Actually I am asking for my neighbor. He had a radiator leak and the driveway now has some ugly rust stains.
Ideas?
TIA
Jim
P.S. I did a search of SMDB for “Concrete & Rust” and found nothing conclusive.
Hmmm, concrete is notoriously porous and when rust hitches a ride on some water, it can do some ugly things when it soaks in. I;d recommend a bleach solution, and a high powered hose. Or even a powerwasher. I know the stains are not permenant, as I grew up with a concrete driveway, and this happened from time to time. Maybe even the same reason the OP mentioned at one time or another.
I have some kitchen cleaner that removes rust stains and the active ingredient is oxalic acid.
I once lived next door to a man who had a big rust stain in his driveway. One day, though, he had a wrecker haul it away.
Citric acid, tartaric acid, oxalic acid – these are all effective at removing rust stains. Oxalic acid is commonly used, and is available in cleaners such as ZUD, and is also sold as “wood bleach.” Chlorine bleach and a powerwasher sound like a good way to clean mildew stains, maybe tire marks, but not much else.
ACE Hardware recommends Jasco® Concrete Clean & Lighten.
I just meticulously powerwashed my entire driveway and sidewalk. It removed everything BUT the rust stains. For the record, those rust stains were produced by Ironite fertilizer if it matters, but I’m thinking rust is rust regardless of the source,
I’ve had good results with Muriatic acid. You should be able to find it at pool supply houses and any company which sells brick, block, and masonry supplies to the trade.
Nothing works as good as Muriatic acid.
Well, 37% Hydrochloric acid is just as good. 
From an Ask The Builder site… note runoff warning.
I can’t believe I just used SciFinder Scholar to look that up. But I have got to use that in lab sometime. Maybe relabel the bottle if people keep walking off with it (like I’ve had in the past.)
I thought it was 31.45% Hydrochloric acid would be just as good.
What’s that figure in Baume?