KINETICS OF THE ALPHA-TO-ALPHA'(H) POLYMORPHIC PHASE-TRANSITION OF CA2SIO4 SOLID-SOLUTIONS

The alpha-to-alpha'(H) transition of Ca2SiO4 solid solutions (C2S(ss)) is a nucleation and growth process. This process was shown on time-temperature-transformation (TTT) diagrams for C2S(ss) with different concentrations of foreign oxides (Na2O, Al2O3, and Fe2O3). The kinetic cutoff temperature and the activation energy for growth of the alpha(H)' phase increase steadily with increasing concentration of impurities. The activation energy for nucleation also increases above 950-degrees-C. The alpha(H)', phase, which exists in equilibrium with the a phase at 1280-degrees-C, is formed at a maximum rate at around 1100-degrees-C regardless of the chemical composition. The TTT diagrams enable us to predict, as a function of cooling rate, the phase constitution of C2S(SS) at ambient temperature.