The problem of moisture movement in old buildings are accompanied by the migration of salts, the moisture problem becoming a dual problem of water and salts.

What are Salts?

Salts are minerals made up of two electro-chemical parts called ions. The electrically positive part (in chemistry called "cation") is normally a metal, the most common ones being: Na, Ca, K and Mg. The electrically negative part (anion) are other chemical groups, the most common ones being: chlorides (Cl), nitrates (NO3) and sulphates (SO4).

These 7 ions - the 4 metals (Na, Ca, K, Mg) and the 3 other ones (chlorides, sulphates, nitrates) - in combination produce 4 x 3 = 12 elementary salts, which account for over 98% of the salts found in old walls. In reality there are more than 12 salts but these are the most important ones. Common table salt (NaCl) is just one of the many existing salts.

Salt crystal

Salts in dry state have an organized crystalline structure

In dry state, the salts are organized in a 3D lattice, the solid salt crystals being held together by intermolecular electrical attraction forces. Small crystals are white or yellowish powders and can have a salty or bitter taste.

In the presence of water, however things change significantly. Salts become diluted by water: the charged salt lattice is surrounded by many charged water ions (positive H+ and and negative O- ions) pulling the salt lattice apart, resulting in the breakdown of its organized crystalline structure. The positive and negative salt ions become separated, travelling independently with the liquid water into the masonry, retaining their individual positive or negative charges in the solution.

salt dissolution

Salts being dissolved by water

Once humidity in the masonry decreases as a result of evaporation, salts re-crystallize - either as the same salt or a new and different salt - depending on how the 7 primary salt ions re-combine in the masonry.

How Salts Break Down the Wall Fabric?

There are 2 key mechanisms on how salts keep the wall fabric damp and how they contribute to the gradual irreversible destruction of the masonry long-term. They are both linked to ambient humidity and temperature changes.

1. Wetting Phase

Every time when ambient humidity increases or the temperature decreases for whatever reason (e.g. wet season, period of rain, day-night variations etc.) salts tend to liquefy. Salts have a very interesting property: they are "hygroscopic" which means they can absorb moisture from the ambient air. One salt molecule in a humid environment can attract and hold many water molecules, making the wall surface look damp. A well-known example of this is the behaviour of common table salt which becomes less crisp or soft in a high-humidity environment by attracting moisture from the surrounding air.

The "soft" liquid salts spread in the masonry by capillary action and other mechanisms. 

As most salts in an old wall, due to evaporation, get deposited either on the surface (known as efflorescence) or 5-10 mm under the surface (subflorescence), salts alone can make a wall fabric look damp even years later, well after the underlying dampness problem has already been remedied.

This brings up an important technical point about the re-decoration of old, salty walls: the old, salt-laden plaster MUST be removed completely and affected wall sections must be freshly re-plastered in order to ensure a salt-free = dry surface. Retaining the old salty plaster and just skimming it over due to financial considerations is a mistake which must be avoided. A salt-resistant lime base coat must also be used to ensure the longevity of the plastering.

2. Drying Phase

When humidity levels decrease or the temperature increases (e.g. during warmer weather, sunshine etc.) salts in the walls crystallize. During crystallization salts expand in volume 5 - 10 times, which is by 500% to 1,000%. The crystallization of salts exert enormous forces inside the capillaries, the internal pressure reaching up to 800 atmospheres, when the resistance of the strongest concrete rarely exceeds 550 atmospheres. Water freezing to ice "only" expands in volume by about 10% yet the induced crystallization pressure cracks iron or steel pipes. In comparison, salts create significantly more damage.

salts microscope

Salt crystallization in mortar under the microscope, leading to cracks

The high intermolecular pressures result in the crumbling and breakdown of paint, plaster and historic wall fabric, leading to crumbling, spalling, flaking, cracking or peeling of the building fabric.

Salts are the primary reason behind the irreversible damage and loss of historic building fabric. They are the main reason behind the damage and breakdown of plasters and why old buildings need to undergo a major replastering cycle every few years.

Here are some real-life examples of salt damages caused in various buildings:

Where Salts Originate From?

In small quantities, salts can be found natively in bricks. Bricks are made of clayey soil and the soil contains salts. However, most salts are deposited into the building fabric through various mechanisms; salts gradually "polluting" the building fabric over time. 

Here are the most common sources of salts:

  • The ground: near ground level the primary mechanism that drives salts into the building fabric is rising damp. The soil contains many salts (minerals) in diluted form. These salts are carried up into the wall fabric through various mechanisms, resulting in high quantities of salts in the lower part of the building fabric. White powdery salt crystallization in the lower meter or so near ground is a common occurrence in old buildings.

  • The air: buildings close to the sea are subject to sea salts by the wind, mist and fog. These salts are washed-in into the building fabric by rainwater, accumulating over time,
    In large cities such as London, air pollution (sulphur gases) combined with rain creates acid rain, which reacting with lime results in salts, leading to the crumbing and decay of stones.

  • Chimneys: interestingly enough, chimneys are very much subject to the erosive action of salts. The black soot in chimneys are very rich in salts. These over the decades or centuries permeate the chimney walls, resulting in a gradual decay of the chimney fabric. Yellow or brown discoloration of chimney walls or ceiling areas nearby is the result of salt damage from the soot.

  • From modern cement plasters: due to the very high firing temperatures (around 1500 °C) used during the production of cement plasters, many chemical reactions that take place in cement are undesirable. The effect of these reactions is mitigated by mixing various additives into the cement mix. Juts one example: to slow down the too fast curing of cement, gypsum (CaSO4) is added into the mix, which is a salt. These salt additives end up in the old wall fabric resulting in salt crystallization and breakdown of the traditional fabric.

The most common types or categories of salts that damage old buildings, are:

Chlorides (Cl) 

Chlorides are plain sea salts (e.g. common table salt). 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 end up in the ground then absorbed up into the building fabric.

Nitrates (NO3)

Nitrates are salts predominantly originating from the decomposition of waste matter. They are prevalent in farming areas. Animal- and farming byproducts such as manure or fertilizers contain high quantities of nitrates, which have been affecting that old farm building or barn walls for centuries. When these buildings are converted to dwellings the very high content of the walls must be known and mitigated.

Sulphates (SO4)

Sulphates are one of the most dangerous type of 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 modern building materials (e..g gypsum, portland cement), or as a result of combustion (e.g. chimneys, power plants, atmospheric pollution etc.).

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

Solutions, Remedies, Best Practices

As a building ages, more and more salts accumulate inside the building fabric, leading to higher and higher salt concentrations. The salts become part and parcel of the building fabric and can't be easily removed from it. 

Various desalination procedures have been developed, but these are cumbersome, complex, costly and require professional application and supervision. Because there are no easy desalination procedure for an old wall fabric, most salt problems are managed instead by using special salt-resistant plasters

One of the most common mistakes during the renovation of old buildings - especially during farm or barn conversions - is not being aware of the potential problems salts can create long term resulting in the decay of plastering. Some problems commonly found in old farms and barns are summarized by the video below.

Replastering an old salty wall with a breathable lime plaster is not a long-term solution. The plastering will only last for a few years until the lime plaster is broken down by the crystallizing salts found in abundance in old farm buildings.

A much better solution to the problem of salts is using special salt-resistant lime plasters. The technology of this plaster originates from ancient Rome. The Romans figured out that if the soft lime is mixed with volcanic sands and ashes from Mount Vesuvius it will result in salt-resistant lime mixes that can even withstand sea water for many decades. These pozzolanic lime plasters have been used in Venice for centuries, performing extremely well in damp and harsh environment.

Applying a salt-resistant lime base coat under the main lime coat will extend the life expectancy of your plaster by about 10X, making your lime plaster last much-much longer, without sacrificing breathability in any way. 

Salt-resistant lime plaster

Rinzaffo MGN salt-resistant lime plaster