Measurement of Relative Humidity to Monitor Salt Migration in Unsaturated Porous Media

Abstract

When aqueous sodium chloride evaporates, crystals can form. Salt crystallisation can take place within voids resulting in subflorescence which can culminate in surface heave, or can lead to efflorescence on exposed surfaces. Evaporation can be measured using relative humidity sensors, and thus salt crystallisation be inferred. A series of laboratory experiments were conducted using vertical flow columns packed with dry medium-grained sand with their exposed surfaces subjected to air at low relative humidity while stood in a shallow container of deionised water or brine. Experiments using deionised water showed that the degree of saturation above the capillary fringe was initially insufficient for transporting salt to the surface through diffusion. Nevertheless, repeated tests using concentrated sodium chloride solution showed that internal changes in relative humidity, and surface heave, were consistent with the upward migration of salt by autogenous internal ‘wicking’. The results indicate that relative humidity sensors can be used as a practical way to detect salt crystallisation and the conditions which promote its transportation.