Everything about Fly Ash totally explained
Fly ash is one of the residues generated in the combustion of coal. Fly ash is generally captured from the chimneys of power generation facilities, whereas
bottom ash is, as the name suggests, removed from the bottom of the furnace. In the past, fly ash was generally released into the atmosphere via the smoke stack, but pollution control equipment mandated in recent decades now require that it be captured prior to release. It is generally stored on site at most
US electric power generation facilities. Depending upon the source and makeup of the coal being burned, the components of the fly ash produced vary considerably, but all fly ash includes substantial amounts of silica (silicon dioxide, SiO
2) (both amorphous and crystalline) and lime (calcium oxide, CaO). Fly ash is commonly used to supplement
Portland cement in concrete production, where it can bring both technological and economic benefits, and is increasingly finding use in synthesis of
geopolymers and
zeolites.
Chemical composition and classification
Two classes of fly ash are defined by
ASTM C618
: Class F fly ash and Class C fly ash. The chief difference between these classes is the amount of calcium, silica, alumina, and iron content in the ash. The chemical properties of the fly ash are largely influenced by the chemical content of the coal burned (for example,
anthracite,
bituminous, and
lignite).
Not all fly ashes meet ASTM C618 requirements, although depending on the application, this may not be necessary. Ash used as a cement replacement must meet strict construction standards, but no standard environmental standards have been established in the United States. 75% of the ash must have a
fineness of 45 µm or less, and have a
carbon content, measured by the loss on ignition (LOI), of less than 4%. In the U.S., LOI needs to be under 6%. The particle size distribution of raw fly ash is very often fluctuating constantly, due to changing performance of the coal mills and the boiler performance. This makes it necessary that fly ash used in concrete needs to be processed using separation equipment like mechanical air classifiers. Especially important is the ongoing quality verification. This is mainly expressed by quality control seals like the Indian ISI mark or the DCL mark of the Dubai Municipality.
Class F fly ash
The burning of harder, older anthracite and bituminous coal typically produces Class F fly ash. This fly ash is
pozzolanic in nature, and contains less than 10%
lime (CaO). Possessing pozzolanic properties, the glassy silica and alumina of Class F fly ash requires a cementing agent, such as Portland cement, quicklime, or hydrated lime, with the presence of water in order to react and produce cementitious compounds. Alternatively, the addition of a chemical activator such as
sodium silicate (water glass) to a Class F ash can lead to the formation of a
geopolymer.
Class C fly ash
Fly ash produced from the burning of younger lignite or subbituminous coal, in addition to having pozzolanic properties, also has some self-cementing properties. In the presence of water, Class C fly ash will harden and gain strength over time. Class C fly ash generally contains more than 20% lime (CaO). Unlike Class F, self-cementing Class C fly ash doesn't require an activator. Alkali and
sulfate (SO
4) contents are generally higher in Class C fly ashes.
Disposal and market sources
In the past, fly ash produced from coal combustion was simply entrained in
flue gases and dispersed into the atmosphere. This created environmental and health concerns that prompted laws which have reduced fly ash emissions to less than 1% of ash produced. Worldwide, more than 65% of fly ash produced from coal power stations is disposed of in
landfills. In India alone, fly ash landfill covers an area of 40,000 acres (160 km²).
The recycling of fly ash has become an increasing concern in recent years due to increasing landfill costs and current interest in
sustainable development. In 2005, U.S. coal-fired power plants reported producing 71.1 million tons of fly ash, of which 29.1 million tons was reused in various applications. If the nearly 42 million tons of unused fly ash had been recycled, it would have reduced the need for approximately of landfill space. Other environmental benefits to recycling fly ash includes reducing the demand for virgin materials that would need
quarrying and substituting for materials that may be energy-intensive to create (such as
Portland cement).
Fly ash reuse
The reuse of fly ash as an engineering material primarily stems from its pozzolanic nature, spherical shape, and relative uniformity. Fly ash recycling, in descending frequency, includes usage in:
Portland cement
Owing to its
pozzolanic properties, fly ash is used as a replacement for some of the
Portland cement content of
concrete. The use of fly ash as a pozzolanic ingredient was recognized as early as 1914, although the earliest noteworthy study of its use was in 1937. Before its use was lost to the Dark Ages, Roman structures such as
aqueducts or the
Pantheon in Rome used volcanic ash (which possesses similar properties to fly ash) as pozzolan in their concrete. As pozzolan greatly improves the strength and durability of concrete, the use of ash is a key factor in their preservation.
Use of fly ash as a partial replacement for Portland cement is generally limited to Class F fly ashes. It can replace up to 30% by mass of Portland cement, and can add to the concrete’s final strength and increase its chemical resistance and durability. Recently concrete mix design for partial cement replacement with High Volume Fly Ash (50 % cement replacement) has been developed. For Roller Compacted Concrete (RCC)[usedin dam construction] replacement values of 70% have been achieved with POZZOCRETE (processed fly ash) at the Ghatghar Dam project in Maharashtra, India. Due to the spherical shape of fly ash particles, it can also increase workability of cement while reducing water demand. The replacement of Portland cement with fly ash also reduces the
greenhouse gas foot print of concrete, as the production of one ton of Portland cement produces approximately one ton of
CO2. Since the worldwide production of Portland cement is expected to reach nearly 2 billion tons by 2010, replacement of 30% of this amount by fly ash could dramatically reduce global carbon emissions.
Embankment
Fly ash properties are somewhat unique as an engineering material. Unlike typical soils used for embankment construction, fly ash has a large uniformity coefficient consisting of
silt-sized particles. Engineering properties that will affect fly ash’s use in embankments include grain size distribution,
compaction characteristics,
shear strength,
compressibility,
permeability, and
frost susceptibility.
Asphalt concrete
Asphalt concrete is a composite material consisting of an asphalt binder and mineral aggregate. Both Class F and Class C fly ash can typically be used as a mineral filler to fill the voids and provide contact points between larger aggregate particles in asphalt concrete mixes. This application is used in conjunction, or as a replacement for, other binders (such as Portland cement or hydrated lime). For use in apshalt pavement, the fly ash must meet mineral filler specifications outlined in
ASTM D242. The hydrophobic nature of fly ash gives pavements better resistance to stripping. Fly ash has also been shown to increase the stiffness of the asphalt matrix, improving rutting resistance and increasing mix durability.
Geopolymers
More recently, fly ash has been used as a component in
geopolymers, where the reactivity of the fly ash glasses is used to generate a binder comparable to a hydrated
Portland cement in appearance and properties, but with dramatically reduced CO
2 emissions.
Roller compacted concrete
Another new application is using fly ash in
roller compacted concrete dams. This has been demonstrated in the
Ghatghar Dam Project in
India.
Bricks
Ash bricks have been used in house construction in
Windhoek, Namibia since the 1970's. There is, however, a problem with the bricks in that they tend to fail or produce unsightly pop-outs. This happens when the bricks come into contact with moisture and a chemical reaction occurs causing the bricks to expand.
In May 2007,
Henry Liu, a retired 70-year old American
civil engineer, announced that he'd invented a new, environmentally sound building
brick composed of fly ash and water. Compressed at 4,000
psi and cured for 24 hours in a 150 °F (66 °C) steam bath, then toughened with an
air entrainment agent, the bricks last for more than 100 freeze-thaw cycles. Owing to the high concentration of
calcium oxide in class C fly ash, the brick can be described as "self-cementing". The manufacturing method is said to save energy, reduce
mercury pollution, and costs 20% less than traditional clay brick manufacturing. Liu intends to license his technology to manufacturers in 2008.
Waste management
Using a proprietary methodology, the US company N-Viro International Corporation uses the alkaline properties of fly ash to process
human waste sludge into fertilizer.
Similarly, the RHENIPAL process owned by DIRK Group utilizes fly ash mixtures for the stabilization of sewage sludge and other toxic sludges. This process was used to stabilize large amounts of
chromium(VI) contaminated leather sludges in Portugal (Alcanena)
Environmental problems
Fly ash, like soil, contains trace concentrations of many
heavy metals that are known to be detrimental to health in sufficient quantities. These include
nickel,
vanadium,
arsenic,
beryllium,
cadmium,
barium,
chromium,
copper,
molybdenum,
zinc,
lead,
selenium,
uranium,
thorium, and
radium. Though these elements are found in extremely low concentrations in fly ash, their mere presence has prompted some to sound alarm.
The
U.S. EPA has said in the past that coal fly ash doesn't need to be regulated as a hazardous waste. However, a revised risk assessment may change the way CCW is regulatedThe EPA's headquarters building in
Washington, D.C. is constructed with concrete containing fly ash. Studies by the
U.S. Geological Survey and others conclude that fly ash compares with common soils or rocks and shouldn't be the source of alarm.
Exposure Concerns
Crystalline silica and
lime are the major components of exposure concern. In and of itself, fly ash is neither toxic or poisonous, nor is it considered hazardous except when it becomes airborne. However, the fine crystalline silica present in fly ash has been linked with lung damage, in particular
silicosis. OSHA allows 0.10 mg/m
3, (one ten-thousandth of a gram per cubic meter of air).
The other fly ash component of some concern is lime (CaO). This chemical reacts with water (H
2O) to form calcium hydroxide [Ca(OH)
2], giving fly ash a pH somewhere between 10 and 12, a medium to strong base. This can also cause lung damage if present in sufficient quantities.
These hazards can be minimised by controlling emissions of fly ash from bulk handling operations via closed pumping systems, and use of storage and handling equipment with approved automated spill containment equipment.
References
Further Information
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