Showing papers on "Calcium aluminates published in 1996"

Control of calcium hexaluminate grain morphology in in-situ toughened ceramic composites

Abstract: The influence of processing conditions on the morphology of calcium hexaluminate (CA6) grains in Al2O3: 30 vol% CaO·6Al2O3 (CA6) ceramic composites was investigated. Specimens were prepared by in-situ reaction sintering using precursor powders of alumina, and either calcium carbonate or calcium oxide. In some samples, 1 vol% anorthite glass was added as a sintering aid. X-ray diffraction was used to study the phase development in the as-calcined and sintered states. The resultant microstructures were characterized using both scanning electron microscopy (SEM), and imaging secondary ion mass spectrometry (SIMS). It was found that the CA6 grains developed a platelike morphology when CaCO3 was used as the starting calcium-rich powder. In contrast, samples prepared using CaO resulted in equiaxed CA6 grains. This result was observed to be independent of the anorthite glass addition. The findings are rationalized in terms of distinct CA6 reaction mechanisms, resulting from differences in the reactivity of the powders during the early stages of calcining.

Hydration of calcium sulphoaluminate cements

Abstract: A range of cements based on calcium sulphoaluminate, 4CaO·3Al2O3·SO3. ‘C4A3S’ are commercially available. The phase contents of some commercial clinkers have been determined. Their hydration behaviour, including calorimetric heat evolutions, are reported. The clinker mineralogies control reactivity and dimensional stability; typically, sulphoaluminate formulations with added lime, gypsum and calcium aluminates are expansive, but those consisting of C4A3S and belite (Ca2SiO4) hydrate rapidly but give rise to dimensionally stable products. The role of accelerating and retarding admixtures is explored. The presence of admixes has a secondary role in controlling the dimensional stability of belite-sulphoaluminate formulations.

Thermal analysis of hydrated calcium aluminates

Abstract: Differential scanning calorimeter (DSC) has been used to study the dehydration characteristics of hydrated calcium aluminates such as CA, CA2 and C12A7 where C and A stand for CaO and Al2O3 respectively. Dehydration of CAH10 and C2AH8 (whereH=H2O) occur ∼ at 160–180°C and 200–280°C respectively. These two phases are unstable and ultimately get transformed to AH3 and C3AH6. Dehydration of AH3 and C3AH6 occur between 290 and 350°C and overlap at lower scanning rate. The activation energy for dehydration of the stable AH3 and C2AH6 phases has been found to be 107.16 and 35.58 kJ mol−1 respectively. The compressive strength of the hydrated calcium aluminates has been determined. The result shows that in the case of CA, almost 90% of ultimate strength has been attained in 1 day whereas in CA2, ultimate strength has been attained in 14 days and in C12A7 in 1 day. DSC results have been correlated with the rate of strength developments.

Determination of Free Lime Contents in Slags by Solution Calorimetry

Abstract: Free lime content in slag was investigated using a solution calorimetric technique in order to clarify hydration behavior of the free lime. After confirmation of the agreement between the heat of hydration of pure CaO obtained by this method and the calculated value, the heats of hydration of slags with different free lime contents were measured. Good correlation between the free lime contents and the heats of the hydration was indicated though small amounts of calcium silicates, calcium aluminates, MgO and calcium ferrite in the slags might be hydrated in addition to the free lime.

Preparation of Ca-β-Al2O3 from alumina gel

Abstract: Using aluminium nitrate solution containing calcium and magnesium cations, and saturated ammonium oxalate solution in concentrated ammonium hydroxide as precipitating agent, alumina gel was prepared. The gel was calcined at different heating rates up to 400 °C and then at different temperatures and soaking times. The calcination products successively appeared as aluminium oxides, calcium aluminate and calcium-beta″-alumina. The obtained calciumbeta″-alumina crystallites were plate-shaped and the direction perpendicular to the crystallite surface was parallel to the Z-axis. It was found that the kinetics of calcium-beta″-alumina synthesis depends on the presence of a molten salt during the gel thermal treatment. The mechanism of calcium-beta″-alumina formation is proposed as a reaction between formerly produced calcium aluminate and aluminium oxide of the alpha form. Sintered bodies prepared from the calcium-beta″-alumina powder were characterized by good ionic conductivity.