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are frequent reports of a lithium shortage in the world [4], much due to the development of lithium consuming batteries in for example electric cars. Therefore, it is of even more importance to provide a lithium free accelerator to achieve acceptable setting behavior at work sites.


The work presented in this paper aims at developing a lithium free deflocculating active compound in order to reduce the temperature dependence and obtain an as regular as possible working time and setting time in the temperature range of 5°C to 35°C. While the temperature dependence of hydration as described in Table 1 is true for most hydraulic phases and type of castables, this work focuses on an alumina-spinel castable using calcium magnesium aluminate cement as binder.


Experimental setup Model Formula


For the purpose of homogeneous measurements and comparisons of applicative performance, a model formula was used as presented in Table 2


Raw Material Tabular alumina Sintered Spinel Reactive Alumina


Calcium Magnesium Aluminate Binder


PCE


T°C Stabilizer Water


Specification 0-5mm 0-1mm P152SB


CMA 72


Peramin® REFPAC®


Table 2: Model formula for a refractory castable


REFPAC 500 contains both mineral phases and organic additives. The chemical composition is mainly Al2O3, CaO, MgO and a LOI<10%).


Ultrasonic measurement The wet mixed castable is placed in the mould and vibrated. A signal is generated from a piezoelectric transducer and the speed of that signal is measured and plotted as a function of time, which generates the kind of graph shown in Figure 1.


Working and Setting time can be identified from analyzing the ultrasonic curves. There are several ways to do this analysis, and perhaps some discrepancies as to how it should be done. One way is to identify the different changes in the curve, and correlate these with the exothermic profile of the concrete. In this work however we have chosen a more pragmatic method by always identifying the time at specific predefined ultrasonic velocities. These velocities are 2750m/s and 4000 m/s. In general, we observe that at the first velocity of 2750 m/s, we have already a hard structure but there is not yet any particular strength. In this paper we refer to this as a Working Time, even if in reality it is usually closer to what would be called a final set on a Vicat measurement (ie zero needle penetration). At the second velocity, 4000 m/s, the castable has developed mechanical strength to a degree where demoulding is possible, typically above 10MPa in compression. Here we call this Setting Time. This point is usually slightly before one reaches the exothermal maximum.


Climate Chambers Special climate chambers were used in order to reproduce reliably different


March 2018 Issue Figure 3: Ultrasonic curves for the reference castable


Al200 500


0.1 0


3.5 %


66 14 11


9


0 1


Technical Paper


temperatures; 5°C, 20°C and 35°C. These chambers are large enough for each step of the manufacturing to be done within the chamber (stocking and mixing of the raw materials and cast into the ultrasonic measuring device on a vibrating table).


Mechanical Strength


Samples of 30x30x160mm where manufactured by vibrocasting and used for measures of MOR and CCS. 40x40x160mm samples where used for measurement of Hot Modulus of Rupture.


Results Ultrasonic curves


The first result to take into account is the analysis of the setting behavior through the use of the UltraSonic measurement. The following graphs where obtained at different temperatures for the reference castable and the one containing REFPAC 500 in Figure 3 and Figure 4 respectively.


Figure 4: Ultrasonic curves for REFPAC 500 material


A quite common setting behavior is observed for the reference castable with a temperature dependence. With REFPAC 500, this dependence is reduced.


Setting as function of temperature


By use of the method of taking a specific ultrasonic velocity of 2750 m/s and 4000 m/s as working time and setting time respectively, as described earlier, we can translate these ultrasonic curves into the Figure 5 and Figure


ENGINEER THE REFRACTORIES 19


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