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

 

my vote is pressure washer...dont bother to buy the small cheap model..takes far too long.. Hire someone with a 'commercial type' to do your driveway/sidewalks/rocks...Miguel
Chlorine,acid is not so good for anything living...

 

I use a pressure washer on mine. If I were living there I would pay someone else to use a pressure washer while I had a beer.

 

I take a good scoop of calcium hypochloride (for my pool) in water and spray. Carful so you don't get it where you don't want.