Showing papers on "Calcium aluminates published in 2003"

Modeling of damage in cement-based materials subjected to external sulfate attack. I: Formulation

Abstract: A chemomechanical mathematical model is presented to simulate the response of concrete exposed to external sulfate solutions. The model is based on the diffusion-reaction approach, and several mechanisms for the reaction of calcium aluminates with sulfates to form expansive ettringite are considered. Fick’s second law is assumed for diffusion of the sulfate ions. A second-order chemical reaction between reacting calcium aluminates and ingressing sulfates depletes the sulfate concentration. The products of the second-order reaction between the aluminates and sulfates are chosen among several competing mechanisms, and a rule-of-mixtures approach is used to relate the expansive nature of the products with the prescribed specific gravity of the compounds. It is furthermore assumed that the crystallization pressure of products of reaction results in a bulk expansion of the solid. The constitutive response of the matrix and the expansive stresses are calculated from the imposed volumetric strain. Microcracks are initiated when the strength of the matrix is reached, leading to changes in the diffusivity and a reduction in the elastic properties of the matrix. The variation of diffusivity is linked to the scalar damage parameter due to cracking of the matrix. Due to the changes in the diffusivity, the problem is treated as a moving boundary problem, and a methodology is proposed to adapt the solution of the 1D case to the 2D problem of a prismatic specimen. Theoretical expansion-time responses are obtained and compared with a variety of data available in the literature.

Modeling of Damage in Cement-Based Materials Subjected to External Sulfate Attack. II: Comparison with Experiments

Abstract: A study is presented to predict the degradation of cement-based materials due to external sulfate attack. Parameters of the model are chosen based on the mix design parameters, degree of hydration, and exposure conditions of concrete. A solution of the diffusion equation with a term for a second-order chemical reaction is proposed to determine the sulfate concentration and calcium aluminate profile as a function of time and space. The crystallization pressure of hydration products such as ettringite can lead to internal stresses. Using the volumetric information, the model predicts the generation of internal stresses, evolution of damage, reduction in stiffness, and thus expansion of a matrix phase. The theoretical expansion-time responses are obtained and compared with a variety of available data in the literature. The most important parameters are the w/c ratio, internal porosity, diffusivity of the cracked and uncracked material, and available calcium aluminates. The importance of controlling the pH of the test solution is clearly observed. Model simulations indicate a reasonable agreement with experimental expansion-time data available in the literature.

Thermodynamic examination of inclusion modification and precipitation from calcium treatment to solidified steel

Abstract: Calcium treatment is a well established method to transform oxide and sulphide inclusions in steel to less harmful inclusions which can have beneficial effects on properties. Transformation of solid alumina clusters to liquid calcium aluminates by calcium is a much used technique to avoid nozzle blocking in continuous casting of aluminium deoxidised steels. Calcium treatment is also a key method to improve machinability, which can be further improved by increased sulphur content. Resulphurised steels, however, tend to have casting problems due to deficient modification of oxides or formation of solid CaS inclusions which also have a tendency to clog the nozzle. In this study, the formation and transformation of liquid and solid inclusions containing oxidic and sulphidic components were examined by thermodynamic calculations. A quasichemical slag model was applied to calculate equilibrium oxide and sulphide inclusions in steel. Calculations were carried out over a wide temperature range to study th...

Effect of Y2O3 on low temperature sintering and thermal conductivity of AlN ceramics

Abstract: The effect of Y2O3 on the densification and thermal conductivity of AlN ceramics sintered at the temperature of 1650 °C was studied using CaF2–Y2O3 system as additives. XRD was employed to identify the phases formed during sintering. The results show that AlN ceramics with Y2O3 addition have a low shrinkage at temperatures below 1600 °C because the solid reaction between Y2O3 and Al2O3 decreases the amount of the calcium aluminates liquid due to the consumption of Al2O3. At 1650 °C, Y2O3 promotes the densification due to the formation of liquid CaYAlO4. The observation by TEM and HREM reveals that the inhomogeneous grain-boundary phases containing different phase compositions move to the three-grain junctions to form discrete pockets during the sintering. The measurement of the lattice parameters and the thermal conductivity of AlN ceramics shows that Y2O3 has no prominent effect on the purification of the AlN lattice at this sintering temperature. The higher thermal conductivity in the presence of Y2O3 mainly comes from the enhancement of the densification.

New spinel-containing refractory cements

Abstract: Use of spinel-containing high alumina refractory concretes has increased significantly over the last few years. Currently the high cost of sintered or electrofused spinels limits their applications. This paper describes new and inexpensive new spinel-containing refractory cements obtained by a reaction-sintering process between dolomite and alumina. The reaction-sintering process occurs in two well defined steps: (a) reaction and (b) sintering. The reaction also happened in two successive stages: (a) dolomite decomposition in the range 700–900 � C, and (b) reaction of lime and magnesia with alumina, in the range from 900 to 1250 � C. A selected composition containing: 43� 5 wt.% CaAl2O4 ,1 5� 3 wt.% CaAl4O7 and 42 � 2 wt.% MgAl2O4, showed a hydration and dehydration behaviour similar to that of commercial calcium aluminate cements. The evolution of the hydrated pure CaAl2O4 cement up to 1500 � C has also been studied. # 2002 Published by Elsevier Science Ltd.

Ettringite binder for dense mortar, comprising calcium sulphates and a mineral compound of calcium aluminates

Abstract: The invention relates to an ettringite binder for dense mortar, comprising calcium sulphates and a calcium aluminate mineral compound, the calcium aluminate mineral compound comprising calcium C and aluminum A oxides, soluble and combined in one or more crystallized and/or amorphous mineralogical phases, in such proportions that: the useful C/A molar ratio of the calcium aluminate mineral compound ranges from 1.2 to 2.7; the sum in weight of the useful (C+A) phases accounts for at least 30% of the total weight of the mineral compound.

Dilatometry as a tool to study a new synthesis for calcium hexaluminate

Abstract: By using a wet chemical route, pure calcium hexaluminate (CA6) was yielded, significantly lowering the reaction temperature and shortening the synthesis time if compared to usual industrial procedures. owever, dilatometric studies performed on compacts made of the as-prepared powder, just after pre-heating at 450°C, has shown a superposition between sintering shrinkage and expansion related to CA2 formation, an intermediate phase formed during calcination and phase evolution to CA6. oupling of such opposite phenomena led to microcracking of the material, mainly if the heating rates (10°C min-1) were high. However, lower heating rates (1-5°C min-1) could quite avoid microcracking but also limit densification.

Modification of non-metallic inclusions in steels with enhanced machinability

Abstract: The objective of this paper is to identify non-metallic inclusions occurring in aluminium-killed steel with regulated sulphur content and modified with calcium, explain the reaction between calcium and non-metallic inclusions and identify secondary metallurgy parameters, which condition obtaining such forms of inclusions to be favourable to improvement in steel machinability. Computer-aided thermodynamic calculations and production experiments were carried out. They enabled to determine technological parameters of treatment in liquid steel ladle deoxidised with silicon and aluminium, with low (0.01%) and increased (up to 0.03%) sulphur content. By comparison of the computer-aided simulations and production experiment results it was found that calcium in the steel both modifies the aluminium oxide inclusions and reacts with sulphur, whereas deep desulphurisation (below 0.01%) is favourable to oxide modification. Non-metallic inclusions of calcium aluminates, which affect positively the machinability of steel, appear in liquid state in the steel bath. During conventional casting and cooling down large ingots of steel with increased sulphur content, aluminates are significantly depleted in calcium as a result of reaction between calcium and sulphur dissolved in these aluminates and steel matrix. To obtain aluminates and (Ca,Mn)S sulphides rich in calcium in finished products accelerated steel cooling is to be applied, which takes place during continuous steel casting. In steels modified with calcium, there is higher homogeneity of distribution and sizes of inclusions on finished products' cross-section and higher globularisation of these inclusions in comparison to non-modified steels. Improved steel machining properties in these steels and improved isotropy of mechanical properties in bars made from it were found.

Persistent Luminescence of Eu2☎ and Na☎ Doped Alkaline Earth Aluminates

Abstract: The luminescence and afterglow properties of the Eu 2+ and Na + doped alkaline earth aluminates, stoichiometric and non-stoichiometric (M x Al 2 O 4 :Eu 2+ , Na + ; M=Ca or Sr, x = 097, 100 or 103, X Na = 002), were studied Broad band luminescence and afterglow of the Eu 2+ ion were observed in the blue (λ max = 440nm) and green (λ max = 520nm) region for the calcium and strontium aluminates, respectively Both Na + co-doping and strontium excess quenched the afterglow efficiently The results supported the mechanism of the persistent luminescence where the cation vacancies act as traps The results for the calcium aluminates were ambiguous, probably due to the slightly larger ionic radius of the Na + with respect to that of Ca 2+ The sodium ions may not fit into the calcium sites and thus form (an) independent compound(s)

On attack product and corrosion mechanism of ordinarily concrete after long-term exposure to salt lakes

Abstract: Attack products of concrete having exposed in salt lake for 20 years were investigated by XRD, DTA-TG, SEM-EDAX and IR method Three types of chemical corrosion on concrete exposed to salt lake bittern are found and reported firstly in this paper The results show that two attack products are corroded by alkali metal ions on calcium silicate hydrate CSH, i e transition pro duct calcium-magnesium silicate hydrate (C 1- x M x ) 1 15—1 70 SH( x =0 22—0 39)of calcium silicate hydrate CSH-magnesium silicate hydrate MSH and another type is the formation of alkali silica gel(C 1- x N x ) 1 85—1 60 SH( x =0 39—0 63) by Na ions substituting Ca ions Besides corrosion mechanism of three types of cement hydration products in salt lakes, i e portlandite CH, calcium aluminates hydrate C 3AH 6 and CSH gel, are proposed

Improved quantum-splitting oxide phosphor, its producing method, and rule for designing the same

Abstract: PROBLEM TO BE SOLVED: To provide an oxide phosphor exhibiting higher quantum efficiency when irradiated by a vacuum ultraviolet ray, its producing method, its designing rule and a light source having higher energy efficiency using the phosphor SOLUTION: Strontium and strontium calcium aluminates and lanthanum and lanthanum magnesium borates activated with Pr and Mn exhibit characteristics of quantum-splitting phosphors Improved quantum efficiency may be obtained by further doping with Gd Refined rules for designing quantum-splitting phosphors include the requirement of incorporation of Gd and Mn in the host lattice for facilitation of energy migration

Mechanical evaluation at high temperatures of hot gunning refractory mixtures

Abstract: The mechanical behavior at high temperatures of three hot gunning refractory mixtures used for coke oven repair was studied. The materials were characterized by chemical, mineralogical, granulometric, and microscopic analyses, exhibiting differences in composition and particle size distributions of both aggregates and bonding phases. The hot adhesion of the mixtures to silica bricks was determined using torsional forces, and the interphase between them was analyzed by scanning electron microscopy. Thermal expansion up to 1200 °C and creep under compressive load (172 kPa at 1200 °C, 3 h) were carried out on prisms prepared by ramming and sintering (1200 °C, 1 h). The creep strain–time data are well fitted using constitutive equations. The mixtures showed differences in both hot adhesion and thermal expansion–creep tests: the material based on calcium aluminates exhibited poorer properties than those based on calcium silicate. The results are explainded taking into consideration the characteristics of both aggregates and bonding phases.

Ettringite binder comprising calcium sulfate and a calcium aluminate composition

Abstract: Ettringite binder for dense mortar comprises calcium sulfates and a mineral compound of aluminates comprising soluble oxides of calcium (C) and aluminum (A) combined in one or more crystalline and/or amorphous phases The molar ratio C/A of the mineral compound is 12-27 The total weight of the phases (C+A) is at least 30% of the total weight of the mineral compound Preferred Features: The weight ratio of the mineral compound of calcium aluminates to calcium sulfate is 05-4, more preferably 15-3 The molar ratio of calcium sulfate to aluminum oxide (A) in the ettringite binder is 05-2, more preferably 06-18, and especially 08-17 The molar ratio C/A of the mineral compound of calcium aluminates is 13-25, more preferably 16-2 The ratio of water to solids, on mixing the ettringite binder with water, is less than 05 The mortar may not comprise Portland cement and/or hydraulic lime, or may comprise Portland cement and/or hydraulic lime in an amount less than 35 weight % with respect to the total weight of the dry mortar The total amount of the phases (C+A) is at least 50% of the total weight of the mineral compound of calcium aluminates Independent claims are given for: (a) a dry mortar containing (in weight % with respect to the total weight of the dry mortar): the invented ettringite binder 15-75; limestone or siliceous sand fillers 25-85; lime and/or Portland cement 0-35; dispersible polymer powder 0-8 and/or a solid-liquid polymer dispersion 0-20; and rheological and/or set-time control additives; (b) a dry mortar containing (in weight % with respect to the total weight of the dry mortar): the invented ettringite binder 20-50; limestone or siliceous sand fillers 50-80; lime and/or Portland cement 0-05; dispersible polymer powder 0-5 and/or a solid-liquid polymer dispersion 0-15; and rheological additives 01-05 and/or set-time control additives 01-05; (c) a wet mortar obtained by mixing the dry mortar with water in an amount such that the weight ratio of water to solids is less than 05 (d) Use of the mineral compound of aluminates in the formulation of an ettringite binder, and in the formulation of the dry and wet mortars

Manganese-aluminum-calcium catalysts prepared by chemical mixing in water

Abstract: X-ray phase and differential thermal analyses, IR spectroscopy, and temperature-programmed reduction method were applied to study the formation of a manganese-aluminum-calcium system prepared by mixing manganese hydroxocarbonate and calcium aluminates in water, which can be used as catalyst in deep oxidation of organic substances. The influence exerted by preparation conditions on the extent of chemical interaction in the system was considered.