Salt Crystallisation in Masonry
|Historic buildings and monuments affected by salt crystallisation include the megalithic temples of Hagar Qim, Malta (left) and Whitby Abbey, Yorkshire (right).|
While salt crystallisation in porous building materials has long been seen as an important cause of deterioration problems, much remains to be understood about how it works, why it causes damage and how best to prevent and treat salt problems. This article reviews the current state of scientific understanding on the problem and potential solutions.
Within the last 15 to 20 years numerous research projects have investigated salt crystallisation problems from theoretical perspectives, as well as based on laboratory experimentation and field monitoring and observations. Only now are we starting to be able to bring findings together from disparate research projects, and to provide a more complete explanation of salt deterioration problems.
SALTS AND MASONRY
Some of the world’s most important historic monuments, such as the Nabatean rock-hewn temples at Petra in Jordan, the Harappan remains at Mohenjo-Daro in Pakistan, the Alhambra in Granada, Spain, and the Sphinx in Egypt, are seriously affected by salt crystallisation. Indeed, the problem affects buildings and monuments around the world, from hyper-arid desert environments to Mediterranean climates and the cooler and wetter conditions found in the UK.
In order to understand the problems of salt crystallisation in masonry, it is important to consider the nature of salts, how they are delivered into the masonry, and the vulnerability of the masonry itself.
Salts are ionic compounds formed between positively charged ions (sodium, for example; Na +) but excluding hydrogen (H +) and the negatively charged ions of acids (chloride, for example; Cl‐). In the natural and built environments many different types of salts are found, including chlorides, sulphates, nitrates and carbonates. Commonly occurring salts are sodium chloride (table salt or ‘halite’; NaCl), gypsum (calcium sulphate; CaSO4) and sodium sulphate (Na2SO4).
The ions making up these salts may be of purely natural origin or may be sourced from de-icing salts and other compounds deliberately applied to roads or walls, or they may come from pollutants in air or water. Alternatively, salts can come from within the masonry itself – they are found, for example, in some mortars.
The reason salts pose a problem to masonry is because they are soluble and can dissolve and recrystallise, often within the pores of the stone at the point of evaporation. Each salt has different solubility characteristics, and some are more problematic than others. Some are able to take up water from the air, and some can change structure as a consequence of their hydration and dehydration. (For example, sodium sulphate can occur as thenardite or mirabilite depending on its hydration state, and the metastable heptahydrate has also been recently observed.) Some salts can dissolve in the water they have taken up from the air, through deliquescence (as is the case for halite). In many situations salts are present in complex mixtures and their behaviour in damp masonry is, as a result, quite complicated.
Salts can be transported onto and into masonry in many ways, such as by capillary rise from groundwater and soil water, splashed from run-off on nearby roads and other impermeable surfaces, in rainfall and driving rain, in fog and dew, and as sea spray. Many buildings will receive salts from multiple sources. Whether or not these salts pose a problem will depend upon the aggressiveness of the environmental conditions and the vulnerability of the masonry.
The nature of the environmental conditions surrounding a building or monument, and those inside a building, are crucial to influencing how far moisture and salts penetrate and how often dissolution/crystallisation and hydration/ dehydration reactions take place. Relative humidity and temperature, and variations in these over daily and annual timescales, are particularly key environmental factors.
Notwithstanding the environmental conditions surrounding and inside a building, different materials will have different levels of vulnerability to such environmental conditions, depending on the nature of their porosity and permeability, and the chemistry of the materials. These factors affect how easily water (containing dissolved salts) can penetrate into the masonry, and how it may interact physically and chemically with the building materials.
Building stones with a high percentage of small pores (<0.5 microns) as well as some larger pores, have been found to be particularly susceptible to salt crystallisation damage. These include many sedimentary rocks (such as some limestones and sandstones) as well as some granites.
|Above left: Salt efflorescence on replacement limestone block within a salt-affected wall in central Oxford. Above right: 2D resistivity survey techniques in use at Byland Abbey, Yorkshire provide non-destructive monitoring of moisture regimes inside the masonry|
HOW DOES SALT MIGRATE AND CRYSTALLISE?
One of the problems associated with understanding the migration and crystallisation of salts within porous materials is the difficulty of seeing what is going on inside a wall. In recent years a number of techniques have been developed which permit visualisation of moisture and salt movements within porous materials, both in the laboratory and in the field, such as nuclear magnetic resonance methods (NMR) and resistivity-based methods. Furthermore, microscope techniques such as environmental scanning electron microscopy (ESEM) allow real-time monitoring of phase changes in salts as a result of changing environmental conditions. These methods have developed hand in hand with new computer modelling techniques, together providing excellent ways to test and develop theoretical explanations.
Water laden with salts can move through masonry in several different ways, depending on the wetting and drying conditions. For example, in unsaturated or partially saturated conditions, salts migrate as a result of capillary processes. Many old walls without damp-proof courses experience clear capillary rise from groundwater. Recent work by Professor Chris Hall (University of Edinburgh) and colleagues, as part of a Leverhulme Trust-funded project on climate change and moisture regimes in stone monoliths, has demonstrated that both the height of capillary rise and the volume of water moving through the wall are related to the evaporation conditions that the wall experiences. Higher capillary rise is found in low evaporation situations, but higher volumes of water are pumped through the wall in higher evaporation environments.
In saturated conditions, salts may migrate through masonry by ionic diffusion. Dr Stephen McCabe (Queen’s University Belfast) has recently been experimenting on this process as part of the ‘Greening of walls’ research project funded by the Engineering and Physical Sciences Research Council (EPSRC). He has found that halite moves more quickly than gypsum through saturated sandstones and that rates of migration vary when mixed salt solutions are involved.
Dissolution and crystallisation occur when the amount of salt changes relative to the capacity of the water to hold dissolved salt. Solutions are described as ‘saturated’ when they contain the maximum possible amount of dissolved salt for the environmental conditions. Changing environmental factors such as temperature and pressure can create ‘super-saturated’ solutions which can rapidly lead to salt crystallisation.
For example, evaporative drying reduces the amount of water present meaning that less salt can be kept in solution and some will crystallise out at the point of evaporation, usually at the surface of the masonry. Cooling can also be important as sodium sulphate, for example, becomes less soluble as temperatures decrease, causing crystallisation without any change in the volume of water involved. In this case, crystallisation may occur within the body of the masonry.
HOW DOES SALT DAMAGE MASONRY
Over time, salts migrate into porous masonry materials and start to clog pore spaces. Where pores become filled with salt, repeated cycles of dissolution/crystallisation and sometimes also hydration/dehydration within pores will lead to the imposition of considerable stress on the surrounding pore walls.
Considerable scientific debate has raged over how salts crystallise and how breakdown of the surrounding material occurs. Recent theory suggests that continued growth of salt crystals in pores requires the presence of a thin aqueous film between the salt crystal and the pore wall. This film of liquid is caused by what is known as ‘disjoining pressure’, or the repulsive force between crystal and pore wall.
Different salt types have different crystal forms (including cubic and needle-like) and some may be more effective in causing deterioration than others. For example, experimental work by Professor Carlos Rodriguez-Navarro and Dr Eric Doehne published in 1999, showed that rapid evaporation caused highly supersaturated sodium sulphate solutions producing irregular-sided (anhedral) crystals of mirabilite which were found to be highly damaging.
Salts can also cause damage to masonry through other processes, for example corrosion of any iron cramps or bars within walls, or chemical weathering of susceptible minerals (as salt solutions are often highly alkaline). Silica becomes highly mobile at pH values greater than nine and thus many sandstones may suffer partial dissolution of silica cement under saturated and salt-rich conditions where the salts are highly alkaline. The presence of hygroscopic salts within masonry (those which can absorb water vapour from the atmosphere) can also accelerate chemical and frost weathering processes by increasing the amount of water present.
|Above left: Typical salt deterioration phenomena, with flaking surfaces at Byland Abbey, Yorkshire. Above right: alveolar weathering at Whitby Abbey|
How might one diagnose salt deterioration problems in a historic building? Often, the presence of salts within walls can be detected through the occasional formation of salt efflorescences on the surface.
Salt crystallising out as efflorescences on the surface of walls causes no direct damage itself (as it is not crystallising out in confined pore spaces), but is symptomatic of the presence of salts within the walls. Efflorescences may only appear under certain climatic conditions, which tip the balance between dissolution and crystallisation, and are often found in distinct zones (often related to capillary rise).
Taking samples of the efflorescence for chemical analysis will help diagnose which salts are present. Some salts are known to be particularly damaging in terms of their crystallisation and hydration effects, such as sodium sulphate.
Other symptoms of salt deterioration include the presence of blistering and flaking of the outer few millimetres of stonework. Such loosening and detachment of surface flakes of stone is consistent with subsurface salt crystallisation (called cryptoflorescence or subflorescence).
In some buildings, alveolar weathering, which creates clusters of depressions in the stone surface, is observed and can be ascribed to salt weathering. In this case, salt crystallisation within near-surface pores causes detachment of individual grains. For reasons which are not yet entirely clear, the process becomes self-organised into a network of near-circular depressions. However, both blistering and alveolar weathering can also be produced by other processes and are not unambiguous indicators of salt deterioration.
HOW CAN SALT DAMAGE BE PREVENTED OR TREATED?
Preventing and treating deterioration problems caused by soluble salts are difficult tasks, and solutions vary widely depending on individual circumstances. What might be effective in one building, may be wholly inadequate somewhere else depending on the sources of moisture, the nature of the salts and the type of building materials. Accurate diagnosis of the problem is vital before any remedial measures are trialled.
Potential approaches to preventing future salt deterioration and treating current problems include: controlling the ingress of salts and water, removing any damaging salts already present (desalination), modifying the environmental conditions to reduce damaging cycles from any remaining salts, and/or using materials known to be less vulnerable to salt attack.
A good example comes from the work of David Watt and Belinda Colston, who examined the causes of and potential solutions to salt deterioration at the redundant church of Walpole St Andrew, Norfolk. The widespread and dominant presence of sodium chloride in this church may be a result of previous stone treatments (some recipes for limewash include sodium chloride), rather than current capillary rise or rainwater.
Desalination and environmental control (to control temperature and relative humidity and thus reduce the likelihood of crystallisation and dissolution cycles) should be effective in this case, because new salt ingress is unlikely. Methods that are unlikely to prevent salt deterioration, and indeed are likely to enhance it, are the application of water repellents. Several recent studies have shown that such techniques encourage subflorescence rather than efflorescence of salts, thus increasing the likelihood of damage.
Some promising future prevention methods are being researched. For example, following research by Professor George Scherer and colleagues at the University of Princeton which identified the disjoining pressure between growing salt crystal and the pore wall as exerting a key control on salt crystallisation damage, methods to reduce this pressure may well be a useful control method in the future.
A number of experiments have already been carried out on the use of surfactants to reduce salt crystallisation damage. Surfactants may help to reduce the contact angle between salt solutions and pore walls, but they themselves can also be affected by salts and thus do not offer a simple solution.
Another possibility is the use of microorganisms: it has been demonstrated that some species have the ability to remove salts from contaminated masonry, and some of the surfactants under consideration are of biological origin.
Salt crystallisation impacts on masonry will undoubtedly be influenced by climate change, as the recent EU-funded Noah’s Ark research project has predicted. Any proposed treatment and preventive strategies for salt-affected masonry should take these predictions on board.
For example, areas in the north west of the UK are likely to be affected by wetter winters over the course of the 21st century. Thus, deeper ingress of moisture into stonework is likely, with concomitant impacts on transport of salts and a potential switch to more chemical weathering impacts of salts. In contrast, in more southerly and eastern parts of the UK, drier summers are likely to reduce the overall annual number of salt crystallisation cycles, potentially reducing damage from this mechanism. However, increasing evaporation in summer in southern and eastern UK may also enhance the capillary uptake of salts.
RM Espinosa-Marzal and GW Scherer, ‘Advances in understanding damage by salt crystallisation’, Accounts of Chemical Research, Vol 43, 2010
AS Goudie and HA Viles, Salt Weathering Hazards, John Wiley, Chichester, 1997
CM Grossi et al, ‘Climatology of salt transitions and implications for stone weathering’, Science of the Total Environment, Vol 409, No 13, 2011
C Hall et al, ‘Moisture dynamics in walls: response to micro-environment and climate change’, Proceedings, Royal Society of London A, doi: 10.1098/rspa.2010.0131, 2010
C Rodriguez-Navarro and E Doehne, ‘Salt weathering: influence of evaporation rate, supersaturation and crystallization pattern’, Earth Surface Processes and Landforms, Vol 24, No 3, 1999
D Watt and B Colston, ‘Investigating the effects of humidity and salt crystallisation on medieval masonry’, Building and Environment, Vol 35, 2000