Masonry Magazine October 1986 Page. 28
EFFLORESCENCE
Contaminated sands with soluble alkali sulfates will cause efflorescence unless the sulfates are removed. Using clean, washed sand will eliminate any efflorescing contribution.
The water used in the mortar and grout during construction can also be a source of contaminants. Clean, potable, salt-free water must be used at all times. Tests of Colorado River tap water show only insignificant amounts of salts in the water. Water from other sources should be checked for their alkali sulfate contents to be sure no efflorescing salts will be introduced into the masonry wall.
In the past, hydrated lime used in the mortar was thought to have been responsible for efflorescence. This did make sense to the layman. After all, anyone can make a logical connection between the white lime and the white efflorescent salts. However, chemical analysis and experimental tests proved otherwise. Hydrated lime (Ca(OH)₂) does not contribute sufficiently toward the soluble alkali sulfates necessary for efflorescence to occur (generally only 4 to is the efflorescing potential of the cements). In addition, the hydrated limes also reduce the voids in the mortar, thus reducing the area for capillary flow of any solutions in the wall.
Potential efflorescent problems can be greatly reduced by using low alkali cements, clean washed sands, and clean, potable, salt-free water.
Controlling Efflorescence
Even if soluble alkali sulfates exist in a masonry wall, before the sulfates can cause efflorescence the salts must be dissolved into solution by water. If no moisture reaches the sulfates then they cannot be rendered into solution and migrate to the surface where the water will evaporate, leaving the sulfate salts on the surface to crystalize and become efflorescence. Attention must be given to preventing any soluble alkaline sulfates from being rendered into solution by water.
This is difficult since during construction water is naturally used in the mortar and grout, therefore the amount of water in the wall should be controlled to only that amount necessary for actual construction. The majority of the water is absorbed into the masonry and used to chemically react with the portland cement and hydrated lime and is thus not free water.
The next critical concern is to prevent any water from penetrating into the masonry wall where it could cause efflorescence to occur. This can be done with good architectural details and quality masonry construction. Designing with overhanging eaves, copings and flashings, and careful attention to landscaping and sprinklers will reduce the chances of water entering the wall. In addition, specifying tooled, compacted mortar joints (concave or "V" type) will also reduce the potential for water infiltration. (See Figures 3 and 4 for good efflorescence controlling design.)
As mentioned earlier, for efflorescence to appear, the alkali sulfates must be able to travel through the pores in the masonry to the surface. If the natural pores in the wall can be reduced, it becomes more difficult for the salts to migrate through to the surface. Consolidating the grout with mechanical vibration will greatly reduce any voids in the grout, as well as improving the bond of the steel and the masonry wall. Dense tooled mortar joints will also reduce the porous nature of the wall and make it difficult for the salts to migrate.
Grout admixtures that claim to inhibit efflorescence are also sometimes used. These chemical additives claim to improve the flow of the grout mix while decreasing the water content. They also claim to reduce voids in the grout due to shrinkage.
Special care must be taken when using these grout admixtures. They have been developed by individual manufacturers and their actual contents are protected trade secrets. The manufacturer's recommendations must be closely followed.
To summarize, three conditions must exist before efflorescence can occur. If these three conditions can be controlled, there should be no efflorescing of masonry walls.
1. Reduce all soluble alkali sulfates.
2. Use good details to prevent water from entering the masonry.
3. Use good construction practices to eliminate migratory paths for moisture.
Remember, it is very difficult to totally control any one of these three conditions, but it is relatively simple to reduce the effect each one has toward efflorescence.
Removing Efflorescence
Despite all efforts, efflorescence may sometimes occur. A detail may have been omitted. Materials may have been incorrectly specified or may not have been used as specified. Sometimes conditions just naturally conspire to generate efflorescence on a wall. The materials may not have been covered or stored on pallets off the ground. Unknown salts could infiltrate masonry materials during a windstorm or simply through ignorance and misuse of the materials. Salt-laden soils could be backfilled against a wall where the salts could be absorbed and cause efflorescence. Whatever the reason, when efflorescence does appear, it has to be removed.
The traditional method of cleaning has been sandblasting, which, of course, works. Unfortunately it removes just about everything else, too. The abrasive action of the sand erodes the surface of the brick and the tooled mortar joints along with any deposited salts. This increases the porous qualities of the masonry and the water absorptive nature of the wall.
Sandblasting will also damage the integrity of the dense tooled mortar joints. A well-tooled and compacted mortar joint readily sheds moisture and provides minimum voids for penetration. After sandblasting, the mortar is more porous, has voids for infiltration, and may even reveal cracks in the mortar. Additionally, the appearance of the masonry wall will be changed since the texture of the brick has been made slightly coarser.
Sandblasting should be used with caution and afterwards the masonry should be sealed with a waterproofing material.
An alternate method which has shown good success when done properly is the use of special chemical cleaners. Most chemical cleaning