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Reducing or Oxidizing
Concrete stains are common, due to the porous nature of the material and the fact that concrete is usually used as the flooring in work spaces such as garages, patios and shops. These are areas where spills are likely. The best method of removing concrete stains varies depending on the type of material that was spilled. Stain removers can be placed into three categories regardless of the material: Reducing bleaches, oxidizing bleaches and acids. Reducing bleaches (hydrosulfite, metabisulfite and sodium bisulfite) remove oxygen molecules from the stain. This alters the color of the stain, usually rendering it colorless or transparent. Oxidizing bleaches (hydrogen peroxide and sodium percarbonate) do the opposite: They actually add oxygen to the stain molecules, again changing the color. Oxidizing cleaners work best on organic stains. Acids simply "eat away" at or consume the stain molecules, but this sometimes also damages the concrete. You can also choose to power wash the stain out. Be aware that power washing concrete can actually damage the concrete itself, so it is best to test the washer on the concrete in a small area first.
Dirt, Oil and Grease
Dirt, grease and graffiti stains can be removed with simple scrubbing or steaming. First, try scrubbing the stain with soap and water. If that doesn't work, add a bit of strong detergent and some hydrochloric acid to make a paste. Let the paste sit on the stain for a few hours, then scrub and rinse. You can rent a steam cleaner to steam dirt, grease and graffiti stains off your concrete if you don't want to spend a lot of time on your hands and knees scrubbing. Oil stains should be first soaked up (not scrubbed) with an absorbent paper. After the oil has been soaked up, the stain should be covered with a dry, absorbent powder such as cornmeal for at least 12 hours. Any remaining stain can be scrubbed with the same detergent mixture as above.
Mildew and Rust
Mildew should be treated with a commercial oxidizing bleach, available at any home center store. Alternately, you can create your own solution with sodium orthophosphate, powdered detergent, sodium hypochlorite, and enough water to make a thick paste. Liberally cover the mildew with the paste, and leave alone for 3 days. Then scrub the stain and rinse. Be careful, as the solution will bleach your clothing. It might also eat away at metal depending on the strength of your solution. Rust stains should be treated with a reducing bleach as described above, or you can make your own by combining oxalic acid and water. Use one part acid and two parts water to start, which should take care of light stains. Cover the rust and wait 3 hours. Then scrub and rinse.