Migration of sodium chloride in dry porous materials

Abstract

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.

Salt creeping as a self-amplifying crystallization process

“…This spreading is very rapid and has even been compared to the phenomenon of superfluid film flow of helium (2). In porous materials, the transport of saline solution by creeping can lead to the physiological aridness of the soil above ground water level and the weathering of cultural heritage (3)(4)(5). The white stains on top of monuments and artworks (e.g., frescoes) are invariably due to salt creeping, and the resulting salt crystallization not only affects the aesthetics but can also even cause damage to the material, such as cracks and delamination.…”

Ion Transport and Precipitation Kinetics as Key Aspects of Stress Generation on Pore Walls Induced by Salt Crystallization

“…Salt crystallization in pores causes damage in porous materials, a major issue in relation with building durability and cultural heritage conservation [1][2][3][4], underground structures [5], road [6] and geotechnical engineering [7]. A better understanding of the associated stress is also important in relation with geomorphology [8], concrete science [9] or the surface heave phenomenon of granular materials [10]. The fact that a growing crystal can generate stress has been known for more than a century [11], [12].…”

Experimental Investigation of Water Migration Characteristics for Saline Soil

“…Zhao [24] modelled the process of coupled water and heat for the soil in freezing and thawing conditions. Robert [25] carried out experiments to study salt crystallization in the process of brine percolating upward and discussed the potential damage to masonry, foundations, and roads. However, only a fraction of the studies concerned the water migration characteristics and soilwater potential of the soil in western Jilin.…”