Fly ash is a product of combustion of coal. It is normally produced while burning coal in a boiler of a power plant and is generally captured before the flue gas goes to the chimney. The other ash from the boiler is bottom ash which is removed from the bottom of coal fired boiler. Depending upon the type of the coal used, the composition of the fly ash vary widely but all the types of fly ash include silicon dioxide (SiO2) (both amorphous and crystalline) and calcium oxide (CaO). Fly ash is a fine glass like powder. The particle size of the ash is in microns. The composition of fly ash in case of bituminous coals is as follows:
SiO2 – 20%- 60%
Al2O3 – 7% – 35%
Fe2O3 – 12% – 40%
CaO – 2% – 12%
LOI – 0%- 12%
Fly ash contains environmental toxins in significant amounts. These toxins include arsenic (45 ppm); barium (805 ppm); beryllium (5 ppm); boron (310 ppm); cadmium (3 ppm); chromium (135 ppm); chromium VI (90 ppm); cobalt (35 ppm); copper (110 ppm); fluorine (30 ppm); lead (55 ppm); manganese (250 ppm); nickel (75 ppm); selenium (8 ppm); strontium (775 ppm); thallium (10 ppm); vanadium (250 ppm) and zinc (180 ppm).
The use of fly ash as an engineering material is due to its pozzolanic nature, spherical shape, and relative uniformity. The following are the various uses of fly ash:
- Portland cement and and in materials for grout
- Embankments and structural fill
- Waste stabilization and solidification
- Raw feed for cement clinkers
- Mine reclamation
- Stabilization of soft soils
- Sub base for road
- Flowable fill
- Mineral filler in asphaltic concrete
- Roofing tiles
- Filler in wood and plastic products
Fly ash is used as a partial replacement for Portland cement. Normally it replaces up to 30% by mass of Portland cement. Recently concrete mix design for partial cement replacement with high volume fly ash (50 % cement replacement) has been developed. For Roller Compacted Concrete [used in dam construction] replacement figures of 70% have been achieved at the Ghatghar dam project in Maharashtra, India.
The replacement of Portland cement with fly ash reduces the greenhouse gas CO2. The production of one ton of portland cement produces approximately one ton of CO2 as compared to zero CO2 being produced while using fly ash.
Properties of fly ash – The following are the properties of fly ash:
- Spherical shape – The difference between fly ash and Portland cement becomes apparent under a microscope. Fly ash particles are almost totally spherical in shape. This allows them to flow and blend freely. This property makes fly ash a desirable admixture for concrete.
- High strength – Fly ash continues to combine with free lime with time. This increases structural strength over time.
- Ball bearing effect– The property of the ball bearing effect of fly ash particles creates a lubricating action in the concrete when it is in its plastic state.
- Decreased permeability – Fly ash increases density and has long term pozzolanic action. This property of fly ash ties up free lime which results in lesser numbers of bleed channels and decrease in permeability. This results into increased durability of concrete. Dense fly ash concrete helps keep aggressive compounds on the surface, where destructive action is lessened. Fly ash concrete is also more resistant to attack by sulfate, mild acid, soft water and seawater.
- Reduction in sulfate attack – Fly ash ties up free lime in concrete so that free lime is no more available to combine with sulfate for creation of destructive expansion.
- Reduced efflorescence – Fly ash chemically combines free lime and salts which can create efflorescence. Dense concrete produced with fly ash holds efflorescence producing compounds on the inside.
- Reduction in heat of hydration – The pozzolanic reaction between lime and fly ash generates lesser heat. This results in reduction in thermal cracking when fly ash is used to replace Portland cement.
- Reduction in shrinkage – The biggest contributor to drying shrinkage is water which binds free lime content. The lubricating action of fly ash otherwise combine with silica from aggregates. Combining of alkalies with silica of aggregate reduces water content and drying shrinkage.
- Reduction in reactivity of alkali silica – Fly ash combines with alkalis from cement that might cause destructive expansion.
- Workability – Concrete with the use of fly ash is easier to place with less effort. It also responds better to vibration to fill forms more completely.
- Ease of pumping – Pumping of concrete with fly ash requires less energy and longer pumping distances are possible.
- Improvement in finishing – Sharp, clear architectural definition is easier to achieve in case of fly ash based cement with less worry about in place integrity.
- Reduction in bleeding – In this case lesser bleed channels decrease porosity and chemical attack. Bleed streaking is reduced for architectural finishes. Improved paste to aggregate contact results in enhanced bond strengths.
- Reduced segregation – Improved cohesiveness of fly ash concrete reduces segregation that can lead to rock pockets and blemishes.
- Reduction in slump loss – More dependable concrete allows for greater working time, especially in hot weather.
Fly ash is also used for making bricks (Fig 1) for building construction. Bricks of fly ash can be made of two types
- One type of brick is made by mixing it with about equal amount of soil and proceeding through the ordinary process of making brick. This type of formation reduces the use of fertile sand in making bricks.
- Second type of brick can be made by mixing soil, plaster of paris and fly ash in a definite proportion with water and allowing the mixture to dry. Because it does not need to be heated in a furnace this technique reduces air pollution.
Fig 1 Fly ash bricks