super heavy rust stain on concrete driveway

My tenant just moved the other week and left me with a large heavy rust stain on the middle of the driveway from the front end of his car. Could have been from his radiator or battery, I really don't know which. I've tried removing stains many times in the past but none of my methods had worked completely. The acid I'd used will eat up the concrete, remove some rust but good part of the rust was still embedded even after a high pressure wash. I've used concrete cleaners, disappointing and none had worked. I've tried all sorts of methods but ended up hiding it with cat litter. I need something that will remove the rust, but not eat up the concrete, then lift the rust out from the concrete pores like magic - too much to ask?? Is there anything that really works well?
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Fill a bucket with iron scrap and a 50-50 vinegar-water mixture. Wait untill the scrap has rusted well, stir to get the vinegar water very oragne and pour it off the scrap. apply the mixture to the rest of the driveway and the rust stains will fade away like magic.
John
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Often paint stores have one part Wood Bleach which has oxalic acid as ingredient listed on the label. Is used to convert black stains on oak from nails rusting.

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