What is the easiest method of removing dark or light discoloration from newly hardened concrete floors or walks?
Phosphoric acid is useful as a good first attempt. It can be bought inexpensively as a concentrate and diluted by adding 30 parts of water to one of acid. Since phosphoric acid is a weak acid it doesn't rapidly expose the aggregate as muriatic acid does. It is also not nearly so dangerous to skin as are stronger acids. The acid should be scrubbed uniformly over the surface. One or more treatments may be needed. A white sheen is likely to remain after drying. This generally disappears in a few months. If phosphoric acid is not available undiluted vinegar may be substituted with almost as good results.