Moisture through Concrete Floor in Finished Basement

Our house is in Puerto Rico where the temperature never drops below 60F. The basement covers only half of the house as the other half sits on the ground. The basement has a 40 ft long cement block wall behind which is solid ground. The opposite wall of the basement has windows to the outside. The basement is one room 14ft wide for a total of 560 sq. ft. It is finished in a style which is typical for PR. A cement plaster finishes the walls and the floor has locally-produced cement tiles which have colored patterns and are polished very smooth on top of a reinforced cement slab. During the summer and fall, rainfall is very heavy. It rains daily in the afternoon. Winter is the dry season.

We have a couple of problems with the basement but the most serious is the floor. Over time it develops a layer of a whitish very thin powder. About two years ago we rented a polisher machine and repolished the tile getting rid of the powder that had built up over several years. We thought this was due to the basement not being used but it is happening again although we have been using the basement the last couple of years.

Recently we bought a dehumidifier (30 pints per day) which is keeping the humidity between 45 and 52% even now in the rainy season. The dry conditions have made the white powder on the floor quite obvious. It seems to have been dissolved in a very, very thin layer of moisture which has now evaporated.

The dehumidifier is working fine in keeping the air dry but I am still worried about the floor. In reading the information on this web site, I learned that the moisture is probably coming up through the concrete floor and not (as I thought) being condensed on the floor from the air. The powder is then probably cement dust which comes up dissolved in the moisture. Have you ever seen this effect? In any case, the dehumidifier will help in making the moisture evaporate immediately upon reaching the surface of the tile but the dust will still probably be brought up and deposited. Am I right?

The information here mentions that there are treatments that can be applied to the cement floors of basements to make them water tight. What are those products? Can they be applied to a polished cement tile and have it be esthetically pleasing? I figure that might be the only solution to our dust problem. It should also diminish the working time of the dehumidifier which will save us money.

 

I have honestly never seen or heard of this powerder..


You asked neighbors or friends if they have the same issues?? maybe the basement don't have drain tiles around the home to help keep the water away?? hard to say.. Sorry I couldn't help you anymore than this.

 

This white powder in my experience with putting up concrete basements, brick walls may be effervescing, I have this occur with foundations and brickwalls, this may not be your problem, but if you wash the powdered area down with a strong mixture of water and regular house vinagar, I think you will find that this powder will go away and stay away for quite a long time, when it comes back and it always does repeat this treatment I think with each treatment the time in between it coming back grows longer and longer, the only thing I could think, you could do would be to seal the tile with polyurthane, but this would leave the tile with a very shiny suface and would eventually flake off and need redoing. I hope I have helped you more then I have confused you.

 

It certainly sounds like efflorescence. It is formed when water soluable salts from the concrete are carried to the surface and are deposited and build up when the capillary moisture evaporates. The source of the moisture is probably the soil under the slab, which gets wetter during the rainy season.

Usually they are calcium salts from the raw materials used in the manufacture of cement. The source could be either the concrete floor ot the concrete tiles. The foot traffic apparently did not remove the salt.

In most outdoor applications, efflorescence is removed by the weathering process. The white deposit is slightly soluable in water. Because it is alkalai, the vinegar(acidic) suggested earlier will remove the deposit.

I suggest not coating the floor. I assume you mop the floors ocassionally. I would make it a practice to add some vinegar to the cleansing water to hopefully remove it as it forms.

I saw the same thing in Spain. In this case, the alkalai ground water and the salts in the concrete were able to pass through the porous clay tile floor. Vinegar solved that problem.

Eventually, the salts will leech out of the concrete, but it can take a long time.

Dick

 

Your responses have been most helpful. I do believe it is the salts and that we now have a way of combating them.