Co-existence of a Cr3+ phase (CaAl2Cr2O7) with hydraulic calcium aluminates at high temperature in the Al2O3–CaO–Cr2O3 system

Abstract Obtaining a stable Cr3+ phase at higher temperatures in the presence of hydraulic calcium aluminates (the phases found in calcium aluminate cement, CAC) in the Al2O3–CaO–Cr2O3 system remains a significant challenge in many application areas, as the formation of toxic, carcinogenic, and water-soluble Cr6+ compounds often occur. To address the issue, recently, we synthesized a high-temperature stable Cr3+ ternary compound (CaAl2Cr2O7) with space group P3 (143). In the present work, we investigated the formation-stability of CaAl2Cr2O7 with in-situ calcium aluminate phases at 1500 °C under the CO2 atmosphere in the Al2O3–CaO–Cr2O3 system through solid-oxide reactions route varying Cr2O3 content (at constant Al2O3:CaO ratio). It co-existed with hydraulic calcium aluminates and other phases over the full investigated composition range of 2.76–68.3 mol% Cr2O3 (5–80 wt%). Apart from CAC phases (CaAl2O4, CaAl4O7), major Cr3+-phases are CaAl2Cr2O7 and (Al,Cr)2O3 while (α,β)-CaCr2O4 and Ca(Al,Cr)12O19 formed as minor phases. At a constant Al2O3:CaO ratio of 5.6:4.4 mol% (7:3 wt%), the formation of the CaAl2Cr2O7 phase increases with Cr2O3 content (up to the investigated composition of 26.43 mol%) and then decreases gradually. The solid solubility of Al and Cr in the CaAl2Cr2O7 phase limited over a narrow range, and presumably dependent more on heat treatment condition rather than composition as reflected from the lattice parameter calculations. Though XRD revealed the presence of only Cr3+-phases in the partial CO2 atmosphere, however, traces of Cr6+ could be detected using XPS and leaching tests. However, leachable Cr6+ content (0.095–1.252 mg/L) were much below the United States Environmental Protection Agency (US-EPA) permissible limit of 5 mg/L. The formation mechanism of the CaAl2Cr2O7 and other phases with plausible reactions were also discussed.