Concrete is a mixture of Portland cement, a fine aggregate (sand), a course aggregate (gravel), water and several chemical additives. The water causes the hardening of concrete through a process called hydration which binds the whole mixture together. In its finished state, concrete appears to be a solid slab, but is actually quite porous. As the excess water in the mix evaporates, it leaves capillaries throughout the slab much like a sponge.