|Description||Sodium chloride crystalli z ation in drying porous media: influence of inhibitor|
|BibTeX||[Gupta:2013]Author: Sonia Gupta|
Note: Salt weathering is one of the biggest threats to the long -term survival of porous building materials, historic structures and stone monuments worldwide. Soluble salts such as chloride, sulphate and nitrates are widely recognized as a cause of damage to porous building materials. Among the various salts present in building materials sodium chloride is one of the most widely distributed. The damage caused by NaCl crystallization inside porous building materials is ubiquitous. In order to stop/prevent damage caused by salt crystallization
, a better understanding of the moisture and ion transport processes inside
these building materials is required , as these processes governs the location of salt crystallization. In addition, effective treatment methods need to
be explored to stop/reduce the damage by salts. In this thesis, the
drying behavior of sodium chloride saturated porous media and the effectiveness of ferrocyanide ions as a preventive method to stop/reduce damage due to crystallization of NaCl has been studied. Ferrocyanide ions are tested as crystallization inhibitor against NaCl damage. The salt ion concentration was measured non - destructively during dynamic measurements using a specially designed NMR set-up.
Initially, the drying behavior of NaCl saturated porous media was studied and is
discussed in chapter 2. It has been shown that the presence of NaCl changes the standard drying behavior of porous media. In the presence of NaCl at
fast drying conditions no critical moisture content is present anymore
. It has been shown that the critical moisture content varies depending on the evaporation rate. In case of NaCl solution saturated fired-clay bricks,
at low humidity conditions a crust like efflorescence blocks
the drying surface and reduces the drying rate. This lower drying rate helps to maintain the hydraulic connections between the drying surface and the solution
which is still present inside the material and as a result no
critical moisture content is present. However, at high humidity conditions, the
non-blocking efflorescence which is formed keeps the system open. This maintains the initial drying rate, followed by the appearance of receding
drying front and a critical moisture content is observed again.
In chapter 3, the influence of inhibitor on the moisture/ion transport and salt
crystallization processes at low humidity conditions is explored.
In the presence of an inhibitora higher supersaturation and dendritic crystal morphology were found for bulk solutions of NaCl. If such a high
supersaturation can be attained inside a building material it can be dangerous for the material. Therefore, the use of inhibitor was tested inside real
porous materials. No supersaturation was attained inside these
materials. Dendritic crystals formed in the presence of inhibitor enhanced the drying rate.
As a result advection remained the governing process during drying and promoted salt crystallization outside the material as non-destructive efflorescence.
The effectiveness of inhibitor has also been test ed at different relative humidity conditions and is discussed in chapter 4. The use of inhibitor is more beneficial in preventing damage at low relative humidity conditions than at high humidity conditions.
At low humidity conditions the presence of inhibitor converts destructive sub
-florescence to non-destructive efflorescence. This is caused by the formation of dendritic NaCl crystals at the material/air interface. For such a case, advection remained the governing process and promotes salt crystallization
outside the material. At high humidity, due to the low vapor flux
at the surface of the material, the drying front is located at the surface of the material.
Therefore,during drying NaCl salt ions crystallize on the surface of the material as efflorescence, even in the absence of inhibitor. For such a case the addition of an inhibitor has no influence either on the formation of efflorescence or on moisture/ion transport processes within the material.
In view of the use of inhibitor in practice, a study was performed where inhibitor solution was sprayed on NaCl contaminated materials. The results are discussed in chapter 5.
It has been shown that for inhibitor to be effective, it is important to give sufficient time for the inhibitor molecules to diffuse into the material
over a sufficiently large distance. This can be done by covering the material after spraying with inhibitor solution.
Once the inhibitor penetrates into the material it will promote efflorescence
formation. The influence of inhibitor has been tested on salt mixtures and is discussed in chapter 6.
This study has been performed in bulk salt solution droplets.
In a salt mixture, NaCl does not significantly supersaturate in the presence of inhibitor, in contrast to single NaCl salt.
The crystal morphology is changed to more fluffy and dendritic for both single salt and salt mixture.
The possibility of NaCl to supersaturate upon repeated crystallization and dissolution cycles has been tested in salt solution droplets and is discussed in chapter 7. An increase in crystal size and supersaturation (1.16)
was seen after repeated cycles.
If such a high supersaturation can be attained inside a porous material it might be dangerous for the material. However, if such a supersaturation
actually occurs inside a porous media still needs to be checked
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crystallization of a solved salt
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