By Nidhi DhullReviewed by Susha Cheriyedath, M.Sc.Jun 26 2024Fly ash, produced in significant quantities by coal power plants globally—over 544 million tons annually—mostly ends up in landfills, presenting severe environmental challenges.1 While reducing fly ash production is challenging due to our reliance on coal energy, the construction sector recognizes its recycling potential. This is vital because using fly ash can substantially decrease the carbon footprint of building materials.2
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Developing eco-efficient concrete using fly ash brings multiple benefits, such as early strength gain, reduced natural resource consumption, versatility in structural applications, and enhanced durability. This study delves into the varied characteristics of fly ash and explores its potential in crafting eco-friendly building materials.
Sources and Production of Fly Ash
Fly ash is a by-product generated when coal is burned in thermal power plants or industrial boilers. It is produced by four main types of coal-fired boilers: fluidized-bed combustion (FBC), pulverized coal (PC), cyclone, and stoker-fired boilers. Each type of boiler serves different purposes in power generation and various industrial processes.
PC boilers are predominantly used in large-scale electricity-generating plants, whereas FBC, cyclone, and stoker-fired boilers are commonly found in various industrial applications. During the combustion process, fly ash accumulates as the exhaust gas stream passes through filters or as flue gases move through electrostatic precipitators, which are part of pollution control systems.
In dry-bottom furnaces, about 80 % of the total ash is released as fly ash in the exhaust fumes. When pulverized coal is burned in wet-bottom furnaces, approximately 50 % of the ash mixes with the flue gas. Cyclone furnaces utilize around 70-80 % of the total ash as fuel, with the remainder appearing as dry ash.
Various treatment techniques are employed to make incinerated fly ash environmentally stable and suitable for specific uses. These include mineralogical and chemical modifications to enhance the reactive phases of the ash. Additionally, processing methods are used to remove harmful contaminants and improve the ash's suitability for industrial applications.1,2
Fly Ash Properties
The properties of fly ash are influenced by its source and the characteristics of the coal being burned. Additionally, combustion methods and particle shapes play a significant role in determining fly ash’s chemical and physical properties. Fly ash is categorized into Class C and Class F based on the proportions of its four main constituents: silica (35-60 %), calcium (1-35 %), iron (4-20 %), and alumina (10-30 %). In Class F fly ash, the combined content of alumina, silica, and iron exceeds 70 %, whereas in Class C fly ash, this content ranges between 50-70 %.1
Fly ash particles are generally irregularly shaped and fine, similar to cement, with an average diameter of less than 10 μm. They contain heterogeneous crystalline and amorphous phases, exhibit low-to-medium density, and have a high surface area. These properties affect the ash’s permeability, compressibility, and strength.1
Key parameters for using fly ash as an inert filler material include particle size distribution, bulk density, specific gravity, and moisture content. Additionally, fineness and specific surface area are crucial for leveraging its pozzolanic properties. Silica in fly ash reacts with alkaline compounds like lime, resulting in pozzolanic reactions.2
The loss of ignition (LOI) is another critical characteristic of fly ash. It must be minimized and is generally restricted to 5-6 % by standards. A higher LOI negatively impacts the mechanical properties, durability, and workability of fly ash-containing concrete.1 Understanding these properties is essential for optimizing the use of fly ash in various construction applications.
Application in Construction
The rapidly growing global construction industry consumes around 260 billion tons of cement annually, a figure expected to increase by 25 % over the next decade. At the same time, the disposal of fly ash in landfills poses significant health and environmental hazards due to contamination with harmful elements.
Utilizing fly ash in construction can significantly mitigate the environmental concerns associated with its disposal. Recycling fly ash into eco-friendly building materials not only reduces the demand for cement but also conserves natural resources like clinker and clay, promoting sustainable development in the industry.
Fly ash is also commonly employed as a pozzolanic material due to the presence of active aluminosilicates, partially or wholly replacing ordinary Portland cement (OPC) in concrete production. Additionally, the fine ash particles are favorable for producing self-compacting concrete with enhanced workability.2
Beyond cement and concrete, fly ash has applications in mine filling, embankment construction, lowland area retrieval, road construction, and various building materials such as aggregates, bricks, blocks, and tiles. Fly ash-based aggregates exhibit high porosity, which reduces the dead weight of structures and provides excellent thermal insulation.
Incorporating fly ash into construction materials and methods thus not only addresses environmental disposal issues but also enhances the sustainability and performance of building practices.2
Latest Developments
Recent advancements have focused on combining fly ash with other waste materials to mitigate the environmental impact of building materials while enhancing their functionality. For example, a study published in Scientific Reports proposed combining agro-forestry waste, construction and demolition (C&D) waste, and fly ash to create eco-friendly bricks. These bricks were evaluated for carbon emissions and thermal behavior in an 8×9×8 ft. building.3
The total CO2 emission for fly ash and burnt clay bricks was 43.28 gCO2 and 290 gCO2 per brick, respectively. Thus, substituting 1-2% of burnt clay bricks with the proposed bricks can reduce 0.5-1.5 million tons of annual CO2 emissions. Additionally, fly ash-based bricks consumed 10-15 times less energy than burnt clay bricks while providing adequate insulation.3
Beyond traditional building applications, fly ash has been used in innovative structures. For instance, an article in the Journal of Materials in Civil Engineering explored the use of fly ash geopolymer concrete (FGC) as a sustainable material for constructing marine artificial reefs. FGC offers high durability with reduced emissions, alkalinity, and cost. Microstructural investigations revealed dense pore structures in the FGC matrix, and the active calcium-containing elements in the FGC enhanced its mechanical strength and durability.4
Conclusion and Future Prospects
Overall, using fly ash as a cement substitute in concrete can address disposal concerns and lower CO2 emissions associated with cement production. However, efforts to reuse fly ash in construction have only been moderately successful.1
Higher concentrations of fly ash in building materials can deteriorate their mechanical properties, such as water absorption and workability. Therefore, either cement replacement should be employed at low concentrations to maximize the benefits of fly ash-containing concrete, or higher incorporation should be managed while ensuring that concrete properties remain within functional standards.2
Moreover, the current sustainable methods are likely to reduce the availability of fly ash, increasing the demand for natural resources like clinker and posing additional environmental challenges. Hence, the focus of sustainable building material development should be on the high-value utilization of fly ash rather than high-volume recycling.2
References and Further Reading
1. Amran, M., Fediuk, R., Murali, G., Avudaiappan, S., Ozbakkaloglu, T., Vatin, N., Karelina, M., Klyuev, S., & Gholampour, A. (2021). Fly Ash-Based Eco-Efficient Concretes: A Comprehensive Review of the Short-Term Properties. Materials, 14(15), 4264. https://doi.org/10.3390/ma14154264
2. Chaudhary, S., Iyer, N. R., Kim, D., & Pijush Samui. (2020). New Materials in Civil Engineering. Butterworth-Heinemann. ISBN: 0128189614,9780128189610
3. Singh, S., Maiti, S., Bisht, R. S., Panigrahi, S. K., & Yadav, S. (2024). Large CO2 reduction and enhanced thermal performance of agro-forestry, construction and demolition waste based fly ash bricks for sustainable construction. Scientific Reports, 14(1), 8368. https://doi.org/10.1038/s41598-024-59012-8
4. Wang, W., Wang, B., Shen, L., & Fan, C. (2024). Properties of Fly Ash Geopolymer Concrete as Marine Artificial Reef Building Materials. Journal of Materials in Civil Engineering, 36(2). https://doi.org/10.1061/jmcee7.mteng-16541
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