FLY ASH CONCRETEA QUALITATIVE APPROACH

MANISH KUMAR MANDALDEPARTMENT OF CIVIL ENGINEERING2ND YEARVEL TECH HIGH TECH ENGINEERING COLLEGE

by

FLY ASH-INTRODUCTIONFly ash is a fine powder produced as a product from industrial plants using

pulverized coal or lignite as fuel. It is the most widely used pozzolan siliceous or aluminosiliceous in nature

in a finely divided form.They are spherical shaped “balls’’ finer than cement particles.

FLY ASH AS A SUPPLEMENTARY:Apart from technical advantages it attributes to the concrete properties, fly ash

has its own benefits

Problem of disposal: In India alone, we produce 75 million tons of fly

ash every year, the disposal of which has become a serious environmental problem. The effective use of fly ash in concrete making is therefore attracting serious considerations of concrete technologists and government departments.

Global CO2 emissions: Global cement production is about 1.3 billion

tons in 1996 and production of every 1 ton of cement emits 0.87 ton of carbon dioxide.

In broader terms, 7% of global CO2 emissions can be attributed to Portland cement industry.

FLY ASH IN CONCRETE: Fly ash could be an expensive replacement for Portland cement in concrete and

using it, improves strength, segregation and ease of pumping concrete.

Fly Ash particles provide a greater workability of the powder portion of the concrete mixture which results in greater workability of the concrete and a lowering of water requirement for the same concrete consistency.

The rate of substitution typically specified is a minimum of 1 to 1 ½ pounds of fly ash to 1 pound of cement

CHEMICAL COMPOSITIONMATERIALS PORTLAND

CEMENT%FLY ASH%

SiO2 21.82 53.39

Al2O3 6.49 16.07

Fe2O3 1.93 13.05

CaO 60.74 6.33

MgO 1.08 5.48

SO3 2.62 1.06

Na2O 0.14 1.59

Free Cao 0.84 0.11

1.fly ash are amorphous (glassy) due to rapid cooling; those of cement are crystalline, formed by slower cooling.

2. Portland cement is rich in lime (CaO) while fly ash is low. Fly ash is high in reactive silicates while Portland cement has smaller amounts

COMPARISON BETWEEN CLASSES OF FLY ASH

Class F is fly ash produced from burning anthracite or bituminous coal, and Class C is produced from the burning of sub-bituminous coal and lignite.

Class F is low in lime, under 15 percent, and contains a greater combination of silica, alumina and iron (greater than 70 percent) than Class C fly ash.

Class C fly ash comes from coals which may produce an ash with higher lime content — generally more than 15 percent often as high as 30 percent. Elevated CaO may give Class C unique self-hardening characteristics.

The fly ash from boilers where mechanical collectorsare used is coarser than fly ash from electrostatic precipitators.

The color varies from light to dark grey depending upon its carbon contents.

The quality of fly ash varies from source to source.

fly ash particles are small, they effectively fill voids

 

PHYSICAL ASPECTS:

Though fly ash offers environmental advantages it also improves the:

performance and quality of concrete. Fly ash affects the plastic properties of

concrete by improving workability Reduces water demand Reduces segregation and bleeding.Lowers heat of hydration. Fly ash increases strengthReduces permeability

MECHANICAL ATTRIBUTES:

The main benefit of fly ash in concrete is that it not only reduces the amount of non durable calcium hydroxide (lime), but in the process converts it into calcium silicate hydrate (CSH), which is the strongest and most durable portion of the paste in concrete. 

REACTION WITH MOISTURE:

HEAT OF HYDRATION:•Fly Ash has a lower heat of hydration.

•Portland Cement produces considerable heat upon hydration.

•In mass concrete placements the excess internal heat may contribute to cracking.

•The use of Fly Ash may greatly reduce this heat build up and reduce external cracking.

COMPRESSIVE STRENGTH:Typically, concrete made with fly ash will be slightly lower in

strength than straight cement concrete up to 28 days, equal strength at 28 days, and substantially higher strength within a year’s time.

Thus, fly ash concrete achieves significantly higher ultimate strength than can be achieved with conventional concrete.

COMPRESSIVE STRENGTH TEST:

FLY ASH CONCRETE: NEED FOR EXTENDING STRENGTH SPECIFICATIONS BEYOND 28 DAYS

Developing sustainable concrete to last more than 100 years requires extending the 28-day specifications.

Extended age parameters can assure more durable concrete.

Proper mix designs can be developed to optimize the projects timeline.

PROJECTS AND THEIR AGE ACCEPTANCE:

project Fly ash% Age strength acceptance

Naval facilitiesengineering

25-40% 28-56 day acceptance

Olivenhain dam San Diego

65% 365 day acceptance

Washbum airport 35% 28-,56-and 90-day acceptance

Caltrans 25-35% 42 days

CONCLUSION:Fly ash thus holds a vast potential for improving the modern

day concrete when it comes to quality in the long term. In spite of being an industrial waste, it can improve durability and reduce heat of hydration.

The prospects of fly ash are still being underused even today. Thus keeping in mind about environmental concerns and its indispensability as a mineral admixture, the values of fly ash should be effectively garnered .

REFERENCES:Concrete technology(theory and practice) –M.S shetty

Activation of fly ash through Nanomodification- ACI spring convention Shiho

Kawashima, Columbia University

Enhancing performance of high volume fly ash concrete –ACI spring convention.

http://www.concretebasics.org/articlesinfo/fa8.php#sthash.r8p7Ivtk.dpuf

magazine of concrete research- volume 64

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