Which cement plants consume fly ash. Recommendations for the use of ash, slag and ash and slag mixture of thermal power plants in heavy concrete and mortars . The use of ash, slag and ash and slag mixture in heavy concrete

fly ash- these are fine particles produced during the combustion of mineral substances of fuel in furnaces, with a high melting point, reaching its maximum up to +800° . Small dusty flakes, due to their low specific gravity, do not fall into the pan, but escape into the surrounding air space.

To collect fine dust, special traps are used that draw in and accumulate combustion waste.

AT CHP fly ash received in greater quantities than anywhere else. In the future, this component is widely used in many sectors of the national economy. In particular, factories engaged in the production of concrete, use fly ash to significantly improve its performance.

Fly Ash for Concrete Concrete

A positive change in the basic characteristics of the concrete mixture occurs due to the correct composition of the proportions of the ingredients included in the composition, and its thorough kneading to a dense, homogeneous mass. Fly ash partly reduces the cost of the composite by replacing more expensive components. In some cases it is 25% from the entire volume of the mixture, partially replacing expensive cement.

Fly ash-mixed concrete saves significant laying labor costs, as it becomes more pliable and easily placed on the surface, or more efficiently and densely fills the required void. When working with enriched concrete, the need for water is significantly reduced, and the mass looks homogeneous and becomes workable compared to other analogues.

Experts note increased strength, durability, improved resistance to water, including aggressive structures made of concrete with the addition of fly ash. The time of the appearance of the first cracks and ruptures is significantly "moved away". That is why such an additive is justified and rational.

It has found wide application in almost all branches of the national economy. It is used to strengthen the coastline, construction of dams, dams, moorings and ports. In the construction industry, slag concrete and blocks and other products are often used, which are much “hardier” than their counterparts.

Our company sells high-quality products for factories engaged in the manufacture of concrete fly ash on price, below that installed to date in the sales markets. All products comply with GOST and are accompanied by the necessary documents.

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fly ash price

Fly ash is supplied in a big bag weighing 400-450 kg. The cost of one big bag is 2000 rubles. With a volume of 20 bags - 1700 rubles.

The composition and structure of ash depend on many factors, such as the type of fuel burned, its ash content, fineness of grinding during its preparation, the chemical composition of the mineral part of the fuel, etc.

Fly ash is applied:

To improve the properties of heavy concretes: instead of part of the sand, as an independent component and instead of part of the cement.

In the production of lightweight concrete. Low-cement concretes are used in the preparation of road foundations. Fly ash is also used in slag-silicate concretes, which are used to repair roads, airfields, bridges, as well as for acid-resistant floors, in chemical shops, livestock complexes, and metallurgical industries.

In the production of foam concrete, its introduction into the foam concrete mixture increases the aggregative stability of the mixture in the period between the beginning and end of the setting of the cement paste, which helps prevent the movement of components and prevent a negative impact on the formation of the structure.

Fly ash easily replaces cement in the production of mortars, ready-mixed concrete, finished products. It is used as an additive to cement, while not reducing its activity, used in the preparation of concrete for road construction, as well as an additive to clay during the manufacture of tiles and bricks.

Workability.

The effect of fly ash is the greater, the smaller the particles (in cements on fly ash, the specific surface area of ​​particles reaches 5000 cm2/g). In each individual case, there is an optimal dosage of ash, which allows you to get the best workability. It is easy to determine the optimum - we build a graph, where the abscissa shows the ash dosage in percent, and the y-axis shows the plasticity of the mixture, determined by the shaking table spread test

Thus, the introduction of ash into the composition of the mixture makes it possible to reduce the consumption of mixing water with the same workability, increase the uniformity and density of the concrete mixture based on M-500 cement and improve its laying, and create the best conditions for stripping.

Adding ash to the concrete mixture (within 30-100 kg/m3) improves its granulometry and ultimately corrects the composition of sands that lack fines. Ash can even replace part of the sand (for example, by 20-30%). The introduction of ash under construction conditions is especially advisable in the presence of a hard, lean mixture with a small consumption of cement. The increased ash content accelerates the setting time, which can also be adjusted with the help of additives in the cold season. Setting times noticeably change at normal temperature (20±5°C) for mixtures of the same plasticity with an ash content of up to 20-30%. Recent studies have shown that there are very effective hardening accelerators used in the cold season, which can reduce the setting time and increase mechanical strength at a very early age. Among them, sodium aluminate 2 Na20-Al203 and caustic soda NaOH proved to be the best. These additives are used in the range of 0.2-0.5% in terms of sodium. A higher content of sodium aluminate or silicate causes a kind of gelation in the spaces filled with water. Such a change in the shear threshold can be useful in the manufacture of concrete products with immediate stripping.

Reducing the heat of hydration.

The heat of hydration released during setting decreases in proportion to the ash content. This property is of interest when concreting massive structures in the hot season.

Capillary absorption and frost resistance.

Capillary water absorption with the addition of fly ash to cement increases by about 10-20% for every 10% of ash. Laboratory tests have found that frost resistance is slightly reduced. This decrease is very insignificant at a content of 20% ash and does not exceed the permissible limits with equal workability of the dough. It is known, however, that frost resistance can be improved by means of air entrainment. The best protection of hardened concrete from freezing is the introduction of air-entraining additives. In general, fly ash cements require a slightly higher addition to achieve the same amount of entrained air. The reason is undoubtedly the absorption of the ash part of the surfactant additive (the carbon of the ash fixes the hydrophobic region of the surfactant molecules).

Resistant to aggressive water.

It can be stated that the use of cements containing 20% ​​ash, or the introduction of fly ash into the concrete mixture, increases the resistance of the material in aggressive waters when completely immersed (in sea or sulfate water). This increase is explained by the fineness of the ash, the increase in the absolute volume of the binder, the presence of lime in small amounts and, most importantly, the decrease in the content of clinker tricalcium aluminate (the main element contributing to the destruction under the influence of sulfates).

Conclusion on the advantages and disadvantages of using goldcarryover.

The use of fly ash gives the following advantages: reducing the cost of the binder; some improvement in grinding; some increase in final strength; improved workability, easier demoulding; decrease in shrinkage and decrease in initial heat release during hydration; lengthening of the period of cracking when tested by the method of the ring; increased resistance to pure and sulfate waters; reduction in the volumetric mass of concrete; increased fire resistance and resistance to thermal shock; lower consumption of clinker and cheaper binder.

Among the disadvantages of the use of ash, one should note the change in the color of the cement (this applies to ash with a high content of underburning, but this content is very low in modern coal-fired power plants); decrease in initial strength, especially at low temperatures, although cement with ash can be subjected to finer grinding, slightly reducing water consumption at the same workability (very effective accelerator additives are currently known); decrease in frost resistance, although there are means to increase it (air-entraining additives). In addition, the use of ash increases the number of components of the mixture to be controlled.

In conclusion, it should be noted that the advantages of fly ash far outweigh the disadvantages mentioned above.

A little about coals.

There are several large coal basins in Russia.

Kuznetsk coal basin (Kemerovo region) is one of the largest coal deposits in the world. The coals are varied in quality and are among the best coals. In deep horizons, coals contain: ash 4-16%, moisture 5-15%, phosphorus up to 0.12%, volatile substances 4-42%, sulfur 0.4-0.6%; have a calorific value of 7000–8600 kcal/kg (29.1–36.01 MJ/kg); coals lying near the surface are characterized by a higher moisture content, ash up to 30% and a lower sulfur content.

East Siberian coals

Irkutsk coal basin located in the southern part of the Irkutsk region.

Khakass coals

The Minusinsk coal basin is located in the Minusinsk basin (Republic of Khakassia). The largest of them are the Chernogorsk and Izykh coal deposits. The basin is dominated by long-flame hard coals with a calorific value of 31-37 MJ/kg. The sulfur content rarely exceeds 1%. Coals are classified as medium ash, while the maximum ash content (11-29.7%) is typical for the coals of the Izykh deposit, the minimum (6.6-17.8%) - for the coals of the Beyskoye deposit.

Kansk-Achinsk coal basin

Located in the Krasnoyarsk Territory, the basin has the most significant reserves of thermal brown coal, which is mainly mined in an open way.
The average ash content is 7 - 14%, the calorific value is 2800 - 3800 kcal / kg, low sulfur.

Ekibastuz coal basin (Kazakhstan) is one of the most significant in terms of reserves and ranks first in the world in terms of coal density: on an area of ​​62 square kilometers, coal reserves are estimated at 13 billion tons or 200 tons per square meter. And for open-pit coal mining is one of the most promising areas in the world. The ash content of hard coal supplied to Russia for energy enterprises reaches 40-50%. The main consumers of coal from this basin are in the Urals.

Depending on the degree of coalification (metamorphism), there are brown coals, hard coals and anthracites. Brown coals have the lowest calorific value, and anthracites have the highest. The most favorable ratio of price and specific heat of combustion is coal. Coal grades D, G and anthracites are used, as a rule, in boiler houses, because. they can burn without blowing. Coal grades SS, OS, T are used to generate electrical energy, because. they have a high calorific value, but the combustion of this type of coal is associated with certain technological difficulties, which are justified only if it is necessary to use a large amount of coal. In ferrous metallurgy, grades G, Zh are usually used for the production of steel and cast iron.

Depending on the type of coal, its deposits, place and method of combustion, completely different ash is obtained at the output.

Ashes obtained from brown coals of the Kansk-Achinsk basin have a very great potential in the construction industry, as they contain a large amount (up to 40-50%) of ash binder. The binder isolated from lignite ash is almost ideal for the production of the entire range of lightweight and extra-light concretes: foam concrete, aerated concrete, etc.

The only disadvantage of the ash binder is the presence of free calcium, which serves as a time bomb. Without removing free calcium, you risk getting self-destructive building materials, the removal of free calcium occurs by adding a 1% CaCl2 solution. Also, brown coal ash contains magnetite up to 3-5%, which is used as a filler for especially heavy concretes, or as a raw material for metallurgy. The products remaining from brown ash have the value of sand, unburned coal can be sent back to the boiler.

Research and practice have established the effectiveness of the introduction of dry dust-like ashes in the manufacture of concrete and mortar mixtures as active mineral additives and microfillers.

Concrete mixtures with ash have greater cohesion, better pumpability, less water segregation and stratification. At the same time, concrete has greater strength, density, water resistance, resistance to certain types of corrosion, and lower thermal conductivity.

Most effective as active additives in concrete acidic ash that does not have astringent properties; their pozzolanic activity is manifested in interaction with the cement binder. Depending on this characteristic in relation to a particular cement, the water demand and workability of the concrete mixture, the conditions and duration of hardening, it is possible to significantly reduce the consumption of cement.

The optimal ash content (kg/m3) for concrete is: steamed - about 150; normal hardening - 100. In accordance with the well-known recommendations, the use of 150 kg of fly ash per 1 m3 of heavy concrete of classes B7.5-VZO saves 40-80 kg of cement. In heat-treated concrete, the use of ash makes it possible to save up to 25% of cement.

Significant practical experience in the use of fly ash in concrete has been accumulated in hydraulic engineering construction. At present, the effectiveness of replacing 25-30% of Portland cement with fly ash for concrete of the internal zones of massive hydraulic structures and 15-20% for concrete in the underwater parts of structures has been proven. In a number of cases, the expediency of increasing the content of fly ash in hydrotechnical concrete to 50-60% by weight of cement has been substantiated. When replacing with ash up to 40% of cement during their joint grinding, the strength of concrete after 28 days is close, and after 60 days it is almost equal to the strength of concrete without additives.

For the first time in 1961, a pilot-production laying of concrete with the addition of 15-20% fly ash was carried out in the body of the dam of the Bratsk hydroelectric power station. Approximately 5,000 m3 of concrete with ash was laid, which, in terms of its basic physical and mechanical characteristics, did not differ from concrete without the addition of ash.

During the construction of the Dniester hydroelectric complex, the introduction of 25% ash into the binder did not reduce the strength characteristics of hydrotechnical concrete at the age of 180 days and made it possible to increase the efficiency of cement use.

Currently, fly ash is increasingly used in the production of prefabricated reinforced concrete structures. Dry ash is introduced into concrete of classes B7.5-B40 in an amount of up to 20-30% by weight of cement. However, with an excessive ash content, swelling of the surface of the steamed products is possible.

One of the essential characteristics of ash as an active mineral additive in concrete is its hydraulic activity. By traditional methods, it is determined by the ability of the ashes to absorb lime from the lime mortar, as well as to exhibit astringent properties in combination with hydrated lime. An accelerated method for determining the activity of ash is the microcalorimetric method, according to which the activity of ash is determined by the value of the heat of wetting in polar and non-polar liquids, taking into account the hydrophilic coefficient and a number of other parameters.

The requirements for ash, as active mineral additives in the concrete mixture, are determined by the physicochemical mechanism of their influence on the processes of hardening and structure formation of concrete. The hydraulic activity of sols, as well as other substances of the pozzolanic type, is largely due to the chemical interaction of the silicon and aluminum oxides included in them with calcium hydroxide released during the hydrolysis of clinker minerals, with the formation of hydrosilicates and calcium hydroaluminates. The hydration of the ashes is promoted by their vitreous phase, the crystalline phase in this process is practically inert. The chemical activity of ashes is also directly related to their dispersion.

According to modern concepts, the strength of cements and concretes with the addition of ash depends on the thickness of the surface layer of the ash particle affected by chemical processes.

The positive effect of ash on the structure formation of concrete is also facilitated by the “effect of fine powders”, which expands the free space in which hydration products are deposited, which accelerates the cement hardening process.

The current regulatory documents allow the use of fly ash as an additive for the preparation of concrete for prefabricated and monolithic structures of buildings and structures, except for structures operated in environments with medium and strong aggressiveness.

Depending on the field of application, ash is divided into types: I - for reinforced concrete structures and products; II - for concrete structures and products; III - for structures of hydraulic structures. Within individual types, ash classes for concrete are additionally distinguished: A - heavy; B - lung.

The specific surface of class A ash must be at least 2800 cm2/g, and class B - 1500-4000 cm2/g. The residue on the No. 008 sieve for class A ash should not exceed 15% by weight. According to the chemical composition, the ash is subject to the requirements indicated in Table. 3.13. Humidity of dry selection ash should be no more than 3%.

For use in concrete, samples from a mixture of ash and cement are tested by boiling in water for uniform volume changes.

The selection of concrete compositions with the addition of ash consists in determining such a ratio of components, including ash, in which the required properties of the concrete mixture and concrete are achieved with a minimum consumption of cement. In the concrete mixture, ash plays the role of not only an active mineral additive that increases the amount of binder, but also a microfiller that improves sand granulometry and actively influences the processes of concrete structure formation. Taking into account the multifunctional nature of the ash additive, its introduction only instead of part of the cement or part of the sand does not allow solving the problem of optimizing the compositions.

Reducing the consumption of cement with the introduction of fly ash is primarily advisable in case of “excessive activity” of cement, i.e., in cases where the brand of cement used is higher than recommended. When using TPP ash, it is allowed to reduce the minimum typical consumption rate of cement for non-reinforced concrete products to 150 kg / m3, and for reinforced concrete - up to 180 kg / m3. The total consumption of cement and ash should be at least 200 and 220 kg/m3, respectively. The amount of ash is assigned in proportion to the required percentage reduction of the "excessive activity" of the cement.

The introduction of fly ash in the optimal amount does not increase the water demand of concrete mixtures, which is explained by the melting and relatively regular shape of the grains. With a high dispersion of ash and a low content of unburned coal in it, the workability of the mixture increases. The plasticizing effect of ash increases if there is a fine aggregate in the concrete mixture with an insufficient amount of fine fractions.

A number of researchers believe that spherical ash particles can be considered as solid “ball bearings” in the mixture; they, similarly to emulsified air bubbles, when using air-entraining additives, have a plasticizing effect on the concrete mixture.

An increase in the dispersity of ashes and a decrease in their water demand can be achieved by selecting them from the last fields of electrostatic precipitators or by grinding, destroying their constituents. organomineral aggregates.

The introduction of fly ash helps to reduce the water separation of the concrete mixture. The plasticizing and water-retaining capacity of ash determines the prospects for its use in cast concrete.

Concrete mixtures with the optimal addition of ash have a sufficiently high viability and are suitable for transportation over long distances.

The effect of ash on the strength of concrete depends on its properties and dispersion, content and chemical-mineralogical composition of cement, age and conditions of concrete processing. To assess the effect of ash on the strength of concrete, the concept of its "cementing efficiency" was introduced, which is characterized by the coefficient Kc e.

The cementing efficiency of fly ash characterizes the amount of cement in kg. replaceable without reducing the strength of concrete 1 kg of ash. It has been established that, similarly to the cement-water (or water-cement) ratio known in concrete technology, which establishes an unambiguous relationship between this parameter and the strength of concrete, the reduced C/W rule is true.

Having determined the value (C/V) pr and setting the optimal ash content with a known value of Kc e, it is possible to find the required (C/V) ash-containing concrete and design their compositions.

Most researchers note the positive effect of increasing the dispersion of ash on the strength of concrete. It has been established that the activity of ash increases significantly when the size of its particles is increased to 5-30 microns. The product of the specific surface of ash and the content of the vitreous phase in it is close to the coefficient K in the Feret formula, with which the strength of concrete is directly proportional. In accordance with the Feret formula, the compressive strength of concrete at the age of 28 days:

where Vu is the volume of cement; VB is the volume of water; A is the volume of air.

Having studied the strength of solutions from cements obtained by mixing clinker and ash, crushed to specific surface values ​​of 2500-6400 and 3000-8000 cm2 / g, respectively, M. Venyua established the necessary correspondence between the granulometric composition of ash and the fineness of grinding of clinker. The most significant increase in the dispersion of ash affects the strength of concrete at an early age.

Compared to separate grinding, the best results were obtained with joint grinding of cement and ash. Joint grinding made it possible to substantiate the possibility of obtaining a three-component binder (35% cement - 25 ash - 40 slag), the compressive strength of which is 84 after 60 days, and 90% of the strength of concrete on cement without additives in tension.

A significant effect from the increase in dispersion is observed after the heat and moisture treatment of concrete, which is weakened by the age of 28 days.

It is characteristic that the effect of ash dispersion on the strength of concrete is much stronger than that of cement. This is due to the plasticizing effect of fine ash fractions on concrete mixtures, despite the possible increase in the normal density of ash-containing elements. Regrinding even low-level ashes up to 4000-5000 cm2D allows saving 20-30% of cement without reducing the class of concrete. More appropriate is wet regrinding, in which the ash is not dried and a higher dispersion is achieved.

In the early stages of hardening (up to 28 days), especially with the introduction of coarse ash, the strength of concrete decreases, although not in proportion to the amount of additive, then leveling is observed, and sometimes higher strength is observed in concrete with an ash additive.

To achieve high strength of ash-containing concrete, the chemical and mineralogical composition of clinker is of certain importance. At an early age, an increase in the strength of concrete is facilitated by an increased content of alkalis in the clinker, which accelerates the chemical interaction of ash and cement; later, for the manifestation of the pozzolanic reaction of ash, cements with a high content of alite are preferable, which, upon hydrolysis, form an increased concentration of Ca (OH) 2.

The strength of ash-containing concrete steamed at 95°C is 12-15% higher than that of concrete steamed at 80°C. Increasing the temperature makes it possible to reduce the heat treatment time by 1-2 hours.

For concretes with the addition of ash, a relatively intensive increase in strength in the late periods of hardening is characteristic. According to Japanese researchers, the compressive strength of concretes containing 190 and 240 kg/m3 of cement and 30% ash addition at the age of 10 is 1.44 and 1.43 times higher than the strength of concrete at the age of 3 months, respectively. The possibility of a more intensive increase in compressive strength is also noted. When testing cores from a concrete pavement in which 30% of the cement was replaced by ash, a compressive strength of 37 MPa was observed after 3 months and 61 MPa after 9.5 years.

From this table it can be seen that in the period of 28-180 days, the intensity of growth in the compressive strength of ash-containing concrete is approximately the same or higher than that of concrete that does not contain ash.

In some works, it is noted that during long-term hardening, the strength of ash-containing concrete increases intensively not only in compression, but also in tension and bending. Samples in the form of rods and bars cut from experimental concrete masonry showed flexural strength of ash-containing concrete after 3 months. - 80, and after 10 years - 150% of the strength of the control concrete. As with other active mineral admixtures, concrete with ash has a higher ratio of tensile strength to compressive strength.

Of interest is the effect of hardening accelerator additives, in particular calcium chloride, on the strength of concrete. In one of the works it is noted that the introduction of 1.2-1.5% calcium chloride by weight of the mixed binder made it possible to increase the strength of ash-containing concrete at the age of 7 days by 18-25%, and at the age of 28 days - by 10-15%.

Replacing part of the cement with ash leads to a decrease in concrete shrinkage, which manifests itself with a decrease in the water demand of the concrete mixture. The decrease in shrinkage is explained by the fact that ash adsorbs soluble alkalis from cement and forms stable, insoluble aluminosilicates.

Ash helps to increase the sulfate resistance of cement concretes in the same way as other active mineral additives. The results of 10 years of testing have shown that concrete containing fly ash cement is more resistant to sea water, even compared to concrete based on Portland slag cement.

The most significant improvement in sulfate resistance was noted for concretes based on Portland cement with a high content of C3A. The best results were noted for concretes with the introduction of ashes with the highest content of SiO2 + Al2O3, i.e., the most acidic in chemical composition. The addition of ash slightly affects the resistance of concrete to carbon dioxide, general acid and magnesia aggression.

According to the recommendations of NIIZhB, when using reactive aggregates containing opal, chalcedony, siliceous schists, volcanic tuffs, etc. in a concrete mixture, ash can be used only if the total content of alkali oxides in the binder in terms of Na20 will not exceed 0.6% by weight. Ashes of dry selection usually contain 1-5% alkaline oxides, their use in mixtures with reactive aggregates is possible when added to practically alkali-free cements. At the same time, a number of studies have shown that the replacement of cement with all types of ash reduces the interaction between alkalis and aggregates. The upper permissible limit of the possible total content of alkaline oxides in the cement-ash binder is recommended to be 1.5%.

Reducing the consumption of cement when ash is introduced into the concrete mixture leads to a decrease in the heat release of concrete and its heating in the initial period. Detailed studies of the use of ash cements in hydrotechnical concretes have shown that heat generation in concrete on cements with 25% ash from the Irkutsk and Krasnoyarsk thermal power plants is 15-25% lower than the heat generation of concrete on cement without additives.

The introduction of a significant amount of mineral additives into the composition of cements or directly into concrete mixtures to reduce heat release is justified only in cases where they do not cause an increase in water demand. These additives, along with blast-furnace slag, include ash. When using fly ash, a 50% decrease in the exotherm of hardening concrete at the age of 28 days is observed.

In the world practice of hydraulic engineering construction, there are many examples when the introduction of ash had a positive effect on the thermal crack resistance of massive concrete structures. When laying a concrete mixture with the addition of 15% ash by weight of the binder, for example, at the construction of the Bratsk hydroelectric power station, the heating of concrete in blocks was approximately 6 ° C lower than without the addition.

Ash, like other active mineral additives, with a moderate content in the concrete mixture, increases the water resistance of concrete. This is due to the hydraulic properties of the ashes and the increase in the density of concrete. Significantly increases the water resistance of the introduction of air-entraining additives START and calcium chloride into the concrete. The most effective was the joint introduction of two additives. The water resistance of concrete in this case increases already at the age of 28 days to W12.

The negative consequences of the introduction of ash into the concrete mix include a decrease in resistance to abrasion and cavitation.

The addition of ash to concrete is not recommended when performing work in the autumn-winter period using the "thermos" method, as it slows down the hardening of concrete at low temperatures. When building in areas with a hot and dry climate, the maintenance of concrete containing ash should be longer than in areas with a temperate climate.

Like other hydraulic additives, fly ash reduces the frost and air resistance of concrete. In concretes with frost resistance F50 and higher or subjected to alternate wetting and drying, the possibility of using ash is established by special studies. The decrease in frost resistance of concrete can be compensated by the introduction of air-entraining additives.

The degree of decrease in the frost resistance of concretes when ash is introduced into them is different and depends on their characteristics. The heterogeneity of the composition and properties of fly ash leads to a significant spread in the basic physical and mechanical properties of concrete, including frost resistance.

The results of long-term tests have shown that when using fly ash, there should be no particular concern due to corrosion of steel reinforcement, if the general requirements for the design and manufacture of reinforced concrete are met.

Tests of concrete with long-term loads have shown that the introduction of ash significantly reduces the creep of concrete. So, when tested for 240 days, the creep of concrete with the addition of fly ash turned out to be 34.5% lower than that of the control concrete. With the introduction of a surfactant additive, the creep deformations of ash-containing concretes differ little from the deformations of concretes without ash. After testing concrete with LST for 300 days, the creep in the absence of ash additives was 59.2 10-5 and 59.5 10~5 at 20% ash.

Research has shown that ash reduces the coefficient of linear thermal expansion of the mortar part of concrete in an air-dry state, bringing it closer to the values ​​that are typical for aggregates. So, at a temperature of 20 ° C, the coefficient of linear expansion for ordinary solutions is 8.8, solutions with 25% ash and the addition of surfactants - 5.8, granite - 3.8. These data show that the introduction of ash into concrete should increase its thermal crack resistance under heating and cooling conditions.

Due to the relatively low water demand of concrete mixtures, the replacement of up to 20% of cement with ash has practically no effect on the shrinkage deformations of concrete during its hardening in air.

Positive experience has been accumulated in the use of cast ash-containing concrete mixtures in monolithic thin-walled reinforced concrete structures. 100-150 kg/m3 of ash and a plasticizing additive are introduced into the concrete composition. Concrete from cast mixtures with the addition of ash have fairly high physical and mechanical properties, and structures made of them have good surface quality. The plasticity of concrete mixtures due to the introduction of ash into their composition increases significantly.

A typical production line for the production of a concrete mixture with the addition of fly ash (3.5) includes a receiving device, a warehouse, a feed hopper and a batcher. Ash is delivered by rail in Hopper type wagons. It can also be delivered by other special vehicles.

After unloading the ash, compressed air is supplied to the tank for aerating and creating the necessary pressure, as well as to the mixing compartment to form an air medium of a certain calculated concentration. The aerated ash loosened by compressed air enters the mixing chamber under the influence of the pressure difference, from where it goes to the warehouse through the transport pipeline. The working pressure of compressed air at the inlet of the pipeline of the pneumatic system depends on the concentration of fly ash and the delivery distance.

With the help of a distributing device, which is included in the installation kit, fly ash is distributed according to the forces of wasps. To clean the air leaving the silos, filters and cyclones are provided, under which dust collectors are installed. The dust is sucked off and transported to the warehouse. With the help of jet or chamber pumps, the ash is fed into the precipitator installed in the over-bunker section of the concrete mixing unit, and then into the feed bins.

The mechanisms of the ash supply path are switched off automatically by the signal of the level indicator installed in the supply hopper. The unsettled ash, together with the air, enters the cyclones, where the mixture is re-cleaned and precipitated. From the dispenser, the ash is fed directly into the concrete mixer. The air entering the receiving device and the jet pump undergoes oil and water purification. When using uncleaned air, ash sticks to the walls of pipelines and the entire system fails.

Thus, for the storage, transportation and dosing of dry ash, basically the same technological equipment and vehicles are used as for cement.

Building solutions. Ash is used as a component of mortars, which combines the properties of a mineral additive, plasticizer and microfiller. Ash improves the plasticity and water-retaining capacity of mortar mixtures, the properties of hardened mortars. When used in solutions of fine ashes taken from the last fields of electrostatic precipitators, the consumption of binders is significantly reduced. The use of ash as an additive is rational in obtaining effective mortars for masonry and building walls from large-sized elements. However, solutions with the addition of ash should not be used in winter due to the slow rate of their hardening at low temperatures.

Both dry ash and hydraulic ash are used in mortars.

In cement mortars, the optimal ash content is recommended to be 100-200 kg / m3, while in "lean" low-cement mortars it is 80-125% of the mass of cement, in more "fat" - 40-50%. With a cement consumption of more than 400 kg/m3, the introduction of ash into the mortar is ineffective. Fine fly ash can be used to replace part of the cement and sand. It is rational to use coarse ash instead of part of the sand without changing the consumption of cement.

When using fly ash in cement mortars, the required cement consumption is usually reduced by 30-50 kg/m3 while improving the workability of the mortar mixture. The excess consumption of cement during the complete replacement of sand with ash is eliminated by adding a small amount of lime paste.

With the complete replacement of sand with ash, shrinkage deformations with time and deformations with alternate wetting and drying increase. They are 2-3 times higher than those of cement-sand mortars.

In cement-lime mortars, ash can replace part of the cement, lime or sand. At the same time, up to 30-50 kg of cement and 40-70 kg of lime paste per 1 m3 of mortar are saved without compromising workability and strength.

Cement-lime-ash mortars are characterized by very low stratification. They are used in the same way as mortars, without the addition of ash, mainly for laying the above-ground parts of buildings.

In lime mortars, the use of fly ash can reduce the consumption of lime paste by 50% without reducing strength and deteriorating other properties. When replacing 50% of lime with twice the mass of fly ash, not only savings in lime are achieved, but also the strength of the solution increases. Without the use of cement on a lime-ash binder, mortars of grade M25 and higher can be obtained.

The selection of compositions of ash-containing solutions is carried out in two stages. First, the consumption of the components of the solution in kilograms per 1 m3 without the addition of ash is determined, and then it is refined, taking into account the introduction of ash, assuming that the average density of the solution increases by 20-40 kg/m3, and the water demand of the mortar mixtures does not change.

The technology for preparing solutions with the addition of ash consists of dosing the initial components by weight and then mixing them in mortar mixers for 3-5 minutes until a homogeneous mixture is obtained.

Ash can also be used in various finishing compositions. For example, for puttying internal surfaces at construction sites on a massive scale, the so-called "bespeschanka" is used, which is a gypsum dough with a setting retarder. Replacing 30 - 50% of gypsum with ash not only does not impair the quality of this putty, but even somewhat reduces the consumption of the moderator.

Ash is used in cement mortars used to seal cracks in reinforced concrete structures, including massive ones. In this case, good pumpability of solutions, their connectivity, stability of properties over time, and a decrease in water separation and segregation (stratification) are of decisive importance. The ash used in such solutions must have certain size restrictions: the residue on a 45 micron sieve must be from 12.5 to 30%; for embedding large cavities, workings, etc., ash can be used, characterized by a residue on a sieve of 45 microns, reaching up to 60%.

Cellular concretes. Slag and ash binders, as many years of experience have shown, can successfully replace lime-silica and lime-cement binders in the production of cellular concrete products. Ground fuel slag and pulverized ash also make it possible to replace finely dispersed quartz sand in the composition of cellular concrete products. Along with autoclave technology, when using high-activity slag-ash binders, it seems possible to obtain cellular concretes under steaming conditions at atmospheric pressure. In the composition with Portland cement, the use of fine ash and slag contributes to the hardening of cellular concrete even without heat treatment.

Ash and ash and slag mixtures in the production of cellular concrete can be used both in dry form and in the form of sludge

On the basis of clinker-free and low-clinker slag binders in wet and dry grinding, as established in MISI them. V.V. Kuibyshev, it is possible to obtain cellular concrete with a compressive strength of 8-12 MPa at a density of 1000-1200 kg/m3, 6-9 MPa at 800-1000 kg/m3, 4-5.5 MPa at 600-700 kg/m3 and 1 -2.5 MPa at a density of 300-500 kg/m3. The upper strength values ​​refer to cellular concretes made on the basis of high- and medium-calcium granulated slags, as well as on the basis of acid granulated slags and fly ash with the addition of 50-75 kg/m3 of Portland cement.

Replacing finely ground lime-sand binder with lime-slag or ash binder allows to reduce lime consumption by 2-3 times.

For the production of non-autoclaved gas-ash-slag concretes, it is desirable to use cement with a high content of active minerals - alite and tricalcium aluminate. In the manufacture of autoclaved cellular concrete, it is possible to use cements with reduced activity, including slag Portland cement and pozzolanic Portland cement.

Cellular ash concrete is a kind of cellular concrete in which ash acts as a silica component. Compared to the usual silica component - ground quartz sand - ash has a higher reactivity, requires significantly less (and does not require at all) costs for grinding and makes it possible to obtain cellular concrete of lower average density. The disadvantages of ash as a silica component are as follows: less than in quartz sand, the content of Si02; the presence of unburned fuel and the instability of the chemical composition. The technological requirements for the ash used in cellular concrete are as follows: the content of vitreous and melted particles must be at least 50%, unburned particles of brown coal - no more than 3%, stone - no more than 5%; specific surface 3000-5000 cm2/g; swelling in water should not exceed 5%.

With the use of fly ash, about 10% of the total volume of production of cellular concrete products is produced, and a significant part of this amount is made up of products made on the basis of oil shale ash. The effective use of oil shale ash is due to its chemical and mineralogical composition (free calcium oxide - 15-25%, clinker minerals - 10-15%, anhydrite - 7-10%, active glass - 30-35%), as well as a set of technological methods, as a result of which hydration of free calcium oxide is ensured in the form of a burn before autoclave treatment (fine grinding of ash, injection molding and keeping raw materials at elevated temperatures in conditions that exclude large temperature differences). Shale pulverized ash must contain calcium oxide in an amount of at least 35%, including free CaO - at least 15-25%, it is unacceptable to have more than 6% SO3 and 3% (K20 + Na20).

Cellular concrete with the use of ash is mainly produced in the form of gas-ash concrete with an average density of 400-1200 kg/m3. They are used to make heat-insulating products, panels, blocks and slabs for external walls, coatings, interfloor ceilings and internal partitions (3.7).

The most common method of forming cellular ash-concrete is casting, when a mixture containing 50-60% water is poured into the molds. The main disadvantages of injection molding are: insufficient gas-holding capacity of the mixture; non-uniform density of products in height; slow hardening; high humidity of products after heat treatment and high shrinkage.

More acceptable for the production of aerated concrete is a complex vibration technology, which allows, due to the effect of dilution of the mixture during vibration during mixing and molding, to reduce the amount of mixing water by 25-30%. At the same time, compared with the casting method, the strength of aerated concrete increases by 15-25%, and shrinkage deformations are reduced by 25-30%. Reinforcement of the cellular structure of aerated concrete with asbestos fibers, mineral wool and other fibers helps to reduce shrinkage and increase the crack resistance of concrete. It is effective to introduce coarse porous aggregate into the composition of cellular concrete mixtures - slag pumice, expanded clay, agloporite, etc., as well as the use of mixtures with additives of surfactants.

The strength of cellular ash concrete in compression is 0.5-15 MPa at an average density of 400-1200 kg/m3, and frost resistance reaches 150 cycles. Cellular ash concrete on cement has a much greater durability than on lime. A negative feature of ash concrete is their ability to high sorption moisture caused by significant microporosity of ash. They are also more sensitive to wet and dry cycling than brick or heavy concrete. To protect against the aggressive effects of the atmosphere, various coatings are applied to products made of cellular ash concrete.

The economic efficiency of cellular ash concrete is due to the replacement of sand with ash, a 1.2-1.5-fold decrease in the consumption of lime binder compared to lime-sand binder, and a reduction in approximately 2 times of capital investments for the extraction and processing of raw materials.

A technology has been developed for obtaining ash-alkali cellular concrete for thermal insulation in civil, public and industrial buildings. As a result of the research, cellular concrete was obtained on liquid glass and causticized soda melt. Fly ash from the Ladyzhenskaya GRES and soluble sodium silicate were used as starting materials. Aerated concrete samples were made using injection technology by mixing fly ash with an alkaline mixing agent, followed by the introduction of a blowing agent into the mixture, which was an aqueous suspension of aluminum powder. Sulfanol and laundry soap were used to emulsify aluminum powder. The swelling rate of the cellular mixture was controlled by adding caustic soda, and the setting time was controlled by adding lime. Products made of cellular concrete were dried at a temperature of 60-80 ° C for 6-10 hours. After drying, the samples acquire water resistance and strength of 40-60% of the branded one. When stored in a dry state, the strength of cellular concrete tends to increase.

For the preparation of cellular concrete, caustic soda melt was used, obtained by boiling soda melt and lime milk with a density of 1.2 g/cm3 at 80-90 °C. To regulate the intensity of the interaction of aluminum powder with an alkaline component, hydrophobic substances (waste machine oil, oleic acid), LST plasticizer, and mineral powder were introduced into the blowing agent. It was found that, unlike cellular concretes on liquid glass, the optimal conditions for hardening concretes on causticized melt are created during heat and moisture treatment.

Electricity is the "lifeline" of modern civilization. In India, coal-fired power plants are the main source of energy. During the production of electricity, they form a by-product - fly ash. This is a very good resource that can be successfully used in the production of cement and concrete, as well as in other areas.

Cement-concrete plants need it especially strongly.

Extraction and storage of fly ash

In India, more than 70% of electricity is produced by coal-fired thermal power plants. Indian coal contains a very high percentage of ash, ranging from 30% to 45%. In total, India produces about 100 million tons of ash every year.

The heavier particles, approximately 20%, which collect at the bottom of the boiler are called "bottom ash". The remaining 80%, smaller ones, are carried away by the gases and collected in electrostatic precipitators (ESPs). This is fly ash.

Fly ash refers to pozzolanic materials. It is transported by pneumatic mechanisms to special towers like silos, where it is stored.

Fly ash is collected in different parts of the ESP. Larger and coarser particles accumulate in the first few areas, and with each subsequent area they become smaller and smaller.

Before fly ash is used in the production of concrete, it is checked for the absence of large particles and unburned coal.

After that, it is loaded into closed tanks and sent to the RMC plant.

Fly ash is also sold in bags at the thermal power plant in Badarpur.

At the RMC plant, fly ash and cement are stored separately in "silo" towers.

Various building materials such as coarse aggregate, sand, etc. are also stored at the factory, and there is no need to store them at the construction site.

Fly ash in concrete production

Portland cement mixed with water produces a cementitious material. Still at the same time a certain amount of "free" lime is produced. It makes concrete porous. However, if there is fly ash in the mixture, the lime will react with it, resulting in additional cement material. It makes concrete thicker, stronger and more reliable. The presence of fly ash also helps to cope with high temperatures and high humidity.

Bureau of Indian Standards IS:456 allows the use of fly ash as a partial replacement for conventional Portland cement up to 35%. This reduces the need for conventional Portland cement, and therefore helps to conserve coal and limestone, which is very economically important. It also reduces carbon dioxide emissions into the atmosphere and is therefore environmentally friendly.

Cement, fly ash, coarse aggregate, fine aggregate and water are automatically mixed. Management takes place in the control room.

Ready mix concrete from the RMC plant is collected in transit mixers and transported to the construction site.

Each batch of concrete mix is ​​tested in laboratories at the plant before being sent to the site. There, the transit mixer unloads the concrete wherever it is required.

We spoke with Mr. Rajesh Agarwal, Chief Engineer of the Delhi Metro Corporation, about his views on the use of fly ash in concrete:

" We, Delhi Metro Corporation, use fly ash mixed with concrete in the construction of underground structures. And naturally, we find it very useful. Fly ash, being a pozzolanic material, reacts with lime. Therefore, concrete becomes stronger than ordinary reactions.Ash mixed with concrete markedly improves the strength of concrete and the effectiveness of low alkaline reactions with aggregates, reduces the impact of sulfates, makes concrete stronger, prevents corrosion of reinforcement and extends the life of buildings.Therefore, we are not limited to sand, traditional fine and coarse aggregates, cement and other common materials available from RMC factories."

We asked Mr. Shiban Reina, CEO of the National Cement and Building Materials Board, what he thinks about the use of fly ash:

"Fly ash, as we all know, is no longer a by-product but a highly valuable resource. Architects and builders can benefit greatly from it. Whether large or small, fly ash is a top quality product. Fly ash can absorb lime, because it contains an active silicon component, which does this.As a result, the 100% potential power of ordinary Portland cement becomes a reality.And this is exactly what you need to pay attention to.We see that ordinary cement turns into something better when it is add fly ash.More fluidity.Now fly ash is widely used in the construction industry.At the construction site, workers need to think about where to store cement and many other things.But there is no need to think about the storage of fillers, including fly ash. You just need to call by phone, make a certain advance payment, and they will be delivered to you right on time."

Today, there are more than 60 RMC plants in India. Fly ash collection has been established at thermal power plants. Indian thermal power plants have a long, long way to go to deal with electricity shortages in this developing country. Fly ash, one of their by-products, has great potential if used wisely and rationally in RMC plants.

In conclusion, it is worth recalling the advantages of using fly ash in concrete:

• High and long-term strength.
• High reliability.
• Low permeability.
• Low heating at high humidity.
• Increased resistance to sulfates and corrosion.
• Reduced reaction between alkali and filler.

It is no exaggeration to call this useful and valuable product a great brainchild of our era.