Showing papers on "Calcium aluminates published in 2001"

Microstructure development in calcium hexaluminate

Abstract: Calcium hexaluminate (CA6) was prepared from alumina and calcium carbonate powders. The influence of processing method and firing temperature on calcium hexaluminate grain morphology was studied. A significant correlation was found between grain morphology and green density, porosity distribution and presence of agglomerates. Platelet grains were observed for low green densities and large pores, while more equiaxed grains were found when green density was increased. A model is proposed for the formation of equiaxed or platelet grains. The model is based on the number of contact areas between alumina and calcium carbonate grains in green specimens as well as on the free space available for calcium hexaluminate to grow.

Historical and present day application of calcium aluminates cements

Abstract: Calcium aluminate cements were originally developed to provide improved durability in sulphate bearing environments and there are many cases of good performance in such environments extending over more than seventy years. Today, due mainly to their high cost, they are not used in situations where Portland cement concretes perform well, but find many specialist applications in severe chemical or abrasive environments. Such applications include sewage pipes, subject to acid corrosion due to bacteriological actions and areas of high abrasive wear in hydraulic dams. In these types of applications calcium aluminate cements effectively extend the range of performance of cementitious materials and compete with materials such as metals or plastics. However, these applications in conventional concrete account for only a minority of the calcium aluminate cements used today. A major use is in refractory concretes, where they combine the advantages of casting complex shapes at ambient temperatures with performance up to temperatures of more than 1600°C. In addition to this long established type of application, they are finding increasing use as a component of complex mixtures, with other cementitious materials such as Portland cement and calcium sulphates, where a vast range of different properties can be obtained including rapid hardening, combination of water, and controlled dimensional variation. This paper reviews the past and present day applications of calcium aluminate cements and explains how their chemistry enables materials with different properties to Portland cements to be obtained.

Metallurgical process engineering for castability-improvement in steel with elevated sulfur contents

Abstract: Trouble-free casting of Al deoxidized steel heats with high sulfur contents necessitates controlled addition of Ca in secondary metallurgy, in order to avoid the nozzle clogging effect. With too little Ca input on the one hand, clogging occurs as a result of the deposition of high melting point calcium aluminates, and at excessively high Ca inputs, on the other hand, precipitation of calcium sulfide occurs. Process engineering for precisely targeted Ca input is developed on the basis of thermodynamic calculations, general steelplant experience, and comprehensive operational tests. The new method reduces the failure rate from casting problems to less than one quarter. Sequence casting of Al killed grades with high sulfur contents is then no longer a problem.

Formation and evaluation of highly uniform aluminate interface coatings for sapphire fiber reinforced ceramic matrix composites (FRCMCs) using carboxylate-alumoxane nanoparticles

Abstract: Sapphire fibers have been dip-coated in aqueous and CHCl3 solutions of carboxylate-alumoxane nanoparticles and calcium-, lanthanum-, and yttrium-doped carboxylate-alumoxane nanoparticles and fired up to 1400°C to form uniform, conformal and contiguous, aluminate coatings. Optimum solvent, dip/dry, and firing sequences were determined for the formation of crack-free coatings. Both carboxylate-alumoxane and ceramic coated fibers were examined by field emission scanning electron and transmission electron microscopy, microprobe analysis and optical microscopy. Coatings produced were stable to thermal cycling in air up to a temperature of 1400°C. The ability of the carboxylate-alumoxanes to provide crack infiltration and repair was demonstrated. Sapphire fiber/alumina matrix FRCMCs have been prepared with calcium-, lanthanum-, and yttrium-aluminate interphase layers. Microscopy and fiber push-out data confirm that the calcium- and lanthanum-aluminate coatings provide a means for controlling failure properties at the fiber-matrix interface. However, FRCMCs containing YAG-coated fibers failed catastrophically before interfacial debonding and/or sliding occurred.

Catalyst for steam cracking reactions

Abstract: Catalyst for steam cracking reactions consisting of crystalline calcium aluminates having a molar ratio CaO/Al2O3 ranging from 1/6 to 3 and of molybdenum and/or vanadium oxides, wherein the molybdenum oxide, expressed as MoO3, or the vanadium oxide, expressed as V2O5, or the sum of said two oxides, is in a quantity ranging from 05 to 10%, preferably from 08 to 5% by weight

Quantum-splitting oxide-based phosphors, method of producing, and rules for designing the same

Abstract: Strontium and strontium calcium aluminates and lanthanum and lanthanum magnesium borates activated with Pr3+ and Mn2+ exhibit characteristics of quantum-splitting phosphors. Improved quantum efficiency may be obtained by further doping with Gd3+. Refined rules for designing quantum-splitting phosphors include the requirement of incorporation of Gd3+ and Mn2+ in the host lattice for facilitation of energy migration.

Effect of Calcium Silicates on Immobilization of Fluorine in Aqueous Solution

Abstract: Immobilization of fluorine in aqueous solution with 2CaO.SiO 2 (C 2 S) and 3CaO.SiO 2 (C 3 S), and the effect of C 2 S and C 3 S on the fluorine immobilization with 3CaO.Al 2 O 3 (C 3 A) and 12CaO.7Al 2 O 3 (C 12 A 2 ) have been studied. The hydration products are identified by X-ray diffraction method and the fluorine-substituted compounds are confirmed by electron microprobe analysis. It is found that C 3 S appears more effective to immobilization of fluorine than C 2 S and (OH - ) site in calcium silicate hydrates is substituted with F - ion. The behavior of fluorine immobilization by calcium aluminates changes with the addition of calcium silicates because calcium silicates affect the formation of Ca 3 Al 2 (OH) 12-x F x , which is the most effective calcium aluminate hydrate in view of the incorporation of F - ion. The fluorine immobilization with C 12 A 2 can be promoted in the presence of appropriate amount of C 2 S or C 3 S.

Thermal conductivity and mechanical properties related to microstructure of a high alumina refractory castable

Abstract: Thermo-mechanical properties and thermal conductivity of a high alumina refractory containing 10 wt% of synthetic spinel have been studied, from room temperature up to 1600°C. Results have been correlated with the microstructural and mineralogical evolutions by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM) as a function of the firing temperature. The refractory is considered as a composite material, formed by coarse aggregates embedded in a fine matrix. For this purpose, dry castable raw materials mixtures were sieved under 125 microns in order to separate the fraction considered as the matrix of this castable. Matrix samples were obtained for thermomechanical characterization and the results were compared to those corresponding to the whole refractory. The different properties are strongly dependent on the temperature since many transformations occur in the binder from the hydration at room temperature to the sintering process up to 1 600°C. Particularly, the dehydration process from 200°C to 900°C is responsible for the great variation of the thermal and thermomechanical properties. At higher temperatures, crystallisation of calcium aluminates plays a role of major importance on these properties.

Shotcrete material and shotcrete method

Abstract: PROBLEM TO BE SOLVED: To provide a shotcrete material, or in particular a dry process shotcrete material capable of reducing the rebound ratio and the amount of dust, immediately after shortcreting dry-mixed cement concrete. SOLUTION: This shotcrete material comprises dry-mixed cement concrete and a quick setting agent slurry which consists of 100 mass parts of a quick setting component containing calcium aluminates and 350-600 mass parts of water, wherein optionally, the quick setting component further additionally contains sulfates, alkali metal aluminates and/or alkali metal carbonates.

Chemical characterization of hydrothermal hot-pressed calcium aluminate-phosphate-silica cement

Abstract: This paper describes another effective use of calcium aluminates in producing a high performance material. The material essentially consists of a calcium aluminate, a phosphate and a siliceous powder and it is fabricated by a hydrothermal hot-press method. The solidified bodies exhibit a high strength, especially for the system calcium hexa-aluminate, sodium or potassium meta-phosphate and silica fume (CAPS). It is necessary to use crystalline calcium hexa-aluminate and silica together with phosphate for preparing a high strength body. Similar to crystalline calcium hexa-aluminate, the other crystalline alkali earth metal hexa-aluminates, such as SrAl 12 O 19 and BaAl 12 O 19 also give a high strength. The strength of CAPS exhibits no strength degradation with time. Moreover, the thermal expansion of CAPS is very small below 500°C. The microstructure of CAPS is assumed to consist of crystalline alkali earth metal hexa-aluminates embedded in a cementitious poly-anion network and it is free from large defects.

Reaction of Manganese(II) Hydroxocarbonate with Calcium Aluminates in Aqueous Medium

Abstract: The reactions of manganese hydroxocarbonate MnCO3·mMn(OH)2·nH2O with calcium aluminates CaAl2O4 and CaAl4O7 in aqueous medium were studied by X-ray phase and differential thermal analyses and by IR spectroscopy.

Process for the regeneration of catalysts for steam cracking

Abstract: Process for the regeneration of exhausted catalysts, containing one or more calcium aluminates, for steam cracking reactions, comprising treating said exhausted catalysts in a stream of water vapour at a temperature ranging from 700 to 950 °C, preferably from 720 to 850 °C, and at a pressure ranging from 0.5 to 2 atm.