Solidification of CaCO 3 containing SrCO 3 by hydrothermal hot-pressing
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Powders of calcium carbonate with strontium carbonate were solidified by mechanical compression under hydrothermal conditions. The effects of hydrothermal conditions such as temperature, pressure, and ratio of Ca/Sr on the reaction process and microstructure of the solidified bodies were examined. Solidification of every powder easily proceeded with shrinkage above 200 °C to 300 °C. The compacts of strontium carbonate were more easily solidified, and compressive strength was 190 MPa and its density was 93.5%. The solidification with shrinkage of compacts consisting of mixed powders of calcium and strontium carbonates was accelerated with an increase in strontium carbonate content. It was revealed that the small amount of strontium carbonate powder acts as a bonding material among large calcium carbonate particles.
I. INTRODUCTION Studies on the densification of powders under hydrothermal conditions have been reported. Roy et al.l produced densified cementitious materials with high mechanical strength by compressing cement paste at 150 °C. Under these conditions, vapor pressure of water must be relatively low, because water is used for the hydration of cement. Hydrothermal sintering was reported by Hirano and Somiya2 and Yoshimura and Somiya.3 This technique is related to reaction sintering in a Pt capsule through which H 2 , generated by the reaction of metal with H 2 O, can penetrate to the outside of the capsule above 800 °C, and results in hot pressing of oxide at high temperature with compression by pressure outside the capsule. Water is only one of the reactants. The new hydrothermal hot-pressing technique is different from these other methods. In this technique, a powder with water is mechanically compressed from outside an autoclave, under relatively mild hydrothermal conditions below 350 °C. It has been applied to the immobilization of radioactive wastes into artificial sedimentary rock.4~6 This technique is useful to densify nonsinterable materials by ordinary sintering, such as hydrate, carbonate, hydroxide, and so on.7-8 A previous report showed that CaCO3 powder was solidified below 300 °C by the linkage of CaCO3 particles with Ca(OH)2 which was formed by the reaction of CaCO 3 with NaOH solution added as a mineralizer.6 In this paper, the effect of mixing SrCC>3 powder with CaCO3 powder is described. This solidification process may be similar to the sedimentary process at the bottom of the sea. However, the sedimentary process as lithification does not need to be clarified. Various inclusions, such as alkaline 1972
http://journals.cambridge.org
J. Mater. Res., Vol. 8, No. 8, Aug 1993
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earth elements, may affect the formation of limestone during the sedimentary process. The results, using the hydrothermal hot-pressing technique, may bring some new information to the sedimentary process and new technology of forming fabrication processes.
II. EXPERIMENTAL Starting carbonate powders were prepared by the precipitation from hydroxide solution with bubbling of CO 2 gas. The preci
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