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Rain Resistant Masonry Construction The Efflorescence Phenomenon
1) General
2) Causes of Efflorescence
3) Prevention of Efflorescence
4) Analysis of Efflorescence Problems
5) Corrections and Solutions
6) Efflorescence Prevention Checklist
1. General
Many research results regarding the causes of efflorescence and ways to eliminate or minimize its occurrence are conflicting and controversial. There are so many potential causes or contributors to efflorescence that this apparent disagreement is not surprising. Some of the researchers obviously tailed to consider all possible factors and limited their investigations to certain combinations of materials or conditions. Nevertheless, it is possible to glean from the technical literature some consensus of opinion.
On one point there is virtual unanimous agreement that, at least, the permanent form of efflorescence is composed primarily of soluble alkali salts, usually sodium and potassium sulfates, but expressed as Na2O and K2O equivalents, that exude from the masonry interior as a solution, and upon drying recrystallize as a supersaturated solution on the masonry facade. Less often these alkali salts are found in carbonate and even bicarbonate forms, such as Na2CO3 and NaHCO3. In addition, in much lesser quantity, soluble chlorides and magnesium sulfate; insoluble calcium carbonate, calcium sulfate, and ferrous sulfate; and even traces of such rare metallic sulfates as vanadium, molybdenum, and others have been detected by chemical analyses in samples collected. There is disagreement on the chemical reactions occurring that perpetrate this phenomena, but not in the end-result.
Virtually all investigators readily agree that efflorescence is caused by multiple factors in combination, usually catalyzed by climatic and environmental conditions. Views are quite disconsonant on which factors are the major culprits in causing this "building bloom". The resulting diagnoses are frequently at odds. This is understandable since it is usually impossible to deduce the exact causes of a specific case with absolute certainty. Following is a complete list of the factors that can contribute to efflorescence.
2. Causes of Efflorescence
2.1. Construction Practices and Design. There appears to be a majority opinion that faulty workmanship, construction practices, and design are the greatest contributors to efflorescence, and at least one or some of the following malpractices are invariably present when efflorescence occurs:
a. Failure to protect piles of masonry units at the building site with tarpaulins or polyethylene film from drenching rains. Units should be kept reasonably dry. Absorption of moisture tends to dissolve traces of soluble salts within the units, so that after being laid in the wall, upon drying, they effloresce. This coincides with the general recommendation that if at all possible, masonry units, even the absorptive types, should not be overly soaked before they are laid. To overcome this absorption problem, plastic mortars of very high water retentivity should be employed to resist the suction of porous units.
b. Failure to cover and protect unfinished walls during construction from rain. Again, the reasons are largely the same as in 1(a): to prevent the bricks and freshly laid mortar from being saturated with water that will stimulate the dissolution of soluble salts from both the units and mortar.
c. At root level from inadequately flashed and unprotected parapet walls. The latter, including chimneys, should be capped to prevent vertical absorption of water into the masonry interior.
d. lack of drips on cornices and sills.
e. Leaky gutters and downspouts, which cause excessive wetting of adjoining wall sections.
f. Failure to tuckpoint cracked mortar joints or settlement cracks. The object is to impede as much as possible the penetration of water from driving rains into the interior of the wall.
g. A common cause is poorly filled mortar joints due to shoddy workmanship and/or the use of a harsh, unworkable mortar that is difficult to spread. The resulting voids in the mortar are most vulnerable to penetration from rains. So, only highly plastic, workable mortars should be employed.
h. Use of dense masonry units and mortars that upon becoming wet in the interior from rain penetration through cracks are paradoxically slow to dry out. In some cases such masonry never completely dries, so that this chronic damp condition is a "breeding ground" for salt concentrations. Masonry materials that are relatively porous dry out much more readily; they tend to "breathe" during wetting and drying cycles. As a result, capillary moisture penetration of masonry is not nearly as inducive to effloresce as water penetration through holes and cracks at the mortar-unit interface.
i. Failure to use dampproof courses, such as metal foil, embedded between the foundation and masonry wall at or just above grade level, may be a cause. Groundwater that enters the foundation, unless impeded, may be absorbed upward vertically into the wall by capillary attraction or a "wicking" action. Again, the object is to keep as much water as possible from penetrating the interior of the wall.
2.2 Masonry Units. In widely varying degrees virtually all types of masonry units will possess, at least, a minuscule amount of efflorescence potential. The varying amounts of this potential are more evident in certain geographic areas than other areas in spite of assiduously attempting to avoid the malpractices described in 2.1 above. This source is described as follows:
a. Certain clay brick derived from clay or shale containing a high total alkali salt content (Na2O and K2O equivalents) that are only soft to moderately burned into high porosity brick have an inherently high efflorescence potential. Clay and shale deposits vary greatly in the amount of alkali salts they contain. Use of chemical additives, like barium sulfate, by the brick manufacturer will reduce the tendency of some of these brick to effloresce, but this is no sinecure. Generally dense to moderately absorptive brick are least troublesome. Other sources of efflorescence may be derived from bits of limestone in the clay. When the brick is fired, the limestone is calcined in to lime (CaO) or the resulting lime reacts with the sulfur from the fuel, forming calcium sulfate, The clay may also contain some gypsum (CaSO4) in the native state. Nodules of lime or gypsum on the surface of the brick will hydrate and disintegrate, causing white streaking and pits to form in the brick surface. These two largely insolute chemical compounds are usually temporary forms of efflorescence and upon a few cycles of wetting and drying are usually washed away. Nevertheless, clay devoid of limestone and gypsum should be used if possible as well as low sulfur fuels. See also ASTM Methods 067 for specifying "No Efflorescence" or "Slightly Efflorescence" quality brick.
b. Concrete products, while generally not as prone to effloresce as clay products, can do so under certain conditions from the free lime that is liberated in the hydration of the cement, the lime carbonating on the surface of the unit. Usually such efflorescence is only temporary and will wash off after the first prolonged rain. Often such efflorescence is not noticeable, due to camouflaging effect of the units. Since it is not usually recurrent, it is not regarded as efflorescence, but simply as "lime streaking." Since in the U.S. most lightweight concrete products that might contain some degree of soluble salts are largely used in the back-up, their efflorescent generating potential would probably be less and more difficult to appraise than clay face brick. Dense concrete facing units generally present no problem because of low capillary action, and they possess only traces of these deleterious salts. Most stone and calcium-silicate facing units would behave similarly, although no masonry unit would be completely devoid of soluble salts -- at least a few parts per million.
The source of efflorescence
The NW Masonry Guide Table of Contents
Masonry Institute of Washington
Washington State Conference of Mason Contractors
