When left exposed to the elements or accidental spills, concrete often becomes discolored if it wasn't properly sealed. If you've tried cleaning it with normal detergents and water to no avail, a few common household chemicals can get rid of the stains. To prevent further discoloration, consider sealing the concrete with a solvent- or silicone-based concrete sealer once you remove the existing stains. If these chemicals don't work, you may need to hire a professional cleaner.
Wet down the concrete with a hose or bucket of water. This helps the baking soda stick to the surface.
Pour baking soda over all of discolored areas. Lightly moisten the baking soda to form a paste. Scrub the baking soda into the concrete with a stiff-bristled scrub brush. Do not use a wire brush, as these can damage concrete.
Allow the baking soda paste to sit on the concrete overnight. This allows the baking soda to absorb any compounds discoloring the concrete. Rinse the concrete thoroughly and allow it to dry.
Apply vinegar or bleach to the discolored areas. If you use vinegar, do not dilute it. Pour it onto the concrete and scrub it into the surface with a scrub brush. If the discoloration starts to fade, repeat until it disappears completely. Add 3 tbsp. bleach to 1 gallon hot water, using the solution to scrub the concrete.