Salts play a key role in the weathering of porous building materials. Salt damage results from phase transformations in the pore space and the associated crystal growth. In the pore space, the enclosed and growing crystals can build up pressures that exceed the mechanical strength of the material in question thus causing their failure. Considerable progress in understanding the actual deterioration mechanisms caused by crystal growth has been achieved in recent years. Regardless of the mechanism, an exact knowledge of the conditions causing the unwanted phase transformation, is crucial for the development of suitable strategies to prevent this damage.
Phase transformation – crystallization
The most important phase transformation process that can lead to damage in building materials, is the crystallization of a salt in the pore solution. For instance, this process can be triggered by the evaporation of water or by temperature fluctuations as the solubility of many salts changes with temperature. The process becomes critical, when it occurs in cycles and under unfavorable conditions, i.e., when salts repeatedly dissolve and crystallize. Such cyclic crystallization processes occur when the humidity level of the material fluctuates continuously. High moisture supply, e.g., wetting, usually dissolves soluble salts in building materials leading to crystallization upon subsequent drying. Condensation can also be a source for moisture and the intermittent dissolution of salts.
Furthermore, properties inherent to the salts determine the moisture content of a building material. Especially the process of deliquescence is of particular importance. When the deliquescence relative humidity (DRH) is exceeded, the salt absorbs moisture from the ambient air and forms a solution. Further increases in humidity lead to more water absorption and the dilution of the solution. Therefore, the hygroscopicity of salts in the material can contribute significantly to the moisture absorption by masonry. If the relative humidity of the ambient air decreases to below the DRH, the salts crystallize. Consequently, just the fluctuation of relative humidity around the DRH can lead to cyclic crystallization processes and in general, can cause severe damage to a material in a relatively short period of time.
Phase transformation – hydration
Crystal growth inside the pores can also take place during hydration reactions. Because the phase in the higher stage of hydration has a lower density, hydration reactions increase the amount of filling of pores. This results in the the build up of hydration pressure with the growth of hydrated crystals against the pore wall. Under unfavorable conditions, cyclic hydration-dehydration changes are also possible. Again, the moisture required for the hydration reaction can originate through liquid water absorption by the material or simple condensation from increases in relative humidity.
Salts and indoor climate
Arnold and Zehnder [Arnold.etal:1991]Title: Monitoring Wall Paintings Affected by soluble Salts
Author: Arnold, Andreas; Zehnder, Konrad
were among the first to investigate the properties of salts with regard to the previously described situations. They correlated their observations on buildings with the properties of the different salts and the indoor climatic conditions. Their findings showed that the dynamics of salt damage processes is mostly determined by the interaction between salt mixtures in the pore spaces of the building material and the ambient relative humidity, i.e., temperature fluctuations in the interior of the building. For instance, the relative humidity varies in the heated indoor environment periodically over the year. During winter, the indoor heating leads to very low relative humidity levels, e.g., around 30-40%. The result is a cycle, where conditions fall above or below the deliquescence or hydration humidity levels of a various salts, inevitably causing damage processes (if salts have accumulated in a building material). Therefore, control of the indoor climate offers a means to safely eliminate or reduce damage processes caused by the above mentioned salt related processes depending on the salt or salt mixtures present [Price:2000]Title: An Expert Chemical Model for Determining the Environmental Conditions Needed to Prevent Salt Damage in Porous Materials, European Commission Research Report No 11, (Protection and Conservation of European Cultural Heritage)
, [Steiger:2005c]Title: Salts in Porous Materials: Thermodynamics of Phase Transitions, Modeling and Preventive Conservation
Author: Steiger, Michael
. This allows as well the possibility of preventive conservation measures.
When a non-hydrating salt is present, e.g., sodium chloride (NaCl, halite), damage can be prevented if the relative humidity is permanently kept below the deliquescence relativ humidity, DRH, of this specific salt. The salt will then remain crystallized and immobilized as long as there is no other water source=. If the relative humidity level is kept constantaly above the RDH of NaCl, the salt will remain in solution and will not crystallize, and therefore not inducing material damage (although other problems may develop because the material is constantly damp). Depending on the kind of salt, its concentration in the building material and the relative humidity, an increased moisture content could be the result of such a measure.
The deliquescence relative humidity of many salts present in building materials varies to a great extent and covers the full relative humidity range. For sodium chloride, however, the deliquescence is almost independent from temperature and amounts to approx. 75% RH, simplifying the prediction for climatic conditions. When looking at other salts, the DRH depends significantly on temperature. If these salts can also occur in different hydrated stages, it can be very difficult to define a suitable indoor air climate. In these cases phase diagrams showing the stability ranges of the different phases as a function of temperature and RH, can be very useful.
In the presence of pure salts and with the particular phase diagram as a basis, it is always possible - through choice of the appropriate indoor air conditions - to prevent phase transformation, i.e., crystal growth. Unfortunately, in building materials it is seldom the case that pure salts are present, rather they are found in more or less complex mixtures. In most cases, the salts found in building materials are mixtures of chlorides, nitrates, sulfates and sodium carbonates. The behavior of salt mixtures is far more complicated than the behavior of pure salts and information can generally not be derived only from the properties of the individual salts present in the building material. For example, salt mixtures cannot be characterized by a single deliquescence relative humidity, rather a range of relative humidity, within which fluctuations lead to phase transformation and crystallization processes, depending on the composition of the mixture. Phase diagrams of salt mixtures are therefore more complex and the prediction of suitable climate conditions can usually only be made with appropriate models. This is discussed in more detail elsewhere salt mixtures.
|[Arnold.etal:1991]||Arnold, Andreas; Zehnder, Konrad (1991): Monitoring Wall Paintings Affected by soluble Salts. In: Cather, Sharon (eds.): The Conservation of Wall Paintings: Proceedings of a symposium organized by the Coutrauld Institut of Art and the Getty Conservation Institute, London, July 13-16, The Getty Conservation Institute, 103-136|
|[Price:2000]||Price, Clifford A. (eds.) (2000): An Expert Chemical Model for Determining the Environmental Conditions Needed to Prevent Salt Damage in Porous Materials, European Commission Research Report No 11, (Protection and Conservation of European Cultural Heritage), Archetype Publications Ltd, London|
|[Steiger:2005c]||Steiger, Michael (2005): Salts in Porous Materials: Thermodynamics of Phase Transitions, Modeling and Preventive Conservation. Restoration of Buildings and Monuments, 11 (6), 419-432|