In chemistry, efflorescence is the migration of a salt to the surface of a
porous material, where it forms a coating. The essential process comprises
the dissolving of an internally-held salt in water, or occasionally in
another solvent. The water, with the salt now held in solution, migrates to
the surface, then evaporates, leaving a coating of the salt.
In what has been described as "primary efflorescence," the water is the invader
and the salt was already present internally. Some people describe a
reverse process, where the salt is originally present externally and is
then carried inside in solution, as "secondary efflorescence." However,
others would give this latter phenomenon another name entirely.
Efflorescences can occur in natural and built environments. On porous
construction materials it may present a cosmetic outer problem only, but can
sometimes indicate internal structural weakness.
Examples A 5 molar concentration aqueous droplet
of NaCl will spontaneously crystallize at 45% relative humidity to form an NaCl
cube by the mechanism of homogeneous nucleation. The original water is
released to the gas phase. Gypsum is a hydrate solid that, in a
sufficiently dry environment, will give up its water to the gas phase and form
anhydrite. Copper(II) sulfate is a blue crystalline
solid that when exposed to air, slowly loses water of crystallization from its
surface to form a white layer of anhydrous copper(II) sulfate.
sodium carbonate deca hydrate will lose water when exposed to air.
Masonry = Primary efflorescence =
Primary efflorescence is named such, as it typically occurs during the initial
cure of a cementitious product. It often occurs on masonry construction,
particularly brick, as well as some firestop mortars, when water moving
through a wall or other structure, or water being driven out as a result of
the heat of hydration as cement stone is being formed, brings salts to the
surface that are not commonly bound as part of the cement stone. As the water
evaporates, it leaves the salt behind, which forms a white, fluffy deposit,
that can normally be brushed off. The resulting white deposits are referred to
as "efflorescence" in this instance. In this context efflorescence is sometimes
referred to as "saltpetering." Since primary efflorescence brings out salts
that are not ordinarily part of the cement stone, it is not a structural,
but, rather, an aesthetic concern. For controlling primary efflorescence,
formulations containing liquid fatty acid mixtures have been commonly used.
The oily liquid admixture is introduced into the batch mix at an early stage by
coating onto the sand particles prior to the introduction of any mix water, so
that the oily admixture is distributed uniformly throughout the concrete batch
mix. = Secondary efflorescence =
Secondary efflorescence is named such as it does not occur as a result of the
forming of the cement stone or its accompanying hydration products. Rather,
it is usually due to the external influence of concrete poisons, such as
chlorides. A very common example of where secondary efflorescence occurs is
steel-reinforced concrete bridges as well as parking garages. Saline
solutions are formed due to the presence of road salt in the winter. This saline
solution is absorbed into the concrete, where it can begin to dissolve cement
stone, which is of primary structural importance. Virtual stalactites can be
formed in some cases as a result of dissolved cement stone, hanging off
cracks in concrete structures. Where this process has taken hold, the
structural integrity of a concrete element is at risk. This is a common
traffic infrastructure and building maintenance concern. Secondary
efflorescence is akin to osteoporosis of the concrete.
For controlling secondary efflorescence, admixtures containing aqueous-based
calcium stearate dispersion are often added at a later stage of the batching
process with the mix water. In a typical batching process, sand is first charged
into the mixer, then the oil-based primary anti-efflorescence admixture is
added with constant mixing to allow the oil to coat the sand. Then coarse
aggregates, colorants, and cement are added, followed by water. If CSD is
used, it is then introduced usually at this point during or after the addition
of the mix water. CSD is an aqueous dispersion wherein fine solid particles
of calcium stearate are suspended in the water uniformly. Commercially available
CSD has an average particle size of about 1 to 10 micrometres. The uniform
distribution of CSD in the mix may render the resulting concrete masonry
unit water repellent, as CSD particles are well distributed in the pores of the
unit to interfere with the capillary movement of water.
= Protecting against efflorescence = The only way to completely and
permanently prevent efflorescence in cementitious materials is by using
special admixtures that chemically react with and bind the salt-based impurities
in the concrete when hydrogen is present. The chemical reaction in these
special additives fuses the sodium chloride on a nanomolecular level,
converting it into non-sodium chemicals and other harmless matter that will not
leach out or migrate to the surface. In fact, the nanotechnology in these
additives can be up to 100,000 times smaller than even the smallest cement
particles, allowing their molecules to literally pass through cement minerals
or sand particles and ultimately become part of the cement or sand with which
they react. And since they require the presence of hydrogen they stop reacting
as the concrete dries out and begin reacting again when the concrete is
exposed to moisture. It is also possible to protect porous
building materials, such as brick, tiles, concrete and paving against
efflorescence by treating the material with an impregnating, hydro-phobic
sealer. This is a sealer that repels water and will penetrate deeply enough
into the material to keep water and dissolved salts well away from the
surface. However, in climates where freezing is a concern, such a sealer may
lead to damage from freeze/thaw cycles. And while it will help to protect
against efflorescence, it cannot permanently prevent the problem.
Efflorescence can often be removed from using phosphoric acid. After application
the acid dilution is neutralised with mild diluted detergent, and then well
rinsed with water. However, if the source of the water penetration is not
addressed efflorescence may reappear. Common rebar protective measures include
the use of epoxy coating as well as the use of a slight electrical charge, both
of which prevent rusting. One may also use stainless steel rebar.
Certain cement types are more resistant to chlorides than others. The choice of
cement, therefore, can have a large effect upon the concrete's reaction to
chlorides. Today's water repellents help create a
vapor permeable barrier; liquid water, especially from wind driven rains, will
stay out of the brick and masonry. Water vapor from the interior of the building,
or from the underside of pavers can escape. This will reduce efflorescence,
spalling and scaling that can occur from water being trapped inside the brick
substrate and freezing during cold weather. Years ago, the water repellents
trapped moisture in the masonry wall creating more problems than they solved.
Condensation in areas that experienced the four seasons were much more
problematic than their counterparts. = Image gallery =
See also Hygroscopy
Deliquescent Hydrate
References