- Effloresce redirects here, for the album by Oceansize, see Effloresce
(album).
Secondary efflorescence - dissolving the cement stone and attacking rebar
Efflorescence, in chemistry, is the loss of water (or a solvent) of crystallization
from a hydrated or solvated salt to the atmosphere on exposure to air.
Examples
- A 5 micrometre aqueous droplet of NaCl will
spontaneously crystallize at 45% relative humidity (298 K) to form a NaCl cube by the
mechanism of homogeneous nucleation. The original water is released to the gas phase.
- Gypsum (CaSO4.2H2O) is a hydrate solid that, in a sufficiently dry
environment, will give up its water to the gas phase and form anhydrous (CaSO4).
- Bluestone (CuSO4.5H2O) 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.
Primary Efflorescence
Primary efflorescence is named such, as it typically occurs during the initial cure of a cementitious product. It routinely
occurs in 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.
Secondary Efflorescence
Secondary efflorescence is named such as it does not occurr 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.
Protecting Against Efflorescence
It is 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 which 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.
Efflorescence can often be removed with diluted phosphoric acid (usually about 1 part acid to 10 parts water, but follow the
directions on the bottle). Always test some of the acid solution on an inconspicuous area first to ensure it will not discolour
the surface material. The acid dilution should then be neutralised with mild diluted detergent, and then be well rinsed with
water.
The source of the water penetration should be addressed. If the water source is groundwater or faulty flashing the
efflorescence may reappear, unless properly sealed.
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.
See also
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