Efflorescence/salt-bloom: The accumulation of salts on a masonry surface such as brick or concrete, this occurs frequently around cracks and is usually a whitish color.
There are two types of efflorescence, one is called primary and the other secondary. Primary efflorescence essentially means that salt is integrated into the masonry, either in the bricks or the mortar itself. Primary efflorescence is more of an aesthetic concern. Secondary efflorescence is typically introduced into the masonry after it was placed. This can be introduced in any number of ways and forms. The general salting of concrete to prevent ice, and fertilizer for agriculture are two of the main contributors. Secondary efflorescence is responsible for many a stalactite in modern tunnels and caves alike. The salt unbinds the cement which binds the concrete/mortar and redeposits it, usually in the form of a stalactite or a flow on the side wall.