Introduction

Marc PRAT

IMFT, CNRS, Université de Toulouse, France

The crystallization of a salt in a porous medium is studied in relation to different applications. First, because of its destructive effect. The latter is easily observable when visiting many monuments and historical sites. As noted in the classic work by Goudie and Viles (1997), this is the case, for example, of the Sphinx in Egypt, the stone monuments of Petra in Jordan, several cathedrals in Mediterranean regions, or even the city of Venice. As discussed in Chapters 7 and 8 of this book, the attack by salts results in different forms of degradation such as cracking, flaking, honeycomb weathering (Figure I.1) and can go as far as pulverizing the material. All of the processes linked to the crystallization of salts that lead to the degradation of porous rocks are defined under the term "haloclasty". Thus, haloclasty is considered to be the main phenomenon causing the alteration of historical monuments in the Mediterranean region.

Although haloclasty has been observed since antiquity (Goudie and Viles 1997), it is only since the 19th century that these phenomena began to be studied seriously. For example, Brard (1828) presented research where the crystallization of salt was studied as analogous to the effect of freezing on porous stones, whereas initial discussions, aiming at explaining that the growth of a crystal in confinement generates constraints, date back to 1853 with the work of Lavalle (1853). Haloclasty has also been identified as an important phenomenon in the evolution of landscapes, contributing to erosion not only on Earth but possibly also on Mars (Malin 1974).

Figure I.1. Examples of damage due to salt crystallization: (a) cracking, (b) flaking (Motomachi Buddha, Oita, Japan) and (c) honeycomb weathering (Santander Cathedral, Spain) (photos (a) and (c), B. Leclère (Leclère 2021); photo (b), H. Derluyn).

As for many other phenomena, the scientific approach to haloclasty and more generally, to the crystallization of salts in porous media and associated phenomena, gained considerable interest in the 20th century. Among the seminal works often cited, we can mention the work of Correns (1949), who proposed an expression of the crystallization pressure, a central concept in the analysis of the generation of stresses due to crystallization (see Chapter 2 of this book). This formulation has since been revisited and we can refer to Steiger (2005) for a more recent presentation according to a purely thermodynamic equilibrium approach. More recent studies (e.g. Gagliardi and Pierre-Louis 2019) indicate, however, that it is important to consider non-equilibrium effects when analyzing the force exerted by a crystal in a pore. The subject of stress generation, due to the crystallization of salt in pores (Scherer 1999; Coussy 2006; Noiriel et al. 2010; Scherer et al. 2014), inducing degradation phenomena in porous stones, as well as the detrimental effects that it can cause on engineering structures (see Chapter 6 of this book), is not the only motivation to study the impact of the presence of one or more salts in a porous medium. Other applications include the following:

• Soil salinization: salinization refers to the accumulation of salts in soils at levels toxic to most plants. It is a main cause of desertification, erosion and land degradation. It makes the soil unsuitable for agriculture. Very likely linked to global warming, the phenomenon is accelerating (Litalien and Zeeb 2020) and threatens ever larger areas of land. Irrigation is one of the main human causes of salinization. In the event of excessive irrigation, the soil is wet at depth, which establishes hydraulic continuity causing the salt to rise to the surface. A good understanding of the transport of dissolved salt in an unsaturated porous medium is therefore key to understanding and analyzing this phenomenon.
• Soil evaporation: the phenomenon of evaporation in the presence of dissolved salt often leads to the formation of a salt crust on the surface of the soil. Depending on the formation conditions, the formation of the crust can severely reduce evaporation (Fujimaki et al. 2010) and therefore affect soil-atmosphere exchanges. Despite advances (see Chapter 3 of this book), the detailed understanding and modeling of this phenomenon remain to be fully understood.
• Ground heave: when crystallization occurs at depth, the soil layers above may heave due to stress caused by crystallization. This mechanism is illustrated in Figure I.2. We find here a mechanical effect in the context of granular media rather than consolidated media such as porous rocks. This uplift phenomenon can lead to major surface disorders (Feng et al. 2020; also see Chapter 6). Although clearly identified (Hird and Bolton 2016), it is still insufficiently described with a lack of predictive models.
• Underground storage of CO2: carbon capture and storage (CCS) in geological formations is a technique aimed at limiting the accumulation of greenhouse gases in the atmosphere. Some of the main issues related to this technique concern the injectivity conditions of the wells used to maintain a high level of injection (Peysson et al. 2014). When this injection takes place in a saline aquifer, salt precipitation can occur, leading to the clogging of pores in the well, a drop in permeability and ultimately the deterioration of injection conditions. A better understanding of these phenomena is therefore desirable to improve the implementation of this technique.
• Impregnation with a saline solution: with the Covid-19 pandemic, it has been shown that individual masks are essential tools to protect against airborne viral particles. In this context, hypertonic saline solution has been shown to effectively reduce infectious viral load on treated masks (Tatzber et al. 2021). The method consists of impregnating the mask with a saline solution of NaCl and then drying it. We can refer to Börnhorst et al. (2016) for a discussion on the impregnation of a porous medium by a saline solution in connection with other contexts.
• The use of crystallization as a cheap method for the recovery of metals: a simple method to recover metals from mine tailings is to soak them in a saline solution and subject them to evaporation (Cala-Rivera et al. 2018). In this situation, as explained in Chapter 3, a flow is induced toward the surface in the porous medium. This flow transports the species in solution to the surface where the salt crystallizes. The metals become trapped in the efflorescence, which then only needs to be extracted and dissolved to recover the metals in question.
• Conservation and restoration of built heritage: considering the damage that crystallization can cause, various techniques have been developed to limit the risk of degradation or restore the affected areas, whether for recent constructions or for the old parts of built or cultural heritage (frescoes, paintings, sculptures, etc). These aspects are discussed in Chapter 7.
• Design of distillation unit evaporators: desalinating seawater is an important solution to meet the growing demand for drinking water, but also potentially in the context of the energy transition and the production of hydrogen by electrolysis, which also uses water as a resource. In this context, solar-thermal water desalination is an interesting solution, which can generate clean water without significantly depending on fossil energy. In devices designed for this purpose (Wang et al. 2020), we find (nano)porous media, evaporation and a saline solution, ingredients common to the applications already mentioned.

Figure I.2. Drying a NaCl solution in a Hele-Shaw cell filled with glass beads. The process leads to the formation of subflorescence causing the granular medium to move upwards and lift off the surface (Diouf 2018).

These different applications illustrate the multidisciplinary nature of the field of research that is the subject of this book. Thus, different communities of researchers contribute to its development including specialists in geophysics, geomorphology, underground or surface hydrology, physicochemistry, physics, chemistry, thermodynamics, civil engineering, transfer in porous media, poromechanics, engineering, or even more directly, in the relevant applications.

Even if each specialist can find their own reasons for their interest in this field of research, it is clear that the analysis of the most complex situations requires a certain level of interdisciplinarity. Consider, for example, the very classic scenario where crystallization results from the evaporation of a saline solution contained in a porous medium. In this scenario, evaporation induces an increase in the salt concentration in the solution within the porous medium by the effect of advection and/or volume reduction (see Chapter 3) until a sufficient concentration is reached for crystallization to occur and subflorescence and/or efflorescence to develop. As shown schematically in Figure I.3, the analysis of this type of situation involves focusing on the transport of dissolved salts in a porous medium subjected to evaporation and often unsaturated (case of drying for example), physicochemical aspects, including the often complex...