An Article from the Division of Building Research, Washington DC
T. Ritchie

According to a dictionary definition efflorescence is a crystalline deposit on the face of a stone wall or other concrete structures. Unfortunately, it is not restricted to concrete (curbing), stone masonry, and concrete flatwork. Notably bricks must be included. It is a common problem in many areas of the United States, as it is in many other countries. The immediate problem raised by its occurrence is that of disfigurement of masonry, stone and cement walkways and concrete borders. Severe defacement of a wall is not uncommon. There is, in addition, the possibility of actual damage to all masonry from the growth of salt crystals near its surface.

Efflorescence before cleaning

Efflorescence after cleaning

Efflorescence usually forms shortly after the construction process, with the result that a brand-new red brick building or concrete wall may become heavily splotched with a white powder, to the dismay of the owner and those associated with its construction.

The problem is not a new one. In an extensive descriptive bibliography prepared in 1925 over 230 references to it are listed for the period between 1877 and 1924. One of the articles traces back the history of the problem at least to the time of the third Napoleon.

Chemical Nature of Efflorescence

Many kinds of salts have been detected in samples of efflorescence. For example, substances named in the following list were said to have been found in efflorescences:

sodium sulfate	potassium sulfate
sodium carbonate	calcium sulfate
sodium bicarbonate	calcium carbonate
sodium silicate	magnesium sulfate

In addition, other salts such as chlorides and nitrates, and salts of vanadium, chromium and molybdenum were mentioned without giving their specific composition. These last, particularly vanadium, were said to produce green efflorescence on white or buff burned clay units, while the other salts produced white or grey deposits and are not uncommon in the Southwestern part of the United States.

For several years, as a part of studies of the Division of Building Research, samples of efflorescence have been taken from buildings and analyzed chemically. Many different types of bricks had been used. In all cases except one, the sample of efflorescence was obtained from the brick surface; in the exception, the bricks of the wall were unmarked by efflorescence, but the mortar joints had heavy salt deposits which projected ¼ to ½ inch from the surface. In all except the mortar joint efflorescence, chemical analysis of the sample indicated that a very large proportion of it was sodium sulfate and potassium sulfate For the mortar joint efflorescence, however, sodium carbonate was indicated to be the main constituent. The samples contained small amounts or traces of other materials as well. The mortar joint being the conduit to efflorescence.

In one sample from the wall of a building in which very high humidity is maintained throughout the winter and unusual efflorescence patterns form beneath windows, it was indicated that magnesium sulfate was present in considerable amounts, in addition to sodium and potassium sulfate In other samples calcium was present in some quantity as carbonate or sulfate Chloride was either not detected in the samples or was present only in very small quantity. The "common denominator" of all the samples analyzed was some salt of sodium and potassium.

The results of these analyses confirm a previous study of masonry efflorescence made by R. K. Robertson who noted that "... in all cases of serious efflorescence, sodium sulfate was invariably present, rarely being under 50% of the total content", an observation based on analysis of a great many samples taken over many years. It was reported also that other salts generally found in efflorescence are sodium carbonate, magnesium sulfate, calcium sulfate, calcium carbonate and sometimes slight traces of sodium chloride.

Weather and Efflorescence

It has been observed that efflorescence is usually a seasonal problem. The cool days and nights of the fall season seem to bring out salts which have not been seen on the surface in the summer. The intensity of efflorescence usually increases throughout the winter season and starts to decrease only in the spring. By summer the salt deposits have generally entirely disappeared. In many cases the amount of efflorescence on brick walls and concrete structures decreases from year to year, so that a building badly affected in the first winter after construction may be much less marked in the second, less still in the third, and entirely unaffected in the following winter.

To account for the "cold weather" development of efflorescence, there is a possibility that the rate of evaporation of moisture from concrete varies from season to season. Under summer conditions the rate may be very high, so that moisture is evaporated within the concrete and the soluble salts are deposited within the concrete rather than on the surface. In colder weather, however, the evaporation rate may be quite slow; this allows moisture to move to the outer surface of the structure before evaporating and leave the salt deposits on the surface.

Sources of Efflorescence

The movement of ground water into building foundations and its passage upwards into masonry by "wicking" action is sometimes the cause of efflorescence when soil moisture carries with it soluble salts which later are deposited on the concrete surface.

Efflorescence frequently forms on brickwork adjoining concrete units. A good example of this is its occurrence beneath concrete window sills. In such a situation concrete frequently is wetted from rain and snow melting on it, and soluble salts of the concrete are dissolved and may be carried into the brickwork beneath.

Initially, the work may be free of salts, but later become contaminated from salts in the adjoining soils. Prolonged dampness also promotes efflorescence. Thus masonry or other concrete structures near defective drains are often marked by efflorescence while other parts of the structures are unaffected; and structures which are splashed with water from nearby horizontal surfaces may be similarly marked.

Treatment of Efflorescence

The usual method of removing efflorescence from brick walls or concrete structures is either to dissolve the salts by rinsing with water, at the same time scrubbing with a brush, or similarly to dissolve them in a dilute hydrochloric acid solution and follow by rinsing the solution from the wall. A 50/50 solution of house vinegar and water will work well in most cases. In both cases it is probable that some of the salts will be carried back into the masonry when the wall is wetted and subsequently again form efflorescence. The acid treatment, however, appears to be successful in a high proportion of the occasions it is used. A second session may be necessary in some cases.

When efflorescence is associated with abnormal wetting of the wall, as from faulty drains or adjacent water tables, it is of course necessary to correct these faults before attempting to remove the efflorescence. When the problem is due to the rise of ground water it is very difficult to correct, since some form of horizontal damp-proof course must be inserted in the base of the wall to prevent continued rise of moisture.

Precautions Against Efflorescence

Since so many factors may contribute to the development of efflorescence on masonry, no one precautionary measure can be expected to take care of all eventualities.

The over-all design to ensure that the concrete structures are kept as dry as possible in service, such as by the use of a roof of considerable overhang and the avoidance of horizontal surfaces adjacent to masonry, unless adequately shielded from splashing and water flow into it, are features that offer protection against efflorescence. If water drains are placed against masonry they should be of ample capacity, and they should be maintained in serviceable condition. In short, the design of a building with respect to obtaining the maximum degree of durability of concrete masonry by protecting it from excessive dampness will also be favorable to the avoidance of efflorescence.

Use of Water-Repellents to Suppress Efflorescence

In recent years there has been developed a new class of materials called silicones, which have come into use in the building field mainly in connection with attempts to correct problems of penetration of rain into unit masonry. Some manufacturers also advocate their use for the prevention of efflorescence on masonry walls.

When a brick or some other type of cementeous unit is treated with silicone, its surface usually becomes highly water-repellent. Formation of efflorescence on that surface is then usually suppressed because solutions of salts in the brickwork are prevented from moving to the surface to evaporate as would normally be the case. The moisture evaporates beneath the treated surface, however, and salt deposits accumulate there rather than on the surface. Localized accumulation of salts and their crystallization may cause the surface of the structure to be spalled or flaked off, so that the use of a silicone treatment to suppress efflorescence may be dangerous in some cases. (In cases where steel is present inside of the cement structure.)

Summary

The problem of efflorescence on concrete masonry is an old one which has been studied for a great many years. Several factors may influence the occurrence of efflorescence in a particular case, but there must be salts in the soil to be taken into solution by water and then wicks to the surface as the moisture dries. The movement of the solutions within concrete masonry is controlled to some considerable extent by seasonal weather, and efflorescence is usually a "cold weather" problem. Possible sources of the salts of efflorescence have been described; as well as "outside" contaminants such as ground water.

Originally published February 1960.