A patio looked great on the day we finished installing it. But within two weeks, it had deteriorated into a gray/white color instead of the terracotta/charcoal patio that was visible on the day it was installed.

The concrete sealant was applied 24 to 48 hours after the concrete was first laid, and within two weeks, it had turned gray/white. The contractor then returned two weeks later, the patio was washed with a jet wash and then a second coat of sealant was applied, making the gray/white colour even worse.

The patio is fine when it is wet (terra-cotta/charcoal), but when dry it becomes gray/white.

The project was completed more than a year ago. The gray/white appearance apparently is salt bloom, which is naturally occurring. However, as you can see, the whole patio is affected and there are no crystalline forms present on the surface.

What could the problem be, why did it occur, and can it be fixed?

Without being able to see the work up close, there are several possible problems. One could be efflorescence of the concrete caused by moisture moving through the slab, dissolving some of the calcium hydroxide in the concrete, and bringing it to the surface where it combines with carbon dioxide to make calcium carbonate, a white colored thin coating on the surface.

But the likely problem is the sealer de-bonding from the concrete surface. If the slab was not cleaned properly in preparation for the sealer, there could be a bonding problem. If a powdered release was used, this could easily be the problem. When the sealer doesn’t bond properly, there is a small air void layer between the sealer and the concrete. This shows as white color when light reflects in the void space.

The moisture content in the concrete at the time of sealing also can cause problems. Some sealers are more sensitive to this than others. I like to stay with all acrylic solvent-based sealers, as they seem to perform best with moisture. Acrylic sealers are best because they allow moisture vapor to pass from concrete to the atmosphere. If a product was used that does not allow moisture vapor to pass through, that also could be the problem. In this case, the sealer will have to be entirely removed and a suitable product installed.

If there is only an attachment problem, your installer can work the white areas with the solvent used for the sealer. The sealer will go back into solution and attach to the concrete.

I don’t know what your temperatures are at this time, but if they are less than 50 degrees F, I suggest waiting until spring before attempting any repairs.

The answer was provided by Joe Nasvik, senior editor of CONCRETE CONSTRUCTION