Microsilica or MgO grain size: Which one mostly affects the in situ spinel refractory castable expansion?

摘要

Microsilica is commonly added to alumina-magnesia castables to counterbalance the in situ spinel expansion. This effect is attained by the generation of a low-melting temperature phase, which also affects the expansive reaction kinetics. Additionally, the MgAl_2O_4 formation depends on the grain size of the reactants. The use of coarse magnesia grains results in lower Mg~(2+) dissolution and could lead, at 1500 deg C, to forsterite development (Mg_2SiO_4). For finer MgO, silica was detected at the edge of the spinel grains. Considering these aspects, this work evaluated the effect of microsilica content for different magnesia grain sizes (<45 or <100 mu m). Due to a faster spinel formation for the fine MgO source, microsilica counterbalanced the MgAl_2O_4 expansion. Conversely, for the coarser MgO, silica increased the Mg~(2+) dissolution, speeding up the spinel formation and expansion. Therefore, microsilica presented opposite roles, pointing out that it does not always counterbalance the spinel expansion. This work also indicated the need for a systemic approach for the expanding design of alumina-magnesia refractory castables.