Technical Paper
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only a minor influence from the additive and an acceptable working time is still achieved. In the case of a classic accelerator, such as a lithium salt, one could potentially have improved the setting time at 5°C, but it is possible that this same addition at 35°C would have resulted in a flash set.
It should be noted however that even if the working and setting times of the castable become more stable and predictable, the ultrasonic curves shows that the setting behavior in reality is still not the same. This is shown by the profile of the curves which are different. There are several reasons why this should be the case. One was mentioned in the introduction of this paper with Table 1, the fact that different types of hydrates are formed as a function of the temperature. We can expect this to still be the case even if we have manipulated the kinetics of these hydration mechanisms.
Figure 11: Working time at 20°C on aged castables
A clear reduction in working time is observed for the reference castable over time, while the behavior with the temperature stabilizer is stable over the 6 month period that was evaluated.
Compatible retarders for REFPAC 500
In order to retard a potentially rapid setting at 35°C, a number of classic retarders where tested together with REFPAC 500 and the impact on the working time was measured at all temperatures. The results are compiled in Figure 12. For each of the retarders, an addition rate of 0.0125% where used on top of the 1% RP500 already in the formula. The results are presented as a multiple of the original working time with RP500 (Figure 5) but which has been normalized to 1. (ie. for example the result “2“ would mean twice as long time)
Measuring the mechanical strength development, the improved setting kinetics is confirmed. While using REFPAC 500, mechanical strength is always achieved in less than a day while the setting time for the reference castable is over three days at 5°C. The results also show that to some smaller degree, an improved mechanical strength is achieved after heat treatments in the range 110°C to 1100°C. At higher temperatures, 1350°C and 1500°C there is no difference anymore between the samples. Understanding the reasons for the improved strength at intermediary temperatures would require a more in depth study. But given that the differences disappear after temperature at which the material sinter, it is possible that some kind of microstructural change occur after hydration as consequence from the use of the additive.
In addition to the Cold MOR measurements, the Hot MOR results show that there is no negative impact on the mechanical properties of the castable when using the temperature stabilizing additive compared to the reference castable.
The ageing study showed a more stable behavior of castables with REFPAC 500. This can probably be attributed to the boosting effect that the additive has. In fact, no significant ageing was observed over the 6 months that were evaluated, while for the reference castable, there was a gradually changing setting behavior over the entire period of time.
Figure 12: Use of different retarders with RP500 Discussion
The typical behavior of a temperature dependent setting is observed for the reference castable but which is completely transformed when using REFPAC 500. When we translate the ultrasonic measurement into a working and setting time as function of the temperature we see an almost flat profile compared to the more linear temperature dependence of the reference. We can note that it is in particular an acceleration at 5°C that is achieved where the working time is reduced from over three days down to about 10h. At the same time, the impact at 20°C is relatively modest. And, at 35°C, though the setting is already very fast for the reference, there is
It is clearly visible and has been commented throughout this paper that the principal action, and thereby stabilizing effect on the temperature by the additive, is an acceleration at 5°C while impacting as little as possible at 20°C and 35°C. In other words, a castable that has a quick setting at 35°C will remain so when REFPAC 500 is used. An important feature for the developed additive is to avoid a flash setting at high temperatures to a system that otherwise is well functioning. But in the interest of managing also the setting behavior at higher temperatures, different typical retarders where evaluated in combination with REFPAC 500. As can be seen in Figure 12, several of the tested retarders permit a retardation of the system at 35°C. This is also known from other studies [5]. What is perhaps more interesting to point out is that for example in the case of retarder B, the setting at 5°C and 20°C remains at acceptable levels even when the retarder is added. In other words, a flash setting can be easily controlled and avoided, while maintaining a relatively stable setting over the entire temperature range thanks to the accelerating power of REFPAC 500 at low temperature, even if the retarder were to be added as standard. The main reason for doing this would be related to ease of use, but obviously, it would foremost depend on the requirements of early strength development.
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ENGINEER THE REFRACTORIES
March 2018 Issue
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