Introduction to Concrete Technology Part-I (Cement)

 Introduction to Concrete Technology Part-I (Cement)

By Md. Yousuf Sk. Amir

Assistant Professor

Definition:-

Cement is defined in many ways as follows,

    ·     Cement, is any material that hardens and becomes strongly adhesive after application of the product.

  ·  Manufactured by burning and crushing powder and well proportion mixture of calcareous (Lime, calcium carbonate) and argillaceous (shate laterite) material is called Cement.

    ·    The Manufactured substance consists of gypsum plaster, or Portland cement.

    ·     Portland cement becomes  hard and adheres after being mixed with water.

History of Cement:

·       The term “Portland cement” was first used in 1824 by Joseph Aspdin, a British cement- maker, because of the resemblance between concrete made from his cement and Portland stone, which was commonly used in buildings in Britain.

·        At that time cements were usually made in upright kilns of rotating kiln where the raw materials are  spread between layers of coke, which was then burnt.

The first rotary kilns were introduced about 1880. Portland cement is now almost universally accepted as structural concrete.

·         In India Portland cement was first manufactured in 1904 by south India industrial Limited at Madras But the venture failed and between 1912 - 1913, The Indian Cement Company formed at porbander and by 1918, three factories were established.

·         After five years plan after the attainment of freedom, the manufacturing of cement increased by leaps and bounds and during some period, the sale and distribution of cement was restricted by the government. But now there are many manufacturers of cement and free and open market for the same. As it is, today India holds the 5^th place in the world in Manufacturing of cement.

Manufacturing Process:

The common two types of manufacturing of cement are

1.      Dry process

2.      Wet process

Basic ingredients used in the manufacturing of Cement are;

1.  Limestone (Calcium)

2.  Clay, shale (Silica/Alumina)

3.  Quarrying (Local resources necessary: no market)

·     Limestone (CaCO₃) and Clay are the two important raw materials used for manufacturing Portland cement.

Dry process

·        In this method totally lime stone and clay are grounded separately and then reduced to very fine powder it is called as raw slurry then it is further blended with correct composition and mixed by means of compressed air. Under this the aerated powder behaves almost like liquid and after certain lime uniform mixture is obtained.

·         This raw material is sieved and is ground to fine powder with the help of granulator by adding water which turn blended meal into pellets.

This process of producing cement is relatively slow  as compare to Wet process and it is tedious.

Wet Process

 

·        In this process the limestone is brought from quarries and then it is taken to ball mill after crushing it to small fragments and is mixed with clay or shale and grounded to make fine slurry by addition of water.

·         The cement slurry formed contain 30 to 50% water as water is added in grinding it is called as wet grinding.

·         The slurry formed is pumped into rotary kiln for burning and cement clinkers are formed due to burning            are cooled and stored.

·         The calcinations of cement slurry is done in a rotary kiln.

When slurry enters into the hot gases and water is removed decomposition occurs and formation of aluminates takes place and formation of clinker takes place.

·         The cooled clinkers are collected from lower end and ground in ball mill with the addition of 3-5 % of gypsum to prevent early setting of cement.

Chemistry of Portland cement:- Chemical Composition of OPC Oxide Composition of Portland cement:

·         Portland cement is a composition of four major oxides: lime (CaO), silica (SiO₂), alumina ( Al₂O₃ ), and iron ( Fe₂O₃).

·        Also Portland cement contains in small amount of magnesia ( MgO ), alkalies (Na₂O and K₂O), and sulfuric anhydrite (SO3)

Approximate Composition Limits of Oxides in Portland Cement

Oxide	Common Name	Content, %
CaO	Calcium Oxide (Lime)	60-67
SiO2	Silica	17-25
Al2O3	Alumina	O3-08
Fe2O3	Iron	0.5-6
MgO	Magnesia	0.1-4
Na2O + K2O	Alkalies	0.2-1
SO3	Sulfuric anhydride	1-2.75
Free lime	-	0-1

 

Oxide Composition

 

	Mass Percentage
Oxide	Cement 1	Cement 2	Cement 3
CaO	66	63	66
SiO2	20	22	20
Al2O3	7	7.7	5.5
Fe2O3	3	3.3	4.5
Others	4	4	4

 

Indian standard specification for 33 grade cement, IS 269-1980, specifies the fallowing chemical requirements’

 

    a)      The Ratio of percentage of lime to percentage of silica, alumina and iron oxide; called as lime saturation factor.

b)      Ratio of percentage of alumina to that of iron oxide should Not less than .66

c)       Weight of insoluble residue should Not less than 4 percent

d)      Weight of magnesia should Not less than 6 percent

e)       Total sulphur content, calculated as sulphuric should Not less than 2.5 %

f)       Total loss on ignition should Not more than 5 percent.

Bogue’s Compound Composition

·           As mentioned earlier the oxides percent in the raw materials when subjected to high clinkering temperature combine with each other to form complex compounds that complex compound called as Bogue’s compound.

·         The identification of major components is largely based on R.H Bogue’s work and hence it is called Bogue’s Compounds” The four major compounds usually regarded as major compounds are listed below:

Bogue’s Compound

Name	Chemical formula	Abbreviation
Tri-calcium silicate	3CaO.SiO2	C3S
Di-calcium silicate	2CaO.SiO2	C2S
Tri-calcium aluminate	3CaO.Al2O3	C3A
Tetra calcium-alumino-ferrite	4CaO.Al2O3.Fe2O3	C4AF

·         It is to be noted that for simplicity take abbreviated notation are used.

o   C stands for Cao,

o   S stands for Sio_2,

o   A stands for Al₂O_3,

o   F stands for Fe₂O_3.

Significance of Compound Composition

	Mass Percentage
Compound	Cement 1	Cement 2	Cement 3
C3S	65	33	73
C2S	8	38	2
C3A	14	15	7
C4AF	4	10	14

Properties of Bogue’s Compounds:

A)     Tri-Calcium silicate (C₃S).

 

i.            Hydration process is fast.

ii.             More heat of hydration.

iii.             Hence develops early strength.

B)     Di-calcium silicate (C₂S).

i.            Hydrate slowly.

ii.             Lesser generation of heat of hydration.

iii.           It contributes little in early development of strength but it provides same ultimate strength.

 

C)     Tri-calcium aluminates (C₃A).

 

i.            It generates large amount of heat of hydration.

ii.             Helps in early setting of cement as it reacts faster with water.

iii.             It does not contribute in development of ultimate strength.

iv.            It helps in initial setting of cement thus early strength can be developed.

D)     Tetra calcium alumino feritte (C₄AF).

i.            It is slow in reaction.

ii.             It is poor in early strength.

iii.             Generates less heat.