Formation of calcium aluminoferrites in the ternary CaO–Al2O3–Fe2O3 system during solid-phase reactions

Abstract

The ferrite phase in the cement composition has a reduced heat release during hydration, improves mechanical properties and increases the corrosion resistance of cement materials. In this article, the formation of C₂A_xF_1–x-type aluminoferrites in the CaO–Al₂O₃–Fe₂O₃ system during solid-phase reactions was studied. A system of empirical equations has been proposed that allows calculating the thermodynamic functions (Δ_fH°₂₉₈, Δ_fG°₂₉₈) of the formation of calcium aluminoferrites of any composition within a continuous series C₂A_0.667F_0.333 (C₆A₂F) – C₂F. Analysis of the various pathways of aluminoferrites formation has been conducted, taking into consideration the formation of intermediate phases in the ternary system. Based on a comparison of experimental data for mixtures of C+F, C+A, and C+A+F, a temperature sequence for phase formation (C₂F, CF, amorphous [CA]_am, C₂(A)F, CA, C₁₂A₇, C₂A_xF_1–x, eutectic liquid) in ternary oxide systems has been established. It has been shown that the amorphous aluminate phase present in the reaction system within the temperature range of 850–1100°C plays a crucial role in the formation of aluminoferrites. Among the investigated ternary oxide mixtures, whose initial composition corresponds to the stoichiometries of C₆A₂F, C₄AF, C₆AF₂, and C₈AF₃, only a certain aluminoferrite (brownmillerite), with a sufficiently high yield (∼53 wt% at 1200 °C), is formed in a 4 C+A+F mixture. An explanation has been proposed for the similarity in morphology between the sintered α-Fe₂O₃ and the calcined products of the C+F and C+A+F oxide mixtures when T>1000°C. The results of this work can be used to improve existing technologies and develop new methods for the production of iron-rich binders and additives based on calcium ferrites and aluminoferrites. The results of this work allow us to clarify the patterns of phase formation of calcium aluminoferrites under conditions of solid-phase reactions.