Efflorescence can be seen on mortar joints, concrete block faces, brick faces. A combination of conditions must be present for efflorescence to form: 1, there must be soluble salts in the masonry and 2, there must be moisture migration through the masonry, carrying the salts to the surface.
On a more technical note; in masonry work, Portland cement is generally used in the manufacture of concrete block, mortar, and stucco. The cement, in the hydration process contains calcium hydroxide. Naturally, water is used to make mortar or block-fill. Moisture, migrating through the body of the feature carries this calcium hydroxide to the surface where it combines with the carbon dioxide in the air forming a new substance….the white powdery material, now calcium carbonate.
Actually, at this point this material is water soluble, so it is usually easily cleaned off during the masonry cleaning effort. If left alone, though, and if there is a continual problem with water migration or leakage from external sources, the efflorescence becomes carbonate deposits or lime runs. These can be quite stubborn to remove, requiring more aggressive cleaning efforts.
The PCA further states that “All masonry and concrete materials are susceptible to efflorescence or staining. Interestingly enough, during periods of slow drying, and cool, damp conditions, such as in the winter, efflorescence can be more prevalent than in the summer.
Most efflorescence, especially on new construction, is temporary, very often called “new construction bloom”. It is most often removed during the masonry cleaning portion of the work. Recurring efflorescence indicates a chronic moisture or water flow problem such as from ground water in retaining walls, around un-caulked window openings, non-full mortar joints. Another source of “salts” can be where the masonry contacts the soil, such as basement and retaining walls. Also, some raw materials found in brick, for example, manganese, can cause brown stains.