Significations et usages de Efflorescence

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  Secondary efflorescence on the Dam of the Robert Moses Niagara Hydroelectric Power Station.

In chemistry, efflorescence (which means "to flower out" in French) is the loss of water (or a solvent) of crystallization from a hydrated or solvated salt to the atmosphere on exposure to air.

Efflorescences can occur in natural and built environments. On porous construction materials it may present a cosmetic problem only, but can sometimes indicate serious structural weakness.

  Examples

1. A 5 molar concentration aqueous droplet of NaCl will spontaneously crystallize at 45% relative humidity (298 K) to form an 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 anhydrite (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 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 (e.g., oleic acid and linoleic acid) 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.^[1]

  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 (CSD) 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 CMU water repellent, as CSD particles are well distributed in the pores of the unit to interfere with the capillary movement of water.^[1]

  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 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.

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  See also

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Look up efflorescence in Wiktionary, the free dictionary.

• Hygroscopy
• Deliquescent
• Hydrate

Brick Industry Association, Technical Note 23A "Efflorescence - Causes and Prevention", June 2006, [1]

  Notes