Probing the surface area of a cement-based material by nuclear magnetic relaxation dispersion

We show how nuclear magnetic spin-lattice relaxation dispersion of 1H water can provide a direct reliable value of the specific surface area of a cement-based material. The remarkable features of the relaxation dispersion support an interpretation in terms of coupled solid-liquid relaxation at pore interfaces, surface diffusion, and nuclear paramagnetic relaxation. The measurement is sufficiently fast to be applied continuously during the progressive hydration and setting of the material. This method is relevant to other chemically reactive porous media in chemical engineering and oil recovery.     

Proton spin-spin relaxation study of hydration of a model nanopore

The hydration pattern of controlled pore glass, with pore diameter of 237 Å, was investigated using nuclear magnetic resonance. Water proton spin–spin relaxation decay curves were monitored and modeled as two-component exponential decays as a function of hydration. The results are consistent with a geometric model involving a surface water layer and a bulk-like liquid fraction in the form of a plug. The amount of surface water increases as the sample hydrates, until hydration reached approximately a monolayer, at which point a water plug starts to form in the pore, and grow in length at the expense of the surface layer. The results are also analyzed in terms of, and compared to, a recently developed puddle pore-filling model [S.G. Allen, et al. J. Chem. Phys. 106 (1997) 7802–7809].     

The effect of silica nanoparticles on the pore structure of hydrating cement paste: a spatially resolved low-field NMR study

ABSTRACT

The hydration process of four different cement-based materials at different depths inside the sample was investigated using localised nuclear magnetic resonance relaxation measurements. All the samples are based on white Portland cement (CEM I 42.5 R), with a water-to-cement ratio of 0.5. One sample is a simple cement paste and the other three additionally contain 5% (by mass of cement) silica nanoparticles (50–200?nm size), silanised silica nanoparticles (with silane A 174) or silica fume (80% silica; 0.15?µm average size). The localised transverse relaxation measurements were performed using a single-sided NMR instrument. The relaxation rates were monitored for 48?h during the hydration process at different depths inside the sample. The results showed that the presence of nanoparticles influences not only the speed of hydration overall, but also the homogeneity of the materials.     

Monitoring the Air Influence on Cement–Lime Mortar Hydration Using Low-Field Nuclear Magnetic Resonance Relaxometry

In the present study, we investigate the relationship between the nuclear magnetic resonance relaxation rate and the hydration time in two types of masonry cement-lime mortar. The studies are performed with the mortars both in an enclosed and a standard atmosphere to monitor the air influence on cement-lime mortar hydration and setting. The constituents of the investigated mortar samples are: cement, slaked lime, sand and water. They were mixed to achieve a flow spread of 10?cm. These types of mortars are usually suitable for historical masonry maintenance, but they can also be used for modern buildings, or even for concrete structures coatings to prevent concrete carbonation. All nuclear magnetic resonance relaxation experiments were performed at 20?°C using a low-field nuclear magnetic resonance instrument operable at 20?MHz proton resonance frequency. A slowing down of the hydration kinetics is demonstrated for the samples kept in closed atmosphere conditions. The results contribute to the understanding of cement–lime mortar hydration, carbonation and setting under closed atmosphere conditions.     

Factors Affecting Early-Age Hydration of Ordinary Portland Cement Studied by NMR: Fineness,Water-to-Cement Ratio and Curing Temperature

The aim of the present study was to apply nuclear magnetic resonance (NMR) relaxation measurements for understanding the microstructure evolution of cement paste during hydration. Ordinary Portland cement powder was mixed with double-distilled water, and hydration process was analyzed via ¹H proton NMR spin–spin relaxation time. In order to induce strong modification of the rate of hydration, water-to-cement ratio, curing temperature and cement fineness were varied. The evolution of the NMR spin–spin relaxation time, T ₂, of hydrating water versus the hydration time was monitored from the very first few minutes after the mixing up to several hours. Authors' address: Marcella Alesiani, Department of Physics, University La Sapienza, Piazzale Aldo Moro 5, 00185 Rome, Italy     

What is the surface specific area of porous cement-based material? A nuclear magnetic relaxation dispersion approach

We propose a new NMR method to measure and follow the evolution of the surface specific area, Sp, of a porous cement-based material. This method, that does not require any preliminary drying process, uses nuclear magnetic relaxation dispersion (NMRD), the measurement of spin-lattice relaxation rate as a function of magnetic field strength or nuclear Larmor frequency. The method is applied for three different mortars samples prepared by mixing cement, sand, silica fume, water and superplasticizer with a water to cement ratio w/c=0.25, 0.38 and 0.65, respectively. The evolution of Sp grows linearly with the degree of advancement of chemical reactions measured by thermal heating and we evidence two relaxation processes independent of the w/c ratio.     

Anomalous surface diffusion of water compared to aprotic liquids in nanopores

1H nuclear magnetic relaxation dispersion experiments show remarkable differences between water and acetone in contact with microporous glass surfaces containing trace paramagnetic impurities. Analyzed with surface relaxation theory on a model porous system, the data obtained for water show that proton surface diffusion limited by chemical exchange with the bulk phase permits long-range effectively one-dimensional exploration along the pores. This magnetic-field dependence coupled with the anomalous temperature dependence of the relaxation rates permits a direct interpretation in terms of the proton translational diffusion coefficient at the surface of the pores. A universal rescaling applied to these data collected for different pore sizes and on a large variety of frequencies and temperatures, supports this interpretation. The analysis demonstrates that acetone diffuses more slowly, which increases the apparent confinement and results in a two-dimensional model for the molecular dynamics close to surface relaxation sinks. Surface-enhanced water proton diffusion, however, permits the proton to explore a greater spatial extent of the pore, which results in an apparent one-dimensional model for the diffusive motions of the water that dominate nuclear spin relaxation.     

Measurement of thermal diffusivity by magnetic resonance imaging

Nuclear magnetic resonance (NMR) may be used for monitoring temperature changes within samples based on measurements of relaxation times, the diffusion coefficient of liquids, proton resonance frequency or phase shifts. Such methods may be extended to the explicit measurement of the thermal diffusivity of materials by NMR imaging. A method based on measuring nuclear spin phase shifts or changes in the equilibrium nuclear magnetization has been developed for measuring transient thermal diffusion effects and thermal diffusivity with potential applications in NMR thermotherapy and materials science. In this method, a thermal pulse is applied to a medium, and the resultant temporal variations of the nuclear spin phase or of the magnitude of the nuclear magnetization produced by the thermal pulse are monitored at a spatial distance. The results obtained on common fluids agree well with the data from other methods.     

Micropore size analysis by NMR in hydrated cement

The understanding of the microstructure of cement remains incomplete. Especially, the progressive setting of the material is still unclear. Micropore size distribution (microstructure) has been investigated by both standard proton nuclear magnetic relaxation (1H-NMR) and field-cycling relaxation in C3S hydrated paste. The non-exponential decay was interpreted as a distribution of discrete relaxation rates. The attribution of T1 is supported by both a spectral and a dispersion curve analyses. These experiments allow us to follow the structuration of the material during setting.     

A field-cycling NMR investigation of resonant spin-lattice relaxation features arising from tunnelling methyl groups

(1)H nuclear spin-lattice relaxation has been investigated in sodium acetate trihydrate and sorbic acid using field-cycling NMR in the solid state. The relaxation is dominated by the reorientation of the methyl groups. Resonant features arising from coherent tunnelling are observed in both the magnetic field dependence of the spin lattice relaxation rate, T(1)(-1)(B(z)) and in the inverse temperature dependence, T(1)(-1)(1/T). The two systems have different barrier heights and tunnelling frequencies, providing different perspectives on the tunnel resonance phenomena. The magnetic field dependence enables different spectral density components to be separately investigated and in the carboxylic acid, sorbic acid, concerted proton transfer in the hydrogen bonds is also identified at low field and low temperature. The methyl hindering barriers and the correlation times characterising the reorientational dynamics has been accurately determined in both materials.     

Surface dynamics of liquids in porous media.

We report remarkable differences in the 1H nuclear magnetic relaxation dispersion data (NMRD) between water and other common aprotic solvents such as acetone when in contact with high surface area calibrated microporous chromatographic silica glasses that contain trace paramagnetic impurities located at or close to the pore surface. All these differences have been related to the particular chemical behaviors and dynamics of these liquids at the pore surface. We apply this technique to probe the structure and dynamics of water and oil at the surface of calibrated macroporous systems, where similar surface dynamics effects have been observed. This technique is also applied to follow the first hydration stage of a white cement-paste. Last, we present an analysis of the magnetic field dependence of 1H nuclear relaxation data to exhibit the microporosity of ultra high performance concretes.     

Application of field-cycling NMR relaxometry to the study of ultrasound-induced effects in the molecular dynamics and order of mesomorphic materials

A salient characteristic of nuclear magnetic resonance (NMR) techniques is the possibility to scan nuclear spin evolutions within a broad Larmor frequency range. Special instrumentation was developed to extend nuclear spin relaxation studies up to proton Larmor frequencies in the sub-kilohertz regime, a technique known as field-cycling NMR relaxometry. This article refers to an experimental version where the sample under study is selectively subjected to ultrasonic irradiation. The fact that ultrasound couples selectively to the collective dynamics of liquid crystals, offers new insights for the study of the molecular dynamics in these materials using NMR relaxation.     

Proton NMR relaxation as a probe for setting cement pastes

We report a 20-MHz proton nuclear magnetic resonance T1 relaxation study of cement paste hydration in the early stages of setting, using different centimeter-sized samples of cements of various origins and different water-to-cement ratios. In every sample, during the first few minutes of hydration, it is found that inverse Laplace processing of inversion-recovery measurements systematically exhibits at least two T1 values: a long one, around 100 ms, whose value correlates well with water content and which may be attributed to bulk water surrounding cement grains; and a short one, around 2 ms, which is quite insensitive to water-to-cement ratio and which may be attributed to water embedded in floculated cement grains before setting occurs. The time evolution of the longest T1 value for several hours is also shown to exhibit a characteristic five-stage behavior that is well correlated with known stages of the hydration process: initial reaction, induction period, acceleration period, deceleration period and slow hydration reaction. These results are compared with calorimetric measurements and electrical conductivity literature.     

Water dynamics in ionomer membranes by field-cycling NMR relaxometry

The dynamic behavior of water within two types of ionomer membranes, Nafion and sulfonated polyimides, has been investigated by field-cycling nuclear magnetic relaxation. This technique, applied to materials prepared at different hydration levels, allows to probe the proton motion on a time scale of the microsecond. The NMR longitudinal relaxation rate R(1) measured over three decades of Larmor angular frequencies omega is particularly sensitive to the host-water interactions and thus well suited to study fluid dynamics in restricted geometries. In the polyimide membranes, we have observed a strong dispersion of R(1)(omega) following closely a 1/sqrt[omega] law in a low-frequency range (correlation times from 0.1 to 10 micros). This is indicative of a strong interaction of water with "interfacial" hydrophilic groups of the polymeric matrix (wetting situation). On the contrary, in the Nafion, we observed weak variations of R(1)(omega) at low frequency. This is typical of a nonwetting behavior. At early hydration stages, the proton-proton inter-dipolar contribution to R(1)(omega) evolves logarithmically, suggesting a confined bidimensional diffusion of protons in the microsecond time range. Such an evolution is lost at higher swelling where a plateau related to 3D diffusion is observed.     

Theoretical estimates for proton-NMR spin–lattice relaxation rates of heterometallic spin rings

Heterometallic molecular chromium wheels are fascinating new magnetic materials. We reexamine the available experimental susceptibility data on MCr₇ wheels in terms of a simple isotropic Heisenberg Hamiltonian for M=Fe, Ni, Cu, and Zn and find in that FeCr₇ needs to be described with an iron–chromium exchange that is different from all other cases. In a second step we model the behavior of the proton spin lattice relaxation rate as a function of applied magnetic field for low temperatures as it is measured in nuclear magnetic resonance (NMR) experiments. It appears that CuCr₇ and NiCr₇ show an unexpectedly reduced relaxation rate at certain level crossings.     

The Effect of Curing Temperature on Early Hydration of Gray Cement Via Fast Field Cycling-NMR Relaxometry

In the present work, we use fast field cycling (FFC) nuclear magnetic resonance relaxometry to evaluate the influence introduced by the curing temperature on the hydration process of gray cement. The main advantage of FFC relaxometry as compared with other relaxation studies performed at a specific frequency is that it is sensitive to a wider range of molecular motions and better separates the surface and bulk contributions from the global measured relaxation rate. In the case of cement hydration, the relaxation process is dominated by the interaction of water protons with the paramagnetic centers located on the surface of cement grains. This allows us in the frame of a two-phase exchange model to monitor the temperature dependence of the transverse diffusional correlation time at the surface of cement grains. An increase of the surface diffusion coefficient of water molecules with the temperature was revealed. Another outcome is that the surface-to-volume ratio of capillary pores continuously increases during the early hydration and this process is strongly enhanced by rising the temperature.     

Agarose as a tissue equivalent phantom material for NMR imaging

Phantoms for evaluation of nuclear magnetic resonance (NMR) imaging systems were made from water-based agarose gels, according to a standard procedure herein described. Copper sulfate (CuSO4) was included in the gels to further affect their proton relaxation characteristics. The proton relaxation rates of each batch of gel are dependent on the concentrations of agarose and copper ions in it, with T1 depending more on copper than on agarose, and T2 depending more strongly on agarose than on copper. The wide range of T1 and T2 which can be covered, and the stability and physical characteristics of the agarose gel material make it well-suited for phantom use.     

The influence of superplasticizers on the first steps of tricalcium silicate hydration studied by NMR techniques

The influence of superplasticizer sulfonated naphthalene formaldehyde (SNF) on the hydration process of tricalcium silicate (C3S) paste was investigated by (1)H nuclear magnetic resonance spin-spin and spin lattice relaxation times. The addition of SNF superplasticizer to C3S paste clearly affects the morphology and growth rates of the hydration products, mainly by increasing the dormant period length, which lasts for several hours more than in conventional C3S hydrated paste, while reducing the acceleration period length. The relaxation data indicated that a pronounced delay occurs in the C3S hardening when sulfonated polymers are added to the makeup water. For all the analyzed samples, prepared with a water-to-C3S ratio of 0.4, the decay of the echo magnetization has been fitted by adopting both a monoexponential and a biexponential relaxation model in order to evaluate the contributions from water in different regimes of hydration.     

Xenon-131 surface sensitive imaging of aerogels in liquid xenon near the critical point

In recent years, optically pumped xenon-129 has received a great deal of attention as a contrast agent in gas-phase imaging. This report is about the other NMR active xenon isotope (i.e., xenon-131, S = 32) which exhibits distinctive features for imaging applications in material sciences that are not obtainable from xenon-129 (S = (1/2)). The spin dynamics of xenon-131 in gas and liquid phases is largely determined by quadrupolar interactions which depend strongly on the surface of the surrounding materials. This leads to a surface dependent dispersion of relaxation rates, which can be substantial for this isotope. The dephasing of the coherence due to quadrupolar interactions may be used to yield surface specific contrast for imaging. Although optical pumping is not practical for this isotope because of its fast quadrupolar relaxation, a high spin density of liquid xenon close to the critical point (289 K) overcomes the sensitivity problems of xenon-131. We report the first xenon-131 magnetic resonance images and have tested this technique on various meso-porous aerogels as host structures. Aerogels of different densities and changing levels of hydration can clearly be distinguished from the images obtained.     

Proton Spin–Lattice Relaxation Study of the Setting Reaction in Conventional and Resin-Modified Glass-Ionomer Dental Cements

The setting processes in KetacCem, AquaCem and Fuji I glass-ionomer dental cements (GIC) as well as in the resin-modified glass-ionomer dental cement (RM-GIC) Fuji Plus have been studied by proton spin–lattice, T ₁, and spin–spin, T ₂, relaxation. The setting time dependence of the degree of hydration was studied as well. In contrast to zinc oxide dental cements, the changes in T ₁ and T ₂ are determined by the interactions of the water with the internal surface. Proton nuclear magnetic resonance relaxation is thus suitable to follow the setting of GIC and RM-GIC and gives valuable information on the GIC hardening dynamics. Authors' address: Tomaz Apih, J. Stefan Institute, Jamova 39, Ljubljana 1000, Slovenia