Strength characteristics of aerated concrete with fly ash filler from Angren Thermal Power Plant

. This article focuses on fly ash aerated concrete, a new and very useful material in the construction industry and is basically a suspension of cement mortar with a maximum content of aluminum powder of 0.2 % by volume. A description of the conducted experimental study on the effect of the complete replacement of river sand with fly ash for various mixtures is given, and compressive strength indicators are also given.


Introduction
In recent years, environmental pollution problems have increased dramatically due to the threat of intoxication in humans and animals.Industrial activities, municipal waste, and agricultural waste are major contributors to the generation of large-scale solid waste.To overcome the accumulation of useless solid waste, supernatural methods must be combined with a systems approach, focusing on the efficient disposal of industrial waste through methodological analysis [1][2][3].
Several million tons of different types of waste are generated every year globally, and this is expected to increase in the future.Most of these wastes are dumped via landfilling or incineration, which creates environmental concerns.One of the possible methods of utilizing these wastes is by incorporating them as alternatives to common concrete constituents.In this regard, foamed concrete could provide an excellent medium for incorporating these wastes in a large volume primarily due to the low strength requirement of foamed concrete.Many researches are carried out to explore the idea of integrating waste materials in pre-foamed concrete [4].
The combustion products of coal, formed during coal combustion in thermal power plants, are one of the solid residues among various industrial wastes.The largest producing countries of coal combustion products in 2016 were China, India, Europe (total production of 140 Mt, which should be considered as utilization factors available only for the EU15), and the USA.The total production estimate for the year was almost 1.2 billion tons.
Usage varies widely in the countries discussed in this paper.Japan recorded the highest effective utilization rate at 99.3 %, while Africa in the Middle East (still) recorded the lowest at 10.6 %.The countries with the highest rates of coal combustion products used were as follows: Japan at 96.3 %, Europe (EU15) at 94.3 %, Korea at 85 %, China at 70%, and Other Asian countries at 67 % or the USA at 56 % [5].
As you know, lightweight concrete is a building material with a density of up to 2000 kg/m 3 , and its density ranges from 900 to 1900 kg/m 3 depending on its compressive strength.By purpose, these materials are divided into heat-insulating -the density does not exceed 500 kg/m 3 ; structural and heat-insulating -500-1400 kg/m 3 ; structural -1400-1800 kg/m 3 .By structure, concrete is divided into dense, porous, and macroporous.According to the method of pore formation, light concretes are divided into concretes porous: foam, gas, and air-entraining additives.
Porous lightweight concrete also includes cellular materials -aerated concrete and foam concrete.It should be noted that the composition of such building materials is not standardized; there are only recommendations.
Aerated concrete is an artificial stone with approximately spherical, closed, but communicating with each other pores 1-3 mm in diameter, evenly distributed throughout the volume.According to the technology of final processing, aerated concrete is divided into autoclaved aerated concrete and "non-autoclaved".
Aerated concrete is obtained by mixing conventional cement, fine aggregates, and specialized blowers.
Pozzolanic materials such as fly ash and ground blast-furnace slag are used to improve cement's durability and strength properties.
Aerated concrete has good properties, such as ease of handling and low cost in preparation.This thermal insulation material has the best freezing and thawing resistance and high sound insulation performance.
The main disadvantages of aerated concrete are the high consumption of river sand used as filler and the setting process.Using industrial waste as a filler in producing aerated concrete significantly reduces the consumption of river sand.
Worldwide, the production of coal-fired electricity generates more than 500 million tons of fly ash per year, of which only 25-30 % is reused in various sectors [10,11].Fly ash is generated mainly from the combustion of sub-bituminous and bituminous coal in a thermal power plant, resulting in fine waste, usually containing the extinct mineral part of the coal.It has siliceous and aluminous properties and has a particle size of about 4000-8000 g/cm 2 [12].This material has been widely publicized.However, the long-term potential of fly ash as a useful material has been ignored.Regardless of the favorable use, the production rate far exceeds the use.For such unsettled waste residues, lakes, landfills, slag heaps, and ponds have been created for processing purposes.All of this is seen as an undesirable, environmentally harmful, and unprofitable use of land assets, as well as imposing a progressive financial burden due to their long-term maintenance.Therefore, conducting more research on the efficient use of fly ash outside the construction sector is desirable.Various studies on fly ash have shown that fly ash can become a valuable building material [13][14][15][16].
To date, several studies have been carried out on the utilization of industrial solid waste for the production of aerated concrete, the replacement of fine aggregate with fly ash, and the influence of the type of filler on aerated concrete; the research results have shown that an increase in the content of fly ash improves the quality of aerated concrete [17][18][19].
The fluidity of preformed aerated concrete mixtures depends on the type of filler.For example, the flow index is higher for mixtures in which sand is partially replaced with fly ash as filler.

Method of experiment 2.1 Cement
In the research work, ordinary general construction Portland cement grade M-500 according to State Standard 31108-2020 was used, the true density of which in laboratory tests was 3.1 g/cm 3 .

Fly ash
Fly ash: Fine, predominantly spherical, vitreous dust resulting from the combustion of finely ground coal, which has pozzolanic properties and/or hydraulic activity.
In construction practice, ash is called solid focal residues with particles up to 0.15 mm in size, formed during the combustion of solid fuels.Large particles belong to slag sand and crushed stone.The properties of the ash depend mainly on the type of fuel burned, the combustion conditions, and the way the ash is removed outside the boiler house or thermal power plant.Since the ash is a product of burning the mineral part of the fuel, the composition of the latter, in the first place, determines the properties of the ash.Ashes are usually classified according to the type and quality of the fuel.On this basis, ashes are divided into coal, shale, and peat.Coals, in turn, are subdivided into anthracite, stone, and ash brown coals.According to this classification, fly ash from the Angren TPP is lignite.Depending on the type of preparation and fuel combustion conditions, pulverized and layered ash is distinguished.On this basis, the fly ash of the Angren TPP belongs to the pulverized combustion ash.
The chemical and granulometric compositions of the ash from the Angren TPP are given in Tables 1 and 2.

Gasifier
Aluminum powder PAP-1 (powder) is a finely divided aluminum particle of a lamellar form.Aluminum particles in PAP-1 powder have a lamellar shape and are covered with a thin oxide and fatty film.PAP-1 is a light-staining silver-gray product that does not contain foreign impurities visible to the naked eye.The bulk density of the powder is about 0.15-0.30g/cm 3 , and the active aluminum content is 85-93 %.The average thickness of the "petals" is approximately 0.25-0.50microns, and the average linear size is 20-30 microns.

Water
For various mixtures, water was used without any chemical compounds and impurities that can affect the cement setting time, hardening rate, strength, frost resistance, and water resistance of concrete above the norm and maintains a pH of 7 and does not contain chlorides.

Production of test samples
In this study, four different mixtures of aerated concrete were considered, namely: Mixture I, a conventional mixture including fine aggregate (sand) 50 %, cement 50 %, blowing agent, and water; Mixture II, which used 60 % cement and 40 % fly ash as fine aggregate; Mixture III, which used 50 % cement and 50 % fly ash as fine aggregate; Mixture IV, which used 40 % cement and 60 % fly ash as fine aggregate.

Sample preparation and testing
A total of four different mixture compositions, details of the composition of the mixtures are given in table 3. Samples of 100 mm x 100 mm x 100 mm were prepared and tested for compressive strength during the period shown in figure 2. Three times were prepared for each mixture sample for testing.Compressive strength tests were carried out following State Standard 10180 Concrete.Methods for determining the strength of control samples after 3 days, 7 days, and 28 days.3 Results of experimental tests and discussion

Density of aerated concrete
The results of experimental tests showed that the density of aerated concrete for mixture (A2) was 734 kg/m 3 compared to all mixtures.It was also noted that all values were closer to the target density of 721 to 734 kg/m 3 after 28 days.A comparative analysis of the results shows that the composition of the mixture (A3) is the most optimal.

Compressive strength of aerated concrete
Figure 2 shows that the increase in the compressive strength of aerated concrete (A3) is 51.7 % observed after 14 days from the start of setting to 28 days, and the rate of increase in the compressive strength of cement-sand aerated concrete over the same period is 10.2 %.As a result, their strength indicators on the 28th day become almost the same.

Conclusions
Based on the experimental results, the following conclusion can be drawn.Increasing the fly ash content in the sample mixture affects the initial hardening period after setting; in the early stages, there is a decrease in strength due to the delay of the pozzolanic reaction compared to the control mixture (A1), the most optimal composition is the mixture (A3) in which 50 % cement was used and 50 % fly ash as fine aggregate.

Table 1 .
Chemical composition of fly ash from Angren TPP

Table 2 .
Granulometric composition of fly ash from Angren TPP Local quarry sand is used for various mixtures, the experimentally determined specific gravity of the sand is 2.71, and the fineness modulus is 1.76, corresponding to class II sand following State Standard 8736-2014.

Table 3 .
Mix designation and Mix proportions.