CEMENT

GENERALLY CEMENT CAN BE DESCRIBED AS : The product obtained by intimately mixing

together calcareous (calcium carbonate or lime) and argillaceous (alumina) or other silica and iron oxide bearing materials, burning them at a clinkering temperature and grinding the resulting clinker.

When it is mixed with water it forms a paste which hardens and binds aggregates (sand, gravel and/or crushed rock) together to form a hard durable mass called concrete.

FOR CONSTRUCTION PURPOSE THE TERM CEMENT IS CONFINED TO THE BONDING MATERIALS USED WITH STONE, BRICKS, BUILDING BLOCKS, ETC.

The functions of cement:

To bind the sand and coarse aggregate together

To fill the voids in between sand and coarse aggregate particles to form a compact mass

Raw materials of cement

Main materials:LimestoneClay

Additional materialsAluminium& Iron Gypsum

CHEMICAL COMPOSITION LIMIT OF CHEMICAL COMPOSITION LIMIT OF PORTLAND CEMENTPORTLAND CEMENT

Name of raw material Chemical formula Percentage limit

LimeSilicaAluminaIron OxideMagnesiumAlkalis (Soda and/or potash)Sulphur trioxide

CaOSiO2

Al2O3

Fe2O3

MgONa2O,K2O

SO3

60-6717-253-8

0.5-60.1-4

0.2-1.31-3

MAIN CHEMICAL COMPOUND OF PORTLAND CEMENTName of compound Chemical

compositionUsual

abbreviationReaction

Tricalcium silicate

Dicalcium silicate

Tricalcium aluminate

Tetracalcium aluminoferrite

3CaO.SiO2

2CaO.SiO2

3CaO.A2O3

4CaO.Al2O3.Fe2O3

C3S

C2S

C3A

C4AF

Quick reaction

Slow reaction

Very quick reaction

Not very important

Properties and roles of the four chief compounds in Portland cement.

Name Abbrev % in OPC

Properties and roles Heat of hydration(J/g)

Dicalcium silicate

C2S 20 Slow strength gain-responsible for long-term strength

260

Tricalcium silicate

C3S 50 Rapid strength gain-responsible for early strength (e.g. 7 days)

500

Tricalcium aluminate

C3A 12 Quick setting(controlled by gypsum); susceptible by sulphate attack

865

Tetracalcium aluminoferrite

C4AF 8 Little contribution to setting or strength; responsible for grey colour of OPC

420

Factors affecting the properties of cement

Chemical composition C3S

C2S

C3A

C4AF

Fineness

Fineness of cement

Finer grinding increases the speed of hydration when reacts with water.

Fineness of grinding is some importance in relation on the workability of concrete mixes.

Greater fineness increases the cohesiveness of a concrete mix

Finer grinding reduces the chances of bleeding of concrete

Fineness increases the chance of shrinkage cracking. Shrinkage cracking are resulted from both cooling and drying out of concrete

In some special type of cement the strength increases slowly than normal though they are finely grounded.

Fineness of cement

Determination of fineness of cement (BS410:1969 and MS 7.13) can be carried out via:

Sieve analysis through a 90 micron sieve. Surface area of cement in cm2 per gram

TYPE OF CEMENT

By changing the chemical composition of the cement and by varying the percentage of the four basic compounds, it is possible to obtain several types of cement, each with some unique characteristic.

Other Types of Portland cement: Rapid-hardening Portland Cement

It is obtained by increasing the C3S content and by finer grinding of OPC clinker. RHPC tends to set and harden at a faster rate than OPC. Early strength development is greater than that of OPC while long-term strength is similar.

It is used where: high speed in construction is needed in cold (winter) countries whereby there is less risk of

concrete freezing. When formwork has to be removed for early use as in case

of precast concrete When roads or air field repairs are to be done urgently so

that the road can be thrown open to traffic fairly quickly

Low-heat Portland Cement The heat output of this cement is low compared to OPC.

At the age of 7 days and 28 days the heat output is 250 J/g and 290 J/g respectively compared to OPC which are 330 J/g and 400 J/g respectively. The reduction in heat is obtained by lowering the quantities of C3S and C3A. Although early strength is slightly less than that of OPC, its long-term strength is similar.

This type of cement is used for mass concrete construction like raft foundation and dams. This is because there is a tendency for the heat to build up internally in such structures causing a difference in temperature between the inner and the outer layers. A temperature differential of as little as 10°C could cause internal cracking as the warmer inner layers eventually cool and are thrown into tension.

Other Types of Portland cement:

Sulphate-resisting Portland Cement (SRPC) This cement has better resistance to sulphate attack

than OPC. The percentage of sulphate-susceptible tricalcium aluminate (C3A) is limited to 3.5 % in order to minimize chemical combination with sulphates in solution. SRPC is obtained by the addition of extra iron oxide. This results in the cement being darker than OPC. Applications include foundations in sulphate-bearing soils, in marine structures since sea water contains sulphates and in mortar for flues in which sulphur may be present from fumes

Other Types of Portland cement:

Portland blast-furnace cement This cement (PBFC) comprises a mixture of OPC

and ground blast-furnace slag, the proportion of the latter not exceeding 65% of the total. The slag contains mainly lime, silica and alumina (in order of decreasing amounts). These exhibit hydraulic action in the presence of calcium hydroxide liberated by the Portland cement on addition of water. Early strength is lower than that of OPC but PBFC generates less heat than OPC and produces better sulphate resistance. Ultimate strength is similar to that of OPC.

Other Types of Portland cement:

Pulverised fuel ash(PFA) in cements PFA is a by-product of coal-powered power

stations and is an example of a pozzolanic material- one which, in the presence of lime(liberated by Portland cement) has hydraulic (cementing) properties. It can be incorporated in cement during manufacturing.

Other Types of Portland cement:

MANUFACTURE OF PORTLAND CEMENT

The process of manufacture of cement consists essentially of:

Grinding the raw materials (treatment of raw materials)

Mixing them intimately in certain proportions Burning in a large-rotary kiln at a temperature of

approximately 1300°C to 1400°C when the materials are partially fused into balls known as clinker.

Then the clinker is cooled and ground to a fine powder with some gypsum added. The resulting product is the commercial Portland Cement so widely used throughout the world

Details of manufacture of Cement:

The mixing of raw materials can be done either in water or in a dry condition. They are called ‘wet’ and ‘dry’ processes. The actual manufacture also depends on the nature of the raw materials used.

Definitions which are involved in the process of manufacturing cement:-

Wash Mill This is circular pit with revolving radial arms carrying rakes,

which breaks up the lumps of solid raw materials Ball mill

A ball mill is a mill where coarse materials are grounded to fine particles

Slurry The slurry is a liquid of creamy assistance with water content

of between 35% and 50% and only a fraction of the material about 2% - larger than a 90µm (No. 170) B.S sieve size. The slurry is kept in a storage tanks such as:

soda ash Na C2O3 (Sodium Carbonate) to reduce viscosity Sodium Silicate Na2SiO3 is used to reduce moisture

Definitions which are involved in the process of manufacturing cement:- Rotary Kiln

This large steel cylinder lined with fire bricks or called refractory lined. This is 2m to 5m in diameter and about 100m to 150m long. The thickness of steel sheeting is 20mm. The cylinder is slightly inclined to the horizontal and rotates about it’s axis at a speed of 1 – 2 times in 2 minutes.

Clinker Clinker is the product of slurry inside a rotary kiln

where slurry undergoes chemical changes due to high temperature and lime, silica and alumina recombine mass the fuses into ball of 3 to 25mm

Grinding and Mixing When chalk is used it is broken up and

dispersed in water in a wash mill. The clay is also broken up and mixed with water in a similar wash mill. The two mixtures are now pumped, mixed in predetermined proportions (normally chalk or stone to clay) and passed through a series of screens. The coarse materials are passed through a ball mill for secondary crushing and re-screened. The resulting cement slurry is then pumped into the large storage tanks. In the storage tanks the sedimentation of the suspended solid is prevented by mechanical stirrers or bubbling by compressed air.

If limestone is used, it has to be blasted with explosives, quarried and crushed in primary and secondary crushers. This may be stored in silos. The crushed limestone is then mixed with clay slurry all fed into a ball-grinding mill, which is wet because of the presence of clay slurry). Here the crushing of limestone is grounded to the fineness of flour, and the resultant cement slurry is pumped into storage tanks. From here onwards the process is the same regardless of the original nature of the raw materials.

Burning in rotary kiln The slurry is then passed into a rotary kiln. The kiln

rotates about its’ axis at a speed of 1 to 2 times in 2 minutes. The rotation is controlled by the inclination of the kiln and by the speed of rotations.

The slurry is fed at the upper thus end of the rotary kiln raising the temperature inside the kiln. The temperature raises from1400°C to 1500°C. The coal, which should not have too high a ash content, deserves a special mention because up to 350 kg of coal is used to make a ton of cement. Oil or natural gas can also be used instead of coal.

The slurry in its’ movement down the kiln, encounters a progressively higher temperature:

At 100°C – all water is driven off At 500°C – there is an evolution of combined water

At 900°C – limestone decomposes, CaCO3 CaO + CO2 (endothermic reaction) CO2 is swept away by current of air.

Between 900C and 1200C – the main reaction between lime and clay takes place (exothermic reaction)

At 1250°C – there is formation of a liquid

1400°C – some 20% to 30% of the material becomes liquid and lime, silica and alumina recombine in the formation of calcium silicates (C2S, C3S) and calcium aluminates (C3A, C4AF) (endothermic reaction). The mass then fuses into greenish block or grey coloured balls to 25mm (1/8 to 1) in diameter. This is known as clinker.

Cooling or Grinding

The clinker is dropped into a cooler situated below the kiln. The cool clinker is characteristically black, glistering and hard.

It is grounded with gypsum (2% - 5%) in order to prevent flash setting of the cement. The grinding is done in a ball mill consisting of several compartments progressively smaller steel balls to a size of 44 microns. The cement discharged by the mill is passed through a separator, and then fine particles being removed to a storage silo by air current while the coarser particles are passed through the mill once again.

Manufacturing Process (Dry)

In the dry and semi dry process, the correct proportion of raw materials are crushed and fed into a grinding mill where they are dried and reduced in size to a fine powder. This dry powder is called ‘raw meal’. The raw meal is pumped to a blending silo where final adjustment in requisite proportions of materials required for manufacture of cement is made. To obtain a uniform and intimate mixture the raw meal is blended by means of compressed air.