Properties of Fly ash

1. What is fly ash?

Fly ash is the ash removed from the exhaust gas of burning coal at power
plants to generate electricity. The ash is removed from the exhaust by air pollution control equipment such as electrostatic precipitators before the exhaust is emitted through stacks or chimneys into the atmosphere. Thus, fly ash is a byproduct of burning coal for heating or generation of electricity.

2. Is fly ash utilized? If so, how much?

Over 70 million tons of fly ash is generated each year in the United States, of which only about 1/3 is used. The remaining 2/3 must be carefully disposed of in order to avoid any damage to the environment. Consequently, fly ash is regarded by the public as a solid waste material, though it is increasingly being used for various beneficial purposes.

3. What is fly ash used for usually?

Fly ash has been used in a number of applications. A major use is as an
ingredient in making concrete, to substitute for a portion of the cement or
aggregates used in the concrete. Another beneficial use is for strengthening road beds. Thirdly, fly ash serves as part of the raw materials for manufacturing cement. It has also been used in many other ways including soil stabilization (improving the foundation of structures), soil improvement (improving crop growth), etc.

4. Is fly ash a hazardous waste?

No! The guardian angel of the nation’s environment – the U.S. Environmental Protection Agency (EPA) – classifies fly ash as “non-hazardous.” Fly ash deserves such a classification for good reasons, one of which is that numerous scientific studies have found that fly ash does not cause water pollution and air pollution. Because fly ash is usually basic rather than acidic, it does not leach out pollutants (heavy metals) when immersed in water. Furthermore, fly ash has been used for more than a century in many countries; yet it has not generated any public health problem or even occupational health problem for those using and
handling fly ash. In spite of that, it is not healthy to ingest or inhale fly ash, just as it is not healthy to ingest or inhale street dust. Eye contact with fly ash should also be avoided just as you should avoid street dust from entering your eyes. Finally, fly ash must be disposed of carefully to avoid spills, which could cause damage to environment, as demonstrated by the fly ash dam failure in Kingston, Tennessee in December 2008.

5. Can fly ash be used for making bricks and/or blocks? How?

Yes! Fly ash can be utilized to make bricks and blocks in one of several ways:

(a) To substitute for a portion of the cement and/or aggregates in making concrete bricks and blocks. This is a common use nowadays not only in India but also in many other countries.

(b) To substitute for a portion of the clay used in making clay bricks. This uses the same process for making clay bricks, requiring heating the adobes (green bricks) in kilns to more than 2,000 oF, which consumes much fossil fuel and generates air pollutants and carbon dioxides due to the combustion of the fossil fuel.

(c) To substitute for all the clay used in making clay bricks, using the same process for making clay bricks which requires burning fossil fuel to heat adobes in kilns/autoclave at over 2,000 oF . This uses the same process and has the same drawback of (b) except that 100% fly ash is used in making bricks. A leading manufacturer of fly ash bricks in Delhi/ NCR is using this technology to manufacture bricks - however their bricks are 'extruded bricks' instead of 'pressed bricks' and hence have no 'frog'. Thus although the brick themselves may be strong - but the walls made of these bricks are weak because of inadequate brick-brick bonding.

(d) Puzzolana Green brick is produced using the FaLG (Fly Ash + Lime + Gypsum) mix to produce non-fired, chemical stabilised brick. We use more than 80% fly-ash in our brick - however the exact ratio of the materials and the binding chemicals used in the process are proprietary.