The Effect of Salts in Old Buildings

The effect of salts in old buildings is a very important yet mostly overlooked topic in building conservation. Due to their importance, salts have already been mentioned several times in previous sections. We mentioned them when we looked at the key differences between sweet and saline water wetting. We discussed the role of salts as a moisture barrier because they significantly affect the drying and breathability of the wall fabric. Salts also make the masonry electrically conductive subjecting the wall fabric to a wide array of electrical effects.

In this section we are going to summarize the most important effects of salts and how they affect old buildings. Understanding the effects of salts is important because salts significantly impact the wall fabric and old walls with high salinity must be renovated differently from walls with low to moderate salinity.

What are Salts?

Salts are minerals consisting of two electro-chemical parts called ions. The electrically positive part (cation) is normally a metal, commonly Na⁺, Ca⁺, K²⁺ and Mg²⁺. The electrically negative part (anion) are other chemical groups, most commonly chlorides (Cl^-), nitrates (NO₃^-) and sulphates (SO₄^2-).

In various combinations, these 7 ions produce 4 x 3 = 12 elementary salts, which account for over 98% of the salts found in old walls. 

Table salt (NaCl) is just one of the many types of salts.

In dry state, 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 which can have a salty or bitter taste.

In the presence of water, however things change. Salts become dissolved by water: the salt lattice gets surrounded by charged water ions (positive H⁺ and and negative OH^- ions) pulling the salt lattice apart, breaking down its organized crystalline structure. The positive and negative salt ions become separated, travelling independently with the liquid water into the masonry, but retaining their individual positive or negative charges in the solution. 

Once humidity evaporates, salts re-crystallize, forming the 12 common salts.

Most Common Type of Salts

The most common types of salts found in the fabric of old buildings are:

Chlorides (Cl)

Chlorides are plain sea salts (e.g. common table salt). Sea salts are transported by wind and fog inland ending up in the ground or washed into the buildings.

Salts sprinkled onto the roads during winter are chlorides that end up in the ground then absorbed into the building fabric.

Nitrates (NO₃)

Nitrate salts are predominantly originating from the decomposition of organic waste matter. Churches, chapels, graveyards and former battlegrounds contain a large amount of nitrates. They are also prevalent in farming areas. Animal- and farming by-products 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 high salt content of the walls must be known and mitigated.

Sulphates (SO₄)

Sulphates are one of the most dangerous or damaging type of salts as they crystallize in long needle-shape crystals which can cause a lot of damage to the old building fabric.

Sulphate salts can be found in modern cement, added to it as an additive to make cement set slower. Under certain conditions the sulphates in cement can react with other salts in the environment, resulting in extreme salt expansion known as sulphate attack, causing severe damage to concrete structures and plasters.

Sulphates are also formed after combustion, being abundantly present in the chimney soot or in emission gases of the polluted atmosphere. When airborne sulphates are washed-down by rainwater, the resulting acid rain containing weak sulphuric acid, dissolves the lime, damaging old limestone statues or façades.

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.

Where Salts Originate From?

In very small quantities, salts can natively be present in bricks. Bricks, made of clayey soil, naturally contain small amounts of salts. However, most salts are deposited over time into the building fabric from various sources. 

Here are the most common sources of salts:

1. 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 by water into the wall fabric, resulting in high quantities of salts in the lower part of the building fabric. White powdery salt crystallization in the lower metre or so near ground is a common occurrence in old buildings.
2. 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 air pollution (sulphur gases) combined with rain creates acid rain, which reacting with the lime creates salts, leading to the crumbing and decay of stones.
3. Chimneys: interestingly enough, chimneys are very much subject to the erosive action of salts. The black soot in chimneys is very rich in salts. These salts over decades and centuries permeate the chimney walls, resulting in the discoloration, crumbing and general decay of chimney walls or ceiling areas. Many "mystery" dampness problems, when no water ingress is present, are caused by salts.
4. From modern cement: 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, negatively affecting the quality of cement. To mitigate them, various additives are mixed into the cement during the manufacturing process. For e.g. to make cement more workable, gypsum salts (CaSO4)  are added into the mix. These salt additives end up in the old wall fabric resulting in irreversible damage of the historic building fabric.

The Effect of Salts – How Salts Damage old Buildings?

Here is a video explaining and demonstrating some of the damages salts can do to old buildings.

Salts have a profound affect onto old masonries, affecting them in many ways. Yet, their effects are commonly understated or mistakenly attributed to the presence of moisture.

Here are some important effects of salts:

1. Salts Make the Masonry Damp (Hygroscopic Action)

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 absorb moisture from the ambient air (hygroscopic action) and liquefy.

One salt molecule in a humid environment can attract and hold many-many water molecules. A well-known example of this is the behaviour of table salt in a salt shaker which in a high-humidity environment goes soft and sticky from the absorbed moisture from the surrounding air.

As a result salty bricks and plasters retain much more humidity than non-salty ones. Lab experiments have demonstrated that the moisture content of salty bricks (green) can easily be about 4.5 times higher than of non-salty bricks (blue), making salty masonries MUCH damper than non-salty ones, under the exact same conditions. 

As most salts in an old wall, due to evaporation, get deposited on the surface (known as efflorescence) or just under the surface (subflorescence), in a high-humidity environment salts can make a wall fabric look damp even if the wall in depth is significantly drier. 

This brings up an important technical point about the re-plastering of old, salty walls. The old, salt-laden plaster MUST be removed completely and affected wall sections must be fully re-plastered in order to ensure a salt-free = dry surface.

Retaining an old salt-laden plaster and just skimming it over in order to save money is a common and often costly renovation mistake which one can easily avoid by understanding what salts do and how they behave. A salt-resistant lime base coat must also be used to protect the longevity of the plastering.

2. Salts Reduce Evaporation and Breathability, Acting as a Moisture Barrier

Just as salts absorb humidity from the environment, they also won’t release it easily when ambient drops or when the wall starts drying out. In comparison to freshwater wetting, it takes a lot more energy to make water evaporate from a salty wall fabric.

In our lab experiment a non-salty brick needed 2 days to become dry, while a salty brick needed 8 days (4 times longer) to dry under the exact same circumstances. This can be seen on the above graph, where the "flattening" time of salty brick (green) is much longer than of the non-salty one (blue).

By hygroscopically trapping the evaporating moisture, salts act as an energy moisture barrier, having a similar behaviour  to cement or other physical moisture barriers.

3. Salt Make Masonry Crumble

When humidity levels decrease or the temperature increases, salts crystallize. During crystallization salts expand in volume by 5 - 10 times (500% to 1,000%). The expansion generates enormous forces inside the masonry (up to 800 atmospheres), capable of breaking down even concrete. 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.

Salt crystallization is similar to water freezing into ice. Ice crystals "only" expands in volume by about 10% yet the induced crystallization pressure can crack iron or steel pipes. In comparison, salts expand many times, thus capable of creating significant damages.

Salts are the primary reason behind the irreversible damage and loss of historic building fabric - not water. Salts are the main reason behind the damage and breakdown of plasters, why old buildings need to undergo a major replastering cycle every few years.

Here are some examples of salt damages, different buildings and walls being damaged by salts.

4. Salts Make the Wall Fabric Subject to Electromagnetic Fields

There is another important, less known aspect of wet salts. Salts are electrolytes (wet solutions with an electric charge) that make the wall fabric electrically conductive. A non-salty wall fabric is an insulator. A salty wall-fabric becomes a good conductor.

Measurements have shown that the electrical resistance of non-salty and salty walls differs significantly. While a non-salty wall fabric (red) has a resistance of hundreds of mega-ohms (being an insulator), the electrical resistance of salty walls (grey) can be 100 to 10,000 times less, decreasing steeply with the increase of salinity and humidity.

Why is this important? Because electrical resistance influences other chemical-physical properties of the wall fabric. Electromagnetic (EM) fields in any conductor can induce energy. For an old wall this translates to the generation and existence of small voltages and currents in the fabric. In other words, an electrically conductive wall fabric becomes susceptible to various EM phenomena from the environment.

The induced electrical charges interfere with the movement of water molecules and salt ions inside the capillaries, affecting wetting and drying cycles. Electrical phenomena in damp masonries are discussed in much more detail here,

Solutions, Remedies, Best Practices

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

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. Some common problems found in old farms and barns are highlighted by the video below.

Various desalination procedures have been developed, but these tend to be complex. As a result most salt problems are managed by using the right type of plasters. 

Replastering an old salty wall with a breathable general purpose lime plaster is not a long-term solution. In the presence of high salinity, the fresh lime plaster is usually short lived as it will be broken down by the crystallizing salts found in abundance in old farm buildings.

Research Data – Scientific Papers on the Effect of Salts in Old Buildings

The destructive effect of salts onto old buildings is of great concern and as such it is being researched by many universities worldwide. For those interested in the research data, here are some international research papers about the importance of salts in building conservation, the salt crystallization process and their effects onto old masonry.

1. The role of sea salts in the occurrence of different damage mechanisms and decay patterns on brick masonry – Construction and Building Materials, March 2004
2. Investigating the effects of humidity and salt crystallisation on medieval masonry – Building and Environment, November 2000
3. Impact of salt crystallization on the mechanical properties of structural masonry: An experimental and numerical study – Construction and Building Materials, July 2022
4. Experimental investigation on bricks from historical Venetian buildings subjected to moisture and salt crystallization – Engineering Failure Analysis, October 2014
5. Effect of salt crystallization on stones of historical buildings and monuments, Konya, Central Turkey – Building and Environment, 2007
6. Characteristics and deterioration mechanisms in coral stones used in a historical monument in a saline environment – Construction and Building Materials, January 2020
7. Experimental research on hygroscopic behaviour of porous specimens contaminated with salts – Construction and Building Materials, June 2004
8. Do environmental conditions determine whether salt driven decay leads to powdering or flaking in historic Reigate Stone masonry at the Tower of London? – Engineering Geology, April 2022
9. Research into the influence of subsoil on sulphates, nitrates and chlorides accumulated in renovation plasters used for rehabilitation of monuments in the Czech Republic – Journal of Cultural Heritage, February 2021
10. Study of nitrate contaminated samples from a historic building with the hygroscopic moisture content method: Contribution of laboratory data to interpret results practical significance – Journal of Cultural Heritage, March–April 2018
11. Electrokinetic desalination of a farmhouse applying a proton pump approach. First in situ experience – Construction and Building Materials, February 2020
12. Electrokinetic removal of Ca(NO₃)₂ from bricks to avoid salt-induced decay – Electrochimica Acta, March 2006
13. The short-term effects of mortar joints on salt movement in stone – Atmospheric Environment, May 1997
14. Influence of paints on drying and salt distribution processes in porous building materials – Construction and Building Materials, May 2009
15. Simulation-based analysis of the differences in the removal rate of chlorides, nitrates and sulfates by electrokinetic desalination treatments – Electrochimica Acta, February 2013
16. 3D-resistivity imaging and distribution of water soluble salts in Portuguese Renaissance stone bas-reliefs – Engineering Geology, July 2012
17. Computational fluid dynamics (CFD) modeling of microclimate for salts crystallization control and artworks conservation – Journal of Cultural Heritage, July–August 2014