Can supersaturation for NaCl crystallization be maintained in a capillary? Combined microscopy and NMR of NaCl crystallization in a capillary

It has been known for a long time that the presence of salts inside the pores of building materials can give rise to damage. Among the various salts present in building materials NaCl is one of the most widely distributed. Sea salt spray, deicing salts are possible sources. Until now, the mechanisms that control the development of damage by crystal growth are poorly understood. The major mechanism generally assumed to be responsible for damage is crystallization pressure, i.e,the damage is caused by the growth of salt crystals in a confined space, e.g., a pore. These crystals exert a pressure on the pore walls, which can exceed the tensile strength of the material and therefore lead to damage. Thermodynamically, the crystallization pressure can be related to the supersaturation (C/Co) of the solution:

Pc=(nRT/Vm) ln(C/Co)

Where, Pc is the crystallization pressure, n is the total number of ions released upon dissociation of the salt (for NaCl = 2), R is the universal gas constant, T is the absolute temperature and Vm is the molar volume and C and Co are the concentration in the vicinity of the crystal and the saturation concentration of NaCl (6.14 m) respectively.
It has been shown that NaCl has a low tendency to supersaturate inside building materials and to develop a high crystallization pressure to generate damage. In addition, it is known that NaCl has a tendency to crystallize on foreign nucleation sites, e.g., impurities on the pore walls. Therefore, crystallization is more likely to occur at low values of supersaturation. Nevertheless, serious decay occurs in the building materials in the presence of NaCl, leaving an open question with respect to the NaCl damage mechanism.
The crystallization of NaCl in capillaries has been recently been studied by Desarnaud et al. They have reported that high supersaturation can be reached for NaCl up to 1.6. The salt ion concentration was calculated indirectly, i.e., it was calculated from images of the decreasing droplet in capillaries using microscopic images. Knowing the time before nucleation and amount of salt, the salt ion concentration was calculated. However this only will give the concentration at the moment of crystallization and does not tell if this high supersaturation will be maintained. Hence we have set up an experiment where we combine time-lapse microscopy with NMR measurements. We are able to follow the dynamics of crystallization by measuring the Na-concentration in a capillary. Hence we are able to measure the concentration once the crystal starts to grow and to measure the supersaturation in time. These measurements have indicated that the high supersaturation is not maintained as the crystal starts to grow and this mechanism cannot explain the damage for NaCl.