minerals Article Effects of Kaolin Additives in Fly Ash on Sintering and Properties of Mullite Ceramics Marta Valášková 1, * , Veronika Blah˚ ušková 1 and Jozef Vlˇ cek 1,2   Citation: Valášková, M.; Blah˚ ušková, V.; Vlˇ cek, J. Effects of Kaolin Additives in Fly Ash on Sintering and Properties of Mullite Ceramics. Minerals 2021, 11, 887. https:// doi.org/10.3390/min11080887 Academic Editor: Hegoi Manzano Received: 13 July 2021 Accepted: 13 August 2021 Published: 17 August 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 Institute of Environmental Technology, CEET, VŠB-Technical University of Ostrava, 17. Listopadu 2172/15, 708 00 Ostrava–Poruba, Czech Republic; veronika.blahuskova@vsb.cz (V.B.); jozef.vlcek@vsb.cz (J.V.) 2 Department of Thermal Engineering, Faculty of Materials Science and Technology, VŠB-Technical University of Ostrava, 17. Listopadu 2172/45, 708 00 Ostrava–Poruba, Czech Republic * Correspondence: marta.valaskova@vsb.cz; Tel.: +420-597-327-308 Abstract: The effective utilization of fly ash (FA) as a raw material for ceramics production is performed on the FA-kaolin mixtures containing kaolins 10% by mass. The mixtures in comparison with FA and three raw kaolins were annealed to mullite ceramics at temperatures of 1000, 1100, 1200 and 1300 ◦ C. The main aims were to contribute to the discussion on the effect of impurity of Na,K- feldspars in kaolins and Fe 2 O 3 in FA on sintering procedure, porous ceramics properties and mullite structural properties. The phases were characterized using X-ray diffraction and thermogravimetry DTA/TGA methods. Mercury intrusion porosimetry was used for characterization of porosity of ceramic samples. Results evidenced the influence of feldspars in kaolins and Fe 2 O 3 in FA on the sintering temperatures and properties of mullite ceramics. The fully FA-based ceramic sintered at 1100 ◦ C exhibited post-sintering properties of bulk density 2.1 g/cm 3 ; compressive strength 77.5 MPa; and porosity, 2% in comparison with the FA/kaolin-based ceramics properties of bulk density 2.2 g/cm 3 ; compressive strength, 60–65 MPa; and porosity from 9.3 to 16.4% influenced by Na,K-feldspars. The best structural and mechanical characteristics were found for the FAK3 sample, supported by the high content of kaolinite and orthoclase in the kaolin K3 additive. The FAK3 annealed at 1100 ◦ C exhibited good compressive strength of 87.6 MPa at a porosity of 10.6% and density of 2.24 g/cm 3 and annealed at 1300 ◦ C the compressive strength of 41.3 MPa at a porosity of 19.2% and density of 1.93 g/cm 3 . Keywords: fly ash; kaolins; mullite; microstructure; porosity 1. Introduction Fly ash (FA) properties depend on the coal source, the method of combustion of power plants, storage, etc. For example, burning in pulverized-coal combustion boiler, the main oxide components of FA are SiO 2 , Al 2 O 3 , Fe 2 O 3 , CaO, MgO, Na 2 O, and K 2 O, and the crystalline phases are mullite (3Al 2 O 3 ·2SiO 2 ) and quartz (SiO 2 )[1]. FA and tradi- tional ceramic raw materials have similar chemical and mineralogical compositions and therefore, make FA a promising ingredient in ceramics [2]. Architectural ceramics are typically prepared using a triaxial formulation of quartz (filler, 5–30 mass%), clay (binder, 30–60 mass%), and feldspar (fluxing agent, 15–40 mass%) [3]. Quartz as a filler can be replaced by FA because of the proper filling properties. Effect of substitution of fly ash for quartz in triaxial kaolin–quartz–feldspar system demonstrated higher density and flexural strength [4,5]. However, FA as a high-temperature product cannot provide plasticity like a clay binder [6]. FA in the range of 5–40 mass% replacing clay improved the bending strength, abrasion resistance, and hardness of porcelainized stoneware tiles but worsened the bending strength of the green compacts [7]. FA containing abundant alkali and alkaline earth metals can partially substitute for feldspar to promote melting, especially in the preparation of ceramic tiles [8,9]. Building mullite-based ceramics can be produced from the recycling of conventional coal combustion ash and clay in the initial mixture, acting as Minerals 2021, 11, 887. https://doi.org/10.3390/min11080887 https://www.mdpi.com/journal/minerals