Raw materials for the production of ceramic slabs are divided into two main groups:
- Clayey (plastic): monomineral (kaolin clays), polymineral (illite clays, bentonite clays, montmorilite clays);
- Non-clayey: waste additives (quartz sand, fireclay, slag, ash), fluxes (feldspar, dolomite, magnesite, chalk).
Plastic (clayey) materials are represented by clays and kaolins.
Clay
Clay is a product of the earth's crust, a sedimentary rock formed as a result of the destruction of rocks during the weathering process. It is an earthy, mineral mass with a particle size of about 0.02 mm and often less than 0.002 mm, capable of forming a plastic body with water, maintaining its shape when dried, and acquiring the hardness of stone after firing. The properties of clays are determined by their composition - chemical, mineralogical and granulometric. The oxides that make up clays include alumina (Al 2 O3), silica (SiO 2 ), iron oxide (Fe 2 O 3 ), sodium oxide (Na 2 O), calcium oxide (CaO), magnesium oxide (MgO), potassium oxide (K 2 O). A change in the chemical composition significantly affects the properties of clays:
- Alumina increases fire resistance, and therefore strength;
- With an increased silica content, the binding capacity of clays decreases, porosity increases, and strength decreases;
- Iron compounds reduce the fire resistance of clays;
- Calcium and magnesium oxides reduce fire resistance, increase firing shrinkage and porosity, which reduces strength and frost resistance;
- Sodium and potassium oxides lower the sintering temperature of clay.
Clays are monomineral and polymineral. Basically, monomineral kaolinite clays consist of the mineral kaolinite.
Kaolinite clays
The term kaolin (kaolinite) comes from the Chinese word "kao ling", which means "High Mountain" - the name of the deposit of this type of clay in China, first used for the production of ceramics. Clays with a predominant content of kaolinite are light in color, swell slightly when interacting with water, are characterized by refractoriness, low plasticity, and little sensitivity to drying. The most famous deposits of kaolin are located in England (Cornwell, Devonshire), Germany (Meissen, Chemnitz, Halle), France, Czech Republic, Ukraine, the Urals, Eastern Siberia, the USA and China (Kao Ling Mountain).
Illite clays
Their name comes from the name of the American state of Illinois, as this is where this type of clay was first found. The most famous deposit is the Apennines. The technological indicators of illite clay can be characterized as an average between the indicators of montmorillonite and kaolin clays, that is, as an equilibrium point for most characteristics. Illite clay does not cause problems during drying; the mechanical bending strength of the dry product ranges between 40 and 60 kg/cm2 , which makes it possible to manipulate the dried material without problems with the appearance of cracks. During firing, the material undergoes a rapid glass transition, starting from 800 o C due to the high K2O content .
Montmorilite clays
Their name comes from the name of the French city of Montmorillon, where this type of clay was first discovered. They consist of minerals of the montmorillonite group. Montmorilite clays are characterized by the ability to swell, especially if Na predominates in the exchangeable cations. They are formed in an alkaline environment during the decomposition of ash (underwater and above ground), weathering of limestone soils, in sea saline basins and lagoons, and during hydrothermal processes.
Technological properties of clays
In addition to clay minerals, clays include non-clay minerals: quartz, mica, feldspars, calcite, magnesite, etc. The mineralogical composition determines the molding ability of clay raw materials, its behavior during firing and other technological properties, characterized by plasticity, attitude to drying and exposure to high temperatures. Very fine clay particles, due to their large surface area, absorb water well, so the clay is hygroscopic and, when wet, acquires the ability to be plastically molded.
Plastic
The characteristic property of clays is to form a dough when mixed with water, which, under the influence of external conditions, can take a given shape without cracking and retain this shape after the force ceases. The degree of plasticity of clay depends not only on the chemical composition, but also on the physical size and shape of clay particles (granulometric composition), and on the amount of water contained in it. Plasticity is the most important technological property of clays, which makes it possible to mold products from them. Plasticity can be increased by adding highly plastic clays and decreased by adding non-plastic materials, such as quartz sand. The natural plasticity of clays can also be increased artificially by increasing its dispersity through freezing, elutriation, aging, and mechanical processing. There are three types of clay:
- Highly plastic - viscous, greasy, require a lot of water in order to become easily molded. Such clays are destroyed at high temperatures;
- Medium-plastic - the golden mean between the other two varieties;
- Low-waisted – also called skinny. To create a plastic body from lean clays, less water is required. Their advantage is fire resistance.
Let's consider such a property of clays as fire resistance.
Fire resistance
This is the ability of a material to withstand prolonged exposure to high temperatures without softening or deforming. Based on fire resistance, clays are classified into refractory, refractory and low-melting clays. Refractory clays (firing at a temperature of 1580 o C) are distinguished by a high alumina content (up to 40% or more) and a low content of impurities that reduce refractoriness (Fe 2 O 3 no more than 3%). Clays with fire resistance from 1350 to 1580 o C are called refractory, below 1350 o C - fusible. When clays are heated, the fusible components melt and, filling the gaps between the solid grains, bring them closer together due to surface tension forces. This causes compaction and reduction in the total volume of the material, measured by fire shrinkage. The higher the firing temperature, the higher the fire shrinkage. The process of compacting clay masses during firing is usually called sintering.
Leaning additives, fluxes (fluxes)
Both fatty and lean clays in their natural form are not suitable for use in production. They require appropriate processing - depending on the ceramic mass, they increase or decrease the plasticity of the clay, improve drying and firing properties, and reduce the duration of drying and firing of the product. For this purpose, so-called leaning supplements are used. These include sand, fireclay, slag, and ash.
Chamotte is fire-resistant clay, fired at a temperature of 1300 - 1400 oC and then crushed. Used as a thinning material.
Clays have the property of shrinkage, that is, a decrease in size without changing shape. A distinction is made between air shrinkage (during drying before firing) and fire shrinkage during firing. Drying properties - the ratio of the material to drying. The most important of them is air shrinkage, that is, a decrease in the volume of the clay body when water is removed. Expressed as a percentage of the original size of the samples, it is usually 2 – 10%. The higher the plasticity of clays, the greater the air shrinkage.
Leaning additives are introduced into plastic clays to reduce shrinkage during drying and firing and prevent deformations and cracks in products. These include fireclay, slag, ash, quartz sand.
Fluxes (fluxes) are used to increase the sintering of clay and reduce the sintering temperature. These include feldspars, dolomite, magnesite, and chalk.
Main types of clays
For the production of ceramic tiles, three main types of clay are used: red, white, porcelain.
Red clay has a greenish-brown color, which is given to it by iron oxide, which makes up 5–8% of the total mass. When fired, the clay acquires a red-brown color; the higher the firing temperature, the darker the shard; The tiles are called red-burning. Red clay can withstand heating of no more than 1150 o Majolica tiles are obtained from red clay .
White clay is light gray when wet, and after firing it becomes whitish or ivory in color. The higher the firing temperature, the whiter the shard. Tiles made of white clay are called white-burning. White clay is less plastic and oily than red clay. The firing temperature is 1150 - 1200 o Earthenware tiles are produced from white clay with various additives - feldspar, quartz sand .
China clay is composed of kaolin, quartz and feldspar. It does not contain iron oxide. When wet it has a light gray color, after firing it is white. The recommended firing temperature is 1200 - 1350 o C. The clay material obtained as a result of high temperatures has fire resistance, water resistance and mechanical strength. So-called porcelain tiles or ceramic granite .
The material from which ceramic products are made after firing is called a ceramic shard or biscuit in ceramic technology. Some types of ceramic tiles are coated with a decorative layer - glaze or engobe - to improve sanitary and hygienic properties, waterproofness, and aesthetic qualities.
The article was based on the following materials:
“Ceramic tiles: a modern encyclopedia” comp. Sergeeva E.E. Estima company.