Showing papers on "Calcium aluminates published in 2009"

Corrosion mechanism of polycrystalline corundum and calcium hexaluminate by calcium silicate slags

Abstract: The chemical reactions involved in the corrosion of polycrystalline alumina (Al 2 O 3 ) and calcium hexaluminate–hibonite (CaAl 12 O 19 ) ceramics by two dicalcium silicate slags with additions of fluorspar (CaF 2 ) were studied using a hot-stage microscopy (HSM) up to 1600 °C. The corrosion mechanism was investigated on post-mortem corroded samples and the phases formed at different stages of the dissolution process were characterised by reflected optical light microscopy (RLOM) and scanning electron microscopy (SEM) with energy dispersive X-ray spectrometry (EDS) microanalysis system. The attack of the fused slags on dense alumina substrates takes place through an interdiffusion mechanism producing successive layers of calcium aluminates. In porous hibonite samples chemical interactions were observed although only a layer of calcium dialuminate was formed. A sintering process in presence of liquid phase was also detected behind the reaction interphase. Thermodynamic calculations, based on the Al 2 O 3 –CaO–SiO 2 , Al 2 O 3 –CaO–SiO 2 –MgO, and Al 2 O 3 –CaO–SiO 2 –CaF 2 phase equilibrium were used to further knowledge of the corrosion mechanism.

Additives for cement

Abstract: The invention provides a belite-calcium sulphoaluminate-ferrite (BCSAF) cement composition comprising: a BC-NAP clinker which clinker has the following mineralogical composition, based on the total weight of the clinker: 5 to 25%, preferably to 20%, of a calcium aluminoferrite phase having the general formula C2A x F(1-X), wherein X is from 0.2 to 0.8; 15 to 35% of a calcium sulphoaluminate phase; 40 to 75% of belite (C2S); from 0.01 to 10% in total of one or more minor phases selected from calcium sulphates, alkali metal sulphates, perovskite, calcium aluminates, gehlenite, free lime and periclase and/or a vitreous phase; and an alkanolamine.

Direct electrolytic reduction of solid alumina using molten calcium chloride-alkali chloride electrolytes

Abstract: Solid alumina was reduced by electro-deoxidation to aluminium metal containing 1.8 and 5.4 at% Ca in molten CaCl2–NaCl and CaCl2–LiCl electrolytes at 900 °C, respectively. The potential-pO2− diagrams for the Al–O–M–Cl (M = Na or Li, or/and Ca) system were constructed to predict equilibrium phase relationships in the electrolytes at 700 and 900 °C. It was found that calcium aluminates were formed as the main intermediate reaction products and were subsequently reduced to form the Al-rich Al–Ca alloys during electro-deoxidation. Calcium and/or lithium, at reduced activities, were created at the cathode especially at 700 °C at the same time as the ionization of the oxygen from the cathode, which resulted in Al2Ca formation. The experimental results were consistent with the thermodynamic predictions.

Intense blue-emitting Ca5Al8O14: Eu phosphor for mercury free lamp

Abstract: The calcium aluminates doped with Eu ions, Ca5Al8O14: Eu, phosphors are prepared by the combustion method. The formation of crystalline aluminates was confirmed by X-ray diffraction pattern. The prepared phosphors were characterized by SEM, TGA, DTA, particle size analyzer and Photoluminescence (PL) techniques. From the UV-excited luminescence spectra it was found that the Eu ions acts as a luminescent centre with luminescence at the blue (λmax = 470 nm) region due to 4f65d1 → 4f7 transition. The excitation spectra show the broad band at 355 nm wavelength (λem = 470 nm). The excitation 355 nm is a mercury free excitation and therefore Ca5Al8O14: Eu may be useful for the solid state lighting phosphor in lamp industry.

Thermodynamics of reaction of alumina during sintering process of high-iron gibbsite-type bauxite

Abstract: The thermodynamics of the reactions of alumina(Al2O3) with CaCO3,CaO,SiO2 and FeO in the sintering process of high-iron gibbsite-type bauxite was investigated.The results show that alumina reacts with calcium carbonate much easier than ferri oxide(Fe2O3) at the industrial sintering temperature of 1 473-1 673 K.Alumina can not react with calcium ferrites(2CaO·Fe2O3 and CaO·Fe2O3) to form 3CaO·Al2O3,but when temperature is over 1 000 K alumina reacts with 2CaO·Fe2O3 to form 12CaO·7Al2O3.SiO2 reacts with CaO much easier than Al2O3.The possibility of alumina silicate from direct reaction of Al2O3 with SiO2 is little.Except for CaO·2Al2O3 and CaO·Al2O3,the other calcium aluminates can transform into calcium silicate by reacting with SiO2.Gehlenite(2CaO·Al2O3·SiO2) can not be formed from the reaction of calcium silicate(CaO·SiO2) with calcium aluminate(CaO·Al2O3),but it can be directly formed from the reaction of CaO,Al2O3 and SiO2.When CaO,Fe2O3,Al2O3 and SiO2 coexist in the sintering process,they are more likely to form ternary compound 2CaO·Al2O3·SiO2 and 4CaO·Al2O3·Fe2O3,which is consistent with the sintering results in laboratory.

Performance of Cement-based Material Containing Ultra-fine Limestone Powder at Early Age

Abstract: Ratio of water demand and mechanical properties of cement-based material containing ultra-fine limestone powder were researched,and mechanism of structure formation during hydration at early age was analyzed by means of electronic resistivity and X-ray diffraction of pasteResults show that ratio of water demand of cement-based material with the increase of different mesh 35% limestone powder first decreases and then increases compared with cement-based material without any limestone powderThe influence to mechanical properties of cement-based material at early performance with low content increase from 27%—44%,and decrease from 47%—350% with high contentFurthermore ultra-fine limestone powder can improve the brittleness of cement-based materialRatio of compressive strength to flexible strength of cement-based material without any of limestone powder at 28 d is 52 more then those adding 5%、15%、35% limestone powder to cement-based materialDuring hydration in 600 min,ultra-fine limestone powder can accelerate the hydration rate,so electronic resistivity of cement paste is higher than that without limestone powderWhen hydrating after 600 min,electronic resistivity of cement pastes with ultra-fine limestone powder increases far slowly than that without limestone powderAdditional,ultra-fine limestone powder in cement paste can react with aluminates to form calcium aluminates monocarbonate which is more stable then calcium aluminates