Let's Make a Mullite Matrix - Elkem

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$ageing of refractory castables; which interactions exist and ... - Elkem$

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 Temperature [°C] 8% 2% 6% Figure 10: R.U.L. of ULCC pre-fired 24 hours at 1000°C. Castables where parts of the m microsilica has been replaced by reactive alumina.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% <strong>Elkem</strong> Materials – page 13
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. <strong>Elkem</strong> Materials – page 14
Page 1 and 2: Let´s Make a Mullite Matrix! Publi
Page 3 and 4: leading to softening and refractory
Page 5 and 6: Figure 3: The phase diagram Al2O3-S
Page 7 and 8: Examples: In this part of the paper
Page 9 and 10: Low cement castables Hot M.O.R. (MP
Page 11 and 12: microsilica, due to placing propert
Page 13: Hot M.O.R. (MPa) 30 25 20 15 10 5 0
Page 17 and 18: hot-MOR [MPa] 35.00 30.00 25.00 20.
Page 19 and 20: Expansion [%] 2 1 0 -1 -2 0 500 100
Page 21: Table 3: Bauxite-based castable com

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