Diagnostics and monitoring of moisture and salt in porous materials by evanescent field dielectrometry

Moisture and salts are the main causes of decay of porous materials, like wall paintings, stones, plasters and cement-based artefacts. Water is the ,driving force' of decay, such as the detachment of the painted layer, the whitening of surfaces due to the crystallization of salts (efflorescence), and the weakening of the cementing binder. Early diagnostics of water content and detection of the presence of soluble salts inside the material is a key issue for understanding the degradation processes in such kind of materials and for improving their schedule maintenance. In this contribution a non-invasive microwave system based on evanescent field dielectrometry is described. The method was tested in the laboratory on moistened plaster samples, some of them containing salts at different concentrations. Measurements on water-saturated and oven-dry samples provide the basis for calibrating the instrument for on-site measurement of masonry structures, wall paintings and concrete historical buildings too. The obtained results prove the usefulness of the method as a tool for diagnostics and for monitoring the effectiveness and durability of restoring interventions