Fly Ash is a fine siliceous (glass-like) powder recovered from the burning of pulverized coal (bituminous or sub-bituminous) in electric generation power plants. The collected fly ash by electrostatic precipitator in form of small particles (0.5 µm to 100 µm). The fly ash particles consist mostly of silicon dioxide (SiO2), aluminum oxide (Al2O3), and iron oxide (Fe2O3). A mixer of Fly ash lime with water is an economic replacement for Portland cement used in concrete, while it actually improves strength, segregation, and ease of pumping of the concrete. Fly ash is also used as the main ingredient in the manufacturing of brick, block, paving, used in Grouting, and structural fills.
FLY ASH BRICKS:
Fly ash-lime bricks are generally manufactured by inter-grinding or blending various raw materials such as Fly ash, sand/stone dust, lime, gypsum, and bonding agent. Which are then molded into bricks or blocks and subjected to curing cycles at different temperatures and pressures (About 272 atm) to produce bricks of higher strength with consistency as well as uniformity? Fly ash bricks are suitable for use in masonry construction just like common burnt clay bricks
USES AND APPLICATIONS
- Fly ash bricks are used in the building industry.
- Fly ash Sand-Lime bricks
- Building industry
Fly ash bricks can be used as an alternative material for burnt clay bricks which is one of the important building materials used for construction of housing and buildings. The fly ash building bricks are unable in all types of brick masonry works and can substitute the conventional burnt clay bricks in nearly all applications.
The manufacturing process of fly ash bricks is semiautomatic of fully automatic with state of the art technology. Though a new-age product introduced in the market, Fly Ash bricks are very well accepted by the organized sectors in heavy industries, high rise buildings, large townships, colonies, etc. because of unique features and merits.
Nearly 190 billion tons of traditional burnt clay bricks are consumed annually. That consumes approximately 350 billion tons of clay which is approximately equal to the top layer of about 5000 acres of soil dug out for brick manufacture. Also, coal burning or fire woods which causes deforestation are other serious problems posed by the bricks industry. The demand for housing units is expected to increase to 90 million units by the year 2020 for low middle- and low-income groups. The demand for housing units will increase to 100 million by 2022. The Indian housing sector currently lacks 20 million housing units for its low middle- and low-income groups which will witness a boom of about 22.5 million housing units. There is ample scope for fly ash brick and block units as the government has an ambitious plan of providing houses for all by 2022. This will increase the demand for fly ash bricks substantially in the coming years.
Fly Ash (60 to 80%):
Fly ash is the major component of the raw materials to manufacture for Fly ash bricks. Therefore, it controls the properties of the finished product to a great extent. As the Fly ash doesn’t have plastic property, a binder material must be added either plastic clay or Portland cement. Fly ash brick (FAB) is a building material, specifically masonry units, containing class C or class F fly ash and water.
Lime (20 to 30%):
It is generally desirable to use a high calcium lime of proper purity as it is the most important ingredient that reacts with silica and alumina etc. present in fly ash so that binder can be formed under hydrothermal conditions other charred limes are not desirable as it does stake readily. Lime particles should be fine 15 enough to deliver well and coat the grain of the mixture. Lime should conform to class C hydrated lime of IS: 712-1973.
Cement / Gypsum (5 to 10 %):
Since fly ash is non-plastic, a suitable additive material such as gypsum, cement, etc. can be used to consideration of durability of bricks. Gypsum (CaSO4.2h2O) is hydrated calcium sulphate. Gypsum acts as a long-term strength gainer.
Sand (20 to 30 %):
Using sand to manufacturing process should be clean and coarse. The presence of harmful materials such as soil and silt in the sand will preferably be less than 5%. As per IS 12894 : 1990.
Potable water use for the manufacture of bricks, generally from a well or a river is required.
- Prepare the raw materials to use for the manufacturing process.
- Weighing all materials as required proportion.
- Dry mix all such materials in a U-Shaft mixer/counter current (costlier).
- Add 8-10 % water into the above dry mix and mix well in pan mixer.
- Transfer this mix proportion by conveyor belt to automatic pressing / manually pressing machine.
- Well mixed material presses by hydraulic/mechanical press in the mould.
- After presses, bricks are allowed to dry in shed air for 24-36 hours depending upon the climate condition.
- After initial curing bricks may autoclaving curing or done by spray water on the dried bricks. Bricks are then covered, to maintain moisture for gaining its maximum strength.
Dimensions and Tolerances
The modular sizes shall be as follows
The standard dimensions and allowable tolerance of pulverized fuel ash-lime bricks as per IS code of shall be within the following limits per 20 bricks:
|Modular Size||Dimensions (In mm)||190||90||90||40|
|720 to 880 (800 ± 80)||760 to 840 (800 ± 40)||760 to 840 (800 ± 40)||760 to 840 (800 ± 40)|
|Non modular||Dimensions (In mm)||230||110||70||30|
|Tolerances (In mm)||520 to 680 (600 ± 80)||160 to 240 (200 ± 40)||360 to 440 (400 ± 40)||560 to 640 (600 ± 40)|
For obtaining proper bond arrangement and modular dimensions for the brickwork, with the nonmodular sizes, the following sizes of the bricks may also be used:
Length (L) mm 70 230
Width (W) mm 110 50
Height (H) mm 70 1/3 length brick 70 1/2 width brick
Compressive Strength –
The classification of pulverized fuel ash-lime bricks on the basis of average wet compressive strength as given in Table. As per given table minimum compressive strength for corresponding class of fly ash bricks, when tested in accordance with the procedure described in IS 3495 (Part 1): 1992. The wet compressive strength of any individual brick should not fall below 20 percent from the minimum average wet compressive strength specified for the corresponding class of bricks. The lot shall be then checked for next lower class of brick.
Classes of Pulverized Fuel Ash-Lime Bricks
|Class Designation||Average Wet Compressive Strength not Less Than|
Water absorption capacity of fly ash bricks tested in accordance with IS 3495 (Part 2): 1992 Immerse completely dried specimen in clean water at a temperature of 27 ± 2°C for 24 hours, water absorption shall not be more than 20 percent by weight up to class 12.5 and 15 percent by weight of higher classes.
Determination of Efflorescence of the bricks test has been done as per IS 3495 (Part 3): 1992, the rating of efflorescence shall not be more than ‘moderate’ up to class 12.5 and ‘slight’ for higher classes.
The average drying shrinkage of the pulverized fly-lime bricks when tested by the method mentioned in IS 4139, being the average of three units, shall not exceed 0.15 percent.
Advantages and Features:
- Fly ash bricks are uniform shape and size thus gives the aesthetic appearance.
- These bricks having fly ash as the main constituent, thus solve the concern of disposal of fly ash.
- The residue of the manufacturing process is also recyclable with raw materials and used again. And no virtual wastage at the construction site also.
- These bricks do not require soaking in water for 24 hours. The sprinkling of water before use is enough.
- About 3 times higher compressive strength as compared to any conventional bricks with the same dry density 1700-1850 kg/m3.
- Water absorption is 6-14% as against 20-25% for clay bricks, reducing dampness of the walls, salinity, and water seepage.
- These are also manufactured at the construction sites, thus reducing transportation costs.
- Greater the thermal insulation.
- Smooth surfaces and uniform shapes and sizes, therefore, require less mortar in brickwork, and also thin plaster thickness requires, thus saving of cement mortar by 30-40%.
- It is also highly resistant to attack by mild acid, water, and sulphate.
- Fly ash bricks are highly fire-resistant.
- The cracking of plaster is reduced due to the lower thickness of joints and plaster.
- Fly -Ash bricks having the great thermal insulating property that’s why it keeps our house cool in summer, and warm in winter hence it’s most suitable for Indian weather conditions.
- Overall, Fly-Ash bricks it’s cost-effective and eco-friendly than clay bricks.
Disadvantages of Fly Ash Bricks
- Only processed fly ash suitable for construction. The presence of unburnt coal will lead to negative undesired impact on the bricks, may affect strength, durability, and also increase permeability.
- Only use automatic machine-made bricks for construction purposes, because manually made bricks did not have sufficient strength due to lower compaction and vibration.
- Bonding with concrete is lower due to the smooth finish.
- It is suitable for subtropical areas (Warm climate) because fly ash bricks do not absorb heat. But in the winter season, it is not helpful.