Hygroscopic salt contamination is very common on chimney breasts and as a result of rising dampness and water ingress.
Ground water and other penetrating dampness introduces Chlorides and Nitrates (salts) into the building fibre. As the water evaporates these salts are left in the building and over many years become concentrated on the wall surface
These salts can also be introduced through the combustion of fossil fuels.
During periods of high humidity (such as the winter months, or periods of prolonged rain), moisture vapour (condensation) is attracted to the salts, causing them to become soluble and therefore damp.
Eradication of Hygroscopic dampness is a combination of ensuring that the path of water is prevented and the salt affected plaster is removed and reinstated with a plaster containing a salt inhibitor and water proofer.
Air Gap Membrane systems are often used to deal with this problem.