K. Kopinga

One-dimensional scanning of moisture in porous materials with NMR

A versatile and modular nuclear magnetic resonance (NMR) instrument is described that is particularly suited for the study of moisture transport in porous media such as various building materials in which moisture can give rise to several kinds of damages. Quantitative measurements of one‐dimensional moisture profiles and their time evolution can be performed on cylindrical samples having a diameter up to 20 mm with a spatial resolution better than 1 mm. Water absorption and drying experiments on various building materials demonstrate that the instrument can also be used for materials containing relatively large amounts of magnetic impurities, which until now were hardly accessible by NMR techniques.

Ion transport and crystallization in inorganic building materials as studied by nuclear magnetic resonance

Salt weathering is a major cause of deterioration of buildings and monuments of cultural heritage. We have determined the underlaying moisture and ion transport within a representative building material by measuring the time evolution of NaCl-saturated samples during one-sided drying using nuclear magnetic resonance. The measured NaCl concentration profiles reflect the competition between advection to the surface and redistribution by diffusion. By representing the measured moisture and NaCl profiles in an efflorescence pathway diagram (EPD) the crystallization is also taken into account. The pathways followed in the EPD indicate that for historical objects in general, crystallization at the surface cannot be avoided.

Determiination of moisture diffusivity in porous media using moisture concentration profiles

A procedure is presented to determine the moisture diffusivity for drying from measured moisture concentration profiles. From error analysis it is shown that the applied space grid for scanning and the one-dimensional resolution of the NMR (nuclear magnetic resonance) method used for the measurements had a minor influence on the accuracy of the determination of the moisture diffusivity. The inaccuracy was dominated by experimental noise and inhomogeneities in the porosity of the materials under investigation. A receding drying front method is presented to determine the moisture diffusivity at low moisture contents: here the moisture transport is dominated by vapour transport, which limits the overall drying rate. The method uses the velocity of the receding drying front to approximate the moisture diffusivity.

Combined NMR cryoporometry and relaxometry

Both cryoporometry and relaxometry probe the surface-to-volume ratio of a porous material. Nuclear magnetic resonance (NMR) relaxometry uses the random motion of molecules, whereas cryoporometry uses the melting-point depression of a confined liquid. An NMR setup has been built to simultaneously perform cryoporometry and relaxometry measurements. Using materials with a well-defined pore size, i.e. silica gels, both methods are compared with the standard N2-adsorption technique, and a good correlation is found. The methods can be used in the pore size range between 1 and 100 nm. By performing NMR relaxometry during cryoporometry, more information about the pore-size distribution can be obtained. Besides for silica gels, this is demonstrated for mortar, which has a complicated pore structure.

Salt transport and crystallization in porous building materials.

Salt weathering is a major cause of deterioration of porous building materials. To obtain information about the mechanisms underlying these damage processes we have studied the moisture and ion transport. We measured the time evolution of NaCl saturated samples of fired-clay brick during one-sided drying using Nuclear Magnetic Resonance. The moisture content and amount of dissolved Na ions could be measured quantitatively as a function of position. The NaCl concentration profiles obtained from these data reflect the competition between advection to the surface and redistribution by diffusion. By representing the measured moisture and NaCl profiles in an efflorescence pathway diagram (EPD) also the crystallization can be taken into account.

Saline absorption in calcium-silicate brick observed by NMR scanning

The absorption of a 4 M NaCl solution in calcium-silicate brick was investigated by nuclear magnetic resonance scanning. This method has the advantage that quasi-simultaneously both the moisture and the Na profile can be measured during absorption. It was found that during the absorption process the Na ions clearly stay behind and hardly any Na is present near the wetting front. This is caused by binding of the Na ions to the pore surface. It is shown that both the moisture and the Na profiles during absorption can be scaled using the Boltzmann-Matano transformation.

Water absorption in a fired-clay brick observed by NMR scanning

The water absorption of fired-clay brick has been investigated by nuclear magnetic resonance scanning of one-dimensional moisture concentration profiles. Because of the large amount of paramagnetic ions present in these materials, the hydrogen nuclei have a very short transverse relaxation time and a broad resonance line width. Therefore standard imaging techniques cannot be applied. This paper presents the first quantitative study on these materials using nuclear magnetic resonance. It is found that the overall behaviour of the moisture diffusivity for absorption of all investigated fired-clay bricks can be approximated by an exponential function of the moisture content.

Water absorption in mortar determined by NMR

Nuclear magnetic resonance (NMR) offers the possibility to determine moisture profiles in porous building materials. Moreover, the relaxation of the nuclear magnetic resonance signal can provide additional information on the water distribution in the microstructure. For mortar, it is shown that the transverse relaxation yields information on the distribution of water in the gel pores and capillary pores. Moisture profiles and relaxation were measured during water absorption. The effect of the drying treatment on the microstructure and the water absorption was investigated.

One-dimensional scanning of moisture in heated porous building materials with NMR

In this paper we present a new dedicated NMR setup which is capable of measuring one-dimensional moisture profiles in heated porous materials. The setup, which is placed in the bore of a 1.5 T whole-body scanner, is capable of reaching temperatures up to 500 °C. Moisture and temperature profiles can be measured quasi simultaneously with a typical time resolution of 2-5 min. A methodology is introduced for correcting temperature effects on NMR measurements at these elevated temperatures. The corrections are based on the Curie law for paramagnetism and the observed temperature dependence of the relaxation mechanisms occurring in porous materials. Both these corrections are used to obtain a moisture content profile from the raw NMR signal profile. To illustrate the methodology, a one-sided heating experiment of concrete with a moisture content in equilibrium with 97% RH is presented. This kind of heating experiment is of particular interest in the research on fire spalling of concrete, since it directly reveals the moisture and heat transport occurring inside the concrete. The obtained moisture profiles reveal a moisture peak building up behind the boiling front, resulting in a saturated layer. To our knowledge the direct proof of the formation of a moisture peak and subsequent moisture clogging has not been reported before.

Ischemia-induced ADC changes are larger than osmotically-induced ADC changes in a neonatal rat hippocampus model

Diffusion‐weighted imaging (DWI) is frequently used to diagnose stroke. However, the origin of the observed reduction in the apparent diffusion coefficient (ADC) in the acute phase following ischemia is not well understood. Although cell swelling is considered to play an important role, it is unclear whether this can completely explain the large ADC decrease. We developed a method to induce in neonatal rat hippocampal slices both osmotic perturbations, which lead to cell swelling, and oxygen/glucose deprivation (OGD), which simulates ischemia. A perfusion system was used to provide the hippocampal slices with nutrients and oxygen to maintain slice viability, which was verified with the use of fluorescent dyes (live/dead staining). Upon induction of OGD, the ADC decreased to ∼57% of the initial value within 2 hr. The ADC reduction cannot fully be explained by changes due to cell swelling, since these led only to a maximum decrease of ∼83%. Therefore, in addition to cell swelling, other changes must contribute significantly to the ADC reduction. Magn Reson Med 53:348–355, 2005.