Combined wicking and drying of a NaCl solution in porous building materials

Abstract

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.