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Ion transport in porous media studied by NMR
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 Department of Mathematics and Computing Science, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands
b
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 M NaCl solution in a calcium silicate brick and the drying of a 3 M NaCl 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 For studies of the moisture and salt transport in masonry it is important to measure the dynamic moisture and ion concentration profiles in a quantitative way. Often destructive methods are used. These methods usually involve drilling or grinding of the sample, gravimetrically determining its moisture content, and chemically determining its salt concentration. These methods have the disadvantage of destroying the sample, which complicates any time-dependent measurements, and usually lack sufficient resolution. It has been shown that Nuclear Magnetic Resonance (NMR) offers a powerful technique to measure the moisture profiles in a non-destructive way [1]. Here NMR is also used to measure the Na content, thereby offering the possibility to measure the combined ion and moisture transport in a nondestructive way. For the experiments described here a home-built NMR scanner is used, which incorporates an iron-cored electromagnet operating at a field of 0.78 T. In order to perform quantitative measurements a Faraday shield is placed between the tuned circuit of the probe head and the sample [2]. In order to measure quasi-simulta-
* Corresponding author. Tel.: ⫹31-40-243-2598; fax: ⫹31-40-2473406/247-4248. E-mail address: [email protected] (L. Pel).
neously both the moisture and Na profile a specially designed RF circuit is incorporated, by which the resonance frequency can be toggled with the help of a step motor between 33 MHz for the moisture measurements and 9 MHz for the Na measurements [3]. Because the sensitivity of chloride is very low, this ion was not considered in this study. A constant magnetic field gradient of up to 0.15 T/m was applied using Anderson coils, giving a one-dimensional resolution of the order of 2 mm for both water and Na. The spin-echo experiments were performed at a fixed frequency, corresponding to the centre of the RF coil. To measure all the unbound water and unbound Na the spin-echo time was kept as short as possible, i.e., 180 s for H and 250 s for Na. The profiles were measured by moving the sample with the help of a step motor with a resolution of 0.25 mm.
2. Experiments First the absorption of a 4 M NaCl solution into a calcium silicate brick was measured. In Fig. 1 the raw data are plotted. The local Na content of the porous material, which 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 absorption the ions will be transported into the material by advection. However, this process competes with the adsorp-
0730-725X/01/$ – see front matter © 2001 Elsevier Science Inc. All rights reserved. PII: S 0 7 3 0 - 7 2 5 X ( 0 1 ) 0 0 3 0 2 - 2
550
L. Pel et al. / Magnetic Resonance Imaging 19 (2001) 549 –550
Fig. 1. The moisture (⫹) and Na profiles (‚) measured during the absorption of a 4 M NaCl 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 observed near the wetting front, which indicates a very large adsorption of Na ions to the pore walls. A detailed interpretation of these measurements can be found in [3]. Secondly, a drying experiment was performed using a 3 M NaCl capillary saturated fired-clay brick. For this fired-clay brick almost no Na ions are adsorbed at the pore walls. Therefore during drying the ions will be transported to the surface by advection. However, this advection now competes 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 accumulation of the Na ions near the surface occurs. Here also the concentration of the salt solution has increased to almost 6 M, which is the saturation concentration. This indicates that salts will crystallize near the surface, which is also found in practice. After 2 days the drying rate decreases and so will the advection. Hence the diffusion will become dominant. After 15 days it was found that the ion profiles had been levelled off completely due to diffusion.
Fig. 2. The moisture (⫹) and Na profile (‚) measured after drying for 1 day of a 3 M NaCl capillary saturated fired-clay brick. The calculated NaCl concentration (E) from the moisture and Na profile. The dashed curves through the points are meant as guides to the eye.
Acknowledgements Part of this project was supported by the Dutch Technology Foundation (STW).
References [1] Gummerson RJ, Hall C, Hoff WD, Hawkes R, Holland GN, Moore WS. Unsaturated water flow within porous materials observed by NMR imaging. Nature 1979;281:56 –7. [2] Pel L. Moisture transport in porous building materials, Ph.D. thesis, Eindhoven University of Technology, the Netherlands, 1995. [3] Pel L, Kopinga K, Huinink HP, Kaasschieter EF. Saline absorption in calcium-silicate brick observed by NMR scanning. J Phys D Appl Phys 2000;33:1380 –5.
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 Department of Mathematics and Computing Science, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands
b
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 M NaCl solution in a calcium silicate brick and the drying of a 3 M NaCl 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 For studies of the moisture and salt transport in masonry it is important to measure the dynamic moisture and ion concentration profiles in a quantitative way. Often destructive methods are used. These methods usually involve drilling or grinding of the sample, gravimetrically determining its moisture content, and chemically determining its salt concentration. These methods have the disadvantage of destroying the sample, which complicates any time-dependent measurements, and usually lack sufficient resolution. It has been shown that Nuclear Magnetic Resonance (NMR) offers a powerful technique to measure the moisture profiles in a non-destructive way [1]. Here NMR is also used to measure the Na content, thereby offering the possibility to measure the combined ion and moisture transport in a nondestructive way. For the experiments described here a home-built NMR scanner is used, which incorporates an iron-cored electromagnet operating at a field of 0.78 T. In order to perform quantitative measurements a Faraday shield is placed between the tuned circuit of the probe head and the sample [2]. In order to measure quasi-simulta-
* Corresponding author. Tel.: ⫹31-40-243-2598; fax: ⫹31-40-2473406/247-4248. E-mail address: [email protected] (L. Pel).
neously both the moisture and Na profile a specially designed RF circuit is incorporated, by which the resonance frequency can be toggled with the help of a step motor between 33 MHz for the moisture measurements and 9 MHz for the Na measurements [3]. Because the sensitivity of chloride is very low, this ion was not considered in this study. A constant magnetic field gradient of up to 0.15 T/m was applied using Anderson coils, giving a one-dimensional resolution of the order of 2 mm for both water and Na. The spin-echo experiments were performed at a fixed frequency, corresponding to the centre of the RF coil. To measure all the unbound water and unbound Na the spin-echo time was kept as short as possible, i.e., 180 s for H and 250 s for Na. The profiles were measured by moving the sample with the help of a step motor with a resolution of 0.25 mm.
2. Experiments First the absorption of a 4 M NaCl solution into a calcium silicate brick was measured. In Fig. 1 the raw data are plotted. The local Na content of the porous material, which 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 absorption the ions will be transported into the material by advection. However, this process competes with the adsorp-
0730-725X/01/$ – see front matter © 2001 Elsevier Science Inc. All rights reserved. PII: S 0 7 3 0 - 7 2 5 X ( 0 1 ) 0 0 3 0 2 - 2
550
L. Pel et al. / Magnetic Resonance Imaging 19 (2001) 549 –550
Fig. 1. The moisture (⫹) and Na profiles (‚) measured during the absorption of a 4 M NaCl 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 observed near the wetting front, which indicates a very large adsorption of Na ions to the pore walls. A detailed interpretation of these measurements can be found in [3]. Secondly, a drying experiment was performed using a 3 M NaCl capillary saturated fired-clay brick. For this fired-clay brick almost no Na ions are adsorbed at the pore walls. Therefore during drying the ions will be transported to the surface by advection. However, this advection now competes 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 accumulation of the Na ions near the surface occurs. Here also the concentration of the salt solution has increased to almost 6 M, which is the saturation concentration. This indicates that salts will crystallize near the surface, which is also found in practice. After 2 days the drying rate decreases and so will the advection. Hence the diffusion will become dominant. After 15 days it was found that the ion profiles had been levelled off completely due to diffusion.
Fig. 2. The moisture (⫹) and Na profile (‚) measured after drying for 1 day of a 3 M NaCl capillary saturated fired-clay brick. The calculated NaCl concentration (E) from the moisture and Na profile. The dashed curves through the points are meant as guides to the eye.
Acknowledgements Part of this project was supported by the Dutch Technology Foundation (STW).
References [1] Gummerson RJ, Hall C, Hoff WD, Hawkes R, Holland GN, Moore WS. Unsaturated water flow within porous materials observed by NMR imaging. Nature 1979;281:56 –7. [2] Pel L. Moisture transport in porous building materials, Ph.D. thesis, Eindhoven University of Technology, the Netherlands, 1995. [3] Pel L, Kopinga K, Huinink HP, Kaasschieter EF. Saline absorption in calcium-silicate brick observed by NMR scanning. J Phys D Appl Phys 2000;33:1380 –5.