a b s t r a c tWe present an experimental study of drying in the presence of dissolved sodium chloride. The process is characterized by the formation of a crystallized salt layer, referred to as efflorescence, at the evaporative surface of the porous medium. By varying the average size of the beads forming the porous medium, we show that the formation of the crystal layer does not affect significantly the drying process and can even enhance the drying rate when the beads are sufficiently large. By contrast, the crystal layer can greatly affect the drying process and even blocks or severely limit the evaporation process for sufficiently small beads. We therefore show the existence of two regimes, namely the blocking regime and the enhanced drying rate regime. It is shown that the two regimes correspond to two different types of efflorescence, referred to as crusty and patchy, respectively. Then by varying the initial salt concentration for a given bead size, we show that the interplay between drying and the efflorescence formation leads to a nonmonotonous variation of the drying rate with the initial salt concentration when the efflorescence is patchy but not when the efflorescence is crusty. The crusty-patchy transition is finally discussed from a simple model of capillary rise in the efflorescence.

We study experimentally the evaporation of an aqueous NaCl solution with efflorescence formation in hydrophilic or hydrophobic two-dimensional model porous media. The efflorescence formation only marginally affects the invasion patterns but greatly modifies the drying kinetics compared to pure water. Two mechanisms are identified for explaining the impact of efflorescence formation on drying kinetics. It is shown that the efflorescence contributes to increase the evaporation rate compared to drying with pure water, a surprising result since the water activity is reduced in the presence of dissolved salt. This effect is explained by the efflorescence liquid capillary pumping effect associated with the porous nature of the efflorescence. Then, we identify a second phase where the efflorescence dries out and acts as a vapour diffusion barrier, leading to a dramatic reduction of evaporation rate.

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Salt crystallisation at the surface or in a porous medium has been recognised as a major mechanism of deterioration of buildings and historical monuments. Often crystallisation occurs when the concentration of salt dissolved in the water contained in the porous medium reaches the saturation concentration as the result of evaporation. In order to predict the evolution of the ion distribution during drying, we develop a simple volume averaged model combining a semi-analytical model of drying with the numerical computation of the ions transport. The model is used to analyse the influence of the drying rate, size of the porous medium, average pore size and initial ion concentration on the ion distribution during drying and therefore the possible location of crystallisation.