By :-Reasat E Noor (REN)

Definition: “Cement is a crystalline compound of calcium silicates and other

calcium compounds having hydraulic properties”.

The product is obtained by pulverizing clinker consisting essentially of hydraulic

calcium aluminates, silicates of varying composition, which hydrates and becomes

hard like stone in contact with water or moist air.

• Lime and clay have been used as cementing material on constructions through many centuries.

• Romans are commonly given the credit for the development of hydraulic cement, the most significant incorporation of the Roman’s was the use of pozzolan-lime cement by mixing volcanic ash from the Mt. Vesuvius with lime.

• Best know surviving example is the Pantheon in Rome (128 AD)

• In 1824 Joseph Aspdin from England invented the Portland cement. The set product of cement , sand and water has got similar color and strength to that of a natural stone obtained at Portland of England.

The first cement factory in Bangladesh established in 1940 at Chattak of Sylhet.

At present there are more than 50 factories in Bangladesh

The total annual production of cement in Bangladesh is more than 5 million metric tones.

Calcareous Materials

Limestone, cement rock, marine shells are example of calcareous materials.

Should contain less than 3.3% MgO.

Limestone containing high percentage of MgO and the percentage is reduced to less than 5%.

Argillaceous Materials

Rich in silica, supply Silica (SiO2), Alumina (Al2O3),Iron Oxide (Fe2O3).

Clay, shell, blast furnace slag, ash, cement rock etc.

CaCO3 = CaO+CO2 ( at 700°-1000° Celsius)

2 CaO + SiO2 = 2CaO SiO2 (C2S) (at 1300° -1500° Celsius)

3 CaO + SiO2 = 3CaOSiO2 (C3S) (“)

3 CaO + Al2O3 = 3CaOAl2O3 (C3A) (“)

4 CaO + Al2O3 +Fe2O3 = 4 CaO Al2O3 Fe2O3 (C4AF)

These materials are combined together to form hard, grayish pellets, which is known as cement clinker.

The lime saturation factor ( CaO/ 2.8 SiO2 +1.2 Al2O3+0.65 Fe2O3) should be in the range of 0.66 to 1.2. This will ensure the formation of C2S,C3S and C3A which are responsible for giving strength of concrete.

Silica modulus (SiO2 / Al2O3 +Fe2O3 ) should be within 2.2 to 3.5

MgO should be below the specific limit which ensures the cement is sound.

These main phases are present in the clinker and in the non-hydrated Portland cement.

They are formed at high temperature (1450°C) in the cement kiln. Compounds referred as C3S, C2S,C3A and C4AF are known as the main crystalline phases of Portland cement.

SI Name of Compound

Oxide Composition Abbreviation % Function

1 Tri Calcium Silicate

3CaO.SiO2 C3S 45-55% Responsible for early and later strength

2 Di Calcium Silicate

2CaO. SiO2 C2S 20-30% Responsible for later strength

3 Tri Calcium Aluminate

3CaO.Al2O3 C3A 6-10% Increases rate of hydration of C3S. It gives flash set in absence of Gypsum

4 Tetra Calcium Aluminoferrite

4CaO.Al2O3Fe3O3 C4AF 15-20% Hydrates rapidly but contribution to strengthis uncertain and very low

Other components of cement are

1. Gypsum

2. Free Lime

3. Chlorides

4. Alkalis

5. Magnesium

Tri Calcium Silicate & Di Calcium Silicate

1. Tri Calcium Silicate hydrates and hardens rapidly and responsible for early strength and initial set.

2. Higher percentage of C3S exhibits higher strength.

3. When C3S increases it decreases C2S.

1. Heat of Hydration Increases:

2Ca3SiO5 +7H2O =3CaO.2SiO2.4H2O +3Ca(OH)2 + 173.6 kj

Tricalcium Calcium Silicate hydrate (Non Cementous compound)Silicate (Cementous glue)

2Ca2SiO4 +7H2O =3CaO.2SiO2.4H2O +3Ca(OH)2 + 58.6 kj

Dicalcium Calcium Silicate hydrate (Non Cementous compound)Silicate (Cementous glue)

1. Reactivity with Supplementary Cementous compound increases

2. Setting Time Decreases

3. Strength Increases

1. Tri Calcium Aluminate hydrates and hardens the quickest. Liberates a large amount of heat and contributes somewhat to early strength.

2. Gypsum is added to retard the hydration of C3A.

3. In absence of Gypsum Portland cement sets immediately after adding water.

1. Bleeding decreases

2. Chloride permeability decreases

3. Heat of hydration increases

4. Setting time decreases

5. Strength increases

6. Sulfate resistivity decreases

7. Water requirement increases

8. Workability decreases

1. It hydrates rapidly but contributes very little to strength. Its use allows lower kiln temperature in Portland Cement Manufacturing.

2. The most Portland cement color effects are due to C4AF

3. The higher the C4AF more darker the color of cement.

1. Gypsum (CaSO4·2H2O) is a setting time retarder, without it cement will set immediately.

2. To prevent Sulfate expansion in cement the Sulfate (SO3) content in cement is limits to 3-3.5%. That’s why Gypsum is kept less than 5% in cement.

3. The higher the Gypsum content the longer is setting time of cement.

4. Excess Gypsum leads to the formation of Calcium Sulfoaluminate,(Ettringite) and it results in a volume increase in the concrete, resulting expansion and cracks.

Lime (CaO) which is not combined with other constituents in clinker is known as free lime.

Free lime is harmful for cement , in presence of higher free lime causes expansion in cement and drop early strength.

Free lime normally generated in burning process

Higher free lime causes cement to change of its color to brown.

Higher lime cement is not a good binder.

Higher chloride content leads to corrosion of steel.

The origin of chloride is mainly from cement raw material.

Higher presence of alkalis leads to internal expansions and cracking in concrete due to reaction between silica and alkali.

The origin of alkali is mainly from cement raw material and fuels.

Air content increases

Bleeding decreases

Heat of hydration increases

Reactivity of SCM increases

Alkali Silica reaction increases

Changes in setting time

Shrinkage decreases

Early Strength up, late strength downs

Water requirement increases

Workability decreases

A material which is not soluble in acid and alkali is known as insoluble residue.

Insoluble residue is non cementing material which is present in Portland cement.

It decreases the early strength of cement.

The limit of insoluble residue is 0.75 percent of cement.

It comes from limestone, fly ash, volcanic clay etc.

Soluble Fluoride decrease the setting time of cement and reduce the strength and density.

Soluble Phosphates retard the setting and strength development of plaster.

These materials come from Gypsum

Fly Ash

• It is the residue that is left from burning coal, and this is formed when the gaseous releases of the coal is efficiently cooled.

Blast Furnace Slag

• It is a byproduct of the iron and steel manufacturing process.

• The chemical reaction results in two products: molten iron metal and molten blast furnace slag.

Advantages

• It is also an environmentally-friendly solution. It improves the strength over time and thus, it offers greater strength to the building.

• Increased density and also the long-term strengthening action of flash that ties up with free lime and thus, results in lower bleed channels and also decreases the permeability.

• Aids to keep aggressive composites on the surface where the damaging action is reduced. It is also highly resistant to attack by mild acid, water and sulfate.

• It effectively combines with alkalis from cement, which thereby prevents the destructive expansion.

• It is also helpful in reducing the heat of hydration. The pozzolanic reaction in between lime and fly ash will significantly generate less heat and thus, prevents thermal cracking.

Advantages

• It chemically and effectively binds salts and free lime, which can create efflorescence. The lower permeability of fly ash concrete can efficiently reduce the effects of efflorescence.

• Make big savings in concrete material prices

Disadvantages

• The poor quality can increase the permeability and thus damaging the building.

• Increases setting time.

Advantages

Concrete containing BFC is less permeable,

It has lower hydration,

Higher ultimate compressive strengths,

It is resistant to sulfate-acid attack and aggressive chemicals, resistant to many forms of deleterious attack.

It has better workability and finish ability than normal concrete.

Disadvantages

Sets slower than normal cement.

Fineness

Soundness

Setting Time

Strength

Specific Gravity

Heat of Hydration

Loss of Ignition

Color

Fineness

1. The fineness of cement effects the hydration rate of cement and thus the rate of strength gain.

2. Greater fineness increases surface area so that increases hydration rate, heat of hydration and greater early strength.

3. The effect on strength can be seen in 7 days.

4. The coarser cement will result in higher ultimate strength and lower early strength.

5. Coarse cement results in creating porosity in concrete thus reduces durability.

6. Fineness can be measured by – Sieve analysis, turbid meter, Blain Air Permeability apparatus

Effect of Fineness

1. Increases water requirement

2. Increases Strength

3. Shrinkages Increases

4. Setting time decreases

5. Heat of hydration increases

6. Bleeding decreases

7. Air Content decreases

8. Workability Decreases

Soundness

The destructive expansion caused by excessive amount of free lime and magnesia.

ASTM C 150 specifies for Portland Cement a maximum autoclave expansion (for presence of MgO) of 0.80 percent.

There is no specification limit for free lime but Lechatelier expansion is restricted to 10 mm .

Setting Time

Initial Set – Occurs when the paste begins to stiff considerably. Should be greater than 45 minutes.

Final Set – Occurs when cement is hardened enough to sustain some load. Should be less than 420 minutes.

Setting Time affects by

1. Fineness ( Setting time Decreases with increasing of fineness)

2. Water Cement Ratio ( Setting time increases when water cement ration increases)

3. Gypsum content increases setting time

4. Admixtures

Strength

ASTM Standard for Compressive Strength of Cement

1. For 3 Days:1890 psi

2. For 7 Days: 2900 psi

3. For 28 Days: 3620 psi

Strength

It depends on various factors

1. Water-Cement ratio

2. Cement-Fine aggregate ratio

3. Type and grading of aggregate

4. Manner of mixing

5. Curing conditions

6. Moisture Content

7. Fineness of Cement

Specific Gravity

1. It is used in mixture proportion calculating.

2. The specific gravity of cement is normally 3.15

Heat of Hydration

1. Increases when water cement ratio, fineness and curing temperature increases.

2. Higher heat of hydration causes considerable loss of strength and regression at later ages.

Loss of Ignition (LOI)

1. The weight loss of a sample at 900° -1000°Celsius.

2. At high LOI indicates pre hydration and carbonation, which may be caused by prolonged storage or adulteration during transportation.

3. Higher LOI causes loss of strength.

Factors affecting choice of cement:

Following factors govern choice of cement.

Durability Characteristics

Functional requirement - Deflection, crack width etc.

Design parameters- Strength, fineness, setting time requirement etc.

Speed of construction- Time for construction etc.

Environnemental Conditions- Ground conditions, Soluble Salt, Sulfate, Chemical plants etc.

1. Ordinary Portland Cement (OPC)

2. Portland Pozzolana Cement (PPC)

3. Portland Blast Furnace Slag Cement (PBSF)

4. Rapid Hardening Cement

5. Low Heat Cement

6. Sulfate Resisting Cement

7. Super Sulfated Cement

8. White Cement

33 grade OPC Cement

It is used for normal grade of concrete up to M-20, plastering, flooring, grouting of cable ducts etc. The fineness should be between 225 and 280 m2/kg.

43 grade OPC Cement

It is the most widely used general purpose cement.

For concrete grades up to M-30, precast elements.

For marine structures but C3A should be between 5 -8%.

53 grade OPC Cement

For concrete grade higher than M-30, PSC works, bridge, roads, multistoried buildings etc.

For use in cold weather concreting.

For marine structures but C3A should be between 5 - 8%.

Portland Pozzolana Cement: (using fly ash):

It gives low heat of hydration and reduces the leaching of calcium hydroxide. This cement should be used only after proper evaluation.

It is used for :

Hydraulic structures- dams, retaining walls

Marine structures

Mass concrete works- like bridge footings

Under aggressive conditions

Masonry mortar and plastering.

Portland Slag Cement (PSC)

OPC + granulated slag: It gives low heat of hydration. The slag should be more than 50% and up to 70%.

It is used for

For concrete grade higher than M-30, PSC works, bridge, roads, multistoried buildings etc.

For use in cold weather concreting.

For marine structures but C3A should be between 5 - 8%.

SRC- Sulfate Resisting Cement: (C3A < 5%)

Sulfate Resisting Portland Cement is a type of Portland Cement in which the amount of tri calcium aluminate (C3A) is restricted to lower than 5% and 2C3A + C4AF lower than 25%.

The percentage of C4AF is comparatively higher in this cement.

The SRC can be used for structural concrete wherever OPC or PPC or Slag Cement are usable under normal conditions.

SRC- Sulfate Resisting Cement: (C3A < 5%)

The use of SRC is recommended for following applications:

Foundations, piles

Basement and underground structures

Sewage and water treatment plants

Chemical factories

Suitable for underground works where Sulfate is present in the Soil and water.

Attention:

Sulfate resisting cement is not suitable where there is danger of chloride attack. This will cause corrosion of rebar.

If both Chlorides and Sulfates are present, Ordinary Portland Cement with C3A between 5& 8 should be used.

Low Heat Cement

Heat liberated during setting is low, due to presence of C4AF and C2S.

Contains low percentage of C3S and C3A.

Shrinkage and cracks are reduced.

This type of cement is used for mass scale concrete work where low liberation of heat is desired such as dams, roofs etc.

Rapid Hardening Cement

Lime saturation factor is relatively higher and the final product is ground to more fineness.

Contains high percentage of C3S and C3A.

More expensive.

This type of cement is used for emergency construction where high early strength is desired.

White Cement

Raw materials are free from iron impurities.

More expensive.

This type of cement is used for fare face concrete , cement paints, colored cements etc.

Portland Cement is a moisture sensitive material

If kept dry it will remain its quality.

When stored in moist and damp condition it will set more slowly and has less strength.

When storing bagged cement, a shaded area or ware house is preferred.

When storing cement bags at outdoor it should be stacked on pallets and covered with waterproof covering.

Storage of bulk cement should be in bin or silo.