Efflorescence is the deposition of mineral salts, usually seen on the surface of masonry and is the cause of unsightly stains or even, in extreme cases, powdery crystal deposits that will harden over time.
Efflorescence is an indication that masonry has been wet for a considerable time, allowing the water to dissolve minerals from the masonry and mortars. These are then deposited on the surface of the wall once the water is given an escape route and can evaporate. They will often occur once impermeable cement renders and plasters are removed and the walls start to dry out.
Once the mineral salts have been mobilized within the masonry, they can be difficult to prevent returning since they are readily dissolved in water/moisture, so can pass into newly applied coats of plaster/render or paint finishes.
… and how to treat them
The key to managing salts effectively is to ensure that the root cause of water ingress into the masonry is identified and corrective action taken. Cementitious renders, plasters and pointing mortars should be removed to give the wall an opportunity to dry out. If allowed to remain wet, the salts can even be deposited as crystals within the masonry, causing it to break apart.
As the wall dries, it should be expected that salts will continue to be deposited. These should be brushed/vacuumed off as they form. Once the wall has dried out, no more salts should appear on the surface.
It is then important to keep the wall as dry as possible to reduce the risk of re-occurrence, so lime plasters/mortars and “breathable” paint finishes are imperative to prevent further build-up of moisture, as well as corrective action on any external defects leading to ingress of rainwater.
Standard lime plastering techniques, however, involve significant amounts of moisture, and so are best avoided. The following process is designed to minimize the risk of re-occurrence of the salt formation, but there are no guarantees:
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Use NHL lime for your base coats – NHL 2 is ideal in a 1:3 mix by volume with a sharp sand.
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Do not pre-wet the wall(s) as normal. Instead, apply a wet coat (the consistency of a wet cowpat) of NHL base coat plaster thrown (harled) on, to give even coverage, with a rough surface.
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Allow this to harden before applying full base coats of the same plaster, incorporating reinforcing fibres.
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If desired, non-hydraulic lime plasters can be used for the final coat.
