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1.
Flávio A Rodrigues 《Cement and Concrete Research》2003,33(10):1525-1529
Rice hull is an agricultural by-product containing about 20% of silica. Usually, this material is burned at the rice fields generating small silica particles, which may cause respiratory and environmental damage. This work describes the use of rice hull ash as a raw material to prepare Ca2SiO4-related cements, which is a component of commercial Portland cement. Rice hull was heated at 600 °C rendering silica with a surface area of 21 m2 g−1. This material was mixed with CaO and BaCl2·2H2O in several proportions, added stoichiometricaly in order to keep a ratio (Ca+Ba)/Si=2. The solids were mixed with water 1:20 (w/w) and sonicated for 60 min. The suspensions were dried and heated at several temperatures (from 500 to 1100 °C). The resulting solids were analyzed by FT-IR spectroscopy and X-ray diffraction. Cements with structure similar to that of β-Ca2SiO4 were obtained at temperatures as low as 700 °C, according to the composition. 相似文献
2.
Rod-like CaxSiOx + 2 catalysts were synthesized by using one-pot hydrothermal method. Catalysts calcined at 550 °C were used in the transesterification reaction of soybean oil with methanol. Under methanol reflux condition, FAME yields of 82% and 95% were achieved on Ca4SiO6 in a reaction time of 1 and 2 h, separately. Also, the FAME yields on different CaxSiOx + 2 catalysts were correlated with their basic properties. Besides, a FAME yield of ca. 80% can be achieved under room temperature over Ca4SiO6 catalyst. 相似文献
3.
Xiaoming Liu Na Zhang Henghu Sun Jixiu Zhang Longtu Li 《Cement and Concrete Research》2011,41(8):847-853
The cementitious behavior of red mud derived from Bauxite-Calcination method was investigated in this research. Red mud were calcined in the interval 400–900 °C to enhance their pozzolanic activity and then characterized in depth through XRD, FTIR and 29Si MAS-NMR techniques with the aim to correlate phase transitions and structural features with the cementitious activity. The cementitious activity of calcined red mud was evaluated through testing the compressive strength of blended cement mortars. The results indicate that red mud calcined at 600 °C has good cementitious activity due to the formation of poorly-crystallized Ca2SiO4. The poorly-crystallized Ca2SiO4 is a metastable phase which will transform into highly-crystallized Ca2SiO4 with the increase of calcination temperature from 700 °C moving to 900 °C. It is the metastable phase that mainly contributes to the good cementitious activity of red mud. This paper points out another promising direction for the proper utilization of red mud. 相似文献
4.
Cubic and orthorhombic crystals of tricalcium aluminate doped with Na2O, Fe2O3 and SiO2 were prepared and examined using an electron probe microanalyzer. The cationic ratios based on six oxygen atoms were derived from the oxide compositions. These data, together with those in previous studies for clinker aluminates containing Mg2+ and K+, provided excellent correlations between Al+Fe and Si (Al+Fe=2.001−1.03Si) and Ca+Mg and Na+K+Si [Ca+Mg=3.006−0.51(Na+K+Si)]. The chemical variation that is constrained by these equations is well accounted for by the general formula (Na,K)2x(Ca,Mg)3−x−y[(Al,Fe)1−ySiy]2O6, where x is the amount of Ca substituted by Na and K (0≤x<0.158), and y is the amount of Al substituted by Si (0≤y<0.136). The crystal system changed from cubic to orthorhombic with increasing x value. The substitution of Si and Fe for Al extended the solid solution range of the orthorhombic phase to lower values of x, while its effect on the solid solution range of the cubic phase was reversed. 相似文献
5.
Barium aluminate cements have been synthesized by barium carbonate, alumina, kaolin and colloidal silica as starting materials. The effects of the source of SiO2 and of firing temperature on phase formation and physical properties of the fired cements have been studied. Cement samples were characterized using XRD, SEM, EDX. The setting time and heat of hydration of cements were also evaluated. The barium aluminate cements were mixed in castables. Cold crushing strengths evaluated, and values compared to those obtained using calcium aluminate cement (Secar 71). Mixtures of BaCO3 and Al2O3 were targeted to produce BaAl2O4; which had fast set time, expansive behavior and lower strength compared to samples with SiO2 additions. SiO2 additions, regardless of source, resulted in BaAl2Si2O8 (celsian) formation. The prepared samples had short setting times and higher mechanical properties in comparison with standard calcium aluminate cement. 相似文献
6.
Formation and stability of hydrogarnet and Al-substituted tobermorite were examined at 175 °C temperature in saturated steam environment processing CaO-quartz and CaO-amorphous SiO2 suspensions. A large quantity of Al2O3 was added to the starting mixtures [molar ratio A/(S+A)=0.10, duration of hydrothermal synthesis—from 0 to 24 h]. It was determined that hydrogarnets always tend to form more rapidly than 1.13 nm tobermorite. However, later, with extension of synthesis duration, they start to fracture and their quantity reduces almost in half during 24 h. CaO is present in the further reaction with SiO2 forming hydrated calcium silicates, and released Al3+ ions are inserted into Al-substituted tobermorite crystal lattice. Using amorphous SiO2·nH2O as SiO2 component, starting raw materials react considerably quicker—the total Ca(OH)2 is joined already while increasing the temperature up to 175 °C. Meanwhile, in the mixtures with quartz when their composition is described by the molar ratio C/(S+A)=1.0, traces of Ca(OH)2 are found even after 24-h isothermal treatment at 175 °C temperature. Moreover, it depends on SiO2 modification the hydrogarnets of what type are to be formed. Si-free hydrogrossular forms in the mixtures with quartz and katoite in the mixtures with SiO2·nH2O. Si4+ ions are inserted into the crystal lattice of the latter compound while the first one remains undisturbed. This is presumably related to the lower solubility of the quartz. It was also noticed that an isomorphic Si4+ ions substitution with Al3+ ions in the hydrated calcium silicate lattice is considerably quicker when an amorphous SiO2 is used as SiO2 component instead of quartz. 相似文献
7.
This work describes the synthesis and hydration behavior of dicalcium silicates doped with manganese. The syntheses were performed using silica obtained from rice hull ash. The solids (SiO2, CaO and MnO) were weighed in stoichiometric proportions to prepare silicates having a ratio (Ca + Mn)/Si = 2. Insertion of manganese varied from 1 to 10% (mol). Solids were grounded and water was added rendering aqueous suspensions. The suspensions were sonicated for 60 min in an ultrasonic bath. After drying, the resulting solids were grounded and burned at 800 °C. The preparation of calcium silicates containing up to 10% of manganese oxide was observed.Hydration degree of a dicalcium silicate and calcium silicate containing 5% of manganese was determined by thermal analysis. Both materials present similar behavior. Hydration degree reaches approximately 70% after 60 days. 相似文献
8.
Wenjuan Hou Baoshan Wu Xia An Tingzhen Li Zhichao Tao Hongyan Zheng Hongwei Xiang Yongwang Li 《Catalysis Letters》2007,119(3-4):353-360
The effects of the ratio of precipitated SiO2 to binder SiO2 (Si(P)/Si(B)) on the reduction, carburization and catalytic behavior of precipitated Fe–Cu–K–SiO2 catalysts for Fischer–Tropsch synthesis (FTS) were investigated by N2 physisorption, temperature-programmed reduction/desorption (TPR/TPD) and Mössbauer effect spectroscopy (MES). FTS performances of the catalysts were tested in a continuous stirred tank reactor (CSTR). It is found that the increase of Si(P)/Si(B) ratio (Si(P)/Si(B) = 0/25 ~ 15/10) decreases the crystallite size of the catalysts, improves the surface basicity, enhances the reduction and carburization of the catalysts, and increases the activity of the catalyst. However, when Si(P)/Si(B) ratio is further increased (Si(P)/Si(B) = 25/0), the catalyst exhibits a restrained reduction and carburization behavior, which may be attributed to the stronger metal–support interaction. Based on the present work, a catalyst with a suitable ratio of Si(P)/Si(B), for example Si(15)/Si(10) displays an optimal FTS performances. 相似文献
9.
SnO2-doped CaSiO3 ceramics were successfully synthesized by a solid-state method. Effects of different SnO2 additions on the sintering behavior, microstructure and dielectric properties of Ca(Sn1−xSix)O3 (x=0.5–1.0) ceramics have been investigated. SnO2 improved the densification process and expanded the sintering temperature range effectively. Moreover, Sn4+ substituting for Si4+ sites leads to the emergence of Ca3SnSi2O9 phase, which has a positive effect on the dielectric properties of CaO–SiO2–SnO2 materials, especially the Qf value. The Ca(Sn0.1Si0.9)O3 ceramics sintered at 1375 °C possessed good microwave dielectric properties: εr =7.92, Qf =58,000 GHz and τf=−42 ppm/°C. The Ca(Sn0.4Si0.6)O3 ceramics sintered at 1450 °C also exhibited good microwave dielectric properties of εr=9.27, Qf=63,000 GHz, and τf=−52 ppm/°C. Thus, they are promising candidate materials for millimeter-wave devices. 相似文献
10.
Homogeneous CaO-SiO2-H2O gels were prepared at Ca/Si molar ratios 0.83, 1.01, 1.21, 1.50, 1.83 and 2.02. These were aged for 12-24 months at 25 °C and subsequently treated in steam, 1 bar total pressure, at 130 or 200 °C; also in water at 55 and 85 °C. Gels with low Ca/Si ratios partially crystallised at 85 °C. At 130 °C in steam, crystalline products included 11 and 14 Å tobermorite, xonotlite, afwillite, portlandite and another incompletely characterised phase. At 200 °C, the gels retained much water but remained amorphous to X-ray powder diffraction (XRD). However, electron microscopy, coupled with diffraction and analysis, disclosed that the “amorphous” product obtained at 85-200 °C had undergone crystallisation with domains typically 10-1000 nm. At higher bulk Ca/Si ratios, 1.83 and 2.02, much nanoscale precipitation of Ca(OH)2 occurs, probably by exsolution, such that the residual C-S-H product has a Ca/Si ratio in the range 1.4-1.5. The complex thermal history of the products is reflected in their pH conditioning ability, measured at 25 °C. The results are applied to predict the evolution of pH in a cement-conditioned nuclear waste repository which experiences a prolonged thermal excursion. 相似文献
11.
12.
The microwave dielectric properties of (BaxMg1−x)(A0.05Ti0.95)TiO3 (A=Zr, Sn) ceramics were investigated with regard to substitution of Ba for Mg of A-site. The microwave dielectric properties were correlated with the Ba content. With an increase in Ba content from 0.01 to 0.1, the dielectric constant and the τf value increased, but the Q×f value decreased. The sintered (BaxMg1−x)(Zr0.05Ti0.95)TiO3 (called BxMZT) ceramics had a permittivity in the range of 19.1−20.6, quality factor from 180,000 to 25,000 GHz, and variation in temperature coefficient of resonant frequency from −35 to −39 ppm/°C with increasing composition x. For sintered (BaxMg1−x)(Sn0.05Ti0.95)TiO3 (called BxMST) ceramics, the dielectric constant increased from 19 to 20.5, Q×f value increased from 120,000 to 37,000 (GHz), and the τf value increased from −50 to −3.3 ppm/°C as the x increased from 0.01 to 0.1. When A=Sn and x=0.1, (Ba0.1Mg0.9)(Sn0.05Ti0.95)TiO3 ceramics exhibited dielectric constant of 20.5, Q×f value of 37,000 (GHz), and a near-zero τf value of −3.3 ppm/°C sintered at 1210 °C for 4 h. 相似文献
13.
Phase equilibria of the CaO-BaO-SiO2-12mol pct. Al2O3-13mol pct. MgO system with a wide substitution range of CaO with BaO have been experimentally determined at 1623 K (1350°C) and 1573 K (1300°C) using high-temperature equilibration followed by X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron probe microanalysis (EPMA) analysis. The (Ca,Ba)3MgSi2O8, BaAl2O4, BaAl2Si2O8 and Ca2(Al0.46Mg0.54)(Al0.46Si1.54)O7 phases have been designated within the phase diagram. The liquidus temperature initially decreased and then increased for the samples substituting CaO with BaO at a constant (C + B)/S ratio between 1.50 and 1.86. For samples with a constant BaO/CaO ratio, the liquidus temperature showed similar trends with a lower (C + B)/S ratio from 1.22 to 0.67. The volume fraction of crystal phases of the samples as-quenched from 1573 K (1300°C) correlated well with the variation of liquidus and the primary phases. In addition, the change in the Gibbs free energy of the reactions and the bond parameter (Xp × Z/Rk) of the cations were analyzed, where the maximum change in the Gibbs free energy was found for the formation of Ca3MgSi2O8; furthermore, the stronger basic tendency of Ba2+ than Ca2+ facilitates Ba2+ substitution for Ca2+ and bonding with acidic tendency cations to form Ba-containing phases. 相似文献
14.
K. Mori T. Fukunaga K. Iwase K. Oishi M. Yonemura M. Sugiyama T. Kamiyama 《Cement and Concrete Research》2006,36(11):2033-2038
Mechanical milling was carried out to synthesize amorphous tricalcium silicate (Ca3SiO5) sample, where Ca3SiO5 is the most principal component of Portland cement. The partial phase transformation from the crystalline to the amorphous state was observed by X-ray and neutron diffractions. Moreover, it was found that the structural distortion on the Ca-O correlation exists in the milled Ca3SiO5. The hydration of the milled Ca3SiO5 with D2O proceeds as follows: the formation of hydration products such as Ca(OD)2 rapidly occurs in the early hydration stage, and then proceeds slowly after about 15 h. The induction time for the hydration of the milled Ca3SiO5 is approximately one half shorter than that for the hydration of the unmilled one. This result means that the mechanical milling brings about the chemical activity of Ca3SiO5 for hydration, and may be particularly useful for increasing the reactivity in the early hydration stage. 相似文献
15.
The compounds formed when the divalent cations Mg2+, Cd2+, Ba2+ and Pb2+ are present during the preparation of Ca3Al2O6 have been studied using X-ray microanalysis and diffraction methods. The smaller Mg cations are found to partially substitute for Ca2+, and structural refinements show that Mg preferentially occupies the smaller six-coordinate sites in Ca3−xMgxAl2O6. When Ba is present, it preferentially occupies the larger eight- and nine-coordinate sites. X-ray microanalysis suggests that Pb and Cd are lost from the samples during the preparation process. The diffraction patterns show a small decrease in the lattice parameters, suggesting that a defect structure of the type Ca3−x(vac)xAl2O6 is formed. The distribution of products formed on hydration of the doped Ca3−xMxAl2O6 is found to be very different than that observed for the undoped material. 相似文献
16.
K. M. Hanna R. L. Cook D. L. Kantro 《Journal of chemical technology and biotechnology (Oxford, Oxfordshire : 1986)》1970,20(11):334-340
Dibarium silicate, Ba2SiO4, was hydrated in two ways: in paste form at 25° using a water/solid weight ratio of 0.7:1, and in a polyethylene bottle rotated on a wheel at 5°, 25° and 50°, using a water/solid weight ratio of 9.0:1. When Ba2SiO4 is hydrated in paste form, the stoicheiometry of the reaction at 25° is the same as in bottle-hydration at 50°: 2BaO.SiO2+2.2H2O = 1.2BaO.SiO2.1.4H2O+0.8Ba(OH)2. The stoicheiometry of bottle-hydration at 5° and 25° is represented by the equation: 2BaO.SiO2+2.2H2O = BaO.SiO.1.2H2O+Ba(OH)2. Barium silicate hydrate, 1.2BaO.SiO2.1.4H2O, is well crystallised and has a specific surface area of ? 3m2/g. The crystals are plate-like and have a tendency to form clusters. The low-baria hydrate, BaO.SiO2.1.2H2O, is poorly crystallised and consists of thin platelets. It has a specific surface area of ? 35m2/g. The thermal dehydration of fully hydrated barium silicate and of the barium silicate hydrates was investigated by thermogravimetric and differential thermal analysis techniques. The similarities and differences between the barium silicate hydrates obtained in the hydration of barium silicate and the calcium silicate hydrates obtained in the hydration of β-dicalcium silicate and Ca3SiO5 are discussed. A mechanism of hydration of barium silicate is proposed which involves solution, precipitation and crystallisation steps. 相似文献
17.
Bulk Cu0.5Tl0.5Ba2(Ca2−xYx)Cu3O10−δ superconductor ceramic samples were synthesized by the conventional solid-state method and characterized by X-ray diffraction, dc-resistivity, ac-susceptibility and Fourier Transform Infrared spectroscopy. The main purpose of this study was to investigate the role of charge carriers and the effect of Y substitution at Ca sites in between the CuO2 planes on superconductivity. The superconducting properties are suppressed by Y substitution at Ca sites in between the CuO2 planes of Cu0.5Tl0.5Ba2(Ca2−xYx)Cu3O10−δ samples. It is most likely that Y3+ may create correlated domains in between the CuO2 planes and localizes the carriers, which lowers the diamagnetic screening and suppresses the superconductivity. Therefore, cationic substitution reduces the three dimensional (3D) mobility of carriers, resulting in the reduction of the Fermi vector and velocity of the carriers, which in turn suppresses the superconducting properties of the material. 相似文献
18.
Yanzhong Wang Anthony ChesnaudEmile Bevillon Guilhem DezanneauJinlong Yang 《Ceramics International》2011,37(8):3351-3355
Ba2SnYO5.5 nanopowders were synthesized by a gel polymerization method. In this process, a gel containing Ba, Sn and Y cations has been obtained by the polymerization of acrylic acid using N,N′-methylene bis-acrylamide as a cross-linking reagent and hydrogen peroxide as an igniting reagent. The gel was calcined at 1200 °C, giving rise to the Ba2SnYO5.5 single-phase nanopowders with the grain size ranging from 50 nm to 70 nm. Nanopowders were sintered at 1150 °C by spark plasma sintering (SPS) to obtain dense nanostructure materials (> 95%) containing grains whose size ranges between 70 nm and 100 nm. Nanostructured Ba2SnYO5.5 shows a good chemical stability in wet atmosphere. However, its protonic conductivity decreases compared with that of microcrystallin Ba2SnYO5.5 ceramics. 相似文献
19.
The oxidation of CH4 with O2 at low pressure was carried out over SiO2 prepared from metal Si. The Si showed only total oxidation activity while the Si partly oxidized to SiO2 showed high selectivities to CH3OH and HCHO. The results on SiO2 prepared from Si were compared with those over commercial silicas. The role of SiO2 in the CH4 oxidation was discussed. 相似文献
20.
According to the results of IR characterization and catalytic study in ethylene hydroformylation, bimetallic Rh–Co catalysts
can be efficiently prepared from [Rh(CO)2Cl]2 and cobalt carbonyls by co‐impregnation on SiO2. The reaction of Co2(CO)8 with [Rh(CO)2Cl]2 (Rh : Co = 1 : 3 atomic ratio) gives rapidly RhCo3(CO)12 on the surface of SiO2. Although Co4(CO)12 is not reactive with [Rh(CO)2Cl]2 on SiO2 to form directly RhCo3(CO)12, an equivalent bimetallic catalyst can be easily obtained from ([Rh(CO)2Cl]2 + Co4(CO)12)/SiO2 or its derivative (Rh+ + Co2+)/SiO2 (Rh : Co = 1 : 3 atomic ratio) under reducing conditions.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献