The term ‘efflorescence’ is used to describe the depositing of soluble salts (in other words, those that dissolve in water) on the surface of finished brickwork. Salts of magnesium, calcium, potassium or sodium may have been present in the clay used to make the bricks or in the sand used to make the mortar.
Fig. 264 Effloresence.
When the brickwork becomes wet, the salts dissolve into a solution. When the brickwork dries out and the moisture evaporates out of the wall, the salts revert to solid form but are left on the outside face of the material, usually in the form of a white powder.
Efflorescence can be seen on brickwork of any age but is most commonly found on new work that is drying out after the construction process. Efflorescence can also originate from the ground, with soluble salt solutions in the earth being absorbed by the brickwork. This explains why efflorescence commonly appears below horizontal DPC level, particularly where bricks with a high rate of water absorption have been used instead of engineering bricks.
The only real solution to efflorescence is periodically to brush off the salt deposits from dry brickwork as they come to the surface until, over time, all the salts have been released. It is a mistake to try to wash off the efflorescence as this will merely re-dissolve the salts and wash them back into the brickwork, ready to emerge again when the brickwork dries out.
New bricks are tested by manufacturers for the extent to which they are likely to effloresce and bricks are graded on the following scales in terms of the bricks’ exposed surface area:
• Nil: no perceptible efflorescence.
• Slight: no more than 10 per cent displays a thin covering.
• Moderate: thin covering affecting between 10 and 50 per cent.
• Heavy: heavy deposits affecting more than 50 per cent but with no flaking.
• Serious: heavy deposits displaying surface powdering/ flaking, which increases during wet weather.
It is rare to encounter bricks that are liable to effloresce any more than slightly.
Crypto-efflorescence is a second, more serious form of the effect, which can cause physical damage to brickwork. It occurs where salt crystals form just beneath the brick surface. These crystals constantly expand or contract according to whether the brickwork is wet or dry. In time, this movement can cause flaking and spalling of the brick surface. This is common in bricks that are already weak through under-firing at the point of manufacture. Crypto-efflorescence and frost damage sometime get confused with each other, as the physical damage to the brickwork resulting from both is much the same.