measurements (hot-M.O.R), it is a good tool. Figure 13 shows the
Elkem Materials – page 17
R.U.L. for the castables of Figure 12. All samples had been pre-fired for 24 hours at 1000°C
prior to R.U.L. testing. We clearly see that all samples soften between 1200 and 1400°C, but
for the castables with more than 6% microsilica, the subsidence comes to a halt around
1400°C after an initial drop. This pattern is very typical for castables with mullite
strengthening and as the mullitization is irreversible, higher pre-firing changes the appearance
significantly, as seen in Figures 5, 9, 14 and 15.
-2
-1
0
1
2
0 500 1000 1500 2000
Temperature[°C]
Expansion [%]
9% MS, 6% cem 1000°C
9% MS, 6% cem 1500°C
Figure 14: R.U.L. of bauxite-based castable with 9% microsilica and 6% cement as a function
of pre-firing. 24 hours at 1000°C or 1500°C.
For the low cement castables based on white fused alumina it was mentioned that pre-firing at
a high temperature lowers the onset of the final subsidence (Fig.5), and the explanation that
was offered was that the mullitization is too fast for equilibrium to follow, and that the bond is
attacked by the cement containing liquid in order to gain equilibrium. This explanation is also
in accordance with the results of Figure 14. The reason why castables with lower cement
content do not show the same tendency (Figures 9 and 15) is probably best explained by the
amount of this attacking liquid. Less liquid gives less attack on larger amounts of mullite
bond.
Elkem Materials – page 18
-2
-1
0
1
2
0 500 1000 1500 2000
Temperature[°C]
Expansion [%]
8%MS, 2.5%cem
1000°C
8%MS, 2.5%cem
1500°C
Figure 15: R.U.L. of bauxite-based castable with 8% microsilica and 2.5% cement as a
function of pre-firing. 24 hours at 1000°C or 1500°C.
Concluding remarks:
The intention behind this paper is to show that the reason for the thermal behavior of castables
can be found and estimated from careful interpretation of established, fundamental principles
like those in phase diagrams. This interpretation is however not always so simple and
although it is regarded more like witchcraft by some, correct use is more likened to an art.
What we have also shown is that the result of use of microsilica in high-alumina castable
systems is very dependent on proper knowledge. Vast differences in behavior are obtained
with only small changes in the cement/silica ratio. Unfortunately there has been a trend
towards promoting what I would consider sub-optimal solutions, with a minimum of
microsilica combined with liberal amounts of cement. Such conditions are promoting rapid
melting, as shown above.
Understanding mullite formation is critically important, and mullite is one of the significant
factors when hot-strength of alumina-silicate systems is considered. It should not be necessary
to get suboptimal results because of misunderstandings connected to the stability and
formation of mullite. The results presented and the proposed mechanisms are not only valid
for fused alumina and bauxite systems; quite a few raw material candidates exist. Members
from the sillimanite group are very prominent, but possibly any high alumina system would
benefit from this (theoretic) treatise.
Elkem Materials – page 19
APPENDIX
Table 1: No/ultralow-cement castables. q-value = 0.25.
Microsilica/reactive alumina ratio (vol%) 100/0 75/25 50/50 25/75 0/100
Weight %:
Alphabond (hydraulic alumina) 0.5 0.5 0.5 0.5 0.5
Cement CAC 71% Alumina 0.5 0.5 0.5 0.5 0.5
White fused alumina:
-74 micron 20 19.5 19.5 19 19
0-0.4mm 22 21.5 21 21 20.5
0.5-3mm 32 31.5 31 30.5 30
2-4mm 10 10 9.5 9.5 9.5
Microsilica 8 6 4 2 0
Reactive alumina, BET 7.5m2/g, D50= 0.8µm. 0 3.5 7 10.5 13.5
Calcined alumina BET 0.8m2/g, D50= 4.5µm 7 7 7 6.5 6.5
Citric acid (retarder) 0.03
Darvan 811D (deflocculant) 0.05 0.05 0.05 0.05 0.05
Water (13 vol%) 4.10 4.02 3.97 3.92 3.85
Table 2: Low-cement castables with 6% cement. q-value = 0.25.
Microsilica/reactive alumina ratio (vol%) 100/0 75/25 50/50 25/75 0/100
Weight %:
Cement CAC 71% Alumina 6 6 6 6 6
White fused alumina:
-74 micron 15 14.5 14.5 14 14
0-0.4mm 22 21.5 21 21 20.5
0.5-3mm 32 31.5 31 30.5 30
2-4mm 10 10 9.5 9.5 9.5
Microsilica 8 6 4 2 0
Reactive alumina, BET 7.5m2/g, D50= 0.8µm. 0 3.5 7 10.5 13.5
Calcined alumina BET 0.8m2/g, D50= 4.5µm 7 7 7 6.5 6.5
Citric acid (retarder) 0.03 0.03 0.05 0.05
Darvan 811D (deflocculant) 0.05 0.05 0.05 0.05 0.05
Water (13 vol%) 4.15 4.10 4.05 3.96 3.93
Elkem Materials – page 20
Table 3: Bauxite-based castable compositions
Sample number
9% MS
6%
cem.
6%MS
6%cem.
3% MS
6% cem.
8% MS
2.5% cem.
Chinese Bauxite: 1-4mm % 35 35 35 35
Chinese Bauxite: 0-1mm % 30 30 30 30
White Fused Alumina: -74micron % 17 17 17 17
Cement :CAC 71% Alumina % 6 6 6 2.5
Microsilica 971U (Elkem Materials)% 9 6 3 8
Calcined alumina BET 0.8m2/g, D50= 5µm 3 6 9 7.5
Additive (SHMP) 0.2 0.2 0.2 0.2
Citric acid 0 0 0.01 0
water wt% 5.0 5.0 5.0 5.0