This is a white powder on the face of masonry, due to the soluble salts in the mortar or concrete being dissolved by the percolating water and being deposited upon the surface when the water evaporates, which frequently disfigures the face of concrete walls. Efflorescence is most likely to occur below the horizontal joints and is particularly noticeable just below the horizontal seam between two successive days' work. The reason for this is as follows: The concrete is placed in layers, and if it is laid dry and tamped, the top surface will be richer and denser, and consequently will stop any percolating water and divert it to the surface of the wall; and this impervious film is more marked between the concrete laid on succeeding days, because the top surface of the set concrete is usually treated with a rich mortar to insure a good bond a 345). Efflorescence appears upon concrete which has been built without stopping work at night and which has been puddled in and not tamped in layers. It appears particularly after a period of wet weather, owing to the saturation of the face of the concrete with water which dissolves the soluble salts an•l later deposits them upon the face of the wall. Efflorescence usually occurs in irregular patches, since in even the best work different portions of the concrete have different densities owing to their being richer, or containing more or less water, or being tamped more or less severely. Different cements contain different proportions of soluble salts, and hence give different amounts of efflorescence.
There are three methods of preventing or at least of decreas ing efflorescence, as follows: 1. Use a cement that has little or no soluble salts in it. There are cements upon the market which are almost entirely free from soluble salts (sulphates and chlorides), and which cause little or no efflorescence.
2. Incline the top surface of all layers, particularly the last one laid each day, toward the back of the wall so the soluble salts will be carried toward the back instead of toward the face of the wall; but this remedy is not applicable with wet or at least with sloppy concrete, unless the last concrete laid at night is mixed drier to permit of thus sloping the surface. If the back of the wall is stepped, as is common in bridge abutments and retaining walls, the top of the step should be given a flat downward slope away from the body of the wall, to prevent pools of water from standing on the top of the step and soaking into the body of the concrete.
3. Make the concrete waterproof, since if the water is kept out it will not dissolve the salts, and consequently efflorescence will be prevented. For a discussion of the several ways of making concrete impervious, see 4 363-83; and for a method of rendering the efflores cence almost invisible, see § 643.
Efflorescence is partially removed by the wind and the rain, and can be entirely removed by scrubbing the surface with dilute hydrochloric acid; but it may return. This scrubbing process is ex pensive in brushes, acid, and time.