Masonry Magazine October 1986 Page. 27
EFFLORESCENCE:
Cause and Control
by Michael W. Merrigan
Efflorescence (ef' la res' ans), noun.
1. a change on the surface to a powdery substance upon exposure to air, as a crystalline substance through loss of water. 2. to become incrusted or covered with crystals of salt or the like through evaporation or chemical change.
Efflorescence. This stubborn problem has caused confusion and trouble for masonry since the first time it appeared thousands of years ago on ancient masonry walls. Efflorescence is the white, powdery scum that sometimes appears on masonry walls after construction. Nobody likes it, nobody wants it, but sometimes this problem persists.
A great deal of time, money and effort have been spent trying to solve the difficulties efflorescence generates. Many test programs have been developed and numerous attempts have been made to eliminate efflorescence. Unfortunately, nothing has proven 100% effective. However, even though no sure-fire cure has been discovered, a great deal has been learned about how efflorescence works and how to prevent it.
What is Efflorescence?
We know that efflorescence is a fine, white, powdery deposit of water-soluble salts left on the surface of masonry as the water evaporates. These efflorescent salt deposits tend to appear at the worst times, usually about a month after the building is constructed, and sometimes as long as a year after completion.
Required Conditions
Efflorescence is not a simple subject. Three conditions must exist before efflorescence will occur.
Michael W. Merrigan has been staff engineer for the Masonry Institute of America, Los Angeles, California, for the past two years. Previously he worked as superintendent for a general contractor building masonry houses for the Army and as an engineering technician for J. H. Kleinfelder's & Associates, a testing laboratory in Nevada.
First: There must be water-soluble salts present somewhere in the wall.
Second: There must be sufficient moisture in the wall to render the salts into a soluble solution.
Third: There must be a path for the soluble salts to migrate through to the surface where the moisture can evaporate, thus depositing the salts which then crystalize and cause efflorescence.
All three conditions must exist. If any one of these conditions is not present, then efflorescence cannot occur. Even though the efflorescence problem is complex, it is not difficult to prevent. Although no economically feasible way exists to totally eliminate any one of these three conditions, it is quite simple to reduce all three and make it nearly impossible for efflorescence to occur.
Source of Efflorescent Salts
A chemical analysis of efflorescent salts in the Southern California area' reveals that they are principally alkalies of sodium sulfates (Na₂ SO₄) and potassium sulfates (K₂ SO₄). These are the main soluble salts to be concerned with in Southern California since these are 90% of the efflorescence found in this area. These alkali sulfates appear because they exist somewhere within the masonry wall itself, either in the brick, the mortar, or the grout, or possibly a combination of all three.
These alkalies combine with sulfates from the masonry to form sulfate salts. The alkali sulfates in the wall are dissolved by water into a solution which then moves through the natural pores in the masonry. The solution migrates to the surface of the wall where the water evaporates, depositing the salts on the wall and generating the white powdery scum.
Research into each of the materials used in masonry walls reveals that the main source of alkalies for the salts is the portland cements used in the mortar and grout. It is recommended that low alkali portland cement be used to reduce the chances of efflorescence occurring. Low alkali portland cement has 0.6% alkali or less, by weight in the cement.
Another culprit is, of course, the clay brick itself. The natural clays used in the manufacture of brick often contain soluble alkali sulfates. Most modern fired clay brick have balanced chemical additives, such as barrium carbonate (Ba CO₂), to immobilize the sulfates and render them insoluble. This prevents the salts from being dissolved into a solution that could migrate through the wall to the surface. Most fired clay brick do not greatly contribute towards the efflorescence problem.
The next source for soluble salts would be the sand used in the mortar.