Calcium magnesium aluminate (CMA) is a novel binder for steel ladle castables.It was reported that CMA has hydraulic properties and is beneficial in the matrix of castables to prevent ladle slag penetration and corrosion 1,2).The CMA grain contains microcrystalline magnesium aluminate spinel which is homogeneously distributed between hydraulic calcium aluminate phases.Applied in castables it develops a unique micro spinel matrix.The present study investigates alumina-spinel castables with matrix spinel employed as fused spinel,as sintered spinel and combined with spinel from CMA in different quantities.It was found that at equal CaO content CMA creates the same strength after drying but higher strength after firing than calcium aluminate cement (CAC) + spinel.This is assumed to be the effect of a higher sintering reactivity of the micro spinel in CMA.When used in alumina magnesia castables CMA enables a reduction of free magnesia leading to an improvement of rheological properties and reduction of cracking risk associated with Brucite formation during drying.Furthermore with less Periclase a lower permanent expansion due to less in-situ spinel formation had been observed 2).The present study shows that this enables a reduction of silica and improves the thermo-mechanical properties.But it is also found that even with CMA and a reduced magnesia content,a small amount of silica should be kept inside the castable matrix as it minimises the magnesia hydration and crack formation during castable drying.
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