Introduction
Soluble salts such as chlorides, sulfates, and nitrates are widely recognized as a cause of destruction in porous building materials. In light of the limited practical options available for the control of salt damage, the use of crystallization inhibitors has been proposed as a potential preventive treatment method. These inhibitors act either by preventing or delaying the onset of nucleation. Here we have looked ferrocyanide (FC) ions as a preventive measure for NaCl crystallization

       Drying of salt solution droplet with/without inhibitor.
Initially we started with the drying of a 3 m salt solution droplet without inhibitor.The measured salt concentration is given in Fig 1A.
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Fig 1 Drying of a 300 μL of 3 m NaCl salt solution droplet without inhibitor A and with inhibitor B.
The concentration in the droplet was measured using NMR

After about 8 h a the sodium concentration remains at 6 M, indicating crystallization. This onset of crystallization was also observed by direct visualization where cubic structured crystals. In order to do the analyses of the advection−diffusion processes in a droplet, the data are plotted in a way that is somewhat similar to a so-called efflorescence pathway diagram. In Figure 2 the data for the 3 m NaCl salt solution droplet are plotted. As can be seen for the salt solution droplet, initially the Pe < 1 path is followed indicating diffusion dominance and the concentration increased until nearly the saturation concentration was reached, after which it stayed constant at approximately the saturation concentration 6.1 m.


Fig 2. Advection−diffusion analysis diagram for the droplet drying experiment: The total amount of dissolved sodium in the droplet is
plotted as a function of the volume of the droplet (V). Both the axes are normalized with respect to the initial volume of the droplet
(Vinitial). The division of both the axes gives the average concentration (Cavg) of Na in NaCl solution droplet shown by solid lines in the
figure. The results for 3 m NaCl salt solution droplet with (△) and without inhibitor (□) are shown.


Next the drying of 3 m NaCl droplet in the presence of 0.01 m inhibitor was studied and the results are given in fig 1B.After approximately 2 h the concentration in the droplet was on the order of 6.1 m but no crystallization was observed. As the drying progresses the concentration increased and the system supersaturated slowly.
After 8h a faster decrease in moisture and sodium content was observed indicating crystallization. This was also observed by direct visualization. A small bunch of dendritic crystals were observed near the edge of the droplet. The corresponding average concentration at this point was nearly 10 m giving a supersaturation of 1.6 (calculated as C/Co). From this moment onward the average concentration in the droplet continued to increase as a rapid increase in the surface area of  the droplet was observed. This was due to the formation of dendritic crystals, the branches of which provided a pathway for the solution to spread over a much larger surface area, this phenomenon being commonly known as “salt creep” (see also Fig 1B). Because of the enlarged surface area for evaporation a similar increase in the drying rate was observed, and the solution concentration increased to nearly 12 m. After about 9.5 h the drying rate decreased and no further spreading of the
droplets was observed. Meanwhile, the concentration returned to the equilibrium concentration (6.1 m). The results from the drying of a salt solution droplet with inhibitor are also plotted in Figure 2; as can be seen in this case the sodium concentration also remains homogeneous until crystallization and the droplet supersaturates, reaching a maximum concentration on the order of 10 m before crystallization. These results show that the ferrocyanide is acting as a strong nucleation inhibitor, which delays the onset of crystallization resulting in higher solution concentration.

    Drying of fired-clay brick with salt droplet with/without inhibitor.

In the previous section we have shown that the presence of an inhibitor significantly increases the supersaturation within salt solution droplets. This
delay in onset of crystallization can promote efflorescence growth by inhibiting NaCl nucleation inside the stone. Also this delay may be an advantage in situations where the object is exposed to a fluctuating environment. For such a case, the conditions for the crystallization may only be temporarily met. Nevertheless when exposed for periods of sustained low RH the critical supersaturation of the NaCl solution may be reached. A higher supersaturation can cause higher crystallization pressure and hence the risk of damage is then greater. Therefore to ascertain the effects of the ferrocyanide inhibitor on the ion transport and crystallization behavior of sodium chloride solution within porous media a series of one-dimensional drying experiments were performed using fired-clay brick. The results of the experiment are given in an EPD in figure 3.



Fig 3. Efflorescence pathway diagram for the brick drying experiment: The total amount of dissolved sodium content is plotted as a function of
time dependent average moisture content (θavg) in the brick. Both the axes are normalized with respect to the initial average moisture content. The
division of both the axes gives the average concentration (Cavg) of Na in the brick as shown by solid lines in the figure.

As in the case of the droplet two extreme situations can be distinguished on this macroscopic scale. In the first case when the system dries very slowly (Pe < 1); diffusion dominates resulting in a homogeneous distribution of ions throughout the sample. For this situation, starting from an initial concentration of 3 m the salt solution
concentration will increase throughout the sample, until the saturation concentration 6.1 m is achieved. From this moment onward any further drying will cause crystallization. Furthermore, the concentration will stay constant at 6.1 m. In thesecond case the system dries very fast (Pe > 1), that is, advection dominates. In this case, ions will be transported alongwith the moisture flow toward the drying surface. Thus, the concentration will increase near the drying surface. If there are
enough nucleation sites, accumulation of ions beyond the saturation concentration will immediately result in crystallization near the surface as efflorescence.
For fired-clay brick with only NaCl after some time as the drying slows down the Pe < 1 path is followed. As salt inhibitor is added this changes. In this case the EPD indicates that Pe>>1. Is due to the inhibitor. Because of dendritic crystal growth morphology in the presence of inhibitor, the effective surface area for evaporation
increases and hence the liquid velocity increases. The salt solution creeps along the branches of the dendrites and transports more and more dissolved salt ions toward the drying surface which was seen as efflorescence by the end of the drying experiment. In this case the concentration of the salt solution in the brick stays ate the initial concentration of 3M.
 

        Conclusion

From the droplet drying experiments it is concluded that the presence of inhibitor makes the drying process faster. Higher supersaturation and change in crystal morphology were observed. After attaining supersaturation the salt concentration returns to the equilibrium concentration (6.1 m). This clearly shows that the system goes from a metastable state to stable equilibrium state and NMR provides a sufficient signal-to-noise ratio to determine this transition. Besides this, a tremendous spreading of the salt crystals in the form of efflorescence was observed. This increase in efflorescence formation was related to the inhibitor concentration and resulted in elevating the drying rate.
From the brick drying experiments it is concluded that the presence of inhibitor changes the drying conditions near the material/air interface due to changes in crystal morphology. Dendrite crystals provide a much larger surface area for evaporation and advection becomes the governing phenomenon throughout the drying process. Advection of dissolved salt ions causes crystallization of salt near the drying surface as nondestructive efflorescence. As a result the concentration inside the material stays almost constant at the initial concentration. This indicates that ferrocyanide ions could potentially be effective against NaCl damage in buildingmaterials as they promote nondestructive efflorescence rather than destructive subflorescence.

 


S.Gupta, Sodium chloride crystallization in drying porous media: influence of inhibitor, Ph.D. thesis, Eindhoven University of Technology (2013)
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