If you have a concrete patio, shaded by a walnut tree, you are probably plagued by stains caused by the sweet nuts which have fallen onto it. Or, maybe your concrete driveway is a luncheon meeting spot for squirrels to relax as they savor fallen walnuts. Upon your arrival home, you drive on the shells left scattered all over the concrete, creating a black and purple eyesore. You could cut down your walnut trees, or pick the nuts from them before they fall onto your concrete structures. Other than such extremes, there's little you can do about the nuts and resulting stains from appearing. However, you can fight back with this cleaning method, which works miracles on those hard-to-remove walnut stains
Put on your eye protection, rubber gloves and rubber boots. Wear these the entire time you are exposed to the chlorine.
Wet the walnut stains on the concrete with a garden hose. Cover the stains with a sprinkling of granular chlorine. Moisten it slightly with a light misting of water. Allow it to sit for about 5 minutes.
Pour a gallon of hot water into a bucket. Add 1/3 cup of liquid dish detergent and 2/3 cup of borax. Stir the solution well.
Scour the stained areas well with the liquid solution and a stiff scrub brush for a minute or two. Rinse very well. The walnut stains should be completely gone now. Allow the concrete to dry out thoroughly in the sun.
Apply a coat of concrete sealer to areas that are typically subject to being stained. Follow the manufacturer's instructions. This will make future stains easier to clean, or even prevent them completely.