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Last edited 26 Nov 2020
 What are hygroscopic salts?
In order to understand the nature of hygroscopic salts, we must first look at the term ‘hygroscopic’ which means to absorb water from the air. Therefore, a substance which is hygroscopic is one which is able to consistently attract water from its surroundings via adsorption or absorption. There are many examples in nature of hygroscopic materials but one of the most commonly known is honey as it is primarily a sugar. Sugar is hygroscopic and in its natural state contains very little water but if the surrounding environment contains moisture it will absorb it.
Salt, like sugar, is also hygroscopic in nature and many salts such as calcium chloride and sodium hydroxide are so hygroscopic that they will dissolve in the water they absorb. Hygroscopic materials also have a tendency to over time become damp and soft when exposed to air that contains a lot of moisture. As such, the level of moisture held by these hydroscopic salts is usually proportional to the level of humidity.
These hydroscopic materials can however lead to a number of problems, particularly in composite materials such as walls. Dampness in walls can arise from a number of problems, most notably as a result of inadequate ground and surface drainage, but it can also be as a result of cracks in foundations, saturated ground- the list goes on. But, when the soluble salts within the walls become concentrated where the water has evaporated from the wall there will be a ‘tide mark’. This line across the wall is not simply an indication that moisture is working its way up through the capillaries of the wall’s building materials. It is in fact a signifier that dampness in the wall has already evaporated to the surface.
In these instances, hygroscopic salts can have a detrimental impact on the very foundations of your home and will continue to wreak havoc, creating tide marks and dampening wall plaster. Wall plaster in any home with hygroscopic salts which are saturated is at risk of being internally and externally dampened. The first step in addressing this issue is to locate the source of the moisture and rectify the problem. Once this has occurred the damage can be assessed and in most cases, it is advisable to re-plaster the affected walls.
Even if the source of the moisture is dealt with, the hygroscopic salts are likely to have retained water and will continue to give the ‘tide’ damp wall appearance. Often, the appearance of the dampness will come and go depending on how dry or humid the air is that day. It is for this reason that the recommendation will always be to freshly plaster as a ‘salt retardant’ additive can be incorporated. This helps to ensure any remaining moisture is unable to evaporate and settle on the plaster surface.
To ensure that any saturated hygroscopic salts found in walls are sufficiently sealed and walls are damp-proofed it is always advisable to use a certified damp-proof specialist who can provide the correct advice and solutions for properties.
--AtlantisDamp 14:19, 25 Jun 2019 (BST)
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