Many civil engineers are attracted to fly ash today, mostly due to commercial considerations. However, if it is looked upon as a performance improver, fly ash can solve many problems.
Fly ash is a by-product obtained while generating electrical power using coal as a fuel. For the last few decades, the demand for electricity has been increasing continually due to growing population and new industries being launched across the world.
During the year 2014-15; the total generation of fly ash has been 184.14 Million Tonnes out of which 102.54 Million tonnes is used, i.e.about 55.69 per cent of generated fly ash. (Information sourced from Central Electricity Authority, Govt. of India) But since fly ash contains a lot of heavy metals, its production causes problems to the environment. Because of this reason, it has become necessary to invent some newer aspects for utilising fly ash, rather than stick to the conventional ways. Fly ash disposal is a serious environmental concern due to its hazardous properties, impact on agriculture and long-term risks to both ecosystems and human beings. Fly ash is a coal-combustion product, composed of the fine particles that are driven out of the boiler with the flue gases. Ash that falls in the bottom of the boiler is called bottom ash. In modern coal-fired power plants, fly ash is generally captured by electrostatic precipitators or other particle filtration equipment before the flue gases reach the chimneys. Together with bottom ash removed from the bottom of the boiler, it is known as coal ash.
Depending upon the source and makeup of the coal being burned, the chemical composition of fly ash will vary considerably; but in general, all fly ash includes substantial amounts of silicon dioxide (SiO2) - both amorphous and crystalline, aluminium oxide (Al2O3) and calcium oxide (CaO), the main mineral compounds in coal-bearing rock strata.
In the past, fly ash was generally released into the atmosphere, but air pollution control standards now require that it be captured prior to release by fitting pollution control equipment. In many countries, fly ash is either stored at coal power plants or placed in landfills. A substantial part of fly ash goes to the cement industry as a cement substituting material, to produce hydraulic cement or hydraulic plaster and a replacement or partial replacement for Portland cement in concrete production. Pozzolans ensure the setting of concrete and plaster and provide concrete with more protection from wet conditions and chemical attack.
Cenospheres are the most important ingredient of fly ash. These are unique free-flowing powders composed of hard-shelled, hollow, minute spheres. Cenospheres are made up of silica, iron and alumina. These have a size range from 1 to 500 microns with an average compressive strength of 3000+ psi. Colours range from white to dark grey.
Fly ash particles are generally spherical in shape and range in size from 0.5µm to 300µm. The major consequence of the rapid cooling is that few minerals have time to crystallise, and that mainly amorphous, quenched glass remains. Nevertheless, some refractory phases in the pulverised coal do not melt entirely, and remain crystalline. In consequence, fly ash is a heterogeneous material. SiO2, Al2O3, Fe2O3 and occasionally CaO are the main chemical components present in fly ash. The mineralogy of fly ash is very diverse. The main phases encountered are a glass phase, together with quartz, mullite and the iron oxides hematite, magnetite and/or maghemite. Other phases often identified are cristobalite, anhydrite, free lime, periclase, calcite, sylvite, halite, portlandite, rutile and anatase. The Ca-bearing minerals anorthite, gehlenite, akermanite and various calcium silicates and calcium aluminates identical to those found in Portland cement can be identified in Ca-rich fly ash. The mercury content can reach 1 ppm, but is generally included in the range 0.01 - 1 ppm for bituminous coal. The concentrations of other trace elements vary as well according to the kind of coal combusted to form it.
The fineness or the particle size of fly ash is a very critical property. The commercial value of fly ash depends on the particles and percentage of unburned carbon. A total of 75 per cent of the ash must have a fineness of 45 -¦m or less, and have carbon content, measured by the loss on ignition (LOI), of less than 4 per cent. The quality of fly ash also depends on the type of technology used in the boiler and the quality of coal. Indian coal generally has high percentage of ash compared to either Australian or South African coal.
The bottom ash, which is still a neglected portion of the same waste, can also find its way into concrete, replacing the stone aggregates. But a lot needs to be done on this material in our country.
Benificiation, the value addition
By using air separators, the fly ash can be divided into different grades depending on the particle size. The best example is that of Dirk India, a company which is only into selling of fly ash, and thriving. Dirk sources its fly ash from state electricity generating companies, and by using air classifiers, produces four different varieties of fly ash which have a good demand in the market.
Challenges in handling fly ash
In a typical fly ash handling system, the material that is generated as a result of combustion is captured by an electrostatic precipitator (ESP) or a bag house before the flue gases reach the stack. These ESPs and bag houses generally have multiple pyramidal hoppers at the bottom, in which the ash is collected by gravity and is then transferred to a storage silo. These storage silos generally have provisions for a truck load-out to carry the fly ash for disposal or reuse. As a result of the frictional nature and fine particle size distribution, fly ash handling systems often experience problems if they are designed without following a prudent engineering approach.
Flow rate limitation
The permeability of fly ash is typically very low, due to its fine particle size distribution. As a result, when de-aerated, fly ash provides considerable resistance to the flow of air or other gases. During discharge from a silo or hopper outlet, air counter-flow through the fly ash bed provides an opposing force to gravity. This air ingress occurs as a result of the natural expansion of the ash bed within the hopper as it flows, or simply due to leakage from the conveying system below. As a result, fly ash hoppers and silos are limited in terms of the maximum discharge rates that they can provide by gravity alone.
Flooding or uncontrolled flow
As a fine powder, fly ash can behave like a fluid when sufficient air is present. Flooding can result, particularly when the handling rate is too high to allow sufficient time for the entrained air to escape. In this case, the fly ash may become fluidised and flush through the outlet unless the feeder can contain it. Flooding not only creates a challenge in metering the discharge, it can also lead to serious environmental, health and safety concerns.
The presence of a significant portion of silicon dioxide makes fly ash very abrasive and frictional. As a result of material sliding and impacting within the handling equipment, wall surfaces undergo tremendous wear.
Dusting can particularly occur at transfer points where the air entrained in the powder is suddenly expelled, carrying these finer particles with it. Dust generation also occurs when local air currents have sufficient velocity to pick up particles from the surface of a pile. Dust by itself is a nuisance and, more importantly, it can result in safety concerns including the health effects of operator exposure and the potential for explosions.
Agglomerated lumps of fly ash and foreign materials can create flow problems, especially when handling fly ash with air slides or aerated bin bottoms. Therefore handling of fly ash always requires proper engineering inputs. The progress made so far has been reasonable, but a lot still needs to be done considering the potential of the material. Let us look at some of the possibilities.
Fly ash as cement replacement and high-volume fly ash concrete
Indian codes permit use of fly ash up to 35 per cent as cement replacement in cement manufacturing. However, in the case of concrete, the percentage will depend on the mix design recipe. If we see the practise abroad, up to 65 per cent of fly ash can be used in concrete.
Two researchers, Dr PK Mehta and Dr VM Malhotra, have made a significant contribution to the engineering community. Dr Mehta is well known for his work on high-volume fly ash concrete. Here is an extract from one of his papers called ´Cement & Concrete Mixtures for Sustainability´:
¨For a variety of reasons, the concrete construction industry is not sustainable. First, it consumes huge quantities of virgin materials. Second, the principal binder in concrete is Portland cement, the production of which is a major contributor to greenhouse gas emissions that are implicated in global warming and climate change. Third, many concrete structures suffer from lack of durability which has an adverse effect on the resource productivity of the industry. Because the high-volume fly ash concrete system addresses all three sustainability issues, its adoption will enable the concrete construction industry to become more sustainable.¨
In this paper, a brief review is presented of the theory and construction practice with concrete mixtures containing more than 50 per cent fly ash by mass of the cementitious material. Dr Mehta is a champion of using more than 50 per cent fly ash by mass of the cementitious material. Mechanisms are discussed by which the incorporation of high volume of fly ash in concrete reduces the water demand, improves the workability, minimises cracking due to thermal and drying shrinkage, and enhances durability to reinforcement corrosion, sulfate attack, and alkali-silica expansion. For countries like China and India, this technology can play an important role in meeting the huge demand for infrastructure in a sustainable manner, says Dr Mehta.
High-performance concrete (HVFA)
As per Dr Mehta, the characteristics defining a HVFA concrete mixture are, minimum of 50 per cent of fly ash by mass of the cementitious materials must be maintained, low water content, generally less than 130 kg/m3 is mandatory and for cement content, generally no more than 200kg/m3 is desirable. For concrete mixtures with specified 28-day compressive strength of 30 MPa or higher, slumps more than 150 mm, and water-to-cementitious materials ratio of the order of 0.30, the use of high-range water-reducing admixtures (super-plasticizers) is mandatory. For concrete exposed to freezing and thawing environments, the use of an air-entraining admixture resulting in adequate air-void spacing factor is mandatory. For concrete mixtures with slumps less than 150 mm and 28-day compressive strength of less than 30 MPa, HVFA concrete mixtures with a water-to-cementitious materials ratio of the order of 0.40 may be used without super-plasticizers. Dr Mehta further concludes in his paper, that throughout the world, waste-disposal costs have escalated greatly. At the same time, the concrete construction industry has realised that coal fly ash is a relatively inexpensive and widely available by-product that can be used for partial cement replacement to achieve excellent workability in fresh concrete mixtures.
Consequently, in the modern construction practice, 15 to 20 per cent of fly ash by mass of the cementitious material is now commonly used in North America. Higher amounts of fly ash in the order of 25-30 per cent are recommended when there is a concern over thermal cracking, alkali-silica expansion or sulfate attack. Such high proportions of fly ash are not readily accepted by the construction industry due to a slower rate of strength development at an early age.
The high-volume fly ash concrete system overcomes the problems of low early strength to a great extent through a drastic reduction in the water-cementitious materials ratio by using a combination of methods, such as taking advantage of the super-plasticizing effect of fly ash when used in a large volume, with the use of a chemical super-plasticizer, and a judicious aggregate grading.
Consequently, properly cured high-volume concrete products are very homogenous in microstructure, virtually crack-free, and highly durable. Because there is a direct link between durability and resource productivity, the increasing use of high-volume concrete will help to enhance the sustainability of the concrete industry.
In conclusion, high-volume fly ash concrete offers a holistic solution to the problem of meeting the increasing demands for concrete in the future in a sustainable manner and at a reduced or no additional cost. At the same time, it reduces the environmental impact of two industries that are vital to economic development - the cement industry and the coal-fired power industry. The technology of high-volume fly ash concrete is especially significant for countries like China and India, where, given the limited amount of financial and natural resources, the huge demand for concrete needed for infrastructure and housing can be easily met in a cost-effective and ecological manner.