Concrete is usually easy to maintain and stands up extremely well to the elements. On occasion, however, concrete can accumulate thick layers of grime, dust and even mineral deposits. When this happens, the solution to the problem may not be as simple as an application of basic soap and water. Rather, a heavy-duty cleaner is in order. Phosphoric acid, which is safer and less potent than muriatic acid, will effectively clean a broad range of surfaces, including concrete and other types of masonry.
Lay a tarp or drop cloth in the area where you will be working. This will help protect other surfaces that phosphoric acid may be damage or discolor. Before you begin working with the phosphoric acid, put on rubber gloves and protective eyewear.
Spray the concrete with clean water. Use a garden hose and spray nozzle to apply the water to the concrete. If the concrete is in an area where a garden hose will not reach, fill a bucket with water and use a sponge to manually soak the concrete. The surface of the concrete must be damp before you begin applying the acid.
Dilute the phosphoric acid with clean water. The strongest mixture will consist of one part phosphoric acid and three parts water. You may, however, dilute the solution all the way down to one part phosphoric acid and 15 parts water if you're dealing with decorative concrete or light-duty cleaning.
Apply the diluted phosphoric acid to the concrete with a nylon scrub brush. Start on an area of concrete that isn't too noticeable. Allow the solution to remain on the surface for about five minutes as you continue scrubbing with the brush. Do not allow the phosphoric acid to dry on the concrete. Rinse with clean water. If the concrete hasn't been discolored or damaged, you may proceed. If the surface is discolored, further dilute the mixture.
Clean the remaining concrete. Work on one section at a time. Apply the diluted phosphoric acid and scrub the section for about five minutes. Thoroughly rinse the phosphoric from the concrete with fresh water. Continue until you've cleaned the entire concrete surface.
Inspect the concrete for any remaining dirt, grime or mineral deposits. If you're not satisfied with the results, reapply the phosphoric acid, scrub and rinse.