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the AS10 (MCC without MP20) occurs at higher temperatures than from the AM6 (LCC without MP20). However, when MP20 is applied in both formulations the water release occurs for both castable types at a very similar low temperature. The thermogravimetric profile of the MCC AS10- MP20 appears to be very similar to the profile of the silica sol/gel bonded castable AS0. The different heating rates in case of AS10 don’t seem to make a big difference in the thermogravimetric profile. However, it has to be taken into account especially for the MP20-free samples that vapor pressure potentially increases to higher levels with higher heating rates and creates a higher risk of explosive steam spalling. With MP20 the risk is lower as water is removed at lower temperatures which minimizes the pressure build-up in the castable as reported by [8].
Effect of MP20 on castable microstructure
Figure 3: Thermogravimetry of MCC with and without MP20 in comparison to a silica sol/gel bonded NCC castable
As the A15 castable consists only of tabular alumina and CAC80 it has been chosen for a more detailed analysis of microstructural differences between the castable with and without MP20. Samples have been cast and cured 24h at 20ºC and 100% rH. Then they have been crushed with a hammer and sieved below 100 µm in order to remove the coarse aggregates and to enrich the binder phase for the XRD and SEM analyses (Figure 5 and Figure 6). The A15 matrix shows the expected crystalline calcium aluminate hydrates. CAH10
and the hexagonale plates of C2 AH8 have been identified.
Very different to that, the matrix of A15-MP20 doesn’t show these characteristic crystalline hydrate phases but has formed a calcium aluminate hydrate gel. Microcracks are visible in the gel but might have been caused by dessication of the gel in the vacuum of the SEM. The XRD analyses confirmed that no crystalline hydrates are present in A15-MP20 while in the reference A15 the classical hydrates CAH10
, C2 AH8 , AH3 and some C4 AH13
Figure 4 Thermogravimetry of AM6-MP20 versus AM6 without and with PP- fiber(300 K/h)
As can be seen in Figure 3 and Figure 4 the MP20 containing mixes have their largest weight reduction in the temperature range between 100 and 150ºC, while those that don’t contain the MP20 release the free and bonded water at significantly higher temperatures. The water release from
were found. To confirm these results, DSC analyses have been conducted on pastes of a 1:1 mix of CAC70 and reactive alumina, with and without MP20. The results in Figure 7 are in line with the findings in the matrix of the A15 castable. The mix without MP20 shows the typical dehydration peaks for CAH10, C2AH8, AH3 (gel and crystalline) and C3AH6. The DSC of the mix with MP20 shows a significantly different profile with much more de-hydration reactions going on in the temperature range of 100 to 150ºC while only a very small peak occurs between 250 and 300ºC. This confirms the findings that the dissolved calcium aluminates phases hydrate in the presence of MP20 rather as a gel than as crystalline phases. EDX analyses indicate that the gel contains both aluminium and calcium oxide.
Figure 5: A15 castable matrix after 24h curing (20°C) November 2017 Issue
Figure 6: A15-MP20 castable matrix after 24h curing (20°C) ENGINEER THE REFRACTORIES 19
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