The Importance of Salts

The importance of salts in rising damp can not be overstated. Salts are an extremely important but often much neglected aspect of rising damp. The rising water flow draws up diluted salts from the ground, depositing them into the building fabric. Following the evaporation of water, salts crystallize, expanding in volume. The exerted mechanical expansion pressure (crystallization pressure) leads to the cracking, crumbling and flaking of the plaster and wall fabric – these damages making up a large percentage of the tell-tale signs rising damp.

The impact of salts onto the building fabric is far more important than the effects of dampness and humidity. Most physical damages related to old buildings - such as crumbling, peeling, detachment of the plaster etc. - can be attributed to a large extent to the effect of crystallizing salts.

The true importance of salts in rising damp and the damages they can make to old buildings are to a large extent unknown or underestimated even in professional circles.

Crystallizing and expanding salts breaking down the pore structure of the masonry

Why are Salts a Problem?

Salts are a major problem for the following reasons:

  • Cumulative effect: as the building ages, salts just keep piling up inside the building fabric. Everything else being equal, a 300-year old wall is saltier than a 100-year old building. The older the building fabric, the saltier and damper it gets. 
  • Hygroscopic effect (wetting): salts are able to pull in and trap humidity from the environment - a property known as hygroscopicity. The bound moisture in salts keeps the building fabric damp. As the building ages and the overall salts content of the building increases, the fabric becomes damper and damper. In their wet phase salt also become mobile, being easily transported by the water flow into other areas of the building fabric.
  • Crystallization and expansion (drying): when the weather becomes dryer, salts dry out and as a result they crystallize. During the crystallization phase salts expand about 10X in volume, generating strong inter-molecular forces (crystallization pressure), which over time through repeated actions break down the building fabric. The more salts, the more crystallization pressure, the more crumbling and breakdown. The crystallization of salts can occur both on the surface (efflorescence) or in-depth under the surface (subflorescence).

Here are some examples of what damages salts can cause in old buildings. If you have something similar ongoing in your own building, then you probably have rising damp. Get it sorted as it will not go away by itself, just gets worse over time.

Some important observations:

  • It won't go away: once salts get into the building fabric, they stay there (often invisibly, hidden). They can be made active by changes in temperature and ambient humidity (wetting & drying cycles).
  • Summer vs winter: it can be observed that in hot climate or during hot summers the damp patches tend to fade (the water evaporates, salts dry out). However during the wet season the problem returns. Over time the problem worsens, the crumbling of the wall fabric increases.
  • Lime and cement plasters behave differently: different building materials react to salts differently. Lime having larger pores ABSORBS (buffers) most salts, giving no indication that there is an ongoing problem until the lime pores fill up with salts and lime starts breaking down. Powdering of the paint or surface are one of the first signs that the plaster is about to fail due to salt attack. Cement, having less and much smaller pores, BLOCKS the salts. It will resist them initially, but the increasing quantity of crystallized salts under the cement and growing crystallization pressure eventually cracks it or detaches it, resulting in a cracked or bossed plaster.
  • More salts = more crumbling, more frequent renovations: the more salts are present in the building fabric, the effect of renovation tends to last less and less. For a Victorian building with less salts in the fabric renovation can last a decent amount of time. In case of older buildings with more porous bricks and higher salts content, the effect of renovation actions last much less, and such buildings need more frequent or ongoing replastering. 

Salts and Moisture are Related

One of the very interesting discoveries we have made about the effect of salts was that the salt content of the building fabric determines primarily the day-to-day moisture content of masonry. The amount and nature of salts found in the building fabric determines to a large extent:

  • The day-to-day moisture content of masonry
  • The time needed for the wall fabric to dry out after the moisture problem has been handled
  • How dry a masonry gets after the moisture problem has been rectified (what the air-dry or equilibrium moisture content of the walls will be)

Generally, the saltier a wall fabric, the higher its day-to-day moisture content will be, and the longer it will take to dry out.

Where Salts Come From?

Salts can originate from a variety of sources. In small quantities they are found in bricks, which being made of clay, contain a small amount of salts.

In smaller quantities, salts can be natively found in bricks made of clay, which contain some salts. However, most of the salts originate from the environment and will gradually "pollute" the aging building fabric via several mechanisms:

  • From the GROUND: most salts migrate into the building fabric through rising damp. In residential buildings they can reach about 1 meter height. In larger buildings with thick walls they can however climb significantly higher.

  • From the AIR: buildings close to the sea are subject to sea salt particles carried and deposited onto the surface by the wind, mist and fog. These salts will be washed-in by the rain into the building fabric. 
    Smog from air pollution (e.g. sulphur gases) combined with rain creates acid rain, which reacting with lime results in salts, leading to the crumbing and decay of stones.

  • From CHIMNEYS: interestingly enough, chimneys are very much subject to the damaging action of salts. Flue gases combined with air + moisture result in salts, which over decades or centuries migrate into the chimney walls, resulting in a gradual decay of the fabric around the chimneys. This damage is not confined to the ground floor, but it can appear on upper floors to on chimney walls.

Most Common Type of Salts Found in Old Buildings

Although salts cover a large group of materials, the most important ones damaging the buildings are:

  • Chlorides (Cl): these are plain sea salts (e.g. common table salt) and the likes. Sea salts are transported by wind and fog inland, where they end up in the ground or washed into the buildings. Moreover, salts sprinkled onto the roads during winter are also chlorides which eventually end up in the ground and the fabric of our buildings.

  • Nitrates (NO3): are organic salts found predominantly in farming areas. Animal- and farming byproducts such as manure or fertilizers contain large amount of nitrates, which have been affecting that old farm building or barn walls for centuries. When these buildings are converted to dwellings this fact must be taken into consideration.

  • Sulphates (SO4): one of the most dangerous salts as they crystallize in long needle-shaped crystals than can do a lot of damage to the building fabric. Sulphates can be found in many building materials (e.g. gypsum, Portland cement), or as a result of combustion or burning stuff (e.g. chimneys, power plants, atmospheric pollution etc.).

The presence of these salts can be measured and assessed using specialist tools. We do this on a regular basis as part of our professional building surveys.

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