PORTLAND POZZOLONA CEMENT

By Austin S. Rodrigues,

Lecturer, Government Polytechnic, Panaji.

PORTLAND CEMENT:

Portland cements are hydraulic cements composed primarily of hydraulic calcium silicate.

During the hydration reaction, cement combines with water to form a stone-like mass, called paste.

When the paste is added to aggregates, it acts as an adhesive and binds them together to form concrete.

THE PORTLAND CEMENT MANUFACTURING

PROCESS:

THE PORTLAND CEMENT MANUFACTURING

PROCESS:

THE PORTLAND CEMENT MANUFACTURING

PROCESS:

THE PORTLAND CEMENT MANUFACTURING

PROCESS:

RAW MATERIALS USED FOR CEMENT:

THE CALCIUM CONNECTION:

During the burning operation in the manufacture of Portland cement clinker, calcium combines with the other components of the raw mix to form four principal compounds that make up 90% of cement by mass.

Gypsum (4% to 6%), or other calcium sulfate source, and grinding aids are also added during grinding.

CALCIUM COMPOUNDS:

Following are the four primary compounds in

Portland cement, their approximate chemical

formulas, and abbreviations:

Tricalcium silicate 3CaOSiO2 = C3S

Dicalcium silicate 2CaOSiO2 = C2S

Tricalcium aluminate 3CaOAl2O3 = C3A

Gypsum, calcium sulfate dihydrate, is the

predominant source of sulfate used in cement.

PORTLAND CEMENT COMPOUNDS:

RELATIVE REACTIVITY OF CEMENT COMPOUNDS:

WHAT THEY DO

Tricalcium Aluminate, C3A, liberates a large

amount of heat during the first few days of

hydration and hardening. It also contributes

slightly to early strength development.

WHAT THEY DO

Tricalcium Silicate, C3S, hydrates and hardens rapidly and is largely responsible for initial set and early strength.

In general, the early strength of Portland cement concrete is higher with increased percentages of C3S.

WHAT THEY DO

Dicalcium Silicate, C2S, hydrates and hardens

slowly and contributes largely to strength

increase at ages beyond one week.

WHAT THEY DO

Tetracalcium Aluminoferrite, C4AF, is the

product resulting from the use of iron and

aluminum raw materials to reduce the

clinkering temperature during cement

manufacture. It contributes little to strength.

WHAT THEY DO

Calcium Sulfate, gypsum is added to cement

during final grinding to provide sulfate to react

with C3A to control the hydration of C3A.

Without sulfate, a cement would set rapidly. In

addition to controlling setting and early

strength gain.

The sulfate also helps control drying shrinkage

and can influence strength through 28 days.

PORTLAND POZZOLONA CEMENT

The Portland Pozzolana Cement is a kind of

Blended Cement which is produced by either

intergrinding of OPC clinker along with gypsum

and pozzolanic materials in certain proportions

or grinding the OPC clinker, gypsum and

Pozzolanic materials separately and thoroughly

blending them in certain proportions.

WHAT IS POZZOLONA.

Pozzolana is a natural or artificial material

containing silica in a reactive form.

It may be termed as siliceous or siliceous and

aluminous material which in itself possesses little,

or no cementitious properties but will in finely

divided form and in the presence of moisture,

chemically react with calcium hydroxide at ordinary

temperature to form compounds possessing

cementitious properties.

FLY ASH:

Fly ash closely resembles volcanic ashes used

in production of the earliest known hydraulic

cements about 2,300 years ago.

Fly ash is the best known, and one of the most

commonly used, pozzolans in the world.

FLY ASH. . TO POZZOLONA

Instead of volcanoes, today's fly ash comes primarily from coal-fired electricity generating power plants.

These power plants grind coal to a powder fineness before it is burned.

Fly ash - the mineral residue produced by burning coal - is captured from the power plant's exhaust gases and collected for use.

Fly ash is a fine, glass powder recovered from the gases of burning coal during the production of electricity.

These micron-sized earth elements consist primarily of silica, alumina and iron.

POZZOLONA PORTLAND CEMENT:

It is essential that pozzolana be in a finely divided state as it is only then that silica can combine with calcium hydroxide (liberated by the hydrating Portland cement) in the presence of water to form stable calcium silicates which have cementitious properties.

The pozzolanic materials commonly used are:

Volcanic ash

Calcined clay

Fly ash

Silica fumes

THE I.S. CODES:

The Indian standards for P. P. C. have been issued in two parts

based on the type of pozzolanic materials to be used in

manufacturing of P. P. C. as given below:

IS 1489 (Part 1), P. P. C. specification (fly ash based)

IS 1489 (Part 2), P. P. C. specification (Calcined clay based)

The quality of flyash or calcined clay to be used in manufacturing

of PPC is also specified by BIS in the following standards:

IS 3812 1981 specification for flyash as pozzolana and

admixture

IS 1344 1981 specification for calcined clay pozzolana

WHY FLY AS AS POZZOLONA.

In view of the availability of good quality fly ash in abundant quantity, the use of calcined clay based pozzolana cement is progressively decreasing.

The flyash is a waste product of Thermal power Plant which creates disposal problems at Thermal power plant site.

The yearly production of flyash in India is about 70 million tonnes per annum.

WHY FLY AS AS POZZOLONA.

Fly ash is the ash precipitated electrostatically from the exhaust fumes of coal fired power station.

The fly ash particles are spherical and are generally of higher fineness than cement so that the silica is readily available for reaction.

As per IS 3812: 1981 the percentage of silica and alumina should be minimum 70% and maximum loss on ignition 12%.

Much superior quality fly ash is available from Indian thermal power plants than specified in IS code.

WHY P. P. C.?

The Portland Pozzolana Cement makes concrete more impermeable and denser as compared to Ordinary Portland Cement.

The long-term strength (90 days and above) of Pozzolana cement is better compared to OPC.

The pozzolanic material reacts with calcium hydroxide liberated by the hydrating Portland Cement and forms cementitious compounds generally known as C-S-H gel.

WHY FLY AS AS POZZOLONA.

The flyash converts Ca(OH)2 in to useful cementitious compound (C3S2H3) thereby increasing the properties of hardened concrete.

The Portland Pozzolana Cement produces less heat of hydration and offers greater resistance to the attack of aggressive waters than normal Portland Cement.

Moreover it reduces the leaching of calcium hydroxide liberated during the setting and hydration of cement.

ADVANTAGES OF P. P. C..

Spherical shape : Fly ash particles are almost totally spherical in shape, allowing them to flow and blend freely in mixtures.

Ball bearing effect :The ball-bearing effect of fly ash particles creates a lubricating action when concrete is in its plastic state.

Higher Strength : Fly ash continues to combine with free lime, increasing structural strength over time.

Decreased Permeability : Increased density and long term pozzolanic action of fly ash, which ties up free lime, results in fewer bleed channels and decreases permeability

ADVANTAGES OF P. P. C..

Increased Durability. 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 (lime hungry) water, and seawater.

Reduced Sulfate Attack : Fly ash ties up free lime that can combine with sulfate to create destructive expansion.

Reduced Efflorescence : Fly ash chemically binds free lime and salts that can create efflorescence and dense concrete holds efflorescence producing compounds on the inside.

ADVANTAGES OF P. P. C..

Reduced Shrinkage : The largest contributor to drying shrinkage is water content. The lubricating action of fly ash reduces water content and drying shrinkage.

Reduced Heat of Hydration :The pozzolanic reaction between fly ash and lime generates less heat, resulting in reduced thermal cracking when fly ash is used to replace portland cement.

Reduced Alkali Silica Reactivity : Fly ash combines with alkalis from cement that might otherwise combine with silica from aggregates, causing destructive expansion.

Workability : Concrete is easier to place with less effort, responding better to vibration to fill forms more completely. Ease of Pumping. Pumping requires less energy and longer pumping distances are possible.

ADVANTAGES OF P. P. C..

Improved Finishing : Sharp, clear architectural definition is easier to achieve, with less worry about in-place integrity.

Reduced Bleeding : Fewer bleed channels decreases 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.

Reduced Slump Loss : More dependable concrete allows for greater working time, especially in hot weather.

A COMPARISON WITH O.P.C.:

Parameter IS:1489 PPC IS:12269 O.P.C.

Setting Time Initial 30 min. (minimum) 30 min. (minimum)

Final 600 min. (maximum) 600 min. (maximum)

Comp. Strength 3 days 16 MPa (minimum) 27 MPa (minimum)

7 days 22 MPa (minimum) 37 MPa (minimum)

28 days 33 MPa (minimum) 53 MPa (minimum)

After 28 days Higher than O.P.C. Normal

Tensile Strength* Lower as compared to O.P.C. Picks up

beyond 90 days and overtakes O.P.C. in the

later stages

Higher in initial stages upto 28

days

Water Demand* Less than O.P.C. hence better workability

and reduced segregation & bleeding

High

Deshuttering period As per IS:456 As per IS:456

Curing period Minimum 10 days Minimum 7 days

Chloride resistance* Better than O.P.C. -

Sulphate resistance* Better than O.P.C. -

Heat of hydration Very low High

Durability Considered better than O.P.C. Generally good

ENVIRONMENTAL ADVANTAGES: 1. Each ton of Portland cement requires

about 1.5 tons of raw material for its production.

2. Each ton of Portland cement that is produced involves the release into the atmosphere of about one ton of CO2. Indeed, the cement industry is responsible for about 7% of global CO2 emissions.

3. PPC cement is reported to show close to zero CO2 emission during its production (only about 100 pounds per ton of cement).

LIMITATIONS:

Less early strength as compared to OPC.

Re-adjustment of removal of formwork is

necessary.

Early strength is due to OPC only and pozzolona

portion joins in strength only later.

THE GOA CONNECTION

As per

IS456:2000, Goa

is in Severe Zone.

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