materials Article Controlling the Thermal Stability of Kyanite-Based Refractory Geopolymers Juvenal Giogetti Nemaleu Deutou 1,2, * , Rodrigue Cyriaque Kaze 1,3 , Elie Kamseu 1,4 and Vincenzo M. Sglavo 2, *   Citation: Deutou, J.G.N.; Kaze, R.C.; Kamseu, E.; Sglavo, V.M. Controlling the Thermal Stability of Kyanite-Based Refractory Geopolymers. Materials 2021, 14, 2903. https://doi.org/10.3390/ma14112903 Academic Editors: Dolores Eliche Quesada and Rui Novais Received: 27 April 2021 Accepted: 24 May 2021 Published: 28 May 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Local Material Promotion Authority(MIPROMALO), P.O. Box 2396 Yaoundé, Cameroon; kazerodrigue@gmail.com (R.C.K.); kamseuelie2001@yahoo.fr (E.K.) 2 Department of Industrial Engineering, University of Trento, Via Sommarive 9, 38123 Trento, Italy 3 Laboratory of Applied Inorganic Chemistry, Faculty of Science, University of Yaoundé I, P.O. Box 812 Yaoundé, Cameroon 4 Dipartimento di Ingegneria dei Materiali e dell’Ambiente, Università di Modena e Reggio Emilia, Via Vignolese 905, 41100 Modena, Italy * Correspondence: giogetti@live.fr (J.G.N.D.); vincenzo.sglavo@unitn.it (V.M.S.) Abstract: The present project investigated the thermal stability of cold-setting refractory composites under high-temperature cycles. The proposed route dealt with the feasibility of using fillers with different particle sizes and studying their influence on the thermo-mechanical properties of refractory geopolymer composites. The volumetric shrinkage was studied with respect to particle sizes of fillers (80, 200 and 500 μm), treatment temperature (1050–1250 ◦ C) and amount of fillers (70–85 wt.%). The results, combined with thermal analysis, indicated the efficiency of refractory-based kyanite aggregates for enhancing thermo-mechanical properties. At low temperatures, larger amounts of kyanite aggregates promoted mechanical strength development. Flexural strengths of 45, 42 and 40 MPa were obtained for geopolymer samples, respectively, at 1200 ◦ C, made with filler particles sieved at 80, 200 and 500 μm. In addition, a sintering temperature equal to 1200 ◦ C appeared beneficial for the promotion of densification as well as bonding between kyanite aggregates and the matrix, contributing to the reinforcement of the refractory geopolymer composites without any sign of vitrification. From the obtained properties of thermal stability, good densification and high strength, kyanite aggregates are efficient and promising candidates for the production of environmentally friendly, castable refractory composites. Keywords: cold setting; kyanite; thermo-mechanical properties; particle size distribution; microstructure 1. Introduction Rapid global population growth and industrialization often conflict with sustainable development and pollution, and environmental drawbacks are typically associated with industrial processes. This particularly applies to the refractory sector, characterized by worldwide production of around 35–40 million tons per year [1–3], with applications in industrial sectors including metallurgy, cement kilns, energy treatment and recovery and energy production [4–7]. The quest to satisfy industrial needs leads to high volumes of refractory production, which is still in conflict with the need for sustainable development in terms of energy demands and carbon footprints. According to the International Energy Agency (IEA) 2008 report and the Global Energy and Climate Outlook 2018 (Report 2018 Global Status), in 2015, the global industrial sector (e.g., Magnesia Spinel Brick Production for production of magnesia refractory raw materials) accounted for 39% of worldwide energy consumption and 21% of total greenhouse gas emissions. To reach the Paris Agree- ment commitment, a total of 9 Gt per annum CO 2 reduction is required from the global building sector alone [3,4]. Common refractory materials are processed at high temperatures (≥1400 ◦ C) for prolonged times, leading to high energy consumption and greenhouse gas emissions. For Materials 2021, 14, 2903. https://doi.org/10.3390/ma14112903 https://www.mdpi.com/journal/materials