Technical Paper
REFRACTORIES ENGINEER
THE
NCCs, ULCC and LCC after HMOR testing at 1400°C and 1500°C, were used for mineralogical phase and morphology analysis using XRD and SEM characterisation. Fig. 7 shows the XRD patterns and Fig. 8 and 9 show the SEM micrographs of etched polished surfaces of specimens.
As shown in Fig. 7, the temperature has a strong influence on the content of mullite. The intensity peak of mullite in samples MSZ-5, MSZ-7 and ULCC-7 after HMOR testing at 1500°C is stronger than that at 1400°C. Moreover, the presence of microsilica has a strong influence on the
Fig. 3: Green strength of microsilica-gel/silica-sol bonded NCCs, ULCC and LCC
microsilca-gel bonded NCC is lower than ULCC and LCC, it is adequate for installation.
3.2 Hot Modulus of Rupture (HMOR)
HMOR vs. test temperature is shown in Fig. 4. At test temperatures in the range of 1200-1500°C, MSZ-7 showed the highest HMOR of the NCCs. At 1400 and 1500°C, the HMOR of MSZ-7 (with 7 mass% microsilica) was 14.3 MPa and 7.7MPa, respectively, while for MSZ-3 (containing 3 mass% microsilica) HMOR was only 3.6 and 2.6MPa, respectively. This indicates that the higher microsilica content, the more mullite forms. Compared to silica-sol bonded NCC (SOL-7), MSZ-7 had surprisingly slightly higher HMOR, probably attributed to the difference in sintering process and morphology/size of the formed mullite. Further investigation on the mechanism is ongoing, and will be reported in the future. MSZ-7 showed lower HMOR than ULCC-7 and LCC-7 at 1200°C. When the temperature was increased to 1400 and 1500°C, the HMOR values of MSZ-7 were higher than ULCC-7 and LCC-7. Especially, at 1500°C, the difference was dramatic. This indicates that liquid phase probably was formed in ULCC-7 and LCC-7, which indeed ruins the hot-properties.
3.3 Refractoriness Under Load (RUL)
Fig. 5 shows the RUL curves for microsilica-gel/silica-sol bonded NCCs, ULCC and LCC after prefiring at 1000°C for 24hrs. It is observed that the specimens softened at various temperatures. For the specimens with sufficient microsilica and low cement content (MSZ-5, MSZ-7, SOL-7 and ULCC-7), they softened at ~1300°C, but around 1400-1500°C the compression came to a halt.
This RUL pattern is typical for castables
with mullite strengthening. This indicates that for the castable with more than 5% microsilica and little cement, mullite formation occurs when the heating temperature is above 1300°C.
The mullite formation is irreversible and takes some time, so if the
samples were pre-fired at a different temperature or duration, the results may have been different, as exemplified in Fig. 6. It can be observed that the typical mullite formation by softening around 1300°C followed by a plateau disappears, if the samples are pre-fired at a temperature above the mullitisation temperature. The pre-firing temperature of 1500°C has shifted the softening point from around 1300°C upwards to around 1500°C.
3.4 Mullite formation
It is important to understand the mullite formation since mullite is one of the significant factors concerning hot-strength of alumina-silicate systems. In this paper, selected specimens of microsilica-gel bonded
Fig. 6: RUL of microsilica-gel/silica-sol bonded NCCs, ULCC and LCC after prefiring at 1500°C
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24 September 2019 Issue
Fig. 4: HMOR of microsilica-gel/silica-sol bonded NCCs, ULCC and LCC
Fig. 5: RUL of microsilica-gel/silica-sol bonded NCCs, ULCC and LCC after prefiring at 1000°C
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