Evaporation from a Porous Medium in the Presence of Salt Crystallization
The interplay between salt transport, crystallization and evaporation from a porous medium is a topic rich in interesting open problems. This is illustrated through the consideration of a few of them from experiments with model porous media. We notably discuss the factors controlling the localization of crystallization spots at the evaporative surface of a porous medium and the impact of surface crystallization on evaporation kinetics.
KeywordsPorous Medium Salt Concentration Evaporation Process Evaporation Flux Salt Transport
Evaporation in a presence of dissolved salt is a key process in several applications such as the injection of CO2 in saline aquifers, the soil salinization problem and the preservation of our cultural heritage (frescos, statues, monuments, etc). The latter application is directly related to the damages that can be induced in a porous material or at its surface by the salt crystallization, i.e. . The damage problem is generally studied in relation with the so-called crystallization pressure, e.g. [2, 3], which is the key concept in order to evaluate the stresses generated within the pores by the crystallization. According to the current theory, the crystallization pressure notably depends on the local supersaturation, that is the amount of dissolved salt in excess compared to the amount corresponding to the thermodynamic equilibrium between the crystal and the solution. It is therefore crucial to predict the salt concentration transient distribution within the system during the evaporation process in order to be able to address the associated poremechanical problem or more modestly to predict where the damages are likely to be generated in the materials. Even in the absence of coupling with poremechanical effects, the study is made complicated by the various couplings existing between evaporation, salt transport and crystallization. The main objective of the present article is to illustrate the interplay between different phenomena (in the absence of structural damages). We indeed believe that it is crucial to develop a much better understanding of the trio: evaporation/transport/crystallization. A few obvious basic questions are: Is evaporation in the presence of salt faster, identical or slower than for pure water? How does the effect of salt on evaporation depend on the initial salt concentration? Does the salt crystallize at the surface of the porous medium or within the porous medium? These are the main points that will be addressed in what follows. The idea is to give an overview of some key results. Details can be found in the references. Also, we do not discuss in details evaporation with pure water, see  in the present book for an introduction to evaporation in porous media. The results we discuss were obtained with sodium chloride, which can be considered as a simple salt, notably because it forms an anhydrous crystal.
19.3 Cauliflower Efflorescence and Crusty Efflorescence
19.4 Efflorescence Is Porous. Efflorescence Growth Mechanism
The case of the crusty efflorescence is much less clear. The model proposed in  for explaining qualitatively the transition from patchy to crusty as the mean pore size of the underlying porous medium is decreased is based on the assumption that the crusty efflorescence is porous. It has been also seen that the vapour can be transported through a salt crust . However, the details of the crust formation and crust growth are a widely open subject. Further experimental investigations are needed.
19.5 Factors Affecting the Localisation of Cauliflower Efflorescence
One striking feature of the patchy efflorescence is the formation of discrete crystallization spots at the porous surface (see Fig. 19.2a). This is interpreted as the signature of the existence of local maxima in the salt concentration field at the porous surface prior and up to the formation of first crystals. The existence of spatial fluctuations in the salt concentration field is explained by the spatial fluctuations in the velocity field existing in the liquid phase. Because the advection effect is significant, the spatial fluctuations in the velocity field induce fluctuations in the concentration field. The factors affecting the salt concentration distribution and thus the localization of crystallization spots at the surface are therefore intimately related to the factors affecting the velocity distribution within the pores. As discussed in [9, 14] and , those factors are the disordered nature of a porous medium (= random distribution of pore sizes), the evaporation flux distribution at the surface, e.g. , and the possible Darcy’ scale heterogeneities of the medium, e.g. . The more detailed analysis reported in  leads to distinguish the impact of surface disorder and internal disorder. The surface disorder refers to the spatial fluctuations in the pore opening at the surface, which induce fluctuation in the evaporation rate at the surface of each meniscus, and thus velocity fluctuations in the adjacent pores. The internal disorder refers to the fluctuations in the pore size within the porous medium, which induce fluctuations in the velocity field, and thus in the salt concentration field.
As discussed in  and , salt concentration gradients are important only in a region of size ξ(t) adjacent to the evaporative surface. This is a consequence of the advection effect on the salt transport, see also . As a result only the internal disorder of this region of size ξ(t) is important. The disorder located further away from the evaporative surface can generally be ignored.
The next question is why the patchy efflorescence continues to grow under the form of well individualized salt structures. As discussed in  and , this is explained by the screening of the porous surface free of efflorescence located between the already growing efflorescence structures and the redirection of dissolved salt present in the porous medium toward the growing salt structures. The screening means that the evaporation flux at the porous medium surface between the growing efflorescence structures tends to zero, which “kills” the advection effect and thus the salt concentration build-up at the surface.
19.6 Impact of Efflorescence Type on Evaporation
Evaporation from a porous medium in the presence of dissolved salt is a particularly interesting problem because of the complex interplay between evaporation, salt transport and crystallisation. This field is widely open and many questions are still to be answered. The detailed understanding of the crusty efflorescence growth and the patchy/crusty transition are just two examples of interesting questions. A more advanced understanding of the transport/crystallization problem is needed not only because the crystallization can greatly affect the evaporation but also because the salt concentrations that can be reached in the solution in contact with growing crystals has a direct impact on the crystallisation pressure, and therefore the possible associated poromechanical effects.
As for evaporation with pure water, e.g. , studies in recent years have mainly focussed on microporous materials (pore size equal or greater than 1 μm). Very little is in fact known for the systems involving nanopores (∼ pore sizes lower than 100 nm).
I am very thankful to the students and colleagues I have worked with over the years on the “salt problem”: S. Ben Nasrallah, P. Duru, H. Eloukabi, F. Hidri, M. Marcoux, N. Sghaier, S. Veran-Tissoires. Their contribution is greatly appreciated.
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