After the concrete becomes firm enough to resist marring, it should be sprinkled with water, then covered with wet canvas, wet burlap, or wet sand. This cover keeps the concrete from drying too rapidly. A chemical reaction between portland cement and water makes concrete harden. For this reason, the longer concrete remains moist, the stronger it becomes. In hot weather, concrete should be kept moist at least three days.
Cold weather slows the rate at which concrete hardens.
Hardening concrete must be protected by canvas or straw when the temperature drops near freezing. Concrete shrinks as it hardens. This results from the loss of moisture as the concrete dries, or from the cooling of the concrete. The chemical reaction of water and portland cement produces heat. When large amounts of concrete are used, as in dams, this heat must be drained away to make the concrete harden properly.