Thermal Activation of a Pure Montmorillonite Clay and Its Reactivity in Cementitious Systems

Clay minerals are potential candidates as raw materials for new supplementary cementitious materials (SCMs) that can partly replace Portland cement and thereby significantly reduce CO₂ emissions associated with cement production. We present the characterization of the complex, disordered structure of a pure montmorillonite clay heated at various temperatures (110–1100 °C), by solid-state ²⁷Al and ²⁹Si MAS NMR methods. The SiO₄ tetrahedra and AlO₆ octahedral sites become progressively more distorted, exhibit a significant degree of disorder upon dehydroxylation (600–800 °C), and do not lead to the formation of any metastable phase. At high temperatures (1000–1100 °C), the layer structure of the clay breaks down, forming stable crystalline phases. The chemical reactivity, measured as the degree of dissolution/precipitation in an alkaline solution, is found to be proportional to the degree of disorder/dehydroxylation. The maximum reactivity as a function of the heating temperature is achieved at 800 °C prior to the formation of inert, condensed Q⁴-type phases in the material. At maximum reactivity the calcium silicate hydrate (C-S-H) phase contains silicate chains with the highest aluminum incorporation, leading to blended cements containing a C-S-H phase with longer chain lengths. Most importantly, by exploiting the differential spin–lattice relaxation behavior of the ²⁹Si spins, evidence of multiple sites and components in both the naturally occurring and heated montmorillonite is being reported for the first time

Structural Investigation of Ye’elimite, Ca₄Al₆O₁₂SO₄, by ²⁷Al MAS and MQMAS NMR at Different Magnetic Fields

Ye’elimite is the principal component in calcium sulfoaluminate cement, which currently attracts significant attention, since it can be produced with lower CO₂ emissions compared to conventional portland cement. The crystal structure of ye’elimite is not well-established, and it has been proposed to exhibit cubic, tetragonal, and orthorhombic structures. The present work reports a comprehensive ²⁷Al magic-angle spinning (MAS) and multiple-quantum (MQ) MAS NMR study of ye’elimite, utilizing six magnetic fields from 4.7 to 22.3 T. These spectra are only compatible with the orthorhombic <i>Pcc</i>2 structure of ye’elimite, implying the presence of eight distinct AlO₄ sites. The ²⁷Al NMR spectra are convincingly simulated by eight distinct sites, and ²⁷Al quadrupole coupling parameters and isotropic chemical shifts are reported for the first time for ye’elimite. These parameters are a prerequisite for a reliable interpretation and quantification of ye’elimite in ²⁷Al NMR spectra of ye’elimite-based cements. Density functional theory (DFT) calculations are used in the assignment of the specific Al sites in ye’elimite. Structural relaxations by DFT, using two proposed <i>Pcc</i>2 structures as starting points, result virtually in the same set of optimized fractional atomic coordinates, which is proposed as a new refined structure for ye’elimite. The refined structure gives the best agreement between experimental and calculated ²⁷Al quadrupole tensor elements for the eight Al sites. Finally, the ³³S MAS NMR spectra for ye’elimite, monosulfate, and anhydrite are reported

Resolution of the Two Aluminum Sites in Ettringite by ²⁷Al MAS and MQMAS NMR at Very High Magnetic Field (22.3 T)

Ettringite (Ca₆[Al­(OH)₆]₂(SO₄)₃·26H₂O) is the first hydration product formed during Portland cement hydration. ²⁷Al MAS NMR has been used in a wide number of studies to detect and quantify ettringite in hydrated cement blends by the observation of a single, narrow resonance at 13–14 ppm. This work reports the first observation of resonances from two distinct Al sites in octahedral coordination for ettringite, employing ²⁷Al MAS and MQMAS NMR at an ultrahigh magnetic field (22.3 T). Thereby, the ²⁷Al NMR spectra are in agreement with the most accepted trigonal model for the ettringite structure. ²⁷Al quadrupole coupling parameters and isotropic chemical shifts for the two Al sites are determined from simulations and least-squares optimization of slow-speed ²⁷Al MAS NMR spectra of the satellite transitions. These data reveal that the local environments for the two octahedral Al sites are very similar, in accord with the most recent XRD refinements of the ettringite structure. Finally, the significant improvement in spectral resolution by the application of an ultrahigh magnetic field is illustrated by the detection of the two Al sites from ettringite in a hydrated cement mimicking the composition of a calcium sulfoaluminate cement

Dynamic Solid-State NMR Experiments Reveal Structural Changes for a Methyl Silicate Nanostructure on Deuterium Substitution

Structural characterizations of three different solid–gas reaction products, recently obtained from abraded solid-state silicate free radicals reacting with two isotopically enriched methane gases, ¹³CH₄ and CD₄ (a possible sink for methane on MARS) and with ¹³CO₂, are derived from various dynamic solid-state NMR experiments. These include cross-polarization/depolarization zero-cross times (ZCTs), variable temperature (VT) NMR to study 3-site jump CH₃/CD₃ activation energies (<i>E</i>_a), and ¹³CO₂/¹³CH₃ molecular species as a spy to determine the approximate diameters for the channel structures for some of these structures. Literature <i>E</i>_a data indicate that l-alanine and 4-CH₃-phenanthrene exhibit the highest known <i>E</i>_a values (= 20–22.6 kJ/mol) for CH₃ 3-site jump motions. The ZCTs for these two compounds are 120 and 162 μs, respectively, indicative of the high <i>E</i>_a values for CH₃/CD₃ groups. Determination of <i>E</i>_a for 4-CD₃-phenanthrene by low-temperature ²H MAS NMR experiments supplemented the previously reported liquid-state <i>E</i>_a value (<i>E</i>_a = 21 kJ/mol) for 4-CH₃-phenanthrene. Finally, such experiments also revealed the structural difference for the free-radical reaction products with ¹³CH₄ and CD₄, i.e, a change from helical to chain structure

Hydrogen Storage Capacity Loss in a LiBH₄–Al Composite

A detailed investigation of the decomposition reactions and decay in the hydrogen storage capacity during repeated hydrogen release and uptake cycles for the reactive composite LiBH₄–Al (2:3) is presented. Furthermore, the influence of a titanium boride, TiB₂, additive is investigated. The study combines information from multiple techniques: in situ synchrotron radiation powder X-ray diffraction, Sieverts measurements, simultaneous thermogravimetric analysis, differential scanning calorimetry and mass spectroscopy, solid-state magic-angle spinning nuclear magnetic resonance (MAS NMR), and Raman spectroscopy. The decomposition of LiBH₄–Al results in the formation of LiAl, AlB₂, and Li₂B₁₂H₁₂ via several reactions and intermediate compounds. The TiB₂ additive appears to have a limited effect on the decomposition pathway of the samples, but seems to facilitate formation of intermediate species at lower temperatures compared to the sample without additive. Solid solutions of Li_<i>x</i>Al_1–<i>x</i>B₂ or Al_1–<i>x</i>B₂ are observed during decomposition and from Rietveld refinement the composition of the solid solution is estimated to be Li_0.22Al_0.78B₂. The intercalation of Li in the AlB₂ structure is further investigated by ¹¹B and ²⁷Al MAS NMR spectra of the LiH-AlB₂ and AlB₂ samples (presented in Supporting Information). Hydrogen release and uptake for LiBH₄–Al reveals a significant loss in the hydrogen storage capacity, that is, after four cycles a capacity of about 45% remains, and after 10 cycles, the capacity is degraded to approximately 15% of the theoretically available hydrogen content. This capacity loss may be due to the formation of Li₂B₁₂H₁₂, as observed by ¹¹B MAS NMR and Raman spectroscopy. Formation of Li₂B₁₂H₁₂ has previously been observed during the decomposition of LiBH₄, but it has not been reported earlier in the LiBH₄–Al (2:3) system

An improvement of legislative base of market of insurance services is in Ukraine

У статті окреслено необхідність реформування діючого законодавства України в сфері функціонування страхового ринку, запропоновані основні заходи щодо покращення його стану.In the article outlined necessity of reformation of current legislation of Ukraine for the sphere of functioning of insurance market, basic measures are offered on the improvement of his state

Synthesis, Crystal Structure, Thermal Decomposition, and ¹¹B MAS NMR Characterization of Mg(BH₄)₂(NH₃BH₃)₂

A metal borohydride–ammonia borane complex, Mg­(BH₄)₂(NH₃BH₃)₂ was synthesized via a solid-state reaction between Mg­(BH₄)₂ and NH₃BH₃. Different mechanochemical reaction mechanisms are observed, since Mg­(BH₄)₂(NH₃BH₃)₂ is obtained from α-Mg­(BH₄)₂, whereas a mixture of Mg­(BH₄)₂(NH₃BH₃)₂, NH₃BH₃, and amorphous Mg­(BH₄)₂ is obtained from γ-Mg­(BH₄)₂. The crystal structure of Mg­(BH₄)₂(NH₃BH₃)₂ has been determined by powder X-ray diffraction and optimized by first-principles calculations. The borohydride groups act as terminal ligands, and molecular complexes are linked via strong dihydrogen bonds (<2.0 Å), which may contribute to the high melting point of Mg­(BH₄)₂(NH₃BH₃)₂ found to be ∼48 °C in contrast to those for other molecular metal borohydrides. Precise values for the ¹¹B quadrupole coupling parameters and isotropic chemical shifts are reported for the two NH₃BH₃ sites and two BH₄^– sites in Mg­(BH₄)₂(NH₃BH₃)₂ from ¹¹B MAS NMR spectra of the central and satellite transitions and MQMAS NMR. The ¹¹B quadrupole coupling parameters agree excellently with the electric field gradients for the ¹¹B sites from the DFT calculations and suggest that a more detailed structural model is obtained by DFT optimization, which allows evaluation of the dihydrogen bonding scheme

Trends in Syntheses, Structures, and Properties for Three Series of Ammine Rare-Earth Metal Borohydrides, M(BH₄)₃·<i>n</i>NH₃ (M = Y, Gd, and Dy)

Fourteen solvent- and halide-free ammine rare-earth metal borohydrides M­(BH₄)₃·<i>n</i>NH₃, M = Y, Gd, Dy, <i>n</i> = 7, 6, 5, 4, 2, and 1, have been synthesized by a new approach, and their structures as well as chemical and physical properties are characterized. Extensive series of coordination complexes with systematic variation in the number of ligands are presented, as prepared by combined mechanochemistry, solvent-based methods, solid–gas reactions, and thermal treatment. This new synthesis approach may have a significant impact within inorganic coordination chemistry. Halide-free metal borohydrides have been synthesized by solvent-based metathesis reactions of LiBH₄ and MCl₃ (3:1), followed by reactions of M­(BH₄)₃ with an excess of NH₃ gas, yielding M­(BH₄)₃·7NH₃ (M = Y, Gd, and Dy). Crystal structure models for M­(BH₄)₃·<i>n</i>NH₃ are derived from a combination of powder X-ray diffraction (PXD), ¹¹B magic-angle spinning NMR, and density functional theory (DFT) calculations. The structures vary from two-dimensional layers (<i>n</i> = 1), one-dimensional chains (<i>n</i> = 2), molecular compounds (<i>n</i> = 4 and 5), to contain complex ions (<i>n</i> = 6 and 7). NH₃ coordinates to the metal in all compounds, while BH₄^– has a flexible coordination, i.e., either as a terminal or bridging ligand or as a counterion. M­(BH₄)₃·7NH₃ releases ammonia stepwise by thermal treatment producing M­(BH₄)₃·<i>n</i>NH₃ (6, 5, and 4), whereas hydrogen is released for <i>n</i> ≤ 4. Detailed analysis of the dihydrogen bonds reveals new insight about the hydrogen elimination mechanism, which contradicts current hypotheses. Overall, the present work provides new general knowledge toward rational materials design and preparation along with limitations of PXD and DFT for analysis of structures with a significant degree of dynamics in the structures

(NH₄)₄Sn₂S₆·3H₂O: Crystal Structure, Thermal Decomposition, and Precursor for Textured Thin Film

Understanding the condensation of the dimeric thiostannate­(IV) [Sn₂S₆]^4– to SnS₂ is of key importance for the development of solution processing of advanced tin­(IV) sulfide based electronic devices such as photovoltaics (e.g., Cu₂ZnSnS₄, CZTSSe) and thin-film transistors. Here, we report the crystal structure of tetraammonium thiostannate­(IV) trihydrate ((NH₄)₄Sn₂S₆·3H₂O), which can be used as a more environmentally friendly alternative to the hydrazinium analogue in solution processed advanced tin­(IV) sulfide based electronic devices, e.g., CZTSSe. Hirshfeld surface analysis shows that crystal bound water molecules play a significant role in the structure and interact strongly with the sulfur atoms in the dimeric thiostannate­(IV) complex [Sn₂S₆]^4–. The thermal decomposition and corresponding condensation of ((NH₄)₄Sn₂S₆·3H₂O) to SnS₂ have been studied by TG/DSC-MS and solid-state ¹¹⁹Sn MAS NMR. It involves the formation of the relatively more condensed thiostannate­(IV) complex [Sn₄S₁₀]^4– at 90 °C via evaporation of ammonia, hydrogen sulfide, and water from the structure. With increasing temperature, more tin is transformed from tetrahedral to octahedral coordination, and at 220 °C, crystalline SnS₂ is formed. In an aqueous ammonium sulfide based solution, the structure of dimeric [Sn₂S₆]^4– is retained, and aqueous solutions of (NH₄)₄Sn₂S₆·3H₂O can be spin coated and thermally decomposed to form crystalline SnS₂ thin films. X-ray scattering techniques show that the solution processed SnS₂ thin film is highly textured with the <i>ab</i> plane parallel to the substrate. Furthermore, AFM and TEM reveal that the thin film is continuous and with an inherent porous surface structure from the gaseous formation byproducts