Samples taken from the core of real-sized castable pieces were studied under TGA, DTA, SEM, and XRD techniques after hydration during normal dry-out process. The TGA was also performed at bulk samples, and the effect of volume on the hydration process was evaluated. The castables consisted of dead-burned magnesia with additions of calcined and/or tabular aluminas, or microsilica to the matrix. The addition of alumina in combination with magnesia increased the hydration rate during the drying of the castable due to a lower permeability of the casted piece. Addition of microsilica, on the other hand, reduced damage due to hydration. The microsilica promotes the formation of a low crystallinity chrysotile precursor, due to its reaction with the magnesium hydroxide formed after the hydration of the surface of magnesia grains. This phase alleviates the stress generally associated to hydroxide formation, which otherwise leads to structural spalling.