Water

The two principal functions of water in a concrete mix are to effect hydration and improve workability. For example, a mix to be poured in forms must contain more water than is required for complete hydration of the cement. Too much water causes a loss of strength by upsetting the water-cement ratio. It also causes water gain on the surface, a condition that leaves a surface layer of weak material called laitance. As previously mentioned, an excess of water also impairs watertightness of the concrete.

Water used in mixing concrete must be clean and free from acids, alkalis, oils, or organic materials. Most specifications recommend that the water used in mixing concrete be suitable for drinking (potable), if such water is available.

Seawater can be used for mixing unreinforced concrete if there is a limited supply of fresh water. Tests show that the compressive strength of concrete made with seawater is 10 to 30 percent less than that obtained using fresh water. Seawater is not suitable for use in making steel reinforced concrete because of the risk of corrosion of the reinforcement, particularly in warm and humid environments.