Technical Paper


to a range suitable for on-site installations and higher cured and dried strength at a constant water demand. Results are summarised in table 3.


Silica fume is known as a “troublemaker” in refractory castables. The quality of silica fume influences the castable properties, e.g. wet out, flow and setting. Myhre [6] reported on the impact of impurities such as carbon and potassium in 97% grade microsilica on flow and setting. Higher impurity levels deteriorated the flowability and significantly increased the working time of castables. Usually these quality variations or even batch to batch variations are often not possible to detect in simple quality control testing, but may have a negative impact on lining installations later.


To increase the reliability of silica fume containing castables CA-470 TI can be used as cement binder. Repeated testing with eight different fume grades at 5°C ambient temperature showed a guaranteed setting start within 24 hours whereas the use of CA-14M resulted in unpredictable setting behaviour from fast to “never setting” [7].


Temperature sensitivity


The ambient conditions, such as temperature and humidity, have an influence on the setting behaviour of calcium aluminate bonded castables. This has taken into account for the installation of refractory castables especially in pre-cast shape production where often more than one casting sequence per day is scheduled and reliable setting behaviour is key.


The working and setting time of castables is adjusted and tested under lab conditions usually at 20°C. However, ambient conditions in practice are not as stable as in the lab due to seasonal temperature variations and varying temperature of the dry mixed material stored in the warehouse or outside. Higher temperatures will shorten the working time and low temperatures will retard the setting time and strength development. Schnabel et al. [8] reported about calorimetric measurements with an 80% tabular alumina and 20% cement mix performed at 23°C and 20°C. The reduction of ambient temperature of only 3°C retarded the main hydration peak by six hours.


To counterbalance the impact of ambient temperature on-site, cold, warm or hot water is sometimes added to the dry mixed castables. The effectiveness of this countermeasure has been systematically investigated in a test series in the laboratory. A low cement vibration castable with ADS 3/ADW 1 as additives and a water demand of 4.1% was used as a test castable. EXO Max at 20°C was adjusted to about 8 hours. Dry mix and


www.ireng.org water temperatures of 5, 20, 35 and 50°C were chosen for the test.


The flow properties over a time of 60 minutes remained stable independent from the dry mix and water temperature (figure 5). The impact of the mixing water temperature on the setting time is negligible. Only very small variations are observed. Conversely, the change in dry mix temperature has a strong influence on the castable setting as shown in figure 6. The comparison of 5°C vs. 50°C dry mix temperature results in a difference of about seven hours for EXO Max. In addition dry mix temperatures ≥ 35°C lead to slightly higher dried strength values compared to low dry mix temperatures.


The investigation showed that a long setting caused by usage of a cold castable mix cannot be counteracted with the addition of hot water. Instead it would be beneficial for low ambient temperatures to increase the dry mix temperature prior to an installation, e.g. by storage in a heated cabinet or warehouse, to reduce the tendency towards extended setting.


A guarantee for reliable setting, especially at low ambient temperatures, is the cement binder CA-470 TI as described by Buhr et al. [7; 9]. For CA-14M tested in three different low and ultra-low cement castables at 5°C ambient temperature a wide range of setting start times from 8 to 50 hours was observed in repeated testing. The replacement of the cement by CA-470 TI resulted in a reduction of the variation range to 6 to 20 hours. Although CA-14M may also occasionally show a reasonable setting start time at 5°C it often can become an extended setting start far longer than 24 hours. This unpredictable behaviour at low temperature is inherent for all regular 70% Al2


cements in the market. In comparison CA-470 TI demonstrated a setting guarantee even at low temperatures.


O3


Aging behaviour Castables


are tailored


to


the


individual


on-site conditions by the


determination of the additive system and the optimum dosage. Usually, after castable production, properties such as flow and setting behaviour are checked by quality control. However, castable material is not used immediately, but first transported and then stored for some weeks or even months. During storage aging can occur by interaction of castable components, e.g. between dispersion and setting control additives and calcium aluminate cement binders, or by ambient conditions. Such interaction can negatively influence workability, setting behaviour and also strength development.


Figure 5: Impact of dry mix temperature on flow at 60 minutes


Figure 6: Impact of dry mix temperature on exothermic reaction (EXO) 14 ENGINEER THE REFRACTORIES


November 2017 Issue


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