What is Efflorescence?
“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 (delaminate), 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 hence 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 instead.
How does it occur?
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 then it is likely that moisture ingress from an external source is occurring, which can present significant ongoing problems.
The solution for efflorescence
Efflorescence will only stop forming when the migration of moisture through the substrate stops. Unless the source of the water (moisture) causing the efflorescence is traced and eliminated (if possible) then it is highly likely that the salts will continue to reappear over time. Failure to remove the source of water ingress allows more efflorescence to form, resulting in subsequent damage to the existing paint coating system.
Some paint coating systems may have varying levels of resistance to efflorescence or alkalinity however there are no guarantees or warranty. Remove all efflorescence where ever possible.
Efflorescence and alkali salts can be removed by dry brushing with a stiff-bristled brush followed by wet sponging the surface with a mild 5% solution of white vinegar (Acetic Acid) in water. The whole area should then be wiped down with a damp cloth and allowed to dry thoroughly.
In severe cases, remove alkaline salts by dry brushing with a stiff-bristled brush followed by a stronger acidic solution (35% commercial-grade calcium chloride mixed 1 part to 3 parts water) should be applied by an experienced contractor, to chemically remove/neutralize the alkaline salts then allow to dry for 48 hours.
Treatment with calcium chloride solution would NOT be necessary on concrete floors that are to be diamond ground, shot blasted or acid etched in order to provide a base for sound adhesion of coatings.
Painted surfaces already damaged by efflorescence must NOT be repainted until the source of the moisture is identified and eliminated or blocked (by waterproofing). It may be necessary to engage the services of a qualified waterproofing specialist.
The damaged paint will need to be totally removed by mechanical means and chemical stripping. The whole surface should then be washed with a mild acid solution as described above, rinsed and allowed to dry. Check that the moisture content of the substrate is less than 10%.
When the surface is sound, clean, dry, free from mould & algae and ready for painting it would be wise to select an alkali-resistant sealer and appropriate topcoats.
This information has been sourced from Dulux Technical Advice on Efflorescence