I am not sure about all the metallurgy behind it, but apparently aluminum doesn't play well with dissimilar metals. Electrons are stolen, reinforcing and conduit is damaged--it's a mess. They call it galvanic reaction. A quick search on this site would give you some more helpful info.
It’s not just bi-metallic corrosion. Aluminium is aggressively attacked by alkaline conditions, and Portland cement is highly alkaline. Put an aluminium embedment in your concrete slab (even if it has no steel reinforcement), and you will soon end up with white powder where the aluminium used to be.
It was a pretty common problem with high-rise apartments built in Australia in the 1970s and 1980s – aluminium handrails were cast into the concrete verandas, or bolted directly onto the surface of the slab, and subsequently corroded off at the surface of the slab. I always check out the handrails carefully when staying at a hotel or holiday apartment before I let the kids out onto the veranda.
The code prohibits the use of aluminum in structural concrete unless it is effetively coated or covered. Aluminum reacts with concrete and, in the presence of chloride ions, may also react electrolytically with steel, causing cracking and/or spalling of the concrete. Aluminum electrical conduits present a special problem since stray electric current accelerates the adverse reaction.
I agree with all of the above. I would like to add, if you must have concrete and aluminum in contact; ensure that you have plenty of tar type material separating the two.
