cement _ 2nd
DESCRIPTION
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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