Cement
Portland cement is called so
because when it hardens, it resembles a stone found in the quarries of Portland
in England.
The basic raw
material of making cement is limestone and clay.
1. Major Types of Cement
·
OPC- Ordinary Portland Cement
·
PPC- Pozzolana Portland Cement
·
PSC- Portland Slag Cement
OPC is further classified as 33 G, 43 G, and 53 G (where G
denotes grade) according to 28 day compressive strength of cement mortar cubes
prepared with cement and standard sand in
the ratio 1:3 , with water added , mould vibrated, cubes cured and tested as per Indian Standard. Thus 43 G means
the cement mortar cubes have 28 day compressive strength of 43 Mpa at 28 days.
PPC is produced by intergrinding predetermined quantities of
Portland cement clinker and pozzolana
(Fly ash, 15 to
35 % by weight), (Fly ash is a byproduct in coal based thermal power plants) If
strict quality control procedures are adopted at the cement plant while
selecting fly ash then PPC can generally be used wherever OPC is used under
normal circumstances.
PPC is
especially suited for mass concrete works like foundation and for structures
subjected to aggressive environments (like sewers, marine construction)
.
Requires longer curing, ideally 14 days,
compared to 10 days for OPC.
As per Indian Standards PPC has to satisfy the strength
requirement of 33 G OPC, but in reality higher strength PPC are available in
the market.
PSC is made by intergrinding 35 to 65 percent of OPC clinker with
Ground Granulated Blast Furnace
slag (GGBFS)
slag (GGBFS)
(an industrial waste product obtained during the
manufacture of pig iron). PSC is most suitable for construction near coastal
areas but is unsuitable in cold weather.
PSC gains
strength less rapidly than OPC, but unlike OPC it continues to gain strength significantly
even after 28 days and hence the ultimate strength gained is higher than same
grade OPC. The curing period for PSC is longer compared to OPC, a minimum of 14
days is recommended. The compressive strength of PSC is equivalent to that of
"33" grade OPC. However, slag cements of strength equivalent to
"43" and "53" grade OPC are available in the market. The
slag constituent shall not be less than 25 per cent and not more than 70 per
cent in PSC.
1. Selection of Cement
Selection of
cement is normally based on:
· Durability
Characteristics
·
Functional requirement - Deflection, crack width etc.
· Design
parameters- Strength, fineness, setting time requirement etc.
· Speed of
construction- Time for construction etc.
· Environmental
Conditions- Ground conditions, soluble salts, sulphates, Chemical plants etc.
For normal
building construction like independent houses and villas any of the three type
of cement (OPC, PPC, PSC) can be used. If using OPC, 43 G or 53 G is suitable,
if found economical use 43 G.
The test certificates
indicate:
·
Grade and type of cement
·
Source of cement
·
Week number for which the test certificate is
applicable (week number is printed on each bag of cement.)
·
Test results obtained.
·
Test results required as per Indian Standards.
·
For some test results there may not be
corresponding requirement in Indian Standard. (Like consistency, etc.)
So while
procuring cement, consumers can check the test certificate corresponding to the
batch of cement (week number printed on cement bags) they are buying.
1.
Points to be observed during purchase of cement
On the cement
bag ensure the following:
·
Type of cement – Colour of lettering on the bag
is Black for OPC, Red for PPC and Orange for PSC
·
IS Certification Mark is present.
·
Net mass of cement is marked.
·
Maximum Retail Price (MRP) is indicated.
·
Week number and year of manufacture of the
cement as marked on each bag
·
Check whether the company's original packing is
intact (stitching should be machine stitching only).
·
Check the weight of cement bag. A bag should
weight 50 kg. A variation of ± 0.5kg in any individual bag is permitted as
packing tolerance. However, 20 bags, when weighed together, should not
be less than 1000 kg (net).
2.
Storage of cement
Proper storage
of cement permits easy access for workers. Cement should be stored in suitable
weather-tight structures to protect it from dampness it should be
stored in a manner that no dampness or moisture is allowed to reach it either
from the ground, walls or from the environment. This becomes particularly
important during the humid season and in coastal regions. Cement strength
deteriorates with passage of time by absorbing moisture directly or indirectly.
·
No cement bags should be stacked in contact with
an external wall.
·
Bags should be piled off-the-floor upon wooden
planks. If, however, the floor is a well-constructed dry concrete floor, the
bags can be placed directly on it after spreading tarpaulin or polythene sheet.
·
Cement bags should be placed closely together in
the stack to reduce circulation of air as much as possible.
·
For extra safety during rainy season, the stacks
of cement bags should be enclosed completely in polythene sheets
Cement bags should not be piled higher than
ten bags in a stack and has to be arranged in header and stretcher fashion as
far as possible. While removing the bags for use, the “First in, first out”
rule should be applied.
Cement required
for a day may be stored at site in the open. In such cases cement bags should
be laid on a dry platform made of wooden planks resting over brick-masonry,
concrete, dry sand or aggregates raised about 15 cm above the ground level. The
number of bags should be kept to a minimum, preferably just sufficient for the
day's consumption. The stack must be kept fully covered with tarpaulin or
polythene sheet and protected against atmospheric moisture.
3.
Reason for cement not used in the
neat form
Cement is not
used in neat form because of both technical and economical reasons. Cement
releases considerable quantities of heat during hydration that creates
shrinkage cracks during cooling. This phenomenon causes tensile stresses
resulting in development of cracks in the matrix. Without using aggregate the
matrix form of cement is not effective hence dimensional stability is at stake.
Aggregates such as sand and crushed stone are less expensive than cement.
Mortar is prepared by mixing cement and sand or any other fine aggregate
together with water. Concrete is prepared by mixing cement, sand, coarse aggregate
together with water.
4.
Impact of color on technical
properties of cement
As a general rule there is no impact of
color on technical virtues of cement. Color consciousness is more of a mind set
and conviction, rather than technical performance. As long as the operating
conditions of kiln and the rate of cooling are maintained within the stipulated
limits, cements with any color behave similarly.
5.
Setting time of cement
Setting time of
cement is of practical implication. Basically the initial setting determines
the length of time in which the cement paste remains plastic and workable. The
term setting implies solidification of the plastic cement paste. The beginning
of solidification, called the initial set marks the point in time when the
paste has become unworkable. Accordingly, placement, compaction, and finishing
of concrete beyond this stage will be very difficult. The paste does not
solidify suddenly; it requires considerable time to become fully rigid. The
time taken to solidify completely marks the final set.
6.
Proper cement content in
concrete
Minimum cement
content in concrete is prescribed under BIS code IS: 456 depending upon the
environment to which concrete will be exposed. IS 456 also limits the maximum
cement content in concrete.
It is best to
leave the design of concrete to an experienced civil engineer.
7.
Reasons for slow setting & quick setting of
cement
Slow setting in cement is due to salts and chemicals in
water, sand and aggregate. Cold weather, less cement, high percentage of
impurities, adulterated cement and improper water cement ratio.
Quick setting of
cement is due to addition of low quantity of Gypsum, hot weather condition,
high cement fineness.
contact rabishnair@yahoo.co.in
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