Spots on Stamped Pool Deck May Be Sealer Diffusion

By Chris Sullivan – ChemSystems, Inc.

Question:

The grayish-white spots on this pool deck
may indicate sealer diffusion, a condition
in which the sealer lifts from the concrete.

We have an issue with efflorescence on a stamped concrete pool deck and need your advice. The last time the pool deck was sealed was about three years ago. About a month ago, we resealed the deck and now it has white spots (see photo). We used a 30%-solids solvent-based acrylic sealer. We applied the sealer to a test area about three months before we resealed, and there did not seem to be any issues. Before resealing, we cleaned the deck with a mild acid wash, neutralized, and then allowed the sealer to dry. We then applied one coat of sealer diluted with xylene followed by a second coat rolled on full strength.

What is causing the white spots, and what percent of acrylic sealer or type of sealer do you recommend for a stamped pool deck?

Answer:

From the picture, it appears that efflorescence may not be the problem. These white spots are actually more gray, and they don't have the look of efflorescence. Instead, this looks like sealer diffusion. This phenomenon, where the sealer is no longer adhered to the concrete, looks a lot like efflorescence from a distance, but on closer inspection is actually more gray or cloudy. Without getting into complex physics, the sealer has lifted (even if only by micro-meters) and it causes the discoloration. The small air gap that now exists between the sealer and concrete causes the light to diffuse and create the grayish-white cloudy look. Moisture-vapor pressure, contamination or over-application of the sealer can cause this problem. To confirm this diagnosis, do a sealer-adhesion scratch test or just lightly scratch the sealer with a key or some other hard blunt-end object. If the sealer holds and does not flake, it may be something else. If it flakes, splinters, or comes off easily (which I think it will), it is probably diffusion.

Another thing it may be is moisture condensation trapped under the sealer. Have you had any major wet weather events in the weeks prior to this occurring? As with most decorative concrete issues, it could be a combination of both of these things, with a little efflorescence thrown in for good measure.

To fix the problem, I would wait until the winter is over and you get some warmer weather. A xylene bath (lightly soaking the problem area with a xylene solvent and back rolling) is the first step. If this does not work, you will probably need to strip off some or all of the sealer.

For any exterior stamped concrete that will be sealed, and especially pool decks, I have a few hard, fast recommendations:

Use light colors for the slab. Dark colors show everything, especially all the chemical residue from pool water, including chlorine, bromine and salt.
Use less-aggressive stamp patterns. Water seeks low spots. If you have lots of "grout" lines and or lots of rough texture, water will puddle in those low areas. Pool decks are wet to start, so the combination can be especially bad. Lighter-texture seamless patterns with simple sawcuts or large light-texture stone patterns work best.
Use a sealer with a solids content of less than 20%. Less is more, and this is especially true when sealing decks around pools and hot tubs. Lower solids allow moisture and air to move through the sealer easier. Lower solids also reduce the risk of slipping accidents. Consider using a solvent-based acrylic sealer with an 18% to 20% solids content. They provide nice gloss, good color development, and greatly reduce callbacks.