Cement Composition & its Functions, Bogue's Compounds I Building Material I Civil Engineering

                                            CEMENT

Introduction

• Cement is an extremely fine grounded material having adhesive and cohesive properties which provide a binding medium for the discrete ingredients.
• The processes used for the manufacture of cement can be classified as dry and wet.
• In the wet process, the limestone brought from the quarries is first crushed to smaller fragments. Then, it is taken to a ball or tube mill where it is mixed with clay or shale as the case may be and ground to a fine consistency of slurry with the addition of water. The slurry is stored in tanks under constant agitation and fed into huge firebrick lined rotary kilns.
• In the dry process, the raw materials are ground, mixed and fed to the rotary kiln in the dry state.

Chemical Composition

• The identification of the major complex compounds is largely based on R.H. Bogue's work and hence these are called Bogue's compounds.

 Functions of Cement Ingredients

The main features of these cement ingredients along with their functions are given below:

1. Lime : 

• Lime is calcium oxide or calcium hydroxide.
• The presence of lime in a sufficient quantity is required to form silicates and aluminates of calcium.
• Deficiency in lime reduces the strength property to the cement.
• Deficiency in lime causes the cement to set quickly.
• Excess lime makes cement unsound. The excessive presence of lime causes the cement to expand and disintegrate.

2. Silica : 

• Silicon dioxide, chemical formula SiO₂.

• The sufficient quantity of silica should be present in cement as dicalcium and tricalcium silicate.

• Silica imparts strength to cement.
• Silica usually presents to the extent of about 20-25 per cent of cement.

3. Alumina : 

• Alumina is Aluminium oxide. The chemical formula is Al₂O₃.
• Alumina imparts quick setting property to the cement.
• Clinkering temperature is lowered by the presence of the requisite quantity of alumina.
• Excess alumina weakens the cement.

4. Magnesia :

•  Magnesium Oxide. The chemical formula is MgO.
• Magnesia should not be present more than 2% in cement.
• Excess magnesia will reduce the strength of the cement.

5. Iron oxide : 

• The Chemical formula is Fe₂O₃.
• Iron oxide imparts greyish colour to cement. 
• It acts as a flux.
• Tricalcium alumino ferrite imparts hardness and strength to cement.

6. Calcium Sulphate :

•  Chemical formula is CaSO₄
• This is present in cement in the form of gypsum (CaSO₄.2H₂O)
• It slows down or retards the setting action of cement.

7. Sulfur Trioxide :

• The Chemical formula is SO₃
• It should not be present for more than 2%.
• Excess Sulfur Trioxide causes the cement to unsound.

8. Alkalies :

• It should not be present more than 1%.
• Excess Alkaline matter causes efflorescence.

Bogue's Compounds

• When water is added to cement, it reacts with the ingredients of the cement chemically (known as hydration of cement) & results in the formation of complex chemical compounds terms as BOGUES compounds, which are not formed simultaneously.

NAME	CHEMICAL FORMULA	PERCENTAGE
Tri-Calcium Silicate (C3S)	3CaO.SiO2	30-50%
Di-Calcium Silicate (C2S)	2CaO.SiO2	20-45%
Tri-Calcium Aluminate (C3A)	3CaO.Al2O3	8-12%
Tetra Calcium Alumino Ferrite (C4AF)	4CaO.Al2O3.Fe2O3	6-10%

 

1. Tri-Calcium Silicate (3CaO.SiO₂ or C₃S)

• Responsible for the early strength of cement.
• A higher percentage of C₃S results in rapid hardening with an early gain in strength with a higher heat of hydration.

2. Di-Calcium Silicate (2CaO.SiO₂ or C₂S)

• responsible for progressive later stage strength.
• A higher percentage of C₂S results in slow hardening, less heat of hydration & greater resistance to chemical attack.

3. Tri-Calcium Aluminate (3CaO.Al₂O₃ or C₃A)

• Max. evolution of heat of hydration & responsible for the flash setting of cement.
• Provides weak resistance against sulphate attack & contribute very less to overall strength.

4. Tetra Calcium Alumino Ferrite (4CaO.Al₂O₃.Fe₂O₃ or C₄AF)

• Formed within 24 hrs of the addition of water
• The high heat of hydration in initial periods.