Exploiting in-situ solid-state NMR spectroscopy to probe the early stages of hydration of calcium aluminate cement

We report a high-field in-situ solid-state NMR study of the hydration of CaAl₂O₄ (the most important hydraulic phase in calcium aluminate cement), based on time-resolved measurements of solid-state ²⁷Al NMR spectra during the early stages of the reaction. A variant of the CLASSIC NMR methodology, involving alternate recording of direct-excitation and MQMAS ²⁷Al NMR spectra, was used to monitor the ²⁷Al species present in both the solid and liquid phases as a function of time. Our results provide quantitative information on the changes in the relative amounts of ²⁷Al sites with tetrahedral coordination (the anhydrous reactant phase) and octahedral coordination (the hydrated product phases) as a function of time, and reveal significantly different kinetic and mechanistic behaviour of the hydration reaction at the different temperatures (20 °C and 60 °C) studied.

A new NMR crystallographic approach to reveal the calcium local structure of atorvastatin calcium

We combine experimental and computational determination of ⁴³Ca solid-state NMR parameters (chemical shift tensors, quadrupolar coupling tensors, and Euler angles) to constrain the structure of the local calcium–ligand coordination environment.

Solid-state NMR and computational insights into the crystal structure of silicocarnotite-based bioceramic materials synthesized mechanochemically

In this work, we report the results of a detailed structural study of a promising bioceramic material silicocarnotite Ca₅(PO₄)₂SiO₄ (SC) synthesized from mechanochemically treated nanosized silicon-substituted hydroxyapatite by annealing at 1000°C. This novel synthetic approach represents an attractive and efficient route towards large-scale manufacturing of the silicocarnotite-based bioceramics. A combination of solid-state nuclear magnetic resonance (NMR), powder X-ray crystallography and density function theory (DFT) calculations has been implemented to characterize the phase composition of the prepared composite materials and to gain insight into the crystal structure of silicocarnotite. The phase composition analysis based on the multinuclear solid-state NMR has been found in agreement with X-ray powder diffraction indicating the minority phases of CaO (5-6wt%) and residual silicon-apatite (7-8wt%), while the rest of the material being a fairly crystalline silicocarnotite phase (86-88wt%). A combination of computational (CASTEP) and experimental methods was used to address the anionic site disorder in the silicocarnotite crystal structure. Distorted [OPO₃] pyramids have appeared as an important structural motif in the SC crystal structure. The ratio between regular [PO₄] and distorted [OPO₃] tetrahedra is found between 2:1 and 3:1 based on XRD experiments and CASTEP calculations. The natural abundance ⁴³Ca magic angle spinning NMR spectra of silicocarnotite are reported for the first time.

Structural roles of calcium in alkaline and alkaline-earth aluminosilicate glasses by solid-state 43Ca, 17O and 27Al NMR

Structural roles of Ca in aluminosilicate glasses have been investigated by solid-state ⁴³Ca, ¹⁷O, and ²⁷Al NMR spectroscopy. In 15Al₂O₃-55SiO₂-15CaO-15RO (R=Mg, Ca, Sr, and Na₂) (mol%) glass systems wherein half of the alkaline plus alkaline-earth cations charge-compensate (AlO₄)^‒ tetrahedra and the other half modify the glass network, with decreasing cation field strength (CFS) in the order of Mg²⁺>Ca²⁺>Sr²⁺>Na⁺, the isotropic chemical shift (δ_iso) of ⁴³Ca moves to a higher frequency and the quadrupolar coupling constant (P_Q) of ⁴³Ca decreases. The change in the δ_iso of ⁴³Ca is more sensitive to the role of Ca than that in the P_Q of ⁴³Ca. The two possible roles (network modifier and charge compensator) of Ca in the glass with R=Ca are not distinguished in the ⁴³Ca 3QMAS and 5QMAS spectra. The ¹⁷O 3QMAS results demonstrate that the cation with higher CFS (e.g., Mg²⁺ in the R=Mg glass and Ca²⁺ in the R=Na₂ glass) dominantly creates non-bridging oxygen, even though there is slight cation mixing. With increasing CFS in the glass, the P_Q of both ⁴³Ca and ²⁷Al also increases, indicating that the cation with higher CFS tends to degrade the structural symmetry.

On the crystal structure of the vaterite polymorph of CaCO3: a calcium-43 solid-state NMR and computational assessment

The vaterite polymorph of CaCO3 has puzzled crystallographers for decades in part due to difficulties in obtaining single crystals. The multiple proposed structures for the vaterite polymorph of CaCO3 are assessed using a combined (43)Ca solid-state nuclear magnetic resonance (SSNMR) spectroscopic and computational approach. A combination of improved experimental and computational methods, along with a calibrated chemical shift scale and (43)Ca nuclear quadrupole moment, allow for improved insights relative to our earlier work (Bryce et al., J. Am. Chem. Soc. 2008, 130, 9282). Here, we synthesize a (43)Ca isotopically-enriched sample of vaterite and perform high-resolution quadrupolar SSNMR experiments including magic-angle spinning (MAS), double-rotation (DOR), and multiple-quantum (MQ) MAS experiments at magnetic field strengths of 9.4 and 21.1T. We identify one crystallographically unique Ca(2+) site in vaterite with a slight distribution in both chemical shifts and quadrupolar parameters. Both the experimental (43)Ca electric field gradient tensor and the isotropic chemical shift for vaterite are compared to those calculated with the gauge-including projector-augmented-wave (GIPAW) DFT method in an attempt to identify the model that best represents the crystal structure of vaterite. Simulations of (43)Ca DOR and MAS NMR spectra based on the NMR parameters computed for a total of 18 structural models for vaterite allow us to distinguish between these models. Among these 18, the P3221 and C2 structures provide simulated spectra and diffractograms in best agreement with all experimental data.

Calcium-43 NMR studies of polymorphic transition of calcite to aragonite

Phase transformation between calcite and aragonite is an important issue in biomineralization. To shed more light on the mechanism of this process at the molecular level, we employ solid-state (43)Ca NMR to study the phase transformation from calcite to aragonite as regulated by magnesium ions, with (43)Ca enrichment at a level of 6%. Using the gas diffusion approach, the phase of Mg-calcite is formed initially and the system subsequently transforms to aragonite as the reaction time proceeds. Our (43)Ca solid-state NMR data support the dissolution-recrystallization mechanism for the calcite to aragonite transition. We find that the (43)Ca NMR parameters of Mg-calcite are very similar to those of pure calcite. Under the high-resolution condition provided by magic-angle spinning at 4 kHz, we can monitor the variation of the (43)Ca NMR parameters of the aragonite signals for the samples obtained at different reaction times. Our data suggest that in the presence of a significant amount of Mg(2+) ions, aragonite is the most stable polymorph of calcium carbonate. The initial precipitated crystallites of aragonite have spine-like morphology, for which the (43)Ca spin-lattice relaxation data indicate that the ions in the lattice have considerable motional dynamics. As the crystallinity of aragonite improves further, the (43)Ca T(1) parameter of the aragonite phase changes considerably and becomes very similar to that obtained for pure aragonite. For the first time, the difference in crystal morphologies and crystallinity of the aragonite phase has been traced down to the subtle difference in the motional dynamics at the molecular level.

A 43Ca and 13C NMR study of the chemical interaction between poly(ethylene-vinyl acetate) and white cement during hydration

(43)Ca and (13)C NMR methods were used to study the chemical interaction of poly(ethylene-vinyl acetate) (PEVAc) admixture in commercial-grade white cement. From (43)Ca NMR it is shown both that PEVAc induces modest changes in the hydrated cement structure, and that hydrated commercial cement is significantly more complex than models that have been used for its structure in past work. The (13)C NMR results show that the PEVAc hydrolysis occurs early in the cement hydration acceleration period, with a rate well-fit by an exponential decay using a time constant of 6±1 days.

A multinuclear solid-state magnetic resonance and GIPAW DFT study of anhydrous calcium chloride and its hydrates

The group 2 metal halides and corresponding metal halide hydrates serve as useful model systems for understanding the relationship between the electric field gradient (EFG) and chemical shift (CS) tensors at the halogen nuclei and the local molecular and electronic structure. Here, we present a 35/37Cl and 43Ca solid-state nuclear magnetic resonance (SSNMR) study of CaCl2. The 35Cl nuclear quadrupole coupling constant, 8.82(8) MHz, and the isotropic chlorine CS, 105(8) ppm (with respect to dilute NaCl(aq)), are different from the values reported previously for this compound, as well as those reported for CaCl2·2H2O. Chlorine-35 SSNMR spectra are also presented for CaCl2·6H2O, and when taken in concert, the SSNMR observations for CaCl2, CaCl2·2H2O, and CaCl2·6H2O clearly demonstrate the sensitivity of the chlorine EFG and CS tensors to the local symmetry and to changes in the hydration state. For example, the value of δiso decreases with increasing hydration. Gauge-including projector-augmented wave (GIPAW) density functional theory (DFT) calculations are used to substantiate the experimental SSNMR findings, to rule out the presence of other hydrates in our samples, to refine the hydrogen positions in CaCl2·2H2O, and to explore the isostructural relationship between CaCl2 and CaBr2. Finally, the 43Ca CS tensor span is measured to be 31(5) ppm for anhydrous CaCl2, which represents only the fifth CS tensor span measurement for calcium.