the untouched surface of concrete is like a sponge and is porous like a sponge. It is a magnet towards moisture. when dry or appears dry, it dusts up, the limestone leeches to the surface. If it's laid directly on the dirt or sand it will absorb ground water and even if it appears dry it is really wet. I would put several coats of a good silicone sealer on raw concrete before I put anything else down on it i.e. a carpet pad and carpet.