CEMENT

Cement is a finely ground, usually greycolored mineral powder. When mixed withwater, cement acts as a glue to bindtogether the sand, gravel and crushed stoneto form concrete, the most widely usedconstruction material in the world

• Cement is typically made from limestone and clay orshale. These raw materials are extracted from thequarry crushed to a very fine powder and thenblended in the correct proportions.

• This blended raw material is called the 'raw feed' or'kiln feed' and is heated in a rotary kiln where itreaches a temperature of about 14000 C to 15000 C.

• The raw feed enters the kiln at the cool end andgradually passes down to the hot end, then falls outof the kiln and cools down.

• The material formed in the kiln is described as'clinker' and is typically composed of roundednodules between 1mm and 25mm across.

• After cooling, the clinker may be storedtemporarily in a clinker store, or it may passdirectly to the cement mill.

• The cement mill grinds the clinker to a finepowder. A small amount of gypsum - a form ofcalcium sulfate - is normally ground up withthe clinker. The gypsum controls the settingproperties of the cement when water isadded.

• The most common raw rock types used incement production are

- Limestone (supplies the bulk of the lime)

- Clay, marl or shale (supplies the bulk of thesilica, alumina and ferric oxide)

- Other supplementary materials such as sand,pulverized fuel ash (PFA), or ironstone toachieve the desired bulk composition

Cement is made by heating limestone (calciumcarbonate) with small quantities of other materials(such as clay) to 1450 °C in a kiln, in a processknown as CALCINATION, whereby a molecule ofcarbon dioxide is liberated from the calciumcarbonate to form calcium oxide, or quicklime,which is then blended with the other materialsthat have been included in the mix. The resultinghard substance, called 'clinker', is then groundwith a small amount of gypsum into a powder tomake 'Ordinary Portland Cement', the mostcommonly used type of cement (often referred toas OPC).

The approximate composition of cement is

• Calcium oxide (CaO) = 50 - 60%

• Silica (SiO2) = 20 - 25%

• Alumina (Al2O3) =5 - 10%

• Magnesium oxide (MgO) = 2 - 3%

• Ferric oxide (Fe2O3) = 1 - 2%

• Sulphur trioxide (SO2) = 1 - 2%

• Tricalcium Silicate - sets quickly and develops considerable strength in a few days.

• Dicalcium Silicate - sets slowly and becomes strong after a month or so.

• Tricalcium Aluminate - sets instantaneously in the presence of water. Cement's internal strength is mainly because of this.

• Tetracalcium Aluminoferrite sets rapidly but not so fast as tricalcium aluminate

Four Principle minerals in OPC are designated as under

C3 A – 4 to 14%

C3 AF – 10 to 18%

C3 S – 45 to 65%

C2 S – 15 to 35%

• On hydration silicates and aluminates ofcalcium get converted to their respectivehydrated colloidal gels.

• At the same time, hydrolysis precipitatescalcium hydroxide and aluminium hydroxide.

• This calcium hydroxide binds calcium silicateparticles together. On the other hand,aluminium hydroxide fills the interstices (anintervening space) rendering the massimpervious (not affording passage to a fluid).

Role of Gypsum - Gypsum reacts with tricalciumaluminate.

The fast-setting tricalcium aluminate is removed toslow down the setting process. A quick setting willgive rise to crystalline hydrated calcium aluminate.A slower setting yields the colloidal gel that impartsgreater strength to the set mass. Thus gypsumhelps in regulating the setting time of cement

• Portland cement is a basic ingredient ofconcrete, mortar and most non-specialitygrout. The most common use for Portlandcement is in the production of concrete.Concrete is a composite material consisting ofaggregate (gravel and sand), cement, andwater. As a construction material, concretecan be cast in almost any shape desired, andonce hardened, can become a structural (loadbearing) element. Portland cement may begrey or white.

Portland Cement Blends

Portland cement blends are often available as inter-ground mixtures from cement manufacturers, butsimilar formulations are often also mixed from theground components at the concrete mixing plant.

• Portland blastfurnace cement contains up to 70 %ground granulated blast furnace slag, with the restPortland clinker and a little gypsum. Allcompositions produce high ultimate strength, but asslag content is increased, early strength is reduced,while sulfate resistance increases and heatevolution diminishes. Used as an economicalternative to Portland sulfate-resisting and low-heat cements

• Portland flyash cement contains up to 30 % fly ash. Thefly ash is pozzolanic, so that ultimate strength ismaintained. Because fly ash addition allows a lowerconcrete water content, early strength can also bemaintained. Where good quality cheap fly ash isavailable, this can be an economic alternative toordinary Portland cement.

• Portland pozzolana cement includes fly ash cement,since fly ash is a pozzolana, but also includes cementsmade from other natural or artificial pozzolana. Incountries where volcanic ashes are available (e.g. Italy,Chile, Mexico, the Philippines) these cements are oftenthe most common form in use.

• Portland silica fume cement. Addition of silicafume can yield exceptionally high strengths,and cements containing 5–20 % silica fumeare occasionally produced. However, silicafume is more usually added to Portlandcement at the concrete mixer.

• White blended cements may be made using whiteclinker and white supplementary materials such ashigh-purity metakaolin.

• Colored cements are used for decorative purposes.In some standards, the addition of pigments toproduce "colored Portland cement" is allowed. Inother standards (e.g. ASTM), pigments are notallowed constituents of Portland cement, andcolored cements are sold as "blended hydrauliccements".

• Very finely ground cements are made frommixtures of cement with sand or with slag orother pozzolan type minerals that areextremely finely ground together. Suchcements can have the same physicalcharacteristics as normal cement but with50% less cement particularly due to theirincreased surface area for the chemicalreaction. Even with intensive grinding they canuse up to 50% less energy to fabricate thanordinary Portland cements

1. BLASTING : The raw materials that are used to manufacture cement (mainly limestone and clay) are blasted from the quarry.

1. BLASTING 2. TRANSPORT

3. CRUSHING AND TRANSPORTATION : The raw materials, after crushing, are transported to the plant by conveyor. The plant stores the materials before they are homogenized.

quarry

3. CRUHING & TRANSPORTATION

2. TRANSPORT : The raw materials are loaded into a dumper.

crushing

conveyor

dumper

storage at the plant

loader

Quarry face

THE CEMENT MANUFACTURING PROCESS

1. RAW GRINDING : The raw materials are very finely ground in order to produce the raw mix.

1. RAW GRINDING

Raw grinding and burning

2. BURNING

2. BURNING : The raw mix is preheated before it goes into the kiln, which is heated by a flame

that can be as hot as 2000 °C. The raw mix burns at 1500 °C producing clinker which, when it

leaves the kiln, is rapidly cooled with air fans. So, the raw mix is burnt to produce clinker : the

basic material needed to make cement.

conveyor Raw mix

kiln

cooling

preheating

clinke

r

storage

at the

plantRaw mill

THE CEMENT MANUFACTURING PROCESS

1.GRINDING : The clinker and the gypsum are very finely ground giving a “pure cement”.

Other secondary additives and cementitious materials can also be added to make a blended

cement.

1. GRINDING

Grinding, storage, packing, dispatch

2. STORAGE, PACKING, DISPATCH

2. STORAGE, PACKING, DISPATCH :The cement is stored in silos before being dispatched

either in bulk or in bags to its final destination.

clinker

storage

Gypsum and the secondary

additives are added to the clinker.

silos

dispatch

bags

Finish

grinding

Following are the three distinct operations involved in manufacture of Portland cement

1. Mixing of raw material

2. Burning

3. Grinding

Packing of Cement

Test on Cement

1. Chemical Composition

2. Fineness of Cement

3. Compressive Strength of Cement

4. Tensile Strength of Cement

5. Consistency of Cement

6. Setting Time of Cement

7. Soundness of Cement

Varieties of CEMENT

1. Acid Resistance Cement2. Blast Furnace Cement3. Coloured Cement4. High Alumina Cement5. Pozzolana Cement6. Quick Setting Cement7. Rapid Hardening Cement8. Sulphate Resistance Cement9. White Cement