Coal bottom ash (CBA) is physically similar to natural aggregates and resembles Portland cement (PC) when pulverized into finer particles.
The increasing trend in coal consumption will continue mainly due to the high demand for electricity. Coal is rapidly gaining favor as an energy source for generating electricity, after gas . The 0.9% increase in world coal consumption in 2019 was driven by Asia (1.8%). The utilization of coal as a global source of electricity generation is expected to increase to 47% by 2030 .
The high demand for coal production has resulted in the generation of a higher amount of industrial waste. Fly ash makes up 70–80% of the total coal ash wastes, and the remaining 10–20% is bottom ash . Of the millions of tons of coal ash waste generated annually, 100 million metric tons (Mt) is bottom ash, and the remainder is fly ash . The World of Coal Ash (WOCA) estimated that coal thermal power plants generate 780 million metric tons of coal bottom ash (CBA), of which 66% is by Asian countries, followed by Europe and the United States . China produces the highest amount of coal ash of 395 million metric tons (Mt), followed by the US (118 Mt), India (105 Mt), Europe (52.6 Mt), and Africa (31.1 Mt). The Middle East and other countries contributed a small amount to the global coal ash generation . Of the 105 million metric tons of coal produced in India , about 35 million metric tons is coal bottom ash produced by the power plants that generate electricity .
The wastes produced in electricity generation are boiler slag, fly ash, clinker, and bottom ash . The physical properties and the chemical composition of bottom ash and fly ash differ because fly ash is lighter than the bottom ash collected in a hopper after falling through the bottom furnace. The bottom ash could be wet or dry bottom ash, depending on the type of boiler.
The disposal of bottom ash landfills has raised a grave environmental concern . The high composition of heavy metal in bottom ash, relative to fly ash, increases the risk of groundwater pollution . One way to deal with the increasing amount of CBA generated and the scarcity of land is by recycling and reusing CBA .
The specific properties of coal bottom ash are dependent on factors such as the coal source and type of coal. There are four types of coal: anthracite, bituminous, sub-bituminous, and lignite . The type of coal is dependent on the types and amounts of carbon, the amount of heat energy the coal can produce, the level of carbon moisture, and other chemical elements . Anthracite has the highest carbon content, followed by bituminous, sub-bituminous, and lignite. Generally, the types of coal used in energy generation are bituminous, sub-bituminous, and lignite. The geological formation of the coal determines its chemical composition; the CBA from the different types of coal have varying silica oxide (SiO 2), alumina oxide (Al 2O 3), and ferric oxide (Fe 2O 3) contents and characteristics that influence the research finding .
Researchers have investigated steel slag, coconut waste, recycled asphalt, recycled concrete, mining waste, glass, crumb rubber, palm oil shell, and palm oil clinker as natural aggregate replacement. Table 1 summarizes the physical properties of the wastes used as aggregate replacement in asphalt pavements and concrete production.
Table 1. The physical properties of the wastes used as aggregates in asphalt pavement and concrete production.
|Waste||Physical Property Parameters||Used As|
|Specific Gravity (No Unit)||Water Absorption (%)||Los Angeles Abrasion (%)||Moisture Content (%)||Fineness Modulus (%)||Fine||Coarse||Reference|
|3.42 *||3.31 *||-||1.56 *||-||√||√|||
|3.58 **||4.23 **||2.8 **|
|Recycled concrete||2.41 *||4.80 *||18.7||√||√|||
|2.42 **||7.40 **|
|2.18 *||2.69 *||24||√||√|||
|2.42 **||4.28 **|
|2.42–2.44 *||6.5–6.8 *||-||-||-||√||√|||
|2.415 **||9 **|
|Palm oil shell||1.37||12.47||-||-||6.53||√|||
|Palm oil clinker||2.08||-||-||√|||
|1.18 *||4.35 *||-||0.28 *||√||√|||
|2.15 **||5.75 **||0.11 **|
* Coarse aggregate, ** fine aggregate.
The physical properties of the wastes used as aggregates in asphalt pavement and concrete production.
Asphalt construction requires a large amount of natural aggregates, namely 100% aggregates for the base and subbase courses, 95% for bituminous, and 87% for concrete pavements. The natural aggregates used to construct one kilometer of a surface course using a bituminous mixture could exceed 15,000 tons . In recent years, natural aggregate replacement with CBA has reduced construction costs and minimized the need to harvest aggregates from natural resources.
|References||Function||Effect on Pavement Performance|
|||Filler in SMA mixture||
|||Fine aggregate in HMA||
|||Cement replacement in roller-compacted concrete pavement||
|||Filler replacement in asphalt mixture||
|||Filler replacement in HMA mixture||
|||An additive in cold recycled mixture||
|||Fine aggregate in HMA||
|||Fine and coarse aggregate replacement||