8.1. General information
Such materials include mortars, concretes, asbestos-cement products, cement-bonded particle boards, etc. The most common building material on Earth is concrete, more precisely, concrete. Today, the annual production of concrete is more than 2 billion m3, which is far superior to the production of other types of industrial products and building materials. This is the most resource-intensive type of human activity in the world.
Concrete is an artificial stone-like composite material resulting from the hardening of a rationally selected and thoroughly mixed mixture of a binder, a solvent (usually water), in some cases fine and coarse aggregates, additives. Today, about 1600 types of concrete are used with the widest range of properties: density from 0.1 to 4.5 t / m3; compressive strength from 0.1 to 1000 MPa; application temperature from –150 to +1500 С. 1. Depending on the main purpose, concretes are divided into:
constructional;
special: heat resistant; chemically resistant; decorative; radiation protective; heat
insulating; straining and others
2. By the type of the knitting concrete can be on the basis of:
- cement binders;
- lime binders;
- slag binders; gypsum binders;
- special binders.
3. By type of aggregates, concrete can be on:
- dense aggregates; porous aggregates;
- special fillers.
4. The structure of the concrete can be:
- dense structure;
- large pore structure;
- porous structure;
- cellular structure;
- fine or coarse-grained structure.
5. According to the conditions of hardening concretes are divided into hardened:
- in natural conditions;
- in conditions of heat and humidity treatment at atmospheric pressure;
- in conditions of heat and humidity treatment at a pressure above atmospheric (autoclave hardening).
6. According to the average density it is possible to divide concretes into:
- especially heavy (over 2500 kg / m3);
- heavy (2200 - 2500 kg / m3);
- lightweight (1800 - 2200 kg / m3);
- lungs (500 - 1800 kg / m3); especially light (up to 500 kg / m3).
The most important characteristic of concrete is its class of strength for axial compression (determined by cubic or prism strength) or axial tension. Concrete class is the value of the unified strength of concrete at the project age in MPa, taken with a guaranteed security of 0.95.
The class of concrete is taken from the condition B ≤ Rg, where Rg is strength guaranteed with a certain probability. Guaranteed strength Rg is calculated by the formula:
R g R (1t),
where R is the average strength of concrete, MPa, which should be ensured in the production of structures; ν - coefficient of variation of concrete strength; t is the student coefficient characterizing the deviations of the sample from the total population. It is accepted according to the tables of reference books depending on the magnitude of significance ( = 1 - Р, where Р is the confidence level) and the number of degrees of freedom k (k = m - 1, where m is the number of tested samples).
Depending on the composition and structure, produce concrete classes of compressive strength from B0.35 to B80. Today, concrete with a strength up to 150 MPa is produced, the so-called High Performance Concrete, by which we mean high (55-80 MPa) and ultra-high (above 80 MPa) concrete, low permeability, increased corrosion resistance and durability, obtained from plastic mixtures.
As with all mineral building materials, brittleness is characteristic of mineral binding materials. Flexural strength of concrete is 5 ... 10 times lower than compressive strength.
An important mechanical property of concrete is creep - the phenomenon of increasing deformation of concrete over time under the action of a constant static load. Creep is realized as plastic deformation, but today it has been established that these deformations are caused by microcracking in the structure of concrete; therefore, they are called pseudoplastic. Creep and stress relaxation associated with it may play a negative role. For example, concrete creep leads to a loss of tension in prestressed concrete structures.
When hardening in air, concrete shrinks , i.e. the concrete shrinks and the linear dimensions of the concrete elements are reduced. Shrinkage is composed of moisture, carbonization and contraction components. Humidity shrinkage is caused by a change in the distribution, movement and evaporation of moisture in the resulting cement stone skeleton. This component plays a leading role in the overall shrinkage of concrete. Carbonization of calcium hydroxide in cement stone with its conversion to calcium carbonate also causes shrinkage, especially noticeable in cellular concrete. The contraction component of shrinkage, caused by a decrease in the absolute volume of the cement-water system, is small and amounts to only about 10% of the moisture shrinkage.
Due to the shrinkage of concrete in reinforced concrete and concrete structures, shrinkage stresses occur, therefore, large structures are cut with shrinkable seams to avoid the appearance of cracks. Uneven shrinkage causes tensile stresses in the outer layers of the structure and the appearance of internal cracks in contact with the aggregate and in the cement stone itself.
In addition to moisture deformations (shrinkage), concrete also undergoes thermal expansion-compression deformations, which are estimated by the value of the coefficient of linear or bulk thermal expansion . The linear coefficient of thermal expansion of concrete is about 10 ∙ 10-6 C, therefore, with a 50 ° C increase in temperature, the expansion reaches about 0.5 mm / m. In order to avoid cracking of structures of great length, they are cut by temperature-shrinkable seams. In addition, large fluctuations in temperature (more than 80 ° C) can cause internal cracking of concrete even in the presence of seams due to different thermal expansion of coarse aggregate and mortar. Internal cracking can be prevented by taking care of the selection of concrete components with similar coefficients of thermal expansion.
In addition, the cyclic temperature effects of heating and cooling will adversely affect the durability of concrete. As well as variable moisture deformations, they lead to concrete fatigue, the appearance of microdefects in it, which over time can develop into large cracks and destroy concrete.
The frost resistance of concrete depends on the quality of the materials used and the capillary porosity of the concrete. The volume of capillary pores has a decisive influence on the permeability and frost resistance of concrete. The frost resistance of concrete increases significantly when the capillary porosity is less than 7%.
The most important technical and economic characteristics of concrete is its homogeneity , which is understood as the spread of the values of its indicators. To assess the homogeneity of the concrete of this brand using the results of control tests of concrete samples. The strength of concrete specimens will fluctuate, deviating from the average value in the larger and smaller side. Such a spread should be minimal - this is the most important requirement. The closer the partial results of the test samples to the average value, the higher the uniformity of concrete. The fluctuations in strength are influenced by fluctuations in the quality of cement and aggregates, the accuracy of the dosing of components, the thorough preparation of the concrete mix, and other factors. The uniformity of strength is taken into account when determining the class of concrete according to the coefficient of variation. In domestic enterprises, the normative coefficient of variation was 13.5
%, and for massive hydraulic structures - 17.0%.
8.2. Raw materials for concrete and mortar
Astringent
Knitting is the main component of the concrete mix, the hardening of which provides the solid-phase state of the artificial stone - concrete. You can do without coarse aggregate, without small, without additives, and without a binder - not to do.
The choice of the type of binder for concrete is predetermined by the conditions of its subsequent operation: the type and amount of power load, temperature and humidity parameters, chemical composition of the operating environment, etc. Sometimes this choice is affected by requirements for the manufacture of building structures (for example, terms, plugging, pre-tensioning). ). The concrete grade of the chosen type of binder and its consumption is determined based on the design class of concrete (the strength of the binder should be at least 10 ... 40% higher than the desired strength of concrete), its grade for frost resistance, water resistance, taking into account the required workability of the concrete mix. Placeholders
Aggregates are granular (sometimes fibrous) materials that, in a rationally composed mixture with a binder and a sealer, form concrete and mortars. The aggregates occupy in concrete up to 80% of the volume and create a rigid skeleton, which increases the strength and modulus of elasticity of the concrete, reduces its creep and shrinkage (approximately 10 times compared with the shrinkage of cement stone). Finally, aggregates can dramatically reduce the consumption of binders - the most expensive and scarce component of concrete.
The composition of the aggregates are divided into organic and inorganic. Mineral aggregates have the greatest application in concretes on mineral binders. In special concretes, metal aggregates are sometimes used, for example, lead shot. The organic waste includes woodworking (sawdust, shavings, shredded (particles up to 40 mm long, 2 and 5 mm wide and thick), wood wool (wood fibers), etc., agricultural products processing waste (cane stalks, cotton, husk of seeds, hemp fiber and flax crops, etc.), waste and products of the industry of polymeric materials (plastics, polymeric fibers, crumb rubber, etc.). Used for the production of chipboard and fibreboard, arbolita , xylolite, fiberboard, cement-bonded chipboard, sawdust concrete, cement concrete and other similar concrete.
By origin, the placeholders are divided into three groups:
1) natural, including from incidentally produced rocks and enrichment wastes. They are used without changing their chemical composition and phase state (sand, gravel, crushed stone from natural stone);
2) artificial , obtained from natural raw materials or industrial waste by thermal or other processing with a change in the chemical composition and phase state (expanded clay, agloporitovy, shungizitovy gravel and sand, slag pumice stone, granulated foam glass);
3) from industrial wastes without changing their composition , which allows not only to expand the cheap raw material base, but also to solve the problem of waste disposal (crushed stone and sand from metallurgical and slag, ashes). According to the form :
1) granular , which are divided into rounded (rounded - gravel, natural sand) and angular forms (crushed stone, crushed sand). Angular aggregates provide a higher strength of concrete due to better adhesion to the cement stone, however, grains of lamellar (flap) shape can adversely affect the quality of concrete.
2) fibrous , as yet limited use of fillers. Such aggregates can significantly increase the tensile strength of a concrete and its crack resistance, that is, reduce the brittleness of the material.
By grain size, the aggregates are divided into:
1) Fine aggregates (sand) is an inorganic bulk material with a grain size from 0.14 to 5 mm.
2) Large aggregates are loose-grained material of rounded (gravel) or acute-angled (crushed stone) form with a size of 5 to 70 mm, and in massive structures up to 150 mm.
By density (in grain):
1) dense with an average density of more than 2 g / cm3, used for ordinary concrete;
2) porous (light) with an average density of less than 2 g / cm3 (usually 0.4 ... 1.6 g / cm3) used for lightweight concrete.
Quartz sand - inorganic natural granular, usually rounded, fine dense aggregate. Formed naturally as a result of weathering of rocks, the place of occurrence is river, mountain, ravine, marine, sand dunes. Accordingly, it is mined hydromechanically from the bottom of reservoirs or excavators in quarries. If necessary, the sand may be subjected to enrichment (removal of harmful impurities, adjustment of the grain composition). In addition to quartz, depending on rock-forming minerals, feldspar and carbonate sands are also used, but quartz sand is most valued because it has a high density (density in the grain is 2.65 g / cm3), strength (quartz strength reaches 2000 MPa), frost resistance, durability, and most importantly - widely distributed. The most important characteristics of the sands are: grain composition (modulus of grain size, sieving curve), voidness, content of harmful impurities, humidity.
As a fine aggregate, crushed sand and from screenings of crushing , made artificially by crushing of rocky rocks and gravel, are also used. There is also sand from the wastes of ore and non-metallic minerals and other industries.
Granite crushed stone is an inorganic natural, angular, coarse, dense aggregate obtained by mechanical crushing of granite rocks with subsequent sieving into fractions, as a rule, 5 ... 10, 10 ... 20, 20 ... 40, 40 ... 70, (70 ... 150) mm. This is the most common aggregate for the manufacture of heavy concrete, since it has a high density (2.6 ... 2.7 g / cm3), strength (100 ... 300 MPa), frost resistance, chemical resistance, durability.
In the production of concrete in factories, it is often not a single fraction that is used, but a mixture of several non-adjacent fractions to reduce aggregate voidness and reduce the consumption of cement-sand mortar and cement.
The aggregate grade should be higher in strength than the designed concrete grade, and not less than in frost resistance. The maximum aggregate size should be, firstly, 3 ... 4 times less than the smallest thickness of the product, and secondly, less than the minimum distance between the reinforcement. When selecting the composition of concrete, the voidness, water absorption, the content of dust and clay particles, harmful impurities, and the radiation-hygienic characteristics of the aggregate are also taken into account.
The most important technical characteristics are: grain size, grain composition, hollowness, content of needle-like and plate-like grains, content of powdered and clay particles, grade by crushability, wear.
Expanded clay gravel - inorganic artificial granular rounded porous large aggregate in the form of porous granules of dark brown color on the outside and black in the fracture. To obtain claydite, the phenomenon of swelling of some clays during firing is used, associated with two processes: gas evolution and transition of clay to the pyroplastic state. Sources of gas evolution are the reduction reactions of iron oxides Fe2O3 + CO = CO2 + 2FeO, burning of organic impurities, dehydration of hydromica and other water-containing clay minerals, dissociation of carbonates, etc. Clay passes into the pyroplastic state when a liquid phase (melt) forms in them at high temperature (up to 1200 ° C), as a result of which the clay softens, acquires the ability to plastic deformation, at the same time it becomes gas-tight and swells out by evolved gases.
The main characteristic of claydite is the bulk density mark, which should be no more than 1200 kg / m3 (the lower the better), the compressive strength in the cylinder. It is a lightweight and durable aggregate used (about 3/4 of the total output of porous aggregates) for the manufacture of lightweight concrete and bulk insulation.
Agloporite crushed stone, gravel and sand - inorganic artificial granular porous aggregate rounded (gravel) or angular (crushed stone) form, obtained by burning clay, unsuitable for the production of expanded clay, mixed with fuel slags, ashes, shale and coal mining waste (8-10% ). The loose mixture is moistened with a binding additive, which is taken as a clay slip or a solution of technical lignosulfonate. The resulting mixture is fed to the granulator, where it is brought to a moisture content of 20 ... 35% and pelletized. The granules are laid on the grate of the belt sintering machine, which is a continuously moving conveyor of pallet carts having a grate at the base. Under the grate in the vacuum chamber, a fan (exhaust fan) creates a vacuum by suction of air, due to which air is sucked through the charge. Top charge is set on fire. When coal is burned, the temperature of the mixture rises to 1400 ... 1500 ° C, the coal burns out, and the particles of the raw material are sintered into a porous vitrified mass (cake). The sintering process is relatively quick.Hot gases, which are sucked down, heat the underlying layers of the charge, and the combustion zone gradually moves to the grate. The upper sintered layers at this time are somewhat cooled by sucked air. When the combustion zone of the fuel reaches the grate and the sintering process is completed, a sintered agloporit cake is obtained, which is subjected to two-stage crushing and fractionated into crushed stone and sand.
Slag Pumice- inorganic porous coarse aggregate from industrial waste in the form of crushed stone, obtained by rapidly cooling the melt of metallurgical (usually blast-furnace) slags with water, leading to swelling. Upon contact of the slag melt (temperature about 1300 ºС) with water, violent boiling occurs with intensive formation of steam. Steam bubbles, penetrating into the melt, can not stand out freely, because when cooled, the viscosity of the melt increases. As a result, it swells up, swells up and solidifies as a porous mass of cellular structure. The chemical composition of slags and the presence of dissolved gases in them, which determine the gas-creation ability, viscosity and surface tension of the slag melts, are of primary importance. Pieces of slag pumice are crushed and scattered, obtaining crushed stone and sand of cellular structure with a pore diameter of 1 ...2 mm (sometimes slag pumice is obtained with a pore diameter of 5 ... 6 mm, similar to a hardened foam). The standard provides for the testing of slag-and-crushed stone rubble for resistance to silicate decomposition. At cost, slag pumice stone is the cheapest artificial porous aggregate. It is made and applied in areas of the metallurgical industry.
Вспученный перлит изготовляют путем обжига водосодержащих вулканических стеклообразных пород (перлитов, обсидианов, витрофира и др.). При температуре 950…1200 °С вода выделяется и перлит увеличивается в объеме в 10…20 раз. Вспученный перлит применяют для производства легких бетонов и теплоизоляционных изделий.
Вспученный вермикулит - пористый сыпучий материал, полученный путем обжига вермикулита (разновидности слюды, магниево-железистый гидроалюмосиликат с содержанием связанной воды 8...18 %). Этот заполнитель используют для изготовления теплоизоляционных легких бетонов.
Асбест – минеральный природный тонковолокнистый мелкий плотный заполнитель, получаемый распушкой природного асбестового камня (в России - минерала хризотил-асбеста, состоящего из силикатов магния) на волокна диаметром 0,02…1 мм и длиной 0,3…10 мм. Хризотил-асбест устойчив к действию щелочей и интенсивно разлагается кислотами, негорюч и обладает значительной теплостойкостью (плавится при температуре около 1550 °С). Природный асбест имеет очень высокую прочность при растяжении вдоль волокнистости 3200…5400 МПа в зависимости от диаметра и длины образцов (выше прочности стали), а модуль упругости – 175…185 ГПа. При распушке асбеста часть волокон разрушается и прочность при растяжении распушенного волокна составляет 700…750 МПа, а модуль упругости снижается до 70…80 ГПа. Волокна асбеста армируют цементный камень. При работе асбестоцементных изделий на изгиб волокна асбеста воспринимают растягивающие напряжения, а цементный камень - сжимающие. Введение гибких волокон в количестве 10…20 % от массы цемента позволяет в 3…5 раз увеличить прочность цементного камня при растяжении и изгибе, а также стойкость к ударным воздействиям. Применяется в производстве асбестоцементных изделий.
For thermal insulating and some types of structural heat insulating lightweight concrete, organic fillers are used that are made from wood, cotton stalks, fires, expanded polystyrene granules (styropeton concrete), fiberglass, polypropylene fibers, etc.
Additives for binders, concretes and mortars
Additives - natural or artificial products introduced into the composition of binders, mortars and concretes during their manufacture in order to improve the physico-chemical and technological properties of the resulting product, as well as reduce its cost. Divided into chemical, fine mineral and complex additives.
Chemical additives are introduced in small quantities (0.1 ... 2% by weight of cement). Depending on the main effect of the action are divided into several types.
1. Regulating properties of concrete mixes :
- plasticizers (increase the mobility of the concrete mix with the same mixing water consumption);
- water reducing (allow to obtain a concrete mix of the required workability with reduced water consumption, which leads to an increase in strength with the same binder consumption, and allow to reduce the binder consumption at the same level of strength);
- stabilizing (reduce the delamination and water separation of the concrete mix, which leads to an increase in the homogeneity of the structure of concrete and its quality);
- regulating the persistence of mobility (slowing and accelerating the setting)
- increasing air (gas) content ( sounding) (substances that contribute to the targeted formation of pores in the concrete body):
- air entraining (surface-active organic substances that contribute to the involvement in the concrete mix when it is mixed with fine air, evenly distributed in the concrete);
- gas - forming (substances capable of emitting gas due to chemical interaction with the components of the binder);
- foaming (surface-active organic substances that provide the possibility of obtaining technical foam of the required multiplicity and durability).
2. Regulating properties of concrete
- regulating the kinetics of hardening (accelerators and moderators allow you to control the processes of concrete hardening due to the influence on the processes of hydration);
- increase strength ;
- reducing permeability (clogging) (substances that contribute to the filling of pores in concrete with water-insoluble products and increasing its water, gas and vapor impermeability);
- enhancing the protective properties of the reinforcement (inhibitors) of corrosion;
- increasing frost resistance
- increasing corrosion resistance
- expanding (for non-shrinkable and expanding concrete). 3. Special properties that give concrete:
- antifreeze (substances that lower the freezing point of water and provide hardening at low temperatures);
- hydrophobic (substances that give the walls of pores and capillaries in concrete hydrophobic (water repellent) properties).
The same substances can be attributed to additives for various purposes. In addition to the main, all additives can have side effects.
Mineral additives are fine powders introduced into concrete in the amount of 2 ... 20%. According to the mechanism of interaction with cement hydration products, they are divided into active mineral additives and inert mineral additives (fillers).
Inert mineral additives (fillers) do not enter into chemical interaction with cement hydration products, play the role of a microfill in the cement stone. Improve the structure of cement stone, save cement. Introducing them in amounts up to 60% by weight of cement usually does not reduce the strength of concrete. In addition, the introduction of such additives can provide acid resistance, alkali resistance and heat resistance of concrete and mortars. These include fine ground quartz sand and limestone, various low-level slags and ashes, diatomaceous earth, bentonite, kaolin, and stone flour, etc.
Active mineral supplements are divided into three types:
- possessing astringent properties (hydraulic additives);
- possessing pozzolanic activity;
- possessing simultaneously astringent properties and pozzolanic activity.
Hydraulic additives are substances that can harden spontaneously after mixing with water. These include mineral binders, burnt earth, sour slags, ashes.
Pozzolanic additives cannot spontaneously harden, but active silica (amorphous type) contained in their composition, and sometimes alumina, can interact with cement hydration products to form strong compounds that increase the strength of cement stone and concrete as a whole, and due to this reduction in consumption cement. These include natural pozzolanic rocks (tripoli, flask, volcanic ash, tuffs, slopes, etc.) in the fine ground form; man-made products (ash, slag); specially made (silica fume, nanosilicates); ground ceramic brick.
It should also be noted that some inert silica mineral additives at elevated temperatures can actively interact with calcium hydroxide, contributing to a significant increase in the strength of hardened binders. This, in fact, is the basis for synthesizing hardening binders.
Although mineral and especially chemical additives are introduced into the composition of mortars and concretes in very small quantities, the effect is very high. Therefore, today, additive-free concretes are not produced: 1 ... 3 types of additives are introduced into domestic concretes at the same time, 5 ... 7 in foreign ones.
8.3. IN IDA CONCRETE
Heavy concrete
This is concrete, that is, an artificial composite stone, in which crushed stone or gravel of solid solid rocks is used as a large aggregate. The release of this concrete is about 70% of the total volume of concrete, so it is sometimes called normal.
Grains of coarse aggregate occupy 40 ... 60% of the body of concrete and form a bearing skeleton, laying of stone and monolithic into a single unit with a cement-sand stone (an analogue of mortar). Cement-sand stone is characterized by the same structure - grains of fine aggregate (20 ... 30% of the total volume of concrete) are homogeneous with hardened cement stone. Cement stone can be considered as micro-concrete, in which the cementitious substance monoliths non-hydrated particles of clinker minerals or filler. Cement stone and cementing substance are largely saturated with pores of different size and origin - macropores, capillary, gel. The average density of heavy concrete is 1800 ... 2500 kg / m3, true - 2.6 ... 2.8 g / cm3. That is, the porosity of heavy concrete is generally low, but the porosity of its main structural component, cement stone (4 ... 6% of the total volume of concrete) is very high. And it predetermines all the properties of heavy concrete - the main structural material in modern construction. It is characterized by strength class from B3.5 to B100. Grades of concrete for frost resistance : F50; F75; F100; F150; F200; F300; F400; F500; F600; F800; F1000. High thermal conductivity. The main requirements for the quality of structural concrete, depending on the conditions of its operation: mark on water resistance, impact resistance, modulus of elasticity, crack resistance, fracture toughness.
These characteristics depend on the structure of concrete: at the macro- and mesoscale levels — on the strength of the matrix (cement-sand mortar and cement stone) and inclusions (particles of coarse and fine aggregate) and their volume fraction, estimated by the expansion factor — ratio of the matrix volume to the volume of intergranular void inclusions. At the micro level - from the porosity of the cement stone.
In turn, these characteristics will depend on the prescription factors - the composition of concrete and technological - the conditions of mixing, molding, hardening.
Lightweight concrete on porous aggregates
These are concrete with an average density not higher than 2000 kg / m3. They have the same structure as heavy concrete, but as coarse (and sometimes fine) aggregates, light porous aggregates are used, usually artificial - expanded clay, agloporite, slag pumice, etc. Concretes are also named by the type of porous aggregate: expanded clay concrete, agloporite concrete, etc. It is the porosity of the aggregate and its volumetric content in the concrete that ensures a lower density and thermal conductivity of the material, low weight of the structures, respectively, reduced transport costs and load on the foundations.
Along with the strength class (B0.35 ... B40), the most important indicators of the quality of lightweight concrete are the grade in average density D200 ... D2000 (after 100), where the figure means
density in the dry state (kg / m3). Depending on the purpose, light concretes are divided into the following groups: functional with a density of up to 500 kg / m3; structurally functional (for enclosing structures - exterior walls, building coatings) with a density of 500-1400 kg / m3; constructional with a density of 1400-1800 kg / m3. Note that light concretes with a density not higher than 1200 kg / m3 are most appreciated - they combine low thermal conductivity with sufficient bearing capacity. True density is the same as that of heavy concrete.
The most advantageous combination of indicators of density, thermal conductivity, strength and cement consumption for lightweight concrete is achieved with the greatest saturation of concrete with porous aggregate, which requires a solid (contiguous) placement of aggregate grains in the volume of concrete and the formation of a dense contact zone near the aggregate. In this case, the concrete will contain less cement stone, the aggregate is placed in a “holder”, and the steel reinforcement will be protected from corrosion. The greatest saturation of concrete with porous aggregate is possible only with the correct selection of the grain composition of the mixture of small and large porous aggregates, as well as with the use of technological factors (intensive compaction, plasticizers, etc.).
Competent selection of the composition, the use of plasticizers allow to obtain concretes on porous aggregates, which are already successfully used in bridge engineering and hydraulic engineering construction.
Cellular concrete
These are concrete, the structure of which is a stone skeleton with macropores uniformly distributed in it in the form of cells up to 3 mm in diameter, without coarse and fine aggregate. The solid phase is in the form of interporous partitions, the material is a cement stone (microgranular concrete) as in heavy concrete with the same average and true density. The porosity of cellular concrete is 90 ... 60%, thanks to this, cellular concrete has a low density and low thermal conductivity. By density, cellular concrete is divided into three groups: thermal insulation D300 ... D400; structural thermal insulation (for enclosing structures) D500 ... D900; constructional (for reinforced concrete)
D1000 ... D1200
Water absorption and frost resistance depend on the size and nature of the porosity of cellular concrete and the density of partitions between macropores. To reduce water absorption and increase frost resistance, they strive to create a cell structure with closed pores.
Heat conductivity coefficients of heat- insulating concrete should not exceed more than 20% 0.08 ... 0.12 W / (m · ° C); construction and heat insulation - 0.12 ... 0.24 W / (m · ° C); structural - 0.29 ... 0.38 W / (m · ° C). The thermal conductivity of cellular concrete depends on the density and humidity, for example, at a density of 600 kg / m3, the thermal conductivity in a dry state is 0.09 W / (m · ° C), and at a humidity of 8% - 0.18 W / (m · ° C).
Depending on the method of manufacturing, cellular concrete is divided into two types:
1) the formation of a cellular structure by swelling the dough binder using gas-forming additives. In a mixture of chemical reactions occur, accompanied by the release of gas, so this method is called chemical.
Most often, the gasifier is aluminum powder, which, reacting with calcium hydroxide, releases hydrogen by the reaction 3Ca (OH) 2 + 2A1 + 6H2O = 3C
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