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

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 Na2SO4-NaCl-H2O system during evaporation, by using two pure solutions and four mixed Na2SO4-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 Na2SO4 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 Na2SO4. 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 x0.8.