Published on 18/10/2011
With rapid growth of industrialization in the last few decades the whole world is facing the problem of environmental pollution. Continuous emissions from industries and anthropogenic practices have hampered the environment substantially, and continue to do so. Environmentalists are much concerned in finding permanent solutions to these problems which are deteriorating the quality of the environmental air, water and soil. As the time is passing a huge giant of such big environmental issues is getting nourished. We must be very much concerned to adopt technological applications to kill the giant for the betterment of the future. One such material is fly ash.
Air pollution is one of the major concerned problems today. Fresh air is getting polluted day by day mainly due to the uncontrolled emissions from industrial stacks, vehicular exhausts, construction activities, forest fires, and other man made sources of air pollution. For the economic development of any nation energy is the prerequisite. Electrical energy is being generated all over the world from thermal power plants in quantities varying from few to thousand megawatts. More than half of the electricity we consume in a day is produced through the power plants fueled by coal. Thermal power plants produce electricity but in addition to electricity these plants also produce a material that is of our major concern and fast becoming a vital ingredient for improving the performance of a wide range of concrete products. The material is called fly ash.
What is Flyash
Fly ash is non-combustible mineral portion of coal which is generated in combustion, and it primarily comprises of fine particles that rise with the flue gases. It is necessary for thermal power plants to grind the coal into the fineness of powder for its efficient burning. When coal is burnt its carbon part is consumed and molten particles rich in silica, alumina and calcium are left behind. With the help of air pollution control devices these particles are captured and solidified later as microscopic, glassy spheres that are collected from the power plant’s exhaust before they can “fly” away hence the product’s name is Fly Ash. Ash which does not rise is termed as bottom ash. In a simple context, fly ash usually refers to the ash produced during combustion of coal.
From time to time practices have been involved for management of fly ash and conversion of this ash into bricks is one of them. Fly ash brick (FAB) are specifically masonry units which are used as building material. These bricks are made up of Class C fly ash and water, compressed at high temperature and pressure and toughened with an air entrainment agent. Due to the presence of high concentration of calcium oxide in class C fly ash, the brick can be described as “self-cementing” because when it is mixed with lime it combines to form cementitious compounds.
Manufacturing Process of Flyash Bricks
The manufacturing of fly ash bricks is quite easy and it is said to save energy because the process involves very little consumption of energy. This also reduces mercury pollution and costs 20% less than traditional clay brick manufacturing system. Raw material required for production are fly ash, gypsum, alum and stone crushing dust. For manufacturing of fly ash bricks, these raw materials have to be mixed as per the ratio specified by the individual industrial establishments. Most of the machine manufacturers suggest the following TWO mixing ratios which are
- Normal Mixing ratio and
- Profitable mixing ratio
The composition of raw materials in the normal mixing ratio is fly ash 62%, sand 25%, lime 8% and gypsum 5% while in case of profitable mixing ratio this ratio becomes 20%, 60%, 15% and 5% respectively. Fly ash manufacturers use profitable mixing ratio to survive in the market if they are facing low availability of fly ash. If a manufacturer is using the profitable mixing ratio for the production of fly ash then at the same he should maintain the quality too.
For the production of fly ash bricks firstly fly ash, gypsum, sand and hydrated limes are fed into a pan mixer manually where water is added in the required proportion for intimate mixing. The proportion of the raw material is generally in the ratio depending upon its quality and availability. Once raw materials are mixed, the mixture is shifted to the hydraulic Brick Making machines. The bricks are carried on wooden pellets to the open area where they are dried and water cured for 14 days. The bricks are tested and sorted before dispatch.
Quality Control and Standards
The Bureau of Indian Standards has formulated and published the following specifications for maintaining quality of the Fly ash Brick and testing purpose.
Quality and Standards: IS 12894:1990 Specification for Fly ash Lime bricks
Types of Flyash Bricks
Fly ash bricks are generally classified into four major groups:
- Clay Fly ash Bricks
- Fly ash – Sand Lime Bricks
- Cold Bonded Lightweight Fly ash Bricks, Blocks and Tiles
- Flux Bonded Fly ash Bricks Blocks and Tiles
Fly ash bricks Vs normal clay bricks
The application of Fly ash bricks in various construction projects leading over traditional normal clay bricks due to its unique properties. A comparative study of fly ash bricks and normal clay bricks is summarized below in the table:
Normal Clay Brick
Fly ash brick
|Color||Varying color as per soil||Uniform pleasing color like cement|
|Shape and size||Uneven shape as hand made||Uniform in shape and smooth in finish|
|Density||Lightly bonded||Dense composition|
|Finishing||Plastering required||No plastering required|
|Weight||Heavier in weight||Lighter in weight|
|Compressive strength||Compressive strength is around 35 Kg/cm2||Compressive strength is around 100 Kg/cm2|
|Porosity||More porous||Less porous|
|Thermal conductivity||Thermal conductivity 1.25 – 1.35 W/m2 ºC||Thermal conductivity 0.90-1.05 W/m2 ºC|
|Absorption of water||Water absorption 20-25%||Water absorption 6-12%|
Advantages of fly ash bricks
The bricks have the appearance which is very pleasant like cement; Due to smoothness and finish on their surface they require no plastering for building work. These are compact, uniformly shaped and free from visible cracks. They are lighter in weight than ordinary clay bricks and are less porous. The color of these bricks can be altered by the addition of admixtures during the process of brick making. The size of these bricks can vary but they are generally available in the same sizes of the normal clay bricks.
2. Structural Capability
These bricks do not cause any extra load on design of structures due to its comparable density and thus provide better resistance for earthquakes and other natural calamities. Compressive strength of fly ash sand lime bricks is av. 9.00 N/mm2 (as against 3.50 N/mm2 for handmade clay bricks). The bricks possess high compressive strength which eliminates breakages/wastages during transport and handling. When a structure is formed using fly ash bricks the possibility of cracking of plaster is reduced due to lower thickness of joints and plaster and basic material of the bricks, which is more compatible with cement mortar.
3. Thermal properties
These bricks have got thermal conductivity around 0.90-1.05 W/m2 ºC (20-30% less than those of concrete blocks). These bricks do not absorb heat; they reflect heat and gives maximum light reflection which causes less heating of huge structures.
4. Sound insulation
It provides an acceptable degree of sound insulation. The sound produced at one side of a wall made using fly ash bricks do not let the sound waves pass easily to the other side of the wall due to its compactness. Hence they may be considered for the abatement of the noise pollution.
5. Fire and vermin resistance
Fly ash bricks have a good fire rating due to the absence of fire catching materials. It has no problems of vermin attacks or infestation.
6. Durability and moisture resistance
Fly ash blocks are highly durable. When their joints are properly joined, the bricks are ready to be directly painted with the paints available in the market or with the cement paint without plaster. The bricks, usually, are rectangular faced having sharp corners, solid, compact and uniformly shaped. The bricks are said to absorb the moisture approximately 6-12% than that of 20-25% for handmade clay bricks thus they help reducing dampness of the walls.
7. Toxicity and Breath-ability
There are no positive evidences and studies that suggest about toxic fume emissions or the indoor air quality of structures built with fly ash bricks. Although scientists in some part of world have claimed about the radioactive emissions by these blocks.
Fly ash as a raw material is very fine so care has to be taken while its handling and transport to avoid any kind of air pollution in the view of occupational safety. Once it is flue, it can remain airborne for long periods of time, causing serious health problems relating to the respiratory system. But block manufactured from fly ash has no such problems.
We can conclude that fly ash is a cocktail of unhealthy and hazardous elements like silica, mercury, iron oxides, calcium, aluminum, magnesium, arsenic and cadmium. It poses serious environment and health hazards for a large population who live in the nearby area of the plants. But the brick is better off, during the process of brick making the toxins associated with fly ash gets changed into a non-toxic product. The mixing of with lime at ordinary temperature leads to the hydration of calcium silicate and formation of a dense composite inert block. Thus it has the potential of being a good building material. In India about 100 million tons of fly ash is produced annually by the numerous thermal power plants, which could cause serious contamination of land, groundwater and air but due to practice of fly ash bricks now it is safe and sound.
9. Build ability, availability and cost
The compressive strength of fly ash blocks is so high that it eliminates breakages/wastage during handling and gives a neat finish, with lower thickness of joints and plaster. The construction technique does not change in the case of fly ash bricks and remains as same as in the case of regular bricks which ensures easy change of material. Masons do not require additional training while construction. Though these bricks are abundantly and widely available closer to thermal power plants all over the country for obvious reasons, finding dealers in all major cities and towns wouldn’t be a problem.
The blocks are easily available in several load bearing grades which are suitable for use in various construction practices
• Load bearing external walls, in low and medium size structures.
• Non-load bearing internal walls in low and medium size structures.
• Non-load bearing internal or external walls in high-rise buildings.