...Once the cure is completed the water is no longer needed in the concrete. It has not been "used up" nor has it "become part of the concrete". What has not drained out or evaporated is still there as water. As far as the concrete is concerned, it makes no difference if the water stays (pillars in the lake) or goes (columns on a building). In fact what eventually happens is that the water content of the concrete comes into equilibrium with its environment. If soil in contact with the concrete is dry, it will dry out the concrete. If the soil is wet, it will add water to the concrete. In fact for houses, foundation walls and often footings are covered with a moisture barrier to help keep the concrete dry -- actually to help stop the migration of water from the soil through the concrete and into the house.
In the same way, air humidity levels will come into balance with the concrete and most dry winter heated basements will dry out the 600 or so gallons of water in the original foundation mix in about two heating seasons or 18 months.
Sealing concrete can slow down water evaporation, but will not stop it. A concrete sealer's primary function is to prevent liquid water from soaking into concrete and secondarily to harden the surface reducing concrete dust formation.
In residential construction, if concrete still has mix water in it (we often call that "green" concrete or "young" concrete), there can be problems caused by that water as it evaporates out of the concrete. We see that with Ontario's bag insulation hung on basement walls with large quantities of water getting caught in the insulation as it tries to move toward the basement but hits that plastic sheet that is holding the insulation in place. With floor coverings put over wet concrete, whether the water comes from the original mix or from a drainage problem, unprotected materials will pick up that moisture and expand, sometimes to the point of destroying the floor.
So that is why we either want to wait for the concrete to dry before covering it, or take precautions to prevent the migration of water or water vapour to other building materials. If you look back at Mike's note at the top, I had mentioned in my database entry about self-levelling concrete that you should let the concrete dry out before putting anything on top of the new compound. The concrete compound couldn't care less, but the new floor covering could be destroyed by all that water, and there is a lot of water in self-levelling concrete that will dry out whether you want it to or not. The whole question is where will it go? I prefer to wait a couple of days and let it evaporate into the room where I can ventilate it outdoors rather than fight trying to protect a floating floor from all that moisture...
