chemistry & maufacture of cement

7/28/2019 Chemistry & Maufacture of Cement

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CEMENT


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Uses of Portland Cement

in Concrete Buildings

Bridges Pavements

Concrete block buildings


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Other Uses of

Cementitious Materials Mortar for masonry

Grout (protection, leveling, bonding, ...)

Shotcrete

Cement board

Soil Stabilization

Railroad ties, countertops, moldings...


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Portland Cement History

Egyptian Pyramid of Cheops (3000B.C.)

First Calcareous Cement (CaO based) Calcined gypsum

Roman and Greek Projects

First Hydraulic Cements (100 B.C.)

calcined limestone and clay


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History of Cement

2000 B.C.: Egyptiansused cement in mortarwhen makingPyramids

27 B.C.: Romancement made of limeand volcanic ash

1756: Smeaton rebuilt

Eddystone Lighthouse 1824: Joseph Aspdin

discovered andpatented Portland

cement


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Portland Cement History

Rotary Kiln Ransome (1886), Edison (1909)

Gypsum and Air-Entraining Admixtures U.S. (1910-1940)


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Cement is a ManufacturedMaterial


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Portland Cement Gypsum+Portland CementClinker (pulverizing)

Portland Cement Clinker Calcareous &

Clayey Materials (burning)

Paste P.C. + Water

Mortar P.C. + Water + Sand

Concrete P.C. + Water + Sand + Gravel


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RAW MATERIALS OF P.C.

1) Calcareous Rocks (CaCO3 > 75%) Limestone

Marl

Chalk

Marine shell deposits

2) Argillocalcareous Rocks (40%


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3) Argillaceous Rocks (CaCO3 < 40%)

Clays

Shales

Slates

Portland cement is made by mixingsubstances containing CaCO3 withsubstances containing SiO2, Al2O3, Fe2O3

and heating them to a clinker which issubsequently ground to powder andmixed with 2-6 % gypsum.


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CLINKER GYPSUM


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PRODUCTION STEPS

1) Raw materials are crushed, screemed &stockpiled.

2) Raw materials are mixed with definiteproportions to obtain raw mix. They are

mixed either dry (dry mixing) or by water (wetmixing).

3) Prepared raw mix is fed into the rotary kiln.

4)

As the materials pass through the kiln theirtemperature is rised upto 1300-1600 C. Theprocess of heating is named as burning. Theoutput is known as clinker which is 0.15-5

cm in diameter.


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5) Clinker is cooled & stored.

6) Clinker is ground with gypsum (3-6%) toadjust setting time.

7) Packing & marketting.


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~100C free water evaporates.~150-350C loosely bound water is lost from

clay.

~350-650Cdecomposition of claySiO2&Al2O3

~600Cdecomposition of MgCO3MgO&CO2

(evaporates)

~900Cdecomposition of CaCO3CaO&CO2(evaporates)


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~1250-1280Cliquid formation & start ofcompound formation.

~1280Cclinkering begins.

~1400-1500Cclinkering

~100Cclinker leaves the kiln & falls into acooler.

Sometimes the burning process of raw materialsis performed in two stages: preheating upto900C & rotary kiln


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CHEMICAL COMPOSITIONOF P.C.

Portland cement is composed of fourmajor oxides (CaO, SiO2, Al2O3, Fe2O390%) & some minor oxides. Minor refersto the quantity not importance.


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Common Sources for RawMaterials

Lime (CaO)- Limestone, shale

Silica (SiO2)

-Clay, sand, shale

Alumina (Al2O3)

- Clay, fly ash, shale

Iron (Fe2O3)

- Clay, iron ore


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Portland Cement

Production 5/8 CaO Limestone or calcareous

rock

1/5 SiO2 Clay or argillaceous rock

1/10 Al2O3 Clay or Ore

1/20 Fe2O3 Clay or Ore

1/20 CaSO4*2H2O Gypsum


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Shorthand Chemistry

C = CaO H = H2O

S = SiO2 S = SO3

A = Al2O3 F = Fe2O3


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Clinker: artificial mineral

containing:C3S tricalcium silicate

C2S dicalcium silicate

C3A tricalcium aluminate

C4AF tetracalcium aluminoferrite


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Clinker Micrographs


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Finish Grinding Interground with

~5% Gypsum

95% material mustpass #325 Sieve


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How are Portland

Cements different?Four Main Compounds

Tricalcium Silicate (C3S) Dicalcium Silicate (C2S)

Tricalcium Aluminate (C3A)

Tetracalcium Aluminoferrite (C4AF)


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C3S

Tri Calcium Silicate 3CaO.SiO2 -Alite

Provides Early strength development 70% reacts by 28 days

Usually present at 40-70% If >65% difficult to burn


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C2S Dicalcium Silicate

2CaO.SiO2 -Belite

Provides late strength development

30% reacts by 28 days

Present at 20-40%

Under-burning can result in higher C2Scontents in cement


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C3A Tricalcium Aluminate

3CaO.Al2O3 -Celite

Provides heat generated in hydration(10 to 15 F per 100 lb. cement)

High C3A not as resistant to sulfate

attack

Little contribution to strength


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C4AFTetracalcium

Aluminoferrite 4CaO.Al2O3.Fe2O3 -Felite

Governs the color of the cement Present at 1-10%

Iron facilitates formation of other

compounds-acts as a flux Little contribution to strength


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Oxide Common Name Abbreviation Approx. Amount (%)

CaO Lime C 60-67

SiO2 Silica S 17-25

Al2O3 Alumina A 3-8

Fe2O3 Iron-oxide F 0.5-6

MgO Magnesia M 0.1-4

Na2O Soda N0.2-1.3

K2O Potassa K

SO3 Sulfuric Anhydride

1-3

CaOlimestone

SiO2-Al2O3Clay

Fe2O3Impurity in Clays

SO3

from gypsumnot from the clinker


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The amount of oxides in a P.C. Depend on theproportioning of the raw materials and how well theburning is done in the kiln. The chemical composition isfound by chemical analysis.

A typical analysis of O.P.C.

Insoluble residue=0.2

Loss on ignition=1.4

C 63.6

S 20.7

A 6

F 2.4

2.1

M 2.6

N 0.1

K 0.9

Free C 1.4

Total 99.8


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CaO (C), SiO2 (S), Al2O3 (A) & Fe2O3 are themajor oxides that interact in the kiln & form themajor compounds.

The proportions of these oxides determine theproportions of the compounds which affect the

performance of the cement.

SO3comes largely from gypsum

P.C. alone sets quickly so some gypsum is

ground with clinker to retard the setting time.


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If too much gypsum is included it leads todistruptive expansions of the hardened

paste or concrete.

ASTM C 150 SO3 3% in O.P.C.

MgO+H2OMH

C+HCH volume expansion & cause

cracking. ASTM C 150 M


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Alkalies (Na2O & K2O) may cause somedificulties if the cement is used with

certain types of reactive aggregates inmaking concrete. The alkalies in the formof alkaline hydroxides can react with the

reactive silica of the aggregate & resultingin volume expansion after hardening. Thisprocess may take years.

Na2O & K2O 0.6%


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Insoluble Residue: is that fraction of cementwhich is insoluble in HCl. It comes mainly from

the silica which has not reacted to formcompounds during the burning process in thekiln. All compounds of P.C. is soluble in HClexcept the silica.

The amount of I.R., determined by chemicalanalysis, serves to indicate the completeness ofthe reactions in the kiln.

ASTM C 150 I.R. 0.75%


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Loss on Ignition (L.O.I.): is the loss inweight of cement after being heated to

1000C. It indicates the prehydration orcarbonation due to prolonged or improperstorage of cement & clinker.

If cement is exposed to air, water & CO2are absorbed & by heating the cementupto 1000C loose these two substances.

ASTM C 150 L.O.I. 3% for O.P.C.


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COMPOUND COMPOSITIONOF P.C. (OR CLINKER)

Oxides interact with eachother in the kilnto form more complex products

(compounds). Basically, the majorcompounds of P.C. can be listed as:Name Chemical Formula Abbreviations

Tri Calcium Silicate 3CaO.SiO2 C3S

Di Calcium Silicate 2CaO.SiO2 C2S

Tri Calcium Aluminate 3CaO.Al2O3 C3A

Tetra Calcium Alumino

Ferrite

4CaO.Al2O3.Fe2O3 C4AF


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The degree to which the potentialreactions can proceed to equilibrium

depends on:

1) Fineness of raw materials & their

intermixing.2) The temperature & time that mix is held

in the critical zone of the kiln.

3) The grade of cooling of clinker may alsobe effective on the internal structure ofmajor compounds.


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There are also some minor compoundswhich constitute few %, so they are

usually negligible. Moreover, portlandcement compounds are rarely pure.

For example in C3S, MgO & Al2O3 replaces

CaO randomly. C3SALITE & C2SBELITE

Ferrite Phase: C4AF is not a true

compound. The ferrite phase ranges fromC2AF to C6AF. *C4AF represents an

average.

Methods of Determining


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Methods of DeterminingCompound Composition

Each grain of cement consists of an intimatemixture of these compounds. They can be determined by:

1) Microscopy2) X-Ray Diffraction

But due to the variabilities involved the

compound composition is usually calculatedusing the oxide proportions.

3) Calculations (Bouges Equations)

I fl f C d


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Influence of CompoundComposition on Characteristics

of P.C. P.C.+waterthe compounds in the cementundergo chemical reactions with the waterindependently, and different products result fromthese reactions.

C3S C2S C3A C4AF

Rate of Reaction Moderate Slow Fast Moderate

Heat Liberation High Low Very High Moderate

Early Cementitious Value Good Poor Good Poor

Ultimate Cementitious Value Good Good Poor Poor


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ASTM Type & Name

of P.C.

Average Compound

CompositionC3S C2S C3A C4AF

Type I - O.P.C. 49 25 12 8 General Purpose

Type II - Modified 46 29 6 12

For Moderate Heat of

Hydration

Type III - High Early

Strength 56 15 12 8

C3S&C3A increased, C2S

decreased

Type IV - Low Heat

P.C. 30 46 5 13 C2S increased

Type V - Sulfate

Resistant P.C. 43 36 4 12

Limit on C3A5%,

2C3A+C4AF25%

chemistry & maufacture of cement

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