Q.: We placed a concrete floor slab for a refrigerated room in a cold-storage facility. The room had an aluminum ceiling that is now pitted. The general contractor says water emitted by the concrete as it dried condensed on the ceiling and caused the pitting, so repairing the problem is our responsibility. Can water from concrete cause this problem?
A.: If the concrete were in direct contact with aluminum, alkali hydroxides from the cement could react with it and perhaps dissolve parts of the metal to produce surface pitting. However, water that evaporates from concrete contains no alkalis. So we don't see how water condensing on the surface could cause the pitting you describe.
If it's a low ceiling and cement paste splattered on the surface, that could perhaps cause surface pitting, but we think that's unlikely because usually superficial contact with concrete only dulls the appearance of aluminum surfaces.