Originally Posted by wagon89
Muratic acid cleans well, might be illegal. wear something you don't care about while doing it
Bingo.
If the driveway is just regular cast concrete... with no exposed aggregate or tile details, use the strongest muratic acid you can find. Hull cleaner (which is usually alkaline) has also worked well for me.
Just be sure to use thick nitrile rubber gloves and eye protection when you use either product.