Magnesium analogues of aluminosilicate inorganic polymers (geopolymers) from magnesium minerals

Attempts to synthesise magnesium-containing analogues of aluminosilicate geopolymers from the 1:1 and 2:1 layer magnesiosilicate minerals chrysotile and talc, as well as the magnesium mineral sepiolite are reported. The effect of pre-treating these starting minerals by grinding and/or dehydroxylation was also investigated by XRD, ²⁹Si and natural-abundance ²⁵Mg solid-state magic angle spinning (MAS) NMR spectroscopy. The products from sepiolite most closely resembled an aluminosilicate geopolymer, setting at 40 °C to an X-ray amorphous product containing a broad characteristic ²⁹Si MAS NMR resonance at ?90 ppm. The ²⁵Mg MAS NMR spectrum of this product also showed evidence that some of the Mg was located in tetrahedral sites, as expected for a conventional geopolymer. A similar ²⁵Mg MAS NMR result was obtained for chrysotile, but talc proved to be extremely resistant to geopolymer synthesis, requiring treatment at 120 °C for 3 days to set to a friable material retaining the XRD and NMR characteristics of the original talc or its crystalline dehydroxylation products. This lack of reactivity may be related to the 2:1 layer-lattice talc structure, or to the fact that a suitably reactive amorphous product is not formed upon dehydroxylation.     

The effect of aluminum additive on structure evolution of silicon oxycarbide derived from polysiloxane

Silicon oxycarbide (SiOC) and aluminum-containing silicon oxycarbide (SiAlOC) glasses were obtained through pyrolysis in argon atmosphere at 1000 °C of a polymethyl(phenyl)siloxane resin and aluminum tri-sec-butoxide-derived siloxane networks. These glasses were further annealed at 1200, 1300, and 1400 °C in vacuum atmosphere to investigate their high-temperature behavior. The two types of glasses were characterized by X-ray diffraction, ²⁹SiMASNMR, ²⁷AlMASNMR, and chemical element analysis. The aluminum incorporated into structure plays a major role on the thermal stability of SiOC by hindering carbothermal reductions. It can be found that introducing aluminum into structure should be an effective way to enhance the thermal stability of SiOC glasses.     

Characterization of alinite cements through X-ray diffraction and mas 29Si NMR studies

Alinite cements have been synthesized using mining and steel plant wastes and pulverized fuel ash (fly ash) as raw materials and a clinkering temperature of 1150°C. The cements possess hydration characteristics comparable to those of portland cements. X-ray diffraction studies on these samples confirm the presence of alinite as the predominant phase. MAS ²⁹Si NMR spectra have been used to distinguish alinite and alite cements. While both show resonances characteristic of Q° type silicate species, the portland cements exhibit three distinct peaks corresponding to three inequivalent SiO₄ units present, while alinite shows a single sharp peak corresponding to the unique Si position.     

Synthesis and characterization of aluminosilicate and gallosilicate sodalites containing acetate ions

One pot synthetic aluminosilicate and gallosilicate sodalites containing acetate guest anion have been synthesized successfully under hydrothermal conditions at 100 °C. This paper deals with the comparison of X-ray powder data with single crystal data for Na₈[AlSiO₄]₆(CH₃COO)₂ and the characterization of a new Na₈[GaSiO₄]₆(CH₃COO)₂ sodalite. The products obtained were characterized by IR, X-ray powder diffraction, MAS NMR, SEM and thermogravimetry technique. Crystal structures were refined in the space group $P\bar 43n$ from X-ray powder data using the Rietveld refinement method with the unit cell parameter a = 9.0679 Å, for aluminosilicate and a = 9.2119 Å, for gallosilicate sodalite. The aluminosilicate and gallosilicate framework shows regular bonding with bond angles of Al-O-Si = 146.241°, Ga-O-Si = 141.294°. ²⁹Si MAS NMR chemical shift values verify Al/Si and Ga/Si ordering of the framework constituents.     

Structural studies of geopolymers by <Superscript>29</Superscript>Si and <Superscript>27</Superscript>Al MAS-NMR

A systematic study of geopolymers by ²⁹Si and ²⁷Al MAS NMR has been carried out in an attempt to understand polymer structural details. ²⁷Al MAS NMR data shows that transient aluminium species are formed during the reaction of metakaolin with NaOH. Interaction of silicate anions with the aluminium sites of metakaolin was evident during the synthesis of geopolymers as observed from low field shift of ²⁹Si MAS NMR resonance lines of silicate centres. As the reaction progresses, the coordination of aluminium (IV, V and VI) in metakaolin changes almost completely to IV. ²⁹Si MAS NMR of selected compositions of the ternary system of sodium silicate, metakaolin and aqueous alkali reveals that geopolymerisation occurs in a distinct compositional region. At high alkalinity [> 30% (mol/mol) overall Na²O content], connectivity of silicate anions is reduced, consistent with poor polymerisation. At low alkalinity [<10% (mol/mol) overall Na²O content], a clear ²⁹Si NMR resonance line due to unconverted metakaolin is observed. NMR spectra were recorded from a series of samples with a fixed Na²O content (20 mol%) and varied SiO²/Al²O³ ratio to observe aluminium substitution in the cross-linked silicon tetrahedra of polymer network. Aluminium insertion into the silicate network is confirmed from the observed ²⁹Si NMR shift as a function of Si/Al ratio. The identification of the presence or absence of metakaolin in the cured geopolymer product is not possible even by ²⁹Si NMR as the signal from metakaolin is indistinguishable from a broad ²⁹Si NMR peak consisting of many resonance lines from the network of cross-linked silicon/aluminium tetrahedra. In an attempt to identify metakaolin signal, we prepared geopolymers with higher SiO²/Al²O³ molar ratios. Since aluminium substitutions in the silicate tetrahedral network are decreased, this results in better-resolved ²⁹Si NMR lines. The ²⁹Si NMR signal due to metakaolin is then distinguishable in the spectra of cured products in a series of samples with 3 to 11 mol% metakaolin. These results indicate that a geopolymer structure is a network of silicon/aluminium tetrahedra with some presence of unreacted metakaolin. The silicon/aluminium tetrahedra might have connectivity ranging from 1 to 4.     

The effect of environment pressure on high temperature stability of silicon oxycarbide glasses derived from polysiloxane

Silicon oxycarbide glasses (SiOC) have been produced by siloxane resin under flowing argon atmosphere at 1000 °C. Those glasses were further annealed at 1200, 1300, 1400, and 1500 °C under the pressure of 0.01 KPa and 101 KPa, respectively, to investigate the effect of environment pressure on their high temperature stability. The two series of glasses were characterized by X-ray diffraction, ²⁹SiMASNMR, and chemical element analysis. The environment pressure plays a crucial role on high temperature stability of SiOC glasses by promoting or hindering bonds redistribution and carbothermal reductions. It can be found that SiOC glasses exhibit enhanced thermal stability in high pressure environment.     

NMR and X-ray diffraction studies of amorphous and crystallized pyrolysis residues from pre-ceramic polymers

²⁹Si MAS NMR and X-ray diffraction studies are presented of black and white pyrolysis residues obtained by initial 1100°C pyrolyses in N₂ and NH₃ atmospheres followed by 1550°C pyrolyses in Ar, N₂ or vacuum atmospheres of a polycarbosilane and four polysilazane precursors to SiC and Si₃N₄ ceramics. Amorphous white pyrolysis residues crystallized under the various conditions to give not only Si₃N₄ but also Si₂N₂O, SiC, SiO₂ and Si, while black amorphous pyrolysis residues crystallized to form only Si₃N₄ or SiC. In general, the crystalline ceramic products observed depended on a variety of factors, i.e. moisture sensitivity of polymer, the initial 1100°C pyrolysis gas (N₂/NH₃), the dryness of the 1100°C-NH₃ pyrolysis gas and the 1550°C pyrolysis atmosphere (N₂, Ar, vacuum). An additional factor of interest affecting product distribution was the choice of crucible (alumina/graphite) employed in the 1550°C pyrolysis. The combined studies suggest that the white amorphous pyrolysis residues are complex silicon oxycarbonitrides (Si_xN_yO_zC_a), while the amorphous black residues are silicon carbonitrides (Si_xN_yC_z). This revised version was published online in November 2006 with corrections to the Cover Date.     

Effect of temperature on C–S–H gel nanostructure in white cement

Different ages of white cement pastes hydrated at 100 % RH and 25 or 65 °C were characterised with ²⁹Si MAS NMR spectroscopy. The findings showed that raising the curing temperature from 25 to 65 °C accelerated hydration of the belite phase considerably, inducing a sixfold rise in its one day degree of hydration, while alite phase hydration grew by a factor of only 1.5 in the first day. Moreover, the C–S–H gel formed at the higher temperature had a longer mean chain length and a higher initial uptake of Al³⁺. Lastly, curing at a higher temperature stabilised only one crystalline aluminate phases, calcium hemicarboaluminate.     

Synthesis and characterization of super-microporous material with enhanced hydrothermal stability

Super-microporouos silicon material with high hydrothermal stability denoted as MCM-41-T has been prepared from mesoporous MCM-41 by high temperature treatment. The structural and chemical property of MCM-41-T has been characterized by X-ray diffraction, transmission electron microscopy, N₂ adsorption-desorption, infrared spectroscopy and ²⁹Si MAS NMR. The characteristic results show that Si-OH groups are forced to condense by high temperature treatment, and the pore size of MCM-41-T is around 1.5 nm in the super-microporous range. Compared with the original material MCM-41, the hydrothermal stability of MCM-41-T has been significantly enhanced.     

Preparation of aluminosilicate precursor by mechanochemical method from gibbsite-fumed silica mixtures

Aluminosilicate (mullite) precursor was prepared by mechanochemical treatment of gibbsite and fumed silica mixtures. The effect of grinding on its structure and thermal behaviour was examined by²⁷Al and²⁹Si MAS NMR, XRD, DTA-TG and FTIR. Thermal treatment of this precursor led to the crystallization of mullite at about 1200°C via a spinel-phase.²⁷Al and²⁹Si MAS NMR spectroscopies show lowering of mullitization temperature, which is associated with increased homogeneity of the precursor. Mechanochemical treatment of gibbsite and fumed silica mixtures resulted in the formation of a more homogeneous aluminosilicate precursor.     

Synthesis and characterization of a basic molecular sieve: Nitrogen-incorporated SAPO-34

Microporous nitrogen-incorporated SAPO-34 molecular sieves were synthesized by thermal post-treatment of SAPO-34 in flowing dry ammonia at elevated temperatures. The results of property characterized by DRIFTS of CO₂ adsorption and CO₂–TPD showed that basic sites existed on the surface of nitridized samples probably due to the presence of nitrogen-containing species incorporating into the framework of SAPO-34 molecular sieves. The changes in structure of nitrogen-incorporated samples were investigated by ²⁷Al MAS NMR, ³¹P MAS NMR spectra X-ray diffraction and BET surface area measurements. It was found that nitrogen-incorporated SAPO-34 still kept good crystallinity and high specific surface area of precursors. And N-containing species had incorporated into the framework of samples during nitridation, in which N atom presumably bounded to Al and P atoms formed N-containing groups, but unfortunately some of them were not ascribed.     

Synthesis and thermal behaviour of potassium sialate geopolymers

The geopolymers potassium polysialate (K-PS) and potassium sialate disiloxo (K-PSDS) have been found to possess extremely good thermal stability. K-PS shows little sign of melting up to 1400 °C, its amorphous structure being replaced by the crystalline feldspars leucite and kalsilite at 1000 °C. ²⁷Al and ²⁹Si MAS NMR confirm the ease of this thermal reaction which involves only slight changes to the tetrahedral Al environment. Silica-rich K-PSDS becomes friable and porous at >1200 °C, with less complete crystallisation of K-feldspar, the formation of some cristobalite and the retention of a degree of amorphous geopolymer. ³⁹K NMR suggests that the charge-balancing alkali ions in the K-PS and K-PSDS geopolymer networks behave similarly to those of Na geopolymers, dehydrating on heating and moving into the feldspar lattice >1000 °C.     

H<Subscript>3</Subscript>PW<Subscript>12</Subscript>O<Subscript>40</Subscript> supported on mesoporous MCM-41 and Al–MCM-41 materials: preparation and characterisation

MCM-41 and Al–MCM-41 has been synthesized using cetyl-trimethylammonium bromide (CTAB) surfactant as template and adding the silica precursor to aqueous solutions containing CTAB. The obtained solids were calcined at 600 °C for 4 h. HPW heteropolyacid supported on the mesoporous were prepared using the incipient wetness method. The characterization of materials was performed by X-ray diffraction, Transmission Electron Microscopy, N₂ adsorption, ²⁹Si Cross Polarization–Magic Angle Spinning and ²⁷Al MAS NMR. Results showed that the hexagonal structure is obtained in both cases. The Aluminium species are located inside an extra-framework. The impregnation reduces the surface area of the mesoporous materials especially of the Al–MCM-41 suggesting a participation of aluminium during the impregnation. HPW is well dispersed in the mesoporous materials and is located inside the pores interacting with the silanol group of the pores wall. ²⁷Al MAS NMR measurements have showed that the impregnation causes the removal of the non-framework aluminium.     

高炉矿渣的组成和结构对其在碱性环境中早期水化特性的影响

以不同组成和结构的高炉矿渣作为研究对象,借助XRD、ICP-MS以及²⁹Si MAS NMR等微观测试手段,探究了高炉矿渣在碱性环境中的离子溶出特性、硅氧四面体聚合状态随水化龄期变化的影响规律。研究表明:高炉矿渣中玻璃体的活性受其化学组成和结晶相含量影响,结晶相及铝氧结构单元的存在和增多降低了玻璃体结构的聚合度;网络改性剂在玻璃体结构中表现为不均匀分布,不同网络形成体结构单元的水解会伴随不同的水化特性;在碱性环境下,高炉矿渣玻璃体结构中硅氧四面体含量较大时,其硅氧结构更易被破坏,溶液中硅离子的浓度较高,溶液pH值维持在较低水平,生成的水化产物中高聚态硅氧四面体较多;高炉矿渣玻璃体结构中铝氧四面体含量较大时,其铝氧结构更易被破坏,溶液中铝离子及钙离子的浓度较高,液相pH值维持在较高水平,生成的水化产物中高聚态硅氧四面体较少。     

Synthesis and characterization of Mu-11: a porous sodium trisilicate Na2Si3O7·H2O with 10-membered ring openings

The title compound (named Mu-11) is a new type of sodium trisilicate which was synthesized by using the quasi non-aqueous route in the presence of ethylene glycol as solvent. This material was characterized by XRD, SEM, ²⁹Si solid-state MAS NMR spectroscopy, elemental and thermal analyses. The structure was determined by single-crystal X-ray diffraction. The symmetry is monoclinic, space group P2₁/c, a=7.3087(8)Å, b=12.7246(13)Å, c=9.0913(11)Å, β=119.01(1)° and V=739.39(10) ų. The structure consists of corrugated sheets built from four-, six- and eight-membered ring units. The connection of such sheets leads to a three-dimensional framework with a monodimensional channel system delimited by a 10-ring aperture. After heating to 300°C another novel phase crystallizes, which is stable up to 600°C.     

Densi cation and crystallisation behaviour of Si–B–O–N ceramics prepared by hot pressing with organic precursors

Abstract

Powders of Si–B–O–N prepared by organometallic polymeric precursor were hot pressed into ceramics at different temperatures. Experimental results indicated that density of ceramics increased slowly with increasing temperature before 1400°C, and increased rapidly after 1400°C. Density of ceramics sintered at 1600°C was 2.11 g cm^-3, which was the highest in this system. Amorphous silicon oxynitride decomposed when sintered at temperatures higher than 1600°C. Differential scanning calorimetry and X-ray diffraction analyses results showed that the amorphous Si–B–O–N began to crystallise at 1350°C and only h-BN precipitates was detected. Observations using SEM showed that particles precipitated at 1400°C were close to equiaxial crystals, however, particles precipitated at 1600°C were well developed platelets. The growth mechanism of precipitates was step like epitaxial growth. Crystals in ceramics sintered at 1700°C were quite small and adhered by molten silica.     

Characterization and electrochemical properties of P2O5–ZrO2–SiO2 glasses as proton conducting electrolyte

The H₂/O₂ fuel cell based on the proton conducting P₂O₅–ZrO₂–SiO₂ glasses was prepared by sol–gel technique. Structural characterization were carried out using X-ray diffraction spectroscopy (XRD), Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (³¹P MAS NMR), N₂ adsorption–desorption analysis, differential thermal analysis (DTA) and thermal gravimetric analysis (TGA) and impedance measurements. The absorption bonds of Si–O–Si, Si–O–P and Si–O–Zr were observed in the P₂O₅–ZrO₂–SiO₂ glasses which were heat treated at 600 °C. A sample (5P₂O₅–4ZrO₂–91SiO₂, mol%) was selected as the electrolyte for the H₂/O₂ fuel cell test and yielded the maximum power density value of 8.5 mW/cm² using electrochemical measurements at 30 °C under relative humidity atmosphere.     

Evidence for a threshold in the biosolubility of aluminosilicate vitreous fibers

Two series of aluminosilicate glasses have been synthesized with the nominal composition (64 − x) SiO₂–x Al₂O₃–36 Na₂O/CaO with x varying from 9 to 19 mol%. They have been corroded in static conditions in a solution that mimics in a simplified manner the intracellular medium of the lung alveolar macrophages (37 °C, pH 4.6, citric acid). The original and corroded glasses have been studied by ²⁷Al and ²⁹Si MAS NMR. Both series display a sharp increase in the silicon dissolution rate with the alumina content. The glass network dissolves extremely slowly, whereas the release of excess sodium is very fast, for the glasses with low alumina content. On the opposite, the glasses with high alumina content dissolve much more rapidly in a nearly congruent manner. The crossover between the two behaviors occurs for x = 13, which corresponds to 33% of aluminum in the glass-former network. The sharp crossover from slow to fast network dissolution is explained in terms of connectivity of the silica sub-network. Above a certain amount of alumina, the silicon sub-network is no more percolating and the corroded glass breaks up into colloids. The sharpness of the transition and the relatively low alumina content required for fast dissolution are related to a structural feature of the aluminosilicate glasses, namely the aluminum self-avoidance that decreases the connectivity of the silica sub-lattice.     

29Si and 17O NMR investigation of the structure of some crystalline calcium silicate hydrates

This paper presents the results of a systematic investigation of the structure of ¹⁷O-enriched, hydrothermally synthesized 1.1-nm tobermorite, 1.4-nm tobermorite, jennite, calciochondrodite, xonotlite, and hillebrandite, using ²⁹Si magic angle spinning (MAS) NMR, ¹H-²⁹Si cross-polarization magic angle spinning (CPMAS) NMR, and ¹⁷O MAS NMR. The ¹⁷O and most of the ¹H-²⁹Si CPMAS results are the first reported for these phases. Six types of oxygen sites were observed in tobermorite and jennite, including both SiOH CaOH linkages. The structure of 1.4-nm tobermorite is similar to that of 1.1-nm tobermorite with about 26% of the Ca²⁺′s in the interlayers. The results support the proposed jennite structure in which silicate chains and rows of OH⁻ groups alternately occur along the CaO layers [1]. Jennite contains long, single silicate chains similar to those in 1.4-nm tobermorite, with SiOH sites primarily occurring on bridging tetrahedra, and there seems to be no interlayer Ca²⁺′s. Although the Si sites in xonotlite and calciochondrodite cross-polarize well, neither contains SiOH linkages.     

Multinuclear solid-state n.m.r. investigation of zeolite MCM-22

In this paper, the results of the first high-field (B₀ = 17.6 T) ²⁷Al MAS n.m.r. measurement on zeolite MCM-22 are reported. The ²⁹Si and ²⁷Al MAS n.m.r. spectra of as-synthesized and dealuminated zeolite MCM-22 were interpreted by signals of framework silicon and aluminum atoms located on at least five crystallographically nonequivalent T sites. After ammonium exchange and calcination of zeolite MCM-22, acidic bridging OH groups identified by a ¹H MAS n.m.r. signal at 4.2 ppm are formed. Using ²⁹Si CP/MAS n.m.r. spectroscopy these bridging OH groups were attributed to hydroxyl protons bonded via oxygen bridges to Si(1 Al) atoms located on all the at least five crystallographically nonequivalent T sites.