Dirty or stained concrete can be cleaned a number of ways, though some stains won't respond to any cleaners. For these stains, wash your driveway with a diluted muriatic acid solution. When working with any acid, take appropriate precautions, including protective clothing and safety gear.
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Muriatic acid is diluted hydrochloric acid and is a very potent chemical. It cleans concrete by removing the topmost layer of your driveway; use it only as a last resort to clean stains. Before washing your driveway with muriatic acid, try a degreaser on oil stains and a solution of one part bleach to three parts water on mold and mildew. Use vinegar or oxalic acid on rust stains and a pressure washer with an aggressive detergent such as trisodium phosphate for general cleaning. Phosphoric acid is less aggressive than muriatic acid and yields similar results. Fall back on muriatic acid if these methods fail, but first thoroughly rinse your driveway with clean water to remove all traces of chemicals.
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Precautions
Some municipalities prohibit draining acid into the sewage system, so consult the local waste-water management agency to stay within the regulations.
Acid can be dangerous to work with, so don appropriate safety gear including acid-rated lung protection, goggles and acid-resistant rubber boots, gloves and coveralls. If you plan to clean the floor of your garage, borrow or rent industrial fans for ventilation. Wet down adjacent lawns to dilute excess acid or make a slurry of baking soda and water to neutralize the acid before it damages your plants.
When mixing acid, slowly add the acid to water to avoid a violent reaction. Because acid reacts with metal, mix muriatic acid in a plastic or glass container.
Washing Your Driveway
Use a solution of one part muriatic acid to ten parts water to clean your driveway. Dilute the muriatic acid and pour it into a plastic watering can. Wet your driveway with water, then start at the highest part of the driveway and sprinkle acid over the full driveway surface. Applying the acid to only the stained parts of your driveway will unevenly discolor the concrete. When the surface is completely covered, scrub stains and spots with a stiff bristled nylon scrub brush. Do not let the acid sit on your driveway for longer than 10 minutes. Neutralize the acid with a generous amount of baking soda or garden lime and rinse the neutralized acid from your driveway.
Take leftover muriatic acid to a household hazardous waste disposal center.