Concrete paths and driveways are both sleek and sturdy, but not indestructible. Mold, rust and other stains can mar the clean beauty of your concrete, lessening the visual impact and even the curb appeal of your property. While some stains can be rinsed away with a pressure washer, tougher stains such as rust require an acid-based cleaner like muriatic acid.
Understanding Muriatic Acid
Muriatic acid is basically hydrochloric acid, which is hydrogen gas dissolved in water. This is the same type of acid found in your stomach. Muriatic acid is the strongest acid-based cleaner available to the public and it must be handled with care. It can and will burn your skin as well as ruin most materials including metal, so it is sold in a specific type of plastic and should not be stored in anything else. Muriatic acid releases strong fumes, so it should never be used indoors or when children or pets are close by.
Gather Your Equipment and Supplies
It's often possible to skip safety equipment because of overzealous warnings, but that is not the case with muriatic acid. To work with this very strong chemical, you need long sleeves and long pants to protect your arms and legs. Wear old clothes that can be discarded when the project is finished, or see if your local hardware or home improvement store has acid-resistant disposable coveralls. Acid-resistant gloves should cover your hands and forearms and you will need a respirator with an acid-resistant filter, and goggles if it is not a full-face respirator.
Cover any foliage or outdoor furniture with plastic before starting. Keep a ready supply of gardening lime and clean, cool water nearby in case of accidental spills or splashes.
Mix and Apply the Solution
Muriatic acid must be diluted with water before use. The easiest dilution to remember is 1 cup of acid to 1 gallon of water. If you need a stronger solution, mix 1 part acid to 10 parts water. Always add the acid to the water, never the other way around or you can cause a heat reaction which can make the solution burp up out of the container. Add cool water to your container and then, wearing full protective gear, slowly pour in the muriatic acid.
Use a sprayer to soak the spots which need cleaning. Stubborn stains might also need to be scrubbed with a stiff, wire-bristled brush. Do not ever mix any other chemicals with muriatic acid.
Once the stains are gone, rinse the area thoroughly with cool water and allow it to dry. If you spill the muriatic acid, cover the spill with a thick layer of garden lime. This will cause a chemical reaction and make the area bubble and fizz. Once the fizzing has completely stopped, the spill can be rinsed away.
Clean Up Safely After Use
Do not pour muriatic acid down the sink, into the toilet or into a storm drain as it can eat through pipes and dissolve rubber seals and solder. Either use your community's hazardous waste disposal system, if it has one, or neutralize the muriatic acid before disposing of it.
This can be done by stirring together 4 cups of garden lime and 1 gallon of water in a 5-gallon bucket. Slowly add the muriatic acid. Make sure you wear your respirator and gloves, and lean as far away from the bucket as you can to minimize any danger of being splashed. Add small amounts of lime and muriatic acid until the leftover acid is all gone and the solution is no longer fizzing. Now that the muriatic acid has been neutralized, it can be poured down any drain.