Migration of sodium chloride in dry porous materials

Groundwater can saturate soil above the water table within the capillary fringe associated with the pore size of the parent soil. External evaporation has been viewed as a mechanism for enhancing upward flow, potentially creating problems of salt heave beneath roads and foundations if the groundwater is saline, analogous to concerns with efflorescence in masonry. The role of internal evaporation in promoting crystallization, and especially in altering the transport process of the pore fluid, has been recognized but is only partially understood. The purpose of this paper is to examine evidence for the upward percolation of brine accompanying salt crystallization inside a porous granular material. A series of experiments are described using vertical flow columns packed with dry sand above a reservoir of saline fluid, to explore whether salt transport could take place by autogenous wicking above the initial capillary fringe. The conditions inside the columns were monitored at specific elevations with sensors measuring bulk electrical conductivity, dielectric constant and relative humidity. Dendritic salt crystallization was observed inside the sand, accompanying surface heave. Ultimately, efflorescence on the surface led to the formation of a salt crust. Some implications for the potential damage to roads and foundations in arid regions, and to masonry subject to rising damp, are discussed.