Hygroscopic salts have the ability to absorb atmospheric water vapour - effectively drawing moisture out of the air and into the wall. Consequently, depending on the relative humidity conditions that prevail, structures which contain such salts may be intermittently ‘damp’, possibly even appearing as damp patches at times of rainfall and/or following rainfall, even though no external source of liquid moisture is present.
Ground salts, such as chlorides and nitrates, will normally be present as a result of rising damp or lateral penetrating damp below ground level. However, widespread problems with chlorides in walls may indicate:
Rising dampness may be controlled by the insertion of a remedial damp proof course, however, these salts alone can cause the wall and any contaminated decorations to remain damp. Problems caused by residual salt contamination are usually dealt with by removing the salt contaminated plaster and re-plastering with a salt resistant plaster mix. It should be noted that the removal of the plaster does not remove the salt accumulated within the masonry itself. There is no economic method of removing this salt and this is the reason why special plastering is used for walls affected by salt contamination.
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