Water plays an important role in the workability, strength and durability of concrete. Too much water reduces the concrete strength, whereas too little will make the concrete unworkable. The water used for mixing and curing should be clean and free from oils, acids, alkali, salts, sugars or organic materials. As per IS 456, potable water is considered satisfactory for mixing as well as curing concrete.
Sea water should not be used for mixing or curing concrete. The permissible limits for impurities as per IS 456 is given below.
In general the amount of water required to be added for cement hydration is less compared to that required for workability. For complete hydration of Portland cement, only about 36 to 42 % of water, i.e., w/c of 0.36 to 0.42 is needed. If a w/c ratio greater than 0.36 is used, excess water which is not required for cement hydration, will remain in the capillary pores or may evaporate in due course. This leads to drying shrinkage.