Technical Paper


In addition, experiences and results with matrix compounds including cements are reported. This can contribute to higher robustness in shelf life and installation of more sophisticated low and ultra-low cement castables.


Introduction of raw materials for robust castable behaviour


CA-470 TI – temperature independent cement is an additive free 70% Al2 O3


cement for use in low and ultra-low cement castables (LCC and ULCC) without or with silica fume. CA-470 TI has a fineness of 90% below 45µm and a d50 of 8µm (Cilas 1090) [4].


Dispersing Aluminas ADS/ADW for silica-free castables and M-ADS/M- ADW for silica-containing castables are a combination of modern organic additives with reactive alumina and other inorganic materials [2]. The ratio of the retarding “S” type to the accelerating “W” type is varied to achieve the control of setting time. The total amount of dispersing alumina is recommended to be about 1% by weight in low cement castables [3].


E-SY 1000 and E-SY 2000 (containing spinel) are bi-modal reactive aluminas with a typical d50 of 1.7µm and specific surface area (BET) of 2.0 m²/g (E-SY 1000) and 1.4µm and 2.3 m²/g (E-SY 2000) [5].


How can robust castable behaviour be achieved?


Different aspects of castable behaviour are highlighted in the paper to demonstrate the effect of different influencing factors and actions which can be taken either by changing ambient conditions or by using less sensitive raw material concepts to improve the robustness and reliability of castables:


• Mixing and flow behaviour • Impurities and inconsistency • Temperature sensitivity • Aging behaviour


Mixing and flow behaviour


On-site installations always start with the mixing of the dry mixed castables with water in order to achieve a good working consistency for proper placing, de-airing and densification without segregation occuring. Usually the water addition range required for the respective castable is given by the refractory supplier. However, there are many factors why accurate water dosage can fail on-site, e.g.:


Table 2: Overview of test castables and water dosage ranges to investigate the impact of water addition on castable properties


• Mixer type (low power) • Mixing batch size (too small or too big) • Improper dosage measuring equipment (low accuracy) • Long castable wet out (excess water addition) • Short working time (early flow decay)


Lab investigation with pure alumina low cement vibration castables and different water dosage levels shows the impact of water addition on the castable properties. An overview of the test castables and the water dosage ranges are given in table 2.


Figure 1 shows the flow values for the different water dosages. A minimum amount of water is required to ensure good flow and placing properties. With increased water addition flow values increase. For the high water addition levels separation on the flow cones and the cast pieces was observed. The tendency for water separation was lowest for VIB-DA and stronger for VIB-STPP especially at a water dosage of 7%.


VIB-DA requires the lowest water addition to achieve good flowability when compared to VIB-PA and VIB-STPP. With 4.5% a vibration flow of 20 cm is achieved even after 60 minutes. For the other castables 5.25 – 5.5% is needed to achieve the same level after 10 minutes. However, these mixes still deteriorate in flow after 60 minutes with 17 cm for VIB-PA and no flow for VIB-STPP. An even higher water addition is needed to obtain a suitable flow at 60 minutes and then the risk of segregation is high.


In a second step the setting behaviour was investigated by measuring the temperature development of the exothermic reaction (EXO). VIB-DA shows a very stable EXO Max over the entire water dosage range with a variation range of less than two hours (figure 2). VIB-PA and VIB-STPP generally show very long setting times due to the use of citric acid as an additive. EXO Max for VIB-STPP increases further with higher water addition. For VIB-PA, EXO Max shows little variation and no trend with different water additions.


The cured and dried cold crushing strength results are shown in figure 3.


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Figure 1: Vibration flow at 10, 30 and 60 minutes for test castables mixed with different water additions


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Figure 2: EXO Max values for test castables mixed with different water additions


ENGINEER THE REFRACTORIES November 2017 Issue


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