Hydrothermal reaction between C6S6H and kaolinite for the formation of tobermorite at 180°C

11 Å tobermorites were made from CSH (Ca/Si = 1.14) and kaolinite with Ca/(Si+Al) = 0.8 and Al/(Si+Al)= 0.15 at 180°C. The CSH was prepared from colloidal silica and lime at 130°C and 180°C for 2 h. Reaction gives in succession CSH, poorly crystalline Al-substituted tobermorite, and highly crystalline Al-substituted tobermorite. The addition of kaolinite markedly accelerates the formation of tobermorite within 4 h, more effectively with CSH prepared at 130°C than with that prepared at 180°C. X-ray fluorescence diffractometry shows that the Al coordination number is a mixture of 4 and 6 in the initial products and shifts to 4 with an increase in processing time. This agrees with the results for the degree of reaction of the kaolinite.     

Hydrothermal reaction between CSH and kaolinite for the formation of tobermorite at 180°C

11 Å tobermorites were made from C---S---H (Ca/Si = 1.14) and kaolinite with Ca/(Si+Al) = 0.8 and Al/(Si+Al)= 0.15 at 180°C. The C---S---H was prepared from colloidal silica and lime at 130°C and 180°C for 2 h. Reaction gives in succession C---S---H, poorly crystalline Al-substituted tobermorite, and highly crystalline Al-substituted tobermorite. The addition of kaolinite markedly accelerates the formation of tobermorite within 4 h, more effectively with C---S---H prepared at 130°C than with that prepared at 180°C. X-ray fluorescence diffractometry shows that the Al coordination number is a mixture of 4 and 6 in the initial products and shifts to 4 with an increase in processing time. This agrees with the results for the degree of reaction of the kaolinite.     

Substituted Tobermorites: 27Al and 29Si MASNMR, Cation Exchange, and Water Sorption Studies

²⁷Al and ²⁹Si magic angle spinning nuclear magnetic resonance spectroscopy revealed that the maximum amount of Al that can be substituted for Si in the tobermorite structure is 15 to 20 mol%. Powder X-ray diffraction and cation exchange studies corroborate the above finding. Anomalous tobermorite structures resulted in all cases, and hydrogarnet appeared beyond the 15 to 20 mol% Al substitution limit for Si. The sorption of water molecules by synthetic [Al³⁺+ Na⁺]-substituted tobermorites and their cation-exchanged forms heated at 200°C under vacuum (≥10⁻³ torr) was measured at 25°C in order to probe the nature of the ion-exchange cavity. Samples with ≤30 mol% of Al substitution for Si showed isotherms of Brunauer, Demming, Demming, Teller (BDDT) type I and gave better linearity with the Langmuir plot than with the BET plot. Samples with ≥35 mol% of Al substitution for Si showed BDDT type II isotherms and gave better linearity with the BET than with the Langmuir equation. The Langmuir monolayer capacity was found to depend on the Al content and on the cationic form. The Li⁺- and Na⁺-exchanged tobermorites with about 20 mol% of Al substitution for Si showed the highest Langmuir monolayer water sorption capacity. The Cs⁺-exchanged tobermorites showed a smaller capacity than the Li⁺- or Na⁺-exchanged samples, which can be ascribed to clogging of the ion-exchange cavity by the large Cs⁺ ions.     

Natural 1.1 and 1.4 nm tobermorites from Fuka, Okayama, Japan: Chemical analysis, cell dimensions, Si NMR and thermal behavior

A natural specimen of tobermorite, which contained both 1.1 and 1.4 nm phases, was studied by optical microscopy, X-ray powder diffractometry (XRD), EPMA, analytical TEM, ²⁹Si NMR and TG-DTA, and the results were compared with those for a specimen from Crestmore, California. The sample, occurring as a vein in contact with metamorphic rocks, could be divided into three zones. In two of the zones containing only the 1.1 nm phase, about 10% of the Si were substituted by Al. The third zone was an intergrowth of 1.1 and 1.4 nm tobermorites and contained almost no Al. The basal spacings of the 1.1 and 1.4 nm tobermorites were changed mainly to 0.9 nm by heating at 300 °C for 24 h but in some parts remained at 1.1 nm. ²⁹Si NMR results showed that silicate anions in the 1.1 nm tobermorite were double chains, which changed mainly into single chains, on heating at 300 °C. XRD results indicated that natural tobermorites are more highly crystalline than the synthetic ones of calcium silicate products.     

Hydrothermal Solidification of Kaolinite–Quartz–Lime Mixtures

The strength development of hydrothermally solidified kaolinite–quartz–lime systems with kaolinite as the aluminum source was studied. The starting materials were mixed so that the Ca/(Si + Al) atomic ratio was in the range 0.23 to 0.25, and the Al/(Si + Al) ratio was between 0 to 0.50. Specimens were formed by uniaxial pressing and hydrothermal treatment under saturated steam pressure at 200°C for 2 to 20 h. For quartz-rich systems with Al/(Si + Al) = 0 and 0.05, strength development by the formation of calcium silicate hydrates, such as C–S–H and tobermorite (Ca₅(Si₆O₁₈H₂)·(4H₂O), was observed. On the other hand, in the case of kaolinite-rich systems with Al/(Si + Al) = 0.24 to 0.50, strength development by the formation of hydrogarnet (Ca₃Al₂(SiO₄)(OH)₈) was recognized, resulting in flexural strengths between 15 to 20 MPa. It is proposed that strength development is related to the formation of mesopores (∼0.04 μm) that accompanied formation of the hydrogarnet.     

Synthesis of normal and anomalous tobermorites

Tobermorites were made from several starting materials at 105 – 180°C and Ca/(Si + A?) 0.8 – 1.0. Reaction gives in succession C-S-H, normal, mixed and anomalous tobermorites, and finally xonotlite. High C/S ratio (1.0), short time, low temperature, stirring, presence of A?, and if quartz is used, small particle size, all tend to stop it at normal tobermorite. In some cases, this effect is due to promotion of crystal growth of normal tobermorite. Low C/S ratio (0.8), long time, high temperature, no stirring, presence of A? plus alkali, and, if quartz is used, large particle size, all tend to give anomalous tobermorite. However, at 180° C this changes easily into xonotlite if C/S > 0.9.     

Synthesis of 11 Å Al-substituted tobermorite from trachyte rock by hydrothermal treatment

The alkaline hydrothermal activation of trachyte rock led to synthesis of technologically important 11 Å tobermorite. Tobermorite synthesis was studied by X-ray diffraction, scanning electron microscopy and ²⁹Si and ²⁷Al high resolution magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy. The influence of the reaction conditions such as different temperatures (150–170 °C), times (5–20 h) as well as different Ca/Si ratios of 0.6, 0.9 and 1.3 on tobermorite formation were investigated. The results showed that the main rock constituents were completely converted into a well crystallized Al-substituted 11 Å tobermorite when hydrothermally activated with 3.0 M NaOH under the optimum hydrothermal conditions of 170 °C for 20 h and using Ca/Si and Al/Al + Si ratios of 0.9 and 0.17, respectively. The local structure of the synthesized tobermorites as determined by MAS-NMR spectroscopy implied an alumino-silicate mean chain length of 5.9 units with 79% of the interlayer cross-links which are of Si–O–Al configuration. The present results show that trachyte rock could be considered as a new economic resource for synthesizing Al-substituted 11 Å tobermorites.     

Sequestration of Cesium and Strontium by Tobermorite Synthesized from Fly Ashes

Tobermorites were synthesized from oxides and from fly ashes under mild hydrothermal conditions. The bulk compositions of the ash-based formulations were adjusted to give the same [Ca]/[Si + Al] ratio as for the oxides. Selective Cs and Sr exchange with these tobermorites was measured. Compared to those prepared from the oxides and from class C fly ash, the tobermorites obtained from class F fly ash exhibited superior Cs and Sr selective properties. This is apparently a result of more extensive Al incorporation with class F fly ash. For example, tobermorite synthesized from class F fly ash exhibited a cesium K _d (mL/g) of 3826 while those prepared from oxides and from class C fly ash showed K _d values of 1112 and 796, respectively, under the same conditions. These results suggest that fly ash waste materials can be converted into tobermorite, which can serve as a resource in the separation, immobilization, and disposal of radioactive species such as Cs and Sr.     

Formation of β-Silicon Nitride Crystals from (Si,Al,Mg,Y)(O,N) Liquid: I, Phase, Composition, and Shape Evolutions

Precipitation, growth, and coarsening of Si₃N₄ crystals in (Si,Al,Mg,Y)(O,N) liquids at 1680°C has been studied. The initial nucleation of β-Si₃N₄ occurs mostly on α-Si₃N₄ because of the very high supersaturation of the liquid. After a brief period of growth, the crystals then undergo accelerated coarsening, decreasing the crystal concentration by almost 100 times with little change in the total crystal volume. Meanwhile, the crystals gradually transform from β-Si₃N₄, by substituting Si-N with Al-O, to β'-SiAlON of various compositions. The evolution of aspect ratio strongly depends on the Si/(Al,Mg,Y) ratio, which is rationalized by cation segregation to the interface driven by the acidity-basicity differential between the liquid and the crystal.     

聚硅酸铝铁絮凝剂的制备及处理工业废水的实验研究

采用粉煤灰、工业废硫酸和硫铁矿渣为原料,通过自制的聚铁和聚铝,制备了不同Si/(Al+Fe)摩尔比的聚硅酸铝铁(PSAF)絮凝剂.实验结果表明:与传统无机盐絮凝剂相比,PSAF的絮凝效果较好,COD及色度去除率能达到78.62%和83.56%以上.实验得到PSAF最佳制备条件:Si/(Fe+Al)=1/20,Fe/Al...     

Mechanical Property Evolution during Autoclaving Process of Aerated Concrete Using Slag: I, Tobermorite Formation and Reaction Behavior of Slag

The chemical reactions in the formation of autoclaved aerated concrete (AAC) block were investigated. The samples were prepared using blast furnace slag at 180°C under saturated steam pressures for various times from 1 to 128 h. Autoclaving for 1 h yielded 1.1-nm tobermorite with higher Al substitution and lower Ca/(Al + Si) ratio than that made without slag, due to the high solubility of the slag during the initial stage of reaction. This suggests that the utilization of slag has an advantage of reducing processing time. On further reaction, the crystallinity of the tobermorite increases and its Ca/(Al + Si) ratio decreases, becoming constant after 64 h. After a long period of autoclaving. thick reaction rims are formed around unreacted slag particles, interrupting the diffusion of Ca from slag to matrix.     

Synthesis and Thermal Stability of Aluminum Titanate Solid Solutions

Aluminum titanate solid solutions with empirical formulas of Al₂Ti_1-xZr_xO₅, Al_6(2-x)(6+x)Si_6x/(6+x)□_6x/(6+x)TiO₅, and Al_2(1-x)Mg_xTi_1+xO₅ were synthesized by reaction sintering and annealed at 900° to 1300°C in air to evaluate the thermal stability. Substitution of Al in Al₂TiO₅ by Si and 2Al by Mg and Ti ions to form solid solutions such as AI_{6(2-x)/(6+x)l-Si}6x/(6+x)□_6x/(6+x)TiO₅, and Al_2(1-x)Mg_xTi_1+xO₅ was effective in controlling the thermal decomposition, but substitution of Ti by Zr had little effect.     

Crystallinity of large single crystals of FAU-type zeolites with a wide range of Si/Al ratios

Large colorless single crystals of FAU-type zeolites were synthesized from gels with the composition xSiO₂ : 2.0NaAlO₂ : 7.5NaOH : 454H₂O : 5.0TEA, where x = 2.0–6.0. FAU-type zeolite with Si/Al = 1.26(4) was nearly pure and the maximum size of the single crystals was ca. 150 μm. In case of FAU-type zeolites with Si/Al = 1.54(5), the maximum size of single crystals was ca. 200 μm and the ratio of FAU/impurity was 0.07. The framework Si/Al ratio of the as-synthesized FAU-type zeolite tended to increase with the Si/Al ratio of gel composition. All of the large single crystals had good crystallinities for single-crystal X-ray diffraction, leading to enough numbers of significant reflections which have strong intensity. The structure of a single crystal of dehydrated zeolite Na-X (Si/Al = 1.41(4)) with composition |Na₈₀|[Si₁₁₂Al₈₀O₃₈₄]-FAU per unit cell was determined by X-ray diffraction methods in the cubic space group \( Fd \bar{3} m; \) a = 24.9434(6) Å at 294 K. The structure was refined by using all intensities to the final error indices (using only the 771 reflections for which F _o > 4σ(F _o)), R ₁ = 0.048 (based on F) and R ₂ = 0.188 (based on F ²). In the crystallographic studies, the Si/Al ratio of the synthetic FAU-type zeolite is 1.41(4) which is quite consistent with the SEM–EDS analysis.     

Influence of aluminium on the conversion of calcium silicate hydrate gels into 11 Å tobermorite at 90°C and 120°C

Mixtures of CaO and colloidal silica with and without γ-A?₂O₃, CaO and alkali-bearing A?₂O₃-SiO₂ gels, or CaO and clinoptilolite were treated hydrothermally at 90°C or 120°C for 4 hr – 4 weeks. Reaction seemed always to proceed through formation of C-S-H gels to 11 Å tobermorite. In the absence of A?, tobermorite crystallized more rapidly at c/S = 1.0 than at C/S = 0.8 but in the presence of A?, it crystallized more rapidly at Ca/(Si + A?) = 0.8 than at Ca/(Si + A? = 1.0. Where the starting materials contained both A? and alkali the tobermorite showed anomalous thermal behaviour similar to that of the natural mineral from Loch Eynort, but where they contained A? but no alkali, the thermal behaviour of the tobermorite was complex.     

Al-substituted tobermorite—The coordination of aluminum as revealed by solid-state 27Al magic angle spinning (MAS) NMR

The coordination of A1 in several synthetic A1-substituted tobermorites was determined to be tetrahedral by solid-state ²⁷Al MASNMR. Earlier results of tetrahedral Al coordination in Al-substituted tobermorites by x-ray emission were confirmed unequivocally by the new high resolution MASNMR technique. However, two ²⁷Al chemical shifts at about 68 and 57 ppm were observed in the Al-substituted tobermorites which probably indicates two different environments for the Al atom in the structure. ²⁷Al MASNMR technique clearly distinguishes the tetrahedral and octahedral coordination of Al in solid samples.     

Formation of 11 Å tobermorite from mixtures of lime and colloidal silica with quartz

Mixtures of lime, colloidal silica and quartz (<10 μm, 10–20 μm) were treated hydrothermally in stirred suspensions at 180°C to prepare 11 Å tobermorite with Ca/Si = 0.8. The runs made using the colloidal silica and lime quickly formed CSH; but did not convert into crystalline tobermorite even after 20 h. The runs with the mixtures of colloidal silica and quartz gave highly crystalline 11 Å tobermorite after 5 h through the reaction of Ca-rich C-S-H and quartz. The reaction of quartz was controlled by its rate of dissolution. The thermal behaviour of the tobermorites was normal, trending to mixed with increase in processing time.     

Ethylene polymerization over (nBuCp)2ZrCl2/MAO catalytic system supported on aluminosilicate SBA‐15 mesostructured materials

Heterogeneous metallocene catalysts were prepared by incipient wetness impregnation of AlSBA‐15 (Si/Al = 4.8, 15, 30, 60, and ∞) mesostructured materials with (nBuCp)₂ZrCl₂/MAO. For comparative purposes commercial silica and silica–alumina (Si/Al = 4.8) supports were also impregnated with the MAO/metallocene catalytic system. A combination of X‐ray powder diffraction, nitrogen adsorption–desorption isotherms at 77 K, transmission electron microscopy, ICP‐atomic emission spectroscopy, and UV–vis spectroscopic data, were used to characterize the supports and the heterogeneous catalysts. Ethylene polymerizations were carried out in a schlenk tube at 70 °C and 1.2 bar of ethylene pressure. The polyethylene obtained was characterized by GPC, DSC, and SEM. Catalysts prepared with mesostructured SBA‐15 supports exhibited better catalytic performance than those supported on amorphous silica and silica–alumina. In general, higher ethylene polymerization activity was achieved if (nBuCp)₂ ZrCl₂/MAO catalytic system was heterogenized using supports with lower pore size in the range of the mesopores and lower Si/Al ratio. All catalysts produced high‐density polyethylene, with high crystallinity values and fibrous morphology when SBA‐15 mesostructured materials were used as supports. POLYM. ENG SCI., 2008. © 2008 Society of Plastics Engineers     

Solid-state high-resolution 29Si NMR spectroscopy of synthetic 14 Å, 11 Å and 9 Å tobermorites

A new method for the investigation of the structure of silicates - solid-state high-resolution ²⁹Si NMR spectroscopy - was used for the investigation of 14 Å, 11 Å and 9 Å tobermorites prepared under different conditions. It is possible to determine selectively the individual SiO₄ structural units in tobermorites (end groups, chain middle groups, branching sites, etc.) and to draw conclusions about the type and structure of the silicate anions in the lattice. Synthetic 14 Å tobermorite consists predominantly of single chains, the 11 Å and 9 Å tobermorites contain, depending on the method of preparation, double chains and/or single chains.     

Preparation and characterization of aluminium – containing MCM-41

Mesoporous aluminosilicate molecular sieves with the MCM-41 structure (Si/Al=20) have been synthesized using tetraethoxysilane (TEOS) and sodium aluminate (NaAlO₂) as Si and Al sources and hexadecyltrimethylammonium bromide (HTMABr) and varying several parameters during the preparation process. All the samples were characterized by X-ray diffraction (XRD), Fourier transformed infrared (FT-IR) and solid state MAS NMR spectroscopy. A significant improvement in the structure by using a novel method for template removal has been evaluated.     

Corrosion behavior of a Mo(Si,Al)2 composite at 1700°C in 95% N2 + 5% H2

A Mo(Si, Al)₂ based composite was pre-oxidized to establish an alumina scale on the material surface. Thereafter, the corrosion behavior of the composite was examined at 1700 °C for up to 24 h in 95% N₂ + 5% H₂.The weight change was followed by recording the material weight before and after exposure. The crystalline corrosion products were analyzed with X-ray diffraction (XRD) and the microstructure of the cross sectioned material was characterized using scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS).It was shown that AlN and Al₅O₆N layers developed on top of the pre-oxidized alumina layer and alumina threads develop out from the specimen surface. The accompanied aluminum consumption converts the substrate Mo(Si,Al)₂ into Mo₅Si₃ immediately below the alumina scale to the extent that the Mo₅Si₃ becomes porous underneath the alumina scale. Corrosion mechanisms are discussed with the support of thermodynamic calculations.