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CAUSES AND MECHANISMS
PART I OF II
It is important for designers to understand the various types of efflorescence which can occur and to
have at least a basic knowledge of the factors influencing the appearance of efflorescence on brick masonry.
This Technical Notes
covers the often very complicated mechanisms leading to the formation of efflorescence,
including the probable sources of soluble salts as well as the sources of moisture needed to activate these salts.
Information is presented as to the composition of each known type of stain, along with research references
describing conditions necessary to cause these stains to appear.
Key Words: admixtures
, carbonates, chlorides, condensation
, manganese, masonry units
, silicates, soluble salts
, sulfates, trim, vanadium.
There are certain simultaneous conditions which must
Efflorescence is a crystalline deposit of water-soluble
exist in order for efflorescence to occur. Soluble salts
salts on the surface of brick masonry. The principal
must be present within or in contact with the masonry
objection to efflorescence is its unsightly appearance.
assembly. These salts may be present in the facing units,
Although efflorescence is unsightly and a nuisance to
backup, mortar ingredients, trim, etc. There also must be
remove, it is usually not harmful to the brick masonry.
a source of water and it must be in contact with the salts
Efflorescence is usually white in color; however, all
for sufficient time to permit them to dissolve. The mason-
white stains on brick masonry are not necessarily efflores-
ry must be such that the migration of salt solutions to the
cence. Also, certain vanadium and molybdenum com-
surface, or other locations, occurs in an environment
pounds, present in some ceramic units, may produce a
which is conducive to the evaporation of water.
green deposit, commonly referred to as "green stain".
It is apparent, from the above discussion, that if
Occasionally, "brown stain" may occur, resulting from
masonry could be constructed to contain no water-soluble
salts, or if no water were permitted to penetrate the
Under certain specific circumstances and conditions,
masonry, efflorescence would not occur. However, in con-
it is possible for the crystals of efflorescence to form with-
ventional masonry exposed to weather, neither of these
in the bodies of the units. When this occurs, it is possible
conditions can exist. Consequently, the practical
that the pressure of crystallization and growth of the crys-
approach to the elimination of efflorescence is to reduce
tals may cause cracking and distress to the masonry.
all contributing factors to a minimum.
This Technical Notes
addresses the mechanisms of
efflorescence, including possible sources of salts and of
Sources of Salts
water. The purpose is to provide a basic understanding of
The chemical composition of efflorescent salts is usu-
the phenomenon of efflorescence for the design profes-
ally alkali and alkaline earth sulfates and carbonates,
sional, specification writer, contractor or owner. Technical
although chlorides have also been identified. The most
23A Revised will present recommendations on how
common salts found in efflorescence are sulfate and car-
to prevent the occurrence of efflorescence, and serve as
bonate compounds of sodium, potassium, calcium, mag-
a guide for its investigation, identification and elimination.
nesium and aluminum. Chlorides may also occur as efflo-rescence. This is usually a result of the use of calcium
MECHANISMS OF EFFLORESCENCE
chloride as a mortar accelerator, contamination of mason-
The mechanisms of efflorescence are many and often
ry units or mortar sand by sea water, or the improper use
complicated. However, simply stated, water-soluble salts
of hydrochloric acids in cleaning solutions.
in solution are brought to the surface of the masonry and
E fflorescence is further complicated by the many avail-
deposited there by evaporation. The salt solutions may
able sources of soluble salts. Soluble salts may be present in
migrate across surfaces of units, between the mortar and
the masonry units, in the mortar, or may result from rain water
units, or through the pore structure of the mortar or the
or ground water, or other sources as discussed hereafter.
*Originally published in May 1985, this Technical Notes
has been reviewed and reissued.
Since efflorescence appears on the
used in wall construction. The mortar is in intimate con-
face of the wall, it is often erroneously assumed to be the
tact with the brick on at least four and sometimes five
fault of the brick. This is not usually the case. There are,
sides. It is applied to the brick in a wet, paste-like condi-
however, soluble salts present in many of the units that
tion which provides ample moisture for the transfer of sol-
uble salts from the mortar to the brick. If any appreciable
- Because of the composition of the raw materi-
soluble material is present in the mortar, it will be carried
als and the high temperatures associated with the manu-
into the brick proportionately to the amount of moisture
facturing process, it is possible for soluble phases to exist
within the finished brick. If water is absorbed by such
"The simplest case of soluble salt contamination of
products, the soluble salts enter into solution and efflores-
efflorescence-free brick is the migration of 'free-alkali'
cence may be formed as evaporation takes place from the
solutions from the mortar to the brick. This situation is not
only the simplest mechanism, but it is also the most com-
In regard to brick which contain water-soluble salts
mon. In the trade, it is known as 'new building bloom'.”
formed during firing, Brownell (W.E. Brownell, ìTheCauses and Control of Efflorescence on Brickworkî,Research Report No. 15, Structural Clay ProductsInstitute, 1969) states: "Products such as these will showefflorescence when placed in distilled water, even thoughall precautions are taken to eliminate outside contamina-tion."
Brick units with low efflorescence potential are readily
available in all parts of the United States and Canada.
The potential for masonry units to effloresce may be easi-ly assessed by the efflorescence test in ASTM C 67,Standard Methods of Sampling and Testing Brick andStructural Clay Tile.
- Masonry materials used as backup or inner
Showing Migration of Soluble Salts from
Backup to Facing Brick
wythes of masonry walls may contain large quantities of
soluble salts. These units may contribute to efflorescenceon the face of the wall, if sufficient water is present to dis-
- The water-soluble alkalies common in mor-
solve the salts and pathways are provided for the solution
tars are sodium and potassium. Alkalies available in port-
land cements vary from one source to another, ranging
A comparison of the various types of concrete mason-
from approximately 0.02 percent to 0.90 percent by
ry units with structural clay tile was made by Young (J.E.
weight of the cement. A survey of masonry cements indi-
Young, “Backup Materials as a Source of Efflorescence,”
cates a range of alkali from 0.03 to 0.27 percent by weight
Journal, American Ceramic Society, 40 (7), 1957). Young
measured the soluble salts content and efflorescent ten-
It is suspected that the sulfate content of the cement
dencies of each of the various units in his experiments. It
may be as significant as the alkali content in contributing
was found that concrete products contain two to seven
to efflorescence. Modern cement manufacturing methods
times as much soluble material as the fired clay material.
which attempt to achieve energy conservation may result
Figure 1 illustrates the transfer of soluble salts from
in larger quantities of sulfates in the finished products.
backup units to facing brick. This result was obtained byplacing the backup block in pans of water with five brick
- Various investigators disagree as to the possi -
on top of each block, as shown. The brick had previously
ble contribution of lime to efflorescence. It has been
been subjected to the efflorescence test and showed no
demonstrated that lime, clay or sand additions to a mortar
efflorescence. The potential for backup units to effloresce
mix-do not generally contribute to efflorescence (T.J.
can be determined by using the same efflorescence test
Minnick, “Effect of Lime on Characteristics of Mortar in
method which is used for facing brick.
Masonry Construction,” Bulletin, American CeramicSociety, 38 (5), 1959.) In fact, these ingredients tend to
- Building trim, such as caps, coping, sills, lin-
dilute the deleterious effects of a high alkali cement.
tels, keystones, etc., are often of materials other than fired
On the other hand, lime is relatively soluble. Its pres-
clay products. These items can be natural stone, cast
ence may serve to neutralize sulfuric acids generated
stone, precast concrete, etc., which may contain soluble
within the masonry. However, a cleaning solution contain -
salts. Such materials may contribute significantly to efflo-
ing hydrochloric acid can produce very soluble calcium
rescence on the face of adjacent brickwork.
chloride which can migrate to the surface. Nevertheless,
Mortar can be a significant contributor to
lime in mortar is very important in establishing good bond
to brick units, and thereby increases the water resistance
"The primary and most obvious source of contamina-
tion of otherwise efflorescence-free brick is the mortar
- Sands used in mortar are primarily silica, and
quent and are limited to highly industrial areas and
as such they are not water-soluble. Sands, however, may
be contaminated with material which will contribute to
Sources of Moisture
efflorescence. This contamination may include: sea
As previously discussed, the mechanism of efflores-
water, soil runoff, plant life and decomposed organic com-
cence is dependent upon the presence of free water in
pounds, among others. Any of these may contribute to
the masonry to dissolve the available soluble salts. Some
of the sources of free water are discussed in the following
Miscellaneous Sources of Salts.
In addition to the
mortar and units placed in the masonry, there are other
. The primary source of moisture for the
outside sources of soluble salts that may contribute to
occurrence of efflorescence is rain water which penetrates
efflorescence. Some of these are discussed here.
or comes in contact with masonry. The exposure of
- A wide variety of admixtures for mason-
masonry to rain water varies greatly throughout the United
ry mortars is available to the masonry industry. Most of
States. Rain water exposure is very severe on the
these products are proprietary and their compositions are
Atlantic Seaboard and Gulf Coast where rains of several
not disclosed. In general, they are classified as grinding
hours duration may be accompanied by high winds. Rain
aids, air-entraining agents, water repellents, wetting
water exposure is moderate in the Midwest and
Mississippi Valley where wind velocities are lower. Rain
The effects of these admixtures on the properties of
water exposure is slight in arid areas of the West.
mortar are generally limited to flow, water retentivity and
Exposure area may be defined roughly in terms of wind
strength. Little information is available as to their effect
pressure and annual precipitation. The maps in Fig. 2
on bond, either between mortar and brick or between
indicate geographic areas of high wind pressures and
mortar and reinforcing. In addition, there is some evi-
heavy precipitation. A Driving Rain Index, which differs
dence, based largely on field experience, that certain
from the maps in Fig. 2, has recently been proposed.
admixtures may reduce the bond between mortar and
See Table 1 and Fig. 2 of Technical Notes
brick. This reduction in bond may make masonry walls
Rain water will penetrate all masonry walls to some
more vulnerable to water penetration.
degree, especially if they are improperly designed or
For these reasons, admixtures with unknown compo-
improperly detailed. The craftsmanship employed in the
sitions are not recommended for use in mortars unless it
construction of a masonry wall also has a significant effect
has been established by experience or laboratory tests
on the amount of water penetrating the wall. Brick
that they will neither materially impair mortar bond nor
masonry with workmanship characterized by partially filled
joints, deep furrowing of the mortar beds and improper
- Calcium chloride is sometimes
execution of flashing and caulking details will be more
added to mortar as an accelerator as permitted by ASTM
subject to rain penetration. See Technical Notes
C 270, Specification for Mortars for Unit Masonry.
Calcium chloride and compounds containing calcium chlo-
In addition to rain water and ground
ride should not be permitted in masonry containing metal
water, water may accumulate within the wall as a result of
anchors or reinforcing, as corrosion of metal embedded in
condensation of water vapor. Frequently, efflorescence
mortar will occur when exposure conditions are favorable.
that appears on masonry walls protected from rain is due
If calcium chloride is used, it should be limited to an
to this accumulation of condensed water.
amount not to exceed 2 percent by weight of the portland
Condensation is usually due to moisture originating
cement or 1 percent of the masonry cement (usually
inside buildings. Cold outside air, entering a building and
about 50 percent portland cement) content of the mortar.
heated for comfort purposes, is invariably low in moisture
See Technical Notes
1. Normally, this amount of calcium
content. Moisture released from cooking, bathing, wash-
chloride will not contribute materially to efflorescence.
ing and other operations employing water or steam, and
- Soluble salts in soil are dissolved by
moisture released by exhalation and perspiration of the
water which penetrates the ground. Consequently, most
occupants humidify this air. This gain in moisture content
ground water contains a high concentration of these salts.
increases the vapor pressure of the inside air substantially
When the earth is in contact with the masonry, ground
above that existing outdoors. This increased pressure
water may be absorbed by the masonry and may rise,
tends to drive the vapor outwardly from the building interi-
through capillary action, several feet above the ground.
or through any vapor-porous materials that may comprise
An accumulation of salts in the masonry is then possible.
When vapor passes through porous and homoge-
- It has been reported by some investi-
neous materials, which may be warm on one side and
gators that sulfurous gases in the atmosphere may conta-
cold on the other, it may pass through the zone of its dew
minate the brickwork. (F.O. Anderegg, “Efflorescence,”
point temperature without condensing into water. But, if
ASTM Bulletin No. 195, 1952). This situation over a peri-
the flow of vapor is impeded by vapor-resistant surfaces
od of time will cause disintegration of the mortar joint sur-
at a temperature below the dew point temperature, the
faces. These acids may also attack the components of
vapor will condense on such cold surfaces. This con-
the brick itself. The reports of such instances are infre-
densed moisture can contribute to efflorescence on the
wall surface. See Technical Notes
7C and 7D.
Another source of moisture which may
The mechanisms of this type of stain are not clearly
cause "new building bloom" and contribute to future
understood, but are often compared with the formation of
occurrences of efflorescence in a building is the water
stalactites in limestone caves. It is apparent that this
which enters the assembly during construction. The
deposit/stain requires a great deal of water traveling a
improper protection of a building during construction may
similar path over an extended period of time. The water
significantly contribute to future problems, including efflo-
takes any of several calcium compounds into solution,
rescence. It is at this stage, when interior assemblies are
and brings them to the surface of the masonry through
exposed, joints are open and foreign materials are pre-
the hole. The source of the calcium compounds may be
sent on the project, that the construction is highly vulnera-
trim, mortar, backup, etc. At the surface, it is thought that
ble to the entry of considerable moisture. Also, in some
the solution reacts with carbon dioxide in the air, thus
cases, additional soluble salts from other sources may
contaminate the masonry wall assembly.
These carbonate stains can be removed using a weak
solution of hydrochloric acid, applied directly to thedeposit. Care must be taken to properly wet the wall areafirst and rinse it thoroughly after cleaning. This is espe-cially true when removing carbonate deposits from light-colored brick. See Technical Notes
20 Revised. Thedeposit is likely to reappear unless the water source isstopped.
Silicate Deposits (Scumming)
Silicate deposits, sometimes called "scumming",
sometimes occur as a general white or gray discolorationon the face of brick masonry. The discoloration mayoccur over all of the face of the masonry or sometimes in
specific locations of 100 to 200 sq ft (9 to 19 m ) in area,irregular in shape. Silicate stain/deposits may also occuradjacent to trim elements, precast concrete and occasion-ally large expanses of glass.
These silicate deposits on brick masonry should not
be confused with the "scumming" that occasionally occurson brick in the manufacturing process. This "scum" willbe evident on the brick units in storage before they areplaced in the wall.
It is known that there is any number of mechanisms
that may precipitate silicate deposits on brickwork. Theirspecific chemistry, however, is not totally clear. Many ofthese stains are related to the cleaning of brick masonrywith hydrochloric acid solutions, especially if proper clean-ing procedures are not carefully followed, i.e., thoroughlywetting the wall, method of applying the cleaning solutionand thoroughly rinsing the wall with clear water.
Silicate deposits are very difficult, if not impossible, to
remove from brick masonry. They are insoluble in mostacids. Often the only practical method of dealing with a
silicate deposit is to disguise it and permit it to weather
Stains will occasionally occur on the surfaces of
away over time. See Technical Notes
masonry structures other than the fairly common white
Vanadium (Green or Yellow Stains)
efflorescence previously discussed. These are carbonate
Some structural clay products develop yellow or green
deposits ("lime run"), silicate deposits (white "scum"),
efflorescent salts when they come in contact with water.
"green stains" and "brown stains".
These stains are usually vanadium salts. They may be
Carbonate Deposits (Lime Run)
found on red, buff or white clay products; however, they
Carbonate deposits, if they occur, usually appear as a
are most objectionable and more readily apparent on the
gray-white, crusty spot in the form of a vertical "run-down"
lighter-colored units. The vanadium salts responsible for
shape on the face of the wall. These deposits are some-
these stains have their origin in the raw materials used for
times referred to as "lime run". This is not really correct,
the manufacture of the clay products. The yellow and
and may be misleading, as "lime run" is not directly a
green stains are usually vanadyl salts, consisting of sul-
result of lime in the mortar. Carbonate deposits nearly
fates and chlorides, or hydrates of these salts.
always occur at a small hole or opening in the face of the
The mechanisms of this type of stain are as follows:
as water travels through the brick, it dissolves both the
1. When a building is under construction using brick
vanadium oxide and sulfates. In this process, the solution
colored with manganese, it should not be cleaned
may become quite acidic. As the solution evaporates
with hydrochloric acid without neutralizing the acid
from the surface of the product, the salts are deposited.
during the rinsing operation. Such neutralization will
The chloride salts of vanadium require highly acidic
tend to reduce the amount of manganese taken into
leaching solutions, and are usually the result of washing
brickwork with acid cleaning solutions. Vanadyl chloride,
2. Application of silicones to brick (if otherwise unob-
one of the most prominent stain compounds, forms almost
jectionable, see Technical Notes
6A) may prevent
exclusively as a result of washing with hydrochloric acid.
staining by retarding water penetration of the brick
As stated by Brownell: “A highly acid condition in the
water leaching through a brick is necessary for the promo-
3. Always request and follow the advice of the brick
manufacturer in cleaning a brown or manganese-col-
Preventing green stain caused by vanadium is impor-
tant, since subsequent efforts at cleaning may turn it into
The removal of manganese stain is a fairly simple
a brown, insoluble deposit that is very difficult to remove.
operation, and is described in Technical Notes
To minimize the occurrence of green stain, the follow-
Revised. However, the permanence of the removal is
often in doubt. Hence, the prevention of the occurrence
1. Store brick off the ground and under protective cov-
of brown manganese stain is of paramount importance.
2. Never use or permit the use of acid solutions to
This Technical Notes
has presented a brief descrip-
3. Seek and follow the recommendations of the brick
tion of the causes, mechanisms and sources of efflores-
manufacturer for cleaning procedures, for all types
cence. Technical Notes
23A Revised will address the pre-
vention of efflorescence, a guide for analysis of efflores-
Green vanadium stains can be difficult to remove.
cence problems and the removal of efflorescent salts from
Some methods and procedures for the removal of green
stain are described in Technical Notes
20 Revised. Never
The information contained in this Technical Notes
attempt to remove green stain with acids.
based on the available data and experience of the techni-
Manganese (Brown Stain)
cal staff of the Brick Institute of America. This information
Under certain conditions, tan or brown, and some-
should be recognized as recommendations which, if fol-
times gray staining may occur on the mortar joints of
lowed with good judgment, should result in brick masonry
brickwork. Occasionally, the brown stain will streak down
onto the faces of the brick. This type of stain is the result
Final decisions on the use of information, suggestions
of the use of manganese dioxide as a coloring agent in
and recommendations as discussed in this Technical
the units. This staining problem is closely related to the
are not within the purview of the Brick Institute of
general efflorescence problem, since it is the sulfate and
America and must rest with the project owner, designer or
chloride salts of manganese that travel to the surface of
the brick and are deposited on the mortar joints.
During the brick firing process, the manganese color-
ing agents undergo several chemical changes, resulting in
For further information on the subject of efflorescence,
manganese compounds that are insoluble in water. They
its causes and mechanisms, the following publications
have varying degrees of solubility in weak acids. As pre-
viously discussed, acid solutions can occur in the brick in
T. Ritchie, "Study of Efflorescence Produced on
a wall. Also, the brick can absorb hydrochloric acid during
Ceramic Wicks by Masonry Mortars", Journal
the masonry cleaning process. It is also possible that in
American Ceramic Society, 38 (10) 1955.
some areas rain water may be acidic (T.J. Minnick, “Effect
W. F. Brownell, J. L. Kenna, and P. P. Wilko, Jr.,
of Lime on Characteristics of Mortar in Masonry
"Staining of Mortar by Manganese Colored Brick",
Construction,” Bulletin, American Ceramic Society, 38 (5),
, American Ceramic Society, 45 (12) 1966.
Technical Notes on Brick Construction
According to Brownell: "The manganese sulfate or
Revised, Brick Institute of America, McLean, Virginia,
chloride solutions from the brick will migrate across the
mortar joints especially during a period of drying. Theseacidic manganese solutions will be neutralized by theinherent basic nature of the mortar. Upon neutralization,insoluble manganese hydroxide is precipitated on themortar joints, and this is converted to brown Mn3O4
To minimize or eliminate manganese staining, the fol-
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