An assessment of the CaO-SiO 2 system
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I.
INTRODUCTION
THE CaO-SiO2 system is one of the most frequently encountered systems in the fields of metallurgy, ceramics, and geology. Its phase diagram has been well established by experimental determination except for the liquidus close to the CaO-rich side. Many researchers have investigated the activities of S i O 2 and CaO in the system. Because of the difficulties in measurement, there is considerable scatter in the results. In recent years, there has been a considerable interest in representing the thermodynamic properties of silicate melts with thermodynamic models. Several assessments of the CaO-SiO2 system have been made: Pelton and Blander tq assessed the system using the modified quasichemical model. Taylor and Dinsdale t21 represented the system using the IRSID cellular model, t3t and Lee t41 estimated the Gibbs energy of formation of ot-fa2SiO4 at high temperatures and calculated the phase diagram using a Redlich-Kister t24t approach. It seems that the greatest difficulty of the assessment is to get a good fit with the miscibility gap. Most of the assessments produced a miscibility gap with a maximum temperature too high. Another question is whether the calculation can be extended to multicomponent systems. Pelton and Blander tq extended their calculation to the SiO2-CaO-FeO ternary system and reported good agreement with measured ternary data. Taylor and Dinsdale t2t tried to introduce sulfur into the oxide system. The purpose of this work is to represent the system using a newly developed model, a two-sublattice model for ionic solutions, tSj and to extend the results of the binary system to the CaO-SiO2-FeO system and other multicomponent systems. The present report deals with the CaO-SiO2 system. II.
E X P E R I M E N T A L DATA
A. Liquid Phase A number of workers have measured activities of SiO2 in the SiOz-CaO system within a composition range from MATS HILLERT, Professor, and BO SUNDMAN, Docent of Physical Metallurgy, are with the Royal Institute of Technology, S-100 44 Stockholm, Sweden. XIZHEN WANG, formerly Research Associate, Royal Institute of Technoloy, is with Wuhan Iron and Steel University, Hubei, People's Republic of China. Manuscript submitted February 3, 1989. METALLURGICALTRANSACTIONSB
40 to 65 mole pct SiO2 at 1773 to 1873 K, but their resuits differ considerably. The present assessment was based upon the measurements of Kay and Taylor, t6] because their evaluation does not depend on knowing the Gibbs energy of formation of any other compound and is accepted by most researchers. For the activities of CaO, the determination by Sharma and Richardson t7] was adopted. B. Solid Phases The system has two congruently melting compounds, Ca2SiO4 and CaSiO3, and two incongruently melting compounds, Ca3SiOs, which is stable only in a temperature range of 1523 to 2323 K, and Ca3Si207, which is stable from low temperature up to 1737 K. The Ce, $298, AG, and AH for these compounds were assessed based upon the same experimental data that Haas et al. t8] selected when making their assessm
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