efflorescence

Effloresce redirects here, for the album by Oceansize, see Effloresce (album).

Secondary efflorescence - dissolving the cement stone and attacking rebar

Secondary efflorescence

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

1. 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.
2. 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).
3. 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

Concrete stalactite

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

Wikimedia Commons has media related to:

Efflorescence

• Hygroscopy

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