Chemical Composition of Cement and Functions of Ingredients Present in Cement

Chemical composition of cement and the functions of ingredients for cement are discussed here.

The chemical composition of cement will depend on the raw materials used in the cement manufacturing. These raw materials are lime, silica, alumina, and iron oxide.

What is the Chemical Composition of Cement?

At high temperatures, the raw materials react with each other in a kiln. The molecular composition is restructured in the kiln by calcination. As a result, a series of complex chemical compounds are formed.

After that, a state of chemical equilibrium is reached except for a small residue of uncombined lime. This is because lime does not have sufficient time to react. But, the equilibrium is not maintained while cooling. And this rate of cooling affects the degree of crystallization.

Alumina and iron produces tricalcium aluminate and tetracalcium aluminate. Alumina and iron reduce the temperature from 2000 °C to 1350 °C (3500°F to 2500°F).

The above temperature reduction is required to form tricalcium silicate. Or else, energy would be needed to reduce temperature and consequently, cost would have increased.

Chemical Composition of Cement and Functions of Ingredients

Oxides of the cement ingredients are formed with the advancement in the chemical reaction. The
relative proportions of these oxides along with the rate of cooling and fineness of grinding affect
the properties of cement.

The composition of cement with percentage is tabulated in the below section.

Chemical Composition of Cement with Percentage

Approximate chemical composition of cement with percentage range is tabulated below:

Oxide	Percentage Range
CaO	60 – 67 %
SiO2	17 – 25 %
Al2O3	3 – 8 %
Fe2O3	0.5 – 6 %
MgO	0.1 – 4 %
Na2O K2O P2O5	0.4 – 1.3 %
SO3	1.3 – 3 %

Functions of Chemical Components of Cement

We discussed cement chemical composition in the above section. Now, let us see the functions
of the ingredients of cement.

1. Calcium Oxide (CaO)

• It controls the strength gain of cement
• It controls the soundness of cement
• Deficiency of CaO in cement reduces strength as well as setting time of cement.

2. Silica (SiO2)

• It provides strength to cement.
• Excess of silica reduces the setting of cement.

3. Aluminium Oxide (Al2O3)

• It is responsible for quick setting of cement
• Excess of aluminium oxide reduces the strength of cement

4. Ferrous Oxide (Fe₂O₃)

• It imparts the characteristic gray colour to the cement
• It also helps in the fusion of different materials

5. Magnesium Oxide (MgO)

• It provides colour to the cement
• It also provides hardness to the cement
• Excess of magnesium oxide causes cracks in mortar and produces unsound concrete.

6. Alkalis (Na₂O, K₂O, P₂O₅)

• Alkalis are present as residues in cement.
• Excess of alkalis causes efflorescence in concrete
• Excess of alkalis may also result into cracking of concrete

7. Sulphur Trioxide (SO₃)

• Sulphur trioxide provides soundness to the cement.

Major Compounds of Cement

The major components of cement formed after the burning of the materials in kiln forms clinker of cement. These compounds can set and harden in the presence of water.

These were mainly identified by the scientist Bogue. Hence, these major compounds are also called Bogue’s compounds.

Major Compounds of Cement, Their Common Name, and Approximate Weight Range

Name of Compound	Formula	Abbreviation	Common Name	Usual Range by Weight
Tricalcium silicate	3 CaO.SiO2	C3S	alite	45 – 60 %
Dicalcium silicate	2 CaO.SiO­2	C2S	belite	15 – 30 %
Tricalcium aluminate	3 Al2O3.SiO2	C3A	–	6 – 12 %
Tetracalcium aluminoferrite	4 Al2O3.Fe2O3	C4AF	ferrite	6 – 8 %

The proportions of the above four elements determine the major properties of cement.

Oxide Composition of Typical OPC & Corresponding Composition of Bogue’s Compounds

Oxide	Composition (%)	Bogue’s Compound	Composition (%)
CaO	63 %	C3S	54 %
SiO2	20 %	C2S	16.6 %
Al2O3	6 %	C3A	10.8 %
Fe2O3	3 %	C4AF	9.1 %
MgO	1.5 %
Na2O K2O	1 %
SO3	2 %
Others	1 %
Loss on Ignition	2 % (Explained below)
Insoluble Residue	0.5 % (Explained below)

If the proportions of these oxides are varied even slightly, it makes great difference to the calculated quantity of the major and minor cement compounds formed.

Therefore, manufacture of cement of a stipulated quantity of a particular compound requires close control on the oxide composition in the raw materials.

Chemical composition test of cement is conducted to determine the proportions of these
constituents in cement.

Insoluble Residue

As per BS EN 197-1, the insoluble residue left should not be more than 5 % of the total mass of cement with filler.

Loss on Ignition

It shows extent of carbonation and hydration of free lime and free magnesia on exposure of cement to atmosphere.

The specified limits of loss on ignition as per ASTM C 150-05 & BS EN 197-1 are as follows-

Type	Loss on Ignition (in %)
ASTM Type I Cement	3 %
ASTM Type II Cement	3 %
ASTM Type III Cement	3 %
ASTM Type IV Cement	2.5 %
ASTM Type V Cement	3 %
Cement with Filler material	5 %

Bogue’s Compounds

Four main compounds of cement are called Bogue’s compounds.

1. Tricalcium Silicate

• It can be considered as the best cementing material.
• It makes the clinkers of cement easy to grind
• It increases the resistance to freezing and thawing
• As it hydrates in the early phase, it provides early strength to the cement
• Its hydration and character of gel developed causes hardness in cement along with 7-day strength of cement
• Increasing its content beyond the specified limits may increase the heat of hydration to a great extent and also increase the solubility of cement in water

2. Dicalcium Silicate

• Its hydration takes time, so hardening occurs in later phase
• The strength is developed after a year or so
• It provides resistance to chemical attack
• Increasing its content makes grinding of clinkers difficult, decreases early strength and heat of hydration
• After a year, the contribution of C₂S to the strength and hardness is almost equal to that of the C₃S

C₃S and C₂S together comprises 70-80 % of cement compounds. Most of the strength of cement is derived from these two compounds only. Upon hydration, they give C₃S₂H₃ & Ca(OH)₂. But C₃S gives early strength while C₂S is responsible for ultimate strength.

3. Tricalcium Aluminate

• It can react with water quickly
• It causes flash setting of finely ground clinker by immediate stiffening of cement paste. To prevent this, 2-3 % gypsum is added at the time of grinding of cement
• It is accountable for initial setting, high heat of hydration, and volumetric changes; thus it is the cause for cracking
• Increasing its content reduces setting time, resistance to sulphate attack, ultimate strength, and heat of hydration

4. Tetracalcium Aluminoferite

• It is also responsible for flash set of cement along with C₃A, but it generates less heat.
• Its cementing value is the least of all
• Increasing its content reduces the strength

Heat of Hydration of Components of Cement

Component	Heat of Hydration (in J/g)
C3S	500 J/g
C2S	260 J/g
C3A	865 J/g
C4AF	420 J/g

Minor Compounds of Cement

Apart from the Bogue’s compounds forming major part of the products, some minor compounds are also produced in the kiln.

There are quite a number of minor compounds in the cement like- CaSO₄.2H₂O, MgO, TiO₂, Mn₂O₃, K₂O, Na₂O, and alkalis.

The major compounds play a significant role in the hydration of cement. To know what hydration of cement is and how these compounds help in gaining strength of concrete, Read Hydration of Cement.

This does not mean that minor compounds can be neglected. In fact, the term minor compound is with respect to its quantity and not importance. This can be understood by the following
instances:

Importance of Alkalis in Cement

Presence of these alkalis increases the pH up to 13.5, which is good for the protection of reinforcement steel from corrosion.

Alkalis react with aggregates in concrete, which causes disintegration of concrete. This phenomenon is known as alkali aggregate reaction and is detrimental for the strength of concrete.

Importance of Gypsum in Cement

The amount of gypsum present in clinker is of great importance.

Gypsum content depends on the C₃A content and alkali content. If C₃A is increased, requirement of gypsum also increases. (Increased C₃A is required if fineness of cement also increases.)

But, addition of gypsum increases the expansion characteristics of cement and causes disruption of cement paste which is set.

Thus, optimum gypsum content needs to be determined. It is determined based on generation of heat of hydration. It is ensured that a desirable rate of early reaction happens so that little C₃A is available for reaction even after all the gypsum is combined.

Chemical Structure of Cement

The two main compounds found in industrial Portland cement are:

• Tricalcium silicate (C₃S)
• ß-Dicalcium silicate (ß-C₂S)

The chemical structure of alite (impure form of C3S) has three major crystalline structures as
follows:

1. Triclinic
2. Monoclinic
3. Trigonal

Structural Arrangement

In C₃S & C₂S ß-C₂S:

The coordination of oxygen ions surrounding the calcium ion is found to be concentrated on one
side of each calcium ion. This irregular arrangement of oxygen ions gives rise to structural voids.
The structural voids are responsible for high reactivity.

The structural voids formed in belite (impure form of C2S) are much smaller in size as compared
to those formed in alite (impure form of C3S). Therefore, the reactivity of C2S is much less than
C3S.

In the above vertical section of the bottom layer of the pseudo-structure of C3S,

• Plain circles – oxygen atoms
• 1, 2, 3 – sections of SiO4 tetrahedron

In ß-C₂S:

ß-C₂S has a regular arrangement of oxygen ions around it, which makes it non-reactive. In the
below section diagram, silicon atoms are not shown, which are present at the centre of silica
tetrahedra.

The crystalline structures of cement components are tabulated below.

SR NO	Chemical Composition	Crystalline Structure
1	C3S	Monoclinic
2	ß-C2S	Monoclinic
3	C3A	Cubic, Orthorhombic
4	C4AF	Orthorhombic

Chemical Requirement of OPC (Grade 33 Cement) Conferring to IS: 269 – 1989

Following are some of the chemical requirements of Grade 33 OPC cement as per IS: 269 -1989:

1. Lime saturation factor is the ratio of percentage of lime to volume of silica, alumina and iron oxide. It should not be more than 1.02.

and

2. Percentage of alumina to percentage of iron oxide

3.

4.

5.

6.

The above listed oxides present in raw material combine with each other at high clinkering temperature and form complex compounds.

Important Points to Note for Variability in Cement Composition (Alteration of Cement Properties)

1. Ratio of silica to alumina and iron oxide controls the rate of setting of cement paste.

2. Heat of hydration can be decreased by

– increasing silica content to 21 %

– limiting alumina content to 6 %

– limiting ferrous oxide to 6 %

3. Resistance to sulphate attack can be increased by

– further increasing silica content 24 %

– limiting alumina content to 4 %

– limiting iron content to 4 %

4. Addition of small percentage of iron oxide to highly siliceous raw materials facilitates their easy burning.

But, excess of this iron oxide produces hard clinkers. These hard clinkers then pose difficulty in grinding.

Iron oxide neutralizes some of the undesirable properties arising because of the reaction of lime with alumina. Also, if lime alone reacts with iron oxide, it causes instability.

Precautions While Altering Raw Materials of Cement:

1. Limit to Lime Content

The quantity of lime should not exceed beyond a certain limit. Else, it will be difficult for lime to combine with other compounds. And lime will remain as free lime in the clinker of cement.

Free lime in clinker will delay the hydration of cement and thus cause unsoundness of cement.

2. Limit to Silica Content

If silica is increased in place of alumina and ferrous oxide, cement will not fuse properly. Formation of cement clinkers will also be difficult.

Increase in total alumina and ferrous oxide is favourable to increase the early strength of cement.

Microstructure of Cement Composition

With the advancement in science and technology, we can now recognize the microstructure of cement concrete before hydration as well as after hydration.

The crystalline/amorphous structure of hydrated/unhydrated cement can be revealed by-

• X-ray power diffraction methods
• X-ray fluorescence method
• Under powerful electron microscope with magnifying power of 50,000 or more

Four different types of crystals were observed in by Le Chatelier and Tornebohm in thin sections of cement clinkers. These four types of crystals were named as alite, belite, celite, and felite by Tornebohm.

The description of these four crystals was found similar to the description of major components by Bogue. Hence, Bogue’s compounds are also referred to with these names in literature.

Bogue’s Compound	Tornebohm’s Name for Crystal
C3S	Alite
C2S	Belite
C3A	Celite
C4AF	Felite

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