Outstanding Problems in the Kaolinite-Mullite Reaction Sequence Investigated by 29Si and 27Al Solid-state Nuclear Magnetic Resonance: 11, High-Temperature Transformations of Metakaolinite

Solid-state ²⁹Si and ²⁷Al NMR spectra of kaolinite fired at 800° to 1450°C, interpreted in light of a newly proposed metakaolinite structure and complementary X-ray diffraction results, lead to the following conclusions about the hightemperature reactions: (1) Removal of the final residual hydroxyl radicals of metakaolinite at ∼9707deg;C triggers the separation of a considerable amount of amorphous free silica and the formation of poorly crystalline mullite and a spinel phase. (2) Mullite and spinel form in tandem, the former originating in the vicinity of AI-0 units of regular octahedral and tetrahedral symmetry randomly distributed throughout the metakaolinite structure. (3) The initially formed mullite is alumina-rich but at higher temperatures progressively gains silica, approaching the conventional 3Al₂O₃· 2SiO₂ composition. (4) The spinel phase contains insufficient Si to be detected by ²⁹Si NMR but has a ²⁷Al NMR spectrum consistent with γ-Al₂O₃. On further heating, the spinel is converted to mullite by reaction with some of the amorpholls silica, the balance of which eventually becomes cristobalite.     

Synthesis and Characterization of a SiAION Glass

Homogeneous SiAION glasses containing up to 1 wt% nitrogen were synthesized via a pressureless method with a controlled quench rate and structurally investigated using ²⁷Al and ²⁹Si magic-angle spinning nuclear magnetic resonance (MAS NMR), Raman, and infrared (IR) spectroscopies. Minor changes occur with the incorporation of nitrogen into the aluminosilicate glass structure as evidenced by modifications to spectra of a nitrogen-free aluminosilicate glass. The ²⁷Al MAS NMR spectrum of the SiAION glass shows the existence of aluminum in 4-,5-, and 6-coordination to oxygen. The ²⁹Si MAS NMR spectra show a distribution of silicon sites in 4-coordination to oxygen. Raman and IR spectra of the SiAION glass show additional features due to incorporation of nitrogen in the structure compared with spectra of nitrogen-free aluminosilicate glasses.     

MAS NMR Studies of Partially Carbonated Portland Cement and Tricalcium Silicate Pastes

²⁹Si, ²⁷Al, and ¹H MAS NMR studies of partially carbonated mature ordinary Portland cement (OPC) and tricalcium silicate (C₃S) pastes have been carried out. The water-to-solid ratios ( W/S ) have been varied between 0 and 1 at hydration temperatures of 23^o and 90^oC. Various Q ⁿi units with n =0, 1,2,3, and 4, and a Q³ (1Al) group have been identified using ²⁹Si NMR. Cross-polarization experiments, in addition, have made it possible to assign the OH groups. Two types of fourfold- and one type sixfold-coordinated aluminum have been distinguished using ²⁷Al NMR. In C₃S pastes for w/s >0.7, progressive carbonation leads to a nearly perfect three-dimensional network consisting of Q³ and Q⁴only. In contrast, in OPC pasted only about 40% of the highly polymerized silicate units are formed, partially copolymerized with AlO₄ tetrahedra.     

Thermal Reactions of Pyrophyllite Studied by High-Resolution Solid-state 27Al and 29Si Nuclear Magnetic Resonance Spectroscopy

Water is lost in two overlapping steps from well-crystallized pyrophyllite from Coromandel, New Zealand. The pyrophyllite structure survives the loss of the first 30% of the total water content, but the loss of a further 60% water in the second step results in the formation of pyrophyllite dehydroxylate, with corresponding changes in both the ²⁹Si and ²⁷Al solid-state NMR spectra. Detailed analysis of the ²⁹Si chemical shift of the dehydroxylate has allowed the silicate layer structure of this phase to be refined. A similarly detailed interpretation of the ²⁷A1 spectra is not possible because of electric field gradient effects which result in the loss of ∼90% of the A1 spectral intensity due to the formation of five-coordinate A1 on dehydroxylation. The loss of further water from the dehydroxylate on further heating results in the formation of mullite and cristobalite and is accompanied by changes in the ²⁹Si and ²⁷Al spectra which can be accounted for in terms of coordination changes in the structural regions which contained the residual hydroxyls.     

Characterization of Synthetic and Naturally Occurring Clays by 27Al and 29Si Magic-Angle Spinning NMR Spectroscopy

Magic-angle spinning nuclear magnetic resonance (MASNMR) spectroscopy (²⁷Al) detected the coordination of Al in clays containing as little as 0.26% Al₂O₃. The ²⁹Si MASNMR of fluorphlogopite showed three distinct Si chemical environments which suggested short-range ordering. Synthetic laponite and mica-montmorillonite showed broad ²⁹Si resonances indicative of short-range disorder. A saponite showed four ²⁹Si resonances. Considerable insight into the short-range ordering of clays can be gained by ²⁷Al and ²⁹Si MASNMR.     

Synthesis, Rietveld Analysis, and Solid State Nuclear Magnetic Resonance of X2-Sc2SiO5

Compounds containing rare earths are of increasing technological interest especially because of their unique mechanical, magnetic, electrical, and optical properties. Among them, rare earth oxyorthosilicates are attractive scintillators for γ- and X-ray spectroscopy and detection. However, there are many structural aspects of those compounds that are not clear. In this research, the structure parameters for Sc₂Si₂O₅, X₂-polymorph, have been refined from powder X-ray diffraction (XRD) data and the ²⁹Si MAS NMR spectrum is reported for the first time. X₂-Sc₂SiO₅ polymorph was synthesized by the sol–gel method and characterized by XRD and ²⁹Si MAS NMR. The XRD pattern was indexed in a monoclinic unit cell with space group I 2/ c ; the resulting unit cell parameters were a =9.9674(2) Å, b =6.4264(9) Å, c =12.0636(2) Å, and β=103.938(1)°. The ²⁹Si MAS NMR spectrum showed a unique signal at −79.5 ppm, compatible with the unique Si crystallographic site in the unit cell. Finally, the band valence method has been applied to the calculation of a "shift parameter," which is correlated with the NMR chemical shift.     

29Si and 27Al MAS-NMR of NaOH-Activated Blast-Furnace Slag

b²⁹Si and ²⁷Al MAS-NMR were performed on NaOH-activated blast-furnace slag to better characterize the amorphous and poorly crystalline phases which occur in this system. The unreacted glass has a mainly dimeric silicate structure represented by a broad ²⁹Si peak (FWHM = 15 ppm) centered at –74.5 ppm [ Q ¹], with aluminum present exclusively in tetrahedral coordination. Upon reaction with 5M NaOH ( w/s = 0.4), three new ²⁹Si peaks with widths of ca. 2 ppm are formed at -78.5 Q ¹, –81.4 [ Q ²(1Al)J, and -84.3 [ Q ²]. Relative peak areas indicate a mostly dimeric silicate structure for the tobermorite-like C─S─H layers, with roughly a third of the bridging sites occupied by aluminum, and less than 10% by silicon. In addition to the tetrahedrally coordinated aluminum substituted in the C─S─H structure, ²⁷Al MAS-NMR reveals the presence of aluminum in octahedral sites, which is attributed to the aluminate phase (C,M)₄AH₁₃. ²⁹Si results indicate rapid initial consumption of the glass, with roughly a third of the glass reacting within the first day and another third consumed over the following 27 days.     

Synthesis of Calcium Silicate Hydrate with Ca/Si = 2 by Mechanochemical Treatment

An amorphous, C-S-H-like phase with Ca/Si = 2 was synthesized from amorphous precipitated silica, calcium oxide, and water by mechanochemical treatment using a vibration mill at room temperature. The product was studied by XRD, ²⁹Si MASNMR, TEM, analytical TEM, and TGA-DTA. After 14 h of treatment, the starting materials react to form an amorphous phase (C-S-H-like phase). The XRD pattern of C-S-H-like phase resembles the C-S-H formed hydrothermally or by cement hydration except it has no reflection at 0.182 nm. The ²⁹Si NMR results revealed a silicate anion structure of C-S-H-like phase consisting mainly of a mixture of a monomer and a dimer. On heating, the C-S-H-like phase decomposed into β-dicalcium silicate below 1000°C.     

Si-Al-O-N Fibers from Polymeric Precursor: Synthesis, Structural, and Mechanical Characterization

Polymeric fibers were produced from a polyaluminocarbosilane obtained by reacting polycarbosilane and an aluminum alkoxide modified with a β-ceto ester. The pre-ceramic fibers were converted into amorphous Si-Al-O-N ceramic fibers after a pyrolysis process under flowing ammonia at 1000°C. ²⁹Si and ²⁷Al magic-angle spinning nuclear magnetic resonance investigations were performed to characterize the conversion process of the polymeric precursor fibers into the ceramic product. However, because of the amount of matter required for the MAS-NMR experiments, the heat treatment applied for fibers was done on powders assuming identical evolution for both materials. Tensile strength was measured at various stages of the pyrolysis process and related to the corresponding structural evolution.     

A Combined 29Si MAS NMR and Selective Dissolution Technique for the Quantitative Evaluation of Hydrated Blast Furnace Slag Cement Blends

Quantification of the C–S–H hydrates and anhydrous material in plain and blended cement systems can be performed by deconvolution of ²⁹Si magic angle spinning nuclear magnetic resonance (MAS NMR) spectra. NMR data are reliable for simple cement systems, but with the incorporation of supplementary cementitious materials, quantification is often uncertain. For example, the overlap of peaks from slag cement and C–S–H in ²⁹Si MAS NMR spectra causes problems with deconvolution. A novel method was developed to address these difficulties. ²⁹Si MAS NMR was combined with a selective dissolution method. The hydrate peaks in slag blends can now be quantified without interference from the slag peak. This new method enables the silica remaining in unreacted slag to be estimated, thus allowing the degree of slag hydration to be quantified. Hydrate Al/Si ratios correlate well with data from analytical transmission electron microscopy (TEM). Analysis of the dissolution residues by TEM and ²⁹Si MAS NMR indicates that they consist of a mixture of unreacted slag, a hydrotalcite-type phase, and small amounts of aluminosilicate gel. The origin of the aluminosilicate gel needs further investigation.     

Synthesis and Characterization of Amorphous Si2N2O

Amorphous silicon oxynitride powder was synthesized by nitridation of high-purity silica in ammonia at 1120°C. The resulting material was X-ray amorphous, and its chemical characteristics were determined by X-ray photoelectron spectroscopy (XPS) and ²⁹Si nuclear magnetic resonance (NMR). The XPS analysis showed a shift to lower binding energies for the Si₂ p peak with increasing nitrogen content. Upon initial nitridation, the full width at half maximum (FWHM) of the Si₂ p peak increased, but decreased again at higher nitrogen contents, thus showing the formation of a silicon oxynitride phase with a single or small range of composition. The ²⁹Si NMR analysis showed the formation of (amorphous) Si₃N₄ (Si–N₄) and possibly two oxynitride phases (Si–N₃O, Si–N₂O₂). It is concluded that while XPS, FT-IR, and nitrogen analysis may show the formation of an homogeneous, amorphous silicon oxynitride (Si₂N₂O) phase, the formation of phase–pure, amorphous Si₂N₂O is extremely difficult via this route.     

Synthesis and Characterization of β'-SiAlON Ceramics from Organosilicon Polymers

A polycarbosilane has been modified with aluminum alkoxide to obtain a new preceramic compound. The pyrolysis in NH₃ flow of this polyaluminocarbosilane leads to the formation of an amorphous Si-Al-O-N phase. The nitridation process has been followed by IR and ²⁹Si MAS-NMR spectroscopies. A fine-grained β-SiAION ceramic is obtained by firing the amorphous phase at 1500°C. The crystallization process has been studied by XRD, ²⁹Si MAS-NMR, and TEM techniques.     

Crystallization Behavior of Novel Silicon Boron Oxycarbide Glasses

Homogeneous silicon boron oxycarbide (Si-B-O-C) glasses based on SiO _x C_{4– x} and BO _y C_{3– y} mixed environments were obtained by pyrolysis under inert atmosphere of sol–gel-derived precursors. Their high-temperature structural evolution from 1000° to 1500°C was followed using XRD, ²⁹Si and ¹¹B MAS NMR, and chemical analysis and compared with the behavior of the parent boron-free Si-O-C glasses. The XRD study revealed that, for the Si-O-C and the Si-B-O-C systems, high-temperature annealing led to the crystallization of nanosized β-SiC into an amorphous SiO₂-based matrix. NMR analysis suggested that the β-SiC crystallization occurred with a consumption of the mixed silicon and boron oxycarbide units. Finally, by comparing the behavior of the Si-O-C and Si-B-O-C glasses, it was shown that the presence of boron increased the crystallization kinetics of β-SiC.     

Investigation of the Structural Characterization of Mesoporous Molecular Sieves MCM-41 from Sepiolite

Mesoporous molecular sieves, MCM-41, were synthesized from sepiolite using acid leaching, followed by hydrothermal reconstruction and then calcinations at 540°C for 5 h. The structures and the porosity of MCM-41 were investigated by means of small-angle X-ray diffraction patterns, Brunaer-Emmett-Teller (BET), ²⁹Si MAS NMR, Fourier transform infrared (FTIR), and high resolution transmission electron microscope (HRTEM) methods. The results showed that the hexagonal MCM-41 was formed in an alkaline solution of pH 12, when crystallization was carried out at 100°C for 24 h. The specific surface area, pore diameter, and pore volumes of MCM-41 from sepiolite were 1036 m²/g, 2.98 nm, and 1.06 cm³/g, respectively. ²⁹Si MAS NMR results revealed that amorphous silica decomposed into Si–O chains consisting of two layers of Si atoms, with Q ³ configurations resulting in an increase in the fraction of Q ³ configuration during the crystallization of post-Mg-extraction sepiolite. The IR results illustrated that the complex of ≡≡SiO⁻–CTA⁺ was formed during the synthesis of MCM-41 from post-Mg-extraction sepiolite.     

Si and Al MAS NMR characterization of non-hydrated zeolites Y upon adsorption of ammonia

Solid-state NMR characterization of zeolite catalysts in the hydrated state is often accompanied by an uncontrolled hydrolysis of the framework. In the present work it is demonstrated that the limitations occurring for ²⁹Si and ²⁷Al MAS NMR spectroscopy of non-hydrated zeolites Y, such as strong decrease of resolution and significant line broadening, can be overcome by loading these materials with ammonia. In the ²⁹Si MAS NMR spectra of non-hydrated and ammonia-loaded zeolites Y, no dehydration-induced high-field shift of Si(nAl) signals (n = 3, 2, 1) occurs, which is generally responsible for the loss of resolution in the spectra of non-hydrated materials. The ²⁷Al MAS NMR spectra of the non-hydrated and ammonia-loaded zeolites Y consist exclusively of signals of the tetrahedrally coordinated framework aluminum atoms with spectroscopic parameters similar to those of framework aluminum atoms in hydrated samples. The framework n_Si/n_Al ratios of the non-hydrated zeolites Y obtained by both ²⁹Si and ²⁷Al MAS NMR spectroscopy upon ammonia-loading agree well with each other.     

Aluminum-27 and Silicon-29 Magic-Angle Spinning Nuclear Magnetic Resonance Study of the Kaolinite-Mullite Transformation

The ²⁷Al and ²⁹Si magic-angle spinning nuclear magnetic resonance (MAS-NMR) study of the kaolinite-mullite transformation has shown the presence of Al in tetra- and pentacoordination in dehydroxylated kaolinite. The ²⁹Si NMR signal analysis of samples heated above 400°C demonstrates that the tetrahedral sheet of kaolinite begins to break down near 600°C and continues to do so to 900°C. From the ²⁷Al NMR signal evolution, it can be deduced that the exothermic peak at 980°C in DTA curves is associated with the modification of the coordination of Al, which changes from the tetra- or pentacoordination to the more stable octahedral coordination. Heating the sample at 880°C for 36 h produces the same transformation in the coordination of Al ions and the elimination of the exothermic peak at 980°C in the DTA diagram. After this transformation, all spectra show two tetrahedral lines characteristic of mullite, indicating that nuclei of mullite with low crystallinity are generated during the exothermic process which are not detected by XRD. At higher temperatures tetrahedral NMR peaks increase in intensity, yielding, at 1200°C, the 3:2 mullite NMR spectrum.     

Alkaline Activation of Fly Ashes: NMR Study of the Reaction Products

Reaction products formed during alkaline activation, with 8 M N_aOH solutions, of fly ashes have been characterized with ²⁹Si and ²⁷Al magic-angle spinning nuclear magnetic resonance (MASNMR) spectroscopy. In particular, the influence of curing conditions (time and temperature of reaction) has been analyzed. NMR results show the formation of amorphous tecto-silicates in which the amount of aluminum decreases in the two consecutive formed phases. The Si/Al ratio of zeolite precursor obtained at 85°C changes from 0.95 to 1.86 when curing time is increased from 5 h to 1 week. The evolution of the mechanical properties of prepared cements has also been discussed in terms of the phases formed and texture and morphology of samples.     

Effect of Physical Structure on the Phase Development of Aluminosilicate Gels

The surface area and density of aluminosilicate xerogels containing a one-to-one Al/Si molar ratio (47 wt% alumina) can be varied dramatically by changing the pore fluid prior to drying. The surface area of ethanol-washed 47 wt% alumina gels was more than 500 m²/g, while gels dried from the mother liquor (approximately 75 vol% ethanol, 25 vol% water) had less than 1 m²/g surface area. Changes in the physical structure of the dried gels had dramatic effects on subsequent phase evolution and densification behavior during heat treatment. NMR, X-ray diffraction, and DTA were used to follow the phase evolution of different gels. Differences in the amorphous gel structure were identified using ²⁷Al and ²⁹Si MAS NMR. Gels of identical composition prepared from the same precursor solutions crystallized to different phases, depending upon the surface area of the gel prior to heating. The high surface material (ethanol washed) formed mullite and amorphous silica, while the low surface area gel (unswashed) crystallized to mullite and cristobalite. These gels were prepared from alkoxide precursors. A low surface area gel with a different degree of chemical homogeneity was prepared by the nitrate method for comparison. Results indicate that the physical structure of aluminosilicate gels, i.e., pore structure and chemical homogeneity, has a dramatic influence on phase evolution.     

Mesoporous AlPO4 Glass from a Simple Aqueous Sol–Gel Route

Transparent and colorless AlPO₄ gel and glass are prepared via a simple aqueous sol–gel route using aluminum lactate and phosphoric acid as precursors. The stoichiometric AlPO₄ glass derived from this sol–gel route has a mesoporous structure with a surface area of 504 m²/g after calcination to 600°C. With increasing gel-to-glass processing temperature, the average degree of P/Al connectivity increases. After sample calcination at 600°C for 4 h, ²⁷Al MAS NMR spectra indicate that aluminum is almost completely converted into AlO₄ units. ²⁷Al{³¹P} rotational echo double resonance and ³¹P{²⁷Al} rotational echo adiabatic passage double resonance NMR experiments as well as ²⁷Al and ³¹P MAS NMR results further confirm that the prepared AlPO₄ glass has a three-dimensional network based on alternating Al(OP)₄ and P(OAl)₄ tetrahedral units in analogy to the local structure of crystalline AlPO₄.     

Location of Aluminum in Substituted Calcium Silicate Hydrate (C-S-H) Gels as Determined by 29Si and 27Al NMR and EELS

Solid-state ²⁷Al and ²⁹Si magic angle spinning NMR spectroscopy has been combined with electron energy loss spectroscopy carried out in the transmission electron microscope to determine the location of Al substituting in a semicrystalline C-S-H gel present in a hydrated synthetic slag glass. The gel is found to contain mainly pentameric silicate chains in which the central silicon is substituted by aluminum.