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Hydration of MgO-Based Cement: Water Dynamics by1H Fast Field-Cycling NMR Relaxometry
Full text link1H fast field-cycling (FFC) NMR relaxometry was applied for the first time to monitor the state of water during the hydration reaction of MgO and silica that leads to the formation of magnesium silicate hydrate (M-S-H), the binder phase of innovative cements with promising applications in the containment of radioactive waste. To this aim, water proton longitudinal relaxation rates (R1= 1/T1) were measured in the Larmor frequency range between 10 kHz and 30 MHz at different hydration times ranging from 0.5 h to â1⁄44 months. The obtained R1versus frequency (NMRD) curves were analyzed considering fast exchange of water molecules between a hydration layer, where dynamics is affected by interactions with the surface of solids present in the reacting mixture, and a bulk phase. For the hydration layer, water molecules undergoing fast local molecular dynamics on the surface gave a constant contribution to R1throughout the investigated frequency range. On the contrary, water molecules undergoing slow dynamics on the surface gave a dispersion of R1and their motions were modeled as "reorientations mediated by translational displacements" in the length scale of a particle and of a cluster of particles, where particles are silica nanoparticles and/or M-S-H globules that form during hydration. The model parameters reflected the different typical steps of cement hydration, showing smooth trends in the induction and diffusion steps and sudden changes during the nucleation and growth period in which water is consumed and M-S-H forms
Traditional Portland cement and MgO-based cement: a promising combination?
No full textMgO/SiO2cements are materials potentially very useful for radioactive waste disposal, but knowledge about their physico-chemical properties is still lacking. In this paper we investigated the hydration kinetics of cementitious formulations prepared by mixing MgO/SiO2and Portland cement in different proportions and the structural properties of the hydrated phases formed in the first month of hydration. In particular, the hydration kinetics was investigated by measuring the free water index on pastes by means of differential scanning calorimetry, while the structural characterization was carried out by combining thermal (DTA), diffractometric (XRD), and spectroscopic (FTIR,29Si solid state NMR) techniques. It was found that calcium silicate hydrate (C-S-H) and magnesium silicate hydrate (M-S-H) gels mainly form as separate phases, their relative amount and structural characteristics depending on the composition of the hydrated mixture. Moreover, the composition of the mixtures strongly affects the kinetics of hydration and the pH of the aqueous phase in contact with the cementitious materials. The results here reported show that suitable mixtures of Portland cement and MgO/SiO2could be used to modify the properties of hydrated phases with potential application in the storage of nuclear waste in clayey disposal
Effect of phosphate additives on the hydration process of magnesium silicate cements
No full textThe role of phosphate additives on the hydration process of magnesium silicate cement pastes was investigated through a multi-technique approach. A MgO/SiO2 mixture was hydrated for 28 days either in the absence or in the presence of sodium hexametaphosphate, trimetaphosphate or orthophosphate. Information on the kinetics of the hydration reaction was acquired by monitoring the free water index by means of differential scanning calorimetry, while the hydration products were thoroughly investigated by X-ray diffraction, thermogravimetric analysis, Fourier transform infrared spectroscopy, scanning electron microscopy and Si-29 solid-state nuclear magnetic resonance spectroscopy. The overall results provide new insight into the effect of phosphates on the hydration reaction and on the structure of magnesium silicate hydrate cements. All additives showed a plasticizing effect and promoted the formation of the binding phase magnesium silicate hydrate (M-S-H), without significantly altering its structure. Sodium orthophosphate was found to be by far the best-performing additive, even better than sodium hexametaphosphate, which is commonly used in these cementitious formulations. For the first time, P-31 solid-state NMR investigation allowed orthophosphate ion to be identified as the effective species
Injectable in situ gelling methylcellulose-based hydrogels for bone tissue regeneration
Full text link: Injectable bone substitutes (IBSs) represent a compelling choice for bone tissue regeneration, as they can be exploited to optimally fill complex bone defects in a minimally invasive manner. In this context, in situ gelling methylcellulose (MC) hydrogels may be engineered to be free-flowing injectable solutions at room temperature and gels upon exposure to body temperature. Moreover, incorporating a suitable inorganic phase can further enhance the mechanical properties of MC hydrogels and promote mineralization, thus assisting early cell adhesion to the hydrogel and effectively guiding bone tissue regeneration. In this work, thermo-responsive IBSs were designed selecting MC as the organic matrix and calcium phosphate (CaP) or CaP modified with graphene oxide (CaPGO) as the inorganic component. The resulting biocomposites displayed a transition temperature around body temperature, preserved injectability even after loading with the inorganic components, and exhibited adequate retention on an ex vivo calf femoral bone defect model. The addition of CaP and CaPGO promoted the in vitro mineralization process already 14 days after immersion in simulated body fluid. Interestingly, combined X-ray diffraction and solid state nuclear magnetic resonance characterizations revealed that the formed biomimetic phase was constituted by crystalline hydroxyapatite and amorphous calcium phosphate. In vitro biological characterization revealed the beneficial impact of CaP and CaPGO, indicating their potential in promoting cell adhesion, proliferation and osteogenic differentiation. Remarkably, the addition of GO, which is very attractive for its bioactive properties, did not negatively affect the injectability of the hydrogel nor the mineralization process, but had a positive impact on cell growth and osteogenic differentiation on both pre-differentiated and undifferentiated cells. Overall, the proposed formulations represent potential candidates for use as IBSs for application in bone regeneration both under physiological and pathological conditions
Dynamics of two glass forming monohydroxy alcohols by field cycling 1H NMR relaxometry
Full text linkThe dynamics of 2,2-dimethylbutan-1-ol (2,2-DM-1-B) and 3,3-dimethylbutan-1-ol (3,3-DM-1-B), two glass forming monohydroxy alcohols, was investigated by field cycling1H NMR relaxometry in their liquid phase, including the supercooled regime. Nuclear Magnetic Relaxation Dispersion (NMRD) curves (i.e. longitudinal relaxation rate R1vs1H Larmor frequency), acquired for the two alcohols at different temperatures in the 0.01-35 MHz frequency range, were analyzed in terms of suitable models for internal motions, overall molecular reorientations, and molecular self-diffusion, and the corresponding correlation times were quantitatively determined. In addition, trends of1H R1as a function of the frequency square root at low frequencies, where the contribution of translational motions dominates, were exploited to achieve an independent determination of the self-diffusion coefficients (D), which does not require the separation of different motional contributions to relaxation. Good agreement was found between D values determined by the two methods, thus corroborating the model used for the description of the NMRD curves. Self-diffusion was found to be slower and more strongly temperature dependent for 2,2-DM-1-B with respect to 3,3-DM-1-B, whereas molecular reorientations were quite similar for the two isomeric alcohols. Correlation times for molecular reorientations were found to be at least one order of magnitude shorter than those reported in the literature for the Debye-like relaxation observed by dielectric spectroscopy in the liquid phase
Unveiling CO2 Dynamics in Perfluorinated Cerium-Based Metal–Organic Frameworks with UiO-66 and MIL-140 Topologies by Solid State NMR
No full textThe dynamics of CO2 was investigated in two ultramicroporous perfluorinated metal-organic frameworks (MOFs), F4_MIL-140A(Ce) and F4_UiO-66(Ce), that share the same linker (tetrafluoroterephthalate) and metal (Ce-IV) but have different topologies. F4_MIL-140A(Ce) displays an S-shaped CO2 adsorption isotherm associated with an adsorption mechanism including CO2 interaction with the Ce-IV open metal site and a structural rearrangement of the linkers. F4_UiO-66(Ce), belonging to one of the most investigated MOF families in the literature, shows a Langmuir-like CO2 adsorption isotherm hinting at no specific strong interactions with the framework. The structural factors influencing CO2 adsorption properties in these systems were found to affect CO2 dynamics, as revealed by line shape analysis of C-13 static solid state NMR spectra of C-13 isotopically enriched CO2 ((CO2)-C-13) adsorbed in the MOFs (1 atm) as well as by the analysis of C-13 longitudinal relaxation times (T-1), at different temperatures. The evolution of the spectral line shapes of (CO2)-C-13 in F4_MIL-140A(Ce) clearly indicated anisotropic dynamics of the gas molecules in the framework, which can be described as a localized wobbling motion on the adsorption site combined with translational hopping from one site to another in the MOF channels. This scenario was corroborated by C-13 T-1 analysis in terms of a relaxation mechanism governed by CO2 reorientations that modulate chemical shift anisotropy. Rates and activation energies were determined for the two motions. On the other hand, spectral line shape and C-13 T-1 analyses of (CO2)-C-13 in F4_UiO-66(Ce) indicated a fast isotropic reorientational motion, reflecting weak gas/framework interactions and high symmetry of the MOF cavities, and allowed the activation energy for the motion to be determine
Studio EPR di membrane tilacoidali sottoposte a stress ossidativo
No full textElectron paramagnetic resonance (EPR) spectroscopy was applied to the study of fluidity in thylakoids of
durum wheat (Triticum durum Desf. cv. Creso) treated with oxygen free radicals. The radicals were generated
by using Fe2+-EDTA and ascorbic acid and let to act either on thylakoid membranes isolated from leaves
or on leaves themselves, from which thylakoids were subsequently extracted. The oxidative treatment was
prolonged for periods ranging from 15 min to 3 h. Line shape analysis of EPR spectra recorded as a function
of temperature on concentrated suspensions of thylakoids labeled with 5-doxylstearic acid (5-DSA) allowed
to get information about the fluidity of differently treated membranes. An axially symmetric rotational diffusion
model, defined by the components of the rotational tensor Dpar and Dperp and by the diffusion tilt angle õ, was
assumed for the dynamics of the spin probes. Good agreement with experimental spectra was obtained with
a diffusion tilt angle continuously increasing with temperature and a high anisotropy ratio N ) Dpar/Dperp of
the spin probe rotational diffusion. High N values have been considered as an indication for the immobilization
of the spin probes in the hydrophobic part of the membranes. The changes of õ with temperature have been
interpreted in terms of chain isomerization within the probes. The differences found between the probe dynamics
in the differently treated thylakoids have been discussed on the basis of the time and type (on the leaves or
on the membranes) of oxidative treatment
Studio della Fluidità di membrane tilacoidali sottoposte a stress ossidativo
No full textElectron paramagnetic resonance (EPR) spectroscopy was applied to the study of fluidity in thylakoids of
durum wheat (Triticum durum Desf. cv. Creso) treated with oxygen free radicals. The radicals were generated
by using Fe2+-EDTA and ascorbic acid and let to act either on thylakoid membranes isolated from leaves
or on leaves themselves, from which thylakoids were subsequently extracted. The oxidative treatment was
prolonged for periods ranging from 15 min to 3 h. Line shape analysis of EPR spectra recorded as a function
of temperature on concentrated suspensions of thylakoids labeled with 5-doxylstearic acid (5-DSA) allowed
to get information about the fluidity of differently treated membranes. An axially symmetric rotational diffusion
model, defined by the components of the rotational tensor Dpar and Dperp and by the diffusion tilt angle õ, was
assumed for the dynamics of the spin probes. Good agreement with experimental spectra was obtained with
a diffusion tilt angle continuously increasing with temperature and a high anisotropy ratio N ) Dpar/Dperp of
the spin probe rotational diffusion. High N values have been considered as an indication for the immobilization
of the spin probes in the hydrophobic part of the membranes. The changes of õ with temperature have been
interpreted in terms of chain isomerization within the probes. The differences found between the probe dynamics
in the differently treated thylakoids have been discussed on the basis of the time and type (on the leaves or
on the membranes) of oxidative treatment
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