6. ARCHITECTURAL AND BUILDING CERAMICS

6.1. Composition, structure and properties of building ceramics

In a general sense, it is accepted to call ceramics refractory inorganic nonmetallic materials possessing a complex of specified properties and obtained by sintering a pre-selected mixture or by other means. In the construction of ceramic called artificial stone materials and products obtained in the process of high-temperature processing (firing) of clay raw materials with additives. There are fine ceramics (fine-grained - faience and porcelain) and coarse, or coarse-grained.

The chemical composition of ceramics: SiO2 - 40 ... 80%; Al2O3 - 8 ... 50%; Fe2O3 - 0 ... 15%; CaO - 0.5 ... 25%; MgO - 0 ... 4%; R2O - 0.3 ... 5%. Depending on the composition, the color of ceramics varies from pale beige to dark brown.

Phase composition: ceramics belongs to crystalline materials, however, it also contains a small (up to 5%) amount of amorphous (vitreous) phase. According to the content of the gas phase (pores) are divided into porous with water absorption by weight more than 5% (ceramic wall bricks and stone, products for roofing and floors, drainage pipes) and dense. Mineralogical composition is polymineral: mainly mullite and quartz, sillimanite and other substances that crystallize during cooling (mainly aluminosilicates).

Unlike fused materials (glass and metals), in its structure, ceramics belongs to the class of composite materials , that is, it is a heterogeneous poly-scale system. At a macroscopic level, ceramics consist of a solid-phase skeleton, which is called a ceramic skull . In porous products, macro- and micropores filled with air and / or moisture are placed in a solid-phase skeleton. At the same time porosity can reach 90% (foam ceramics). A sintered (dense) skull has only micropores in a small amount (therefore, its water absorption by weight is less than 5% - this is a clinker brick, facing slabs). In turn, the ceramic skull of coarse ceramics is a composite in which the pulverized and sandy particles are monolithic by a cooled melt. The cooled melt, in turn, is a microcomposite material consisting of crystal grains of minerals that crystallize during cooling (mainly aluminosilicates), combined into a single whole by a layer of continuous vitreous phase. The vitreous, amorphous phase (supercooled liquid) is represented in the microstructure by low-melting components that did not have time to crystallize at a given rate of cooling of the melt. That is, in contrast to glass, in the production of ceramics, not the whole mass passes into the melt, but only a small part of it from clay particles. Actually glass in the composition of ceramics is not more than 5%, but basically it gives connectivity to this material.

At the atomic-molecular level, the structure is a combination of silicon-oxygen and aluminum-oxygen tetrahedra (anions) combined with metal cations of calcium, magnesium, iron, etc.

The true density of ceramic materials is 2.5 ... 2.7 g / cm3; the average density of the dense skull is 2000 ... 2300 kg / m3; thermal conductivity of an absolutely dense shard 1.16 W / (m · ° C). Water absorption of ceramic materials, depending on the porosity varies from 0 to 70%. The heat capacity of ceramic materials is 0.75 ... 0.92 kJ / (kg · ° C). Compared to metals, ceramics has an increased refractoriness up to the highest refractoriness.

The compressive strength of the most common representative of building ceramics is from 7.5 to 70 MPa. And the strength of technical ceramics can reach 1000 MPa. Flexural strength of 0.7 ... 5 MPa - an order of magnitude less than compressive strength due to the fact that ceramics has a low crack resistance. This is its main drawback compared with metals.

6.2. The basics of ceramic technology

The main stages of production: extraction of raw materials (clay raw materials consisting of clay, dust and sand particles), preparation of the mass, molding products (raw), their drying at 60 ... 70 ° C and roasting at 900 ... 1100 С. In the firing process, sintering of the skull occurs - low-melting clay particles melt, the melt formed flows around unmelted particles and solidifies as it cools, combining all the particles into a solid, durable, water-resistant stone.

The raw material for the production of building ceramics is clay raw materials with or without additives. Clay raw materials (clays and kaolins) are a product of weathering of igneous feldspathic rocks, containing impurities of other rocks. The granulometric composition includes clay particles with a size of up to 0.005 mm, dust particles with grain sizes of 0.005 ... 0.05 mm and sand particles with grain sizes of 0.05 ... 2 mm. The mineralogical composition of clays: clay particles consist of secondary minerals (kaolinite Al2O3 · 2SiO2 · 2H2O, montmorillonite Al2O3 · 4SiO2 · 4H2O, hydromica); Dusty and sandy include quartz, feldspar, mica, etc.

The most valuable component of clay raw materials are clay particles, which are the smallest scales of minerals. It is clay particles that, when mixed with water, form a plastic dough that retains the given shape during drying (the particles are in the form of plates, between which thin layers of water form when wetted, causing the particles to swell and slip against each other without loss of cohesion). When fired, melt and form a solid, waterproof, stone when cooled. Technological properties of clay raw materials: plasticity, air and fire shrinkage, density, porosity, strength, fracture and color of the ceramic shard, which depend on the content of clay particles.

Clay additives

To reduce high shrinkage during drying and roasting of "fat" clays and prevent deformations and cracks in the products, emaciated additives are introduced into the raw materials. These include: dehydrated clay, fireclay, slag, ash, quartz sand, skull (marriage of baked products).

In order to increase the plasticity of "lean" clay with less water consumption, plasticizing agents are introduced. These include highly plastic clays, bentonites, surfactants.

To increase the porosity of the crock and improve the insulating properties of ceramic products, pore-forming additives are introduced. These include: burnable additives (sawdust, coal powder, peat dust, these additives are at the same time emaciated), gas formers and blowing agents.

In order to reduce the firing temperature of ceramics, fluxes are introduced. These include: field wheat, iron ore, dolomite, magnesite, talc, sandstone, pegmatite, cullet, perlite.

Special supplements. To increase the acid resistance of ceramics, sand mixtures that have been closed with liquid glass are added to the raw material mixtures. To obtain some types of colored ceramics, metal oxides (iron, cobalt, chromium, titanium, etc.) are added to the raw material mixture.

Some types of ceramic products to improve sanitary and hygienic properties, water resistance, improve the appearance of covered with a decorative layer - glaze or engobe. Glaze is a glassy coating with a thickness of 0.1 ... 0.2 mm applied on the product and fixed by firing. Glazes can be transparent and deaf (opaque) of different colors. For the manufacture of glaze used: quartz sand, kaolin, feldspar, salts of alkali and alkaline earth metals. Raw mixes are ground to powder and applied to the surface of products in the form of a powder or suspension before firing. Angobe is a thin layer of a white-burning or colored clay deposited on a product that forms a colored coating with a matte surface. The properties of the engobe should be close to the main shard.

Clay preparation and molding

Clay extraction is carried out in quarries, usually by an open-cut method, by excavators and transported to the ceramic enterprise by rail, road or other means of transport. The development of the quarry is preceded by preparatory work: geological exploration with the establishment of the nature of the occurrence, useful stratum and reserves of clay; clearing the surface of plants for a year or two before the start of development, removal of rocks that are unsuitable for production

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The quarry clay in its natural state is usually unsuitable for the production of ceramic products, so the clay is prepared by natural and / or mechanical processing. Natural treatment is the drying out of clay for 1-2 years when moistened with atmospheric precipitation or artificial soaking and periodic freezing and thawing. When this clay is crushed, clay particles are separated from each other. Washed harmful impurities. The machining of clays is carried out with the aim of further destroying their natural structure, removing or grinding large inclusions, removing harmful impurities, crushing clays and additives, and mixing all the components until a homogeneous and easily moldable mass is obtained using: clay clappers; stone separating, perforated, disintegrating, coarse and fine grinding of rollers; runners, clay paste machines, basket disintegrators, rotary and ball mills, single and twin shaft mixers, propeller mixers, etc.).

Drying products

Before firing, the products should be dried to a moisture content of 5 ... 6% in order to avoid uneven shrinkage, distortion and cracking during firing. Previously, in the production of bricks, raw materials were dried mainly under natural conditions in drying sheds for 2 ... 3 weeks, depending on climatic conditions. At present, drying is carried out predominantly artificially in tunnels of continuous action or chamber periodic action of dryers for several to 72 hours, depending on the properties of the raw materials and the moisture content of the raw material. Drying is carried out at the initial temperature of the coolant — waste gases from kilns or heated air — 120–150 ° C. The temperature in the dryer is about 90 ° C.

Roasting products

Roasting is the most important and final process in the manufacture of ceramic products. The firing temperature range of most ceramic products ranges from 900 ° C to 1100 ° C. At this temperature, low-melting ceramic compounds and minerals melts into a melt, which envelops the non-melted particles, tightens and after cooling, gives the product a stone-like condition, water resistance and strength.

This process can be divided into three periods: heating the raw material, roasting itself, and controlled cooling.

After cooling, the product acquires stone-like state, water resistance and durability.

ness This phenomenon is called sintering, and the property of clays to thicken during firing and form a stone-like shard is called the sintering of clays.

Firing is carried out, as a rule, in furnaces of continuous action (tunnel, earlier - ring) and rarely - of periodic action. Ring furnaces are not being built now due to the high labor intensity of loading and unloading operations, but the existing ones are still being exploited.

6.3. Nomenclature and use of products of building ceramics. For the intended purpose, ceramic products are divided into the following types:

1. Wall for masonry: brick and ceramic stones ; large-size blocks and panels of brick and ceramic stones .

2. Finishing (facing) for external and internal facing of buildings and structures with the purpose of decorative and artistic decoration and increasing their durability: - brick and stones facing

- large-size cladding ceramic plates

- ceramic tiles front and carpets of them

- tiles for interior wall cladding

3. For floors : floor tiles; porcelain stoneware.

4. Roofing (shingles).

6. Fillers for concrete (expanded clay, agloporite).

7. Thermal insulation (cellular ceramics, foam ceramics).

8. Road (brick bricks, road tiles).

9. Sanitary-technical (sinks, toilets, flush tanks, bidets, urinals, sinks).

10. Ceramic pipes (sewer and drainage).

11. Special (brick for chimneys and acid resistant products)