Five years ago we supplied the concrete for an elevated slab that was later covered with vinyl composition floor tiles. We recently got a letter from the building owner telling us that the floor tiles were coming loose and that the flooring installer had inspected the problem, measured the pH at the surface, and found that it exceeded 9. A representative from the manufacturer of the tile and tile adhesive told the installer that the high pH had eaten the adhesive away in 5 years, causing the flooring failure. The manufacturer seems to be placing the blame on our concrete.

The building owner contacted us and asked how to treat the concrete so the problem wouldn't recur if the tiles were removed and replaced. What information can we give the owner?

First, you should explain to the owner that concrete is by nature a high-pH material. The pore water in newly placed concrete can have a pH as high as 12 to 13 because a significant amount of calcium hydroxide is produced as a byproduct of cement hydration. Smaller amounts of sodium and potassium hydroxide are also present. After a floor surface cures, atmospheric carbon dioxide reacts with some of the calcium hydroxide, which may reduce the pH to somewhere around 9, but generally not much lower because the absolute low pH for fully carbonated concrete is about 8.4, and seldom is concrete fully carbonated. Thus it isn't surprising that the installer measured a pH above 9.

The problem may also be related to moisture vapor coming to the concrete surface and carrying dissolved alkalis with it. This can result in an accumulation of high-pH water immediately beneath the tile, causing the adhesive failure. The important point to note here is that adhesives must be compatible with the surface they're placed on, so when making adhesives to be used on concrete, the flooring manufacturers need to design them to perform in a high-pH environment.