Efflorescence

L’efflorescence is a property of some sali losing water molecules crystallization.  The phenomenon is due to the difference in vapor pressure between the hydrate and the atmospheric water vapour pressure. This results in the tendency of ’ water of crystallisation of the compound to evacuate the solid passing in vapour in air ’ (quando si è in condizioni di bassa moisture relativa o di buona evaporazione).  As a result of this, the mixture will tend to become dry.

Classic examples of efflorescenti compounds are:
the Chalk, (Case4·2H2OR),
cupric sulfate pentahydrate, CuSO4·5H2OR (that, losing water, switches from a blue color to a white.

Other compounds are also some efflorescenti chlorides and nitrates hydrates.

The process can be considered as a case opposed to deliquescenza (tendency to absorb atmospheric moisture).

The term is used in construction in the broadest sense side where, for efflorescence, are those visible Crystal deposits that form on the surface of masonry in stone or brick or other architectural elements like floors or plaster. These crystals are generated as a result of the simple evaporation of ’ water in which were dissolved in the surface.
The efflorescence are commonly, and erroneously, also called salnitri (term that actually describe only the ’ effect of potassium nitrate).
The efflorescence can be formed both inside and outside of the building damaging the painting and l’plaster, When these are applied before construction has had time to dry or items made from operation as a result of rising damp phenomena or marine aerosols.

Types

The most common forms of efflorescence consist of whitish deposits to crystalline appearance, pulvurulenti or sticky.
More rarely (and in humid environments) also in cellars, caves and barns where you can the ’ action of nitrifying bacteria that convert l urea ’ of organic compounds (such as droppings) in nitrate salts (These have a filamentous appearance efflorescence similar to white fluff).

Formation and effects

Prerequisite to the formation of efflorescence (as well as building understanding) is the simultaneous presence of soluble salts and their water.
The efflorescence is caused by soluble salts that migrate to the surface when the water content evaporates in the resulting drying of the wall.
A time dry the wall, concentrate salts on the surface of the masonry and crystallize.
The passage to the solid phase with repeated accumulation of crystals results in a strong increase in volume. If the artifact, During his lifetime, is subject to repeated wetting and drying cycles, the ’ alternation of dissolution and recrystallization of superficial deposits of salt leads to exfoliation of plaster, malte, paintings etc.
Normally the ’ efflorescence is a surface phenomenon, Since the water ’, evaporating, Drag the dissolved salts to the external ’.
Evaporative crystallization takes place usually on the surface and in the early 15 – 20 mm beneath the surface.
Deeper the salts remain in solution and are not normally able to do damage. Are therefore void-efflorescence behind the surface that determines the detachment of the most superficial parts of masonry, Similarly to phenomena due to frost action ’.

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