White marks on bricks — and other masonry items — are typically mistaken for mold, soil leaching or water stains. However, efflorescence is normally the cause of the white marks. Efflorescence occurs when dissolved salt runs down the brick, leaving an unsightly mark. Salt is a common raw material found in masonry items such as brick. When water runs across the brick, the trapped salt dissolves and leaves white streaks, trails or marks on the brick. This efflorescence will make your brick look old and dirty.
Protect plants, shrubs and other vegetation near the brick by covering them with a plastic tarp. Brush the brick clean with a broom to remove any surface dirt or debris. Rinse the brick with a water hose.
Fill a 5-gallon bucket with 1 part muriatic acid and 10 parts cool water. Muriatic acid is a dangerous chemical and you must dilute it before using. Alternatively, use undiluted white vinegar for a safer cleaning solution.
Neutralize the muriatic acid by sprinkling baking soda over the bricks and scrubbing with a clean scrub brush. Rinse the bricks clean with the water hose.
Dip a scrub brush in the mixture and scrub the bricks clean. Continue scrubbing the bricks until you have removed the white marks. Let the acid sit on the bricks for 10 minutes. Rinse the bricks clean with the water hose.