Ash of entrainment: description, composition, GOST, features of application and reviews

During the combustion of fuel waste is formed, which is called as fly ash. Near the fireboxes are installed special devices that capture these particles. They are a dispersion material having components of less than 0.3 mm in size.

What is fly ash?

Fly ash is a finely dispersed material with small particle sizes. It is formed when burning solid fuel at elevated temperatures (+800 degrees). It contains up to 6% of unburned substance and iron.

Formation of fly ash when burning mineral impurities that are in fuel. For different substances, its content is not the same. For example, in wood, the fly ash content is only 0.5-2%, in fuel peat 2-30%, and in brown and coal coal 1 - 45%.

Receiving

Ashes of entrainment are formed during the combustion of fuel. Properties of the substance obtained in boilers are different from those created in the laboratory. These differences affect the physicochemical characteristics and composition. In particular, when burning in a furnace, the mineral substances of the fuel melt, which leads to the appearance of components of the unburnt composite. Such a process, called mechanical underburning, is associated with an increase in temperature in the furnace to 800 degrees and above.

To trap fly ash, special devices are required, which can be of two types: mechanical and electrical. When working GZU consumes a large amount of water (10-50 m ^{3 of} water per 1 ton of ash and slag). This is a significant drawback. To get out of this situation, use a circulating system: water after purification from the particles of ash re-enters the main mechanism.

Main characteristics

• Workability. The smaller the particles, the greater the effect of fly ash. The addition of ash increases the uniformity of the concrete mix and its density, improves stacking, and also reduces the flow of mixing water with the same workability.
• Reducing the heat of hydration, which is especially important in the hot season. The ash content in the solution is proportional to the decrease in the heat of hydration.
• Capillary absorption. When 10% ash is added to the cement, the capillary absorption of water increases by 10-20%. This, in turn, reduces frost resistance. To eliminate this drawback, it is necessary to slightly increase the air entrainment by means of special additives.
• Resistance to aggressive water. Cements, which are 20% ash, are more resistant to immersion in aggressive water.

Pros and cons of using fly ash

Addition to the mixture in the form of fly ash entails a number of advantages:

• The consumption of clinker is reduced.
• Improves grinding.
• Strength increases.
• Improves workability, which makes it easier to remove.
• Reduces shrinkage.
• Reduces the release of heat during hydration.
• Increases the time until cracks appear.
• Improves resistance to water (both clean and aggressive).
• The mass of the solution decreases.
• Increased fire resistance.

Along with the advantages, there are some disadvantages:

• Adding ash with a large amount of underburning changes the color of the cement solution.
• Reduces the initial strength at low temperatures.
• Reduces frost resistance.
• The number of components of the mixture that need to be controlled increases.

Types of fly ash

There are several classifications according to which it is possible to divide fly ash.

By the type of fuel that is burned, the ash can be:

• Anthracite.
• Carboniferous.
• Brown coal.

According to their composition, the ash is:

• Acidic (with a calcium oxide content of up to 10%).
• Basic ( calcium oxide content is higher than 10%).

Depending on the quality and further application, four types of ash are isolated: from I to IV. And the ash of the latter kind is used for concrete structures that are used in harsh conditions.

Processing of fly ash

For industrial purposes, unprocessed fly ash (without grinding, sifting, etc.) is most often used.

When the fuel burns, ash forms. Light and small particles due to the motion of the flue gases are carried away from the furnace and trapped by special filters in the ash collectors. These particles are the ash of entrainment. The remainder is referred to as fly ash.

The relationship between these fractions depends on the type of fuel and design features of the furnace itself:

• At a solid removal in the slag remains 10-20% ash;
• When liquid slag removal - 20-40%;
• In cyclone-type furnaces - up to 90%.

When processing into the air, particles of slag, soot and ash can get into the air.

The fly ash of dry selection is always sorted by fractions under the influence of electric fields, which are created in the filters. Therefore, it is the most suitable for use.

To reduce the loss of material during calcination (up to 5%) fly ash is necessarily homogenized and sorted by fractions. The ash, which is formed after the combustion of low-reactive coals, contains up to 25% of the combustible mixture. Therefore, it is further enriched and used as energy fuel.

Where are fly ash used?

Ash is widely used in various spheres of life. This can be construction, agriculture, industry, sanitation

In the production of individual types of concrete, fly ash is used. The application depends on its kind. Granular ash is used in road construction for the foundation of parking lots, storage areas for solid waste, bicycle paths, embankments.

The fly ash of dry trapping is used to strengthen the soils as an independent astringent and rapidly hardening agent. It can also be used for the construction of dams, dams and other hydraulic structures.

For the production of hydraulic concrete, ash is used as a substitute for cement (up to 25%). As a filler (small and large), ash is included in the process in the production of cinder and blocks used in the construction of walls.

Widely used in the production of foam concrete. Adding ash to the foam concrete mixture increases its aggregative stability.

Ashes in agriculture are used as potash fertilizers. They contain potassium in the form of potash, which is readily soluble in water and is available for plants. In addition, the ash is rich in other useful substances: phosphorus, magnesium, sulfur, calcium, manganese, boron, micro- and macro elements. The presence of calcium carbonate makes it possible to use ash to reduce the acidity of soils. The ashes can be brought under different crops in the garden after plowing, fertilizing it with tree trunks of trees and shrubs, and also pouring in meadows and pastures. It is not recommended to use ash simultaneously with other organic or mineral fertilizers (especially phosphoric).

Ash is used for sanitation in the absence of water. It increases the pH level and kills microorganisms. It is used in latrines, as well as in places of sewage sludge.

From all of the above, we can conclude about the widespread use of such a substance as fly ash. The price for it varies from 500 r. Per ton (at large wholesale) to 850 rubles. It should be noted that when using self-transportation from distant regions, the cost can vary significantly.

GOSTs

Designed and operating documents that control the production and processing of fly ash:

• GOST 25818-91 "Ashes of entrainment of thermal power stations for concrete".
• GOST 25592-91 "Mixtures of ash and slag TPP for concrete".

To control the quality of produced ash and mixtures with its use, other additional standards are used. In this case, sampling and all types of measurements are also conducted in accordance with the requirements of GOST.