Abstract
The evaporation of salt (NaCl) solutions from porous media is studied in the presence of surfactants, because surfactants are often used as cleaning agents for salt-contaminated stones. We show that, contrary to what is commonly assumed, the presence of the surfactant and the changed wetting properties do not affect the drying kinetics: The impact of the surfactants is rather that of a crystallization modifier for the salt. Upon adding a cationic or nonionic surfactant to salt solution, the drying rate is unchanged initially, but can slow down dramatically at later times due to the formation of a salt crust at the surface. When this happens, the total drying time increases compared to pure NaCl solutions without surfactants, at least for very porous stones for which the pores become completely blocked. Surprisingly, for a low-porosity stone the small pores at the surface remain open. The longer drying time for the large porosity stone increases the risk of, e.g., frost or fungal damage to the stones. Consequently, the use of surfactants in conservation treatments should be done with caution.
Highlights
Drying of saturated porous media is a widely studied phenomenon owing to its relevance to soil mechanics, food production, carbon dioxide sequestration, etc. (Rad et al 2015; Eloukabi et al 2013; Shahidzadeh-Bonn et al 2007; Yiotis et al 2003; Coussot 2000)
Zooming on the surface reveals that salt precipitation at the surface does not clog the pores of the stone, which is in agreement with previous studies (Veran-Tissoires et al 2012; Desarnaud et al 2015; Shokri-Kuehni et al 2017)
The mechanism of salt crust formation is shown to be due to the lateral steps growth of crystallites at the surface with defective junctions between them
Summary
Drying of saturated porous media is a widely studied phenomenon owing to its relevance to soil mechanics, food production, carbon dioxide sequestration, etc. (Rad et al 2015; Eloukabi et al 2013; Shahidzadeh-Bonn et al 2007; Yiotis et al 2003; Coussot 2000). For drying at higher relative humidity, different salt precipitation dynamics at the surface enables continuous hydraulic conductivity and can even enhance the evaporation process (Desarnaud et al 2015; Gupta et al 2014; Shokri-Kuehni et al 2017; Veran-Tissoires et al 2012). This leads to the counterintuitive result that the total drying time can be longer at a low relative humidity. We test the influence of different surfactant types on the drying of sodium-chloride-contaminated sandstones with a high (Mesne sandstone) and a low porosity (Fontainebleau sandstone)
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