Damage processes: Difference between revisions
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== Introduction == | == Introduction == | ||
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|deterioration | 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|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 == | == 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 temperature fluctuations | 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, | Furthermore, properties inherent to the salts determine the moisture content of a building material. Especially the process of [[Deliquescence humidity|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 == | == Phase transformation – hydration == | ||
Crystal growth inside the pores can also take place during [[Hydration|hydration reactions]]. Because the phase in the higher stage of hydration has a lower density, hydration reactions increase the filling | Crystal growth inside the pores can also take place during [[Hydration|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]]crystals of the hydrated stage of the salt, growing against the pore wall and the build up of [[hydration pressure]]. Under unfavorable condition, cyclic hydration reactions are also possible. | ||
The moisture needed for the hydration reaction can be introduced through precipitation or condensation. Also an increase in relative humidity can be sufficient to trigger the reaction. | The moisture needed for the hydration reaction can be introduced through precipitation or condensation. Also an increase in relative humidity can be sufficient to trigger the reaction. | ||