This went on for a week or so, here is what I found.
I had some on concrete in a place where I could experiment. Pressure washing
was largely ineffective. I suppose eventually it might do something. Bleach was
The thing that works instantly, muriatic acid, cut about 4:1 with water.
Using that as a guide I figured out "Lime away" will also work but it requires
a little scrubbing with a brush. That is a less aggressive chemical. CLR would
probably do the same.