Over time, researchers have discovered materials (cementitious materials) which can partially replace cement and can achieve similar and/or higher compressive strengths whilst reducing the carbon emissions (Barbour, 1991). Amongst these cementitious materials FA, which can be defined as a fine powdered residue generated in coal fired power stations, have been proven to be a fine cement-replacement material in concrete (Ahmaruzzaman, 2010). Originally, FA was used as a cement-replacement material to improve the rheological characters, reduce the alkali-aggregate reactions and most
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importantly to reduce the carbon footprint by reducing the amount of cement in concrete. However, over time FA has been rapidly attracting attention to being used as a source material in the production of GPs due to high compositions of silicon and aluminium (Davidovits, 2008).
During the combustion process of coal three main products are formed, namely FA, bottom ash and gas/vapour. FA is identified as the fine part of ash and the bottom ash is identified as the heavier residue having coarser particles. The gas/vapour is partly condensed onto the surface of the FA particles and the remainder is discharged into the atmosphere (Joshi and Lohtia, 1993). FA is considered as fine, mostly spherical, hollow glassy particles having a diameter ranging from 1μm–150μm, which is finer than Portland cement and lime particles (Brahammaji and Muthyalu, 2015, Siddique, 2008). Figure 2.9 shows the collection of FA from coal fired electrical generating station.
Figure 2.9 – The collection of FA from flue gases (Davidovits, 2008)
When considering the chemical composition of FA, it is mainly comprised of silica, alumina and ferric oxide and other minor constituents such as calcium, sulphur, magnesium, phosphorus, titanium, alkaline and manganese (ASTM, 2003). However, the chemical and the physical compositions of FA vary and are dependent on the type of coal, the method of combustion and the particle
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shape from which the FA is produced. Fly ash can be categorised mainly into high calcium class C FA or low calcium class F FA. Table 2.1 shows the main categories of FA as given by Davidovits (2008). Hardjito and Rangan (2005) states that more than 10% Calcium oxide (CaO) can be found in FA which is produced from burning sub-bituminous coals These types are identified as high calcium class C FA containing high levels of calcium and low levels of both silica and alumina which provides cementitious and pozzolanic properties. FA having less than 10% CaO is considered to be low calcium class F FA and is formed from the bituminous and anthracite coals. These are found to contain high levels of silica and alumina and low levels of calcium resulting in only pozzolanic properties (Ramachandran, 1996, Davidovits, 2008, ASTM, 2003).
Table 2.1 – Main categories of FA (Davidovits, 2008)
Sub bituminous coals are generally brown to black in colour and contain a carbon percentage of around 42-52%. Records show that an estimated 50% of the worlds’ coal reserves are of sub bituminous or lignite coals, including deposits which can be found in Australia. Bituminous coals are found to have around 77-78% of a carbon percentage and elements such as water, sulphur, hydrogen and few other impurities. The production of bituminous coals is found to occur when sub bituminous coals undergo a more organic process of metamorphism. Anthracite coals on the other hand, have the highest carbon percentage and therefore lesser impurities. Anthracite coals do not ignite easily and produce blue smokeless flames upon ignition for a short time. These types of coals are recorded to be comparatively rare and hard to find. Heidrich (2002) stated that a majority of the FA found in Australia contains 80%–85% silica and alumina and can be categorised as Class F low calcium
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FA. Table 2.2 gives the chemical composition for class C and class F FA (Davidovits, 2008).
Table 2.2 – Range of chemical compositions for low and high class FAs (Davidovits, 2008)
In 1998, the annual ash production was estimated to be more than 390 million tonnes and this value was estimated to massively increase to about 780 million tonnes annually by the year 2010 (Mehta, 2004). In the year 2000, FA production in Australia was calculated to be approximately 12 million tonnes out of which only 5.5 million tonnes had been utilised (Heidrich, 2002). FA production in the United Sates was about 68 million tonnes in the year 2001 of which only 32% had been utilised (Brahammaji and Muthyalu, 2015). A summary of the production of coal combustion products (CCPs) in the United States from 1991 to 2016 as given by the American Coal Ash Association is shown in Figure 2.10 (Association, 2017). The production of CCPs were seen to decrease in the years 2014, 2015 and 2016, however the usage remained somewhat constant. This report further gives information that out of the 107.4 million tonnes of CCPs produced in the year 2016 in the United States, 37.8 million tonnes was FA. Though the production of FA is seen to decrease from the year 2001 to 2016 in the United States, Harris (2017) stated that within the next 30 years, countries such as China, India and other South East Asian countries will experience an increase in the production of FA.
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Figure 2.10 – Production and usage of all CCPs from 1991 to 2016 in the United States (Association, 2017).
After an extensive review, Izquierdo and Querol (2012) provides information that because FA is a heterogeneous material (diverse in character) and the elements are not equally distributed, it imposes a big threat on the environment in terms of land and water pollution whether it is used as recycled ash, sent off to landfills or disposed in surface impoundments.
Several advantages in both fresh and hardened concretes have been identified through the use of FA in concrete. These can be identified as improvements in workability, reduction of water consumption, reduction of bleeding and slower setting time in fresh concrete, together with higher strength readings, reduced permeability, increase durability in hardened concretes (Oner et al., 2005). Additionally, the use of FA contributes greatly into reducing the carbon footprint on the environment.