Efflorescence is white, streaking and sometimes powdery in appearance. It can form on the surface of brick, stone, concrete, and mortar. Efflorescence is formed when moisture within the substrate migrates to the surface bringing along naturally embedded salts and minerals within the substrate. When the moisture evaporates into the atmosphere, these salts and minerals remain deposited on the surface. When there are sufficient accumulations of these minerals, they become visible in this white appearance, and are known as Efflorescence.
Efflorescence is usually tricky to remove. Even when removed, it is not unusual for the white appearance to reappear within weeks or months. It may need to be removed again. The reason for this is there can be still more salts/minerals within the substrate and they will continue to migrate to the surface until no longer present within the brick, stone, concrete or mortar.
Sometimes the buildup is so heavy, these salts and minerals achieve a hard, thick calcification build up, and resemble the composition of stalactites and stalagmites. In this case this buildup must be removed mechanically, such as by wet sandblasting.