Hi,Ron. Sorry that I don't have a definative answer but I can think of two things to try. The first, particularly if you have real hard water, is Lime Away, or something similar and the rings are white. The rings could be from the lime or magnesium that is left behind when water evaporated. Since your concrete probably has lime in it, I would not leave this on too long. Try a small section and then rinse heavily with water. The second suggestion is chlorine bleach, particularly if the rings are reddish. I don't really know why or if this will help, but there might be some pigment that has leached out of the pots.