- Introduction
Fly ash is finely divided residue resulting from the combustion of powdered coal and transported by the flue gases and collected by electrostatic precipitator. Also known as “pulverised fuel ash”, it is a coal combustion product that is composed of the particulates that are driven out of coal-fired boilers together with the flue gases. Ash that falls to the bottom of the boiler is called bottom ash and fly ash is generally captured by electrostatic precipitators. In India, Fly ash was used first in Rihand dam construction replacing cement up to about 15 per cent.
Several factors which are responsible for low level of utilization of fly ash includes absence of standards and specifications for fly ash products , lack of reliable quality assurance for fly ash products , poor public awareness about the products and their performance.
But in the recent time, the importance and use of fly ash in concrete has grown so much that it has almost become a common ingredient in concrete, particularly for making high strength and high performance concrete. In India alone, we produce about 75 million tons of fly ash per year, the disposal of which has become a serious environmental problem. Therefore, effective utilization of fly ash has become necessary. By using fly ash, disposal costs are minimized and also less area is reserved for disposal, enabling other uses of the land.
Other important uses of fly ash include in manufacturing of low cost bricks, in the construction of road and embankment, in soil stabilization which increases the physical and chemical properties of soil, etc.
- Types of fly ash
- CLASS F FLY ASH
Class F fly ash has pozzolanic properties only. It is normally produced by burning anthracite or bituminous coal. It usually has less than 5% CaO. Due to its pozzolanic property, Class F fly ash requires a cementing agent, such as Portland cement, quicklime, or hydrated lime mixed with water to react and produce cementitious compounds. Adding a chemical activator such as sodium silicate to a Class F fly ash can form a geopolymer.
- CLASS C FLY ASH
Class C fly ash has pozzolanic properties as well as cementitious property. It is normally produced by burning lignite or sub-bituminous coal. Class C fly ash may have CaO content in excess of 10%. Unlike Class F, self-cementing Class C fly ash does not require an activator. Alkali and sulfate (SO4) contents are generally higher in Class C fly ashes.
- Major use of fly ash
- Use in concrete
- In fresh concrete
The use of good quality fly ash with a high fineness and low carbon content reduces the water demand of concrete and consequently the use of fly ash should permit the concrete to be produced at lower water content when compared to a portland cement concrete of the same workability. A gross approximation is that each 10% of fly ash should allow a water reduction of at least 3%.
Generally fly ash will reduce the rate and amount of bleeding primarily due to the reduced water demand. High levels of fly ash used in concrete with low water contents can virtually eliminate bleeding. The impact of fly ash on the setting behavior of concrete is dependent not only on the composition and quantity of fly ash used, but also on the type and amount of cement. During hot weather the amount of retardation due to fly ash tends to be small and is likely to be a benefit in many cases. During cold weather, the use of fly ash, especially at high levels of replacement can lead to very significant delays in both the initial and final set. Practical considerations may require that the fly ash content is limited during cold-weather concreting. The reduction in the rate of the heat produced and hence the internal temperature rise of the concrete has long been an incentive for using fly ash in mass concrete construction.
- In hardened concrete
Fly ash, when used in concrete, contributes to the strength of concrete due to its pozzolanic reactivity. However, since the pozzolanic reaction proceeds slowly, the initial strength of fly ash concrete tends to be lower than that of concrete without fly ash. Due to continued pozzolanic reactivity concrete develops greater strength at later age, which may exceed that of the concrete without fly ash. The pozzolanic reaction also contributes to making the texture of concrete dense, resulting in decrease of water permeability and gas permeability. Since pozzolanic reaction can only proceed in the presence of water enough moisture should be available for long time. Therefore, fly ash concrete should be cured for longer period. In this sense, fly ash concrete used in under water structures such as dams will derive full benefits of attaining improved long term strength and water-tightness. Properly proportioned concrete containing fly ash should create a lower cost because of the reduced permeability and reduced calcium oxide in properly selected fly ash, it should be less susceptible to the alkali-aggregate reaction, sulfate and other chemical attacks. Reduced permeability helps to protect the concrete from chloride penetration, the cause of rebar corrosion. A superplasticizer combined with fly ash can be used to make high-performance and high-strength concrete.
- Use in manufacture of bricks
Fly ash lime bricks are chemically bonded bricks manufactured by utilizing 80-82% of fly ash, which is a major waste bye-product of pulverised coal fired in Thermal Power Stations, 9-10% of lime, 9-10% of sand and 0.2% of Chemical accelerator. For manufacturing fly ash lime bricks no firing is needed. Curing in steam for predetermined period is employed to enable the bricks to gain desired strength. Thus, fly ash lime bricks satisfy the basic parameters of building units, moreover the bricks are also suitable for the construction of building in coastal areas where normal red clay burnt bricks are found to be affected.
Various special features of fly ash lime bricks are being machine finished these are uniform in size and shape, consumes 20-25 percent less cement mortar, stronger than Class-I burnt clay building bricks, outside wall plastering is not essential as these bricks have cement gray colour, smooth surface and low water absorption capacity, resistance to salinity, being lighter in weight in comparison to the conventional red bricks, the dead building load and the transportation cost will be less, adoption of this process helps to conserve invaluable top soil of agricultural land ,by consuming 80-82% fly ash, the cause of environmental pollution and hazards due to disposal is minimized as firing of the bricks is not needed thus pollution due to firing is eliminated.
Fly ash bricks weigh, on average, one-third less than conventional clay-fired bricks, enabling a truck to carry more bricks per load, thus reducing shipping costs and improving profit margins. The second economic reason is an abundance of low-cost fly ash available to make the bricks, yielding an excellent product. Even though pulverised fly ash brick at first instance may appear to be costlier than conventional products, ultimate financial benefit can be evaluated in terms of its increased physical and chemical properties. Fly ash bricks manufacturing units can be set up nearby thermal power station the entrepreneur has to bear only transportations charges from thermal power stations to the fly ash bricks manufacturing unit.
In fly ash clay burnt brick fly ash generally contains 5 to 6% of unburnt carbon, incorporation of fly ash, therefore, results into a better burnt product together with an economy in fuel consumption. It has been experimentally verified that saving of about five tons of coal per Lac bricks could be achieved by mixing 40% flash ash by volume with the clay for making bricks. Reduction in drying shrinkage and about 15 to 25% of the weight of the bricks with better thermal insulation is observed in these bricks.
Fly ash for Roads
Fly ash utilization in construction of roads has many advantages over conventional methods as it saves top soil which otherwise is conventionally used, avoids creation of low lying areas (by excavation of soil to be used for construction of embankments),avoids recurring expenditure on excavation of soil from one place for construction and filling up of low lying areas thus created, does not deprive the nation of the agricultural produce that would be grown on the top soil which otherwise would have been used for embankment construction, reduces the demand of land for disposal/deposition of fly ash that otherwise would not have been used for construction of embankment.
- Used as Fill Material
Fly ash can be used as a borrow material to construct fills and embankments. When fly ash is placed in thin lifts and compacted, a structural fill is constructed that is capable of supporting highway buildings or other structures. Fly ash has been used in the construction of structural fills/embankments that range from small fills for road shoulders to large fills for interstate highway embankments. Non-structural fly ash fill can be used for the development of parks, parking lots, playgrounds and other similar lightly loaded facilities. One of the most significant characteristics of fly ash in its use as a fill material is its strength. Well-compacted fly ash has strength comparable to or greater than soils normally used in earth fill operations. In addition, fly ash possesses self-hardening properties which can result in the development of shear strengths. The addition of cement can induce hardening in bituminous fly ash which may not self-harden alone. Significant increases in shear strength can be realized in relatively short periods of time and it can be very useful in the design of embankments. When used in structural fills and embankments, fly ash offers several advantages over soil and rock as it is cost effective because it is available in bulk quantities, eliminates the need to purchase, permit, and operate a borrow pit, can be placed over low bearing strength soils. PFA (Pulverized Fuel Ash) embankments should be constructed on a free draining layer that acts as a capillary break. Materials like concrete that may be attacked by sulfates should also isolated from PFA with a capillary break and metallic items should not be placed closer than 500mm from PFA.
- Fly ash for Soil Stabilization
Fly ash is an effective agent for chemical and/or mechanical stabilization of soils, soil density, water content, plasticity, and strength performance of soils. Soil stabilization is the permanent physical and chemical alteration of soils to enhance their physical properties. Stabilization can increase the shear strength of a soil and/or control the shrink-swell properties of a soil, thus improving the load-bearing capacity of a sub-grade to support pavements and foundations. Stabilization can be used to treat a wide range of sub-grade materials from expansive clays to granular materials. Stabilization can be achieved with a variety of chemical additives including lime, fly ash, and Portland cement. Benefits of the stabilization process can include higher resistance (R) values, reduction in plasticity, lower permeability, reduction of pavement thickness, elimination of excavation – material hauling/handling – and base importation, aids in compaction, provides “all-weather” access onto and within projects sites.
Problem Associated With Fly Ash Disposal
The rapid economic growth and the resultant increased standard of living of the population calls for huge increase in supply of power. Beside this High ash content in Indian coal and inefficient combustion technologies contribute to India’s emission of air particulate matter and other trace gases, including gases that are responsible for the greenhouse effect. India is the third-largest producer of coal, but Indian coal is of poor quality with high ash content (35-50%) and low calorific value (gross heat of combustion). A major portion of the ash is inherent in the coal, aggravating the difficulty in removing it.
Ash disposal can have adverse impacts on the environment due to land use diversion, resettlement, water resources allocation and air pollution. Construction of large ash disposal areas results in resettlement issues, loss of agriculture/grazing land/ habitat. When the ash gets dried in the absence of water or vegetation cover, fugitive dust from ash pond pollutes the air thereby increasing local concentration of respirable particulate. Once-through slurry disposal systems place additional strain on scarce fresh water resources.
Problems associated of increasing fly ash
India has billion tons of coal reserves, which is known to be the largest resource of energy but the coal available in India is of poor quality, with very high ash content and low calorific value, and most of the coal mines are located in the eastern part of the country so whatever good quality coals available are used by the metallurgical industry, like steel plants. The coal supplied to power plants is of the worst quality. High ash content in Indian coal and inefficient combustion technologies contribute to India’s emission of air particulate matter and other trace gases, including gases that are responsible for the greenhouse effect. Fly ash water also affects the scale structure because it is a directly in contact with water.
- Future research and prospects
Generally, raw fly ash has low adsorption capacity. Modification of fly ash would enhance its adsorption capacity .Fly ash contains aluminosilicates and a potential source for the synthesis of zeolites. Zeolite has a variety of applications as adsorbents and ion exchangers and exhibits much higher capacity than the raw fly ash. Some investigations have been made in the hydrothermal conversion of fly ash into different types of zeolites using the extracted supernatant solution. Few researchers have been succeeded in the conversion of the bulk fly ash into pure zeolite. The presence of silico-aluminate phases in fly ash makes it a suitable raw material for the synthesis of geopolymer.
Repeated harvesting of foodstuffs depletes trace elements in the soil. Although, the use of fly ash as soil amendment has been studied, the full scale application of this technology has not been implemented. In future, farmers may use fly ash rather than lime to enrich their soil. The trace elements in fly ash might be used to replace trace elements in the soil, and to increase mineral content in the soil. Therefore, more research should be conducted in these areas. Unburned carbon is an important component of fly ash, whose composition in fly ash varies with combustion efficiency. In fly ash, unburned carbon contents generally range between 2% and 12%. The higher percentage of unburned carbon in fly ash will lead to efficiency loss and poor marketability for cement production.Unburned carbon is similar to the precursors for production of premium carbon materials, such as, activated carbon. Activated carbon made from unburned carbon of fly ash has a significant potential of cost advantage over other activated carbon. Therefore, separation of unburned carbon from fly ash will be beneficial to fly ash application, either for carbon recycling or mineral fly ash application in cement production and zeolite synthesis.
Autoclaved cellular concrete (ACC), is a light weight building product with high insulating value, which can be manufactured using 60–75% fly ash by weight. Fly ash, with carbon content up to 12% may be used, thereby, allowing the high volume use of ash from sources that do not meet ready-mix concrete specifications. Further research in this area is needed.