Composition of Cement Clinker

          The various constituents combine in burning and form cement clinker. The compounds formed in the burning process have the properties of setting and hardening in the presence of water. They are known as Bogue compounds after the name of Bogue who identified them. Le-Chatelier and Tornebohm have referred these compounds as Alite (C3S), Belite (C2S), Celite (C3A) and Felite (C4AF). The following Bogue compounds are formed during clinkering process.

The properties of Portland cement varies markedly with the proportions of the above four compounds, reflecting substantial difference between their individual behaviour.

Tricalcium Silicate:- It is supposed to be the best cementing material and is well burnt cement. It is about 25-50% (normally about 40 per cent) of cement. It renders the clinker easier to grind, increases resistance to freezing and thawing, hydrates rapidly generating high heat and develops an early hardness and strength. However, raising of C3S content beyond the specified limits increases the heat of hydration and solubility of cement in water. The hydrolysis of C3S is mainly responsible for 7 day strength and hardness. The rate of hydrolysis of C3S and the character of gel developed are the main causes of the hardness and early strength of cement paste. The heat of hydration is 500 J/g.

Dicalcium Silicate:- It  is about 25-40% (normally about 32 per cent) of cement. It hydrates and hardens slowly and takes long time to add to the strength (after a year or more). It imparts resistance to chemical attack. Raising of C2S content renders clinker harder to grind, reduces early strength, decreases resistance to freezing and thawing at early ages and decreases heat of hydration. The hydrolysis of C2S proceeds slowly. At early ages, less than a month, C2S has little influence on strength and hardness. While after one year, its contribution to the strength and hardness is proportionately almost equal to C3S. The heat of hydration is 260 J/g.

Tricalcium Aluminate:- It is about 5-11% (normally about 10.5 per cent) of cement. It rapidly reacts withwater and is responsible for flash set of finely grounded clinker. The rapidity of action is regulated by the addition of 2-3% of gypsum at the time of grinding cement. Tricalcium aluminate is responsible for the initial set, high heat of hydration and has greater tendency to volume changes causing cracking. Raising the C3A content reduces the setting time, weakens resistance to sulphate attack and lowers the ultimate strength, heat of hydration and contraction during air hardening. The heat of hydration of 865 J/g.

Tetracalcium Alumino Ferrite:- It is about 8-14% (normally about 9 per cent) of cement. It is responsible for flash set but generates less heat. It has poorest cementing value. Raising the C4AF content reduces the strength slightly. The heat of hydration is 420 J/g.

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 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 grayish-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
Silicon dioxide (silica)	SiO2	S
Aluminum oxide (alumina)	Al2O3	A
Iron oxide	Fe2O3	F
Water	H2O	H
Sulfate	SO3	S

Chemical composition of clinker 

The cement clinker formed has the following typical composition:
 

Compound	Formula	Shorthand form	% by weight1
Tricalcium aluminate	Ca3Al2O6	C3A	10
Tetracalcium aluminoferrite	Ca4Al2Fe2O10	C4AF	8
Belite or dicalcium silicate	Ca2SiO5	C2S	20
Alite or tricalcium silicate	Ca3SiO4	C3S	55
Sodium oxide	Na2O	N	Up to 2
Potassium oxide	K2O	K
Gypsum	CaSO4.2H2O	CSH2	5