Concrete is poured within excess of water typically called water of convenience that does not bind with the cement particles. This excess water takes many years to remove itself. In addition there are number of mechanisms that bring water under the slab. One such mechanism is clays suction from around the perimeter of a foundation. I document that more fully in my previous blog, "Evidence of MoistureLevel Smart Foundation System Performance." In addition through the “stack effect”, water vapor along with other gases is pulled in from around the perimeter of a structure to replace gases that are transmitted up through the foundation into the structure and by heat convection drawn up into the attic. This is a well-documented process for one of those gases, radon.
Particularly here in the arid Southwest, air-conditioning dries the surface of the slab promoting vapor transmission as well as condensation of water vapor as it is pulled through the slab. As water condenses the pH level is usually around seven which can promote dissolution and attacking of cement particles causing great greater voids in the concrete thus exacerbating the problem. Once this process has begun de-bonding of most surface treatments becomes more problematic over time.
Most solutions to this problem involve installing sealers on the surface of the concrete. This is problematic at best for the following reasons:
The problem here in the Southwest is exacerbated by the reluctance of installers to install a vapor barrier under the slab. Because of the arid hot curing conditions, placing a vapor barrier underneath a slab promotes extreme slab curling due to the difference of evaporation on the sealed bottom versus the open top.