Battery acid stains may be removed from concrete by cleaning the area, covering the stain with a cleaning agent like acid or bleach, letting it sit and then rinsing away the cleaning agent. It is safest to try a milder cleaning agent first before moving on to something more caustic.
The first step is to sweep the concrete area clean with a regular broom, rinse it with water and let it dry. Next, the cleaner should be applied. Oxalic acid may be purchased at a supermarket or drugstore. It is a powder that dissolves in warm water, and it should be mixed in the exact ratio according to the instructions on the package. The acid should be brushed onto the stain and allowed to sit for 15 to 20 minutes. Alternatively, chlorine bleach may be spread over the area, allowed to sit for five minutes and then scrubbed thoroughly with a bristle brush. After either of these cleaners is applied, the area should be flushed with clean water. As a last resort, muriatic acid may be applied. It is extremely caustic and should be used in a solution of one part muriatic acid to four parts water.
All of these cleaning agents are dangerous, so anyone using them should wear a long-sleeve shirt, long pants and protective gear such as boots, goggles, a mask and rubber gloves. Hands should never be dipped directly into an acid solution.