The change in strength of hydratable alumina-bonded castable refractories in the presence of magnesia powder and magnesia aggregate, after heat treatment at 110 degrees, and 816 degrees C, and the relationship between the strength and morphology and thermal decomposition behavior of the hydrates in the castables are investigated. The mechanism of bonding facilitated by the presence of magnesite in hydratable alumina-bonded castables after drying is discussed. The contribution of polycondensation process occurring after dehydroxylation of the hydrates in the castables after firing at 816 degrees C is proposed as the principal mechanism for strength development. (c) 2005 Springer Science + Business Media, Inc.