Turkey is an important producer and exporter of ceramic sanitary ware in Europe. However, it produces nearly 10% of the world’s production [1]. The main products of ceramic sanitary ware are sinks, shower trays, toilet bowls, and urinals. These products should be hygienic in terms of human health since they are used in bathrooms and toilets with high humidity. In addition, it is desirable that the strength values of these products are high, as they are constantly in contact with humans and exposed to human weight [2]. Two different slip is used for the products produced in the ceramic sanitary ware sector. These slips are vitreous china (VC) and fine fire clay (FFC) slips. FFC is used in the production of large-size products in ceramic sanitary ware products, while vitreous china is used in the production of other products such as small sinks and toilet bowls [3]. In recent years, it is seen that in today's modern life, vitrified products have become an important visual element in the bathroom, and as a result, design studies have been given importance in the sector [4, 5]. For this reason, the demand for special design products in bathrooms is gradually increasing and large-sized, straight and sharp-line products are generally preferred. In the production of such large-sized products, FFC slips with a high content of fired clay are used due to their low deformation characteristics. The main raw materials used in the production of FFC are clay, kaolin, quartz, and chamotte [6].
Chamotte is the fired state of clay until it loses its binding properties [7]. The FFC body in the production of ceramic sanitary ware typically contains 30% chamotte material. Kaolin is calcined and used as fireclay, especially since its firing color is white. In hard and soft porcelain as well as bone porcelain, kaolin's calcination behavior is crucial. [8–10]. Since chamotte is a fired raw material, it provides strength to the product and minimizes deformation when it is in semi-product and fired products in FFC bodies. However, Turkey is entirely dependent on foreign suppliers for the supply of chamotte material due to the high investment requirements of the kaolin enrichment and grinding processes [11]
While natural raw materials are becoming scarce, millions of tons of waste are generated every day in mining and mineral processing, the disposal of which is subject to increasingly stringent environmental legislation. However, some wastes are similar in composition to the natural raw materials used in the manufacture of ceramics and often contain materials that are also useful in the manufacturing process. The conversion of waste into alternative raw materials is technologically, economically and environmentally important. Therefore, in this study, as an alternative to the imported chamotte material used in FFC ceramic sanitary ware, it was investigated to obtain chamotte by synthesizing kaolin and boron waste.
Researches on the calcination of kaolin have been conducted mainly to determine the thermal behavior of kaolin and to use it as raw material, especially in the ceramic sector, by trying to remove sulfur in kaolin with alunite [12–14].
The assessment of boron waste in ceramic bodies, glazes, and the production of construction materials has been the subject of several research in recent years. It has been determined that boron waste-added clays can be used in the preparation of frit, glaze and masse in the ceramic sector [15]. Genç et al. [16]. showed that boron wastes cannot be used directly in tile glaze due to their high expansion coefficients, but boron wastes can be used as glaze material in tile products if a recipe is prepared to reduce the expansion coefficient. In another study on adding boron waste to floor tile masses, it was stated that products with low porosity, high strength values and low water absorption were obtained [17]. Sağlam and Emrullahoğlu [18] investigated the usability of boron waste in hard porcelain as a result of calcination by mixing it with various raw materials. When the experimental results were examined according to the reference sample, it was seen that results close to the reference sample were obtained by using chamotte-containing boron waste. However, although quite a lot of studies have been conducted on the use of boron waste in ceramic bodies, the calcination of boron waste together with kaolin has not been studied in the researches.
Mullite is formed by heating kaolin-type minerals and is also obtained as a result of the reaction of alumina-silica mixtures. Michel et al., [19] in their research on alumina-silica systems, determined that the mullite phase provides strength in the ceramic body. In his study on ceramic mullites, Schneider [20] stated that mullite can be used in high-temperature applications with its properties such as low thermal expansion, high melting point, good frictional strength, low thermal conductivity and chemical stability. When studies on the use of chamotte and/or mullite-based chamotte in the FFC body are examined, it is understood that the mullite phase positively affects the properties of the ceramic body. Kong et al. [21] obtained mullite phase from an alumina-silica mixture at low temperatures such as 1200 oC by high energy grinding. Although the mullite phase is often formed at temperatures above 1500 oC, it has been revealed in this work that the grinding effect and the influence of impurities may produce the mullite phase at low-temperature values. In addition, it has been determined that alternative materials should be used instead of chamotte used in the ceramic sanitary ware industry, the mullite phase should be increased in alternative materials or new phases with similar properties to the mullite phase should be created. Ak [22] investigated how the mullite phase affected the mechanical characteristics of anorthite-based vitrified ceramics. By developing two different recipes, the research examined the mechanical characteristics of the samples fired at an increase in temperature from 1290 oC to 1390 oC. The maximum values were obtained from the samples fired at 1360°C, and it was concluded as a consequence that the mechanical characteristics behaved parallel with the materials' densities. Additionally, it has been found that the body containing mullite is stronger and has a higher elastic modulus than the anorthite body. Kunduracı et al. [23] synthesized chamotte material by applying a sintering process to the powder mixture obtained from Symlox powder (commercial mullite) and alumina-silica-tungsten carbide mixture, which has a high mullite content. In the study, it was seen that the synthesized fireclay could be used instead of the imported fireclay material.
In this study, the production of mullite-based chamotte as a result of the calcination of kaolin waste instead of imported chamotte used in the production of sanitary ware products was investigated. Boron waste was added to reduce the calcination temperature of kaolin and to support mullite formation. The obtained kaolin and kaolin + boron waste chamotte were used in the FFC body. The interactions of the parameters affecting the synthesis method, both individually and with each other, were determined by statistical experimental design. The process is modeled with a mathematical model that is a function of independent variables. In addition, the properties of the vitrified product produced with the reference chamotte and the chamotte produced with the optimization study were compared.