 Concrete is an artificial material obtained by mixing together cementing
material, coarse aggregate, fine aggregate and water.
 If cement is used as cementing material in the mix, it is known as plain
cement concrete.
 If steel rods/bars are embedded in the plain cement concrete, then it is
called reinforced cement concrete abbreviated as R.C.C
 Instead of cement, lime is used as cementing material in the mix, the
resulting mass is termed as lime concrete.
INGREDIENTS OF CONCRETE
 The ingredients of concrete are as follows:
 Cement
 Fine aggregates (sand)
 Coarse aggregate (broken stone or gravel)
 Water
 And additives are also used to improve the quality of concrete like
plasticizers, retarders, accelerators, air-entraining agents etc.
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CEMENT
Cement is a very important binding material.
The name cement refers to the material manufactured from limestone
and clay and made available in powder form, which when mixed with
water can set to a hard durable mass even under water.
It is always measured by weight,
It is available in bags
MANUFACTURE OF PORTLAND CEMENT
Cement is manufactured from limestone and clay.
The limestone is crushed and the clay is made into a liquid form by
addition of water.
They are again mixed together and finely ground.
The mixture is called the slurry.
It is then conveyed into tanks and then to a long cylinderical rotary
kiln where it is gradually heated to high temperature of 1300 to 1500
deg.
In this process, it is converted to clinker (fused limps), which is then
ground in ball mills and tube mills to an extremely fine powder to
form cement.
USES OF CONCRETE
 Concrete is used to make pavements, pipes, architectural structures,
foundations, walls, beams, columns, slabs etc.
INGREDIENTS AND FUNCTIONS OF CEMENT
a)
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b)
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c)
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Alumina or Clay
It is responsible for the setting action of the cement.
Larger the amount of alumina present in the cement it will start setting.
Excess quantity of alumina weekens the cement.
Silica
It is also responsible for imparting strength to the cement.
Lime
It is also responsible for the strength characteristics of the cement.
d)
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e)
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Iron Oxide
This ingredient mainly imparts colour to the cement
It also goes into chemical combination and helps increase strength
and hardness to the cement.
Magnesium Oxide (MgO)
It also imparts strength and hardness to the cement, but only present
in small amount.
Calcium Sulphate(CaSO4) OR Gypsum
This ingredient is used to retard or prolong the setting action of the
cement.
CONSTITUENTS OF CEMENT
 During burning and fusion in rotary kiln, all the ingredients of cement
undergo chemical combination and form what are known as Bogue
Compounds in the finely ground cement.
S.No. Bogue compound
Abbrev. as
1.
Tricalcium silicate (3CaOSiO2)
C3S
2.
Di calcium silicate (2CaOSiO2)
C2S
3.
Tricalcium Aluminate (3CaOAl2O3)
C3A
4.
•Tetracalcium Alumino Ferrite (4CaOAl2O3Fe2O3)
C4AF
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ii.
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iii.
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After the addition of water in cement Tri-Calcium Aluminate
(C3A) is the first compound which starts hydrating and is
responsible for early setting cement.
C3A does not contribute any strength to the cement but it is mainly
responsible for setting action of the cement.
After C3A, hydration of Tri-Calcium Silicate (C3S) starts.
It is C3S whose hydration os responsible for the initial strength of
the cement.
The strength acquired during first 7days is mostly due to hydration
of C3S.
Di-calcium silicate (C2S)
The strength of concrete after 7 days, mainly due to hydration of
C2S.
C2S starts contributing strength after 7days of its placement and
continues for about one year.
iv.
Tetra-calcium alumino ferrite is more or less inactive compound and
does not play any significant role in setting and hardening
properties of the cement.
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WHAT IS HYDRATION?
Chemical reaction between water and cement compound is
known as hydration.
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1.
SETTING OF CEMENT
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Is changing plastic state of cement to stiff solid state which is not so
strong enough and has a very small compressive strength.
2.
HARDENING OF THE CEMENT
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Is the rate of gain of strength. With the passage of time as the
hydration proceeds, the compressive strength of the product goes on
increasing.
3.
INITIAL SETTING
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Initial setting is a stage in the process of hardening, (stage when the
cement started gaining strength)
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In short it can be said,
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Initial setting of the cement is controlled by tricalcium aluminate,
C3A.
Early strength is controlled by tricalcium silicate, C3S
Strength of cement developing after 7-8days of its placement is
dependent on the amount of dicalcium silicate.
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 LABORATORY TESTS FOR PORTLAND CEMENT
1.
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Fineness Test
Is done to verify the standard of grinding of cement. The rate of
hydration of cement depends on its fineness
Fineness of cement is determined by sieve test.
100gms of cement is taken and sieved through
The residue left over the sieve should not be more than 10% of the
original weight.
2.
COMPRESSIVE STRENGTH TEST.
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Test is carried out to determine the compressive strength of the
cement.
Cement and sand are taken in 1:3 ratio and paste is prepared using
water.
Paste is filled in 76mm cube mould and vibrated on a vibrating
machine for 2min.
Freshly moulded cubes are kept aside for 24 hous.
After 24 hrs they are shifted to water bath for curing.
The cubes are tested under compression machine (3 cubes tested)
The average of three will give the comp. strength of cube at 1, 7, 14
and 28th day.
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3.
TENSILE STRENGTH TEST
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To determine the tensile strength of the cement.
4.
INITIAL SETTING TIME TEST AND FINAL SETTING TIME
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Vicate’s apparatus is used.
An apparatus consists of a frame to which a movable rod is attached.
An indicator is attached to the movable rod with the help of which
penetration of the needle can be determined.
a.
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Initial Setting Time
Mould is filled with cement paste.
The needle is gently brought in contact with the cement paste.
In the beginning the needle penetrates completely.
After sometime the needle is again brought in contact with the paste
and released.
The process is repeated until the needles penetrates upto 5mm
measured from bottom of the mould.
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Definition of initial setting time – the interval between the
addition of water to cement and the stage when the needle ceases to
penetrate 5mm layer of paste measured from the bottom, is known
as initial setting time of cement.
In about 30 minutes, it is said to have attained its initial set
b)
Final Setting Time
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Cement paste of standard consistency is prepared.
A special needle F is fixed at the bottom of vicate apparatus.
The needle is brought in contact and released.
The time upto which the needle makes an impression on the test
block of paste, but it will not penetrate more than 0.5mm.
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Definition of final setting time–
the time interval between the moment water was added to cement
and the moment, the stage is reached, when the needle will not
penetrate more than 0.5mm is taken as the final set.
In about 10 hours, it becomes rock hard and is said to have reached
its final set.
 TYPES OF CEMENT
 Cement is specified by its grade ie th mortar cube strength in
N/mm2 in 28days.
1.
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ORDINARY PORTLAND CEMENT
The initial setting time of OPC is not less than 30minutes and final
setting time is not more than 10 hours.
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The rate of gaining strength, heat of hydration, resistance to
cracking are moderate.
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Used in pavements, RCC works.
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OPC is available in three grades
I.
Grade 33 – designated as C-33
II.
Grade 43 – Designated as C-43
III. Grade 53 – designated as C-53
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E.g Grade 33 Cement designated as C-33 where C stands for cement
with standard mortar cube strength of 33N/mm2 in 28days.
2.
RAPID HARDENING CEMENT (HIGH EARLY STRENGTH)
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Manufacturing is same as that of OPC but it differs only in chemical
composition, i.e. clinkering at high temperature and high degree of
fineness in grinding is achieved.
It attains high strength in less time.
Advantage of this – it enables the removal of form work at earliest.
Used for highway slabs, which is to be opened to the traffic at the
earliest.
Also used for precast elements.
3.
LOW HEAT CEMENT
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In case of mass concrete structures such as dams, bridges etc. rate of
loss of heat of hydration from the surface is much lower than it is
generated.
This causes rise in temperature in the mass of concrete, may develop
cracks.
Under such circumstances low heat cements are used.
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 This cement is proportioned in such a way that C3A and C3S are formed in
lesser amount, C2S is formed in increased amount, which result in reduced
rate of evolution of heat of hydration.
 Low heat cement has not only slower rate of heat generation, but also
greater resistance to cracking.
4.
AIR ENTRAINING AGENT.
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Just the ordinary cement in which air entraining agents have been added,
such as vinsol, resins etc.
Air entraining agents are added during the process of grinding the
clinkers.
Concrete made with such a cement contains minutes, well distributed air
bubbles.
Such a cement is resistant to severe frost action.
5.
WATER REPELLENT/WATER PROOFING CEMENT
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This cement is used where water tight conditions are predominant.
Small amount of water proofing compounds (Alor Ca) stearate is mixed
with an ordinary cement.
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6.
WHITE AND COLOURED CEMENT
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It is an ordinary cement having pure white colour.
Iron oxide is limited to less than 1%.
Coloured cements are produced by addition of suitable mineral
pigment to ordinary or white cement.
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7.
SULPHATE RESISTANT CEMENT
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It is resistant to sulphate attack .
In this the amount of C3A is reduced.
8.
QUICK SETTING CEMENT
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When concrete has to be laid under water, this cement is
advantageous.
Setting action of this cement starts within 5mins. And it becomes
stone hard in less than one hour.
This property is achieved by adding small % of aluminium sulphate
and also by grinding this cement much finer than ordinary cement.
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