I know this is from a year ago but for future reference:
Hydro-Chloric Acid is my go to for stubborn concrete stains.
CLR or Calcium/Lime/Rust remover is another possibility.
Be sure to
If using Hydro:
1. Wear thick commercial rubber gloves to elbows.
2. Wear a gas mask, rated for acidic chemical filtration & safety goggles. This stuff will burn your aiways and eyes
3. Wear good high rubber boots (Gum Boots - Aussie) & possibly full length leg chaps.
4. Splash over stains
5. Use hard bristled broom / brush to cover stain
6. Hydro will be bubbling and gassing as soon as you splash. Rinse off before bubbling finishes with a lot of cold water. If you miss doing this, the concrete will turn yellow. Which means reapplying to remove the yellow stain, completing all wash & rinse steps.
Also rinse yourself off - Gloves, Boots, Chaps, Mask, Goggles, Arms, Legs etc.
Hydro-chloric acid is also an additive to pool water for cleaning. Very little is added. It is highly corrosive, as you are probably aware.