Effects of quartz particle size and water-to-solid ratio on hydrothermal synthesis of tobermorite studied by in-situ time-resolved X-ray diffraction

Hydrothermal synthesis process of tobermorite (5CaO·6SiO₂·5H₂O) has been investigated by in-situ X-ray diffraction using high-energy X-rays from a synchrotron radiation source in combination with a purpose-build autoclave cell. Dissolution rates of quartz were largely affected by its particle size distribution in the starting mixtures. However, the composition (Ca/Si) of non-crystalline C-S-H at the start of tobermorite formation was identical regardless of the quartz dissolution rate. An effect of water-to-solid ratio (w/s) was investigated for samples using fine particle quartz. Tobermorite did not occur with w/s of 1.7 but occurred with w/s higher than 3.0. Surprisingly, however, the dissolution curves of quartz were nearly identical for all samples with w/s from 1.7 to 9, indicating that the dissolution rate is predominated by surface area. Possible reaction mechanism for tobermorite formation will be discussed in terms of Ca and/or silicate ion concentration in the liquid phase and distribution of Ca/Si in non-crystalline C-S-H.     

Aluminum 2-Methoxyethoxide: An Internally Coordinated Aluminum Alkoxede

Abstract

Aluminum 2-methoxyethoxide was isolated and characterized by ¹H, ¹³C and ²⁷Al NMR. The ²⁷Al NMR and mass spectra show that the compound is an internally coordinated dimer. This aluminum alkoxide is less susceptible to hydrolysis in comparison to other aluminum alkoxides.     

Solid-state Al and Si NMR characterization of hydrates formed in calcium aluminate-silica fume mixtures

Partially deuterated Ca₃Al₂(SiO₄)_y(OH)_12−4y-Al(OH)₃ mixtures, prepared by hydration of Ca₃Al₂O₆ (C₃A), Ca₁₂Al₁₄O₃₃ (C₁₂A₇) and CaAl₂O₄ (CA) phases in the presence of silica fume, have been characterized by ²⁹Si and ²⁷Al magic-angle spinning-nuclear magnetic resonance (MAS-NMR) spectroscopies. NMR spectroscopy was used to characterize anhydrous and fully hydrated samples. In hydrated compounds, Ca₃Al₂(OH)₁₂ and Al(OH)₃ phases were detected. From the quantitative analysis of ²⁷Al NMR signals, the Al(OH)₃/Ca₃Al₂(OH)₁₂ ratio was deduced. The incorporation of Si into the katoite structure, Ca₃Al₂(SiO₄)_3−x(OH)_4x, was followed by ²⁷Al and ²⁹Si NMR spectroscopies. Si/OH ratios were determined from the quantitative analysis of ²⁷Al MAS-NMR components associated with Al(OH)₆ and Al(OSi)(OH)₅ environments. The ²⁹Si NMR spectroscopy was also used to quantify the unreacted silica and amorphous calcium aluminosilicate hydrates formed, C-S-H and C-A-S-H for short. From ²⁹Si NMR spectra, the amount of Si incorporated into different phases was estimated. Si and Al concentrations, deduced by NMR, transmission electron microscopy, energy dispersive spectrometry, and Rietveld analysis of both X-ray and neutron data, indicate that only a part of available Si is incorporated in katoite structures.     

Gelation of Aluminosilicate Systems Under Different Chemical Conditions

Optically clear aluminosilicate gels of different chemical compositions (0–0.9 mole ratios of total Al/(Si + Al)) were prepared directly from solutions of inorganic aluminum salts, tetraethoxysilane, water and alcohol without the time-consuming sol forming. However, in these gels only 0–75% of total Al content was incorporated by chemical bonding into the gel network depending on the compositions of gels and the preparation conditions. The incorporation of aluminum atoms into the gel framework and the structure of wet gels were investigated by chemical analysis, ²⁷Al magic angle spinning nuclear magnetic resonance, and small angle X-ray scattering. The present method may be most favourable for the preparation of aluminosilicate gels with 0.30–0.70 mole ratios of total Al/(Si + Al). At lower Al content acidic catalysis is required. Above 0.70 mole ratio homogeneous gels cannot be obtained by this method. The highest aluminum incorporation in homogeneous gel structures of various mole ratios of total Al/(Si + Al) was 0.53 mole ratio of bonded Al/(Si + Al) in contradiction to 0.1 mole ratio of Al/(Si + Al) achieved by traditional melting process of glass.     

Aluminum-27 NMR Investigations of Aqueous and Methanolic Aluminosilicate Solutions

Aluminum-27 NMR spectroscopy was used to characterize aqueous and methanolic alkaline solutions of tetramethylammonium (TMA) aluminosilicates. Aluminosilicate solutions have been prepared with different concentrations of silicon [0.577–1.24% (w/w)], aluminum [0.0022–0.239% (w/w)], methanol [0.0–0.70% (w/w)] and H₂O [0.23–90% (w/w)]. All solutions contain the same ratio of Si/TMA = 1 and Si/Al molar ratios between 0.5 and 25.²⁷Al NMR spectra of TMA aluminosilicate solutions are characterized by a variety of aluminosilicate species such as q¹(Al1OSi), q²(Al2OSi), q³(Al3OSi) and q⁴(Al4OSi). Aluminum-27 NMR spectra of TMA aluminosilicate solutions indicate that considerable changes occurred by changing the Si/Al ratio. The distribution of aluminosilicate species was affected by the presence of the methanol and the method of mixing the silicate and aluminosilicate solutions. A methanolic aluminosilicate solution needs about twice the time required for an aqueous aluminosilicate solution to reach a steady state, i.e., the latter takes 36 h to reach steady state. Results with the same concentration of silicon and aluminum show that the formation and distribution of aluminosilicate species are strongly dependent on the solvent comprising the silicate and aluminate solutions.     

MAS-NMR investigations of the crystallization behaviour of lithium aluminum silicate (LAS) glasses containing P2O5 and TiO2 nucleants

Lithium aluminum silicate (LAS) glass of composition (mol%) 20.4Li₂O-4.0Al₂O₃-68.6SiO₂-3.0K₂O-2.6B₂O₃-0.5P₂O₅-0.9TiO₂ was prepared by melt quenching. The glass was then nucleated and crystallized based on differential thermal analysis (DTA) data and was characterized by ²⁹Si, ³¹P, ¹¹B and ²⁷Al MAS-NMR. XRD and ²⁹Si NMR showed that lithium metasilicate (Li₂SiO₃) is the first phase to c form followed by cristobalite (SiO₂) and lithium disilicate (Li₂Si₂O₅). ²⁹Si MAS-NMR revealed a change in the network structure already for the glasses nucleated at 550 °C. Since crystalline Li₃PO₄, as observed by ³¹P MAS-NMR, forms concurrently with the silicate phases, we conclude that crystalline Li₃PO₄ does not act as a nucleating agent for lithium silicate phases. Moreover, ³¹P NMR indicates the formation of M-PO₄ (M=B, Al or Ti) complexes. The presence of BO₃ and BO₄ structural units in all the glass/glass-ceramic samples is revealed through ¹¹B MAS-NMR. B remains in the residual glass and the crystallization of silicate phases causes a reduction in the number of alkali ions available for charge compensation. As a result, the number of trigonally coordinated B (BO₃) increases at the expense of tetrahedrally coordinated B (BO₄). The ²⁷Al MAS-NMR spectra indicate the presence of tetrahedrally coordinated Al species, which are only slightly perturbed by the crystallization.     

Understanding of bonding and mechanical characteristics of cementitious mineral tobermorite from first principles

This paper reports density functional theory study of the structural and mechanical properties of tobermorite mineral (9 Å phase) as one of the main component of cementitious materials in a concrete chemistry. Calculated bulk modulus and elastic constants reflect a relatively high resistance of the tobermorite structure with respect to external isostatic compression. Moreover, the elastic constants proved the anisotropic character of the tobermorite structure. The directions parallel to the axb plane are more resistant to the compression than the perpendicular direction. The largest contribution to this resistance comes from the “dreierketten” silicate chains. The bonding analysis linked macroscopic mechanical properties and the atomic structure of the tobermorite. It was found that polar covalent Si? O bonds are stiffer than iono‐covalent Ca? O bonds. The SiO₄ tetrahedra are resistant with respect to the compression and the effect of external pressure is reflected by the large mutual tilting of these tetrahedra as it is shown by changes of the Si? O? Si bridging angles. Polyhedra with the seven‐fold coordinated Ca²⁺ cations undergo large structural changes. Especially, axial Ca? O bonds perpendicular to the axb plane are significantly shortened. Besides, it was shown that structural parameters, more or less in parallel orientation to the axb plane, are mainly responsible for the high resistance of the tobermorite structure to external pressure. The main mechanism of a dissipation of energy entered to the structure through the compression is proceeded by the tilting of the tetrahedra of the silicate chains and by large shortening of the axial Ca? O distances. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2011     

Incorporation of aluminum in the calcium silicate hydrate (C-S-H) of hydrated portland cements: a high-field 27Al and 29Si MAS NMR investigation

The calcium silicate hydrate (C-S-H) phase resulting from hydration of a white Portland cement (wPc) in water and in a 0.3 M NaAlO(2) solution has been investigated at 14 and 11 hydration times, respectively, ranging from 6 h to 1 year by (27)Al and (29)Si MAS NMR spectroscopy. (27)Al MAS NMR spectra recorded at 7.05, 9.39, 14.09, and 21.15 T have allowed a determination of the (27)Al isotropic chemical shift (delta(iso)) and quadrupolar product parameter (P(Q) = C(Q)) for tetrahedrally coordinated Al incorporated in the C-S-H phase and for a pentacoordinated Al site. The latter site may originate from Al(3+) substituting for Ca(2+) ions situated in the interlayers of the C-S-H structure. The spectral region for octahedrally coordinated Al displays resonances from ettringite, monosulfate, and a third aluminate hydrate phase (delta(iso) = 5.0 ppm and P(Q) = 1.20 MHz). The latter phase is tentatively ascribed to a less-crystalline aluminate gel or calcium aluminate hydrate. The tetrahedral Al incorporated in the C-S-H phase has been quantitatively determined from (27)Al MAS spectra at 14.09 T and indirectly observed quantitatively in (29)Si MAS NMR spectra by the Q(2)(1Al) resonance at -81.0 ppm. A linear correlation is observed between the (29)Si MAS NMR intensity for the Q(2)(1Al) resonance and the quantity of Al incorporated in the C-S-H phase from (27)Al MAS NMR for the different samples of hydrated wPc. This correlation supports the assignment of the resonance at delta(iso)((29)Si) = -81.0 ppm to a Q(2)(1Al) site in the C-S-H phase and the assignment of the (27)Al resonance at delta(iso)((27)Al) = 74.6 ppm, characterized by P(Q)((27)Al) = 4.5 MHz, to tetrahedrally coordinated Al in the C-S-H. Finally, it is shown that hydration of wPc in a NaAlO(2) solution results in a C-S-H phase with a longer mean chain length of SiO(4) tetrahedra and an increased quantity of Al incorporated in the chain structure as compared to the C-S-H phase resulting from hydration of wPc in water.     

27Al and 29Si solid-state NMR characterization of calcium-aluminosilicate-hydrate

Calcium silicate hydrate (C-S-H) is the main constituent of hydrated cement paste and determines its cohesive properties. Because of the environmental impact of cement industry, it is more and more common to replace a part of the clinker in cement by secondary cementitious materials (SCMs). These SCMs are generally alumina-rich and as a consequence some aluminum is incorporated into the C-S-H. This may have consequences on the cohesion and durability of the material, and it is thus of importance to know the amount and the location of Al in C-S-H and what the parameters are that control these features. The present paper reports the (29)Si and (27)Al MAS NMR analyses of well-characterized C-A-S-H samples (C-S-H containing Al). These samples were synthesized using an original procedure that successfully leads to pure C-A-S-H of controlled compositions in equilibrium with well-characterized solutions. The (27)Al MAS NMR spectra were quantitatively interpreted assuming a tobermorite-like structure for C-A-S-H to determine the aluminum location in this structure. For this purpose, an in-house written software was used which allows decomposing several spectra simultaneously using the same constrained spectral parameters for each resonance but with variable intensities. The hypothesis on the aluminum location in the C-A-S-H structure determines the proportion of each silicon site. Therefore, from the (27)Al NMR quantitative results and the chemical composition of each sample, the intensity of each resonance line in the (29)Si spectra was set. The agreement between the experimental and calculated (29)Si MAS NMR spectra corroborates the assumed C-A-S-H structure and the proposed Al incorporation mechanism. The consistency between the results obtained for all compositions provides another means to assess the assumptions on the C-A-S-H structure. It is found that Al substitutes Si mainly in bridging positions and moderately in pairing positions in some conditions. Al in pairing site is observed only for Ca/(Si+Al) ratios greater than 0.95 (equivalent to 4 mmol.L(-1) of calcium hydroxide). Finally, the results suggest that penta and hexa-coordinated aluminum are adsorbed on the sides of the C-A-S-H particles.     

New synthesis of mullite. Structural evolution study by 17O, 27Al and 29Si MAS NMR spectroscopy

Mullite has been prepared from a new combination of precursors. An aluminum alkoxide, aluminium isopropoxide, and silicon tetrachloride, are hydrolysed in tetrahydrofuran solution by ¹⁷O enriched water. The resulting powder is chemically homogeneous, crystallizing into mullite at 980°C. The structural evolution has been studied by DTA, TGA, XRD and ¹⁷O, ²⁷Al and ²⁹Si MAS NMR spectroscopy.     

Effect of Solution Chemistry on Preparation of Boehmite by Hydrothermal Assisted Sol-Gel Processing of Aluminum Alkoxides

Various aluminum alkoxide precursors have been used for the preparation of boehmite by hydrothermal assisted sol-gel processing. The coordination status of aluminum in solution for all precursors employed for the preparation of boehmite phase was determined by ²⁷Al NMR and correlation between coordination status of aluminum atoms of precursors and development of boehmite phase has been investigated. Hydrothermal assisted hydrolysis of aluminum alkoxides where the aluminum atoms are four or five coordinated in solution resulted in the formation of boehmite. In contrast, hydrothermal hydrolysis of aluminum alkoxides where the aluminum atoms are six coordinated resulted in the formation of amorphous gel. Development of boehmite phase by hydrothermal hydrolysis of aluminum alkoxides at various temperatures was pursued by X-ray diffraction (XRD) and Fourier Transform Infrared (FTIR) spectroscopy.     

Hydrogen bonds and local symmetry in the crystal structure of gibbsite

First‐principles quantum mechanical calculations of NMR chemical shifts and quadrupolar parameters have been carried out to assign the ²⁷Al MAS NMR resonances in gibbsite. The ²⁷Al NMR spectrum shows two signals for octahedral aluminum revealing two aluminum sites coordinated by six hydroxyl groups each, although the crystallographic positions of the two Al sites show little difference. The presence of two distinguished ²⁷Al NMR resonances characterized by rather similar chemical shifts but quadrupolar coupling constants differing by roughly a factor of two is explained by different character of the hydrogen bonds, in which the hydroxyls forming the corresponding octahedron around each aluminum site, are involved. The Al‐I site characterized by a C_Q = 4.6 MHz is surrounded by OH? groups participating in four intralayer and two interlayer hydrogen bonds, while the Al‐II site with the smaller quadrupolar constant (2.2 MHz) is coordinated by hydroxides, of which two point toward the intralayer cavities and four OH‐bonds are aligned toward the interlayer gallery. In high‐resolution solid‐state ¹H CRAMPS (combination of rotation and multiple‐pulse spectroscopy) four signals with an intensity ratio of 1:2:2:1 are resolved which allow to distinguish six nonequivalent hydrogen sites reported in the gibbsite crystal structure and to ascribe them to two types of structural OH groups associated with intralayer and interlayer hydrogen bonds. This study can be applied to characterize the gibbsite‐like layer—intergallery interactions associated with hydrogen bonding in the more complex systems, such as synthetic aluminum layered double hydroxides. Copyright © 2010 John Wiley & Sons, Ltd.     

Aluminum Solvate Complexes Forming in Acidic Methanol-Acetone Mixtures Studied by <Superscript>27</Superscript>Al NMR Spectroscopy

²⁷Al NMR spectroscopy is a powerful tool for the study of coordination and solvation in both aqueous and nonaqueous solutions. In this study, the complexes coexisting upon dissolution of AlCl₃ in acidic acetone + methanol solutions are shown to consist essentially of mixed hexacoordinated species of the general formula [Al(CH₃OH)_6−n (CH₃COCH₃) _n ]³⁺ (n=1,2 and 3), all exhibiting distinctly different ²⁷Al shielding effects. The relative populations of the various mixed species are found to be highly dependent upon the acetone:methanol mole ratio that in the more acetone-rich mixtures with aluminum become appreciably coordinated by acetone. The results demonstrate that the key factor for the formation of acetone-containing species in acidic methanolic solutions is having the CH₃COCH₃:CH₃OH mole ratio at 3:1.     

Modification Study of Aluminum sec-Butoxide by Acrylic Acid

A new way of modifying aluminum sec-butoxide (Al(OBu^s)₃) is proposed. This synthesis is carried out by reacting Al(OBu^s)₃ dissolved in tetrahydrofuran with an unsaturated acid, viz. acrylic acid.The structure evolution of Al(OBu^s)₃ with increasing acrylic acid amounts is investigated by infrared, ¹H NMR, ¹³ C NMR, and ²⁷Al NMR spectroscopies and viscosity measurements. Information obtained suggests that the exchange reaction occurring between butoxy groups and acrylate ligands is stopped for an acid/alkoxide molar ratio within the range 1.6–1.7. This value leads us to assume that the dominant trimeric species of precursor is preserved after modification. Moreover, ²⁷Al NMR analysis only reveals the presence of hexacoordinated Al sites in the structure of the modified Al(OBu^s)₃.Evidence of the acrylic acid reaction with sec-butanol released during the alkoxide modification is also proved by the infrared and ¹³ C NMR data. However the produced ester amount can be considered as negligible.     

Substitution clustering in a non-stoichiometric celsian synthesized by the thermal transformation of barium exchanged zeolite X

The thermal transformation of Ba exchanged zeolite X to celsian has been studied by ²⁷Al and ²⁹Si MAS NMR spectroscopy. Evidence for the degradation of the zeolite framework is present in the ²⁹Si NMR spectra after thermal treatment at 850 °C. Confirmation is provided by the ²⁹Si NMR data that synthesis of celsian via the decomposition of Ba exchanged zeolite leads to a single defect phase. Clustering of the isomorphous replacement of aluminium by silicon must occur to explain the observed ²⁹Si chemical shifts. The ²⁷Al NMR data show distorted aluminium co-ordination sites upon the thermal transformation of Ba exchanged zeolite X. The distortions present in the amorphous matrix are greater than those present in the monoclinic and hexagonal crystalline phases of celsian.     

Formation of functional phosphosilicate gels from phytic acid and tetraethyl orthosilicate

Phosphosilicate gels with high phosphorus content (P mol% > Si mol%) have been prepared using phytic acid as the phosphorus precursor, with tetraethyl orthosilicate (TEOS). It is shown that the structure of phytic acid is maintained in both the sols and those gels dried at a low temperature (i.e. ≤120 °C). Solid state ²⁹Si and ³¹P NMR suggest that the gel network is primarily based on tetrahedral silicon and that phosphorus is not chemically incorporated into the silicate network at this point. X-ray diffraction shows the gel to be amorphous at low temperatures. After heat treatment at higher temperatures (i.e. up to 450 °C), P–O–Si linkages are formed and the silicon coordination changes from tetrahedral to octahedral. At the same time, the gel crystallizes. Even after this partial calcination, ³¹P NMR shows that a large fraction of phytic acid remains in the network. The function of phytic acid as chelating agent is also maintained in the gels dried at 120 °C such that its ability to absorb Ca²⁺ from aqueous solution is preserved.     

“NMR invisible” aluminum species in dealuminated mordenite zeolite: a1H/27 AI TRAPDOR NMR study

Various kinds of aluminum species in dealuminated mordenite were investigated in detail, and the quadrupole coupling constants (QCCs) for aluminum atoms associated with these species were obtained by means of the newly introduced¹H/²⁷ AI TRAPWR method as well as²⁷Al magic angle spinning (MAS) nuclear magnetic resonance (NMR). QCC values of 11.3, 15.3, 13.3 and (14.0± 0.6) MHz were determined from the TRAPDOR profiles for Lewis acid sites, Bronsted acid sites (SiOHAl) and two kinds of non-framework aluminum species Al(OH) _n , respectively. The source of the “invisible Al” is discussed on the basis of the NMR experimental results.     

Synthesis and In Vitro Behavior of Organically Modified Silicate Containing Ca Ions

Organically modified silicates containing calcium ion have a potential to bond to bone via an apatite layer deposited on their surfaces in the body environment. In this study, we examined the relationship between apatite deposition and the microstructure of the organically modified silicates synthesized from tetraethoxysilane (TEOS) and poly (dimethylsiloxane) (PDMS) with a different amount of calcium nitrate tetrahydrate (Ca(NO₃)₂·4H₂O) and hydrochloric acid (HCl). Apatite deposition was evaluated in vitro using a simulated body fluid (Kokubo solution). Copolymerization was confirmed between TEOS and PDMS even if PDMS free from —SiOH termination are used as one of the starting materials. The porosity and Ca content incorporated in the structure depended on the amount of HCl, whereas analysis of ²⁹Si MAS NMR spectra indicates that it caused few effects on the local structure around Si atoms. Apatite-forming ability is enhanced by optimal amounts of HCl and Ca (NO₃)₂·4H₂O. The difference in apatite-forming ability among the hybrid gels was attributed to both Ca(II) contents in the structure and aggregation states of the Si—OH groups. Better bioactivity of the hybrid gels is achieved by the release of Ca(II) ions trapped in structure at gelation and the formation of hydrated silica rich in Si—OH.     

Magic-Angle-Spinning NMR (MAS-NMR) Spectroscopy and the Structure of Zeolites

After outlining the chemical features and properties which make zeolites such an important group of catalysts and sorbents, the article explains how high-resolution solid-state NMR with magic-angle spinning reveals numerous new insights into their structure. ²⁹Si-MAS-NMR readily and quantitatively identifies five distinct Si(OAl)_n(OSi)_4-n structural groups in zeolitic frameworks (n = 0, 1,….4), corresponding to the first tetrahedral coordination shell of a silicon atom. Many catalytic and other chemical properties of zeolites are governed by the short-range Si, Al order, the nature of which is greatly clarified by ²⁹Si-MAS-NMR. It is shown that, as expected from Pauling's electroneutrality principle and Loewenstein's rule, both in zeolite X and in zeolite A (with Si/Al = 1.00) there are no ? Al? O? Al? linkages. In zeolite A and zeolite X with Si/Al = 1.00 there is strict alternation of Si and Al on the tetrahedral sites. Ordering models for Si/Al ratios up to 5.00 (in zeolite Y) may also be evaluated by a combination of MAS-NMR experiments and computational procedures. ²⁹Si-MAS-NMR spectra reveal the presence of numerous crystallographically distinct Si(OSi)₄ sites in silicalite/ZSM-5, suggesting that the correct space group for these related porosilicates is not Pnma. ²⁷Al-MAS-NMR clearly distinguishes tetrahedrally and octahedrally coordinated aluminum, proving that, contrary to earlier claims, Al in silicalite is tetrahedrally substituted within the framework. In combination, ²⁹Si- and ²⁷Al-MAS-NMR is a powerful tool for monitoring the course of solid-state processes (such as ultrastabilization of synthetic faujasites) and of gas-solid reactions (dealumination of zeolites with silicon tetrachloride vapor at elevated temperatures). They also permit the quantitative determination of framework Si/Al ratios in the region 1.00 < Si/Al < 10 000. Since most elements in the periodic table may be accommodated within zeolite structures, either as part of the exchangeable cations or as building units of the anionic framework, there is immense scope for investigation by MAS-NMR and its variants (cross-polarization, multiple pulse and variable-angle spinning) of bulk, surface and chemical properties. Some of the directions in which future research in zeolite science may proceed are adumbrated.