You must be at your wits' end – or at least you should be to turn to muriatic acid to clean concrete. This means that you should have tried hot, soapy water and then trisodium phosphate to remove the tough stains on your concrete patio, sidewalk or driveway. Muriatic acid is a dangerous chemical that can cause instant skin burns. And its strong vapors require face protection, including a respirator. Muriatic acid should be used only outdoors – where there is ample ventilation – and never indoors. Duly warned, proceed with extreme caution with this chemical. The tradeoff is that it is usually a highly effective cleaning agent.
Ensure Your Safety
Wear a respirator, eye mask, face mask and strong rubber gloves. Put on old clothing that fully covers your arms and legs. Wear socks and old shoes or boots; never go barefoot while working with muriatic acid.
Keep a pressure washer right next to your work area in case of accidental spills. You might substitute a hose for the pressure washer, but only if it has strong water pressure. You'll also need the pressure to dilute and wash away the acid when you're finished.
Protect grass and other foliage near your work area since muriatic acid can damage and even kill your greenery. Douse them with water, then cover these areas with a tarp.
Keep flames, heat and sparks far away from the acid. Muriatic acid is not flammable, but it can form a flammable hydrogen gas if it comes into contact with metal.
Wait for a day with no wind so you do not run the risk of acid spatters. Follow the instructions on the muriatic acid for the manufacturer's guidance on how to mix it with water. In general, you might be instructed to fill a plastic bucket with 1 gallon of cool water; then slowly and carefully add 1 cup of muriatic acid to the water with a disposable measuring cup. Mix the solution in this sequence – water first, then acid, and not the other way around – to reduce the chance of acid spatters.
Hose down your concrete patio, sidewalk or driveway. Do not let the water evaporate.
Test the acid on an inconspicuous portion of the concrete to make sure it won't damage the surface.
Dip the masonry brush into the bucket. Scrub it into the concrete. If the stains don't lift right away, dip the brush back into the bucket and apply some of the acid solution onto the concrete. Let it sit on the stains for no more than five minutes. Then scrub the concrete again.
Rinse the concrete thoroughly with cool water. Call your local municipality or recycling center to find out how to dispose of any unused muriatic acid. Do not pour it down a sewage drain.