Contents lists available at ScienceDirect Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc Original Article Macroporous high-temperature insulators physical properties by in situ CA 6 formation: Does the calcium source matter? O.H. Borges a,b, *, T. Santos Junior a,b , R.R.B. Oliveira b , V.R. Salvini c , V.C. Pandolfelli a,b a Federal University of Sao Carlos, Graduate Program in Materials Science and Engineering, Brazil b Materials Microstructure Engineering Group (GEMM), FIRE Associate Laboratory, Federal University of São Carlos, Materials Engineering Department, Rodovia Washington Luis, Km 235, São Carlos, SP, 13565-905, Brazil c College of Technology (FATEC), Jordão Borghetti Street 480, Sertãozinho, SP, 14160-050, Brazil ARTICLE INFO Keywords: Calcium sources Macroporous insulators Direct foaming Linear shrinkage ABSTRACT Aiming the in situ formation of CA 6 (CaO·6Al 2 O 3 ) at alumina-based macroporous insulators, distinct Ca 2+ sources and contents were used and their efect on some of the refractories properties were investigated. Adding CaCO 3 , Ca(OH) 2 or CaO resulted in the decrease of the onset strengthening temperature (T S ) and also of the linear shrinkage. However, a higher amount of Ca(OH) 2 and CaO could not be used because of their efect on reducing the insulator total porosity. The composition prepared with 12.9 wt% of CaCO 3 was the most promising one, leading to an expansion of 0.81 % after fring at 1600 °C for 5 h, T S of 680 °C and low thermal conductivity. These results point out the potential reduction of sintering temperatures and to the possibility of in situ fring the ceramic insulator. These features enable the development of a macroporous refractory composition with a higher thermal insulating efectiveness, which can help industries to decrease their energy demand. 1. Introduction Constant eforts are being made aiming at cost reduction in all economic sectors. Due to the strong competition, the industries are even more susceptible to this scenario and to cope with that, heavy invest- ments in research and development of new materials and processes are required [1]. Energy-related costs are one of the most relevant for in- dustries, especially for those operating at high temperatures (> 1000 °C) [2]. Such industries are called energy-intensive and were re- sponsible for 12 % of all energy consumed worldwide in 2018 [2]. In this context, macroporous refractory ceramics have been drawing attention as they can be used as thermal insulators, reducing the amount of energy demanded and decreasing production costs. However, some challenges should be overcome to increase macro- porous ceramics applications. Among them, the control of shrinkage after fring and the reduction of thermal conductivity can be cited. Additionally, for macroporous insulators produced by direct foaming methods, the control of aging phenomena, which take place before foam solidifcation, is another key issue. These phenomena act by in- creasing the average size of bubbles and making their distribution wider [3], which makes it more difcult to achieve the target micro- structure designed for the thermal insulator. Regarding the reduction of shrinkage after fring, a promising alternative could be the in situ formation of expansive phases, such as hibonite (also known as calcium hexaluminate, CaO·6Al 2 O 3 , or just CA 6 ), which induces a signifcant volumetric expansion due to its low density and asymmetrical crystals [4,5]. Thus, this expansive efect due to CA 6 formation could counteract the material shrinkage. Furthermore, plain CA 6 presents high refractoriness, incongruent melting close to 1800 °C [6] (however, impurities source and content can reduce this refractoriness signifcantly) and low thermal conductivity, so its in- corporation as light aggregate in thermal insulators has already been tested, inducing signifcant reductions in their efective thermal con- ductivity [7,8]. Distinct Ca 2+ sources can be used to produce CA 6 in situ [6], such as calcined calcium oxide (CaO), calcium carbonate (CaCO 3 ), calcium hydroxide [Ca(OH) 2 ], and calcium aluminate cement (CAC). Besides promoting CA 6 formation when heated up to 1400 °C in the presence of Al 3+ sources, recent studies have shown that CaCO 3 and Ca(OH) 2 in- duced lower strengthening temperature in dense alumina-based re- fractories when compared to samples containing just CAC as Ca 2+ source [9,10], as can be seen in Fig. 1. Therefore, using these Ca 2+ sources in alumina-based macroporous refractories is of great technological interest as the material could be thermally treated at lower temperatures to be applied in low thermal demand environment (e.g. in aluminum industries) or to attain enough https://doi.org/10.1016/j.jeurceramsoc.2020.04.001 Received 6 February 2020; Received in revised form 31 March 2020; Accepted 1 April 2020 ⁎ Corresponding author. E-mail address: otavio.borges@dema.ufscar.br (O.H. Borges). Journal of the European Ceramic Society 40 (2020) 3679–3686 Available online 09 April 2020 0955-2219/ © 2020 Elsevier Ltd. All rights reserved. T