For quality concrete all that is required of the water is that it be at least potable. But has enough attention been paid to the water ingredient? If water is "hard" do the minerals affect the cement? What effect do chlorine, and the fluorides and occasionally alum (materials usually added by the water supply facilities for health reasons) have on the strength and durability of concrete?
It is still a good rule that any water that is "potable" that is, fit to drink is suitable for use in concrete. The minerals that contribute to "hardness" would seldom if ever be present in the kind and amount that would have any significant effect on the hydration of the cement. Chlorine has no effect because it does not combine with cement compounds. The fluorides are not harmful in the amounts used in drinking water nor would the products of alum purification be harmful. This whole subject is treated in detail in a pamphlet available from the Portland Cement Association for about $1.00 called "Concrete Mix Water How Impure Can It Be?" by Harold H. Steinour. (Portland Cement Association, Old Orchard Road, Skokie, Illinois 60076)