Fly ash is the finely divided residue generated during the combustion of pulverized coal, which is carried by flue gases and subsequently collected through electrostatic precipitators. Globally, fly ash is predominantly utilized as a pozzolanic material (Shetty, 2000). In reinforced concrete production, its incorporation is generally restricted to 25% or less by weight (Berryman et al., 2005). Excess or unused fly ash is typically disposed of in landfills, where it contributes to soil, water, and air pollution (Diaz-Loya et al., 2011; Palomo et al., 1999).
The properties of fly ash are strongly influenced by the type of coal from which it is derived (Ram Chandar et al., 2022). Combustion of anthracite and bituminous coals yields low-calcium fly ash, which exhibits predominantly pozzolanic behavior due to its high silica content (Ram Chandar et al., 2022). Conversely, lignite and sub-bituminous coals produce high-calcium fly ash, which possesses both cementitious and pozzolanic characteristics. The latter contains relatively lower silica and alumina, but higher CaO concentrations (Dhir et al., 1984).
Fly ash can be incorporated into construction materials in two principal ways: (i) blending with cement clinker to manufacture Portland pozzolana cement, or (ii) direct addition as an admixture during concrete production at construction sites (Shetty, 2000). According to the American Society for Testing and Materials (ASTM C618), fly ash is broadly categorized into two classes: Class F and Class C. Class F is produced from the combustion of anthracite or bituminous coal, while Class C is derived from lignite or sub-bituminous coal.
Chemical composition also plays a critical role in determining performance. Excessive SO₃ content in fly ash may induce volume instability in concrete due to ettringite formation, which compromises long-term durability. To mitigate this risk, Indian Standards (IS 3812-Part 1:2003) specify a maximum permissible SO₃ content of 3% for fly ash intended for use as a binder in concrete. Furthermore, particle fineness is a key parameter influencing the development of mechanical strength in fly ash–blended systems (Fernández-Jiménez & Palomo, 2003).
Reference
Berryman, C., Zhu, J., Jensen, W., & Tadros, M. (2005). High-percentage replacement of cement with fly ash for reinforced concrete pipe. Cement and Concrete Research, 35(6), 1088–1091. https://doi.org/10.1016/j.cemconres.2004.06.040
Dhir, R., Munday, J., & Ong, L. T. (1984, June 1). INVESTIGATIONS OF THE ENGINEERING PROPERTIES OF OPC/PULVERISED FUEL ASH CONCRETE: STRENGTH DEVELOPMENT AND MATURITY. https://www.semanticscholar.org/paper/INVESTIGATIONS-OF-THE-ENGINEERING-PROPERTIES-OF-OPC-Dhir-Munday/a8a01682c37872a6d285298fe9e4ab714f2d5a5c
Diaz-Loya, E. I., Allouche, E. N., & Vaidya, S. (2011). Mechanical Properties of Fly-Ash-Based Geopolymer Concrete. ACI Materials Journal, 108(3). https://trid.trb.org/View/1103895
Fernández-Jiménez, A., & Palomo, A. (2003). Characterisation of fly ashes. Potential reactivity as alkaline cements☆. Fuel, 82(18), 2259–2265. https://doi.org/10.1016/S0016-2361(03)00194-7
Palomo, A., Grutzeck, M. W., & Blanco, M. T. (1999). Alkali-activated fly ashes. Cement and Concrete Research, 29(8), 1323–1329. https://doi.org/10.1016/S0008-8846(98)00243-9
Ram Chandar, K., Gayana, B. C., & Shubhananda Rao, P. (2022, May 27). Mine Waste Utilization. https://doi.org/10.1201/9781003268499
Shetty, M. S. (2000, November 30). Concrete Technology: Theory and Practice. https://www.semanticscholar.org/paper/Concrete-Technology%3A-Theory-and-Practice-Shetty/6eb1a6e7cdb03e5f2e84f5b303b3429b665abfe7
