Efflorescence - Removal and Prevention
By Frederick M. Hueston, stoneforensics.com
The following article discusses historic masonry but the
causes and principles are the same for any stone, brick or concrete surface.
The crystallization of soluble salts in Historic masonry
causes severe deterioration of the substrate. This crystallization is called
subflorescence. These salts are derived from several sources, including salts
within the substrate, pollution, deicing salts and improper cleaning chemicals. Problems associated with subflorescence can
be diagnosed and identified by visual clues, such as spalling and rising
damp. Accurate diagnosis of
subflorescence can be confirmed with laboratory testing. Once the salts are
detected there are several methods which can remove these salts. Water washing,
surface rendering and poulticing are a few of the methods used. If these salts are successfully removed it is
important to prevent reoccurrence. Preventative applications might include installation
of dampproof barriers, chemical injection and coating the masonry with a sealer
or impregnator.
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Subflorescence
The deposit of water
soluble salts in the pores of Historic masonry is the major cause for
deterioration of these surfaces. These
salts originate from several sources. Gauri, Holdren and Vaughan (1986) report that
these salts are inherent in brick, concrete and natural stone. Boyer(1986) contributes polluted rain water,
roof salts, deicing salts and adjacent materials as the source of salt
deposition. Ashurst(1994) reports that
careless cleaning using improper chemicals can deposit salts causing
deterioration. Regardless of the source
all agree that water soluble salt deposition contributes significantly to the
rapid deterioration of historic masonry.
Grimmer (1984) defines Subflorescence as follows:
Subflorescence is a potentially harmful accumulation, or
build-up of soluble salts deposited under or just beneath the masonry surface
as moisture in the wall evaporates.
Particularly during the freeze-thaw cycle, the moisture and salts in the
wall freeze and expand, building up pressure within the masonry, which if
sufficient, may cause parts of the outer surface to spall off or delaminate.
(P. 22)
In other words, Subflorescence is the deposit of salt
crystals beneath the surface of the masonry.
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Subflorescence and Efflorescence
Subflorescence should not be confused with
efflorescence. Efflorescence is a
deposit of soluble salts on the surface of masonry. Efflorescence can be
identified by a whitish haze on the surface of the masonry. Efflorescence can be a precursor to
subflorescence since its indicates the
presents of salts. Efflorescence is
common and often harmless on newly constructed buildings, but if it appears on
historic masonry, it should serve as a warning that moisture has found a way
into the masonry.
Diagnosing and Identifying Subflorescence
Subflorescence should be suspect when the following
conditions are observed on a masonry surface:
1. Spalling- Spalling
of masonry surfaces can be identified by the breaking off of the surface layer.
Small pieces of the outer layer will flake off in both small and large
portions. Often the surface is very brittle and can easily be removed by prying
with a knife. Spalling can also be called delamination when referring to
manmade masonry. The term delamination
is general used for stone masonry.
2. Rising Damp- Along
the base of most masonry buildings, a wet, darken outline can be detected. This
darkened area usually extends from the ground to several feet above the
ground. This darken is the result of
water being carried through capillary action into the porous masonry. This
condition is known as rising damp.
Rising damp can lead to efflorescence and ultimately subflorescence
since salts can be dissolved and carried by the water. Rising damp is a concern in Northern climates
where deicing salts are used. If the
moisture remains during freezing months, the water will freeze, expand and cause
spalling of the masonry surface.
3. Post Cleaning
Operations- Frequently masonry surfaces will develop efflorescence within
several days after cleaning. The masonry surface should be checked carefully,
since efflorescence can be a precursor to subflorescence. This is especially troublesome if the
cleaning operation used copious amounts of water. Excessive use of water can saturate the
masonry, dissolved salts within the masonr, causing spalling.
4. Chemical Cleaning Operations- Certain chemicals can
deposit soluble salts within the pores of the masonry if not rinsed throughly. Alkaline cleaning chemicals contain alkaline
salts which can crystallize causing spalling of the masonry surface. If spalling occurs shortly after cleaning
check the type of cleaners used.
Testing for Soluble Salts
If soluble salts are suspected, a simple field test can be
used to determine if salts are present. To determine the type of salts, thus
leading to a possible source, laboratory tests must be conducted. The following
describes these tests.
Field Testing- London (1988) describes a simple qualitative
test for determining the presents of soluble salts using a protimeter. The test uses the principle that dissolved
salts have a higher capacity to conduct an electrical current. A filter paper is saturated with distilled
water, placed on a rubber block and a reading taken with the protimeter. A second filter paper is saturated with
distilled water, placed on the suspect masonry and a reading taken. If the reading on the masonry is higher than
the reading on the control, then soluble salts may be present. This test does
have a margin of error and can be misleading. It is advised to verify with
laboratory testing.
Laboratory Testing- Laboratory testing can determine
quantitative as well as the exact identification of the salts. It is important to know which type of salts
are present in order to determine the origin.
Most all salts in historic masonry can be classified as follows:
Chlorides- Chloride salts are found in deicing salts.
Ashurst (1988) also reports that chlorides can be hygroscopic, taking moisture
directly from the atmosphere. Chlorides can also be found in high
concentrations in environments near oceans and salt marshes. Chlorides can also
be present in the make up of the masonry itself.
Nitrates- If nitrate salts are detected, ground water should
be suspected. Nitrates are commonly found in fertilizers and in soils.
Carbonates- Carbonates are found in high concentrations in
masonry mortars continuing lime. Carbonates are also present in pointing mortar
and will usually present a problem when a building is repointed.
Sulfates- Sulfates are found in air pollution from vehicle
exhaust, industrial pollutants etc. Sulfates are an increasing problem in areas
where pollution is high.
Alkaline and Acid Salts- Alkaline and acid salts are deposited
into historic masonry when cleaners are used and not rinsed probably. If
efflorescence and/or subflorescence occurs shortly after cleaning, these salts
should be suspect. Hydrofluoric acid is
commonly used to clean historic masonry since it is one of the few chemicals
that do not deposit soluble salts(Ashurst, 1988).
Subflorescence
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Removal of Soluble Salts
Before any attempt is made to treat historic masonry and to
remove both efflorescence and/or subflorescence it is necessary to find the
origin of the salts. If this is not found, further damage may result. Once the origin is found the first step in
salt removal is to eliminate the source.
There are several methods for desalination of historic
masonry. The following methods have proved successful but are not fool proof.
It is vital that these procedures be performed by trained and experienced
individuals.
Ashurst (1988) describes two possible procedures for removal
of soluble salts, poulticing and sacrificial rendering. Poulticing uses an absorbent clay mixed with
distilled water into a paste. This past
is applied to the masonry, which has been saturated with water. As the clay
dries it will absorb the moisture from the masonry carrying the salts with it. Poulticing can be an effective treatment for
removing soluble salts but caution must be exercised since salts can be drawn
to the surface and crystallization may occur. Sacrificial Rendering uses an
application of a thin coat of a lime and sand mixture. This mixture is trowel on the wall as if applying
stucco. The principle is that the rendering will absorb moisture and transfer
the soluble salts to it. Gauri et al
(1986) describes two suction techniques for removal of soluble salts. The suction method uses a vacuum pump
attached to a funnel. The masonry is saturated with water and the vacuum pump
pulls the moisture out with the soluble salts.
This method can be time consuming on larger surfaces but industrial
equipment is available that can process several hundred square feet per day.
Grimmer (1984) suggests the use of water washing to remove efflorescence. The water washing method employs the use of
water sprayed on the surface of the masonry. The water can be applied with
high pressure, low pressure, intermittent with an endless
number of spray patterns. Water washing is the most common method employed for
cleaning historic masonry but does have its shortcomings. Chemical cleaning is
often used in an attempt to remove soluble salts with little to no result.
Hydrofluoric acid is the most often used chemical on masonry surfaces. The acid works by dissolving the salts which
are rinsed away after a timed dwell period. Hydrofluoric acid also is damaging
to the masonry itself since it dissolved the substrate.
Which ever method is used it is important to perform a test
on the masonry and monitor it closely for any harmful effects.
Preventing Subflorescence
Damaging salts are carried into historic masonry by
moisture. To prevent salt deposition it is necessary to eliminate the moisture.
Grimmer (1984) suggests using a dampproof course. This dampproof course
consists of placing a material such as plastic, horizontally in a masonry wall
to prevent moisture from rising into the substrate. Injecting chemical plastics into the masonry
have also been used to eliminate moisture as well as the application of sealers
and impregnators.
Whichever preventive treatment is used it is important to
remove the existing salts. This can be accomplished by the methods mentioned
above or the masonry should be left alone and the salts allowed to migrate out.
This is especially important with chemicals treatments. Sealers,
consolidants and impregnators should not be applied to
masonry containing salts. The
application of these sealers, etc will prevent moisture from escaping,
accelerating the chance of deterioration.
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Conclusions
The deterioration of Historic masonry through the
crystallization of soluble salts is a growing problem in the preservation
community. Improper cleaning techniques and air pollution both contribute to
the build up of these salts. It is
important that the proper diagnosis be formulated so that further damage is not
caused. Desalination practices and
procedure must be tested and performed by experience individuals.
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References
Ashurst, J. (1988).
Practical Building Conservation.
New York: Halsted Press.
Ashurst, N.
(1994). Cleaning Historic
Buildings Volume I. London:
Donhead.
Boyer, D.W.
(1986). Masonry Cleaning-The
State of the Art. American Society
for Testing and Materials. 25-51.
Gauri, K.L.
(1986). Cleaning Efflorescence
from Masonry. American Society
for Testing and Materials. 3-13.
Grimmer, A.
(1984). A Glossary of
Historic Masonry Deterioration Problems and Preservation Treatments. Washington: Department of Interior.
London, M.
(1988). How to Care for Old
and Historic Brick and Stone.
Washington: Preservation
Press.
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