Cement

CementThe term cement is commonly used to refer

to powdered materials which develop strong adhesive qualities when combined with water

These materials are more properly known as

Dr.V.S.Gayathri & Dr.K.Yamuna

These materials are more properly known as hydraulic cements

Gypsum plaster, common lime, hydraulic limes, natural pozzolana, and Portland cements are the more common hydraulic cements, with Portland cement being the most important in construction.

Definition

Cement is an inorganic binding material possessing adhesive and cohesive properties and

Port land cement is a hydraulic cement produced by pulverizing clinker consisting essentially of


cohesive properties and capable of bonding materials like stones , bricks , building blocks etc.,

consisting essentially of hydraulic calcium silicates, usually containing one or more of the forms of calcium sulfate (gypsum)

Cement was first invented by the Egyptians. Cement was later reinvented by the Greeks and the Babylonians who made their mortar out of lime


Later, the Romans produced cement from pozzolana, an ash found in all of the volcanic areas of Italy, by mixing the ash with lime

Portland cement

Portland cement is composed of four major oxides

CaO SiO2


2

Al2O3Fe2O3 & some minor oxides. (Minor refers to the quantity not importance)

CaO (C), SiO2 (S), Al2O3(A) & Fe2O3 (F) interact in the kiln & form the major compounds that constitute cement

The proportions of these

compounds Chemical Formula Abbreviations

Tri Calcium Silicate 3CaO.SiO2 C3S

Di Calcium Silicate 2CaO.SiO2 C2S


The proportions of these oxides determine the proportions of the compounds which affect the performance of the cement

Tri Calcium Aluminate 3CaO.Al2O3 C3A

Tetra Calcium Alumino Ferrite

4CaO.Al2O3.Fe2O3 C4AF

Types of Portland cementASTM Type & Name

of P.C.

Average Compound Composition

C3S C2S C3A C4AFType I - O.P.C. 49 25 12 8 General Purpose

Type II - Modified 46 29 6 12For Moderate Heat of Hydration


Type II - Modified 46 29 6 12 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%

Chemical Composition of PCCement Compound Weight % Chemical Formula (symbol)Tricalcium silicate 45% 3CaO.SiO2 or Ca3SiO5 (C3S)

Dicalcium silicate 25 % 2CaO.SiO2 or Ca2SiO4 (C2S)

Tricalcium aluminate 10 % 3CaO .Al2O3 or Ca3Al2O6 (C3A)


Tetracalcium aluminoferrite 9%

4CaO.Al2O3.Fe2O3 or Ca4Al2Fe2O10(C4AF)Gypsum 5 % CaSO4.2H2O

Calcium Oxide 2% CaO

Magnesium Oxide 4% MgO

Production / Manufacture of

Port land Cement (PC)


Port land Cement (PC)

RAW MATERIALS

1) Calcareous Rocks (CaCO3 > 75%) Limestone Marl Chalk


Chalk Marine shell deposits

2) Argillocalcareous Rocks (40%

Argillaceous Rocks (CaCO3 < 40%)

Clays Shales Slates


Slates

Steps Raw materials are crushed, screeRaw materials are crushed, screenned & ed & storedstored

Raw materials are mixed Raw materials are mixed inin definite definite proportions to obtain raw mixproportions to obtain raw mix


They are mixed either dry (dry mixing) or They are mixed either dry (dry mixing) or made as slurry with made as slurry with water (wet mixing).water (wet mixing).

Dry Mixture or Slurry Dry Mixture or Slurry mix is fed into the mix is fed into the rotary kilnrotary kiln

As the materials pass through the kiln their temperature is rised upto 1300-1600 C. The process of heating is named as burning

The output is known as clinker which is 0.15-5 cm in diameter

Clinker is cooled & stored


Clinker is cooled & stored

Clinker is ground with gypsum (3-6%) to adjust setting time

Packed & marketted

Burning in Rotary Kiln The raw mix enters at the

upper end of the kiln and slowly works its way downward to the hottest area at the bottom over a period of 60-90 minutes, undergoing several different reactions as the temperature increases.


temperature increases.

Dehydration zone (up to ~ 450C): This is simply the evaporation and removal of the free water. Even in the dry process there is some adsorbed moisture in the raw mix.

Calcination zone (450C 900C): The term calcination refers to the process of decomposing a solid material so that one of its constituents is driven off as a gas. At about 600C the bound water is driven out of the clays, and by 900C the calcium carbonate is decomposed, releasing carbon dioxide. By the end of the calcination zone, the mix consists of oxides of the four main elements which are ready to undergo further reaction into cement minerals


Solid-state reaction zone (900 - 1300C): CaO and reactive silica combine to form small crystals of C2S (dicalcium silicate), one of the four main cement minerals. In addition, intermediate calcium aluminates and calcium ferrite compounds form.

Clinkering zone (1300C 1550C): This is the hottest zone where the formation of the most important cement mineral, C3S (alite), occurs. C3S forms by reaction between C2S crystals and CaO.

The clinkering process is complete when all of silica is in the C3S and C2S crystals and the amount of free lime (CaO) is reduced to

65-125C Free water evaporates

400-650C Clays decompose

Si2Al2O5(OH)2 (kaolinite) 2 SiO2 + Al2O3 + 2 H2O (g)

KAlSi3O8 (orthoclase) + SO2 + O2 3SiO2 + Al2O3 + K2SO4

500-650C Dolomite decomposes

Chemical Changes occurring in Kiln


500-650C Dolomite decomposesCaMg(CO3)2 CaCO3 + MgO + CO2

650-900C Calcium carbonate reacts with silica to form "incipient belite"

2 CaCO3 + SiO2 Ca2SiO4 + 2 CO22 MgO + SiO2 Mg2SiO4

Ca5(PO4)3OH + SiO2 1Ca3(PO4)2 + Ca2SiO4 + H2O(g)

700-900C Calcium carbonate reacts endothermically with alumina and iron oxide to form incipient aluminate and ferrite.

12 CaCO3 + 7 Al2O3 Ca12Al14O33 + 12 CO24 CaCO3 + Al2O3 + Fe2O3 Ca4Al2Fe2O10 + 4 CO24 CaCO3 + Al2O3 + Mn2O3 Ca4Al2Mn2O10 + 4 CO2

900-1050C When all available silica, alumina and iron oxide have reacted, the remaining calcium carbonate decomposes endothermically to calcium oxide.

CaCO3 CaO + CO2


CaCO3 CaO + CO2

1300-1425C Aluminate, ferrite and part of the belite melt endothermically, and belite reacts with calcium oxide to form alite.

Ca2SiO4 + CaO Ca3SiO5Ca12Al14O334 Ca3Al2O6 +3 Al2O3

Reactions in nut shell (12 to 4)

1. Belite is formed by the reaction between lime and silica (700-900oC)

2CaO + SiO2 Ca2SiO4 (2C+S C2S)


2.Belite (Ca2SiO4) reacts with calcium oxide to form Alite (Ca3OSiO4) (1300-1450oC)

Ca2SiO4 + CaO Ca3SiO5 (C2S + C C3S )

[Belite] + CaO [Alite]

3. Calcium oxide reacts with alumina to form Calcium aluminate (1300-1450oC)

3CaO + Al2O3 Ca3Al2O6 (3C+A C3A)


4. Calcium oxide reacts with alumina and iron oxide to form Calcium alumino ferrite

(700-900oC)4CaO + Al2O3 + Fe2O3 Ca4 Al2 Fe 2O10 (4C+A + F C4AF)

Portland cement is made by mixing clinker with 2-6 % gypsum

GypsumClinker


If too much gypsum is added it leads to distruptive expansions of the hardened paste or concrete

Properties of cement

Cement forms paste like mass with water resulting in hydration of compounds of cement. This mass then becomes stiff and hard. The process of solidification comprises three stages

Hydration: Interaction between cement and water


Setting: formation of tobermorite gel

Hardening: formation of hard crystalline mass

Hydration & heat of hydration

As water comes into contact with cement particles, hydration reactions starts immediately at the surface of the particles

It is an exothermic process


It is an exothermic process

The process of hydration is complex and results in reorganization of the constituents of original compounds to form new hydrated compounds

As the hydration proceeds the deposits of hydrated products on the original cement grains makes the diffusion of water to unhydrated nucleus more & more difficult


Hence the rate of hydration decreases with time & as a result hydration may take several years.

The heat of hydration of PC is on the order of 85-100 cal/g

About 50% of this heat is liberatedwithin 1-3 days & 75% within 7 days

By limiting the


By limiting the percentage of C3S &C3A heat of hydration can be reduced as too much heat may lead to expansion and crack in concrete

Chemistry of setting and Hardening

Setting is stiffening of the plastic mass

Hardening is crystalisation

In practice, the terms initial set & final set are used

Initial set ( with 24 hours) indicates the beginning of a noticeable


Initial set ( with 24 hours) indicates the beginning of a noticeable stiffening . It is mainly due to the hydration of C3A , C4AF.

Final set ( 7 28 days) is mainly due to the formation of Tobermorite gel and crystallization of Calcium hydroxide (CH) .

CH does not contribute very much to the strength of portland cement.

on hydration both calcium-aluminate and calcium alumino ferrite C3A and C4AF forms the same product - Calcium-Alumino hydrate

C3A + 6H C3A .6HC4AF + 7H C3A .6H + CFH

on hydration both calcium-silicates , C S and C S forms the same


on hydration both calcium-silicates , C2S and C3S forms the same product - Calcium-Silicate-Hydrate (C-S-H gel) It is similar to a mineral Tobermorte. Hence it is named as

TOBERMORITE GEL

2C3S+6H C3S2H3 + 3CH (7 days)2C2S+4H C3S2H3 + CH (28 days)

Chemical reactions during setting

3CaO.Al2O3 + 6H2O 3CaO.Al2O3.6H2O

4CaO.Al2O3.Fe2O3+7H2O 3CaO.Al2O3.6H2O+CaO.Fe2O3.H2O

Tobermorte Gel formation


Tobermorte Gel formation

2[Ca2SiO4] + 4H2O 3CaO.2SiO2.3H2O + Ca(OH)2

3[Ca3SiO4] + 6H2O 3CaO.2SiO2.3H2O + 3Ca(OH)2

70-80% of Portland cement is C2S & C3S

Therefore the strength of cement is mainly depend on hydrates of these compounds ie Tobermorite gel


C3A and is characteristically fast reacting with water & may lead to a rapid stiffening of the paste (with few hours) with a large amount of the heat generation (Flash-Set)-(Quick-Set)

In order to prevent this rapid reaction ,gypsum is added to the clinker


Gypsum, C3A & water react to form relatively insoluble Calcium-Sulfo-Aluminates

Calcium-Sulfo Aluminates are prone (less resistant) to sulfate attack & does not contribute much for strength

C4AF: The hydration of ferrite phase is not well understand. Ferrite phase has lesser role in development of strength

C3S hydrates at a moderate rate with greater heat generation.It is also responsible for early strength ( compared to C2S)

C2S hydrates & hardens slowly and heat of hydration is also less & and contributes to ultimate strength


Higher C3Shigher early strength-higher heat generation (roads, cold environments)

Higher C2Slower early strength-lower heat generation (dams)

P.C.+waterthe compounds in the cement undergo chemical reactions with the water independently, and different products (hydrates) are formed

C3S C2S C3A C4AF

Rate of Reaction Moderate Slow Fast Moderate

Heat Liberation


Heat Liberation( heat of hydration) High Low Very High Moderate

Early Cementitious Value Good Poor Good Poor

Ultimate Cementitious Value Good Good Poor Poor

During hardening the solid cement material begins to gain strength

It depends on the chemical combination of cement and water

Hardening starts with great speed and the rate decreases with time

Hardening stops if the cement becomes dry


Hardening stops if the cement becomes dry

Abnormal setting reactions : Flash setting and False setting

Flash setting

Immediate stiffening of cement paste in a few minutes after mixing with water. It is accompanied by large amount of heat generation upon reaction of C3A with water

Gypsum is placed in cement to prevent flash-set


However too much gypsum will cause undesirable expansion after hardening and lead to cracks

False setting It is a rapid development of rigidity of cement paste without generation of

much heat

This rigidity can be overcome & plasticity can be regained by further mixing without addition of water

Thus cement paste restores its plasticity & sets in a normal manner without any loss of strength


Probable Causes of False-Set: When gypsum is ground with hot clinker, gypsum may be dehydrated into hemihydrate (CaSO4.1/2H2O) or anhydrate (CaSO4).

The hemi hydrate and anhydrate calcium sulphate on reacting with water forms gypsum , which results in stiffening of the paste

Factors Affecting Setting Time

Temperature & Humidity

Amount of Water


Chemical Composition of Cement

Fineness of Cement (finer cement, faster setting)

Special cements

White Cement

Water resistant Cement / Water Proof cement


Proof cement

White Cement It is colorless due to the absence of iron and manganese

compounds (Fe2O3 + MnO 0.8%)

It is expensive and is used in marble work

If required , coloring agent can be added and the cement can be made as colored cement


Uses

It acts as seal for pores and prevents the seepage of water

It is used to make water resistant concrete

It is also used to join ceramic tiles and mosaic tiles

Stone masonry

Fixing tiles on walls and floors

Construction of domes

White washing of walls ( replacement of lime)

Plastering monumental walls


Manufacture of cement paint

Manufacture of toys and house hold articles

Construction of garden furniture like benches and tables

Water resistant cement It is hydrophobic in nature

They are impermeable to water and other fluids

Fatty acids are used to block pores within the concrete, preventing water passage

It is prepared by mixing ordinary or rapid hardening cement and some


It is prepared by mixing ordinary or rapid hardening cement and some percentage of metal stearate ( Ca, Al etc)

It is resistant to water and oil penetration.

It is also resistant to acids, alkaline and salt discharged by industrial water

It is used for water retaining structure like tanks, reservoir, retaining walls, pool, dam etc

Cement

Documents

Transcript of Cement