Salt weathering is one of the major causes of the damage both in cultural heritage as well as in civil engineering constructions. A special case develops when there is a continuous wicking of a salt solution into a material in combination with evaporation of the moisture at its surface. In this study we are interested in the case where the absorption rate is much higher than the evaporation and as a result a salt concentration will build up at the drying surface resulting in crystallization. To this end we propose a simplified model to describe this mechanism. In order to check the model the NaCl concentration profiles were measured non-destructively by Nuclear Magnetic Resonance during a combined wicking and evaporation experiment with limestone. A good correlation was found between the model and the measured NaCl concentration profiles.

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Salt weathering is a major cause of destruction of many valuable monuments. The salt damage of historical porous materials is mainly due to the crystallization of soluble salts due to drying. A special situation which occurs often in marine environments, the case where there is a permanent supply of sea water at one side of a material, whereas the other side is exposed to continuous drying in the open air. A well-known example is the historic city of Venice. Evaporation from the air exposed side provides a continuous moisture sink which is compensated by capillary suction, i.e., wicking of the sea water. As a result there will be a continuous flux of NaCl ions towards the surface. As soon as the concentration at the surface reaches the maximum solubility, crystallization will start which can give rise the damage. A simple analytic model was developed to describe this process. In order to verify this model non-destructive measurements were made of the moisture and ion transport during an experiment with limestone. In order to do so, we make use of a specially designed Nuclear Magnetic Resonance setup where we are able to measure quasi-simultaneously the 1H and 23Na content. Hence this method gives us the possibility for real-time monitoring of transport processes of the ions during experiments. It is seen that the concentration rise at the drying surface can be described by a simple analytic model.

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