The presence of salts significantly impact the condition and appearance of the building fabric. The effect of salts is largely unknown to many builders or building professionals. Due to their importance, during the past few years a lot of research went into this field, however due to the complexity of the subject, many aspects of salts migration are still poorly understood.
Salts are one of the primary causes behind the irreversible breakdown of the building fabric. Why are salts such a big problem? Because they affect the building on an ongoing basis, regardless if the weather is dry or wet.
Salts impact the structural integrity of the masonry in many ways:
- CUMULATIVE effect: as the building ages, salts just keep piling up inside the building fabric. Everything else being equal, a 200-year old wall will be saltier than a 100-year old one and so on. The older the building, the saltier and the damper it gets.
Drying out very old buildings thus becomes a challenge. Many remedy actions that work well on not too old properties give much less- or no results in very old buildings.
- HYGROSOPIC effect (during wetting): salts are able to trap and pull in humidity from the environment – a property known as hygroscopicity. The bound moisture in salts keeps the building fabric damp, resulting in a hazy, damp appearance of the salts affected areas. The more salts are trapped in the building fabric, the more pronounced these damp patches tend to be. As the building ages and the overall salts content of the building increases, the fabric tends to become damper and damper.
In their wet phase salt also become mobile and are transported by the water flow into other areas of the building fabric.
- CRYSTALLIZATION and erosion (when drying): when the wetter becomes dryer, salts dry out and as a result they crystallize. During the crystallization phase salts expand about 5 - 10X in volume, creating strong intermolecular forces (crystallization pressure), which over time, through repeated action, breaks 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).
Crumbling, spalling, flaking, cracking or peeling of the building fabric is largely related to the crystallization of salts.
Where Do Salts Come From?
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 erosive action of salts. Flue gases combined with air and moisture will result in salts, which over the decades or centuries migrate into the chimney walls, resulting in a gradual decay of the fabric around the chimneys.
Type Of Salts
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 (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.
Solutions, Remedies, Best Practices
There is no simple, cost effective solution that would permit the easy removal of salts from the depth of the building fabric. The few specialist solutions that exist are expensive, time consuming and messy, they are limited to relatively small surface areas, and salts are removed from near the surface only (e.g. 10-20 mm depth).
As a result, when salts become a problem, it is easier and more cost-effective to remove them with the plaster or just live with them and manage them long term. The most common solutions are re-plastering or plasterboarding and we also highlight some of the costly pitfalls to avoid while dealing with them.
Because of the complexity of such situations, it is best to seek the advice of an expert.
Finally, here are some real buildings with salts-related problems along with some comments.