Let's Make a Mullite Matrix - Elkem

Let´s Make 

a Mullite Matrix! 

Published in Refractories Applications and News, in press. 

The fine art of

Introduction: 

Let’s make a mullite matrix! 

Bjørn Myhre, Elkem Materials 

P.O. Box 8126 Vaagsbygd, 4675 Kristiansand, Norway 

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 1

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.

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. 


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 heating 6 . 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 


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 


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 

Refractoriness Under Load (R.U.L): 

many cases where standards dictate a tooshort 

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 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. 


In this picture, the experimental set-up is 

shown. The sample is the cylinder placed 

Castables based on white fused alumina: 

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. 

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 papers 6, 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 


Low cement castables 

Hot M.O.R. (MPa) 

30 

25 

20 

15 

10 

5 

0 

1100 1200 1300 1400 1500 1600 

Temperature (C) 

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 detected 7 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. 


Expansion [%] 

3 

2 

1 

0 

-1 

-2 

-3 

-4 

0 200 400 600 800 1000 1200 1400 1600 1800 

Temperature [°C] 

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. 

Expansion [%] 

2 

1 

0 

-1 

-2 

0 200 400 600 800 1000 1200 1400 1600 1800 


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% 


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, (


30 

25 

20 

15 

10 

5 

0 

1100 1200 1300 1400 1500 1600 


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. 



30 

25 

20 

15 

10 

5 

0 

1100 1200 1300 1400 1500 1600 


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. 


Expansion [%] 

2 

1.5 

1 

0.5 

0 

-0.5 

0 200 400 600 800 1000 1200 1400 1600 1800 2000 

Temperature[°C] 

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. 

Expansion [%] 

1.5 

1 

0.5 

0 

-0.5 

-1 

-1.5 

0 200 400 600 800 1000 1200 1400 1600 1800 2000 


8% 

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. 

4% 

0% 


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. 


hot-M.O.R [MPa] 

20 

18 

16 

14 

12 

10 

8 

6 

4 

2 

0 

0 5 10 15 20 25 30 

Time [h] 

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. 


hot-MOR [MPa] 

35.00 

30.00 

25.00 

20.00 

15.00 

10.00 

5.00 

0.00 

1000 1100 1200 1300 1400 1500 

Temperature[C] 

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 prefired 

24 hours at test temperature. 

Expansion [%] 

2 

1 

0 

-1 

-2 

-3 

-4 

0 500 1000 1500 2000 


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 


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. 

Expansion [%] 

2 

1 

0 

-1 

-2 

0 500 1000 1500 2000 


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. 


Expansion [%] 

2 

1 

0 

-1 

-2 

0 500 1000 1500 2000 


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. 


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 7.5m 2 /g, D50= 0.8µm. 0 3.5 7 10.5 13.5 

Calcined alumina BET 0.8m 2 /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 7.5m 2 /g, D50= 0.8µm. 0 3.5 7 10.5 13.5 

Calcined alumina BET 0.8m 2 /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 


Table 3: Bauxite-based castable compositions 

9% MS 

6% 6%MS 3% MS 8% MS 

Sample number 

cem. 6%cem. 6% cem. 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 0.8m 2 /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

Let's Make a Mullite Matrix - Elkem

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?