Admixtures 1

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ADMIXTURES

Definition

Admixtures are chemical/mineral substances (other than fine and coarseaggregates, cement, or water), which are added in small amounts just before or

during the mixing stage to concrete products. They can also be blended during the grinding stage of cement manufacturing.

Use

They should normally only be used where it is desirable to modify the propertiesof either fresh or hardened concrete (or both) for particular reason when such achange cannot be effected by changes in the composition or properties of the

normal mix.

Mineral Admixtures

Fly ash or Pulverised Fuel Ash (PFA).

Silica Fume or Micro silica. Rice Husk Ash. Ground Granulated Blast Furnace Slag (GGBS). Metakaolin.

Reason: Why Fly Ash?

Annually, more than 110 million tonnes of Fly Ash is being generated in India. Requires approsimately 65,000 acres of land for disposal

Generation of Fly ash:

Fly ash is a solid, fine-grained material resulting from the combustion of

pulverized coal in thermal plant/furnaces

Collection of Fly ash: Bottom ash: Collected from boiler bottom Ponded Fly ash: ESP Product Mixed with water Dry Fly ash : Separated from field B & C and stored in silo for loading in bunkers

Electrostatic Precipitator


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Morphological Characteristics

The spherical particles in a flyash samples are its outstanding features specially inthe context of its use as a pozzolana or admixture in concrete.

Morphology of the particles influenced by modes of collection Smooth, Glassy surface show higher reactivity.

Smooth, glassy surface


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SEM of Pond Ash.

Chemical Composition Major constituents of most fly ashes are: SiO2, Al2O3, Fe2O3, CaO and Loss on Ignition (LOI). Other elements are MgO, Na2O, K2O, SO3, MnO, TiO2. The early reaction is due to the presence of silica and alumina in the reactive form

(amorphous).

Long term strength is due to the crystalline form. Presence of alkalies may affect durability adversely although they may act asactivators during the early stage reaction.

Activity of the ash is influenced by the presence of important chemical constituentcalcium.

un burnt Carbon Is the Most Important Component of LOI The higher the carbon content of a fly ash, the more water is needed to produce a

paste of normal consistency.

IS: 3812 Specifications

CHEMICAL PROPERTIES for Grade I & II of FA (percentage by mass)

Characteristic Bituminous Coal Ash Lignite Ash1. SiO2 + Al2O3 + Fe2O3 (min.) 70.0 50.0

2. SiO2 (min.) 35.0 25.03. Soluble silica (min) 25.0 20.04. MgO (max) 5.0 4.0

5. Total Sulphur as SO3 (max) 2.75 2.5

6. Alkalis as Na2O (max) 1.50 1.507. Total chlorides (max)0.05 0.05


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8. Loss on ignition (max)5.0 5.0

Types of fly ash Low-lime fly ash (CaO < 10% )

Exhibit Pozzolanic properties produces cementitious properties with the help of an

activator (cement or lime). High-lime fly ash (CaO > 10% ) Exhibit Cementitious properties itself.

Chemical Composition

Mineralogical composition

Fly ashes consist of non-crystalline particles or glass, and a small amount ofcrystalline material.

Major crystalline phases: Quartz, Mullite, Magnetite, and Hematite in sub-bituminous fly ashes.

X-RD Scan of Fly Ash(Amorphous Phase)

Chemical ASTM ASTM INDIA RTPS

Composition C-618 C - 618 FLY ASH PROCESSED

% CLASS - F CLASS - C RANGE FLY ASH

SiO 2 44 41 37-66 61

Al2O3 22 20 20-31 25

Fe2O3 6 5 2--12 5.0

SO 3 0.5 2 0.3 - 1.0 0.3

CaO 4 17 1.3 - 5.0 4.0

MgO 1.5 3 0.3 - 2.9 0.3

Moisture 0.6 0.3 1.0 - 12 1.6

LOI 1.0 1.0 0.3 - 8.8 1.2

SP-GR 2.18 2.5 2.10 - 2.52 2.2

Glass Content 80 - 84 80 - 86 --- > 84

Phases: Quartz Quartz --- Quartz

Mullite Periclase --- Mullite

Hematite Ferrite --- Hematite

Megnetite Spinel --- Megnetite

Carbon Melilite ---

Merisinite ---

LR Mpa --- --- 3.0 - 8.0 > 5.0

CR % --- --- 60-85 > 80

Carbon % --- --- 1.5 - 12 < 1.5


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X-RD of RTPS Pond Ash.

PHYSICAL CHARACTERISTICS OF FLY ASH

Grain size distribution

Grain size distribution plays an important role in determining the activity of flyash. The particles in the size range of 5 to 30 m plays a decisive role in

determining the activity of fly ash.

For fly ashes with low glass content and normal carbon, the reactivity was foundto be directly proportional to the amount of particles smaller than 10 m, and

inversely proportional to the amount of particles greater than 45 m for low

calcium fly ashes. The particles below 45 m, which are mostly present in an amorphous form or as

glass in the ash, are responsible for the pozzolanic effect.

But the enhancement in compressive strength appears to be related more toparticle sizes below 10 20 m.

Fineness

One of the principal parameters to define the aptitude of the ash to be added tocement as it influences the rate of development of mechanical strength and

relative values to be attained. However, there is an optimal fineness above which the increase in strength

becomes less significant because of the increase in the specific surface.

Researchers have also stated that finer the fly ash, the higher is the pozzolanicactivity.

Fineness can be expressed in terms of specific surface area determined byBlaines Air Permeability method.

The values are seen to range from 200 600 m2/kg


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Specific Gravity Specific Gravity varies significantly for particles of different shape, color and

chemical composition

Irregular shaped, black, coal particles, which contain few mineral grains, have a

specific gravity between 1.3 and 1.6 - are non-reactive and dilute the activePozzolanic material in fly ash

Variations in the particle densities is due to the presence of bubbles in the glassyparticles and voids in the agglomerated particles

In general, it appears that specific gravity does not have any direct influence onthe reactivity of the ash

IS: 3812 Specifications

PHYSICAL PROPERTIESCharacteristic Grade I Grade II

1.Fineness-specific surface in m

2

/kg byBlaines permeability method, minimum 320 2502.Particles retained on 45 micron IS sieve

(wet sieving) in percent, maximum. 34 40

3.Lime reactivity average compressive

strength in N/mm2 , minimum 4.5 3.5

4.Compressive strength at 28 days --- Not less than 80%, minimum of the strength of core.Plain cement mortar cubes

Properties of Concrete with Pozzolans

Hydration ProcessSio2+Ca(OH)2 CSHA- Cement

B- Water

C- CSHD-Calcium Hydroxide

Pozzolanic activity of fly ash

Hydration Process

Sio2+Ca(OH)2 CSH

A- CementB- Water

C- CSH

D- Calcium Hydroxide

Reaction products may include

Calcium aluminate hydrate (C4AH19) Ettringite (C3A.3CS.H32)


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Gehlenite (C2A.S.H8)

Calcium mono sulpho aluminate hydrate(C3A.CS.H12)

Provides reduced settling and more homogeneous concrete

The mix is strongly cohesive and, in consequence, there is very little bleeding oreven none.

Blocking of the capillary pore/ channels both by physical action due to fineparticle size and due to the formation of new products of pozzolanic reaction results in

lower permeability of concrete

Increased Compressive and Tensile strength due to lack of Bleed pockets andformation of dense hydrated cement paste.

Permeability The use of pozzolanic material reduces the permeability by refining the capillary

pores(regarded as flaws in concrete)


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Increased surface area, as determined by air permeability tests, in many casescorrelates well with quality of a fly ash from a single source

Reduced Permeability means more resistance to chemical attack, (Sulphate

attack, Acid attack, Chloride induced corrosion etc.,)


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Applications

Innovative Applications

Blended cements Fly ash Concrete High Volume Fly Ash Concrete Self Compacting Concrete Roller Compacting Concrete

Geo-technical Applications Building blocks Structural fills Fine Aggregate Agricultural applications Waste stabilization

Others (as filler in paints, metal castings etc.,)

Concluding Remarks Fly ash is produced in large quantities in India and requires to be utilised for

sustainability of resources. Indian fly ashes are mostly Class F fly ashes containing low calcium content. It contributes to strength and durability by way of filler effect and pozzolanic

action. It enhances workability and cohesion due its spherical nature.

It has become an important component of cement concrete.

Reference:

1. Properties of Concrete, A.M. Neville, Fourth Edition, Pearson Education Asia pvt.,

Ltd., 2000.

2. Concrete- Microstructure, Properties and Materials, P.K. Mehta and Palulo J.M.Monteiro, Tata Mcgraw Hill., 2006.

3. Concrete Technolgy Theory and Practice, M.S. Shetty, S. Chand and Company

Ltd.,, 2005.

4. Current Literature..,

Documents

Admixtures 1