Based on their composition and properties, ceramic products are divided into types, types and varieties.
The ceramic type is determined
composition and ratio of individual phases
Their processing, especially the fineness of grinding,
composition of glazes,
temperature and duration of firing.
The composition of masses of all types of ceramics includes plastic clay substances (clay, kaolin), waste materials (quartz, quartz sand), fluxes (feldspar, pegmatite, perlite, bone ash, etc.) When firing molded products as a result of complex physico-chemical transformations and interactions of the components of masses and glazes, their structure is formed.
Based on the nature of their structure, ceramics are divided into coarse and fine.
Products of coarse ceramics (pottery, bricks, tiles) have a porous, coarse-grained shard of heterogeneous structure, colored yellowish-brown by natural impurities.
Fine ceramic products are distinguished by a fine-grained white or light-colored, sintered or finely porous shard of uniform structure.
According to the degree of sintering (density) of the shard, ceramic products are distinguished:
Dense, sintered with water absorption less than 5% - porcelain, fine stone products, semi-porcelain;
Porous with water absorption of more than 5% - earthenware, majolica, pottery.
Depending on the structure there are:
Rough is a porous, coarse-grained, fractured shard of heterogeneous structure, colored by natural impurities in yellowish-brown colors (porosity 5-30%) - pottery ceramics - pottery, bricks, tiles. Many building ceramic materials, such as facing bricks, are classified as coarse ceramics
Fine ceramics are distinguished by fine-grained white or light-colored, sintered glassy or finely porous shards of uniform structure (porosity<5%) - фарфор, полуфарфор, фаянс, майолика, керметы.
A special group includes the so-called highly porous ceramics (porosity 30-90%), which usually includes heat-insulating ceramic materials.
The properties of ceramic products depend both on the composition of the masses used and on the technological features of their production.
Ceramics are needed where high resistance to external influences is required: high temperature, abrasion, aggressive environments, etc.
The stability of structure and properties is ensured by strong chemical bonds.
Due to the unique properties of ceramics, they have received well-deserved recognition in various fields of technology.
Physical and mechanical properties ceramics are determined by the nature of the chemical bond and crystal structure.
Depending on the purpose of ceramics, obtaining the specified properties of products is achieved by selecting raw materials and additives and by technology features.
The main properties include density, mechanical strength, hardness, porosity, thermal resistance, chemical stability, whiteness, translucency, speed of propagation of sound waves.
Ceramics are characterized by high hardness, rigidity, relatively high compressive strength and lack of ductility.
Hardness. Even porous pottery clay scratches glass, because... contains quartz particles (Mohs 7). Technical ceramics contain aluminum oxide (Mohs 9) - sapphire, ruby. This property is most fully used in abrasive ceramic materials - silicon carbide, aluminum oxide, boron and carbon nitride - hard and superhard materials.
Mechanical strength- one of the most important properties on which the durability of the product depends. It has quite high strength. Strength is highly dependent on the porosity of the ceramic. clay pot, porcelain cup with thin walls... Specific mechanical strength, i.e. the ratio of the applied force to a unit of bottom thickness, is determined by the method of free fall of a steel ball along the bottom of the product. In faience it is higher than in porcelain. On the contrary, the impact strength of earthenware products using the pendulum method is lower than that of porcelain products.
It withstands compressive stress well, worse than bending stress and very poorly with tensile stress (35-350 MPa, ordinary brick 5 MPa, piano steel wire 3100 MPa, leather 40 MPa, human hair 190 MPa). When designing the shape of a product, the shape is calculated so that the forces arising during operation lead to compression or bending stresses (picture).
Density b depends on the composition and porosity of porcelain is 2.25-2.4 g/cm³, and earthenware - 1.92-1.96 g/cm³.
Porosity determined by the method of water absorption, which for porcelain is 0.01-0.2%, and for earthenware - 9-12%.
Fire resistance – resistance to high temperatures. In demand in furnaces and units for smelting metals. T 1000-3000. At T more than 1000, it is stronger than any alloy. Depends on the composition, i.e. on the melting point of its main components. Not all ceramic materials are fireproof; all building and household ceramics have low operating temperatures. They will withstand the fire, but the glaze coating will be covered with scratches.
Fire resistance is the property of ceramic materials and products to withstand high temperatures without melting. An indicator (quantitative measure) of fire resistance is the temperature at which a sample of this material, having the shape of a triangular truncated pyramid (conventionally called a “cone”), is deformed under the influence of its own gravity, touching the ceramic stand with its top.
Heat resistance characterizes the product’s ability to withstand sudden temperature changes. For glazed tiles = 125-150 C, which means the possibility of a sharp drop from this temperature to 20 C without the formation of cracks.
Heat-resistant materials must have a low temperature coefficient. lin. ext., high thermal conductivity and mechanical strength.
The most heat-resistant are quartz ceramics, ceramics based on cordierite, and spodumene.
The most heat-resistant art ceramics are porcelain and stone ceramics - they make teapots and cups. The thermal resistance of porcelain products is higher than that of earthenware. Thus, in accordance with the current GOSTs 28390-89 and 28391-89, the heat resistance of porcelain products should be 185°C, earthenware - from 125°C (for colorless glazes) and 115°C (for colored glazes).
The chemical bonds in ceramics are very strong, which is why ceramics are also characterized by high melting temperatures and chemical resistance.
Ceramics mp.,°С
Titanium carbide TiC 3120
Titanium boride TiB2 2980
Tungsten carbide WC ~2850
Aluminum oxide Al2O3 2050
Chromium oxide Cr2O3 1990
Thorsterite 2MgO SiO2 1830
Mullite 3Al2O3 2SiO2 1810
Silicon oxide (cristobalite) 1715
Titanium oxide TiO2 1605
The lack of free electrons is why ceramics are generally poor conductors of electricity and heat. Therefore, ceramics are widely used in electrical engineering as dielectrics.
The needs of vacuum technology in ceramics are primarily related to their high dielectric properties, high chemical resistance (including at high temperatures) and high temperature resistance.
lack of hygroscopicity in most materials,
good electrical (piezoelectric, ferroelectric)
and magnetic characteristics with sufficient mechanical strength, stability of characteristics and reliability,
resistance to high-energy radiation and the use of fairly cheap and accessible raw materials have ensured their widespread use in various fields.
Hygroscopicity - ceramics is an environmentally friendly product and has a capillary structure that allows the wall to “breathe”. A wall made of such material acts as a natural air conditioner: it absorbs moisture when there is an excess of it and releases it when there is a deficiency, maintaining a healthy temperature and humidity balance in the living space. The wall surface remains dry at any time of the year, which, in turn, prevents the formation of fungus and mold.
In Europe, ceramic blocks are well known and loved. Today, more than half of buildings are built from this material. Now this material has come to the Russian market and confidently continues to conquer it thanks to its undeniable advantages.
Aesthetic properties It is difficult to characterize ceramic materials unambiguously, since the compositions, surface textures and methods of decoration are too different.
For pottery ceramics and terracotta, the texture of the surface and the warm tones of natural colors play an important role. terracotta color.
The decorative effect of majolica, earthenware, and porcelain is associated primarily with glazing and painting. Faience - noticeable thickness, roughness of form, porcelain - graceful coldness, translucency.
When assessing the aesthetic properties of ceramic products, one can emphasize their plasticity and naturalness of shape, variety of textures and colors, i.e. high decorative capabilities.
Ceramics is one of the most environmentally friendly materials.
Whiteness is the ability of a material to reflect light falling on it. Whiteness is especially important for porcelain products. Whiteness is determined visually by comparing the test sample with a standard or using an electric photometer, as well as on a "Spekol".
Spread speed sound waves for porcelain products are 3-4 times higher than for earthenware, therefore, when hitting the edge with a wooden stick, porcelain products produce a high-pitched sound, and earthenware - a dull sound.
Translucency characteristic of porcelain, which is translucent when the product is thick because it has a sintered shard. Earthenware products are not translucent due to the porous shard.
Hardness of the glaze layer The mineralogical scale for porcelain is 6.5-7.5, and for earthenware - 5.5-6.5, microhardness is determined by indentation of a diamond pyramid. Porcelain glazes are considered hard, majolica are soft, and earthenware are considered medium.
The chemical resistance of glazes and ceramic paints used for household porcelain and earthenware products must be high, since they should not be destroyed when treated with weak acids and alkalis at ordinary temperatures or when heated to 60-65°C.
Color “live” clay” is deceptive. When dried in air, it usually becomes only a little lighter. But when fired, most clays change their color dramatically: green becomes pink, brown becomes red, blue and black become white. For example, craftsmen from the village of Fnlimonovo near Tula sculpt their famous toys from black and blue clay, which after firing takes on a white, slightly creamy color. Here in the kiln, during firing, all the organic particles that gave it a “living” black color burn out. Only white clay remains white after firing.
Ceramic are building materials and products obtained by firing various clay and similar masses to a stone-like state.
3.1. Raw materials for the production of ceramic products
3.1.1. Clays . Clays are a group of sedimentary rocks common in nature, composed of various clay minerals - hydrous aluminosilicates - with a layered crystalline structure. The most important clay minerals are kaolinite (Al 2 O 3 2 SiO 2 2H 2 O); halloysite (Al 2 0 3 2SiO 2 4H 2 O) montmorillonite (Al 2 O 3 4SiO 2 n H 2 O); beidellite (Al 2 O 3 3SiO 2 nH 2 O) and products of varying degrees of hydration of mica.
If the clays are dominated by kaolinite and halloysite, then the clays are called kaolinite; if montmorillonite and beidellite predominate - montmorillonite; if products of different degrees of hydration of micas predominate - hydromicas. Finely dispersed rocks with a predominance of montmorillonite are called bentonites
Clay minerals determine the main feature of clays - to form a plastic dough with water, capable of maintaining its given shape during the drying process and, after firing, acquiring the properties of stone.
Along with clay-forming minerals, clays contain quartz, feldspar, sulfur pyrites, iron hydroxides, calcium and magnesium carbonates, titanium compounds, vanadium, organic impurities and other impurities that affect both the production technology of ceramic products and their properties.
The ceramic properties of clays are characterized by plasticity, cohesion and binding ability, air and fire shrinkage, fire resistance and the color of the shard after firing.
Plasticity of clays. The plasticity of clays is the ability of clay dough, under the influence of external forces, to take a given shape without the formation of cracks and to maintain it stably.
Impurities contained in clays reduce the plasticity of clays, and to a greater extent, the higher their content. The plasticity of clays increases with increasing amount of water in the clay dough, but up to a certain limit, beyond which the clay dough begins to lose its workability (sticks to the surface of clay processing machines). The more plastic the clay, the more water they require to obtain a moldable clay dough and the greater their air shrinkage.
A technical indicator of plasticity is the plasticity number:
Pl = W T – W r , 3.1
Where WT And Wr moisture content values in% corresponding to the yield strength and rolling limit of the clay rope.
Highly plastic clays have a water requirement of more than 28%, a plasticity number of more than 15, and an air shrinkage of 10...15%. Products made from these clays greatly decrease in volume when drying and crack. Excessive plasticity is eliminated by introducing leaning additives.
Clays of average plasticity have a water requirement of 20...28%, a plasticity number of 7...15 and air shrinkage of 7...10%.
Low-plasticity clays have a water requirement of less than 20%, a plasticity number of less than 7, and an air shrinkage of 5...7%. Products made from these clays are difficult to mold. Insufficient plasticity is eliminated by removing sand (elutriation), aging (natural weathering), grinding in special machines, steam treatment or adding plastic clay.
Connectivity – the force required to separate clay particles. Cohesion is due to the small size and lamellar shape of the particles of the clay substance. The higher the amount of clay fractions, the higher the cohesion.
The binding ability of clay is expressed in the fact that clay can bind particles of a non-plastic substance (sand, fireclay, etc.) and form a fairly strong product when dried - raw.
Shrinkage of clays. When clays are wetted with water, clay minerals swell due to the fact that the water they absorb is located between the individual layers of their crystal lattices; in this case, the interplanar spacing of the gratings increases significantly. When drying clays, the reverse process occurs, accompanied by shrinkage.
Under air shrinkage(linear or volumetric) understand the decrease in the linear dimensions and volume of a clay dough sample upon drying. The higher the plasticity of the clay, the greater the air shrinkage.
When firing clays, after removing hygroscopic moisture and burning out organic impurities, decomposition of clay minerals occurs. Thus, kaolinite at a temperature of 500 - 600°C loses chemically bound water; in this case, the process proceeds with complete decomposition of the crystal lattice and the formation of an amorphous mixture of alumina A1 2 O 3 and silica SiO 2. With further heating to temperatures of 900 - 950 ° C, new metal silicates appear, for example mullite 3Al 2 O 3 2SiO 2, and a certain amount of melt (liquid phase) is formed due to the melting of the most fusible minerals that are part of the fired clay masses. The more flux oxides Na 2 O, K 2 O, MgO, CaO, Fe 2 O 3 are in the clay composition, the lower the temperature of the liquid phase formed. During the firing process, under the influence of surface tension forces of the liquid phase, the solid particles of the fired material come closer together, and its volume decreases, i.e., fire shrinkage occurs.
Fire shrinkage (linear or volumetric) is the reduction in linear dimensions and volume of dried clay samples during the firing process.
The transition of clay masses during firing and subsequent cooling into a stone-like body is due to the adhesion of particles as a result of diffusion processes, leading to the formation of new crystalline silicates due to topochemical reactions, and the formation of a glassy melt that binds individual refractory grains into a strong monolithic shard. The process of compacting clay masses during firing is commonly called sintering.
The firing temperature at which the water absorption of the fired product is 5% is taken as beginning of clay sintering. The temperature interval between refractoriness and the beginning of sintering is called sintering interval clay It depends on the composition of the clays: pure kaolin clays have a sintering range of more than 100° C, the presence of calcite CaCO 3 in the clay composition reduces the sintering range. When producing dense ceramic products, only clays with a large sintering interval can be used.
Fire resistance clays depend on their composition. For pure kaolinite, the fire resistance is 1780° C. According to fire resistance, clays are divided into fireproof - with a fire resistance of more than 1580 ° C, refractory - with a fire resistance of 1350 - 1580 ° C, and low-melting clays - with a fire resistance of less than 1350 ° C.
To obtain ceramic building materials, predominantly low-melting (brick) clays are used, containing a significant amount of quartz sand, iron compounds and other fluxes.
Clay shard color , after firing, depends on the composition of the clays, in particular on the presence of oxides in them; gland. Iron compounds color the ceramic shard red when fired in an oxidizing environment and dark brown or black when fired in a reducing environment. The color intensity increases with increasing Fe 2 O 3 content in the clay.
3.1.2. Skinny materials. Leaning materials are added to plastic clays to reduce shrinkage during drying and firing and prevent deformation and cracks in products.
Quartz sand and pulverized quartz (natural materials), dehydrated clay (obtained by heating clay to 600...700 o C - in which case the clay loses plasticity), fireclay (obtained by firing refractory or refractory clays at 1000...1400 o C) are used as depleting materials. subsequent grinding to 0.16...2 mm), ash and slag (industrial waste).
3.1.3. Pore-forming materials. Pore-forming materials are introduced into the raw material to produce lightweight ceramic products with increased porosity and reduced thermal conductivity.
To do this, use substances that dissociate during firing (for example, chalk, ground dolomite, etc.) with the release of gas (for example, CO 2), or burn out (sawdust, coal powder, peat dust, etc.). These supplements are also fattening.
3.1.4. Plavni. Fluids are added to clay in cases where it is necessary to lower its sintering temperature.
For this, feldspars, iron ore, dolomite, magnesite, talc, etc. are used. When producing colored ceramics, metal oxides are added to the raw material as fluxes: iron, cobalt, chromium, etc.
1.5. Glazes and engobes. To impart resistance to external influences, water resistance and a decorative appearance, the surface of some products (facing bricks, ceramic tiles, ceramic pipes, etc.) is coated glaze or engobe.
Glaze is a glassy layer applied to the surface of a ceramic material and fixed to it by firing at high temperatures. Glazes can be transparent or opaque (dull), and have different colors.
For the production of glaze, the following are used: quartz sand, kaolin, feldspar, salts of alkali and alkaline earth metals, lead or strontium oxides, boric acid, borax, etc. The composition of the glaze, as a rule, is the know-how of the enterprise. The raw material mixture is ground into a powder (either raw or after fusion into a frit) and applied as a slurry before firing.
Engobe is made from white or colored clay and applied in a thin layer to the surface of a raw product. Unlike glaze, engobe does not produce a melt during firing, i.e. does not form a glassy layer, and therefore the surface is matte. The properties of the engobe should be close to the main shard.
3.2. Fundamentals of ceramic production technology
The production process of all ceramic products includes the extraction of clay, the preparation of clay masses for molding, molding of products, drying, and firing.
For some ceramic products, the process of obtaining them (after firing) ends with external finishing.
In the production of ceramic tiles, ceramic pipes, sanitary products, the technology additionally includes glazing before firing or after primary firing, and sometimes applying a pattern using various methods (most often by decoration).
Extraction and transportation of clay. In most cases, clay is mined by open-pit mining using single- and multi-bucket excavators, scrapers and other mechanisms. Clay is delivered to the plant by rail, road transport, overhead roads and conveyors.
Preparation of ceramic mass. Quarry clay in most cases is not suitable for producing ceramic products. Therefore, the technology of any ceramic production begins with the preparation of ceramic mass.
The purpose of this stage of production is to destroy the natural structure of clay raw materials, remove harmful impurities, crush large pieces and obtain a homogeneous, moldable mass.
In preparation for molding clays of high (excessive) plasticity, thinning and pore-forming additives are introduced into their composition, and, if necessary, fluxes. If there are rocky inclusions in the clay with a particle size of over 5 mm, it is passed through stone separation rollers or these inclusions are crushed by processing the clay on runners.
Then, in a clay mixer, the clay is mixed with water to obtain a clay dough of molding moisture.
Depending on the type of product being manufactured and the properties of the raw material, the ceramic mass is produced by plastic, semi-dry and slip (wet) methods and the molding method is selected accordingly.
Molding of products.
Plastic molding method. With the plastic method In preparing the mass and molding, the starting materials at natural moisture or pre-dried are mixed with each other with the addition of water until a dough is obtained. The moisture content of the resulting mass ranges from 15 to 25% or more. The prepared clay mass enters a molding press, most often a regular belt press or one equipped with a vacuum chamber (Fig. 3.1).
Vacuum helps remove air from the clay and bring its particles closer together, which increases the homogeneity and moldability of the mass, as well as the strength of the raw material. A clay beam of the required cross-section coming out through the mouthpiece of the press is cut by a cutting machine into products (raw products). The plastic method of mass preparation and molding is most common in the production of mass materials (solid and hollow bricks, tile stones, facing tiles, etc.).
Semi-dry and dry molding methods.
With the semi-dry method During preparation, the raw materials are first dried, crushed, ground into powder, and then mixed and moistened with water or, better yet, steam, since this makes it easier to transform the clay into a homogeneous mass and improves its swelling and molding ability. The ceramic mass is a low-plastic press powder with low humidity: 8...12% for semi-dry and 2...8% (usually 4...6%) for dry molding. Therefore, products from such masses are molded under high pressure (15...40 MPa) on special automatic presses. Products after pressing can sometimes be fired immediately without pre-drying, which leads to faster production, reduced fuel consumption and cheaper products. In contrast to the plastic molding method, low-plasticity clays can be used, which expands the raw material base for production. Solid and hollow bricks and facing tiles are produced using the semi-dry pressing method, and dense ceramic products (floor tiles, road bricks, earthenware and porcelain materials) using the dry method.
Slip method . By slip method the starting materials are pre-crushed and thoroughly mixed with a large amount of water (mixture humidity up to 40%) until a homogeneous fluid mass (slip) is obtained. The slip is used directly for the manufacture of products (casting method) or for the preparation of press powder, drying it in spray tower dryers. The slip method is used in the technology of porcelain and earthenware products, facing tiles.
Slip with a moisture content of 35-45% is poured into plaster molds (or into molds made of special porous plastic). Water from the slip is absorbed by the porous material, and a raw product is formed on the surface of the mold. Depending on the type of product, its shape and purpose, the slip can be completely dehydrated in the mold (pouring method) - this is how products of complex shapes are made, for example, sanitary ceramics, etc., or partially dehydrated. In this case, during the molding process, the slip is added to the required level, and after a certain time has elapsed, it is completely poured out of the mold. In this case, a thin-walled product remains on the surface of the mold.
Drying products.
Drying is a very important stage of the technology, since cracks usually appear at this stage, and during firing they are only finally revealed. Usually, drying the raw material to a residual moisture content of 6...8% is sufficient.
During the drying process, the movement of moisture from the thickness of the ceramic product to the outer layers occurs much more slowly than moisture loss from the surface, this is especially evident in the ribs and corners of the product. In this case, different degrees of shrinkage of the inner and outer layers occur, and consequently, stresses are created that can lead to cracking of the material. To prevent this, thinners are added to fatty clays, which form a rigid skeleton that prevents the clay particles from approaching each other, and increase the porosity of the product, which promotes the movement of water from its inner layers to the outer ones. To reduce the sensitivity of clays to drying, steam heating and evacuation of clays are also used, and some organic substances are used in small doses - lignosulfonates (LST), tar and bituminous substances, etc.
Previously, raw meat was dried mainly under natural conditions (in drying sheds). Natural drying, although it does not require fuel, is largely dependent on the weather and lasts a very long time (10...20 days). Currently, drying of raw materials, as a rule, is carried out artificially in special periodic (chamber) or continuous (tunnel) dryers. Flue gases from kilns or hot air from heaters are used as a coolant. The drying time is reduced to 2...3 days, and sometimes to several hours.
Firing of products.
Firing is an important and final stage of the technological process of ceramic products. The total costs of firing reach 35...40% of the cost of commercial products. When raw material is fired, an artificial stone material is formed, which, unlike clay, is not washed away by water and has relatively high strength. This is explained by physicochemical processes occurring in clay under the influence of elevated temperatures.
When raw ceramic products are heated to 110°C, free water is removed and the ceramic mass becomes non-plastic. But if you add water, the plastic properties of the mass are restored. With an increase in temperature to 500...700 °C, organic impurities burn out and chemically bound water found in clay minerals and other compounds of the ceramic mass is removed, and the ceramic mass irreversibly loses its plastic properties. Then the decomposition of clay minerals occurs until the complete disintegration of the crystal lattice and the formation of an amorphous mixture of Al 2 O 3 and SiO 2. With further heating to 1000°C, due to reactions in the solid phase, the formation of new crystalline silicates is possible, for example sillimanite Al 2 O 3 -SiO 2, and then at 1200...1300°C its transition to mullite 3Al 2 Oz-2SiO 2. At the same time, low-melting compounds of the ceramic mass and flux minerals create a certain amount of melt (liquid phase). The melt envelops the unmelted particles, partially fills the pores between them and, having the force of surface tension, pulls them together, causing them to approach and compact. After cooling, a stone-like shard is formed.
Firing of products from “brick clay” is carried out at a temperature of 900...1000 o C. When obtaining products with a sintered shard from refractory and refractory clays, firing is carried out at a temperature of 1150...1400 o C.
For firing ceramic materials, special furnaces are used: tunnel, ring, slot, roller, etc.
After firing, the products are cooled gradually to prevent the formation of cracks.
Fired products may vary in the degree of firing and the presence of defects.
3.3. Types of ceramic materials and products
All ceramic materials are divided into two groups (depending on porosity) - porous(with water absorption more than 5%) and dense (with water absorption less than 5%).
According to their intended purpose, ceramic materials and products are divided into wall materials, bricks and stones for special purposes, hollow products for floors, materials for cladding building facades, products for interior cladding, roofing materials, pipes (sewage and drainage), fireproof materials, sanitary products .
The group of wall materials includes ordinary clay brick, hollow brick, porous-hollow brick, light and hollow ceramic stones.
According to the average density in a dry state, wall materials are divided into classes A (ρ o = 700 - 1000 kg/m 3), B (1000-1300 kg/m 3), B (1300-1450 kg/m 3) and D (more 1450 kg/m 3):
The lower the average density of wall materials, the greater their porosity and the lower their thermal conductivity. The minimum porosity of ceramic wall materials is limited by relevant standards and controlled by their water absorption. The water absorption of clay, ordinary and hollow semi-dry pressed bricks must be at least 8%. and hollow plastic molding and hollow ceramic stones - no less than 6%.
All ceramic wall materials must be sufficiently frost-resistant (at least 15 cycles of alternating freezing and thawing in a water-saturated state). Lightweight building bricks must withstand at least 10 cycles.
Construction brick. Ordinary clay brick is an artificial stone in the shape of a rectangular parallelepiped. It is made single with a size of 250x120x65 mm or modular with a size of 250x120x88mm. The average density of brick in a dry state, depending on the manufacturing method, ranges from 1600 to 1900 kg/m3. Semi-dry pressed brick has a higher average density, and therefore thermal conductivity.
By ultimate compressive strength; and bending is divided into seven grades: 75, 100, 125, 150, 250 and 300. Ordinary clay brick is used for laying internal and external walls, pillars, vaults and other parts of buildings in which its high strength is fully used.
Ordinary building bricks have a fairly high thermal conductivity, so it is necessary to build external walls of greater thickness than required by strength calculations. In such cases, it is more effective to use less durable, but less thermally conductive hollow, porous-hollow and lightweight bricks.
Hollow brick has slot-like voids or round holes that are formed during the process of plastic molding of the brick when the clay beam passes through a special die with metal cores. Using semi-dry pressing, hollow bricks are made with through and non-through voids. Porous hollow brick is produced similarly to hollow brick, but burnable additives are added to the clay composition. Lightweight porous bricks are made both from clays with burnable additives, and from diatomite (tripoli) with or without burnable additives.
Hollow Ceramic Stones They are made in the same way as bricks - by plastic pressing. The stones have the following dimensions: length 250 or 288, width 120, 138, 250 or 288 and thickness 138 mm. The average dry density ranges from 1300-1450 kg/m3. According to the compressive strength of the gross cross-section (without deduction of void area), stones are divided into grades 75, 100, 125 and 150.
According to their purpose, ceramic stones are distinguished for laying load-bearing walls of one-story and multi-story buildings and for internal load-bearing walls and partitions.
Bricks and stones for special purposes
This group of ceramic materials includes patterned clay bricks, stones for sewer structures and bricks for road pavements.
Clay pattern brick are manufactured by plastic molding in four types with different radii of curvature. It is intended for laying industrial chimneys. Based on compressive and bending strength, bricks are divided into grades 100, 125 and 150. The requirements for patterned bricks in terms of frost resistance and water absorption are the same as for ordinary bricks.
Stones for sewer structures They have a trapezoidal shape and are intended for the installation of underground collectors. They must have a compressive strength of at least 200 kgf/cm2 (20 MPa).
Bricks for road pavements , otherwise called clinker, is produced by firing before sintering, therefore, refractory clays with a large sintering interval (about 100°C) are used for its production. Clinker bricks are divided into grades 400, 600 and 1000 with water absorption and frost resistance, respectively, for M400 - 6% and 30 cycles; M600 – 4% and 50 cycles; M1000 – 2% and 100 cycles. In addition, this brick is subject to requirements for abrasion and impact resistance.
Clinker bricks are used for laying roads, floors of industrial buildings, as well as for laying foundations, plinths, pillars, walls of critical structures and sewer collectors.
Hollow ceramic products for floors. This group of products includes:
Stones for frequently ribbed floors of grades 50, 75, 100, 150 and 200 with an average dry density of no more than 1000 kg/m 3 ;
Stones for reinforced ceramic beams of grades 75, 100, 150 and 200 with an average density of no more than 1300 kg/m 3;
Rolling stones of grades 35, 50 and 75 with an average density of no more than 1000 kg/m 3.
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| Rice. 3.3. Ceramic stone flooring |
Ceramic products for cladding building facades
Both unglazed and glazed ceramic products are used for cladding building facades. Ceramic products for cladding building facades are divided into facing bricks and facing ceramic stones, carpet ceramics, small-sized facade tiles, ceramic facade slabs.
Bricks and ceramic facing stones must not have any fading, efflorescence, large inclusions or other defects. The front surfaces of brick and stone can be smooth, embossed or textured.
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| Rice. 3.4. Brick sizes according to EU standards. |
Based on their compressive and bending strength, bricks and stones are divided into grades 75, 100, 125, 150, 200, 250, 300. Their water absorption should be no less than 6 and no more than 14%. In a water-saturated state, they must withstand at least 25 cycles of alternating freezing and thawing without any damage.
The facing brick can have dimensions of 250x120x65 mm or be of other sizes (European and American standards).
Carpet ceramics called a set of small-sized (from 20x20 to 46x46 mm) thin-walled glazed or unglazed tiles glued to a paper base. The requirements for tiles in terms of frost resistance and water absorption are approximately the same as for facing ceramic stones.
Small-sized facade tiles They are made both glazed and without glaze.
Ceramic facade slabs are divided into embedded slabs, installed simultaneously with the laying of the walls, and leaning slabs, installed on the mortar after the walls are erected and settled. The slabs may be unglazed or coated with glaze. Unglazed slabs are called terracotta. They are made from clays that have a white or lightly colored shard after firing.
The frost resistance requirements for facade slabs are the same as for other ceramic materials used for cladding buildings: their water absorption should not be more than 14%.
Ceramic products for interior cladding
This group of products includes wall tiles and floor tiles.
Tiles for wall cladding are divided into majolica, made from fusible clays with colored shards and the front side covered with a dull (opaque) glaze, and earthenware, made from refractory white-burning clays with the addition of lean materials (quartz sand and ground tile scrap) with the front side , covered with transparent white or colored glazes. A design can be applied to the glaze using various methods (silk-screen printing, decoration, etc.)
Previously, square (150x150 mm and 100x100 mm), rectangular (150x25, 150x75, 150x100 mm) and shaped tiles were produced.
Now most factories in Ukraine and Russia have switched to the European standard - rectangular 300x200 mm (sometimes 250x200, 400x225 mm). However, in elite collections other tile sizes may be used. In modern technologies, high-precision stamping equipment is used to obtain the correct geometry of products, as well as laser trimming of finished products.
The thickness of the tiles should not be more than 6 mm.
The tiles must be thermally resistant, i.e., chipping and surface hairline cracks should not appear on the glaze when heated to 125°C followed by rapid cooling in water at room temperature. Both majolica and faience tiles have a porous crock; their water absorption should not exceed 16%.
Tiles are used for internal cladding of walls of sanitary facilities, as well as rooms with high humidity.
Floor tiles are made by semi-dry pressing and fired until sintered. Based on the type of front surface, tiles are divided into smooth, embossed and embossed, and based on color - into single-color and multi-color. By shape, tiles are divided into square, rectangular, triangular, hexagonal, tetrahedral (half hexagonal), pentagonal and octagonal. Floor tiles are characterized by high density (water absorption no more than 4%) and low abrasion (mass loss during testing should not exceed 0.08 g/cm2).
Roofing materials (clay tiles)
Clay tiles are one of the oldest roofing materials. Despite this, clay tiles are one of the best roofing materials. Its main advantages are durability (more than 100 years) and fire resistance. In addition, due to the absorption - evaporation of water and high heat capacity, the tiles regulate the microclimate of the room, increasing the comfort of the building.
They produce stamped groove tiles, groove strip tiles, flat strip tiles, wavy strip tiles, S-shaped strip tiles and ridge grooved tiles. Low-melting plastic clays are used to make tiles.
Strip tiles are produced according to a scheme similar to the scheme for producing bricks using the plastic molding method. However, the clay mass is processed more carefully before molding, usually using runners. The outlet holes of the press mouthpiece are shaped to match the shape of the tiles coming out of the press in the form of a belt; The clay mass is cut on cutting machines into individual tiles. Stamped tiles are pressed in metal or gypsum molds on eccentric presses, fired in ring or tunnel kilns at a temperature of 1000-1100 ° C.
The following requirements are imposed on clay tiles: the breaking load when testing tiles for fracture in an air-dry state must be at least: 100 kg for S-shaped, 80 kg for grooved stamped tiles and 70 kg for all other types of tiles. The weight of 1 m2 of tile covering in a water-saturated state should be no more than 65 kg for flat strip tiles, and no more than 50 kg for other types (with the exception of ridge tiles, the weight of 1 m2 of which should not exceed 8 kg). When saturated with water, the tiles must withstand at least 25 cycles of alternating freezing and thawing.
Ceramic sewer and drainage pipes
Sewer pipes are made from refractory and refractory clays. Pipes are formed on vertical belt presses from a plastic, well-prepared clay mass. After drying the pipes, fusible materials are applied to their internal and external surfaces.
compositions (glaze) that form a glassy film during the firing of pipes. The presence of a thin layer of glaze on the surface of the pipes determines their high resistance to acids and alkalis. Sewer pipes are made of a round cross-section with a socket at one end. Pipes must withstand hydraulic pressure of at least 2 atmospheres (0.2 MPa) and have a water absorption of the shard of no more than 9% for the first grade and 11% for the second. The high chemical resistance of ceramic pipes allows them to be effectively used for draining industrial waters containing alkalis and acids, as well as when laying sewer pipes in aggressive environments.
Ceramic drainage pipes are manufactured either unglazed without sockets or glazed with a socket of various diameters. They must withstand at least 15 cycles of alternating freezing and thawing in a water-saturated state without any signs of destruction. Drainage pipes are used mainly for draining swampy soils,
Refractory ceramic materials
Refractory are ceramic materials with a fire resistance of at least 1580 ° C. Materials obtained from refractory clays, thinned with the same clay, but pre-fired to sintering and crushed (chamotte), are called fireclay products.
Fireclay products in the form of bricks are called fireclay bricks. It is made from refractory clays by semi-dry pressing or plastic molding, followed by firing until sintering at a temperature of 1300-1400 ° C. Shaped refractory products, including large blocks, are also made from refractory clays thinned with fireclay. The fire resistance of fireclay products is approximately 1670-1770° C.
Fireclay refractories are characterized by high thermal resistance, the ability to well withstand the action of acidic fuel slag and molten glass at temperatures up to 1500 ° C. They are used for laying walls and vaults of furnaces, lining fireboxes, chimneys, etc.
Sanitary products
Equipment for sanitary facilities in residential and industrial premises (baths, sinks, etc.) can be made from earthenware, semi-porcelain and porcelain.
Porcelain is a dense ceramic material with a white shard, obtained by firing a raw material mixture that includes refractory clay, kaolin, feldspar, quartz and porcelain scrap.
Earthenware are called ceramic materials with a finely porous shard, usually white, for the production of which the same raw materials are used as for porcelain, but with a different recipe. So, to produce earthenware, the composition of the raw material mass can be as follows (%): kaolin-clay part 45-50, quartz sand 35-45, feldspar 2-5, chalk 10 and broken products or fireclay 10-15. Porcelain differs from earthenware in being more dense and durable.
Semi-porcelain in its properties it occupies an intermediate position between earthenware and porcelain.
The technology for the production of sanitary ceramic products will include all the main stages. The stage of preparing the raw mixture is, as a rule, more complex. Sanitary ceramic products are usually produced by casting a liquid mass (slip) into molds, then drying and firing the products. Firing can be one-time or two-time. To make sanitary products waterproof and look better, they are coated with glaze. The glazing composition (glaze) is applied to the molded products after drying or first firing. During firing, the glaze melts and covers the product with a thin shiny film.
Literature
- Domokeev A.G. Construction materials. – M. Higher. school, 1989. – 495 p.
- Gorchakov G.I. Bazhenov Yu.M. Construction materials. – M. Higher. school, 1986.
- Sheykin A.E. Construction materials. – M. Higher. school, 1978. – 432 p.
- Savyovsky V.V., Bolotskikh O.N. Repair and reconstruction of civil buildings. – Kharkov: Spirit level, 1999 – 290 s
General information about ceramic building materials and products
Classification of ceramic building materials and products. Properties, application
Raw materials for the production of ceramic materials and products. Classification, technological properties
Production of ceramic building materials and products. General technological processes
Ceramic materials are artificial stone materials obtained from natural clays or clay mixtures with mineral additives by molding, drying and subsequent firing. The word "ceramics" (Greek ceramos) means fired clay. Burnt bricks, roofing tiles, water pipes, and architectural details were made from it. Ceramic materials are the most ancient of all artificial stone materials. Shards of crude pottery are found at the sites of Stone Age settlements. Traces of ancient ceramics (dishes, vases, etc.) were preserved in Ancient Egypt and Greece. In Rus' there are ancient Russian cathedrals of the X-XV centuries. (Vladimirsky, Novgorodsky, the church in Kolomenskoye and St. Basil's Cathedral (Pokrovsky Cathedral, 1561). In Moscow, during the construction of which colored and ordinary bricks, tiles and other ceramic products were widely used).
Ceramics developed greatly in Central Asia, Ancient India, China and Japan. The Greeks and Romans used clay to make baked bricks, roofing tiles, architectural details and other products, and adobe dwellings (IV-III millennium BC).
Russian tiled art of the 15th-18th centuries was also noted for its high artistic merit. Terracotta and glazed samples were made in Moscow and Yaroslavl. Terracotta (from Italian terra – earth, cotta – burnt) is an unglazed plain ceramic with a characteristic colored porous shard.
Brick appeared more than 5,000 years ago and was first used as a structural material in Ancient Egypt and Babylonia. And now, during the period of rapid development of the construction industry, clay brick has not lost its importance. The ubiquity of the raw material - clay, ease of manufacture and long service life allow us to consider it one of the main local building materials.
Classification of ceramic building materials and products. Properties, application
Ceramic building materials and products according to their purpose in the decoration of buildings and individual elements are divided into:
façade products – facing bricks, various types of tiles;
products for interior decoration - glazed and unglazed tiles, shaped products, carpet and mosaic ceramics;
floor tiles;
products made of earthenware and porcelain for decorative purposes.
Finishing ceramics (tiles for walls and floors, ceramic carpet mosaics, architectural details, terracotta, majolica) have valuable universal consumer properties:
water resistance
resistance to aggressive influences;
high environmental friendliness;
simplicity of manufacturing techniques;
variety of raw materials;
strength;
durability;
hygiene;
decorativeness.
Ceramic products have different properties, which are determined by the composition of the raw materials, methods of processing, and firing conditions.
Application - in all elements of buildings and structures, in prefabricated ceramic housing construction, in the construction of ceramic wall products, for the production of facade ceramics, porous aggregates for concrete, sanitary ceramics, floor tiles, ceramic sewer pipes, etc.
Thus, ceramic materials meet modern trends in construction technology and are competitive with other building materials for the same purpose. The material from which ceramic products are composed is called a ceramic shard in ceramic technology.
Depending on the porosity of the structure ceramic building products are divided into two groups:
porous(water absorption by weight of 5 or more than 5% - ceramic bricks and stones, roofing tiles, facing tiles and ceramic pipes);
dense(water absorption by weight - less than 5% - floor tiles and road bricks);
Sanitary ceramics can be porous (faience) and dense (sanitary porcelain).
Raw materials for the production of ceramic materials and products. Classification, technological properties
Clay – raw material for the production of ceramic materials
The quality of raw materials is determined by the mineralogical composition, physical properties, depending on the deposit and the conditions of occurrence. The main raw materials for the production of ceramic products are clay And kaolins; quartz and slag sands, fireclay, burnable additives of organic origin (sawdust, coal chips, etc.) are used as auxiliary raw materials to improve technological properties.
Clay is one of the most common types of sedimentary rocks of polymineral composition. Oxygen, silicon and aluminum by their total mass make up about 90% of the composition of the earth's crust, therefore the vast majority of minerals are aluminosilicates, silicates and quartz, the basis of naturally occurring ceramic raw minerals. The sizes of clay particles range practically from colloidal dispersion to 5 microns. The main mineral of kaolin clays is the mineral kaolinite.
Clays are earthy sedimentary rocks consisting of clay minerals with significant impurities: kaolinite, halloysite, montmorillite, beidellite, quartz particles, feldspars, hydromica, iron oxide hydrates, aluminum, magnesium carbonates, calcium, etc.
The plasticity of clay raw materials, determined by the plasticity number (by rolling out a clay rope with a diameter of 3 mm), depends on the content of clay minerals and the moisture content of the mass. Depending on the content of clay minerals clays are divided into:
fatty (more than 60%);
ordinary (30... 60%);
heavy loams (20... 30%);
medium and light loams (less than 20%).
According to plasticity Clay materials are divided according to their plasticity number into:
highly plastic (less than 25);
medium plastic (15... 25);
moderately plastic (7... 15);
low plasticity (3... 7).
Water adsorbed by the surface of clay particles during the preparation of the raw material mixture plays the role of a hydrodynamic lubricant, which largely ensures its plastic characteristics. At the same time, the removal of water both from the clay particles themselves and from their surface during the drying and firing process causes the phenomenon of air and fire shrinkage.
Shrinkage deformations cause internal stresses in the product, which ultimately affects their quality indicators.
To reduce shrinkage during drying and firing, as well as to prevent the formation of cracks, artificial or natural clays are introduced into plastic clays. leaning supplements. These include dehydrated clay, fireclay, boiler slag, ash, quartz sand, etc.
The introduction of flux into the raw material mixture ensures a lower sintering temperature. Floods include feldspars, pegmatite, dolomite, talc, magnesite, barium and strontium carbonates, nepheline syenites (for earthenware masses). An artificial ceramic material, molded from clay raw materials, is obtained as a result of complex physical, chemical and physicochemical changes that occur during firing, i.e. when exposed to high temperatures.
Kaolins– these are pure clays, consisting mainly of the clay mineral kaolinite (Al 2 O 3 · 2SiO 2 · 2H 2 O). Kaolins are fire-resistant, low-plasticity, and white in color. They are used for the production of porcelain, earthenware and thin facing products, since after firing they produce a white shard.
Regular clays differ from kaolins in a wide variety of mineralogical, chemical and granulometric composition. Changes in the chemical composition significantly affect the properties of clays. With an increase in A1 2 O 3, the plasticity of clays and fire resistance increases, and with an increase in the SiO 2 content, the plasticity of clays decreases, porosity increases, and the strength of fired products decreases. The presence of iron oxides reduces the fire resistance of clay, and the presence of alkalis impairs the moldability of products.
In the manufacture of ceramic materials main technological properties of clays are:
plastic;
air and fire shrinkage;
fire resistance
ceramic shard color
sinterability.
Clay plasticity is the ability of clay dough to take a given shape under the influence of external forces and retain it after the action of these forces ceases. By degree of plasticity clays are divided into:
highly plastic, or “fat”,
medium ductility
low-plasticity, or “skinny”.
Fatty clays They mold well, but when dry, they crack and shrink significantly. Skinny clays do not mold well. To increase the plasticity of clays, the operation of aging them in a wet state in air, freezing, and rotting in dark basements is used, which loosens the material and increases its dispersion. Plasticity can also be increased by adding highly plastic clays. The most common way to increase ductility is their mechanical processing. To reduce the plasticity of clays, additives of various non-plastic materials (lean additives) are introduced.
Shrinkage– reduction in the linear dimensions and volume of raw clay during drying (air shrinkage) and firing (fire shrinkage). Shrinkage is expressed as a percentage of the original size of the product.
Air shrinkage occurs when water evaporates from the raw material during its drying in air and amounts to 2...10%.
Fire shrinkage It turns out due to the fact that during the firing process, the low-melting components of the clay melt and the clay particles at the points of their contact come closer together. Fire shrinkage is 2...8%.
Complete shrinkage is defined as the arithmetic sum of the values of air and fire shrinkage. The value of total shrinkage ranges from 4...18%. Complete shrinkage is taken into account when molding products.
Fire resistance– the ability of clay to withstand high temperatures without deformation. According to the melting temperature, clays are divided into:
low-melting (with a melting point below 1350°C),
refractory (with a melting point of 1350...1580°C)
fireproof (over 1580°C).
Refractory clays are used for the production of refractory products, as well as porcelain and earthenware. Refractory clays are used in the production of floor tiles and sewer pipes. Low-melting clays are used for the production of ceramic bricks, hollow stones, and tiles.
The color of the shard after firing depends on the composition and amount of impurities in the clay. Kaolins produce white shards. The color of fired clay is influenced by the content of iron oxides, which give the color from light yellow to dark red and brown. Titanium oxides cause a bluish coloration of the shard. Using mineral dyes, you can produce ceramic products of various colors and shades.
The sinterability of clay is its ability to compact during firing and form a stone-like material. When sintering, strength increases and water absorption of products decreases.
Production of ceramic building materials and products. General technological processes
The performance characteristics of ceramic products are largely determined by both the composition of raw materials and the technological methods of their manufacture. In the production of a wide range of modern building ceramics, related technological processes are used, which allow us to briefly summarize the basics of the production of ceramic materials.
The following general technological processes can be distinguished:
1. clay mining;
2. preparation of raw materials;
3. molding of the product (raw material);
These five production stages are common to all types of ceramic products. For certain types of products, various molding methods can be used (plastic and semi-dry molded bricks), different drying methods (air or in drying chambers), as well as additional production processes - coating products with glaze or engobe.
Clay extraction: The extraction of raw materials is preceded by geological exploration, determination of the chemical and mineral composition, physical properties of the raw materials, useful thickness of the deposit, its homogeneity and nature of occurrence, scope of work, etc. Clay usually occurs at shallow depths. Raw materials are mined in open-pit mines using single-bucket, multi-bucket or rotary excavators. Factories for the production of ceramic products are usually built near clay deposits, i.e. The quarry is an integral part of the plant. We strive to extract clay in the warm season, creating a supply of material in the warehouse for work in winter. Clay is transported from the quarry to the factories by rail transport in tipping cars, belt conveyors and dump trucks.
Preparation of raw materials. Clay extracted from a quarry and delivered to a factory is unsuitable for molding products, and it is necessary to destroy the natural structure of the clay, clean it of harmful impurities, crush large fractions, mix with additives, and moisten it to form a moldable mass. In covered warehouses or open areas, clay materials are aged for up to two years. During this time, organic residues decompose and under the influence of atmospheric factors (moistening and drying, freezing and thawing) and pre-treatment (loosening, stone removal, etc.) it is possible to achieve comparative homogeneity of the mass, both in granulometric and mineral composition. Further preparation of the mass is carried out depending on the type of products and the intended technology for their production.
At this stage, with the help of stone releasing machines, rollers, mills of various types, additive and water dispensers, clay mixers or dispersants, it is possible to obtain a mass suitable for molding products. The molding mass is prepared using plastic, semi-dry or wet methods, depending on the properties of the raw materials and the requirements for the quality of the resulting product.
Product molding– one of the important operations in the manufacture of ceramic products. Manufacturing methods are determined by the molding properties of the raw material mixture and, above all, plasticity, which largely depends on the amount of water in the molding mixture. Depending on the moisture content of the molding mass, the methods are divided into dry, semi-dry, plastic and casting (slip).
In the dry method, the press powder has a humidity of 2...6%, at which mechanical or hydraulic presses are used, developing pressure over 40 MPa. This method is used to produce dense ceramic products: floor tiles, some types of bricks, earthenware and porcelain products.
The semi-dry method involves the use of working mixtures with a humidity of 8... 12%. Therefore, bricks, shaped products, and tiles are made using this method.
The most economical and widespread is the method of plastic molding at a mass humidity of 18... 24%. The main mechanism used in this case is a belt press. The press auger with variable blade pitch grinds the mass, simultaneously compacting it to the outlet. Vacuuming at the last stage of pressing allows you to further compact the mass. The outlet of the press - the mouthpiece - ensures the production of a continuous clay bar of the required geometric dimensions. The shape of the mouthpiece and its dimensions determine the type of products produced: bricks, stones, tiles, tiles, pipes, shaped products. Void formers installed in front of the mouthpiece make it possible to mold perforated products, with slotted voids, etc.
The casting method produces ceramic products of complex geometric shapes: sanitary products (sinks, toilets, urinals, etc.), some decorative products, tiles for interior decoration. The components of the working mixture are thoroughly stirred, dosed, and mixed with water. The humidity of the mass in this case is from 40 to 60%. The homogeneous mass thus prepared is poured into plaster molds. The developed microporous structure of gypsum stone causes the removal of part of the water in the wall layers. As a result, depending on time, the required thickness of the compacted layer is achieved. Excess mixture is then removed. After drying, the individual elements are mounted.
Drying and firing of products. Depending on the manufacturing method, the moisture content of raw mixtures varies within very wide limits from 2 to 60%. Removal of water from molded products is accompanied by shrinkage deformations and, accordingly, the occurrence of internal stresses. The latter, under harsh drying conditions, can cause curvature and cracks, which reduce the quality of products. Drying of products is carried out to a residual moisture content of 4... 6% in tunnel or chamber dryers. Coolant temperature 120...150°C.
Firing ceramic products is one of the most critical technological stages, which largely determines the properties of the resulting materials.
In the production of building ceramics, continuous tunnel kilns are mainly used; dried products on firing trolleys, moving through the tunnels, are gradually heated to the sintering temperature in the fuel combustion zone, and then slowly cooled by a counter flow of air.
At a temperature of about 100...120 °C, physically bound free water is removed. At temperatures of 450...600 °C, clay substances irreversibly lose their plastic properties. A further increase in temperature leads to the destruction of the crystal lattice of aluminosilicates and their decomposition into individual oxides: when the temperature rises to 1000 ° C, the compound sillimanite is formed, at a temperature of 1200-1300 ° C - a new mineral mullite. These minerals provide high strength and resistance of ceramic shards to various environmental factors.
After firing, the resulting products are cooled slowly, since sudden cooling may cause cracks to form. Before shipment to the consumer, ceramic products are sorted in order to check quality indicators for their compliance with the requirements of state standards.
Ceramic materials are obtained from clay masses by molding and subsequent firing. In this case, an intermediate technological operation often takes place - drying of freshly molded products, called “raw”.
Based on the nature of the structure of the shard, ceramic materials are distinguished between porous (unsintered) and dense (sintered). Porous ones absorb more than 5% of water (by weight), on average their water absorption is 8...20% by weight. Brick, blocks, stones, tiles, drainage pipes, etc. have a porous structure; dense - floor tiles, sewer pipes, sanitary products.
Based on their intended purpose, ceramic materials and products are divided into the following types: wall - ordinary brick, hollow and porous bricks and stones, large blocks and panels made of brick and stones; For floors - hollow stones, beams and panels made of hollow stones; For external cladding - ceramic facing bricks and stones, carpet ceramics, ceramic facade tiles; For internal lining Andbuilding equipment - slabs and tiles for walls and floors, sanitary products; roofing -tiles; pipes - drainage and sewerage.
Raw materials
The raw materials for the manufacture of ceramic materials are various clay rocks. To improve the technological properties of clays, as well as to give products certain and higher physical and mechanical properties, quartz sand, fireclay (crushed refractory or refractory clay fired at a temperature of 1000...14000°C), slag, sawdust, coal dust are added to the clays. .
Clay materials were formed by the weathering of igneous feldspathic rocks. The process of rock weathering consists of mechanical destruction and chemical decomposition. Mechanical failure occurs as a result of exposure to variable temperature and water. Chemical decomposition occurs, for example, when feldspar is exposed to water and carbon dioxide, resulting in the formation of the mineral kaolinite.
Clay is the name given to earthy mineral masses or clastic rocks that are capable of forming a plastic dough with water, which, when dried, retains its given shape, and after firing, acquires the hardness of stone. The purest clays consist predominantly of kaolinite and are called kaolins. The composition of clays includes various oxides (AI2O3, SiO 2, Fe 2 O3, CaO, Na 2 O, MgO and K2O), free and chemically bound water and organic impurities.
Impurities have a great influence on the properties of clay. Thus, with an increased content of SiO 2 not associated with Al 2 Oz, the binding capacity of clays in clay minerals decreases, the porosity of fired products increases and their strength decreases. Iron compounds, being strong fluxes, reduce the fire resistance of clay. Calcium carbonate reduces refractoriness and sintering interval, increases firing shrinkage and porosity, which reduces strength and frost resistance. Oxides Na2O and K2O lower the sintering temperature of clay.
Clays are characterized by plasticity, cohesiveness and binding ability, and attitude to drying And to high temperatures.
The plasticity of clay is its property of forming a dough when mixed with water, which, under the influence of external forces, is able to take a given shape without the formation of tears and cracks and retain this shape during subsequent drying and firing.
The plasticity of clay is characterized by the plasticity number
P =W T - W r ,
Where W t and W p - moisture values corresponding to the yield strength and rolling limit of the clay rope, %.
According to plasticity, clays are divided into highly plastic (P>25), medium plastic (P = 15...25), moderately plastic (P = 7... 15), low-plasticity (P <7) and non-plastic. For the production of ceramic products, moderately plastic clays with a plasticity number P = 7... 15 are usually used. Low plasticity clays are difficult to mold, while highly plastic clays crack during drying and require thinning.
In the production of firing materials, along with With The clays used are diatomites, tripoli, shale, etc. Thus, in the production of light bricks and products, diatomite and tripoli are used, and intumescent clays, perlite, and vermiculite are used to produce porous aggregates.
Many ceramic factories do not have raw materials suitable in their natural form for the manufacture of corresponding products. Such raw materials require the introduction of additives. Thus, by adding thinning additives up to 6...10% (sand, slag, chamotte, etc.) to plastic clays, it is possible to reduce the shrinkage of clay during drying and firing. Fractions smaller than 0.001 mm have a great influence on the binding ability of clays and their shrinkage.
The higher the content of clay particles, the higher the plasticity. Plasticity can be increased by adding highly plastic clays, as well as by introducing surfactants - sulfite-yeast mash (SYB), etc. Plasticity can be reduced by adding non-plastic materials called slag agents - quartz sand, fireclay, slag, sawdust, coal chips.
Clays containing an increased amount of clay fractions have higher cohesion, and, conversely, clays with a low content of clay particles have low cohesion. With an increase in the content of sand and dust fractions, the binding capacity of clay decreases. This property of clay is of great importance when molding products. The binding ability of clay is characterized by the ability to bind particles of non-plastic materials (sand, fireclay, etc.) and form a sufficiently strong product of a given shape upon drying.
Shrinkage is the reduction in linear dimensions and volume during drying of a sample (air shrinkage) and firing (fire shrinkage). Air shrinkage occurs when water evaporates from the raw material during its drying process. For various clays, linear air shrinkage ranges from 2...3 to 10...12%, depending on the content of fine fractions. Fire shrinkage occurs due to the fact that during the firing process, the low-melting components of the clay melt and the clay particles at the points of their contact come closer together. Fire shrinkage, depending on the composition of the clays, can be 2...8%. Complete shrinkage equal to the algebraic sum of air and fire shrinkage, it ranges from 5...18%. This property of clays is taken into account when manufacturing products of the required sizes.
A characteristic property of clays is their ability to turn into a stone-like mass when fired. In the initial period of temperature increase, mechanically mixed water begins to evaporate, then organic impurities burn out, and when heated to 550...800 ° C, dehydration of clay minerals occurs and the clay loses its plasticity.
With a further increase in temperature, firing occurs - some low-melting component of the clay begins to melt, which, spreading, envelops the unfused clay particles, and upon cooling hardens and cements them. This is how the process of turning clay into a stone-like state occurs. Partial melting of the clay and the action of surface tension forces of the molten mass cause its particles to approach each other, and a reduction in volume occurs - fire shrinkage.
The combination of processes of shrinkage, compaction and hardening of clay during firing is called clay sintering. With a further increase in temperature, the mass softens - melting of the clay occurs.
The color of fired clay is influenced mainly by the content of iron oxides, which color ceramic products red when there is an excess of oxygen in the kiln, or dark brown and even black when there is a lack of oxygen. Titanium oxides cause a bluish coloration of the shard. To obtain white brick, firing is carried out in a reducing environment (in the presence of free CO and III in gases) and at certain temperatures in order to convert iron oxide V nitrous.
Processes that occur during firing and drying of clays
ceramic products production diagram
Despite the extensive range of ceramic products, the variety of their shapes, physical and mechanical properties and types of raw materials, the main stages of the production of ceramic products are general and consist of the following operations: extraction of raw materials, preparation of the raw material, molding of products (raw materials), drying of raw materials, firing of products, processing of products (trimming, glazing, etc.) and packaging.
Raw materials are extracted in open-pit mines using excavators. Transportation of raw materials from the quarry to the plant is carried out by dump trucks, trolleys or conveyors at a short distance from the quarry to the molding shop. Plants for the production of ceramic materials are usually built near a clay deposit, and the quarry is an integral part of the plant.
The preparation of raw materials consists of destroying the natural structure of the clay, removing or grinding large inclusions, mixing the clay with additives and moistening until a moldable clay mass is obtained.
The molding of the ceramic mass, depending on the properties of the initial raw material and the type of product being manufactured, is carried out using semi-dry, plastic and slip (wet) methods. At semi-dry method In production, clay is first crushed and dried, then crushed and with a moisture content of 8...12%, it is fed for molding. At plastically During molding, the clay is crushed, then sent to a clay mixer (Fig. 3.2), where it is mixed with lean additives until a homogeneous plastic mass with a moisture content of 20...25% is obtained. The molding of ceramic products using the plastic method is carried out mainly on belt presses. In the semi-dry method, the clay mass is molded on hydraulic or mechanical presses under pressure of up to 15 MPa or more. By slip method the starting materials are crushed and mixed with a large amount of water (up to 60%) until a homogeneous mass is obtained - slip. Depending on the molding method, the slip is used both directly for products obtained by casting, and after drying it in spray dryers.
A mandatory intermediate operation in the technological process of producing ceramic products using the plastic method is drying. If the raw material, which has high humidity, is fired immediately after molding, it will crack. When drying raw materials artificially, flue gases from kilns and special furnaces are used as a coolant. In the manufacture of fine ceramic products, hot air generated in heaters is used. Artificial drying is carried out in batch chamber dryers or continuous tunnel dryers (Fig. 3.4).
The drying process is a complex of phenomena associated with heat and mass transfer between the material and the environment. As a result, moisture moves from the inside of the product to the surface and evaporates. Simultaneously with the removal of moisture, the particles of the material come closer together and shrinkage occurs. The reduction in the volume of clay products during drying occurs up to a certain limit, despite the fact that the water has not yet completely evaporated at this point. To obtain high-quality ceramic products, the drying and firing processes must be carried out under strict conditions. When the product is heated in the temperature range O...15O°C, hygroscopic moisture is removed from it. At a temperature of 70°C, the pressure of water vapor inside the product can reach a significant value, therefore, to prevent cracks, the temperature should be raised slowly (5O...8O°C/h) so that the rate of pore formation inside the material does not outstrip the filtration of vapors through its thickness.
Firing is the final stage of the technological process. The raw material enters the oven with a humidity of 8...12%, and in the initial period it is completely dried. In the temperature range of 550...800°C, dehydration of clay minerals and removal of chemically bound constitutional water occurs. In this case, the crystal lattice of the mineral is destroyed and the clay loses its plasticity, at which time shrinkage of the products occurs.
At a temperature of 200...800°C, the volatile part of the organic impurities of clay and burnable additives introduced into the mixture during the molding of products is released, and, in addition, the organic impurities are oxidized within the limits of their ignition temperature. This period is characterized by a very high rate of temperature rise - 300...350°C/h, and for efficient products - 400...450°C/h, which contributes to the rapid burnout of fuel pressed into the raw material. Then the products are kept at this temperature in an oxidizing atmosphere until the carbon residues are completely burned out.
A further rise in temperature from 800°C to the maximum is associated with the destruction of the crystal lattice of clay minerals and a significant structural change in the shard, therefore the rate of temperature rise is slowed down to 1OO...15O°C/h, and for hollow products - to 200...220° S/h. Upon reaching the maximum firing temperature, the product is held to equalize the temperature throughout its entire thickness, after which the temperature is reduced by 1OO...15O°C, as a result the product undergoes shrinkage and plastic deformation.
Then the cooling intensity at temperatures below 800°C increases to 250...300°C/h or more. The temperature decline can only be limited by the conditions of external heat exchange. Under such conditions, brick firing can be carried out in 6...8 hours. However, in conventional tunnel kilns, high-speed firing modes cannot be implemented due to the large unevenness of the temperature field across the cross section of the firing channel. Products made from low-melting clays are fired at a temperature of 900...1100°C. As a result of firing, the product acquires a stone-like state, high water resistance, strength, frost resistance and other valuable construction qualities.
) and their mixtures with mineral additives, manufactured under high temperature followed by cooling.
In the narrow sense, the word ceramics means clay that has been fired. However, modern use of the term expands its meaning to include all inorganic non-metallic materials. Ceramic materials can have a transparent or partially transparent structure and can be made from glass (see glass ceramics). The earliest ceramics were used as dishes made from clay or mixtures of it with other materials. Currently, ceramics is used as an industrial material (mechanical engineering, instrument making, aviation industry, etc.), as a building material, artistic material, and as a material widely used in medicine and science. In the 20th century, new ceramic materials were created for use in the semiconductor industry and other fields.
The word "ceramic" also comes from the Indo-European Kerry, meaning heat. Where "Ceramic" can be used as an adjective to describe a material, product or process; or as soon as the plural noun "ceramics".
Story
Historically, ceramics were hard, porous, and brittle. The study of ceramics is leading to the development of more and more new methods to solve these problems, paying particular attention to the strengths of the materials, as well as their unusual uses.
Ceramics have been known since ancient times and are perhaps the first material created by man. The appearance of ceramics dates back to the Mesolithic and Neolithic eras. Different types of ceramics are terracotta, majolica, earthenware, stoneware, porcelain, glass ceramics.
Based on the origin of the word ceramics, we understand those products for which clay (in case of kaolin), mixed with feldspar, quartz or lime, serves as the main raw material. These raw materials are mixed and processed into a mass, which is either shaped by hand or on a turntable and then fired.
Certain types of ceramics were formed gradually as production processes improved, differing depending on the educational properties of the shard and the incandescent heat. Most of them are still held today. The oldest type is a common pottery product with an earthy, colored and porous shard. These are typical household ceramics or products that were ennobled in various ways - by stamping and engraving (for example, Bucchero nero), a thin facing layer (Greek ceramics and Roman Terra - sigillata), colored glaze (Haffnerceramics of the Renaissance). Initially, ceramics were shaped by hand. The invention of the potter's wheel in the third millennium BC was a great advance, which made it possible to produce dishes with thinner walls.
By the end of the 16th century, ceramics transitioned to majolica in Europe. Possessing a porous shard of iron- and lime-containing but white earthenware or tile clay, it is covered with two glazes: an opaque tin-containing glaze and a transparent, shiny lead glaze. Majolica, originally from the Trans-Alpine countries, is called faience. The decor was painted on majolica using wet glaze before firing the product at a temperature of about 1000 °C. Paints for painting were taken of the same chemical composition as the glaze, but a significant part of them were metal oxides that could withstand high temperatures (the so-called fire-resistant paints - blue, green, yellow and violet). Starting from the 18th century, they began to use so-called muffle paints, which were applied to already fired glaze. With their help, especially on porcelain, good results are achieved.
In the 16th century, the production of stoneware spread in Germany. White (for example, in Siegburg) or colored (for example, in Rehren), a very dense shard consists of clay mixed with feldspar and other substances. When fired at a temperature of 1200-1280 °C, stoneware is very hard and practically non-porous. In Holland, following the example of Chinese ceramics, they began to produce it red, and the same feature is revealed by Böttger's stoneware.
Stoneware was also made by Wedgwood in England. Fine faience as a special type of ceramics was born in England in the first half of the 18th century with a white porous shard covered with a white glaze. Depending on the strength of the shard, it is divided into soft, thin faience with a high lime content, medium - with a lower content, and hard - without lime at all. This latter shard often resembles stoneware or porcelain in composition and strength.
Cement is widely used in construction - one of the types of ceramics, the raw materials for which are clay and limestone mixed with water.
The history of the appearance of ceramics in Rus'
Ceramics in Russia
Ceramics have been known since ancient times and are perhaps the first material created by man. Russia worthily occupies a leading place in the world in the field of ceramics, despite the fact that in international literature the issue of the emergence of porcelain and ceramic production is often belittled. Using the example of the appearance of black ceramics, it has been archaeologically proven that already in the 3rd millennium BC. e. black polished ceramics were used for ritual and ceremonial purposes. Significant damage to the development of ceramics in Russia was caused only by one Mongol-Tatar invasion, which destroyed much of the achievements of Russian potters of the 9th-12th centuries. For example, two-handled amphora pots and vertical lamps disappeared, the ornament, the art of cloisonné enamel, and glaze became simpler (the simplest one was yellow, and survived only in Novgorod).
Only in the 15th century did the development of ceramics in Rus' continue. In Russia and nowadays, especially in rural areas, every ceramic vessel is irreplaceable. Food in ceramic pots is the most flavorful and has a long shelf life.
The production of ceramic dishes on a potter's wheel was and is of particular interest. The so-called kvass (vessels for sour cabbage soup, mash, beer, yeast or fruit kvass) appeared in Moscow in the 19th century.
Transparent ceramics
Historically, ceramic materials are opaque due to the nature of their structure. However, sintering nanometer-sized particles has made it possible to create transparent ceramic materials with properties (range of operating wavelengths, dispersion, refractive index) that lie outside the standard range of values for optical glasses.
See also
- Welded ceramics
Links
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