Ceramics International 47 (2021) 2624–2630 Available online 11 September 2020 0272-8842/© 2020 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Co-existence of a Cr 3+ phase (CaAl 2 Cr 2 O 7 ) with hydraulic calcium aluminates at high temperature in the Al 2 O 3 –CaO–Cr 2 O 3 system Mithun Nath a, b, * , Shengqiang Song a, b , Ning Liao a, b , Tengteng Xu a, b , Hang Liu a, b , Himansu Sekhar Tripathi c , Yawei Li a, b, ** a The State Key Laboratory of Refractories & Metallurgy; Wuhan University of Science and Technology, Wuhan, 430081, China b National-provincial Joint Engineering Research Center of High-Temperature Materials & Lining Technology, Wuhan University of Science & Technology, Wuhan, 430081, China c Refractory and Traditional Ceramics Division, CSIR–Central Glass & Ceramic Research Institute, Kolkata, 700 032, India A R T I C L E INFO Keywords: Al 2 O 3 –CaO–Cr 2 O 3 phase diagram CaAl 2 Cr 2 O 7 A Cr 3+ phase Cr 6+ XRD Leaching tests ABSTRACT Obtaining a stable Cr 3+ phase at higher temperatures in the presence of hydraulic calcium aluminates (the phases found in calcium aluminate cement, CAC) in the Al 2 O 3 –CaO–Cr 2 O 3 system remains a signifcant challenge in many application areas, as the formation of toxic, carcinogenic, and water-soluble Cr 6+ compounds often occur. To address the issue, recently, we synthesized a high-temperature stable Cr 3+ ternary compound (CaAl 2 Cr 2 O 7 ) with space group P3 (143). In the present work, we investigated the formation-stability of CaAl 2 Cr 2 O 7 with in- situ calcium aluminate phases at 1500 ◦ C under the CO 2 atmosphere in the Al 2 O 3 –CaO–Cr 2 O 3 system through solid-oxide reactions route varying Cr 2 O 3 content (at constant Al 2 O 3 :CaO ratio). It co-existed with hydraulic calcium aluminates and other phases over the full investigated composition range of 2.76–68.3 mol% Cr 2 O 3 (5–80 wt%). Apart from CAC phases (CaAl 2 O 4 , CaAl 4 O 7 ), major Cr 3+ -phases are CaAl 2 Cr 2 O 7 and (Al,Cr) 2 O 3 while (α,β)-CaCr 2 O 4 and Ca(Al,Cr) 12 O 19 formed as minor phases. At a constant Al 2 O 3 :CaO ratio of 5.6:4.4 mol% (7:3 wt%), the formation of the CaAl 2 Cr 2 O 7 phase increases with Cr 2 O 3 content (up to the investigated composition of 26.43 mol%) and then decreases gradually. The solid solubility of Al and Cr in the CaAl 2 Cr 2 O 7 phase limited over a narrow range, and presumably dependent more on heat treatment condition rather than composition as refected from the lattice parameter calculations. Though XRD revealed the presence of only Cr 3+ - phases in the partial CO 2 atmosphere, however, traces of Cr 6+ could be detected using XPS and leaching tests. However, leachable Cr 6+ content (0.095–1.252 mg/L) were much below the United States Environmental Pro- tection Agency (US-EPA) permissible limit of 5 mg/L. The formation mechanism of the CaAl 2 Cr 2 O 7 and other phases with plausible reactions were also discussed. 1. Introduction The system CaO–Al 2 O 3 and Al 2 O 3 –Cr 2 O 3 have been investigated meticulously over the years [1–4]. Owing to their versatile properties, they possess a wide range of applications in pyrometallurgy, coatings, catalysis, pigments, sensors, optical-magnetic, solid oxide fuel cells, and so on [5–11]. However, the Al 2 O 3 –CaO–Cr 2 O 3 phase diagram is amongst one of the most understudied [12–15] systems until our recent investigations [16–20] despite its immense potential [18], because of the generation of toxic and carcinogenic Cr 6+ compounds at high-temperature in the presence of oxygen, alkali and alkaline earth elements [21–23]. The Cr 3+ phases like Cr 2 O 3 , (Al,Cr) 2 O 3 , and Mg(Al, Cr) 2 O 4 tend to convert into Cr 6+ at high temperatures in the presence of various hydraulic calcium aluminates [19,20,24]. There were several studies carried out relating to the removal of Cr 6+ but only after its formation during industrial applications [25–28]. Besides, many Abbreviations: CAC, Calcium aluminate cement; XRD, X-ray diffraction; SEM, scanning electron microscopy; EDS, energy dispersive spectrometer; XPS, X-ray photoelectron spectroscopy; TRGS, Technische Regeln für Gefahrstoffe; DPC, 1,5-diphenylcarbazide; US-EPA, United States Environmental Protection Agency; GoF, Goodness of Fit. * Corresponding author. The State Key Laboratory of Refractories & Metallurgy, Wuhan University of Science and Technology, Wuhan, 430081, China. ** Corresponding author. The State Key Laboratory of Refractories & Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, China. E-mail addresses: mithunnath@wust.edu.cn, mithunnathn@gmail.com (M. Nath), liyawei@wust.edu.cn (Y. Li). Contents lists available at ScienceDirect Ceramics International journal homepage: www.elsevier.com/locate/ceramint https://doi.org/10.1016/j.ceramint.2020.09.109 Received 30 July 2020; Received in revised form 8 September 2020; Accepted 10 September 2020