1. Introduction
Obtaining high-quality and durable ceramic products - porcelain stoneware based on local natural mineral raw materials is an urgent task. The quality and durability of high-strength porcelain stoneware is characterized by increased compressive strength and frost resistance. Especially porcelain stoneware, consisting of quartz, feldspar and kaolin, has a unique combination of mechanical strength and chemical inertness. The microstructure of porcelain stoneware, characterized by large quartz grains, mullite crystals and an amorphous silicate phase, determines its unique properties.
Currently, there are several ceramic materials that meet the needs of interior wall and floor coverings: decorative stones, including granite, marble, slate and others [
1]; quartz-resin composites, also called engineered or agglomerated stone [
2,
3]; ceramics, porcelain tile and porcelain stone [
4,
5]; hard surface composites based on aluminum trihydroxide [
6,
7]; concrete and wood [
8]. These materials must meet several performance requirements, the most important of which are resistance to heat, stains, scratches and chips, as well as ease of maintenance [
9]. All these requirements, combined with high durability and high aesthetic standards, are met in the most demanding applications - which, in addition to countertops, include suspended coverings, interior and exterior coverings, as well as various furniture elements - by means of rigid materials such as granite, engineered stone or porcelain stoneware [
9,
10].
The significant growth of the ceramic industry has led to a huge consumption of clay raw materials, which results in their overuse and harm to the environment. For example, the production of ceramic tiles requires a significant amount of fluxes, which is about 50-60% by weight. Therefore, it is important to recycle and reuse industrial by-products such as metakaolin, fly ash and silica, which have a good ratio of alumina to silica, making them suitable as raw materials for ceramic production. Therefore, the ceramic industry has a good opportunity to use industrial secondary raw material as an alternative raw material in a sustainable manner.
The production of porcelain stoneware consists of the following stages, usually using tunnel roller kilns, which have a pre-kiln zone, or drying, a heating zone, or preliminary heating, firing zones, rapid cooling, slow cooling and final cooling [
11,
12]. In some cases, during the firing and production of porcelain stoneware, a defect called a crater is formed.
According to [
13], when raw materials are sintered in a furnace with the participation of a liquid phase, the carbonaceous material acts as a reducing agent at high temperatures. At a firing temperature of materials starting from 700°C, the formation of new crystalline phases consisting of SiO
2, silicates and complex aluminosilicates is observed. With an increase in temperature to 800 - 950°C, decomposition of carbonates and dolomite occurs with the release of carbon dioxide and thermal decomposition of sulfates and fluorides. Upon reaching a temperature of 1150-1200°C, a liquid phase is formed due to the presence of feldspars containing a large amount of alkali metals. Feldspars are fluxes, and the resulting liquid phase fills the pores, increasingly dissolving the oxides of clay minerals, leading to noticeable shrinkage and compaction of the mass.
In the earth's crust, the most common rock-forming silicate minerals are feldspars and kaolin - clay consisting mainly of the mineral kaolinite - Al
4[Si
4O
10](OH)
8 [
12]. Among the most common minerals in nature is quartz, a rock-forming mineral of most igneous rocks [
13].
There is natural mullite i.e. a mineral from the class of silicates in the form of mAl
2O
3·SiO
2. Mullite is formed by heating kaolinite to 950°C [
14]. Mullite is the main component of synthetic porcelain stoneware. The iron content in the mineral – iron oxide of various modifications with other metal oxides – gives the porcelain stoneware a shade from pink to brown [
15].
As a result of high-temperature firing, part of the quartz remains unchanged, and metakaolinite, formed during the dehydration of kaolinite, is transformed into mullite (3Al₂O₃·2SiO₂). At the same time, part of the quartz is preserved in the structure of the final product [
16].
Kaolinite, as the main component of the raw mix, undergoes dehydration and subsequent condensation, forming mullite. The resulting mullite is the main component of porcelain stoneware. With an increase in temperature to 1200ºС, other chemical transformations occur, leading to the formation of new crystalline phases and determining the final composition of the ceramic product [
17].
Due to the lack of aluminum oxide in the composition of the original material, unbound SiO
2 (amorphous) is formed during the heat treatment. For greater mullite formation, an additional amount of Al
2O
3 is added to the mixture. When fired in the temperature range of 25-1200ºC, reactions of formation of 3Al
2O
3·2SiO
2 and other compounds occur. This reaction leads to the formation of mullite, which is a highly durable and hard mineral. Mullite gives porcelain stoneware its characteristic properties, such as increased strength, hardness, heat resistance and abrasion resistance [
18,
19,
20,
21,
22,
23,
24,
25].
Thus, from the above-mentioned known methods, natural minerals are mainly used, which contain carbon-containing minerals dolomite, quartz-resin composites, during the firing process, a defect is formed, the so-called black chips (cracks) or craters of which reduces the strength of porcelain stoneware and frost resistance. Such characteristics significantly affect the quality of porcelain stoneware. To eliminate such defects and improve the quality of porcelain stoneware, we offer the use of waste silicon production containing active silicon oxide, which leads to the formation of a durable solid-phase mineral mullite due to aluminum oxide and microsilicon-silicon oxide, allows the removal of carbon dioxide and eliminates the formation of defects, cracks and chips in porcelain stoneware.
At present, the production of porcelain stoneware using the active component of microsilica secondary raw material from silicon production and its effect on the physical and mechanical properties of porcelain stoneware have not been studied. Optimization of the process of obtaining porcelain stoneware from a mixture of raw materials using microsilica as a silicon-containing component allows us to determine the phase composition and microstructure of the material, increasing its strength and hardness. The results of the study demonstrate the innovativeness of the technology in comparison with the known method, the prospects and scientific interest of using micro-silica in the production of high-quality porcelain stoneware.