The corrosion product of aluminum appearing in mortar has been studied with XRD, EPMA and EDX methods. The corrosion product consisted of two layers which were the mixtures of bayerite, hydrargillite and nordstrandite. The inner layer was gel-like in which sodium and chloride ions concentrated. The outer layer was crystalline and flake-like. Sodium ions in the inner layer reacted with Al(OH)4- ions to form hydrated sodium aluminate, which easily dissolved and changed to crystalline trihydrated aluminum oxide and kept the high concentration of hydroxide ions, causing severe corrosion.
