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How to remove rust from cement
Rinse and wash the concrete with soap and water before beginning.
Dirt and dust will just get between your cleaner and the stain, making your work less effective. Once you've cleaned the surface of the concrete you should let it dry before moving on.
Pour or spray lemon juice on the rusted surface. Almost all rust remover uses acid to lift and scrub away the stain, and the high concentration of citric acid in pure lemon juice makes it a worth-while cleaning candidate.