Q.: We have lived in the tropics in Central and Latin America where concrete tile floors are common and all I've ever seen done to maintain them is to swab and mop the floors with diesel fuel. However, we now have a 25-year-old house in the Bahamas in which the 3/4-inch-thick concrete tiles set on the concrete slab have a hard, white calcareous deposit on them, especially under appliances, rugs and furniture. The ground under the slab is mostly limestone and the floor probably has no moisture barrier under it. The tiles are scratched from many years of abuse and no maintenance. The tiles themselves are beautiful, however, and I would like to restore and maintain them.
I was advised to acid wash a test area, clean with proprietary cleaners and degreasers, then apply a urethane sealer and finally liquid wax. The acid wash discolored the test area unevenly, so I removed the deposits with dry abrasive scrubbers, then applied the prescribed materials. After two weeks the whole surface discolored from the urethane film, so I removed everything again with the abrasive scrubbers. Then the white deposits started building up again during the rainy season.
Can you tell me how to remove the white deposits and how to seal the tiles with an oil or breathable sealer?
A.: It is possible that moisture is continually moving up from the soil, through the slab and tiles, bearing dissolved materials and leaving them on the surface when the water evaporates. You could determine if this is happening by making the following test during a dry period of the year when the relative humidity is low enough so that atmospheric moisture does not condense on the floor. Remove a tile and whatever mortar or adhesive is sticking to the concrete surface. Darken a small portion of the slab surface with a black, nonfading felt-tip marking pen to provide good color contrast. Observe the marked area periodically to see if a white deposit is building up on it. If so, moisture is moving up from below and bringing with it salts from the soil or slab or both. (In some locations near the ocean, seawater has been used in the original concrete mix proportioning, thus loading the concrete with sea salts.) You may have to find some method of stopping this continual upward flow of moisture with its virtually inexhaustible supply of salt.
On the other hand, the moisture movement may be mainly or exclusively the result of the moisture that condenses on the floor when the humidity is high and is then absorbed by the tiles. This moisture dissolves some salts from within the floor and then when the humidity goes down the moisture moves up to the surface again. There it evaporates and leaves its dissolved materials behind as the white deposit. Any calcium hydroxide and calcium bicarbonate present in the deposit react with atmospheric carbon dioxide to form calcium carbonate. This won't be redissolved and carried back down by subsequent condensation because it is practically insoluble in water. In this way an increasingly thick white deposit builds up.
If this is what is happening, the main effort might be to clean the top surface thoroughly during the dry season, preferably with a scrubbing machine that doesn't require abrasive. Then you could treat the tile surface with a material capable of preventing the absorption of the moisture that condenses during humid weather. A good wax might work, or possibly even a treatment with a generous amount of the diesel fuel or mineral oil that appeared successful in the other tropical areas where you've lived. If the moisture doesn't go in, it won't be bringing back its load of salt.