Small amounts (e.g. 0.25 wt%) of MgO in water greatly accelerate the hydration of Al powder, attributable to increased basicity of the solution. This leads to dissolution of the native Al2O3 film and early hydration products, decreasing the thickness of these protective layers and/or exposing new Al surface. The effect of this is exacerbated by additional heat released by the accelerated hydration of Al and MgO. The significance of this on the use of Al powder in refractories castable matrices is discussed.