Masonry Magazine June 1982 Page. 16
soluble salts present in many of the units that may make up a wall assembly.
Brick. Because of the composition of the raw materials and the heat treatment used in the process of manufacturing, it is possible for soluble phases to result in the finished product. If water is absorbed by such products, the soluble salts enter into solution and efflorescence may be formed as evaporation takes place from the surface of the brick.
However, as stated by Brownell (Ref. 1): "Products such as these will show efflorescence when placed in distilled water, even though all precautions are taken to eliminate outside contamination."
Brick units which will not contribute to efflorescence are readily available in all parts of the United States. The liability of masonry units to effloresce may be easily determined by the wick test for efflorescence, which is included in "Standard Methods of Sampling and Testing Brick," ASTM C 67.
Backup. Many masonry materials used as backup or inner wythes of masonry walls contain large quantities of soluble salts. These units may contribute to efflorescence on the face of the wall, if sufficient water is present to dissolve the salts and pathways are provided for the solution to reach the surface of the masonry. A comparision of the various types of concrete blocks with structural clay tile was made by Young (Ref. 2).
Young measured the soluble-salts contents and efflorescent tendencies of all units in his experiments. In the soluble-salts determinations, it was found that concrete products contain two to seven times as much soluble material.
Figure 1 illustrates the transfer of soluble salts from back-up units to facing brick. This result was obtained by placing the back-up blocks in pans of water with five brick on top of each block as shown. The brick had previously been subjected to the wick test for efflorescence and showed no efflorescence. As in the case of facing units, the liability of back-up units to effloresce can be determined by the wick test.
Trim. Building trim, such as caps, coping, sills, lintels, keystones, etc., are often of other than burned clay products materials. These items are often natural stone, caststone, precast concrete, etc., which may contain considerable amounts of soluble salts. Such materials may contribute significantly to efflorescence on the facing of adjacent brickwork.
Mortar
W. E. Brownell states (Ref. 1): "The primary and most obvious source of contamination of otherwise efflorescence-free brick is the mortar used in wall construction. The mortar is in intimate contact with the brick on at least four and sometimes five sides. It is applied to the brick in a wet, paste-like condition which provides ample moisture for the transfer of soluble salts from the mortar to the brick. If any appreciable soluble material is present in the mortar, it will be carried into the brick proportionately to the amount of moisture transferred..... "The simplest case of soluble salt contamination of efflorescence-free brick is the migration of 'free-alkali' solutions from the mortar to the brick. This situation is not only the simplest mechanism, but it is also the most common. In the trade, it is known as 'new-building bloom."
Cement
The free-alkali solutions common in mortars are sodium and potassium hydroxides. Free alkalies available in portland cements vary from one source to another. The range approximately 0.02 per cent to 0.90 per cent for the combined alkalies. A survey of masonry cements indicated a range of free alkali of from 0.03 to 0.27 per cent.
According to Brownell (Ref. 1): "Experience has shown that 0.1 per cent free alkali in a portland cement used in common mortars will cause 'new-building bloom'; therefore, if such efflorescence is to be avoided, the free alkali of the cement should be less than this and should be specified as low as possible.".
Lime
Experience and laboratory data indicate that lime is seldom a contributor to efflorescence. It has been demonstrated (Ref. 6) that lime, clay or sand additions to a mortar mix do not generally contribute to efflorescence. In fact, these ingredients tend to dilute the deleterious effects of a high free-alkali cement.