HYGROSCOPIC AND DELIQUESCENT SALTS
Hygroscopic and Deliquescent salts have the ability to absorb atmospheric water vapour. Consequently, depending on the relative humidity conditions prevailing, structures, which contain such salts, may be intermittently damp. Even though no external source of liquid moisture is present.
Ground salts such as chlorides and nitrates may be present as a result of rising damp or penetrating damp below ground level.
However, widespread problems with chlorides in walls may indicate;
• The use of unwashed sand during construction.
• Overuse of chloride-based mortar additives.
• Salt water exposure eg seawater, de-icing salts on roads.
SALTS FOUND IN SOIL
Soil is the main organic consisting of decayed vegetable material with an abundance of nitrogen (nitrates) and the chloride salts of various elements notably calcium.
Water taken from the soil is not pure. But contains dilute solutions of soluble nitrates and chlorides. It can be seen therefore that the presence of nitrates and chlorides (particularly nitrates) in building structures can give a clear indicator that rising damp is or was present.
Nitrates and Chlorides are usually hygroscopic and will absorb moisture from the air. In this way, they can keep the surface of the wall wet. Even though the rising damp, which deposited them, has been cured.
They are also electrically conductive, even when dry, and can give misleading readings on conductivity type moisture meters.
SALTS FOUND IN BUILDING MATERIALS ABOVE GROUND
Carbonates and sulphates are found in most traditional building materials. They are usually efflorescent (flower) and appear as crystalline growth on the surface of a wall. Where they are seen during the drying out of new construction. Or on renewed plaster or can be as a result of evaporation of the moisture of penetrating damp. They are seldom hygroscopic.
Hygroscopic – Tending to absorb moisture from the atmosphere and in the case of solids, without liquefaction.
Deliquescence – The liquefaction of hygroscopic salts as they absorb water vapour.
Efflorescence – On the surface of clay bricks, powdery crystals that grow from salts are dissolved in the brick. It is unsightly, although usually harmless unless it lifts paint, plaster or tiling. The salts come mainly from gypsum or pyrite in the clay and coal that bricks are fired with. Mortar plasticisers generally contribute no salts. If efflorescence cannot be removed by light brushing or chemical neutralising and breaks up the plaster. The plaster must be removed and replaced with sulphate resistant plaster. It can be greatly reduced if not prevented by keeping bricks dry during laying, which includes covering them at night.
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