Rust spots in concrete can be stubborn and tough to get rid of especially if they have been there for long time. Oxalic acid has been proven successful for eliminating most rust spots. There are some household cleaners such as ZUD and Bar Keepers Friend that contain oxalic acid. Wet the surface and spray the cleaner on. If it comes in powder form, just sprinkle it on the stain, wet it and brush it around. Let it stand for a few minutes and then scrub it with a brush. Keep repeating this until the stain is gone. For best results, rinse with a pressure washer.
If oxalic doesn't work then try TSP (tri sodium phosphate). This is a powder that can be found at most home improvement stores and paint stores. Mix 1/2 cup of TSP with 1/2 gallon of hot water, pour it on the stain and let it sit 15-20 minutes. Then scrub with a stiff bristled brush and rinse with water. Repeat a few times if necessary.
If rust spots in concrete are so stubborn that this method doesn't work, then you can use an etching solution or diluted muriatic acid (4 parts water:1 part acid) as a last resort. This actually eats away the cementitious material on the surface. So you're not actually cleaning the concrete, but removing the cement that the rust has bonded with. This is a last resort because, by removing some of the cement, the surface is being weakened and becomes more susceptible to chipping and scaling. However, keeping a coat of clear concrete sealer on the surface can eliminate these risks. Use caution with products that contain acid. Wear rubber gloves and safety goggles and keep clean water nearby in case of accidental splashes.
Another idea which we have discovered recently is using toilet bowl cleaner. What's good about this is toilet bowl cleaner is thick and you can concentrate it on the stains without it drying out quickly. It can be especially useful on vertical concrete surfaces. Toilet bowl cleaner contains acid which actually etches away cement as it removes the stain.
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