Over the last 10 years, there has been a growing interest in NMR
research on fluids confined in porous materials. The so-called
longitudinal (T1) and transverse (T2) relaxation times of hydrogen nuclei in a fluid
depend in principle on the pore size. Because we are interested in
the moisture transport through building materials, we look at the
hydrogen nuclei of the water molecules in the pores of the building
material. The interpretation of these NMR relaxation times in terms
of a pore-size distribution is called relaxometry and will be
briefly described below.
Figure 1: Intensity of the spin echo of a mortar sample and
bulk water as a function of the spin echo time.
In Fig. 1 the NMR signal intensity is plotted logarithmically as a
function of time for both bulk water and water in a mortar sample.
Mono-exponential relaxation behavior would show up as a straight
line in this plot. As can 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 these exponentials
are much shorter than the relaxation time of bulk water. To
understand this phenomenon, one has to look in more detail at the
relaxation behavior of water which is confined in the restricted
geometry of a porous material (like mortar).
Figure 2: Schematic representation of an isolated pore
with surface S and volume A. A water molucule travels in
random directions through the pore due to Brownian motion, which
is characterized by the self-diffusion coefficient D.
Consider an arbitrary isolated pore with volume V and surface S, as
shown in Fig. 2. During an NMR experiment, the water molecule and
therefore also the spin moment of the hydrogen nuclei move within
the pore due to Brownian motion. The velocity of this motion is
characterized by the self-diffusion coefficient. Solving the
equation which is describing the nuclear magnetization for this
where a is the typical pore size and rho is called the surface
relaxivity. The latter is a material constant.
It is also shown that this result will hold for a whole porous
material with a broad pore size distribution. In that case, the
magnetization is described as the sum of all individual decaying
pores: An Inverse Laplace transform, which can be performed
numerically, yields the relaxation-time distribution which can be
transformed into a pore-size distribution using the above shown
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
scale) 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 schematical picture of the pore structure of mortar.
The smallest relaxation time, hence pore size, is attributed to the
water in the gel pores. The highest relaxation time, hence pore
size, is attributed to the water in the capillary pores.
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 resolved.
- 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
R.M.E. Valckenborg, NMR on technological porous materials,
Ph.D. thesis, Eindhoven University of Technology, the
L. Pel, K. Kopinga and K. Hazrati, Water distribution and
porestructure in concrete as determined by NMR, 9. Feuchtetag,
Sept 17/18, Weimar, Germany, 294-300 (1997).
L. Pel, K. Hazrati, K.Kopinga and J. Machand, Water absorption
in mortar determined by NMR, Mag. Reson. Imaging 16,
K. Hazrati, Étude des mécanismes de transport de l'eau
absorption capillaire dans des matériaux cimentaires usuels et
de haute performance, Ph. D. thesis, Université Laval, Québec,
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