The white powder visible on many outdoor brick surfaces, known as efflorescence, occurs when the water soluble salt in masonry products is extracted by water. Once the water evaporates, the salt hardens and forms unsightly white streaks on the brick. Efflorescence is not limited to brick--it can appear anywhere where masonry paste has been used to embed or assemble stones or rocks. Though efflorescence sounds severe, it can removed using a bit of elbow grease and simple household products.
Hose down the brick surface with a high-pressure hose or attach a nozzle which can direct a powerful jet of water at the surface. Efflorescence is water soluble, so most of it can be removed with high pressure.
Treat the surface with a light duty concrete cleaner. Allow it to saturate the surface and remaining efflorescence for 5 to 10 minutes.
Scrub the area clean with wire brushes and/or scouring pads. Add more concrete cleaner as needed.
Hose down the clean surface and allow it to air dry completely.
Prevent regrowth of efflorescence by spraying a silane/siloxane water repellent sealer onto the brick surface.
As collecting moisture is the main cause of efflorescence, make an effort to keep your brick surfaces dry. After rainfall or exposure to a garden sprinkler, pat the brick down with an old towel. All supplies can be found at your local hardware store or home improvement retailer.
Do not use muriatic acid to clean away efflorescence, as is sometimes recommended. It is an extremely corrosive chemical, and is hazardous to you as well as the environment.