There are several options. You can use bleach, ammonia (Note: never mix the two) and other commercial solutions. For a natural solution to stubborn stains on interior concrete floors, mix flour and hydrogen peroxide into a paste the consistency of peanut butter. Spread it over the stained area. Let sit overnight. Then, scrape with a plastic-edged scraper tool.
Trisodium phosphate (TSP) mixed with water and scrubbed onto the stained area with a nylon bristle brush can remove tough stains. Scrub until the stain is gone and rinse with a hose. TSP is available at home improvement stores.
Muriatic acid can be used on outdoor floors to clean the toughest stains, like rust or dried grout. It’s a powerful, toxic material that should be used with extreme caution. Wear protective eyewear and gloves when using it and be sure to follow manufacturer instructions on how to clean concrete with muriatic acid.
For indoor concrete floors that are polished or stamped, you’ll need to take a gentler approach to how to clean it. You’ll just use a mop and a bucket of water mixed with a mild cleaner. Don’t use ammonia, bleach or any highly acidic substance on polished or stamped floors. Some good cleaners include Castile soap, liquid dish detergent, stone cleaners and mild floor cleaners. Mix a solution, apply to floor with a wet mop and then rinse with a mop dipped in clean water.