How to get rid of water stain on concrete?
I had a similar problem recently and I successfully used a solution of muriatic acid. I wouldn't use full strength because of its potency. You can purchase a mild muriatic solution in most hardware stores.
Make sure you wear protective gloves and eyewear. Pour it on according to directions, lightly scrub the area with a brush being careful not to splatter the solution. After the time indicated on the bottle rinse the area thoroughly.