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I had this car battery that flipped in my car trunk. When I got it out of there it spilled some acid on the pavement. It's a sulfuric acid battery. I added some baking soda on it and poured some more water+baking soda to make sure the acid was neutralized.

However there is still a brown-rusty stain on the pavement. I tried water and a big broom brush but it still there.

Any idea what I can use to clean up the pavement to its original color? image of pavement image 2 of pavement

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    $\begingroup$ Sorry about your mishap. I don't think the organosulfur tag applies. $\endgroup$ – electronpusher Jun 17 at 6:53
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    $\begingroup$ "a big broom brush" - A short bristle/wire hand brush would be far more abrasive. $\endgroup$ – MrWhite Jun 17 at 7:55
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You did the right thing by neutralizing with a base.

The best solution is to let time heal the stain. The surface is obviously damaged, but there is no way to reverse that damage. The stain will only go away if someone keeps on scrubbing the area until a new surface is exposed. The good news is that dirt, soil, and water will slowly remove and even scrub away the (temporary) surface stain. Maybe keep a flower pot covering the area for the time being.

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The brown stain may be iron oxides that were dissolved from the stone and precipitated by the sodium bicarbonate, or possibly a sulfonation product of the asphalt which is not soluble in water, but coats the asphalt. Three techniques could reduce the staining:

1) Put a few spoonfulls of vinegar on the spot and let it stand several minutes before washing it off with water. If it works a little, you can do it again.

2) Put some 3% H2O2 on the spot and let it stand. If it foams quickly, insoluble ferric compounds will be reduced to soluble ferrous compounds. H2O2 and acetic acid (vinegar) are compatible in this application.

3) Make a fairly concentrated solution of a laundry detergent, put it on the stain, and abrade with a stiff broom or brush. If you are able to brush thru the stain layer, the detergent may help suspend the brown stuff. Warm water should be more effective.

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    $\begingroup$ I wouldn't put any more acid on the concrete. Even though vinegar is a weak acid, it will pit the concrete. $\endgroup$ – MaxW Jun 17 at 16:56
  • $\begingroup$ James, I am afraid that point no 2 is not correct "% H2O2 on the spot and let it stand. If it foams quickly, insoluble ferric compounds will be reduced to soluble ferrous compounds". H2O2 will not reduce ferric to ferrous, nor it will help in dissolution. $\endgroup$ – M. Farooq Jun 17 at 17:13
  • $\begingroup$ Sure it does. It does it for blood and for ferric oxides. You may have to acidify the H2O2 to keep the ferrous ions in solution, but H2O2 does reduce Fe+++. $\endgroup$ – James Gaidis Jun 18 at 13:10
  • $\begingroup$ If the sulfuric acid hasn't pitted the black concrete already, vinegar won't do any more. It seems that the driveway is asphalt concrete, not portland cement concrete, like the much lighter colored garage floor visible in the upper right. If it were my driveway, I would use vinegar + H2O2. You couldn't hurt it any more, and could help. Of course, time is the easiest cure, or just forget about it, but that requires patience or wisdom. $\endgroup$ – James Gaidis Jun 18 at 13:16

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