cement _ 2nd

Concrete Technology

Dr. P. DINAKAR

Department of Civil Engineering

Portland Cement

Portland Cement

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Portland Cement

Portland cement is a type of binder that glue individual filler particles together to form into concrete.

A binder can be classified as organic and inorganic binder.

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Organic Binder

Epoxy: Resin + hardener

Asphalt : Petroleum asphalt – original form or cutbacks

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Inorganic Binder Use water for mixing

Non-hydraulic cement: cannot harden in water examples: Gypsum and Lime

Hydraulic cement: can harden in water examples: Hydraulic lime, Pozzolan cement, Portland

cement

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Gypsum Plaster Boards

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Gypsum Plaster Boards

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Inorganic Binder - Gypsum

Natural gypsum, CaSO4.2H2O

Heated to about 130-160°C, it loses a part of water of

crystallization, is known as half-hydrate gypsum (plaster

of Paris) （ CaSO4.0.5H2O)

At about 200°C, gypsum loses all its water of

crystallization, turn out into anhydrate gypsum.

Gypsum-based items should be used only in dry state

and in premises of not more than 60% relative humidity

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Inorganic Binder - Cement

Raw material:

Limestone CaCO3

Shell, coral, chalk

Manufacture: burning limestone at a temperature of

about 900~1000°C ， lime stone is decomposed to CaO

and CO2

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Cement

Portland cement is the major binder for civil engineering.

Manufacture of Portland cement

Chemical composition

Hydration

Types of Portland cement

Porosity of hardened cement paste and the role of

water

Basic tests of Portland cement

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Manufacture of Portland Cement

Raw materials:

Main materials:

Limestone(CaCO3) (1000oC) →CaO + CO2

Clay (600oC) →SiO2+ Al2O3+ Fe2O3+ H2O

Additional materials

Aluminium & Iron (Al2O3+ Fe2O3)

Gypsum (2CaSO4•2H2O)

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Manufacture of Portland Cement

Limestone(CaCO3) (1000oC) →CaO+ CO2

(100) (56) (44)

One ton cement contains about 620kg CaO

So that CO2= 620 x 44 / 56 = 487kg

Fuel burning produces CO2 450kg

One ton cement produces one ton CO2!!

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Manufacture of Portland Cement

CLINKER

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Clinker composition and performance

Clinker is a sintered material produced by burning a blend of raw materials at high temperature in a kiln

The clinker minerals are formed by chemical reactions between CaO (calcium oxide) and SiO2, Al2O3, Fe2O3

The main minerals formed by these reactions are

C3S (tricalcium-silicate) or Alite – 3CaO.SiO2

C2S (dicalcium-silicate) or Belite – 2CaO.SiO2

C3A (tricalcium-aluminate) or Aluminate – 3CaO.Al2O3

C4AF (tetracalcium-alumoferrite) or Ferrite – 4CaO.Al2O3.Fe2O3

(In cement chemistry, oxides are conventionally identified by the initial letter of their formula; e.g. C, S, A, F)

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Chemical Composition

Abbreviation

CaO = C SiO2= S

Al2O3= A Fe2O3= F

H2O = H SO3 = S

Examples: Ca(OH)2= CH

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Chemical Composition of Clinker

Compounds Oxide Colour Per

Tricalcium silicate

C3S White 50 ‘Alite’

Dicalcium silicate

C2S White 25 ‘Belite’

Tricalcium Aluminate

C3A White / grey

12 ‘Aluminate’

Tetracalcium Alumino Ferrite

C4AF Black 8 ‘Ferrite’

Major Compounds

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Influence of Clinker on Cement PerformanceClinker

phaseEarly strength

(Hydration

heat)

Final

strength

Water

demand

Setting

time

C3S

C2S

C3A

C4AF

Beneficial effect (positive influence)

Indifferent effect (no influence)

Detrimental effect (negative influence)

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Chemical Composition

Minor components

Gypsum, MgO, and alkali sulfates

Gypsum: To avoid flash set (barrier effect)

Alkalies (MgO, Na2O, K2O, free lime) :

Increase pH value upto13.5

Potential Problems - Soundness problem

- Alkali aggregate reaction

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Chemical Composition

MgO: limited to 4-5%,

Free-lime: behaves similarly with MgO

SO3: typically <3.5%, if excessive, expansion

Alkalis (K2O and Na2O): Limit content 0.6%

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Raw Materials for Cement

Calcareous material – Containing CaCO3 (primary source – limestone); impurities such as iron and alumina are sometimes present

Argillaceous material – Containing clayey matter, source of SiO2, Al2O3 and Fe2O3

Gypsum – Added in the final stages of manufacture as a set regulator

Sometime, ground limestone is also added to cement

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Location of Cement Plants

Outskirts of the city

Primarily, where raw material sources are easily available

Necessary infrastructure (power, equipment, manpower, access) should be available

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Cement Production

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Schematic Depiction of Process

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Limestone is quarried

near the cement plant

Quarry rock is trucked to

the primary crusher

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Rotary Kiln

Clinker

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Concrete Batching Plant

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Concrete Batching Plant

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Pulverization

Raw material feedstock should be pulverized to the right size

Reduces overall power consumption

Better blending and burning possible with reduced size of material

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Blending of raw materials

• Choice of blending process

- Wet or dry• Wet process – more uniform mixing• Dry process – higher output, lower power consumption• Dry process with precalciners are the order of the day

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Blending – Wet Vs. Dry When moisture content of raw materials is > 15%, wet blending (in slurry form) is preferred

When MC < 8%, dry blending is done

For 8% < MC < 15%, dry blending with precalciners used

Wet blending – better blend

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Picture of a cement plant, showing a precalciner and rotary kiln

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Burning in kiln

• Only rotary kilns used nowadays

• Typical kilns are long ~ 30 – 40 m

• Length of kiln also depends on blending process

• Temperature inside kiln varies from 850 (at inlet) to

1450 oC (at the outlet)

• Reactions are not completed inside kiln; some

require cooling to occur

• What comes out of kiln is called ‘clinker’

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Reactions in Kiln

Upto 700oC:activation of silicates to remove moisture &

change of structure

Between 700-900oC : removal of CO2, initial combination

of aluminates and ferrites, start of reaction between

CaO and SiO2

900-1200oC : formation of Belite (C2S)

> 1250oC: reaction between C2S and CaO leading to

formation of Alite (C3S)

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Intergrinding with gypsum

• Final step in cement manufacture• Gypsum added as a set regulator (absence

flash set)• Strict control on temperature required• Done in ball mills• Cement of required fineness produced

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Hydration of Calcium Silicates

The chemical and Physical processes between Portland cement and water:

Hydrations of pure cement compounds

Calcium silicates

2 C3S + 6H = C3S2H8+ 3CH + heat

2 C2S + 4H = C3S2H8+ CH + heat

C3S2H8 (C-S-H Gel) : 50-60% small Size

CH—Ca(OH)2 : 20-25% pH value 12

Hydrated PC contains about 50 - 60% of C-S-H and 20 - 25% of CH

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Hydration of Calcium Silicates

C-S-H Gel

Calcium silicate hydrate constitutes 50-60% of the solids in the paste. It forms a continuous binding matrix. It is amorphous and fibrous and hence has a large surface area. It is an important factor for the strength development of cement paste.

CH

Calcium hydroxide makes up about 20% of the solids in the paste. It exists in the form of thick, crystalline hexagonal plates and is embedded in the C-S-H matrix. Its growth fills the pore spaces. It does not significantly contribute to strength. Its leaching causes white patches and efflorescence.

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Hydration of the Ferrite Phases

Yet not well understood !

Similar reactions as for the C3A, but slower.

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Hydration

Kinetics and Reactivities

Hydration rate order : C3A > C3S > C4AF > C2S

Reactivities:

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Heat of Hydration

Calorimetric Curve of Portland Cement

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Hydration – Five Stages

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Thermal influence on heat of hydration

0 4 8 12 16 20 24 28 32 360

20

40

60

80

100

Time [hours]

Hea

t evo

lutio

n r

ate

Q/t

[J/(h

.g)] 50°C

40°C

30°C

20°C

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SO3 influence on heat of hydration

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 1500.0

2.5

5.0

7.5

10.0

12.5

15.0

Time [hours]

Hea

t evo

lutio

n r

ate

Q/t

[J/(h

.g)]

3.6% SO

2.7% SO

4.1% SO

3.2% SO

3

3

3

3

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Influence of cement fineness

0 5 10 15 20 25 30 35 40 45 500.0

2.5

5.0

7.5

10.0

12.5

15.0

Time [hours]

Hea

t evo

lutio

n r

ate

Q/t

[J/(h

.g)]

2500 cm²/g

Specific surface = 5500 cm²/g

3500 cm²/g

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Types of Portland Cement

ASTM BSI

Type I OPC (ordinary)

Type II ---

Type III RHPC

Type IV LHPC

Type V SRPC

OPC -----Ordinary Portland cement

RHPC ---Rapid Hardening P.C.

LHPC ---Low Heat P.C.

SRPC ---Sulfate Resistance P.C.

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Types of Portland Cement

Applications:

Type I ---General purpose

Type III ---High early strength

Type IV ---Mass concrete (dam, pile)

Type V ---Ocean structure

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Temperature rise for the different types of cement

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Hydration

Setting and hardening

Setting :Losing plasticity and starting solidification. Initial setting: ~2 hours after mixing. Final setting: ~5-10 hours after mixing.

Hardening: Process of gain of strength.

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The role of water

Hydration & Workability

Three types of water

i. Chemically reacted water-nonevaporable water

ii. Absorbed water-on surface of CSH gel

iii. Free water-water held by capillary tension

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The role of water - Changes in hydration

100% hydration

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Changes in hydration

Unhydrated cement

Hydration products (gel)

Gel pores

Capillary pores

= 0 = 0.25 = 0.5 = 0.75 = 1.0

Increasing hydration

Evaporablewater

Total “solid”volume

Constant w/c 0.50

Completehydration

Nohydration

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Basic Tests of Portland Cement

Test standards

ASTM-American Society for Testing and Materials BSI-British Standard Institution

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Basic Tests of Portland Cement

Fineness (=surface area/weight):

Represent average size of cement grain

Typical value of 350 m2/kg

Controls rate of hydration

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Normal Consistency

Water requirement of cement paste

Definition

The quantity of mixing water to form a standard paste

for setting and soundness test It depends upon the compound composition and

fineness of cement About 24%~30% for Portland cement