Technical Paper


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TEMPERATURE STABILIZING EFFECT ON SETTING


PROPERTIES IN ALUMINA- SPINEL CASTABLES


Carl Zetterström*, Eric Frier, Magali Szepizdyn, Christoph Wöhrmeyer Kerneos Research and Technical Center, Lyon, France. www.kerneos.com


Abstract


Setting behavior of calcium aluminate cement based refractory castables usually vary with temperature. Commonly, setting times for deflocculated castables can be excessively long at low temperatures, and inversely, too short at high temperatures. This leads to various difficulties at work sites as well as in manufacturing of refractory pre-shaped blocks.


In this paper, a mineral based deflocculating active compound for use in calcium magnesium aluminate bonded castables is evaluated. By use of ultrasonic measurements, the reaction kinetics of different compositions are evaluated across the temperature range 5°C to 35°C with the aim to obtain a stable setting behavior across the complete range.


The active compound is further evaluated in terms of the ageing behavior in a formulated castable over a six months period, as well as the early strength development and the strength development after various heat treatments. Results are compared using a model alumina-spinel refractory formula containing calcium magnesium aluminate as binder.


Introduction


The hydration of calcium aluminate phases is dependent on time and temperature. This dependence is shown in Table 1 Temperature < 10°C


10 – 27°C >27°C f(T°C + time)


Hydration CA + 10H 2CA + 11H CA + 10H 3CA + 12H


2CAH10 3C2AH8


Reaction


® CAH10 ® C2


® CAH10 ® C3 ® C2 ® 2C3


AH8


AH6 AH8


+ AH3


+ 2AH3 + AH3


AH6 + AH3 Table 1: Hydration mechanisms of CAC as function of temperature


Figure 1: Impact of temperature on the velocity profile of an ultrasonic measurement on a refractory castable [3]


The different hydration mechanisms have in turn an impact on the setting behavior and the mechanical strength development of deflocculated refractory castables containing calcium aluminate phases [1, 2].


There are several ways for observing the setting, but one useful method and instrument, is the measurement of the speed of propagation of an ultrasonic wave through the castable [3]. An example of this is shown in Figure 1 and further described in the experimental setup.


The temperature dependent setting translates into difficulties while casting on work sites or on pre-cast shop floors, either due to extended demoulding times in cold countries or flash setting in warm countries.


+ 9H + 9H


In many cases it is possible to influence the setting with retarders or accelerators, but this requires the additional time and effort of re-adjusting the castables. There are a number of typical additives that are used for this purpose. For retarding it is for example common to use different acids. On the accelerating side one of the most applied additive types are lithium salts. Even if this usually works, and can achieve the desired effect of accelerating at low temperature, it could impact the castable properties at high temperatures. Since ambient temperature at jobsites is difficult to predict sometimes the retarding or accelerating additive has even to be added manually at the jobsite during mixing with water. In addition, there


18 ENGINEER THE REFRACTORIES March 2018 Issue


A MINERAL BASED ACTIVE COMPOUND WITH


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