Q.
What is portland cement?
Portland cement is composed of calcium silicates and aluminate
and aluminoferrite It is obtained by blending predetermined
proportions limestone clay and other minerals in small
quantities which is pulverized and heated at high temperature –
around 1500 deg centigrade to produce ‘clinker’. The clinker is
then ground with small quantities of gypsum to produce a fine
powder called Ordinary Portland Cement (OPC). When mixed with
water, sand and stone, it combines slowly with the water to form
a hard mass called concrete. |
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Q.
Is there any shelf life of cement?
Cement is a hygroscopic material meaning that it absorbs
moisture In presence of moisture it undergoes chemical reaction
termed as hydration. Therefore cement remains in good condition
as long as it does not come in contact with moisture. If cement
is more than three months old then it should be tested for its
strength before being taken into use. |
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Q.
What are Supplementary Cementations Materials (SCM)?
Supplementary Cementations Materials (SCM) like silica fumes,
meta-kaolin, fly ash, slag are the materials which improve the
properties of concrete and enhance its durability, by reducing
pore size in concrete through better particle distribution and
through increased packing density of the concrete. |
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Q. How fineness of cement affect strength gain?
Fineness defines the surface area of cement particles present in
per unit weight, which implies that more fineness means more
particles in unit weight. This enhances the reaction rate which
in turn will result in faster gain of strength at earlier
stages. |
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Q.
What is blended cement?
Blended cement is obtained by either intergrinding pozzolanic
material or slag with clinker along with Gypsum or by blending
ground pozzolana or slag with Portland cement. They are also
termed as composite cements. |
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Q.
How does blended cement affect durability?
Pozzolana combines with lime and alkalies in the cement and when
water is mixed forms compounds which contribute to strength,
impermeability and sulphate resistance It also contributes to
workability, reduced bleeding and controls destructive expansion
from alkali-aggregate reaction. Leaching of free lime is also
reduced. |
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Q.
What is setting of cement?
When water is mixed with cement, the paste so formed remains
pliable and plastic for a short time. During this period it is
possible to disturb the paste and remit it without any
deleterious effects. As the reaction between water and cement
continues, the paste loses its plasticity. This early period in
the hardening of cement is referred to as ‘setting’ of cement. |
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Q.
What is initial and final setting time of cement?
Initial set is when the cement paste loses its plasticity and
stiffens considerably. Final set is the point when the paste
hardens and can sustain some minor load. Both are arbitrary
points and these are determined by Vicat needle penetration
resistance. |
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Q.
What are the reasons for slow or fast setting of concrete or
mortar?
Slow or fast setting normally depends on the nature of cement.
It could also be due to extraneous factors not related to the
cement. The ambient conditions play an important role. In hot
weather, the setting is faster, in cold weather, setting is
delayed Some types of salts, chemicals, clay, etc if
inadvertently get mixed with the sand, aggregate and water could
accelerate or delay the setting of concrete. |
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Q.
What are the different grades of Ordinary Portland Cement (OPC)?
The Bureau of Indian Standards (BIS) has classified OPC in three
different grades The classification is mainly based on the
compressive strength of cement-sand mortar cubes of face area 50
cm2 composed of 1 part of cement to 3 parts of standard sand by
weight with a water-cement ratio arrived at by a specified
procedure. The grades are |
(i) |
33 grade |
(ii) |
43 grade |
(iii) |
53 grade |
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| The grade number
indicates the minimum compressive strength of cement sand mortar
in N/mm2 at 28 days, as tested by above mentioned procedure. |
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Q.
What is Portland pozzolana cement?
Portland Pozzolana Cement (PPC) is obtained by either
intergrinding a pozzolanic material with clinker and gypsum, or
by blending ground pozzolana with Portland cement.
Nowadays good quality fly ash is available from Thermal Power
Plants, which are processed and used in manufacturing of PPC. |
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Q.
What are the advantages of using Portland pozzolana cement over
OPC?
Pozzolana combines with lime and alkali in cement when water is
added and forms compounds which contribute to strength,
impermeability and sulphate resistance It also contributes to
workability, reduced bleeding and controls destructive expansion
from alkali-aggregate reaction. It reduces heat of hydration
thereby controlling temperature differentials, which causes
thermal strain and resultant cracking n mass concrete structures
like dams. |
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Q.
Why is the colour of PPC sometimes different from OPC?
The colour of PPC comes from the colour of the pozzolanic
material used. PPC containing fly ash as a pozzolana will
invariably be slightly different colour than the OPC. |
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Q.
Does the shade of cement affect quality?
No. The quality of cement depends upon the raw materials used
and the quality control measures adopted during its manufacture,
and not on the shade of the cement. The cement gets its colour
from the nature and colour of raw materials used, which will be
different from factory to factory, and may even differ in the
different batches of cement produced in a factory. Further, the
colour of the finished concrete is affected also by the colour
of the aggregates, and to a lesser extent by the colour of the
cement. Preference for any cement on the basis of colour alone
is technically misplaced. |
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Q.
What is slag?
Slag is a non-metallic product consisting essentially of glass
containing silicates, alumino-silicates of lime and other bases
and is obtained as a by-product in the manufacture of pig iron
in a blast furnace or electric furnace. Granulated slag is used
in the manufacture of Portland Slag Cement (PSC). |
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Q.
How is PSC made?
PSC is made by intergrinding clinker, granulated blast furnace
slag and gypsum, or by blending ground slag with Portland
cement. |
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Q.
Where can PSC be used?
Slag cement can be used for all plain and reinforced concrete
constructions, mass concreting structures such as dams,
reservoirs, swimming pools, river embankments, bridge piers,
etc. It is used with advantage where low heat of hydration and
resistance to alkali-silica reaction are desired, for structures
in aggressive environments where chemical and mildly acidic
waters are encountered (where the use of OPC is not
recommended), for marine constructions, dykes, wharves, etc
where sulphatic water is encountered. In short, PSC can be used
wherever OPC is used. |
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Q.
What is the effect of long storage periods on cement?
Cement which is in the form of a fine powder has a tendency to
absorb moisture present in the atmosphere. When it absorbs
moisture it hydrates, and when subsequently used does not
contribute to the strength development. Jute bags (gunny bags)
in which cement is bagged are neither airtight nor damp-proof
and do not prevent absorption of moisture. Cement deteriorates
in quality on long storage.
Cement bagged in woven polythene bags or paper bags are not
likely to deteriorate to the extent mentioned above. The loss of
strength also depends on the condition of the godown. It is
advisable to use cement within three months of its bagging, or
to test the cement for its strength if stored for longer
periods. Hence cement bought first should be used first. |
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Q.
How should cement be stored?
Precautions that must be taken in the storage of Portland cement
are given below in a series of DON’Ts. |
(i) |
Do not
store bags in a building or a godown in
which the walls, roof and floor are not
completely weatherproof. |
(ii) |
Do not store bags in
a new warehouse until the interior has thoroughly
dried out. |
(iii) |
Do not be content with
badly fitting windows and doors, make sure
they fit properly and ensure that they are
kept shut. |
(iv) |
Do not stack bags against
the wall. Similarly, don’t pile them
on the floor unless it is a dry concrete
floor. If not, bags should be stacked on
wooden planks or sleepers. |
(v) |
Do not forget to pile
the bags close together |
(vi) |
Do not pile more than
15 bags high and arrange the bags in a header-and-stretcher
fashion. |
(vii) |
Do not disturb the
stored cement until it is to be taken out
for use. |
(viii) |
Do not take out bags
from one tier only. Step back two or three
tiers. |
(ix) |
Do not keep dead storage.
The principle of first-in first-out should
be followed in removing bags. |
(x) |
Do not stack bags on
the ground for temporary storage at work
site. Pile them on a raised, dry platform
and cover with tarpaulin or polythene sheet. |
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Q.
How to identify the time for which the cement was stored before
use?
On the cement bag, week number, month and year of manufacturing
are being mentioned and this can be checked before use. |
