Download - 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%