Most people do not realize that concrete is porous – more porous than Swiss cheese! The pores constitute 12 – 18% or more of the concrete but they are invisible, much smaller than the diameter of human hair. After new concrete is poured, almost half of the water has to evaporate. As this surplus water pushes to the surface, it leaves behind a network of capillary pores in the concrete.
The pores are orders of magnitude larger than a water molecule. Water vapor and soil gases, including radon gas, flow easily right through, drawn in by the lower pressure inside buildings. But liquid water is initially held back by its surface tension inside the dry microscopic pores as the globs of water get stuck. However, once the pores get wet, the liquid water breaks through. Moreover, the wet pores start actively pulling in water from the ground by capillary action.
Cement paste, which holds concrete together, contains platelets of calcium. Since this calcium is soluble in water, the pores in concrete get gradually larger and larger in the presence of water. A previously perfectly dry basement will start smelling musty, feeling humid, and eventually will start seeping water.
Even if your basement or concrete slab looks perfectly dry, deep-seal it now with RadonSeal® Penetrating Concrete Sealer against invisible water vapor and gases, and protect it against water infiltration, efflorescence, and deterioration.