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Based on a kinetic model that describes the hydration reaction via nucleation and growth mechanisms, formic, acetic or propionic acids were analyzed as hydration site activators on magnesia particles with different reactivity and granulometry used as a main binder source in high-alumina refractory castables. These additives induced the hydroxide generation in MgO suspensions and the hydration degree was proportional to the size of the carboxylate chain. Moreover, a more controlled growth of the brucite crystals could be achieved when formic acid was added to the refractory castables. By adjusting the amount of formic acid in the castable mixtures, large crack-free samples could be prepared, which pointed out the role of this additive to prevent the drawbacks associated with MgO hydration. Therefore, the suitable kinetic control of the nucleation and growth of brucite crystals on the magnesia surface allowed for the development of high-alumina magnesia-bonded castables. [ABSTRACT FROM AUTHOR]