One way of combating problems caused by fly ash is by optimizing the uses of fly ash so that it could become a valuable raw material. This can be achieved through precisely defining and controlling physical and chemical characteristics of fly ash; so that a uniform and reproducible material can be supplied for reuse (Foner et al., 1998). Some of the uses of fly ash are as follows:
2.9.1 Cement and Construction Industry
Fly ash is used in the construction industry in the making of cement or concrete because of its cementious and pozzolanic properties which improves the workability, durability and strength in hardened concrete (Jaturapitakkul et al., 2003). The loss on ignition (LOI), which is a measurement of the amount of unburned carbon remaining in the fly ash (Styszko-Grochowiak et al., 2004), is one of the most significant chemical properties of fly ash, especially as an indicator of suitability for use as a
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cement replacement in concrete. According to ASTMC618, a loss-on-ignition (LOI) greater than 6 % renders fly ash unusable for cement or concrete manufacture, because the presence of carbon can influence air entrainment which is an important property of concrete. Surfactants, or air entraining admixtures that are normally used in the formulation of concrete, can be adsorbed onto the surfaces of the porous residual carbon particles resulting in a reduced resistance of the concrete to freeze and thaw (Senneca, 2008).
Fly ashes can certainly be beneficially used in the concrete and construction industries. It is used as a partial replacement for Portland cement in concrete manufacture (Ahmaruzzaman, 2010) and used as a sand supplement in the manufacture of building bricks, blocks and pavers (Manz, 1997). It is also used as a replacement for the fine aggregate (sea sand or machine-ground sand) in concretes and mortars. In addition it is also used as a constituent of light-weight aerated concrete, especially for construction of insulating building blocks. These could replace many of the low fines concrete blocks used presently. Furthermore it is used as a constituent of “flowable fill” for filling trenches, and surrounding insulation in building basements, shelters, foundations etc. (Foner et al., 1999).
2.9.2 Treatment of Acid Mine Drainage (AMD)
Acid mine drainage (AMD) is formed when sulphide minerals, such as pyrite, found in association with the coal or overburden come into contact with oxygen and water during mining and oxidize. Sulphide minerals undergo further bacterially-catalysed oxidation reactions which accelerate acidity generation and increases Fe and sulphate concentrations in recipient water bodies. Acid mine drainage is characterized by high acidity (pH 2–4), high sulphate concentrations (1–20 gL-1) and contains high concentration of heavy metals such as Fe, Mn, Al, Cu, Ca, Pb, Mg, Na and Ni (Gitari et al., 2008). Acid mine drainage, often at a pH 2 or less, that contains a wide range of heavy metals is a major source of ground and surface water pollution near abandoned mines (Dutta et al., 2009). Fly ash can be used in the treatment of acid mine drainage because it contains relative high concentrations of SiO2, Al2O3 and CaO, which is
considered as a liming agent to neutralise acid mine drainage. The neutralisation of
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acid mine drainage is usually attained by the addition of chemicals such as CaO, Ca(OH)2, CaCO3, NaOH and Na2CO3. The use of fly ash for acid mine drainage
neutralisation, involves a process whereby the pH of the acid mine drainage is increased from 2–3 up to a neutral pH of 7, or even over pH 10. Fly ash contains considerable amounts of total alkalinity in the form of CaO, MgO, K2O and Na2O,
thus increasing the neutralisation potential of fly ash. Calcium oxide (CaO) is formed in fly ash by the oxidation of calcium during coal combustion process in a coal-fired power station. Fly ash may therefore be a substitute for limestone or lime treatment in the neutralisation of acid mine drainage (Somerset et al., 2005).
2.9.3 Removal of Heavy Metals in Water Treatment
Heavy metals are among the most important pollutants in wastewater, and are becoming a severe public health problem due to the toxicity of some heavy metals. Removal of heavy metals and metalloids from aqueous solutions is usually carried out by a number of processes such as, chemical precipitation, solvent extraction, ion exchange, reverse osmosis or adsorption etc. Among these processes, the adsorption process may be a simple and effective technique for the removal of heavy metals from wastewater. Fly ash has been widely used as a low-cost adsorbent for the removal of heavy metal such as Ni, Cr, Pb, As, Cu, Cd and Hg from wastewaters. The major chemical composition of fly ash (alumina, silica, ferric oxide, calcium oxide, magnesium oxide and carbon), and its physical properties such as porosity, particle size distribution and surface, highlights its potential as an adsorbent in waste water treatment (Cetin and Pehlivan, 2006).
Heavy metal adsorption on fly ash depends on the initial concentration of the heavy metal, contact time and pH. The initial concentration of heavy metal has a strong effect on the adsorption capacity of the fly ash. The adsorption capacity of fly ash depends on the surface activities, such as specific surface area available for solute surface interaction. In a certain pH range, most metal adsorption increased with increased pH up to a certain value, and then decreases with further increase in pH (Krishnan and Anirudhan, 2003).
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Fly ash can be regenerated after the adsorption, using suitable reagents. Batabyal et al., 1995) reported the regeneration of the used saturated fly ash with 2% aqueous H2O2 solution. The regenerated fly ash was dried, cooled and used for further
adsorption. The adsorption rate and equilibrium time were found to be the same as the fresh fly ash particles.
2.9.4 Soil Amendment
Fly ash is also used in agriculture as a soil amendment, in composting and as a source of nutrients for plants based on its chemical compositions and physical properties. Fly ash contains almost all the necessary plant nutrients i.e., macronutrients including P, K, Ca, Mg and S and micronutrients like Fe, Mn, Zn, Cu, Co, B and Mo, except organic carbon and nitrogen. It can replace lime, a costly amendment for acid soils; agricultural lime application contributes to global warming through emission of CO2
to the atmosphere. Use of fly-ash instead of lime as soil ameliorant can reduce net CO2emission and thereby lower global warming (Basu et al., 2007). According to
Kishor et al., (2010), the beneficial and harmful effects of fly ash application to soil are as follow
Beneficial effects
(1) Improvement in soil texture; (2) reduction in the bulk density of soil; (3) improvement in the water holding capacity of the soil; (4) optimization of the soil pH value; (5) increases the soil buffering capacity; (6) improvement in the soil aeration, percolation and water retention in the treated zone (due to dominance of silt-size particles in fly ash); (7) reduction in crust formation; (8) provision of micro-nutrients like Fe, Zn, Cu, Mo, B etc.; (9) provision of macro-nutrients like K, P, Ca, etc.; (10) reduction in the consumption of soil ameliorants (fertilizers, lime); (11) fly ash can also be used for insecticidal purposes and (12) decreases the metal mobility and availability in soil, due to an increased pH.
Harmful effects
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(1) High pH results in the reduction in bioavailability of some nutrients (generally from 8 to 12); (2) high salinity and (3) high content of phytotoxic elements, especially boron.
Fly ash may be suitable for use on agricultural land where food crops are produced, although potential trace element enrichment in plants from certain types of fly ash may make it more suitable for non-food chain end uses (Punshon et al., 2002).