Over the last 10 years, there has been a growing interest in NMR
on fluids confined in porous materials. The so-called longitudinal (T1)
and transverse (T2) relaxation times of hydrogen
in a fluid depend in principle on the pore size. Because we are
in the moisture transport through building materials, we look at the
nuclei of the water molecules in the pores of the building material.
interpretation of these NMR relaxation times in terms of a pore-size
is called relaxometry and will be briefly described below.
Figure 1: Intensity of the spin echo of a mortar sample and bulk
as a function of the spin echo time.
In Fig. 1 the NMR signal intensity is plotted logarithmically as a
of time for both bulk water and water in a mortar sample.
relaxation behavior would show up as a straight line in this plot. As
be seen, the relaxation behavior of the bulk water is mono exponential
(with a very slow decay). On the other hand, the relaxation behavior of
the water in the mortar sample is not mono exponential at all, but was
found to be double exponential. Moreover, the relaxation times of
exponentials are much shorter than the relaxation time of bulk water.
understand this phenomenon, one has to look in more detail at the
behavior of water which is confined in the restricted geometry of a
material (like mortar).
Figure 2: Schematic representation of an isolated pore with
S and volume A. A water molucule travels in random directions
the pore due to Brownian motion, which is characterized by the
Consider an arbitrary isolated pore with volume V and surface S, as
in Fig. 2. During an NMR experiment, the water molecule and therefore
the spin moment of the hydrogen nuclei move within the pore due to
motion. The velocity of this motion is characterized by the
coefficient. Solving the equation which is describing the nuclear
for this situation yields:
where a is the typical pore size and rho is called the surface
The latter is a material constant.
It is also shown that this result will hold for a whole porous
with a broad pore size distribution. In that case, the magnetization is
described as the sum of all individual decaying pores: An Inverse
transform, which can be performed numerically, yields the
distribution which can be transformed into a pore-size distribution
the above shown equation.
Figure 3: Pore water distribution of mortar determined from NMR
relaxation measurements. The dotted lines show the result from a double
exponential fit. The inset shows a schematic representation of mortar.
Figure 3 shows both the relaxation-time distribution (bottom x-axis
and the pore-size distribution (top x-axis scale) of the magnetization
measurement in mortar as shown in Fig. 1. The inset of Fig.3 gives a
picture of the pore structure of mortar. The smallest relaxation time,
hence pore size, is attributed to the water in the gel pores. The
relaxation time, hence pore size, is attributed to the water in the
This technique can be applied to all porous building materials. One
has to keep in mind that for different materials a different rho has to
be determined. This relaxometry can be done spatially resolved and time
An extensive description can be found in:
R. Valckenborg, L.Pel, K. Hazrati, K.Kopinga and J. Marchand, Pore
water distribution in mortar during drying as determined by NMR, Materials and Structures 34, 599-604
R.M.E. Valckenborg, NMR on technological porous materials, Ph.D.
Eindhoven University of Technology, the Netherlands (2001).
L. Pel, K. Kopinga and K. Hazrati, Water distribution and
in concrete as determined by NMR, 9. Feuchtetag, Sept 17/18, Weimar,
L. Pel, K. Hazrati, K.Kopinga and J. Machand, Water absorption in
determined by NMR, Mag. Reson. Imaging 16, 525-528,
K. Hazrati, Étude des mécanismes de transport de
absorption capillaire dans des matériaux cimentaires usuels et
haute performance, Ph. D. thesis, Université Laval,
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