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‘Efflorescence’ is the term most commonly used to describe the deposit of crusty white mineral salts that appear on a masonry surface (concrete, render, brick or mortar) that have leached out from within the substrate when moisture migrates through it.
Any efflorescence on the surface must be removed prior to painting as it is regarded as a poor and friable base that prevents paints and coating systems from adhering effectively.
Paint systems adhering normally to the surface of the substrate can also be forced off (delaminated), when the pressure caused by the growth of salt crystals builds up beneath the paint film, resulting in its gradual but irreversible destruction. Since the mineral salt crystals are not fluid, the pressure is therefore not uniform, so the coating does not form smooth rounded blisters.
The paint film may stretch to form the outline of the growing crystals or the paint film may just rupture, crack, flake or peel.
Efflorescence in masonry appears as white alkaline salts on the surface and consists mainly of calcium carbonate. A reasonable amount of moisture is required for it to form.
The carbonation process occurs when alkaline salts such as Lime (calcium hydroxide) within the pores of the substrate react with atmospheric carbon dioxide to form calcium carbonate, which is transported to the surface by moisture. Concrete or mortar with high levels of Portland cement contains high concentrations of calcium that, in turn, can produce excess salts.
It should be recognised that there must be water (moisture) present to dissolve and/or transport these salts so the source of the moisture needs to be either blocked (if it’s external to the substrate, such as groundwater) or exhausted (if moisture is emanating from within the substrate, such as rising damp).
As moisture enters and moves through the substrate it dissolves mineral salts (mainly calcium carbonate) that are present in the cement (or plaster). When the mineral salt solution finds its way to the surface of the substrate, the water evaporates, leaving behind a white deposit of crystalline salts.
Efflorescence may occur within the first year of a new construction project and may only be an aesthetic concern on an unpainted substrate.
Efflorescence due to residual moisture within the substrate will cease naturally as the masonry dries and cures. However, when the efflorescence continues unabated it is likely that moisture ingress from an external source is occurring, which can present significant ongoing problems.
Without doubt, the best way to prevent efflorescence from occurring in the first instance is to put the following practices into effect at the design and construction phase:
Salt, generally alkaline, may be transported to the surface of plaster by the movement of moisture to a surface from which it can evaporate. Efflorescence due to residual moisture will cease naturally as the plaster cures, whereas efflorescence caused by moisture ingress may continue indefinitely while there are salts present to be transported, unless the source of moisture ingress is detected and eliminated, whereupon the decline in moisture levels and cessation of efflorescence will proceed as for residual moisture.
The cessation of efflorescence may be ascertained by brushing or wiping an affected surface free of existing efflorescence, then waiting a reasonable period to ensure its continuing absence.
Moisture ingress generally becomes apparent in new plasterwork as a failure of moisture levels to decline as expected despite conditions being adequate for such moisture loss to occur – i.e., in terms of time, temperature, relative humidity and ventilation rate. The source of moisture should then be traced and rectified before moisture levels can decline to a normal equilibrium. Any side effects of the elevated moisture levels, such as disintegration of the plaster or mould growth, should also be rectified before proceeding to paint.
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