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Combined wicking and evaporation of NaCl solution with efflorescence formation: The efflorescence exclusion zone

An experiment combining wicking and evaporation of a NaCl solution and leading to the formation of salt efflorescence is presented. The experiment shows that efflorescence develops over the porous medium surface exposed to evaporation except in the bottom region of the sample. This region remains free of efflorescence and is called the exclusion zone. It is shown that the exclusion zone extent depends on the solute concentration in the bottom reservoir. A model is developed, and it helps understand the exclusion phenomenon. The arch shape of the exclusion zone upper boundary is explained and modeled. The study is also seen as a successful test for the model of efflorescence growth driven by evaporation and salt precipitation presented in a previous study. The modeling approach is expected to help develop better models of salt transport with crystallization at the surface of porous media in relation with soil salinization issues or the salt weathering of porous materials.