What is Cement?
“Cement” is a material
with adhesive and cohesive properties that makes it capable of
bonding mineral fragment into a compact and rigid mass. The word
cement seems to have been derived from the middle age English “cyment”, and Latin
“caementum”. The latter word “ caementum “ meant rough
quarried stone or chips of marble from which a kind of mortar was
made more than 2000 years ago in Italy. During the Middle Ages term
“cement” or “sement” generally was made for a mortar. Common lime,
hydraulic lime, gypsum plaster,
“pozzolana”, natural and Portland cement are few of the
material, which are used for cementing purposes. These cementing
material may be classified into two groups:
do not have the ability to set and harden under water but requires
carbon dioxide from air to harden e.g. non-hydraulic lime and
plaster of Paris. Their cementing prosperity arises from the re
absorption of gases that were expelled during their processing.
Their products of hydration are not resistant to water.
Hydraulic cement is
defined as cement having the ability to set and develop strength in
air or under water and which are insoluble in water after they have
set. Such cement harden even in the absence of air and form a solid
product which is stable in water and can be safely used in all
structures in contact with water. Hydraulic cement includes
hydraulic limes, Portland cement (both basic and blended), oil-well
cement, white cement, colored cement, high alumna cement, expensive
cement regulated and hydrophobic cement etc.
Quarrying and crushing
The primary raw
material for cement manufacture is calcium carbonate or limestone.
This is obtained from the quarry where, after the removal of
overburden, the rock is blasted, loaded into trucks and transported
to the crusher. A multistage crushing process reduces the rock to
stone less than 25 mm in diameter. Most modern cement factories are
located close to a source of limestone as about 1.5 tons of
limestone is needed to produce one ton of cement.
Blending and storage
milling and homogenization
Carefully measured quantities of the various raw materials are fed,
via raw mill feed silos, to mills where steel balls grind the
material to a fine powder called raw meal. Homogenizing silos are
used to store the meal where it is mixed thoroughly to ensure that
the kiln feed is uniform, a prerequisite for the efficient
functioning of the kiln and for good quality clinker.
The most critical step
in the manufacturing process takes place in the huge rotary kilns.
Raw meal is fed into one end of the kiln, either directly or via a
preheater system, and pulverized coal is burnt at the other end. The
raw meal slowly cascades down the inclined kiln towards the heat and
reaches a temperature of about 1 450 °C in the burning zone where a
process called clinkering occurs. The nodules of clinker drop into
coolers and are taken away by conveyors to the clinker storage
silos. The gas leaving the kiln is cleaned by electrostatic
precipitators prior to discharge into the atmosphere.
The cement mills use
steel balls of various sizes to grind the clinker, along with a
small quantity of gypsum to a fine powder, which is then called
cement. Without gypsum, cement would flash set when water is added
and gypsum is therefore required to control setting times. The
finished cement is stored in silos where further blending ensures
Extensive sampling and testing during the manufacturing process
ensures the consistency and quality of the end product. Testing
takes place at the stages of the manufacturing process indicated by
Cement is despatched
either in bulk or packed in 50 kg bags and distributed from the
factory in rail trucks or road vehicles. The 50kg bags are either
packed directly onto trucks or can be palletised. The pallets can be
covered by a layer of plastic to offer further protection from the
Types Of Cement:
The types of special
cement now being produced can be roughly classified in the following
six categories according to the special purpose for which these have
been designed. These are:
to chemical attack of certain soil and aggregates.
Low heat of
cement for steel reinforcement.
and whether resisting cement.
and other special cement.
Under this category
following two cement have the desired properties of fast development
of strength viz. the Portland Rapid Hardening Cement and High
Aluminum Cement. “Their specific characteristics are as follows:
Rapid Hardening Cement
(Type III of A.S.T.M):
This cement has high
early strength, its equal to or better than 3 Days’ strength of OPC.
This is achieved by having high contents of tricalcium silicates in
its composition. It is mostly used in intended to release the
framework within 24 hours or so for subsequent use in the mass
production of RCC elements.
The respective 1,2,3,7
and 28 days’ strengths (in equivalent P.S.I figures) of this cement
under British specifications, German standard, and A.S.T.M. Japanese
and Pakistan standards for mortar cubes are as follows:
1 day strength
The difference in
strengths given above is basically due to difference in all
standards the 3 days strength is nearly 1-1/2 to 2 times of O.P.C.
The disadvantages of this cement beside its higher cost are its high
heat of hydration, which renders it unsuitable for mass concreting
This cement is used
where very rapid setting and very high early strength are required.
This cement has strength at one day nearly equal to 28 days strength
of O.P.C. Its setting is so fast that it must be put in place within
a few minutes of its mixing. It is generally used in plugging
leakage in dams etc. or putting in pile foundations where limited
time is available for setting of cement before the seepage water
build up occurs. In Pakistan it has been used in some specific
locations in terbela dam. Abroad it has been used in buildings where
it was found essential to remove the framework after one or two
This cement besides
its high cost has the disadvantages of the high heat of hydration
and retrogression in strength in time. There have been some
structural failures due to miscalculation of its final strength
after some years of use especially in humid and hot atmosphere. This
cement has however excellent heat registering qualities and is
therefore extensively used in Kilns, boilers and Furnace linengings.
Cement resistant to chemical attack especially of Sulphate and
Organic acids or Soil and active Silica of aggregate. In this
category the following cement can be included:
2-A Highly Sulphate Resistant Cement
2-B Moderately Sulphate Resisting Cement
2-C Portland Blast France Slag Cement.
2-D pozzolana Cement
2-E Low Alkali Cement
Highly Sulphate Resisting Cement H.F.R.C
The most important and
the most widely used chemical resistant cement is H.S.R.C cement
High concentration of sulphate salts is present in seawater and in
the soil near seashores. These salts are sometimes present in soil
and in the submit water even thousands of miles away from the sea.
Even Terbela Dam site was found not free from sulphate and H.S.R.C
had to be used in foundations at the site. The sulphate salt
severally attack concrete can start within months. This cement has
lower 3 days strength than OPC and also its capacity to protect
reinforcement steel in structures exposed to atmosphere action is
lower than OPC and hence not recommended for usual R.C.C. work in
Moderately Sulphate Resisting Cement (M.S.R.C.)
This cement has been
developed as a compromise Cement having the good properties of
sulphate resistance to some extent and of good alkalinity like that
of OPC which useful for reinforcement protection and also of early
strength development better than Highly Sulphate Resisting Cement.
No standard exists for this cement in the B.S and P.S.
specifications but under ASTM it is designed as Type II. The three
days minimum strength of OPC, H.S.R.C & H.S.R.C is 1800, 1200 and
1500 PSI respectively under ASTM.
This cement though
better than H.S.R.C in many respects is not as resistant to sulphate
as H.S.R.C and should not be used in foundations near the seashore.
Incidentally the O. P. cement produced in most of the Cement
Factories around Karachi are confirming to the ASTM specifications
for this type II and hence more resistant to mild sulphate attack
than the cement produced in the factories in other parts of country.
Portland Blast Furnace Slag Cement:
This cement has well
to moderate resistance to sulphate attack from soil. This cement has
some other very desirable qualities of stability details of which
follow below under item 3C.
This cement has well
to moderate resistance to sulphate attack from seawater or soil
Low Alkali Cement:
This is a variety of
ordinary Portland cement in which the total alkali contents of
cement has been controlled to remain below 0.6%. With this reduced
percentage of alkali contents the danger of alakie of cement
attacking the active silica contents of aggregate is eliminated.
Generally we do not have active aggregate of this type in Pakistan
but on each large scale-concreting project, test of alkali aggregate
reaction must be performed to enshore safety of the project. Certain
varieties of Chert-stone found in Pakistan contain active Silica and
would require low alkali for making concrete.
Low Heat of Hydration Cement:
Normal and Rapid
Hardening Cement generate lot of heat during the setting and
hardening process so much so that the structure under concreting can
crack. This can occur specially while poring large messes of
concrete in confine spaces like those of Dam and Bridge pier
foundations. In order to avoid this problem cement of low Heat of
hydration have been developed some of which are as listed below:
Low Hear of Hydration Cement (type IV of A.S.T.M.)
Portland blast Furnace Slag Cement
Super Sulphate Cement
Low Hear of Hydration Cement (type IV of A.S.T.M.):
This is cement
specially meant for the concreting of structures where large masses
of concrete have to be poured at one time. Generally it is specified
that heat of hydration on 7 days will not exceed 250Kg. This is
achieved by making this cement with larger percentage of di-calcium
silicates in its contents than normally presents in OPC. In A.S.T.M
this cement is designated as type IV and under BS as LHP. Under
German standards its type is LAHORE with symbol as N.W. while the
Japanese equivalent is type L.H.P with symbol as M.H.C. The
disadvantage of this cement is its slow development of strength and
is therefore not used at sited where rapid hardening or other
specific qualities are required.
This is another
variety of low heat cement. Grinding 35% to 65 % of granulated blast
furnace slag with ordinary Portland cement clinker produces it. The
higher the slag contents, the lower are the 3 and 7 days strengths
but better is the resistance to chemical attack. Under German
standards as much as 90% slag can be used with type HOZ cement and
under Japanese standard up to 70% in the type BSCC. This cement is
slower hardening compared to OPC- its strength compared to OPC in
ASTM being as follows:
This cement besides
being a low heat cement has also the advantage of being medley
sulphate resistant although it cannot replace the highly sulphate
resistance cement for marine piles and foundations. This cement has
excellent resistance to weak organic acids present in the soil and
has also the very desirable quality of protecting reinforcement
steel better than any other cement.
proportions of natural pozzolana, tars or volcanic ash with ordinary
Portland clinker makes this cement. It is very good cement in the
sense that it has good workability properties in addition to having
low heat and moderate sulphate resisting properties. It has been
used extensively in the 37KM long causeway connecting Dhahran with
Bahrain in the gulf. So far in Pakistan we have not been able to
locate useful pozzolana deposits, but there are indication that
there may be good deposits of this material in the overburden of
coal deposits of Sindh.
Super Sulphate Cement:
This is another
variety of low heat cement. Its standards exist under B.S but not
under A.S.T.M. it is made by grinding about 70-80% B. F. Slag with
about 10% gypsum and 1-2 % Portland clinker or lime. This cement is
also middy resistant to sulphate attack. It is very finely ground
cement and its early strength at 3 day is comparable to OPC although
under the BS its 7 days strength is required to be comparable to at
least the 3 days strength of OPC. This cement is also good masonry
cement due to its good workability but it can be used in RCC and
other construction work in the same manner as OPC is used with
Cement For Better
Protection of Reinforcement against Corrosion:
The basic steps for the
prevention of resulting of steel in concrete is to use such cement
aggregate and mixing water as are basically free from chlorides,
maximum contents of chlorides in concrete being limited to 0.02 % by
weight. The following properties in cement are essential for greater
protection of steel:
Cement to be with minimum percentage of
Chlorides says not exceeding 0.01 per cent.
Portland Cement preferably having about
6 to 8 per cent Tricalcium Aluminates.
Cement made with slag as additive.
If the Tricalcium
Aluminates is less than 5 per cent as per ASTM the cement will not
have the capacity to neutralize the stray Chloride entering into
concrete and thus fail toward off the effects of Chlorides. The
properties of slag cement with about 35 percent slag and 60 per cent
Portland Clinker are superior to other cement in this respect.
Cement for Better
Workability and Wealth E.W. Resistance
These are cement,
which are render the corresponding concrete more workable than other
normal cement. This quality is necessary where high compatibility
and better weather resistance is demanded. Because of greater
workability the concrete made from this cement can achieve much
higher strength due to lower water cement ratios achievable compared
with other cement.
This property is given
to the cement by addition of the entraining agents like lime or
other plasticisers so that the remix and to place in position. Some
of the cement belong to this category are:
Air Entraining Cement:
Under ASTM four
different types of cement have been classified as air-entraining
version for normal Rapid-hardening and Sulphate Resisting Cement and
each concrete thus made is more workable and attains higher weather
resisting property compared to their non-air entrained versions.
Blended Hydraulic and
Masonry Cement and Grouts:
This cement is
basically made for plastering and grouting. Standards have been laid
down for this cement both in the British and ASTM specifications. In
fact under ASTM there are at least 10 versions of Masonry cement.
Some of the standards are for OPC based cement with or without air
entraining agents while others are for sulphate resistance and low
heat versions. This masonry cement is made by addition of
plasticizing materials like lime, ground silica, slag or Pozzolana
and air- entraining agents’ etc. Portland Cement clinkers during
Decorative Cement Oil
Well and other Special Purpose Cement:
This cement is
basically of properties similar to O.P.C except that it is made from
such raw materials, which contain the least amounts of coloring
pigments like traces of iron, manganese and chrome. Basically this
cement has higher Tricalcium Aluminates in its contents than OPC and
therefore subjects to sharp attack by sulphate from any source.