Microstructure of Cement/Clay paste with Aluminum bar after 1 year storage in Fresh Water and in 6% NaCl
Harald Justnes, Tobias Danner
Appears on pages(s):
Aluminum, calcined clay, chloride, durability, hydration, microstructure, reinforcement
The long-term stability of aluminum metal in binders intended for concrete has been studied. Aluminum was cast in paste where 55% cement was replaced by calcined smectitic clay. After 7 days of curing several samples were submersed in either distilled water or in a 6% NaCl solution. After 1 year of submersion, the microstructure of samples submitted to both exposure conditions was analyzed. Chloride corresponding to 1.5-2% Cl- of cement mass had reached the Al-bar. The surface of the Ø 10 mm aluminum bar had corroded to a depth of less than 10μm, irrespectively if it was submerged in water or in 6% NaCl. There was no sign of intergranular corrosion. The hydrated binder consisted of amorphous calcium silicate hydrate gel and crystalline layered double hydroxides of hydrocalumite-type. When cured in water, the crystals were a mix of hemi- and mono-carboaluminate, but when stored in 6% NaCl, the carbonate/hydroxide was partly replaced by chloride. There was a dense binder zone formed around the ≈ 15 μm oxide layer of the aluminum bar richer than the overall binder in aluminum. Some of the alumina formed on the metal surface may have been partly dissolved by alkalis and precipitated/reacted with the nearest binder region and densified it and thus preventing further reaction.