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Efflourescence in Concrete Floors

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Joined: Apr 17 2007

We live in a 1958 Eichler in Lower Lucas Valley. We inherited floors that were a mish-mash of carpet, tile over VCT/linoleum, and artificial wood. The radiant heating system works and the boiler pressure has never varied from 10lbs.

We decided to remove everything, wet grind the floors smooth, impregnate the floors with an architectural sealer called QC Cemguard, apply two coats of acrylic-based sealer, and then polish with a water-soluble finish. The whole project took place over four weeks, and we finished this past Sunday.

The day after completion, I started to notice efflourescence coming through the floor. It started as powder that came through some of the visually rough areas. As of yesterday, it got worse and was starting to bubble up under the acrylic sealer in several spots. Overall, it affects about 5-10% of the floor. This was the first time that our floor finisher used QC Cemguard, which is formulated to make concrete floors impenetrable to water. (The spec sheet noted that it can be used with existing concrete floors.) Before grinding, I did not notice any wet areas anywhere on the floor, and very minor efflourescence.

Can anyone offer any advice on how you have been able to permanently seal your slab and prevent continued efflouresence? Is it possible that grinding the slab encouraged the development of efflouresence? Does anyone know if the efflourescence presents a safety risk? (Our 1.5 year old puts everything she can find in her mouth.) We are installing baseboard soon, and I'd like to avoid skimcoating (which may also delaminate), and I'm not comfortable laying down other floor materials over an unstable slab. Thanks for your input!

Joined: Aug 6 2006

I'm not an expert in this subject but I have my suspicions as to why this may be happening.
Efflorescence can occur when there is some type of water penetration through the slab and salt deposits occur (which cause the milky-looking color). Sometime blisters will also result in efflorescence. I'm not quite sure if this is due to any sort of condensation from the radiant heat pipes or ground water penetration through the slab. You may want to do a moisture test on the slab to see if this is occuring. You can do this by taping down a small square of plastic over popped blister areas and seeing if any condensation builds up.

When you wet ground the floors did you wait until it was dry and wipe it down again/vacuum to get rid of any concrete dust? IT may be that the concrete dust combined with any moisture from the sealer may create a chemical reaction that causes efflorescence. Once it is trapped under the sealers, the resulting moisture may also cause blistering to occur. It is also advisable to not apply the sealers during rainy, foggy, or humid weather. The best time to apply the sealers is probably during the drier times of the year.

I have not heard of QC Cemguard but my office has used Scofield Clearcoat with success (as long as it can counteract the hydrostatic pressures through the slab.) I said before in another post that many Eichlers were constructed without a proper vapor barrier under the slab so it's almost as if you'd have to treat your slab like a swimming pool project and seal it accordingly.

If you choose to skimcoat again, make sure that you prep the surface so that it is porous enough for another sealant application to avoid delamination. I believe that some professional installers will acid wash existing application to put a new one on (They are professionals so they are more apt to handle these chemicals more safely than any lay-person).

As far as I know, efflorescence is not much of a safety risk but chipped off blister caps are (if not chemically dangerous, then most likely choking hazards). If you are looking at putting other flooring materials over the slab, you can mitigate the moisture penetration by laying down 30# roofing felts over it first (lapping at least 12") for wood floors and hardiboard over mortar/omnigrip for ceramic tiles. If you do this, then you probably wouldn't have to reseal the slab.
If you don't want anything else on the slab to get the best effect out of your radiant floor heating, you may want to ponder just refinishing.

I don't know if this helps but hopefully it will give you a clearer direction.

"Any intelligent fool can make things bigger, more complex, and more violent. It takes a touch of genius - and a lot of courage-to move in the opposite direction."-E. F. Schumacher.

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Joined: Apr 17 2007

I did a moisture test in the worst looking room, and got some condensation after two days. During those two days I did turn up the heat up from 58 to 62 degrees. Needless to say, I'm dissapointed that our floors are not water tight.

To clarify, we are not getting efflorescence through a skim coat; it is coming through two layers of clear solvent-based acrylic and one layer of polish. Prior to the acrylic, we wet-ground the floors, and let them sit for almost four weeks. They were cleaned several times after grinding, using both wet and dry pads. We then applied five treatments of QC Cemguard that is supposed to prevent water penetration through concrete. If this is also what Scofield Clearcoat is supposed to do, perhaps it would make sense to try it. Surfer-Architect, thanks for the insight and suggestion.

The effect we were trying for was to simply refinish the floors and live on them. We are not concerned with the old carpet glue stains, outlines from the original linoleum tile, repairs to tacking strip nails, or cracks in the slab. At this point, I'm open to suggestions. If a skim coat or paint would help resist further efflorescence, that would be great. We can't afford ceramic tiles, and we're not crazy about linoleum. Thanks for the help so far!

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Joined: Apr 19 2007

in our old place, we had concrete floors on the bottom level. it was a newly poured slab, tinted/stained an enviable charcoal with a neat, organic patina (pic / pic)... the floor seemed barely coated with anything save a good burnish (i.e.: you could not "feel" a sealer)... it powdered up, especially under rugs and around furniture. we got used to it and damp-mopped it up from time to time. we have a cat who didn't seem to mind, but not a toddler. eventually, it subsided to the point it was hardly and issue. we had a similar situation in a rental in sf with a heavier coating in which we had blisters pop up -- which was much more of a pain imho... my suggestion would be to give it a few months to see if it subsides before doing anything more drastic like coating it with something else that might blister... it might still be the moisture from the wash/wet-grind and not from the ground itself. either way, the efflorescence should be easy enough to mop up (as opposed to multiple coatings of various sealers which would be tough to remove)

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