Masonry Magazine August 1972 Page. 22
Note 3. Cement containing not more than 0.60 per cent alkali calculated as the percentage of Na2O plus 0.658 times the percentage of K2O may be specified when the cement is to be used in concrete with aggregates that may be deleteriously reactive. Reference should be made to the Specifications for Concrete Aggregates (ASTM Designation: C 33) for suitable criteria of deleterious reactivity.
The alkalies referred to in this note include both water-soluble and insoluble alkalies. In general, the water-soluble alkali content will be of the order of 60 per cent of the total. As suggested by 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."
Other ingredients for mortar, lime, sand and water should also be selected with care; although their contribution to efflorescence may be less frequent (see Technical Notes 23).
Mortar types and proportions should be selected on the bases of structural and exposure requirements for the particular project.
Recommendations for mortar are contained in Technical Notes 8, "Mortar for Clay Masonry" (Ref. 3).
Admixtures for mortar are in general not recommended because of their unknown ingredients, and the lack of data on their effect on bond strength and, consequently, watertightness of masonry walls.
Several investigators have studied the effect of additions of barium carbonate to mortar on the tendency of the mortar to contribute to efflorescence. Data resulting from these studies indicate that additions of barium carbonate to the mortar, in the proportions of 3 to 6 lb of carbonate per sack of cement, will reduce efflorescence which appears in the form of sulfates, but are ineffective in reducing efflorescence resulting from carbonates. The effectiveness of barium carbonate in reducing the efflorescence of any particular mortar can best be judged by efflorescence test of the mortar, both with and without the barium carbonate.
Laboratory tests, as well as field experience, indicate that calcium stearate, when added to mortars in the proportion of 2 per cent of calcium stearate by weight of the combined cement and lime in the mortar, has an insignificant effect on mortar bond. This addition, however, reduces the absorption of the mortar and retards penetration of moisture by capillary action. For mortars high in alkaline content or in locations where the atmosphere has a high sulfur content which may combine with rain water to form dilute sulfuric acid, the addition of stearate to the mortar may reduce the tendency of the wall to effloresce.
Design
The most meticulous design and detailing may be thwarted by the selection of inappropriate materials or by poor workmanship. The converse is also true; the use of the best possible materials and craftmanship will not in themselves ensure a successful and permanent structure.
Wall Sections. The design of a masonry wall and the selection of materials for its construction should, from the standpoint of resistance to rain penetration, be based upon the exposures to which it may be subjected.
There are two principal methods employed for preventing penetration of wind-driven rain into the body of the masonry. One is to provide channels back of the exterior wythe to conduct water that may penetrate the exterior to the outside. A second method is to provide a barrier to water penetration back of the exterior wythe. These two wall types are generally referred to as "drainage" and "barrier" wall types, respectively, and are discussed in detail in Technical Notes 7B, "Moisture Control in Brick and Tile Walls Rain Penetration". In general, the "drainage" type walls are recommended for maximum resistance to rain penetration.
Details. As previously stated, one of the necessary conditions for the occurrence of efflorescence is the presence of moisture in the wall assembly. The preclusion of this moisture will thwart the mechanism for efflorescence. Much, therefore, depends on the design and attention to certain critical details. Of primary concern are those details concerned with the prevention of the entrance of moisture into the masonry assembly. Also of importance are details that will direct water collection away from wall tops and horizontal surfaces.
Design recommendations, wall types, workmanship characteristics, detailing, flashing, drips and weep holes are some of the points to which careful attention must be paid in order to prevent the occurrence of efflorescence. These subjects are discussed with recommendations in Technical Notes 7A, "Flashing Clay Masonry" and 7B,