There's a product advertised called CLR which may be of help. I have no experience with it but its supposed to remove Calcium, Lime, and Rust stains, hence the name CLR.

I've seen it at home centers and even at the grocery store near the bleach & Drano. Give it a shot, most you'd lose is about 5 bucks!

~Piz
ps: let us know if it works!!!

Gee, it only took 3¬Ω months to get a reply!
Good luck,
~Piz

You deserve another reply after waiting so long!!

It's worth noting that for most stains and the general cleaning of concrete/cement, Muriatic acid is very effective and inexpensive.

Important - follow the directions exactly to avoid an accident. It is an acid and can be dangerous - but it does work!

I also would suggest trying use of Muriatic Acid....also purchase special gloves that resist chemicals and acids....I think you can get them at Home Depot.

If this is truly from your battery, then try baking soda. If it is rust, try a product called Heavy Duty Concentrate. It can be purchased online at www.shopEZ.unfranchise.com. Hope you get it off!

CLR does work and it works well but if you have cheap concret it may damage it.
I have a concret brick driveway(interlocking)The CLR did not damage my bricks except for one i gusse something was wrong with but onlt that one bricks was damaged. And the damage was not big, only the colour faded and i only noticed it after getting down on my hands and knees to look at it.

Home Depot sells an acid-based rust remover for concrete in the paint department under the Behr brand name for about $13 a gallon. You will need some formula with an acid base in it. Ordinary concrete cleaners will nto suffice.