  Refractory producers face many challenges in terms of producing
MgO-containing castables due to the high likelihood of magnesia to hydrate in
contact with water, resulting in Mg(OH)2 generation. The expansive feature of
this transformation affects the performance of such refractories, as (i) if
this hydrated phase is not accommodated in the formed microstructure, ceramic
linings with cracks and low green mechanical strength will be obtained; and
(ii) if crack-free pieces are prepared, they should present low porosity and
reduced permeability, which require special attention when heating these
materials. This work investigated the ability of various additives in the
optimization of the drying behavior of Al2O3-MgO castables. Vibratable
compositions were tested after incorporating polymeric fibers (PF), an organic
salt (OAS), SiO2-based additive (SM) or permeability enhancing active compound
(MP) into the dry-mixtures. Various experimental measurements were performed to
infer the role of the drying agents to prevent the samples explosion and
whether they would also influence other properties of the castables. As
observed, OAS and MP helped to inhibit the MgO-bonded samples explosion even
under severe heating conditions (2-20C/min) and increased their green
mechanical strength and slag infiltration resistance when compared to the
additive-free composition. On the other hand, the addition of polymeric fibers
(PF) or silica-based compound (SM) to the formulations was not able to prevent
the castables explosion when using a high heating rate and other side effects
could also be observed when testing these materials. Thus, the selection of
suitable drying agents is a key issue, as they may allow the development of
MgO-bonded castables with enhanced properties and lower spalling risk during
their first thermal treatment.
