Magnetic Resonance Imaging 19 (2001) 549 550
Ion transport in porous media studied by NMR
L. Pela,*, H.P. Huininka, K. Kopingaa, L.A. Rijniersa, E.F. Kaasschieterb
a
Department of Applied Physics, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands
b
Department of Mathematics and Computing Science, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands
Abstract
Moisture and salt transport in masonry can give rise to damages. Therefore a detailed knowledge of the moisture and salt transport is
essential for understanding the durability of masonry. A special NMR apparatus has been made allowing quasi-simultaneous measurements
of both moisture and Na profiles in porous building materials. Using this apparatus both the absorption of a 4 MNaCl solution in a calcium
silicate brick and the drying of a 3 MNaCl capillary saturated fired-clay brick have been studied. It was found that during the absorption
process the Na ions clearly stay behind, which this is caused by adsorption of these ions to the pore surface. For the drying it was found
that at the beginning of the drying process the ions accumulate near the surface. As the drying rate decreases, diffusion becomes dominant
and the ion profile levels off again. © 2001 Elsevier Science Inc. All rights reserved.
Keywords: NMR; Moisture transport; Ion transport
1. NMR Set-up neously both the moisture and Na profile a specially de-
signed RF circuit is incorporated, by which the resonance
For studies of the moisture and salt transport in masonry frequency can be toggled with the help of a step motor
between 33 MHz for the moisture measurements and 9 MHz
it is important to measure the dynamic moisture and ion
for the Na measurements [3]. Because the sensitivity of
concentration profiles in a quantitative way. Often destruc-
chloride is very low, this ion was not considered in this
tive methods are used. These methods usually involve dril-
study. A constant magnetic field gradient of up to 0.15 T/m
ling or grinding of the sample, gravimetrically determining
was applied using Anderson coils, giving a one-dimensional
its moisture content, and chemically determining its salt
resolution of the order of 2 mm for both water and Na. The
concentration. These methods have the disadvantage of de-
spin-echo experiments were performed at a fixed frequency,
stroying the sample, which complicates any time-dependent
corresponding to the centre of the RF coil. To measure all
measurements, and usually lack sufficient resolution. It has
the unbound water and unbound Na the spin-echo time was
been shown that Nuclear Magnetic Resonance (NMR) of-
kept as short as possible, i.e., 180 s for H and 250 s for
fers a powerful technique to measure the moisture profiles
Na. The profiles were measured by moving the sample with
in a non-destructive way [1]. Here NMR is also used to
the help of a step motor with a resolution of 0.25 mm.
measure the Na content, thereby offering the possibility to
measure the combined ion and moisture transport in a non-
destructive way. For the experiments described here a
2. Experiments
home-built NMR scanner is used, which incorporates an
iron-cored electromagnet operating at a field of 0.78 T. In
First the absorption of a 4 MNaCl solution into a cal-
order to perform quantitative measurements a Faraday
cium silicate brick was measured. In Fig. 1 the raw data are
shield is placed between the tuned circuit of the probe head
plotted. The local Na content of the porous material, which
and the sample [2]. In order to measure quasi-simulta-
is the property actually measured by NMR, is the product of
the Na concentration within the liquid in the pores, CNa, and
the moisture content , and therefore denoted by CNa . For
the moisture a sharp wetting front is observed. During the
* Corresponding author. Tel.: [phone]; fax: [phone]-
absorption the ions will be transported into the material by
3406/247-4248.
E-mail address: [e-mail] (L. Pel). advection. However, this process competes with the adsorp-
0730-725X/01/$  see front matter © 2001 Elsevier Science Inc. All rights reserved.
PII: S0730-725X(01)00302-2
550 L. Pel et al. / Magnetic Resonance Imaging 19 (2001) 549 550
Fig. 1. The moisture ( ) and Na profiles ( ) measured during the absorp-
tion of a 4 MNaCl solution. The dashed curves through the points are
meant as guides to the eye, whereas the times are given as an indication of
the elapsed time.
tion of Na to the pore walls. As can be seen, the Na profiles
clearly lag behind the moisture profiles and almost no Na is
Fig. 2. The moisture ( ) and Na profile ( ) measured after drying for 1 day
observed near the wetting front, which indicates a very large of a 3 MNaCl capillary saturated fired-clay brick. The calculated NaCl
concentration (E) from the moisture and Na profile. The dashed curves
adsorption of Na ions to the pore walls. A detailed inter-
through the points are meant as guides to the eye.
pretation of these measurements can be found in [3]. Sec-
ondly, a drying experiment was performed using a 3 M
NaCl capillary saturated fired-clay brick. For this fired-clay
Acknowledgements
brick almost no Na ions are adsorbed at the pore walls.
Therefore during drying the ions will be transported to the
Part of this project was supported by the Dutch Tech-
surface by advection. However, this advection now com-
nology Foundation (STW).
petes with the diffusion of the ions, which tends to level off
the accumulation of ions. In Fig. 2 an example is given of
the measured profiles after 1 day. Obviously, an accumula-
References
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