Expanding on what x3n0ph3n3 said, concrete doesn't harden because the water in it evaporates away. Rather, the water is consumed in a chemical reaction. In fact

Expanding on what x3n0ph3n3 said, concrete doesn't harden because the water in it evaporates away. Rather, the water is consumed in a chemical reaction. In fact, you have to be careful that the water doesn't evaporate too much during the curing process. It's very common to hose down a curing sidewalk and keep it covered for 30 days. Incidentally, the chemical reaction is exothermic. That's one of the reasons why you can't pour a concrete dam all at once: the heat generated would be enormous.