Silicate wall materials. Autoclaved silicate products Properties of silicate materials

K category: Construction materials

Silicate materials and products

Silicate products are an artificial stone material made from a mixture of lime, sand and water, molded by pressing under high pressure and autoclaved.

Sand-lime bricks are widely used in construction; silicate dense concrete and products made from it; cellular silicate concrete and products; silicate concrete with porous aggregates.

Sand-lime brick is pressed from a lime-sand mixture of the following composition (%): pure quartz sand 92-94; air lime 6-8 and water 7-8. The lime-sand mass prepared in mixers is molded on presses under a pressure of 15-20 MPa and steamed in autoclaves at a saturated steam pressure of 0.8 MPa and a temperature of approximately 175 °C.

When steaming, lime, sand and water react, resulting in the formation of calcium hydrosilicate, which cements the mass and gives it high strength. The duration of the autoclave treatment cycle is 10-14 hours, and the entire process of making sand-lime bricks is 16-18 hours, while the process of making ordinary clay bricks lasts 5-6 days.

Sand-lime brick is available in two types: single size 250 X 120 X 65 mm and modular size 250 X 120 X 88 mm. The volumetric mass of sand-lime brick is 1800-1900 kg/m3, frost resistance is not lower than Mr3 15, water absorption is 8-16% by weight. In terms of compressive strength, sand-lime brick is divided into five grades: 75, 100, '25, 150 and 200. In terms of thermal conductivity, sand-lime brick differs slightly from ordinary clay brick and completely replaces the latter when laying the walls of any buildings, except for walls made in high humidity conditions or exposed to high temperatures (ovens, chimneys). The color of sand-lime brick is light gray, but it can also be colored, colored in the mass by introducing mineral pigments into it.

Products made of dense silicate concrete. Fine-grained dense silicate concrete - autoclave-hardening cementless concrete based on lime-siliceous or lime-ash binders - is produced according to the following technological scheme: part of the quartz sand (8-15%) is mixed with quicklime (6-10%) and finely ground in ball mills, then crushed lime-sand binder and regular sand(75-85%) is mixed with water (7-8%), mixed in concrete mixers and then the mixture goes to the molding stand. Molded products are steamed in autoclaves at a temperature of 175-190 ° C and a steam pressure of 0.8 and 1.2 MPa.

Products made of dense silicate concrete have a volumetric mass of 1800-2200 kg/m3, frost resistance of 25-50 cycles, and compressive strength of 10-60 MPa.

Large solid wall blocks, reinforced floor slabs, columns, beams, foundation and plinth blocks, staircase and partition structures are made from dense silicate concrete.

Silicate blocks for external walls and walls in wet rooms must have a grade of at least 250.

Products made of cellular silicate concrete. According to the method of formation of the porous structure, cellular silicate concretes are foam silicate and gas silicate.

The main binder for the preparation of these concretes is ground lime. Ground sand, volcanic tuff, pumice, fly ash, tripoli, diatomite, tras, and slag are used as siliceous components of the binder and fine aggregates.

In the manufacture of cellular silicate products, a plastic lime-sand mass is mixed with stable foam prepared from a HA preparation, soap root, etc., or with gas-forming agents - aluminum powder, and then the mixture is poured into molds and subjected to autoclave processing.
The volumetric mass of foam silicate products and gas silicate products is 300-1200 kg/m3, compressive strength is 1-20 MPa.

According to their intended purpose, cellular silicate products are divided into heat-insulating products with a volumetric mass of up to 500 kg/m3 and structural-thermal-insulating products with a volumetric mass of more than 500 kg/m3.

Thermal insulating cellular silicates are used as insulation materials, and external wall blocks and panels, as well as complex building coating slabs, are made from structural and thermal insulating silicates.

Products made of silicate concrete on porous aggregates. Finely ground lime-silica mixtures are used as binder silicate concrete on porous aggregates, and expanded clay, pumice, porous slag and other porous light natural and artificial materials in the form of gravel and crushed stone. After autoclave processing, such concretes acquire a compressive strength of 3.5 to 20 MPa with a bulk weight of 500 to 1800 kg/m3, and blocks and panels of external walls of residential and public buildings are mainly made from them.

TO autoclaved silicate materials These include materials whose production is based on the hydrothermal synthesis of a mineral mixture (main raw materials, binders and fillers), carried out at elevated pressure (up to 1.5 MPa) and temperature (174...200 °C) water vapor.

The main raw materials for autoclave hardening materials are predominantly lime-sand mixtures and industrial waste - blast furnace slag, fuel ash, nepheline sludge, etc. The most common are lime-sand mixtures. (silicate) materials.

The main binding component of autoclaved materials is lime. For the production of silicate products, it is recommended to use fast-quenching lime with a total content of active calcium and magnesium oxides of more than 70%. In this case, the MgO content should be no more than 5%. Along with lime, Portland cement can be used, in particular in the production of cellular concrete. The use of Portland cement helps to increase the frost resistance of products.

Most common filler silicate materials- quartz sands. When using feldspathic and carbonate sands physical and mechanical properties products deteriorate.

When heat treating the main raw materials in autoclaves, there is an interaction between calcium hydroxide, silica and water, accompanied by the formation of sparingly soluble reaction products - calcium hydrosilicates:

A Ca(OH) 2 + Si0 2 + ( n-a)H 2 0 → a CaO. Si0 2 . n H20,

and the value of the coefficient A is determined by the ratio of the concentrations of CaO and Si0 2 in the liquid phase.

Amorphous and glassy raw materials have high reactivity during autoclave processing. These include volcanic effusive rocks, granular slags, fuel ash, etc.

Intensification of hardening and improvement of the basic properties of autoclave materials are achieved by using highly dispersed raw materials. In the manufacture of high-strength lime-sand products, quicklime is ground with sand to a specific surface area of ​​3000...5000 cm 2 /g and used as a binder.

According to their intended purpose, products made from silicate materials differ in structural And thermal insulation products, and according to the form of manufacture - on piece And large-sized products.

In terms of production volume of products made from autoclave-hardening materials, the leading place is occupied by sand-lime brick, and behind him - wall products from dense and cellular concrete.

Sand-lime brick is an artificial non-firing wall building material made by pressing from a mixture of quartz sand (90...92%) and slaked lime (8...10%) followed by hardening in an autoclave.

In the composition of the raw material mixture for the production of sand-lime brick, the lime content ranges from 7 to 10% in terms of the active role of CaO. To increase the strength of sand-lime brick, finely ground lime-silica, lime-slag and lime-ash mixtures are used as a binder component.

In the production of sand-lime bricks, the most desirable are quark sands with grains of 0.2...2 mm in size and having a minimum number of voids. The content of clay impurities is allowed no more than 10%, since with a higher clay content, water absorption increases and the strength and frost resistance of the brick decreases. The presence of organic impurities in the raw material mixture for brick production reduces its strength and can lead to the formation of cracks due to the release of gases during autoclave hardening.

Sand-lime brick is used along with ceramic bricks for laying stone and reinforced stone exterior and internal structures in the above-ground part of buildings with normal and wet operating conditions. Due to lower resistance to water and substances dissolved in it, sand-lime brick, unlike ceramic brick, cannot be used for laying foundations and plinths of buildings below the waterproofing layer. It is not allowed to use sand-lime brick for the walls of buildings with wet operating conditions (baths, laundries, etc.) without special measures to protect the walls from moisture. It is not allowed to use for laying stoves, pipes, because it cannot withstand prolonged exposure to high temperatures.

Silicate concrete is a compacted mixture hardened in an autoclave, consisting of quartz sand (70...80%), ground sand (8...15%) and ground quicklime (6...10%). It is characterized by lower corrosion resistance of reinforcement, which is due to the weak alkalinity of the environment. The durability of the reinforcement is reliably ensured at an air humidity of 60%. Like cement, silicate concretes are classified depending on density, structural features, maximum size and type of aggregates, as well as area of ​​application.

According to gas-dynamic parameters, they distinguish laminar and turbulent flame.

Laminar(from Latin lamina - layer, plate) is called a calm, irrotational flame of a stable geometric shape.

Turbulent(from Latin turbulenze - whirlwind) is a restless, swirling flame of constantly changing shape.

You have all observed both of these modes many times. Remember an ordinary lighter: when the gas flow rate is set to low, the flame is calm, like a candle flame, this is a laminar flame; when the flow rate increases, the flame changes its shape and becomes restless, swirling with vortices, constantly changing shape, this is a turbulent flame.

This behavior of the flame in a turbulent mode is explained by the fact that much large quantity combustible gas, that is, at a given moment more and more fuel must be oxidized, which leads to an increase in the size of the flame and its further turbulization.

The gas-dynamic combustion regime depends on the linear speed of the combustible substance or mixture and is characterized by Reynolds criterion (measure of the ratio of inertial forces and internal friction in the flow):

× (to remember: "bucket of milk")

where v is the linear velocity of the gas flow, m/s;

d is the characteristic flow size, m;

r - gas density, kg/m3;

m - dynamic viscosity coefficient, N×s/m 2

The laminar regime is observed at Re< 2300, при 2300 < Re < 10000 режим переходный, а при Re >10000 - turbulent. In all cases, the thickness d of the combustion zone (front) of the flame is d lams< d п epex < d т yp .

Due to restrictions imposed by the rate of diffusion, flammable gases and vapors often do not have time to react completely with oxygen in the air, and combustion products, in addition to volatile gases and vapors, contain small hot condensed particles of unburned carbon organic matter in the form of soot, which emit light and heat.

The radiation of a flame is determined by the radiation of combustion products in different states of aggregation.

Flame structure

The flame has its own structure, knowledge of which is extremely necessary for understanding the combustion process as a whole.

The chemical redox reaction itself occurs in a thin surface layer limiting the flame, called flame front .

Flame Front- a thin surface layer that limits the flame, directly in which redox reactions occur.

The thickness of the flame front is small; it depends on gas-dynamic parameters and the mechanism of flame propagation (deflagration or detonation) and can range from tenths of a millimeter to several centimeters. Inside the flame, almost the entire volume is occupied by flammable gases (GG) and vapors. Combustion products (PG) are present in the flame front. There is an oxidizing agent in the environment.

Diffusion flame diagram gas burner and changes in the concentrations of combustible substances, oxidizer and combustion products along the flame cross section are shown in Fig. 1.2.

The thickness of the flame front of various gas mixtures in laminar mode is 0.5 - 10 -3 cm. The average time for complete conversion of fuel into combustion products in this narrow zone is 10 -3 -10 -6 s.

Maximum temperature zone located 5-10 mm above the luminous cone of the flame and for a propane-air mixture is about 1600 K.

Diffusion flame occurs during combustion when the combustion and mixing processes occur simultaneously.

As noted earlier, the main difference between diffusion combustion and the combustion of pre-mixed combustible mixtures is that the rate of chemical transformation during diffusion combustion is limited by the process of mixing the oxidizer and fuel, even if the rate of the chemical reaction is very high, the intensity of combustion is limited by the mixing conditions.

An important consequence of this idea is the fact that in the flame front the fuel and the oxidizer are in stoichiometric ratio. Whatever the ratios of the separately supplied oxidizer and fuel flows, the flame front is always set in such a position that the flow of reagents occurs in stoichiometric ratios. This has been confirmed by many experiments.

Driving force diffusion of oxygen into the combustion zone is the difference in its concentrations inside the flame (CO = 0) and in the surrounding air (initial CO = 21%). As this difference decreases, the rate of oxygen diffusion decreases and at certain oxygen concentrations in the surrounding air - below 14-16%, combustion stops. This phenomenon of spontaneous attenuation (self-extinguishing) is observed during combustion in closed volumes.

Each flame occupies a certain volume in space, the outer boundaries of which can be clearly or vaguely limited. When gases burn, the shape and size of the resulting flame depend on the nature of the initial mixture, the shape of the burner and stabilizing devices. The influence of fuel composition on the flame shape is determined by its influence on the combustion rate.

Flame height is one of the main characteristics of flame size. This is especially important when considering the combustion and extinguishing of gas fountains and the combustion of petroleum products in open tanks.

The height of the flame is greater, the larger the diameter of the pipe and the greater the flow rate, and the smaller, the greater the normal speed of flame propagation.

For a given mixture of fuel and oxidizer, the flame height is proportional to the flow speed and the square of the jet diameter:

where is the flow speed;

Jet diameter;

Diffusion coefficient.

But at the same time, the shape of the flame remains unknown and depends on natural convection and temperature distribution in the flame front.

This dependence persists up to a certain flow rate. As the flow speed increases, the flame turbulizes, after which further increase in its height stops. This transition occurs, as already noted, at certain values ​​of the Reynolds criterion.

For flames, when there is a significant release of unburned particles in the form of smoke, the concept of flame height loses its definition, because it is difficult to determine the combustion limit of gaseous products at the top of the flame.

In addition, in flames containing solid particles, compared to flames containing only combustion gases, radiation increases significantly.

Chemical and physical processes in a flame

In a flame, chemical and physical processes simultaneously occur, between which there are certain cause-and-effect relationships.

Chemical processes in a flame include:

on the approach to the combustion zone:

Thermal decomposition of starting substances with the formation of lighter products (hydrogen, carbon oxides, simple hydrocarbons, water, etc.);

in the flame front:

Thermal-oxidative transformations with the release of heat and the formation of products of complete (carbon dioxide and water) and incomplete combustion (carbon monoxide, soot, soot, resins, etc.);

Dissociation of combustion products,

Ionization of combustion products.

Physical processes in a flame include:

Heat and mass transfer in the flame front;

Processes associated with the evaporation and delivery of volatile combustible substances to the combustion zone.

The rate of transfer (diffusion) of substances is critical, for example, in heterogeneous systems, where it is much less speed chemical oxidation reactions. The ratio of the rate of chemical transformations and physical processes determines the mode of the combustion process.

Spread of flame in space

The occurrence of combustion or ignition is only the initial stage of the combustion process, its initiation. This stage is certainly important from the point of view of preventing fires and explosions. But it is not always possible to prevent them, so for practical fire workers great importance has the ability to predict the dynamics of combustion development, namely, in what mode and with what parameters a fire or explosion will develop on real objects. Besides, in practical activities we have to face the need to restore the picture of the development of fires and explosions that have already occurred. To do this, it is necessary to know the basic laws of the processes of propagation and development of combustion. This information is also necessary for the right choice the most effective type and method of using a fire extinguishing agent in specific conditions.

The simplest combustion scheme is the combustion of gases and vapors. When mixed with an oxidizing agent (in most cases, atmospheric oxygen), they form a flammable mixture. As mentioned above, combustion can be diffusion and kinetic.

With diffusion combustion of gases, the flame spreads as the fuel mixes with the oxidizer, we discussed this above.

During kinetic combustion of gases, flame propagation can occur through the mechanism of deflagration (normal combustion) and detonation.

Normal or deflagration combustion- this is the spread of flame through a homogeneous flammable medium, in which the flame front moves due to its layer-by-layer heating according to the mechanism of thermal conductivity.

A deflagration flame propagates at a low speed, on the order of several meters or tens of meters per second. In this case, heat transfer occurs layer by layer using the mechanism of thermal conductivity.

In deflagration combustion, the flame spreads at a speed called normal flame propagation speed.

The essence of the mechanism of thermal flame propagation, as established above, is the transfer of heat from the combustion zone by thermal conductivity and heating of the adjacent layer of fresh combustible mixture to the auto-ignition temperature.

The danger of deflagration combustion, in addition to what was mentioned above, also lies in the fact that under certain conditions deflagration can turn into detonation.

Detonation –This is a combustion mode in which the flame front propagates due to the self-ignition of the combustible mixture in the front of a shock wave traveling ahead.

The speed of flame propagation during detonation is entirely determined by the speed of propagation of the shock wave.

The rate of detonation in real flammable gas systems is much higher than deflagration. It can reach 3 km/s. This determines the greater destructive ability and danger of the detonation wave.

The phenomenon of spontaneous occurrence of detonation combustion is of great professional interest to fire specialists. It is quite often observed during the combustion of homogeneous steam and gas-air mixtures in pipelines, various narrow spaces between equipment, in cable tunnels, containers, etc. In these places, the normal, deflagration combustion mode can turn into detonation.

Like deflagration, detonation of gas systems is possible only in a certain range of fuel and oxidizer concentrations.

Production of silicate materials

Silicate materials are called materials from mixtures or alloys silicates, polysilicates and aluminosilicates. These are solid crystalline or amorphous materials, and silicates sometimes include materials that do not contain silicon oxides.

Silicates are compounds of various elements with silica (silicon oxide), in which it plays the role of an acid. The structural element of silicates is the tetrahedral orthogroup -4 with a silicon atom Si +4 in the center and oxygen atoms O -2 at the vertices of the tetrahedron. Tetrahedra in silicates are connected through common oxygen vertices into silicon-oxygen complexes of varying complexity in the form of closed rings, chains, networks and layers. Aluminosilicates, in addition to silicate tetrahedra, contain tetrahedra of the composition [A1O 4 ] -5 with aluminum atoms A1 +3, forming aluminum-silicon-oxygen complexes with silicate tetrahedra.

Chains, ribbons and layers are interconnected by cations located between them. Depending on the type of oxosilicate anions, silicates have a fibrous (asbestos) or layered (mica) structure.

In addition to silicates, they are widely distributed in nature aluminosilicates, in the formation of which, along with SiO 4 tetrahedra, AlO 4 tetrahedra take part.

In addition to the Si +4 ion, complex silicates include:

cations: Na + , K + , Ca ++ , Mg ++ , Mn ++ , B +3 , Cr +3 , Fe +3 , A1+ 3 , Ti +4 and anions : O 2 -2, OH –, F –, Cl -, SO 4 2-, as well as water. The latter can be present in silicates in the form of constitutional, included in the crystal lattice in the form of OH -, crystallization H 2 O and physical, absorbed by silicate.

The properties of silicates depend on their composition, the structure of the crystal lattice, the nature of the forces acting between the ions, and are largely determined high binding energy between atoms of silicon and oxygen, which makes up 450-490 kJ/mol. (For contact C-O energy is 314 kJ/mol). Most silicates are refractory and fire-resistant; their melting point ranges from 770 to 2130 °C. The hardness of silicates ranges from 1 to 6-7 units. according to the Mohs scale. Most silicates are slightly hygroscopic and resistant to acids, which is widely used in various fields of technology and construction.

The chemical composition of silicates is usually expressed in the form of formulas composed of the symbols of elements in increasing order of their valence, or from the formulas of their oxides in the same order. For example, feldspar K 2 Al 2 Si 6 O 16 can be represented as KAlSi 3 O 8 or K 2 O×A1 2 O 3 ×6SiO 2.

Silicate materials count big quantity various species, present large scale product chemical production, are used in many areas technology and industry.

On rice. 11.1 given classification of silicates.

Rice. 11.1. Production of silicate materials

All silicates are divided into natural (minerals) and synthetic (silicate materials). Silicates are the most common chemical compounds in the Earth's crust and mantle. making up 82% of their mass, as well as in lunar rocks and meteorites. The total number of natural known silicates exceeds 1500. Based on their origin, they are divided into crystallization (igneous) rocks and sedimentary rocks. Natural silicates are used as raw materials in various fields National economy:

In technological processes based on roasting and smelting (clays, quartzite, feldspar, etc.);

In hydrothermal treatment processes (asbestos, mica, etc.);

In construction;

In metallurgical processes.

Silicate materials have a large number of different types, represent a large-scale product of chemical production and are used in many areas of the national economy.

Raw materials for their production serve:

– natural minerals (quartz sand, clays, feldspar, limestone),

– industrial products (sodium carbonate, borax, sodium sulfate, oxides and salts of various metals)

– waste (slag, sludge, ash).

In terms of production scale, silicate materials occupy one of the first places.

11.1 Typical processes silicate materials technologies

In the production of silicate materials, standard technological processes are used, which is due to the similarity of the physical and chemical principles of their production.

In the very general view the production of any silicate material consists of next successive stages (rice. 11.2):

Rice. 11.2. Schematic diagram production of silicate materials

The first stage is the preparation of the charge.

This stage includes mechanical operations for the preparation of solid raw materials: grinding, (sometimes fractionation), drying, mixing components.

The second stage is the molding stage.

The molding operation must ensure the production of a product of a given shape and size, taking into account their changes in subsequent drying and high-temperature processing operations.

Molding includes:

a) moistening the material (charge);

b) briquetting or giving the material a certain shape depending on the purpose of the product.

The third stage is drying the product.

Drying of the product is carried out to preserve the product's given shape before and during the high-temperature treatment operation.

The fourth stage is high-temperature processing of the product or charge.

1) At this stage, minerals of a certain nature and composition are synthesized from the components of the charge.

2) Depending on the purpose and properties of the resulting material, high-temperature treatment consists of firing the product or boiling the charge.

During high-temperature treatment in the charge, with increasing temperature, the following processes occur sequentially:

Removal of water, first physical, then crystallization;

Calcination of charge components, i.e. separation from them of constitutional water (entering the crystal lattice in the form of OH - ions) and carbon monoxide (IV);

Polymer transformations in the components of the charge and their restructuring crystal lattice;

Education of new chemical compounds in the form of solid solutions.

At this stage, the components of the charge - metal carbonates, metal hydroxides and aluminosilicates are converted into acidic oxides: SiO 2, B 2 O 3, Al 2 O 3, Fe 2 O 3 and basic oxides: Na 2 O, K 2 O, CaO, MgO , which react with each other;

Sintering of charge components.

Sintering can occur:

in the solid phase at a temperature below the melting point of the components;

or in the liquid phase, at a temperature above their melting point.

Cooling of the mass with the formation of liquid and amorphous phases.

11.2 Ceramics

Ceramic materials or ceramics are polycrystalline materials and products made from them, obtained by sintering natural clays and their mixtures with mineral additives, as well as metal oxides and other refractory compounds.

Ceramic products are very diverse and can be classified according to several criteria.

By application:

Construction (brick, tile);

Refractories;

Fine ceramics (porcelain, faience);

Special ceramics.

According to structure and degree of sintering: - porous or coarse-grained (brick, refractories, earthenware);

Sintered or fine-grained (porcelain, special ceramics).

According to surface condition: glazed and unglazed.

11.2.1 Raw materials

Substances with sintering properties are used as raw materials for the production of silicate ceramic materials.

Caking ability is the property of a loosely poured or compacted (molded into a product) powdery material to form when heated to certain temperature polycrystalline body - shard.

Such raw materials are:

Plastic materials (clay);

Non-plastic and thinning additives (quartz sand);

Fluxes and mineralizers (calcium and magnesium carbonates).

The most important and large-capacity ceramic materials are: building bricks and refractories.

11.2.2 Production of building bricks

Raw materials. The raw materials for the production of building bricks are low-melting clays of the composition Al 2 O 3 ∙nSiO 2 ∙mH 2 O, sand and iron (III) oxides.

The addition of quartz sand eliminates the appearance of cracks due to shrinkage of the material during drying and firing and allows you to obtain higher quality products.

The technological process of brick production can be carried out in two versions:

By the plastic method, in which a mixture of prepared raw material components is converted into a plastic mass containing up to 25% water;

Semi-dry method, in which the raw material components are moistened with steam (up to 10%), which ensures the necessary plasticity of the mass.

In fact, both methods differ in the amount of water and the method of water supply.

Technological scheme for the production of building bricks

1) A charge prepared by one method or another containing
40 - 45% clay, up to 50% sand and up to 5% iron oxide are pressed into a belt press using the plastic method, or a mechanical press operating under a pressure of 10-25 MPa using the semi-dry method. In Fig. 11.3 shows a schematic diagram of the production of building bricks using the semi-dry method.

Rice. 11.3. Belt press: 1 - loading funnel; 2 – rollers; 3 – auger; 4- press mouthpiece; 5 – humidifier; 6 – clay mass in the form of a ribbon; 7 – support rollers.

2) The molded brick is sent to dry in a tunnel dryer continuous action and then fired at a temperature of 900 - 1100 ºС. To speed up drying, electrolyte is added to the clay.

11.2.3. Production of refractories

Refractory materials (refractories) are non-metallic materials characterized by increased fire resistance, that is, the ability to withstand high temperatures without melting.

Application area.

Refractories are used:

In industrial construction for laying metallurgical furnaces, lining equipment operating at high temperatures;

Manufacturing of heat-resistant products and parts (crucibles, neutron absorber rods in nuclear reactors, rocket fairings).

The following requirements apply to materials used as refractories:

Thermal resistance, that is, the ability to retain mechanical characteristics and structure under single and multiple thermal influences;

Low coefficient of thermal expansion;

High mechanical strength during temperature operation;

Resistance to molten media (metals, slag).

The range of refractories is very wide. Depending on their composition, they are divided into several groups.

In Fig. 11.4 presents the classification of refractory materials according to their composition:

Rice. 11.4. Classification of refractories by composition

1. Aluminosilicate refractories are among the most common refractories.

They are based on the “Al 2 O 3 -SiO 2” system with different ratios of aluminum and silicon oxides, on which their properties, in particular, resistance to melts of varying acidity, largely depend.

2. Dinas refractories contain 95% silicon oxide with an admixture of calcium oxide. They are resistant to acidic slags and fireproof up to 1730 ºС.

Used for coke and glass furnaces. They are obtained from quartzite and calcium oxide by firing at 1500 ºС.

3. Semi-acid refractories contain up to 70-80% silicon oxide and 15-20% aluminum oxide. They are relatively resistant to acidic slags and silicate melts and are used in metallurgical furnaces and thermal power plants.

4. Fireclay refractories contain 50-70% silicon oxide and up to 45% aluminum oxide. They are resistant to the action of both basic and acidic slags, fireproof up to 1750 ºС and thermally stable. Obtained according to the scheme (Fig. 11.5):

Rice. 11.5. Production of fireclay refractories.

When firing kaolin, the following reactions occur:

Al 2 O 3 ∙2SiO 2 ∙2H 2 O = Al 2 O 3 ∙2 SiO 2 + 2H 2 O

3(Al 2 O 3 ∙2SiO 2) = 3Al 2 O 3 ∙2SiO 2 + 4SiO 2 ∙

5. Magnesite refractories contain magnesium oxide as a base. For example, dolomite refractories consist of 30% magnesium oxide, 45% calcium oxide and 15% silicon oxides.

All types of magnesite refractories are resistant to the action of basic slags, fireproof up to 2500 ºС, but their thermal resistance is low.

They are used for lining steel converters, in electric induction and open-hearth furnaces.

Obtained by firing natural minerals, for example, dolomite:

CaCO 3 ∙MgCO 3 = MgO + CaO + CO 2; (MgO + CaO – refractory).

6. Corundum refractories consist mainly of aluminum oxide. They are fire-resistant up to 2050 ºС and are used in devices for heating and melting refractory materials in radio engineering and quantum electronics.

7. Carborundum refractories consist of silicon carbide (carborundum) SiC. They are resistant to acidic slags, have high mechanical strength and heat resistance.

They are used for lining metallurgical furnaces, making casting molds, and thermocouple covers.

8. Carbon refractories contain from 30 to 92% carbon and are manufactured:

Firing a mixture of graphite, clay and fireclay (graphite refractory materials);

By firing a mixture of coke, coal tar pitch, anthracene fraction of coal tar and bitumen (coke refractories).

Carbon refractories are used for lining the hearths of blast furnaces, non-ferrous metallurgy furnaces, electrolyzers, and equipment for the production of corrosive substances.

11.3. Production of binding materials

Binding materials are single- and multi-component powdered mineral substances that, when mixed with water, form a plastic, moldable mass that hardens into a durable stone-like body upon aging.

Depending on the composition and properties, binders are divided into three groups (Fig. 11.6):

Rice. 11.6. Classification of binders

1. Air binders are materials that, after mixing with water (mixing), harden and retain strength for a long time only in air.

2. Hydraulic binders are materials that, after mixing with water and pre-hardening in air, continue to harden in water. In other words, they retain strength both in air and in water.

3. Acid-resistant cementitious materials include those that, after hardening in air, retain strength when exposed to mineral acids.

This is achieved by using aqueous solutions of sodium silicate to mix them, and acid-resistant fillers (diabase, andesite, etc.) are introduced into the mass of the material.

The raw materials for the production of silicate materials used as binders are:

Natural materials – gypsum stone, limestone, chalk, clay, quartz sand;

Industrial waste – metallurgical slag, pyrite cinder, nepheline processing sludge.

Application. Cementing materials in construction are used in the form of:

Cement paste (binder + water);

Mortar(binder + sand + water).

The action of the binder material can be divided into three successive stages:

Mixing (adding water) or forming a plastic mass in the form of a dough or solution by mixing a binder with an appropriate amount of water or silicate solution;

Setting or initial thickening and compaction of dough with loss of fluidity and transition to a dense but weak connection;

Hardening or gradual increase in mechanical strength during the formation of a stone-like body.

The most important species binding materials are: Portland cement (hydraulic cement) and air (construction) lime.

11.3.1 Portland cement production

Portland cement called a hydraulic binder material consisting of silicates and calcium aluminosilicates of different compositions.

The main components of Portland cement are the following compounds:

- alite (tricalcium silicate) 3CaO∙SiO 2 ,

- whitewashes (dicalcium silicate) 2CaO∙SiO 2 ,

- tricalcium aluminate 3CaO∙Al 2 O 3 .

The characteristic of Portland cement is its “grade”.

Brand of cement is the compressive strength of a cement sample after hardening for 28 days, expressed in kg/cm2. The higher the grade of cement, the higher its quality.

There are brands 400, 500 and 600.

The production of Portland cement consists of two stages: obtaining clinker and grinding it.

11.3.1.1 Preparation of clinker

Receipt clinker can be carried out two ways – wet And dry , which differ method of preparing raw material mixture for firing.

Wet method. According to the wet method, raw materials are crushed in the presence of a large amount of water. This creates pulp, containing up to 45% water.

In this method provided:

high uniformity mixtures;

is decreasing dustiness;

But increase expenses energy for water evaporation.

Dry method. By dry method raw materials components dried, crushed and mixed into dry form.

Such technology is energy saving , That's why specific gravity cement production dry method continuously increases.

On rice. 11.7 presented scheme Portland cement production wet method:

Rice. 11.7. Schematic diagram of the production of Portland cement.

Production clinker includes operations:

- crushing, grinding, adjusting the composition of raw materials;

- subsequent high temperature treatment the resulting mixture is fired.

Raw materials. The raw materials in the production of Portland cement are:

Various calcareous rocks - limestone, chalk, dolomite;

Marls - representing homogeneous finely dispersed mixtures limestone and clay.

When firing the charge, the following processes occur sequentially:

- evaporation of water(100 ºС);

- dehydration crystal hydrates and burnout organic substances:

MeO∙nH 2 O = nMeO + nH 2 O (500 ºС);

thermal dissociation carbonates:

CaCO 3 = CaO + CO 2 (900-1200 ºС);

Interaction main And acid oxides with education silicates, aluminates and calcium aluminoferrites:

CaO + SiO 2 = 2CaO∙SiO 2 (belite)

2CaO∙SiO 2 + CaO = 3CaO∙SiO 2 (alite)

3CaO + Al 2 O 3 = CaO∙Al 2 O 3 (tricalcium aluminate)

The process ends at a temperature of 1450ºC, after which the clinker is sent for cooling.

The composition of the product formed after firing is as follows: alite
40-60 %; whitewashes 15-30%; tricalcium aluminate 5-14 % .

To roast the charge, drum rotary kilns with a diameter of 3.5-5.0 m and a length of up to 185 m are used (Fig. 11.8):

Rice. 11.8. Rotary kiln for producing cement clinker:
1 – rotary kiln; 2 – bandages; 3 – support rollers; 4 – electric motors;
5 – gears; 6 – screw feeder; 7 - fridge; 8 - chimney

The raw material components entering the furnace sequentially pass through the zones of drying, heating, calcination, exothermic reactions of silicate formation, sintering and cooling.

Coming out of the oven clinker is cooled in drum refrigerators, and the heated air is used to heat the air and gaseous fuel entering the furnace.

11.3.1.2 Grinding clinker

For grinding cooled clinker :

- maintained in stock within 10-15 days For hydration free calcium oxide air moisture;

- mixed With additives And crushed in crushers and multi-chamber mills up to particles 0.1 mm and less.

Hardening Portland cement based on reactions hydration, included in its composition silicates And aluminosilicates , education crystalline hydrates various composition:

3CaO∙SiO 2 + (n+1) H 2 O = 2CaO∙SiO 2 ∙nH 2 O + Ca(OH) 2

2CaO∙SiO 2 + nH 2 O = 2CaO∙SiO 2 ∙nH 2 O,

3CaO∙Al 2 O 3 + 6H 2 O = 3CaO∙Al 2 O 3 6H 2 O

When mixing cement powder with water ( retreat ) the mass hardens.

To give cement certain properties, additives are added to it:

- hydraulic, increasing water resistance due to the binding of calcium hydroxide contained in cement:

Ca(OH) 2 + SiO 2 = CaSiO 3 + H 2 O;

- plasticizing, increasing the elasticity of the mass;

- acid-resistant, giving cement corrosion resistance To acidic environments (granite );

- inert, For reduction in price products ( sand );

- regulating time setting of the mass (gypsum ).

The bulk of Portland cement is used to make concrete and products made from it.

Concrete called an artificial stone obtained by hardening a mixture mixed with water cement , sand And filler .

As fillers use:

IN ordinary concrete – sand, gravel, crushed stone;

IN lungs concrete - various porous materials – pumice, slag;

IN cellular concrete – closed pores formed in concrete during the decomposition of substances introduced into the concrete mixture gas And foaming agents ;

IN fireproof concrete – fireclay powder;

In reinforced concrete - metal fittings.

11.3.2 Production of puffed lime

Air or construction lime is a silicate-free binding material based on calcium oxide and hydroxide.

There are three types of air lime:

- boiling pot(Not slaked lime) – calcium oxide CaO;

- fluffy(slaked lime) – calcium hydroxide Ca(OH)2;

IN the greatest number The earth's crust (lithosphere) contains free silicon anhydride or silica Si0 2. It is found in most minerals in the form of silicates -> chemical compounds with basic oxides. Free, naturally occurring crystalline silica occurs in the form of quartz, one of the most common minerals in the earth's crust. Its crystals have the shape of hexagonal prisms with hexagonal pyramids at the ends (bases). Quartz is usually opaque, more often it is white, milky. Quartz has no cleavage, its fracture is conchoidal, it has a greasy sheen; It does not combine with alkalis at ordinary temperatures and is not destroyed by acids (except hydrofluoric acid). Quartz has a specific gravity of 2.65 and a hardness of 7 on the hardness scale. Quartz has high compressive strength (about 20,000 kg/cm2) and has good abrasion resistance. When heated to a temperature of 575° C, quartz goes from the β-modification to the α-modification (high-temperature), abruptly increasing in volume by about 1.5%. At a temperature of 870° C, it begins to transform into tridymite (specific gravity 2.26), significantly increasing in volume (the tridymite mineral crystallizes in the form of thin hexagonal plates). These changes in the volume of quartz at high temperatures must be taken into account in the production of refractory silica products. At a temperature of 1710° C, quartz turns into a liquid state. With rapid cooling of the molten mass (melt), quartz glass is formed - amorphous silica with a specific gravity of 2.3.

In nature, the mineral opal has an amorphous structure, which is a silica hydrate (Si0 2 *nH 2 0). Amorphous silica is active and can combine with lime at normal temperatures, while crystalline silica (quartz) acquires this ability only under the influence of high-pressure steam (in an autoclave) or during fusion.

GROUP OF ALUMINOSILICATES

Alumina A1 2 O 3 occupies second place after silica in the earth's crust. Free alumina occurs naturally as the minerals corundum and other aluminous minerals.

Corundum is one of the hardest minerals. It is used to produce highly refractory materials and is a valuable abrasive.

Another aluminous material - diaspore - is alumina monohydrate A1203. H20 and contains 85% A1203. Diaspore is part of bauxite - finely dispersed rocks, often red or purple in color, rich in alumina (from 40 to 80%) and used as raw materials for the production of aluminous cement.

Alumina is usually found in the form of chemical compounds with silica and other oxides called aluminosilicates. The most common aluminosilicates in the earth's crust are feldspars, which by weight account for more than half of the total mass of the lithosphere. The same group of minerals includes micas and kaolinites.

GROUP OF IRON-MAGNESIAN SILICATES

The minerals included in this group are dark in color, therefore they are often called dark-colored minerals. Their specific gravity is greater than that of other silicates, the hardness is in the range of 5.5-7.5; they have significant viscosity. With a high content of them in rocks, they give the latter a dark color and greater viscosity, i.e., increased resistance to impact. The most common rock-forming minerals of the ferruginous-magnesian group are pyroxenes, amphiboles and olivine.

GROUP OF CARBONATES

The most common rock-forming minerals in sedimentary rocks are carbonate minerals, the most important of which are calcite, magnesite and dolomite.

Calcite, or crystalline lime spar CaC0 3 is one of the most common minerals earth's crust. It easily splits along cleavage planes in three directions, has a specific gravity of 2.7 and a hardness of 3. Calcite is slightly soluble in pure water (0.03 g per 1 l), but its solubility increases sharply when the water contains aggressive carbon dioxide CO 2 , since acidic calcium carbonate Ca(HC0 3)2 is formed, the solubility of which is almost 100 times greater than that of calcite.

Magnesite MgC0 3 occurs for the most part in the form of earthy or dense aggregates with a cryptocrystalline structure. It is heavier and harder than calcite.

Dolomite CaC0 3 -MgC0 3 is close in physical properties to calcite, but is harder and more durable and even less soluble in water.

GROUP OF SULFATES

Sulfate minerals (sulfates), like carbonates, are often found in sedimentary rocks; the most important of them are gypsum and anhydrite.

Gypsum CaS0 4 *2H 2 0 is a typical mineral of sedimentary rocks. Its structure is crystalline, sometimes fine-grained, the crystals are lamellar, columnar, needle-shaped and fibrous. Gypsum occurs primarily in the form of solid granular, fibrous and dense rocks, together with clays, shales, rock salt and anhydrite. Gypsum is white, sometimes transparent or colored with impurities in various colors. Its specific gravity is 2.3, hardness 2.

Gypsum dissolves relatively easily in water at a temperature of 32-41 ° C, its solubility is 75 times greater than calcite.

Anhydrite CaS0 4 has a specific gravity of 2.8-3, hardness of 3-3.5; By appearance looks like plaster. It occurs in layers and veins along with gypsum and rock salt. Under the influence of water, anhydrite gradually turns into gypsum, and its volume increases.

ROCKS OF CHEMICAL ORIGIN

Magnesite MgC03 is used to produce refractory materials and magnesium lowering - caustic magnesite.

Dolomite consists mainly of the mineral of the same name CaC03 MgC03. The properties of dolomites are close to dense limestones, and sometimes they have even more high qualities. They are used as building stone and crushed stone for concrete, as well as for the production of fire-resistant materials and binders (caustic dolomite). Dolomites are widespread.

Gypsum CaS0 4 *2H 2 Q, consisting of the mineral of the same name, is used mainly for the manufacture of gypsum binders and as an additive in the production of Portland cement.

Anhydrite CaS0 4, consisting of a mineral of the same name, is used to obtain binders, as well as for the manufacture of slabs for internal cladding. Externally, anhydrite does not differ noticeably from gypsum and usually occurs together with it.

Calcareous tuffs were formed as a result of the precipitation of CaCO 3 from cold and hot underground carbon dioxide waters. Very porous calcareous tuffs are used as a material for decorative buildings (grottoes, etc.) and as a raw material for making wicker, and dense ones with small evenly spaced pores and a compressive strength of up to 800 kg/cm 2 - for external cladding buildings

CONCRETE. BASIC INFORMATION ABOUT CONCRETE

Concrete is an artificial stone obtained by hardening a rationally selected mixture consisting of a binder, water and aggregates (sand and crushed stone or gravel). The mixture of these materials before hardening is called concrete mixture.

Grains of sand and crushed stone make up the stone framework in concrete. Cement paste, formed after closure concrete mixture with water, envelops the grains of sand and crushed stone, fills the gaps between them and initially plays the role of lubricating the aggregates, giving mobility (fluidity) to the concrete mixture, and subsequently, when hardening, binds the grains of the aggregates, forming an artificial stone - concrete. Concrete combined with steel reinforcement is called reinforced concrete.

CLASSIFICATION OF CONCRETE

Concrete is classified according to the following main characteristics: volumetric weight, type of binder, strength, frost resistance and purpose.

The main classification is based on volumetric weight. Concrete is divided into extra-heavy with a volumetric weight of more than 2500 g/m3, heavy - with a volumetric weight of 1800 to 2500 kg/m3 inclusive, light - with a volumetric weight of 500 to 1800 kg/m3 inclusive, extra-light - with a volumetric weight of less than 500 kg/m3. m 3.

Depending on the largest size of the aggregates used, a distinction is made between fine-grained concrete with aggregate up to 10 mm in size and coarse-grained concrete with the largest aggregate size of 10-150 mm.

The most important indicators of the quality of concrete are its strength and durability. Based on compressive strength, concretes are divided into grades R in kg/cm2. Heavy concrete based on cement and ordinary dense aggregates have grades 100-600, extra-heavy concrete 100-200, light concrete based on porous aggregates 25-300, cellular concrete 25-200, dense silicate concrete 100-400 and heat-resistant concrete 100-400.

The durability of concrete is assessed by the degree of frost resistance. Based on this indicator, concrete is divided into frost resistance grades Mrz: for heavy concrete Mrz 50-300 and for light concrete Mrz 10-200. Based on the type of binder, concrete is distinguished: cement concrete, made with hydraulic binders - Portland cement and its varieties;

silicate - on lime binders in combination with silicate or aluminate components;

gypsum - using gypsum anhydrite binders; concretes on organic binding materials.

Heavy concrete is made from cement and conventional dense aggregates, and light concrete is made from cement using natural or artificial porous aggregates. A type of lightweight concrete is cellular concrete, which is a hardened mixture of binder, water, finely dispersed silica component and a blowing agent. It is characterized by high porosity (up to 80-90%) with evenly distributed small pores. Silicate concrete is produced from a mixture of lime and quartz sand, followed by hardening of the molded products in an autoclave at a pressure of 9-16 atm (g) and a temperature of 174.5-200 ° C.

According to their intended purpose, concrete can be of the following types:

normal - for concrete and reinforced concrete load-bearing structures buildings and structures (columns, beams, slabs);

hydraulic - for dams, locks, canal lining, etc.;

for buildings and light floors;

for floors and road surfaces and foundations;

special purpose: acid-resistant, heat-resistant, especially heavy for biological protection.

The latter are made using cement with special types high bulk density aggregates.

Cement

For the preparation of heavy concrete, ordinary Portland cement, plasticized and hydrophobic, Portland cement with hydraulic additives, Portland slag cement, etc. are used. The characteristics of these cements and the requirements for them are set out in the fourth chapter.

Mixing water

For mixing concrete mixtures and watering concrete, water is used that does not contain harmful impurities that interfere with the normal hardening of concrete - acids, sulfates, fats, vegetable oils, sugar, etc. You cannot use swamp and waste water, as well as water contaminated with harmful impurities, having a pH value less than 4 and containing sulfates (calculated as SO3) more than 0.27%. Sea and other waters containing mineral salts can only be used if the total amount of salts in them does not exceed 2%. The suitability of water for concrete is established chemical analysis and comparative tests of the strength of concrete samples made with this and with clean drinking water and tested at the age of 28 days. when stored in normal conditions. Water is considered suitable if samples prepared with it have a strength no less than samples prepared with clean drinking water.

Sand

Sand is a loose mixture of grains with a particle size of 0.14 to 5 mm, formed as a result of the natural destruction of massive rocks or their crushing (natural sands). In addition to natural sands, artificial sands are used, obtained by crushing or granulating metallurgical and fuel slags or specially prepared materials - expanded clay, agloporite, etc. Fractionated and unfractionated sands can be used.

Coarse aggregate

Gravel or crushed rock from rocks, less often slag and crushed brick, are used as coarse aggregates for heavy concrete.

Gravel is an accumulation of grains measuring 5-70 (150) mm, formed as a result of the natural destruction of rocks. The gravel grain has a rounded shape and smooth surface. For -concrete, the most advantageous are low-rounded, crushed stone-shaped grains, worse are ovoid (rounded), and even worse are lamellar and needle-like grains, which reduce the strength of concrete. The content of lamellar and needle grains in gravel is allowed no more than 15%, and grains of weak (porous) rocks - no more than 10%. According to the grain size, gravel is divided into the following fractions: 5-10, 10-20, 20-40 and 40-70 mm.

Often gravel occurs along with sand. When the gravel contains 25-40% sand, the material is called a sand-gravel mixture.

Crushed stone is produced by crushing massive rocks, gravel, boulders or artificial stones into pieces measuring 5-70 mm. To prepare concrete, crushed stone obtained by crushing dense rocks, crushed stone from gravel and crushed stone from blast furnace and open-hearth slag are usually used.

BASIC PROPERTIES OF CONCRETE MIXTURE AND CONCRETE

Heavy concrete is most often made from Portland cement, quartz sand and gravel or crushed stone from dense rocks. Concrete must acquire design strength by a certain date and have other qualities corresponding to the purpose of the structure being manufactured (water resistance, frost resistance, density, etc.). In addition, a certain degree of mobility of the concrete mixture is required, which would correspond to the accepted methods of its laying.

Each of these components affects the viscoplastic properties of the mixture. So, if you increase the content of aggregates, the mixture becomes more rigid; if cement paste is more plastic and fluid. Significantly affects the properties of the concrete mixture and the viscosity of the cement paste. The more water in the cement paste, the more plastic the dough is and, accordingly, the more plastic the concrete mixture.

One of the main properties of a concrete mixture is thixotropy - the ability to liquefy under periodically repeated mechanical influences (for example, vibration) and thicken again when this influence ceases. The mechanism of thixotropic liquefaction is that when vibrating, the forces of internal friction and adhesion between particles decrease and the concrete mixture becomes fluid. This property is widely used when laying and compacting concrete mixtures.

Figure 9.1. Determination of the mobility of plastic concrete mixtures by cone settlement (OC):

1-supports;2-handles;3-cone shape;4-concrete mixture.

Workability - generalized technical specifications viscoplastic properties of concrete mixture. Workability is understood as the ability of a concrete mixture, under the influence of certain techniques and mechanisms, to easily fit into a mold and be compacted without delamination. The workability of mixtures, depending on their consistency, is assessed by mobility or rigidity.

Mobility serves as a characteristic of the workability of plastic mixtures that can deform under the influence of their own weight. The slump is characterized by the settlement of a standard cone formed from the concrete mixture being tested. To do this, a metal cone mold mounted on horizontal surface, filled with concrete mixture in three layers, compacting each layer with bayonet. The excess mixture is cut off, the cone mold is removed and the settlement of the cone from the concrete mixture is measured - OK (Fig. 9.1), the value of which (in centimeters) serves as an indicator of mobility.

Rigidity- characteristics of the workability of concrete mixtures in which no cone settlement is observed (OK = 0). It is determined by the vibration time (in seconds) required to level and compact a pre-formed cone of concrete mixture using a special device (Fig. 12.3), which is a metal cylinder with a diameter of 240 mm and a height of 200 mm with a stand and rod 6 and a metal disk 4 with six holes. The device is fixed on a standard vibrating platform 1, a cone mold 3 is inserted into it. The cone is filled with concrete mixture in three layers, each layer being bayoneted 25 times. Then the cone shape is removed and, turning the tripod, lowered metal disk 4 onto the surface of the concrete mixture. After this, turn on the vibrator. The time during which the mixture is distributed evenly in the cylindrical form 2 and laitance begins to be released through at least two holes of the disk is taken as an indicator of the hardness of the mixture (W).

Rice. 9.2. Scheme for determining the hardness (H) of a concrete mixture:

a - the device in the initial position; b - the same, at the end of the tests; 1 - vibration platform; 2 - cylindrical shape; 3- concrete mixture; 4 - disk with holes; 5- bushing; b - rod; 7 - concrete mixture after vibration

Depending on workability, rigid and flexible concrete mixtures are distinguished (Table 9.1).

Rigid concrete mixtures contain a small amount of water and, accordingly, a reduced amount of cement in comparison with mobile mixtures of concrete of equal strength. Rigid mixtures require intense mechanical compaction: prolonged vibration, vibratory compaction, etc. Such mixtures are used in the manufacture of prefabricated reinforced concrete products in factory conditions (for example, at house-building factories); In construction conditions, rigid mixtures are rarely used.

Table 9.1. Classification of concrete mixtures by workability

Movable mixtures are characterized by high consumption of water and, accordingly, cement. These mixtures are a thick mass that easily liquefies when vibrating. Mixtures of grades PZ and P4 are fluid; under the influence of gravity they fill the mold without requiring significant mechanical effort. Movable mixtures can be transported by concrete pumps through pipelines.

Cohesion is the ability of a concrete mixture to maintain a homogeneous structure, that is, not to delaminate during transportation, laying and compaction. Under mechanical influences on the concrete mixture, as a result of its thixotropic liquefaction, part of the water, as the lightest component, is pressed upward. Coarse aggregate, the density of which is usually greater than the density of the mortar part (a mixture of cement, sand and water), sinks down (Light aggregates (expanded clay, etc.), on the contrary, can float. All this makes the concrete heterogeneous, reducing its strength and frost resistance.

STRENGTH, GRADE AND CLASS OF CONCRETE

Heavy concrete- the main structural building material, therefore, much attention is paid to assessing its strength properties. The strength characteristics of concrete are determined strictly in accordance with the requirements of the standards. Several indicators are used to characterize the strength of concrete. The heterogeneity of concrete as a material is taken into account in the main strength characteristic - the class of concrete.

Strength. Like all stone materials, the tensile strength of concrete in compression is significantly (10...15 times) higher than in tension and bending. Therefore, in building structures, concrete, as a rule, works in compression. When people talk about the strength of concrete, they mean its compressive strength.

Portland cement concrete gains strength gradually. At normal temperatures and constant humidity, the growth of concrete strength continues for a long time, but the rate of strength gain fades over time.

The strength of concrete is usually assessed by the arithmetic average of the test results of samples of this concrete after 28 days of normal hardening. For this purpose, cube samples measuring 150 x 150 x 150 mm are used, made from a working concrete mixture and hardened at (20 ± 2) ° C in air at relative humidity 95% (or under other conditions that ensure moisture retention in concrete). Methods for determining the strength of concrete are regulated by the standard.

Brand of concrete. Based on the arithmetic average value of the strength of concrete, its grade is determined - the rounded strength value (and the rounding always goes down). For heavy concrete, the following compressive strength grades are established: 50, 75, 100, 150, 200, 250, 300, 350, 400, 450, 500, 550, 600, 700 and 800 kgf/cm2. When designating a brand, use the index “M”; for example, the concrete grade M350 means that its average strength is at least 35 MPa (but not more than 40).

Distinctive feature concrete - significant heterogeneity of its properties.

This is explained by variability in the quality of raw materials (sand, coarse aggregate and even cement), violation of the regime for preparing the concrete mixture, its transportation, and laying

(degree of compaction) and hardening conditions. All this leads to variations in the strength of concrete of the same brand. The higher the production culture ( better quality preparation of materials, preparation and laying of concrete, etc.), the smaller the possible fluctuations in the strength of concrete will be. For the builder, it is important to obtain concrete not only with a given average strength, but also with minimal deviations (especially downward) from this strength. An indicator that takes into account possible fluctuations in the quality of concrete is the concrete class.

Concrete class- this is a numerical characteristic of any of its properties (including strength), accepted with guaranteed security (usually 0.95). This means that the property specified by the class, for example the strength of concrete, is achieved in at least 95 cases out of 100.

The concept of “concrete class” allows you to assign concrete strength taking into account its actual or possible variation. The less the variability of strength, the higher the class of concrete at the same average strength.

GOST 26633-85 establishes the following classes of heavy concrete in terms of compressive strength (MPa): 3.5; 5; 7.5; 10; 12.5; 15; 20; 25; thirty; 32.5; 40; 45; 50; 55 and 60. The compressive strength class is designated by the Latin letter B, to the right of which its guaranteed strength in MPa is assigned. Thus, class B15 concrete has a compressive strength of at least 15 MPa with a guaranteed strength of 0.95.

The relationship between classes and grades of concrete is ambiguous and depends on the homogeneity of concrete, assessed using the coefficient of variation. The lower the coefficient of variation, the more homogeneous the concrete. The class of concrete of the same grade increases noticeably with a decrease in the coefficient of variation. So, with a concrete grade of M300 and a coefficient of variation of 18%, the concrete class will be B15, and with a coefficient of variation of 5% - B20, i.e. a whole step higher. This shows how important it is to carefully carry out all technological operations and improve production standards. Only in this case is high homogeneity of concrete and a higher class of its strength achieved with constant cement consumption.

Construction standards have adopted a standard coefficient of variation in concrete strength equal to 13.5% and characterizing the technology concrete works as satisfactory.

The relationship between classes of concrete in terms of compressive strength and its grades with a standard coefficient of variation equal to 13.5% is given in Table. 9.2.

Table 9:2. The relationship between brands and classes of heavy concrete by strength with a coefficient of variation of 13.5%

BASIC PROPERTIES OF HEAVY CONCRETE

The main properties of heavy concrete, in addition to strength, include: porosity, deformability (modulus of elasticity, creep, shrinkage), water permeability, frost resistance, thermophysical properties, etc.

Deformability concrete. Concrete under load does not behave like a perfectly elastic body (for example, glass), but like an elastic-visco-plastic body (Fig. 9.3). At low stresses (no more than 0.2 of the ultimate strength), concrete deforms like an elastic material. Moreover, its initial modulus of elasticity depends on porosity and strength and is 10 4 MPa for heavy concrete (2.2...3.5) (for highly porous cellular concrete the elastic modulus is about 10 4 MPa).

Fig.9.3. Stress curve Fig. 9.4. Development of concrete deformations

in coordinates σ - ε in time: ε initial - initial deformation of concrete

at the moment of loading; ε p - def. creep

At high stresses, plastic (residual) deformation appears, developing as a result of the growth of microcracks and plastic deformations of the gel component of the cement stone.

Creep- the tendency of concrete to increase plastic deformations under prolonged action of static load. Creep of concrete is also associated with the plastic properties of the cement gel and micro-crack formation. It has a decaying nature over time (Fig. 9.4). The absolute values ​​of creep depend on many factors. Creep develops especially actively if concrete is loaded in early age. Creep can be assessed in two ways: as a positive process, helping to reduce stresses arising from thermal and shrinkage processes, and as a negative phenomenon, for example, reducing the effect of prestressing reinforcement.

Shrinkage- the process of reducing the size of concrete elements when they are in air-dry conditions. The main reason for shrinkage is compression of the gel component due to loss of water.

The higher the volume of cement paste in concrete, the higher the shrinkage of concrete (Fig. 9.5). On average, the shrinkage of heavy concrete is 0.3...0.4 mm/m.

Rice. 9.5. Shrinkage curves when hardening in air: 1-cement stone, 2-mortar, 3-concrete

Due to concrete shrinkage in concrete and reinforced concrete structures Large shrinkage stresses may occur, so the elements long distance cut with shrinkage seams to avoid cracks. If concrete shrinkage is 0.3 mm/m in a structure 30 m long, the total shrinkage will be 10 mm. Shrinkage cracks in concrete at the contact with the aggregate and in the cement stone itself can reduce frost resistance and serve as sources of concrete corrosion.

Porosity. Strange as it may seem, such a dense-looking material has noticeable porosity. The reason for its occurrence, as has been said more than once, lies in the excess amount of mixing water. Concrete mixture after correct installation is a dense body. During hardening, part of the water is chemically bound by the minerals of the cement clinker (for Portland cement, about 0.2 by weight of cement), and the remaining part gradually evaporates, leaving behind pores. In this case, the porosity of concrete can be determined by the formula

P = [(V - ώ C)/1000] 100,

where V and C are the consumption of water and cement per 1 m 3, ώ is the amount of chemical bound water in fractions of the mass of cement.

Thus, at the age of 28 days, cement binds 17% of its mass of water; The water consumption in this concrete is 180 kg, and the cement consumption is 320 kg. Then the porosity of this concrete will be:

P = [(180 - 0.17-320)/1000] 100 = 12.6%.

This is the total porosity, including gel micropores and capillary pores (we do not consider the volume of entrained air). From the point of view of the influence on the permeability and frost resistance of concrete, the number of capillary pores is important. The relative volume of such pores can be calculated using the formula, %:

P k = [(V -2 ώ C)/1000]100

For our case, the number of capillary pores will be 7.3%.

Water absorption and permeability. Thanks to its capillary-porous structure, concrete can absorb moisture both upon contact with it and directly from the air. Hygroscopic moisture absorption in heavy concrete is insignificant, but in lightweight concrete (and especially in cellular concrete) it can reach 7...8 and 20...25%, respectively. "

Water absorption characterizes the ability of concrete to absorb moisture in a drop-liquid state; it depends mainly on the nature of the pores. The greater the number of capillary interconnected pores in the concrete, the greater the water absorption. The maximum water absorption of heavy concrete with dense aggregates reaches 4...8% by weight (10...20% by volume). For lightweight and cellular concrete this figure is significantly higher.

High water absorption negatively affects the frost resistance of concrete. To reduce water absorption, they resort to hydrophobization of concrete, as well as vapor and waterproofing of structures.

The water permeability of concrete is determined mainly by the permeability of the cement stone and the contact zone “cement stone - aggregate”; In addition, microcracks in the cement stone and defects in the adhesion of reinforcement to concrete can be the paths for filtration of liquid through concrete. The high water permeability of concrete can lead to its rapid destruction due to corrosion of the cement stone.

To reduce water permeability, it is necessary to use fillers of appropriate quality (with a clean surface), as well as use special sealing additives (liquid glass, ferric chloride) or expanding cements. The latter are used for concrete waterproofing.

Based on water resistance, concrete is divided into grades W2; W4; W6; W8 and W12. The mark indicates the water pressure (kgf/cm2), at which a 15 cm high cylinder sample does not allow water to pass through during standard tests.

Frost resistance- the main indicator that determines durability concrete structures in our climate. The frost resistance of concrete is assessed by alternately freezing at minus (18 ± 2) ° C and thawing in water at (18 ± 2) ° C, samples of the tested concrete previously saturated with water. The duration of one cycle is 5... 10 hours depending on the size of the samples.

The frost resistance grade is taken to be the greatest number of “freezing-thawing” cycles that the samples can withstand without reducing the compressive strength by more than 5% compared to the strength of the control samples at the beginning of the tests. The following frost resistance grades of concrete have been established: F25, F35, F50, F75, F100...1000. The standard also provides for accelerated test methods in salt solution or deep freezing to minus (50 ± 5) ° C.

The reason for the destruction of concrete under the conditions under consideration is capillary porosity (Fig. 12.16). Water enters the concrete through capillaries and, freezing there, gradually destroys its structure. Thus, concrete, the porosity of which we calculated higher, in accordance with Fig. 12.16 must have frost resistance F150...F200.

To obtain concrete with high frost resistance, it is necessary to achieve a minimum capillary porosity (not higher than 6%). This is possible by reducing the water content in the concrete mixture, which in turn is possible by using:

Rigid concrete mixtures that are intensively compacted during installation;

Plasticizing additives that increase the workability of concrete mixtures without adding water.

Thermophysical properties.

Of these, the most important are thermal conductivity, heat capacity and temperature deformation.

The thermal conductivity of heavy concrete, even in an air-dry state, is high - about 1.2-1.5 W/(m K), i.e. 1.5...2 times higher than that of brick. Therefore, heavy concrete can only be used in enclosing structures in conjunction with effective thermal insulation. Lightweight concrete (see § 12.7), especially cellular concrete, has a low thermal conductivity of 0.1...0.5 W/(m K), and their use in enclosing structures is preferable.

The heat capacity of heavy concrete, like other stone materials, is in the range of 0.75...0.92 J/(kg K); on average - 0.84 J/(kg K).

Temperature deformations. Temperature coefficient of linear expansion of heavy concrete (10...12) Yu DS1. This means that with an increase in concrete temperature by 50° C, the expansion will be approximately 0.5 mm/m. Therefore, in order to avoid cracking, long-term structures are cut with expansion joints.

Large temperature fluctuations can cause internal cracking of concrete due to the different thermal expansion of coarse aggregate and cement stone.

LIGHTWEIGHT CONCRETE

Significant disadvantage usually heavy concrete - high density (2400...2500 kg/m3). By reducing the density of concrete, builders achieve at least two positive results: weight is reduced building structures; their thermal insulation properties increase.

Lightweight concrete (at the beginning of the 20th century they were called “warm concrete”) - concrete with a density of less than 1800 kg/m3 - a universal material for enclosing and load-bearing structures of residential and industrial buildings. Most wall panels and blocks, roofing slabs and stones for laying walls are made from them. The term “lightweight concrete” unites a large group of concretes with different composition, structure and properties.

Based on their intended purpose, lightweight concrete is divided into:

structural (strength class - B7.5...B35; density - 1800 kg/m3);

structural and thermal insulation (strength class not less than ВЗ,0, density -600...1400 kg/m3);

thermal insulation - especially light (density< 600 кг/м3).

Based on the structure and method of obtaining the porous structure, lightweight concrete is divided into the following types:

continuous concrete with porous aggregates;

cellular concrete, which contains neither coarse nor fine aggregate, and their role is played by small spherical pores (cells);

large-porous, in which there is no fine aggregate, as a result of which voids are formed between the particles of the coarse aggregate.

For lightweight concrete, the following strength classes (MPa) are established: from B2 to B40. The strength of lightweight concrete depends on the quality of the aggregates, the brand and amount of cement used. In this case, naturally, the density of concrete also changes.

For lightweight concrete, 19 density grades (kg/m3) are established from D200 to D2000 (with an interval of 100 kg/m3). A reduced density of lightweight concrete can be achieved by porous cement stone.

The thermal conductivity of lightweight concrete depends on its density and humidity (Table 9.3). An increase in volumetric humidity by 1% increases the thermal conductivity of concrete by 0.015...0.035 W/(m K).

Table 9.3. Average thermal conductivity values ​​of lightweight concrete

Frost resistance of lightweight concrete when it is porous

Silicate materials are materials made from mixtures or alloys of silicates, polysilicates and aluminosilicates. Silicates are compounds of various elements with silica (silicon oxide), in which it plays the role of an acid. The structural element of silicates is a tetrahedral orthogroup -4 with a silicon atom Si +4 and oxygen atoms O -2 at the vertices of the tetrahedron, with edges 0.26 nm long. Tetrahedra in silicates are connected through common oxygen vertices into silicon-oxygen complexes in the form of closed rings, chains, networks and layers. Aluminosilicates, in addition to silicate tetrahedra, contain tetrahedra [AlO 4 ] -5 with at.Al +3.

Complex silicates also include cations: Na+, K+.Ca++, Mg++, Mn++, B +3, Cr +3, Fe +3, Al +3, Ti +4 and anions: O 2 –2, OH-, F- ,Cl-,SO 4 – 2, as well as water.

Most silicates are refractory and fire-resistant; their melting point ranges from 770 to 2130 0 C. Chem. The composition of silicates is usually expressed in the form of formulas, comp. From the symbols of their molecules, arranged in order of increasing valency, or from the formulas of their oxides: feldspar K 2 Al 2 Si 6 O 16.

All silicates are divided into natural (minerals) and synthetic (silicate materials). Synthetic ones are divided into: binders, ceramics, non-silicate materials, glass, glass ceramics. Natural silicates In decomposition Areas of the national economy: In technological processes based on roasting and smelting (clay, quartzite, feldspar, etc.); in hydrothermal treatment processes (asbestos, mica, etc.); in construction; in metallurgical processes.

The raw materials for the production of silicate materials are natural minerals (quartz sand, clays, feldspar, limestone), industrial products (sodium carbonate, borax, oxides and salts of decomposed metals) and waste (slag, sludge, ash).

In the production of silicate materials, standard technological processes are used, which is due to the similarity of the physical and mathematical principles of their production. Stage diagram:

Raw materials - preparation of the charge - formation of the product from the charge - drying ed. – high temperature Processing – material.

Batch preparation is necessary to ensure high efficiency of subsequent high-temperature preparation processes and consists of conventional mechanical operations for the preparation of solid raw materials: grinding, classification, drying, mixing components.

The molding operation must ensure the production of a product of a given shape and size, taking into account their changes in subsequent drying and high-temperature processing operations. Forming involves moistening the charge and giving the material a certain shape.

Drying is carried out to maintain the product's given shape before and during the high-temperature processing operation, which is the final stage of the production of silicate materials. High-temperature treatment involves firing or cooking the charge (product). Processes of high molecular weight processing: 1) removal of water, first physical, then crystallization; 2) calcination, i.e. separation of water and CO 2 from the charge components; 3) charge components - metal carbonates, metal hydroxides and aluminosilicates are converted into acid oxides: SiO2, B2O3mAl2O3, Fe2O3 and basic oxides: Na 2 O, K 2 O, CaO, MgO, reacting with each other; 4) sintering of charge components. It can leak into TV. Phase, at a temperature below the melting point, or in the liquid phase, at a temperature above the melting point. In the second case, due to the diffusion process, the process speed is higher; 5) cooling of the mass with the formation of crystalline and amorphous phases.

Production of ceramics. Ceramic materials are polycrystalline materials and products made from them, obtained by sintering clays and their mixtures with mineral additives, as well as metal oxides and other refractory compounds. Classification: By composition - oxygen-containing (silicate), oxygen-free (carbide, nitride, boride, silicide); By application: construction, refractories, thin ceramics, special. Ceramics; according to the degree of sintering - porous (brick, refractories, sanitary ware), sintered (porcelain, special ceramics); according to surface condition - glazed and unglazed. Raw materials for production must have the property of sintering - the property of a powdered material to form a polycrystalline body - a shard - when heated. Raw materials - clays, quartz sand, calcium and magnesium carbonates.

The technological process for brick production is 2 options: plastic method and semi-dry. The charge, containing 40-50% clay, 50% sand and up to 5% iron oxide, is pressed into a belt press (plastic method) or into a mechanical press, works. under pressure 10-25 MPa (semi-dry method). The formed brick is sent for drying in a tunnel dryer and then fired at a temperature of 900-1000 0 C.

Plastic molding is carried out on a belt press. It consists of 1. a loading funnel; 2. rollers; 3.auger;. As the mass moves toward the mouthpiece 4. of the press, it is additionally mixed and compacted. Water is supplied from the humidifier 5. to wet the mouthpiece, acting as a lubricant. The clay mass in the form of a ribbon 6. is cut into bricks using a cutting machine. 7. support rollers.

Semi-dry brick production scheme:

Refractories are non-metallic materials characterized by increased fire resistance, that is, the ability to withstand high temperatures. Refractories are divided into: 1. aluminosilicate; 2. Dinas refractories - comp. No less than 95% silicon oxide; 3. semi-acid - up to 70-80% silicon oxide and 15-25% aluminum oxide. 3. Fireclay refractories - up to 50-70% silicon oxide and up to 46% aluminum oxide. Fireproof up to 1750 0 C.

Scheme and leveling.

4. High-alumina refractories - more than 45% aluminum oxide.

5. magnesite - magnesium oxide as a base. Fireproof up to 2500 0 C.

CaCO 3 + MgCO 3 = MgO + CaO + 2CO 2

6.corundum refractories; 7.Carborundum - composition. Silicon carbide;7. zirconium and thorium; 8. carbon.