Damage processes: Difference between revisions
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== Salts and indoor climate == | == Salts and indoor climate == | ||
Arnold and Zehnder <bib id="Arnold.etal:1991"/> | Arnold and Zehnder <bib id="Arnold.etal:1991"/> 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 <bib id="Price:2000" />, <bib id="Steiger:2005c"/>. This allows as well the possibility of [[Preventive Conservation| preventive conservation measures]]. | ||
When a non-hydrating salt is present, e.g., sodium chloride ([[Halite|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 above the | 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|moisture content]] could be the result of such a measure. | ||
== [[Phase diagrams]] == | == [[Phase diagrams]] == | ||
The [[Deliquescence humidity|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 occur in different | The [[Deliquescence humidity|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. | ||
== [[Salt mixtures]] == | == [[Salt mixtures]] == | ||
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, pure salts are | 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 salts present in the building material. For example, salt mixtures cannot be characterized by a single deliquescence relative humidity, rather a range of relative humidities depending on the composition of the mixture there is a relative humidity range, wherein fluctuations lead to phase transformations and crystallization processes. Phase diagrams of salt mixtures are therefore more complex and the prediction of suitable climate conditions can usually only be made with appropriate models. Elsewhere[[Salt mixtures| salt mixtures]] are discussed in more detail. | ||
== Literature == | == Literature == | ||