Duckback 1 Gal Mason's Select Concrete Cleaner 6500Duckback Mason's Select Concrete Cleaner is formulated to remove dirt, grime, oil, grease, mildew stains and algae stains from concrete and masonry surfaces. Concrete cleaner is a non flammable, fast acting, easy to use concentrate that is 100% biodegradable and contains no phosphates. Neutralizes concrete and masonry surfaces after acid etching. One gallon yields up to 4 gallons of cleaning solution.