From: "Roland Valckenborg" <R.M.E.Valckenborg@tue.nl>
Sent: Thursday, October 04, 2001 2:28 AM
Nuclear Magnetic Resonance (NMR) is a very powerful technique. In the research described in this thesis, NMR is used to study technological porous materials, with emphasis on building materials. Since these materials contain a large amount of magnetic impurities, the research leads to a lot of challenging physical problems.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 are influenced by the porous material. Water is the most studied fluid, but also research of oil in rock and of fat in the human body is performed. Very frequently studied porous materials are: biological tissue, rock, plants, heterogeneous catalysts, clay, and building materials. The latter materials have our special attention, because we would like to describe the moisture transport within them. With this knowledge, the most important damage processes can be understood. Moisture in a building material can lead to frost damage, salt crystallization, and growth of algae. NMR Imaging (MRI) is the best imaging technique for this moisture.
The building materials described in this thesis are mortar, clay, and fired-clay brick. Mortar differs from clay in various respects. The pore structure of mortar contains relatively small pores (1-100 nm), which are badly interconnected, whereas clay contains relatively large pores (100 nm - 10 µm), which are interconnected very well. The NMR relaxation times T1 and T2 depend in principle on the pore size. One would therefore expect shorter relaxation times for mortar than for clay. However, the NMR dephasing behavior, caused by magnetic impurities in clay, shortens T2 to such an extent that this relaxation time for clay is on the same order as for mortar. The interpretation of relaxation times in terms of a pore-size distribution is called relaxometry and is not trivial for the materials we used, but leads to the research as described in this thesis.
Another technique to determine the pore-size distribution is cryoporometry, which measures the volume-to-surface ratio, just as the relaxometry technique. This technique is based on the fact that the melting point of a fluid confined in a porous material is increasingly decreased for smaller pores. To measure this so-called melting-point depression, a specialized NMR setup has been built including a cryostat, that can control the temperature of the sample for a long period (2 days) within a range of -100 °C to room temperature. Measurements on a series of silica-gel samples with well-known pore-size distributions showed a good correlation between the results of relaxometry and cryoporometry. Next, a combined measurement was performed on mortar, to study the complex pore structure of this material. From these measurements it appears that a layer of water is present on the pore surface. Also the dense-gel and open-gel pores can be distinguished. Apart from that, the water in the capillary pores is clearly discernible from the water in the gel pores, because of the low melting-point depression.
The same mortar was used for a spatially resolved one-dimensional drying experiment, in which relaxometry was performed simultaneously. This experiment demonstrates the big advantage of a relaxation-weighted MRI-experiment: it can distinguish the water in the large (capillary) pores from the water in the small (gel) pores. It is concluded that the water in the gel pores cannot be extracted from mortar during a drying experiment at room temperature with a relative humidity of maximum 5 %, in contrast to the water in the capillary pores that does evaporate.
The above mentioned interpretation of the relaxometry results in terms of a pore-size distribution is one of the many examples which lead to the earlier mentioned research on the dephasing behavior. NMR Hahn and CPMG spin-echo measurements showed such a complex spin-echo decay, that it was decided to develop a numerical model. In this model, the magnetic moments of nuclei accumulate a certain phase, depending on their random trajectory through an inhomogeneous magnetic field. The results of this numerical model for classical situations, e.g., free diffusion in a uniform magnetic-field gradient) are in perfect agreement with the well-known analytical solutions. Therefore, we continued with simulations of the dephasing behavior of nuclear spins in a spherical pore. First, a uniform gradient was assumed in this pore. Using these simulations the existing theory, which predicts two asymptotical dephasing regimes (`motional averaging' and `localization'), has been verified and extended with an `intermediate regime'. Next, in the model a dipolar magnetic field was created inside the pore, by putting a magnetic point-dipole in the solid matrix surrounding the pore. The decay of the simulated spin-echoes can be predicted with scaling laws. For the `motional averaging regime' with a homogeneously distributed nuclear magnetization in the pore, this relation could be derived directly from existing theories. However, for the `localization regime' with an inhomogeneously distributed magnetization, these scaling laws were derived from basic principles by ourselves.
The NMR spin-echo measurements on a series of magnetically doped clay samples are explained using a uniform magnetic-field gradient inside a pore. The dephasing regime was determined and even a rough estimate of the value of this internal gradient could be obtained. Further analysis of Hahn and CPMG spin-echo measurements with a much better signal-to-noise ratio revealed that deviations occur compared to the mono-exponential decay observed for the uniform gradient. The measured spin-echo decay, however, agrees with the results of the model of a dipolar magnetic field inside a pore. Therefore, in the model, the magnetic impurities are described as additional magnetic point-dipoles with a strength depending on the doping fraction. The experimental results agree well with the predictions from this model. This holds for both Hahn and CPMG measurements, up till spin-echo times for which the diffusion length becomes comparable to the pore size. In conclusion, it can be posed that this thesis provides the limitations for interpreting NMR relaxation-time measurements of a water-saturated porous building material in terms of a pore-size distribution.
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