Pat , a mixture of muriatic acid and water should work, I used it on the clay pavers at my pizza place when they got moldy. Use about 1/5th gallon of acid to 1 gallon of water. Hose the area first with water and work the solution in with a hard stubbly brush (available at city hardware, white bristles and blue plastic). Do about a 10ft by 10 ft area then rinse it off. Continue until done, should work great.. Good luck. I can come help you if needed..09082117077


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