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Reactive processing of ceramic binding systems for refractory castables Ye, Guotian

Abstract

The general applied objective of this project is to develop hydratable alumina-bonded castables of properties comparable to the high-temperature fired basic bricks. This work is focused on improving the sintering of the ceramic binding systems at relatively low temperatures (~1300°C) through incorporation of ultrafine powders. Ultrafine magnesium aluminate spinel powders were synthesized using already known and original methods. Combustible ingredients were incorporated to prevent the direct contacts of the precursor particles during drying and the combustibles leave a continuous pore network, when they burn off during calcination at 700-800°C. Mechanical activation of the spinel precursor prepared using a heterogeneous sol-gel process decreased the activation energy for spinel formation from 688 kJ/mol to 468 kJ/mol, and lowered the incipient temperature of spinel formation from 900°C to 800°C, and the temperature of complete spinellization from >1280°C to 900°C. A new method for preparing homogeneous spinel precursor was presented and completely crystallized spinel with specific surface area of 105 m² /g, and crystallite size of 13 nm was formed from the precursor after calcination at 900°C. This work established the influence of ultrafine-sized powders on the sintering and strength development of multi-sized systems rich in large particles at relatively low temperatures, with emphasis on the temperatures around 1280°C. It was found that the relationship between the strength and linear shrinkage for the powder compacts consisting of uniform particle size distribution does not apply to the binding systems with wide particle size distributions (90 nm - 90 μm). The addition of the ultrafine powders (≤0.5 μm median) up to 10% had no significant influence on strength in two-component binding systems after firing at 1280°C and 1450°C. However, the ultrafine powders increased the strength of three-component binding systems presumably because the addition of ultrafine powders increased effective contacts for sintering. The particle size of the ultrafine power plays an important role in enhancing the strength of the castables after heat treatment at 816°C and 1280°C. Aggregate types have substantial influence on the strength of the castables after firing at 1280°C, eventually due to the cracks generated by in-situ spinel formation and mismatch in thermal expansion coefficients between the aggregate and the matrix. Spinel-based castables with high bending strength (>15 MPa) were obtained with addition of 2% ultrafine spinel (0.2 μm median) after firing at 1280°C. This research work also contributes to the reaction mechanisms of hydratable alumina with various forms of MgO. After hydration at 20°C for 48 h, a hydrotalcite hydrate was formed in the mixture of hydratable alumina and reactive magnesia, while such a hydrate was not observed in the mixtures of hydratable alumina and deadburnt or fused magnesite under the same hydration conditions. After hydration at room temperature for 48 h at 20°C and then for 12 h at 110°C, hydrotalcite compounds were formed in all three mixtures. The presence of deadburnt/fused magnesia in hydratable alumina-bonded castables improved the strength of the castables after drying at 110°C because of the hydrogen bonding of hydrotalcite and Mg(OH)₂ on magnesia particles. The polycondensation accompanying dehydroxylation of hydrotalcite and Mg(OH)₂ up to 816°C contributed to the higher strength of hydratable alumina-bonded castables containing magnesia.

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