Cement

Introduction: Cement is a material which possesses adhesive and cohesive properties and capable of
bonding materials like bricks, stones, building block etc. When mixed with water.

(i) Natural Cement: It is made by calcining a naturally occurring argillaceous lime stone at a high
temperature and then Pulverizing the calcinated product. Calcium silicate and aluminates are
formed by the combination of silica and alumina with calcium oxide during calcination.
Properties:
(i) It possess hydraulic properties
(ii) It is a quite setting cement
(iii) It has low strength
Applications:
(a) Combination of sand with natural cement (Known as Mortars) is used for laying bricks and
setting stones.
(b) It is also used in large masses of concretes e.g. dams and foundation

1
Classification of cement continued........

(ii) Pozzolana cement: It is the oldest cement which was invented by Romans and was used for
construction of dams etc. It is prepared by mixing and grinding pozzolana and slaked lime.
Natural pozzolana is deposit of volcanic ash produced by rapid cooling lava. Lava is
a molten mixture of silicates of calcium, iron and aluminium.
Properties: They posses hydraulic properties.
Application: Their mixture with Portland cement is used for different applications
(iii) Slag cement: It is prepared from blast furnace slag and hydrated lime. At first a mixture of
calcium, aluminium silicates (blast furnace slag) is granulated by pouring it into the streams of
cold water. Now dried the mixture and mixed it with hydrated lime. Then pulvurized the
mixture. Sometimes acceleratory like clay, salt or caustic soda are added to accelerate the
hardening process.
Properties:
(i) They are low setting
(ii) They are poor in abrasion resistance.
(iii) They have low strength.
Applications:
(a) It is used for making concrete in bulk construction.
(iv) Portland Cement: It is made by calcining (at 1500 °C) an intimate mixture of clay and
lime containing raw material in correct proportion. After calcinations, retarder like gypsum
is added.

2
Classification of cement continued........
Portland cement:
It is the most common type of cement in general use around the world as a basic ingredient of
concrete and mortar. It was developed from other types of hydraulic lime in England in the early 19th
century by Joseph Aspidin, and usually originates from limestone. It is a fine powder, produced by
heating limestone and clay minerals in a kiln, to form clinker, grinding the clinker, and adding 2 to 3
percent of gypsum. Several types of Portland cement are available. The most common, called ordinary
Portland cement (OPC), is grey, but white Portland cement is also available. Its name is derived from
its resemblance to Portland stone.

3
 Raw materials of Portland cement
The raw materials used in the manufacture of Portland cement are:

(a) Calcareous materials (which supply lime) e.g., lime stone, cement rock (a soft argillaceous
lime stone), chalk, marine shells, and waste calcium carbonate from industrial processes.
A lime stone high in magnesia cannot be used unless its magnesia content is reduced by some
means, as by flotation, or dilution with low-magnesia rock so that the product will not contain
more than 5% MgO. Similarly, chalk containing flint has to be freed from that impurity, and
seams of gypsum or other materials such as pyrite may require selective handling before use.

(b) Argillaceous materials (which supply silica, alumina and iron oxide) e.g., clay, shale,
blast furnace slag, ashes and cement rock. Clay or shale are most commonly used. Cement rock
was sometimes used as such without any further addition, since it contains both limestone and
clay minerals. The modern demands, however, have made necessary more precise control of
composition and hence, such a simple procedure is rarely employed today.

4
 Methods of Manufacturing Cement

 Dry Process: It is slow and costly process. Cost of production of cement is less, as the fuel
consumption is low.The quality of cement produced is inferior. This process is adopted when the
raw materials are quite hard and devoid of moisture . A shorter kiln is sufficient.

 Wet process. This process was in predominant use in India and Europe until recently. In this
process, the raw materials are finely ground and blended in the desired proportion and the mix is
brought to the condition of a free flowing slurry containing 30 – 40% water. It is comparatively faster
and cheaper process. Cost of production is higher because of the higher fuel consumption.The quality of
cement produced is somewhat superior. This process is preferred when the raw materials
are soft. Longer kiln needed to drive off the excess water.

5
Percentage composition of main constituents and their function on
properties of cement
Chemical Percentage Functions
composition
CaO 60 to 66% Too little lime reduces strength of cement if its content is high. It gives
high early strength but generally increases setting time. Too high
percentage of lime produces unsound quality of cement making it liable
of disintegration.

SiO2 17 to 25% Its higher % increases strength and usually prolongs setting time . High
silica cement do not attain their full strength for a considerable period

Fe2O3 0.5 to 6% It imparts characteristic grey colour strength and hardness to Portland
cement

Al2O3 3 to 8% Its higher % increases the strength and reduces setting time

Na2O and 0.5 to 1.5% If present in excess causes cement efflorescent

K2O

SO3 1 to 3% It imparts soundness to cement when present in small amount

6
 Characteristics of the constitutional compounds of cement
(1) Tricalcium aluminate (C3A): The strength developed by different constitutional compounds
in cement with time is represented
in Fig.
 Tricalcium aluminate
undergoes hydration at a very fast
rate.
 It is responsible for the initial
set or flash set.
 Its early strength is
good but the ultimate strength is
quite low as shown in Figure.
 Its heat of hydration is about 210 cals /
gram (879 KJ/Kg), which is the
Strength developed by different constitutional
highest amongst all the constitutional compounds in cement
compounds of cement.
 Its rate of hydration is 82.5%, as followed by
X-ray diffraction studies.

7
 Characteristics of the constitutional compounds of cement..............

(2) Tricalcium silicate (C3S):

 It develops very high strength quite early and the ultimate strength is also the highest.
 Its rate of hydration is medium (73.5%).
 Its heat of hydration is about 120 cals /gram (502 KJ/Kg).

(3) Tetracalcium aluminoferrite (C4AF):

 It does not contribute much to the strength of cement because both its early strength and the
ultimate strength are poor and the lowest among the constitutional compounds.
 It acts as a filler.
 Its rate of hydration is slow (57%) and hence it is slow setting.
 Its heat of hydration is about 100 cals /gram (418.4 KJ/Kg).

(4) Dicalcium silicate (C2S):

 This hydrates very slowly. Its rate of hydration is 37.5%.
 Its heat of hydration is the lowest among all the constitutional compounds of cement and is about 60
cals/gram (251 KJ/Kg).
 Its early strength is quite low but develops ultimate strength almost of the same order as C3S.

8
 Additives of cement
Any material entering into concrete other than cement, water and aggregate is known as an
admixture. Any material inter-ground with the cement clinker (other than gypsum normally used
in the manufacture of cement) is called an addition. Admixtures and/or additions are classified as
under
1. Accelerators:
 These are added to increase the early strength development.
 Chemical accelerators commonly used include common salt, CaCl2, some organic compounds
such as triethanol amine, some soluble carbonates, silicates and fluorosilicates.
 CaCl2 is the most widely used accelerator.

2. Air-entrainment agents:
 Air-entraining agents or pore-forming agents are compounds that entrain microscopic air bubbles
in cement compositions,
which then harden into concrete having microscopic air voids.
 Air entraining admixture is usually introduced when it is desired to
increase concrete workability but it results in the reduction of
compressive strength.
 Vinsol resin and Darex are the commonly used commercial
air-entrainment agents which are introduced as “additions” during
grinding of the clinker.

9
 Additives of cement continued......

3. Retarders:
These are used to offset the accelerating effect of temperature from hot weather concreting or hot water
flows in grouting, to prevent the premature stiffening of some cements, or to actually delay the
stiffening under difficult placing conditions. Admixtures of very small quantities.

4. Water repelling agents:

 These are used in 0.1 to 0.2% of the weight of the cement and are usually present in waterproofed
Portland cements and many masonry cements.
 The commonly used water-repelling agents include soaps or other fatty acid compounds such as
calcium–,ammonium–, aluminium, or sodium stearates or oleates and petroleum oils or waxes.
5. Workability agents:
 These are usually employed to offset deficiencies in grading that tend to produce harshness or
segregation and jeopardize successful placement under inaccessible difficult conditions.
 Examples are bentonite clay and diatomaceous earth which are used upto 3 to 5% by weight of cement.
Other examples are fly ash, clay, finely divided silica, fine sand, hydrated lime, talc
and pulverized stone, some of which are added even upto 20% by weight. Some of the commonly
used air-entraining agents also increase workability.
6. Gas forming agents:
 Aluminium powder is the widely used gas forming agent.
 It reacts with the hydrating hydroxide in concrete to permeate the mass with minute hydrogen bubbles.
Amounts added are of the order 0.005 to 0.02% by weight of the cement. However, larger quantities are
used to produce the light weight, low strength, sound or heat insulation filler concrete known as
10
Acrocrete.
Additives of cement continued......
7. Pozzolanic materials: Finely divided siliceous and aluminous substances e.g., fly ash (fine
flue dust, which is obtained as a by product of thermal power-plants), volcanic ash, heat treated
diatomaceous earths, heat treated raw clays and shales which are not cementitious in themselves,
combine with hydrated lime and water to form stable compounds of cementitious value.
These are generally used upto 10 to 35% of the cement (as cement substitutes) in large hydraulic
structures (mass concrete works) to lower the heat of hydration and to instill greater resistance to sea
water, sulfate bearing soils, or natural acid waters. The specific gravity is lower than that of cement
and substitution by equal weights increases the relative bulk of fine material thereby improving
Work ability and reducing bleeding and segregation. The rate of gaining strength is slower but under
Favourable curing conditions, the later strengths are higher with most of the pozzolanic admixtures.
8. Natural cementing materials:
These are the natural cementing materials such as hydraulic lime, water quenched blast furnace slag
and lime. These are used upto 10 to 25% by weight of Portland cement. These may increase
workability, decrease the bleeding and segregation, decrease the heat of hydration and usually
decrease the strength when used in larger quantities. Some of them may contribute to the strength of
the concrete through their own chemical activity. Natural cementing materials generally require a
longer curing period for the development of their potential strength.
9. Miscellaneous admixtures. These include colouring pigments, integral floor hardeners, pore
fillers and additives for resistance to wear and decrease of dusting

11
composition of the constitutional compounds of
cement

12
 Properties of cement
1) Setting and hardening of cement:

13
Setting and hardening of cement.........

14
 Properties of cement
2) Heat of hydration :

3) Fineness: It affects the hydration of cement.Hydration process occurs more rapidly in a fine
ground cement as water is in contact with more surface area. The cement should not be ground
too finely because there is a possibility of prehydration due to accidental contact with moisture
vapour during manufacturing and storage. In general ,finer the cement is ground , higher the heat
of hydration and resulted accelerated strength gain.

4) Soundness: It is the ability of a cement to maintain a stable volume after setting. A sound
cement resist cracking, disruption and eventual disintegration of material mass.An unsound
cement has excess amount of free lime which is enclosed in cement particles . After the cement
has set, when the moisture reaches the lime , lime expands with considerable force disrupting the
set cement.

15