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Efflorescence Causes, Removal, and Prevention
Efflorescence is a crystalline deposit on surfaces
of masonry, stucco or concrete. It is whitish in
appearance, and is sometimes referred to as
"whiskers". Efflorescence has been a problem for
many years, and is a topic of much controversy. The
formations of these salt deposits are not mysteries.
They are, for the most part, water-soluble salts
that come from many possible sources to mar and
detract from an otherwise beautiful and serviceable
structure. First of all, there must be water present
to dissolve and transport the salts. Groundwater is
often a source of efflorescence. For water to carry
or move the salts to the surface there must be
channels through which to move and migrate.
The more dense the material, whether it be brick,
stone, stucco or concrete, the more difficult for
the water to transport salts to the surface.
Conversely, the more porous the material, the
greater the ease with which salts are transported
and deposited. Salt-bearing water, upon reaching the
surface of a structure, evaporates the air to
deposit the salt. When humidity is low, the water
may evaporate before reaching the surface of the
structure, leaving the salt deposit beneath the
surface, and unseen. When the humidity is high,
water evaporation is slower allowing more
opportunity for whisker growth. Growths which
project 1/4 to 1/2 inch below the surface have been
reported in some areas of the country.
Since humidity has a definite effect on whether or
not the salts appear, it can be assumed that
efflorescence is a seasonal problem. The intensity
of efflorescence increases after rainy winter
seasons, decreases in spring, and by summer has
practically disappeared. This cycle may repeat for
months or years, but generally the intensity of the
efflorescence decreases in all but very extreme
cases, and by about the third year it should be
practically eliminated.
The mechanics by which efflorescing salts are
carried to the surface of structures by moisture and
capillary action through porous materials is
understood. The amount and character of the material
deposited varies considerably, depending on the
nature and source of the soluble materials.
Composition of Efflorescence
The problem of efflorescence, or the deposit of
water-soluble salts on the surfaces of masonry,
stucco or concrete, is an old one, and one that has
been studied and reported on as early as 1877. These
reports are all in common agreement that
efflorescence originates from more than one source,
and may be made up of more than one or two
compounds.
In addition, other salts such as chlorides and
nitrates, and salts of vanadium, chromium and
molybdenum are mentioned without giving specific
composition. These last, particularly vanadium, are
said to produce green efflorescence on white or buff
burned clay units, while other salts produce white
or gray deposits. Efflorescence derived from complex
vanadium compounds contained in the clay used in
brick manufacture is not uncommon in the
southwestern part of the United States.
There are many sources for water-soluble salts with
some salts more soluble than others. The movement of
groundwater into building foundations and by
capillary action, or wicking, upwards into masonry,
stucco or concrete, is very often the cause of
efflorescence. In the case where soil conditions
exhibit water soluble sulfates, precautions should
be taken to preclude the passage of this
sulfate-bearing water to the structure. Low
absorption is the best assurance against
efflorescence. Properly graded aggregates, low
water-cement ratio, good compaction and proper
curing practices will produce concrete of maximum
density and low water absorption.
Sand and gravel, in their natural state, may or may
not have been associated with salt bearing water or
soil. If they have, and these salts are not removed
by washing, this can be a possible source for
efflorescence. Most rock, sand and gravel plants,
however, are conscientious in washing material so
that any contribution made to efflorescence from
this source is negligible.
When mixing-water used for mortar, stucco or
concrete is obtained from a natural source which has
been in contact with a sulfate-bearing soil, the
resulting structure may exhibit efflorescence. The
adherence, again, to a good concreting practice of
low water-cement ratio, will help reduce the
appearance of salts from this source.
Another potential source of soluble salts are clay
products, such as building brick and face brick.
Generally, in the present day manufacture of these
products, the highly soluble salts are washed from
the clay, and a barium salt such as barium carbonate
is added to the product to react with the calcium
sulfate which may be present. In this reaction, the
product is two fairly insoluble compounds-barium
sulfate and calcium carbonate. When produced in this
manner, clay products exhibit little tendency to
efflorescence.
Building brick must be stored in a dry place off the
ground to prevent absorption of moisture or dampness
from possible salt bearing soil. A standard test may
be made to show the capacity of brick to contribute
to efflorescence through soluble salt content. A
brick is placed on end in a pan of distilled water
for seven days, in which time water is drawn upward
and through the brick and then evaporated from the
surface. Soluble salts are taken into solution by
the water and deposited on the surface.
It has also been noted that the occurrence of
efflorescence bears a relationship to the type of
mortar used. With a particular type of brick and a
certain mortar no efflorescence may occur, whereas,
the same brick with different mortar may produce a
wall heavily coated with salt deposits. The
appearances of sodium and potassium salts (as
sulfates) usually suggest Portland cement mortar as
the origin. The use of low alkali cement in mortar
and grout will minimize efflorescence, at least from
this source.
Since, for the most part, concrete masonry is
somewhat porous, evaporation of the salt bearing
water usually takes place before reaching the
surface when exposed to a drying atmosphere. The
hydroxides are converted by reaction with the carbon
dioxide of the air to alkali and calcium carbonates.
Efflorescence in the form of alkali chlorides and
sulfates is formed when the structure is surrounded,
exposed, or in contact with salt-bearing water or
soil and appears as columnar or whisker-like
crystals.
Some of the sources of water-soluble salts have been
covered. These may be deposited on stucco, masonry
or concrete walls as efflorescence. Practically any
building materials in direct contact with the earth
are potential sources for water-soluble salts. This
fact has been recognized by the various producers of
building materials, and steps have been taken to
reduce their presence to a great degree.
Several methods are suggested. One is to use water
under pressure or one of a number of products
available from stone dealers; another is muriatic
acid with subsequent flushing with water. Acid
applied to brick masonry, without previous wetting,
may cause "burning" or discoloration of the brick
and may also eat into the mortar. The Handbook on
Reinforced Grouted Brick Masonry Construction
suggests the use of light sandblasting for removal
of stubborn efflorescence (after many months).
Allowing the surface to dry thoroughly and then
using a stiff brush, prior to washing with water,
has helped prevent re-penetration of the surface by
the salt.
Various methods have been used in attempts to remove
efflorescence from masonry structures. It has been
found that when efflorescence is caused by soluble
alkali salts, the salts will dissolve in water
applied to the structure and migrate back into it.
These salts would then reappear on the surface as
the structure re-dried. It was learned accordingly,
that the best way to remove these soluble salts was
to brush the surface thoroughly with a stiff brush.
Water, however, has been satisfactory for removing
efflorescence from the face of concrete structures,
since concrete is fairly well saturated with water.
In fact, efflorescence in the form of alkali salts
will be washed from the surface of concrete
structures, if exposed to rain, over some period of
time.
If the coating is largely calcium carbonate or
calcium sulfate, it adheres rather strongly and is
difficult to remove by brushing. The practice
developed in this case for masonry surfaces, has
been to saturate the structure as thoroughly as
possible with water, and then wash with diluted
muriatic acid, followed immediately with an alkaline
wash, then washed with water. The acid recommended
is five (5) parts hydrochloric to one hundred (100)
parts water, or twenty (20) parts vinegar to one
hundred (100) parts water. The alkaline wash
recommended is diluted household ammonia.
Much care must be taken in applying acid to Portland
cement products. The acid will attack, not only the
calcium carbonate and calcium sulfate efflorescence,
but also other calcium compounds to produce calcium
salts such as calcium chloride. It is, therefore,
very important to neutralize the acid before it can
attack other compounds.
Rust is one of the most difficult stains to remove
from marble, granite and other stone surfaces. Many
of the rust removers available in stores can damage
stone. You may want to give the following suggestion
a try:
There is a product called "Iron Out" that is
available from most home centers and hardware
stores. This product is a white powder that is
designed for removing iron from water softeners, but
also works great for removing iron and other metal
stains from stone. To remove iron stains mix 1 part
"Iron Out" to 2 parts poultice powder (diatomaceous
earth). Add water until you get a soupy consistency.
Apply this mixture to the iron stain, cover with
plastic and let it sit for 24 hours. After 24 hours
remove the poultice and reapply if necessary.
As with any stone treatment it is always good
practice to try this out on a small section of stone
that is not visible to make sure it will not do any
damage.
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