Effect of the Mineralogical Composition of Clay on Microwave Sintering of Aluminosilicates
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ct of the Mineralogical Composition of Clay on Microwave Sintering of Aluminosilicates I. A. Zhenzhurist* Kazan State University of Architecture and Civil Engineering, Zelenaya ul. 1, Kazan, 420043 Tatarstan, Russia *e-mail: [email protected] Received November 12, 2019; revised December 1, 2019; accepted December 27, 2019
Abstract—This paper presents results of firing of bentonite clay, refractory clay, and metakaolin materials with NaCl additions using convective and microwave heating. Phase structures of microwave-sintered aluminosilicates have been examined and the formation of a nanometer-scale aluminosilicate structure has been demonstrated. The microwave-fired materials have been found to have an increased strength. The mineralogical composition of clay has been shown to influence the microwave sintering behavior of aluminosilicates. Kaolinite and kaolinite-based materials exhibit increased activity in the temperature range 450–550°C during microwave sintering. The role of NaCl as a mineralizing salt has been found out. Keywords: bentonite clay, refractory clay, metakaolin, NaCl, convective heating, microwave firing DOI: 10.1134/S0020168520080166
INTRODUCTION Sintering is the main step in heat treatment of a wide range of inorganic fired materials, among which aluminosilicate ceramics hold an important place. Sintering produces the final structure of a material, which determines its engineering performance. In addition, the formation of the structure of a material is influenced by parameters of raw materials, specific features of their processing and fabrication of an article, and the heat treatment process. At present, energy efficiency and environmental safety of heating are of special importance in the technology of fired materials. In this respect, microwave heating has obvious advantages over convective heating, which typically involves fuel combustion. As shown earlier [1–3], the mechanism of hightemperature processes and the final properties of materials are significantly influenced by the heating rate [1–3]. As a result of electric field-assisted flash sintering, one can obtain a material with a microcrystalline structure and good mechanical properties [1–3]. In a number of studies [4–10], microwave energy was used for heat treatment of materials. In a study of interaction of ceramics with alumina in a microwave field, Binner et al. [4] observed the formation of an intergranular glass phase, good diffusion, homogeneity of the material, lower strain in the materials in comparison with traditional convective heating, and increased strength of the compound. Microwave processing of ceramic materials, kaolin, alu-
mina, and talc [5] was reported to ensure rapid (30 min) heating to temperatures from 1200 to 1400°C and a considerable decrease in reaction time (by three to five times). It was proposed that convective heating should be conducted first, until the maximum dielectric loss tangent of the starting-mixture components would be reached, followed by microwave sintering. Based on analysis of reports
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