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Paradoxical drying due to salt crystallization

Gupta, Sonia and Huinink, Hendrik P. and Prat, Marc and Pel, Leo and Kopinga, Klass Paradoxical drying due to salt crystallization. (2014) Chemical Engineering Science, 109. 204-2011. ISSN 0009-2509

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Official URL: http://dx.doi.org/10.1016/j.ces.2014.01.023

Abstract

We investigated how salt crystallization inside a porous building material influences the formation of a receding drying front. Nuclear Magnetic Resonance (NMR) is used to measure non-destructively both hydrogen and dissolved sodium ions simultaneously during drying experiments. In this study we focused on the influence of NaCl on the drying. The results show that salt changes the drying process. At low relative humidities (RH ~ 0%), the drying rate of a brick saturated with NaCl solution is much lower than the drying rate of water saturated brick. Moreover, the presence of salt suppresses the development of a receding front. In this case homogenous drying of the material continues till very low saturation values. This is due to salt crystallization near the surface of the brick that causes blockage of the pores. This blockage reduces evaporation rate at the surface and allows maintaining a continuous hydraulic connection between the surface of the porous medium and the liquid present inside the material till low saturation values. Increasing the relative humidity at 55% and 70% RH with the brick saturated with the salt solution leads to a paradoxical situation with evaporation rates greater than for 0% RH. The paradox is explained by the impact of evaporation rate on the efflorescence microstructure, leading to the formation of a blocking crust for sufficiently high evaporation rates and non-blocking efflorescence for sufficiently low evaporation rates. The fundamental difference between the two types of efflorescence is demonstrated from a simple imbibition experiment. Using a simple continuum scale model of drying, critical moisture content was determined and all the essential features of the experimental results are validated. It is shown that critical moisture content can be very low in case of fired-clay brick due to the low threshold of the pore space, which is consistent with the relatively large pore size distribution of the fired-clay brick.

Item Type:Article
Additional Information:Thanks to Elsevier editor. The original publication is available at http://www.sciencedirect.com
HAL Id:hal-00999299
Audience (journal):International peer-reviewed journal
Uncontrolled Keywords:
Institution:French research institutions > Centre National de la Recherche Scientifique - CNRS (FRANCE)
Université de Toulouse > Institut National Polytechnique de Toulouse - INPT (FRANCE)
Université de Toulouse > Université Toulouse III - Paul Sabatier - UPS (FRANCE)
Other partners > Eindhoven University of Technology - TU/e (NETHERLANDS)
Laboratory name:
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Deposited By: Marc PRAT
Deposited On:03 Jun 2014 12:03

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