Paradoxical drying due to NaCl cystallization
and influence of inhibitor
We investigated how salt
crystallization inside a porous building material influences the
formation of a receding dryingfront. Initially,
the drying behavior of fired-clay brick samples vacuum saturated
with water and salt solution (3m NaCl) was studied. The samples
were dried at 0% RH and 1 l min-1 air flow rate. Fig. 1a and 1b
show the measured moisture profiles during drying of water and
salt saturated samples, respectively.
Fig 1 The measured moisture profiles for (a) water
saturated (b) 3m NaCl saturated fired-clay brick plotted
as a function of position. The profiles are given for every 0.45
h and 2.26 h for water and salt saturated bricks. The samples
were dried using dry air with a flow of 1 l min-1 and 0%
relative humidity. The drying surface is at 0 mm. The vertical
arrow shows the homogenous drying of the sample (externally
limited) and the horizontal arrow shows the penetration of the
receding drying front (internally limited).
In case of water saturated samples
the first few profiles are almost horizontal, representing the
first (externally limited) drying stage (shown by a vertical
arrow in fig. 1a). Afterwards, a drying front develops which
recedes below the sample surface (shown by a horizontal arrow
in fig. 1a). This represents the second (internally limited)
drying stage. The addition of salt changes the drying behavior
of the fired-clay brick (fig. 1b). Two effects were seen.
Firstly, the presence of NaCl reduces the drying rate compared
to the drying rate of water saturated fired-clay brick.
Complete drying of water saturated fired-clay brick took about
one day, in comparison to more than one week for NaCl
saturated fired-clay brick of the same dimensions and at the
same drying conditions. Secondly, the receding drying front
vanishes and homogenous drying of the material continued till
low saturation values. Hence, drying stage-1 prolongs and
homogeneous drying is maintained till low saturation values.
To investigate this, additional
drying experiments were performed on samples saturated with
salt solution at 55% and 70% relative humidity. The samples
were vacuum saturated with 3m NaCl solution and dried inside
NMR at room temperature and 1 l min-1 air flow rate. The
results are shown in fig. 2 rate of volume change (dV/dt) is
plotted as a function of moisture content.
Fig 2.The rate of
volume change (dV/dt) as a function of moisture content (m3m-3)
for salt saturated bricks
dried at different relative humidity conditions. The
dashed lines are a guide to the eye.
For the brick dried at 0% RH a
continuous decrease of the flux with decreasing moisture
content is seen. Thus, there is no constant rate period in
this case. However, at high humidities initially a constant
drying rate is maintained (stage-1) and later a falling drying
rate period is observed (stage-2). This leads to a paradoxical
drying situation since the evaporation rate is greater for 55%
RH and 70% RH than for 0% RH. Thus, in the presence of NaCl a
receding front develops again at high humidities. At the end
of the experiment, the efflorescence formed on the surface of
the fired-clay brick was collected and weighed. At 0% relative
humidity 6% -7% of the NaCl crystallized as efflorescence.
This efflorescence had the form of a very thin crust
on the outer surface of the fired-clay brick and was strongly
adhered to the substrate. It was not easy to remove the
efflorescence from the substrate. On the other hand, at 55%
and 70% RH, a significant amount of NaCl crystallized as
efflorescence. About 48% and 40% of the salt crystallized as
efflorescence at 55% and 70% RH, respectively. The type of
efflorescence formed at high humidities was rather fragile and
was easy to remove from the substrate by rubbing. Pictures of
the efflorescence formed on the surface of the materials are
shown in fig. 2.5. The efflorescence is clearly quite
different at 0% RH compared to the efflorescence at 55% and
70% RH. Tis suggests distinguishing two types of
efflorescence, referred to as “patchy” and “crusty” and
that can be referred to as well as “non-blocking” and
“blocking”. The efflorescence obtained at 0% RH is blocking
whereas the efflorescence at 55% and 70% RH is non-blocking.
Figure 3: Pictures of the efflorescence formed at the end of
drying experiment in the case
of 3m NaCl saturated brick dried at 0%, 55% and 70% relative
humidity. The amount of
efflorescence increases at higher humidities
Effect of inhibitor
In the absence of inhibitor after
approx. 15 hours, the saturation concentration was achieved in
the top few mm of the sample . This causes a dramatic drop in
the drying rate for salt saturated brick. Since, most of the
salt crystallized as sub-florescence, it causes a more severe
blockage of the pores near the drying surface. However, in the
presence of inhibitor the crystal morphology changes from
cubic to dendritic. The salt solution creeps along
the branches of the dendrites and transports more and more
dissolved salt ions towards the drying surface causing
the efflorescence observed at the end of drying experiment.
Pictures of the materials with efflorescence are shown in fig.
4. Approx. 26% and 69% of the salt crystallized as
efflorescence in the presence of 0.001 m and 0.01 m inhibitor
respectively. Because of the formation of efflorescence
in the presence of inhibitor the average salt ion
concentration inside the brick remained below saturation.
Therefore, the system remained open and less blockage occurred
compared to the salt saturated system without inhibitor. As a
consequence of this no dramatic drop in drying rate was seen
and the paradoxical disappears.
. Pictures of the efflorescence formed at the end
of a drying experiment in case ofsalt saturated brick with and
without inhibitor dried at 0% RH.
The amount of efflorescence increases significantly with
the addition of inhibitor.
In case of water saturated fired-clay brick
two drying stages were observed, i.e., a continuous drying rate
period followed by a receding drying front period. These results
are in accordance with the standard drying behavior of water
saturated porous media. However, at 0% RH, NaCl suppresses the
formation of a drying front. This is due to the extremely low
drying rate, which is mainly caused by pore blockage near the
drying surface. For NaCl salt saturated fired-clay brick the
evaporation rate is higher at high relative humidities and salt
ions crystallize as efflorescence on the surface of the brick.
Because of the higher evaporation rate the water transport cannot
be maintained. This leads to the penetration of front at high
humidities, also in the presence of salt. Hence, drying with
salt leads to a paradoxical situation in which increasing the
relative humidity in the external air and thus reducing the
external evaporation demand can increase the evaporation
rate. Sufficiently high evaporation rates lead to the
formation of blocking efflorescence (crust) whereas lower rates
can lead to non-blocking efflorescence. Addition of
inhibitor was found to be useful at low humidity conditions. At
low humidity, due to the crystal morphology in the presence of
inhibitor salt crystallizes as nondestructive efflorescence.
- An extensive description can be found in:
Sonia Gupta, Hendrik
P.Huinink, Marc Prat, Leo Pel, Klaas Kopinga, Paradoxical
drying of a fired-clay brick due to salt crystallization, Chemical Engineering
Science 109 204–211
Sonia Gupta, Hendrik P.
Huinink, Leo Pel, and Klaas Kopinga, How ferrocyanide
influences NaCl crystallization under different humidity
conditions, dx.doi.org/10.1021/cg4015459 Cryst. Growth
chloride crystallization in drying porous media: influence of
inhibitor, Ph.D. thesis, Eindhoven University of Technology (2013)
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