Let's Make a Mullite Matrix - Elkem

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

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 Temperature [°C] 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% <strong>Elkem</strong> Materials – page 9
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, (
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: Low cement castables Hot M.O.R. (MP
Page 13 and 14: Hot M.O.R. (MPa) 30 25 20 15 10 5 0
Page 15 and 16: The characteristics of these castab
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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