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Technical Paper


Figure 5: Working time as f (temperature)


high permeability at 110ºC (Figure 7) which is essential for a low steam pressure build-up (Figure 8). In each of the three model formulations, 2.5% CAC (either CAC70 or CAC80) are replaced by 2.5% MP20. When MP20 is used in the recipes, no additional deflocculant was added to the mixes. Trials have also been conducted with PP-fibers in the formulation AM6. The formulations that contain MP20 have not used PP-fibers. Pastes of mixes of calcium aluminate cement, reactive alumina, and MP20 have been used to investigate the underlying mechanisms that lead to the change in permeability and dry-out capability. A cement-free silica sol/gel bonded model castable AS0 (Table 6) has been used as reference for a castable- type that is known for its rapid dry-out capability, but at the expense of very low green strength [4]. With a thermocouple in the center of a 10x10x10 cm3


castable cube thermogravimetric analyses have been started after a


curing period of 24h at 20°C and 100% rH. Samples of AS10 and AS10- MP20 have been heated with 3 different heating rates of the furnace, 20, 120 and 300 K/h (Figure 9). For the silica sol/gel bonded castable AS0 only the fastest heating schedule has been tested (300 K/h). Figure 10 shows the thermogravimetry of 10x10x10 cm3


cubes of castable AM6 without and


with PP-fibers, and castable AM6-MP20 without PP-fibers. The heating rate is 300 K/min. The weight evolution of the cubes during heating has been plotted as a function of the temperature recorded in the center of the cube.


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 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


Raw Material Tabular Alumina


Sintered Spinel Reactive alumina Sintered MgO Silica Fume


Calcium Aluminate Cement


Deflocculant & Permeability Enhancing Active Compound


Deflocculant (Permeability enhancer) Water (%) Vibration Flow (%) Table 5: Model Alumina-Spinel-Magnesia LCC-formulations (m-%) September 2017 Issue ENGINEER THE REFRACTORIES 17


Specification 0-5mm


0-1mm P152SB


0-0.09mm


Elkem Microsilica® Secar®


71 (CAC70) Refpac® Mipore 20 (MP20)


Peramin® (PP-fiber)


AL200 (PCE) 971U


Figure 6: Setting time as f (temperature)


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 [3]. The impact of the PP-fiber addition to AM6 is visible above 150ºC in the thermogravimetric profile (Figure 10) but remains relatively small compared to the advancement with MP20 in the AM6 mix.


The impact of ambient temperature on the MCC-permeability has been studied on samples cured at 5, 20 and 35ºC for 24h at 100% rH. Prior to the permeability measurement at room temperature they have been dried for 24h at 110ºC. At all tested temperatures very low permeability has been found when MP20 is not present in the formulation. With the introduction of MP20 the permeability is for all curing temperatures more than 10 times higher than for the MP20-free mixes.


To evaluate the impact of sample size on the dry-out behaviour cubes of three different dimensions and cured for 24h (20ºC, 100% rH) have been heated with a rate of 300 K/h. Figure 12 shows the temperature in the center of the castable at which 2/3 of the initially added water have been removed from the sample. While the removal temperature increases with the sample size for the MP20-free castables it remains almost constant in the range between 120 and 140ºC for all 3 tested MP20-containing castables. Without MP20 the A15 formulation shows the highest water removal temperature and the 12x12x12 cm3


AM6 68


13 9


3.5 0.5 6


-


+0.05 (+0.1) +5.3


110 (100)


AM6-MP20 68


13 9


3.5 0.5 3.5


2.5


- -


+5.3 115


cube exploded during the test when the temperature


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