Crystallization Behavior and Damage Potential of Na₂SO₄–NaCl Mixtures in Porous Building Materials

Abstract

Understanding salt crystallization is critical to uncover the mechanism of salt damage in building materials. Although the behavior of several single salts commonly found in building materials is well characterized, few studies have been carried out to investigate the behavior of salt mixtures that are normally present in building materials. Employing the phase diagram, this study focuses on the crystallization behavior and damage potential of the Na 2 SO 4 −NaCl−H 2 O system during evaporation, by using two pure solutions and four mixed Na 2 SO 4 − NaCl solutions with different NaCl molar ratios x of 0.33, 0.5, 0.8, and 0.97. A series of droplet drying experiments were carried out, and in situ Raman spectroscopy was employed to determine the crystallization sequence. The supersaturation of solutions induced by evaporation was measured through visualization of evaporation in capillary tubes to estimate the maximum crystallization pressure. The results show that the presence of a small amount of halite does not influence the crystallization pathway of sodium sulfate nor the crystallization pressure. In particular, mirabilite nucleation was hindered in mixed salt solutions as well as in the pure Na 2 SO 4 solution under room condition. However, a high molar ratio of NaCl (x 0.8 ) results in remarkable reduction of the evaporation rate, leading to a lower supersaturation of the solution with respect to thenardite, and the solution crystallization follows an equilibrium pathway. NaCl nucleation is observed at a high supersaturation, which is hardly affected by the presence of Na 2 SO 4 . In addition, the influence of halite on the morphology and crystal size was observed by comparing the precipitates of solutions of the same sodium sulfate content. Large thenardite twins are formed in solutions x 0.33 and x 0.5 , while small bipyramidal prisms and extensive creeping are found in solution x 0.8 .

Salt-Induced Doping and Templating of Laser-Induced Graphene Supercapacitors

“…As such, the Na 2 SO 4 particle size was measured from the spherical particles and is smaller than NaCl particles, with an average size of 2.4 ± 1.3 μm. These results appear consistent with the known crystallization behavior of these saltsNaCl is known to form only the anhydrous halite salt via evaporation above 0 °C, while the crystallization behavior of Na 2 SO 4 is fairly complex and can result in the formation of hydrates such as mirabilite (Na 2 SO 4 ·10H 2 O) and the heptahydrate (Na 2 SO 4 ·7H 2 O), as well as a number of different anhydrous phases, including thenardite (Na 2 SO 4 phase V), which is the stable anhydrous phase at room temperature, and the metastable phase III …”

NaCl-related weathering of stone: the importance of kinetics and salt mixtures in environmental risk assessment

“…The volume of the solution in the capillary is calculated based on equations. 5 and 6 in Shen et al [25]. Unlike Shen et al we do not present results on the supersaturation at the onset of crystallization, but the total time of completed crystallization/dissolution with respect to the distance from the capillary entrance of the solution in the capillary.…”

“…Furthermore, fundamental research on salt mixtures can only be found in recent literature and is rare, e.g. [25][26][27][28].…”

Patterns during Evaporative Crystallization of a Saline Droplet

“…Crystallization in porous structures is affected by the transport of liquid in the medium that is in turn influenced by its permeability and wettability. The wicking of saline solution in conjunction with the morphology of the precipitated crystals and the interaction of the salt crystals with the underlying substrate often result in salt precipitation at locations beyond the initial solid–liquid–air interfacea phenomenon termed creeping. , Creeping is influenced by the rate of evaporation which, in turn, depends on substrate wettability and ambient temperature and humidity. ,− The wettability of the substrate also affects the orientation of the growing crystal that, in turn, results in the self-amplifying growth of crystals, at times resulting in beautiful patterns …”