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Technical Paper
Figure 3: Cured and dried cold crushing strength for test castables mixed with different water additions
A loss in strength with increasing water addition is observed for all test castables. Due to the lower water addition required and the earlier cement reaction for VIB-DA, a significantly higher strength level is achieved.
The risk of “overwatering” is high for castables which show a very long wet out behaviour after water addition. This is typical for modern castable concepts with an optimised matrix composition. Kockegey-Lorenz et al. [5] reported on the wet out and mixing behaviour of E-SY alumina containing castables. The mixing behaviour of different castable formulations has been tested with different mixers. All test castables with E-SY 1000/2000 as matrix alumina showed very short wet out times of one minute or less, regardless of the mixing equipment used. That excellent wetting behaviour eliminates the risk of overwatering a castable during on-site installations. Also fast wet out mixes with E-SY alumina guarantee smooth lining operations as they enable low energy mixing at very low water demand due to non-dilatant rheology and soft mixing consistency.
It is common practice in castable development and modification to use well established formulations as the basis and adjust the recipes to the requirements of a similar application. Small changes are often done without extended lab testing as the effort involved is time-consuming. However even small changes may influence placing properties such as wet out and flow. The modification of just the cement content was part of a lab test series. Low cement castables with and without silica fume were tested at cement contents of 2 – 10% (mix with silica fume) and 3 – 11% (mix without silica fume). Included in the test series was the modification of the dispersing alumina dosage in order to verify if the same additive dosage can be used for all cement contents or has to be adjusted to achieve the optimum dispersion and flow. The lab tests showed that in general a lower dispersing alumina dosage (0.7–1.1%) is required for lower cement contents (2-5%) and has to be increased (1.3–1.5%) for higher cement contents (10%) to achieve good flow properties. As an example wet out and flow results of a silica fume containing castable are shown in figure 4. For cement contents of 2% and 5%, good flow at 60 minutes is achieved with M-ADS/W additions of 0.7% and 1%. The mix with 10% cement suffers in flow if the additive dosage is 1% and below. Comparable flow to the LCC and ULCC castable types is achieved at a higher M-ADS/W addition of 1.3%. The dispersing agents are attaching to the surface of the cement particles. The more cement particles that are present the more additives that are required for good dispersion. The need to adjust the amount of dispersants may also be required if other fine matrix components are modified. The test series shows a trend towards higher M-ADS/W dosages for higher silica fume
Figure 4: Impact of M-ADS/W dosage on wet out and flow of a silica fume containing vibration castable with different calcium aluminate contents
contents but more data is required to confirm that trend.
Impurities and inconsistency Aggregate raw materials can contain trace impurities from their processing such as small residues from floatation or washing processes in Andalusite or other natural high alumina materials, or carbides and metallic inclusions in brown fused alumina. Synthetic alumina based raw materials usually do not have that problem, although white fused alumina fines may contain higher amounts of Na2
O, depending on material selection during crushing
and sizing. These trace impurities are difficult to analyse but in many cases negatively influence the workability and setting properties of castables.
A low cement castable formulation containing 5% silica fume, 15% calcined alumina and 10% chamotte Mulcoa 60 in the matrix fines was tested both with Mulcoa 60 and Andalusite as coarse aggregates. Using dispersing aluminas M-ADS 1 and M-ADW 1 it is possible to adjust the formulation to obtain good flow properties and a reasonable setting time of 10.6 hours for EXO Max for the Mulcoa based castable at a water addition of 5.9%. The Andalusite mix requires only 5.3% water addition. However flow values are slightly worse when compared to the Mulcoa castable. The adjustment of setting is limited to EXO Max of 13 hours by adding 1% M-ADW 1 as an accelerator.
The replacement of CA-670 by CA-470 TI and use of M-ADS 3 and M-ADW 1 results in considerably improved flow properties, acceleration of setting
Table 3: Mulcoa and Andalusite based test castables and properties November 2017 Issue ENGINEER THE REFRACTORIES
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