CEMENT

Cement was invented by Joseph Aspdin of UK 1824. It is a material which has a cohesive and adhesive properties in the presence of water.

Standard density 1440 kg/m3.

Volume of 1 bag of cement approximately 35 liters.

It is a product obtained by pulverizing clinker formed by calcinating raw materials primarily consisting of lime (CaO), Silicate (Sio2), Alumina (Al2O3), Iron Oxide (Fe2O3).

It can be classified as hydraulic and non-hydraulic cement. Hydraulic cement which can set and harden extremely fast in the presence of water Ex- Portland Cement. Non-Hydraulic cement are derived from calcination of gypsum or limestone because their product of hydration is not resistant to water Ex- Plaster of Paris.

Manufacture of cement

Portland cement is manufactured by crushing, milling, and proportioning the following materials:

• Lime or calcium oxide, CaO: from limestone, chalk, shells, shale or calcareous rock
• Silica, SiO₂: from sand, old bottles, clay or argillaceous rock
• Alumina, Al₂O₃: from bauxite, recycled aluminum, clay
• Iron, Fe₂O₃: from clay, iron ore, scrap iron and fly ash
• Gypsum, CaSO₄.2H₂0: found together with limestone

The materials, without the gypsum, are proportioned to produce a mixture with the desired chemical composition and then ground and blended by one of two processes – dry process or wet process. The materials are then fed through a kiln at 2,600º F to produce greyish-black pellets known as clinker. The alumina and iron act as fluxing agents which lower the melting point of silica from 3,000 to 2600º F. After this stage, the clinker is cooled, pulverized and gypsum added to regulate setting time. It is then ground extremely fine to produce cement.

Chemical shorthand

Because of the complex chemical nature of cement, a shorthand form is used to denote the chemical compounds. The shorthand for the basic compounds is:

Compound	Formula	Shorthand form	%
Calcium oxide (lime)	Ca0	C	60-65
Silicon dioxide (silica)	SiO2	S	17-25
Aluminum oxide (alumina)	Al2O3	A	3-8
Iron oxide	Fe2O3	F	0.5-6
Sulphate	SO3	S	1-2
Magnesia	MgO	M	0.5-4

Chemical composition of clinker

The cement clinker formed has the following typical composition:

Compound	Formula	Shorthand form	% by weight1
Tricalcium aluminate	Ca3Al2O6	C3A	5-11
Tetracalcium aluminoferrite	Ca4Al2Fe2O10	C4AF	8-14
Belite or dicalcium silicate	Ca2SiO5	C2S	25-40
Alite or tricalcium silicate	Ca3SiO4	C3S	25-50
Sodium oxide	Na2O	N	–
Gypsum	CaSO4.2H2O	CSH2	5

Properties of cement compounds

These compounds contribute to the properties of cement in different ways-

 Tricalcium aluminate, C3A: –
It liberates a lot of heat during the early stages of hydration but has little strength contribution. Gypsum slows down the hydration rate of C3A. Cement low in C3A is sulfate resistant.
 

 Tricalcium silicate, C3S: –
This compound hydrates and hardens rapidly. It is largely responsible for Portland cement’s initial set and early strength gain.
 

 Dicalcium silicate, C2S: –
C₂S hydrates and hardens slowly. It is largely responsible for strength gain after one week.
 

 Ferrite, C4AF: –
This is a fluxing agent which reduces the melting temperature of the raw materials in the kiln (from 3,000^o F to 2,600^o F). It hydrates rapidly but does not contribute much to the strength of the cement paste.

By mixing these compounds appropriately, manufacturers can produce different types of cement to suit several construction environments.

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