Hydration produces calcium-silicate-hydrates (CSH) crystals that make concrete hard. Excess water in the mix eventually evaporates, leaving behind tiny pores where the water previously took up space.
A by-product of this reaction is calcium hydroxide (Ca[Oh]2), also known as lime. Lime adds no structural value to concrete. Lime born by water can move through the slab and become efflorescence. It is the “dusting” that occurs on concrete that has not been densified.