Types of cement
Cement is
classified into various categories based on its composition and specific end
uses. Primarily cement is classified into portland
Portland cement
Portland cement
is the most common type of cement in general usage, as it is a basic ingredient
of concrete. A mixture of limestone and clay is ground and burnt at
a very high temperature to form clinker. The clinker is ground to a fine powder
with the addition of gypsum (up to 5 per cent) to form portland cement.
Essential components of Portland cement are lime, silica, alumina and iron
oxide.
There are different types of portland cement, which differ based on
their chemical composition; however, the manufacturing process remains the
same. Portland cement consists of tricalcium silicate or C3S,
dicalcium silicate or C2S, tricalcium aluminate or C3A,
and tetracalcium aluminoferrite or C4AF [C = CaO - calcium
oxide (calcia), S = SiO2 - silicon dioxide (silica), A = Al2O3
- aluminium oxide (alumina), F = Fe2O3 - iron oxide
(ferric oxide)]. The varying proportion of these constituents imparts diverse
properties to the different types of portland cement.
Ordinary portland cement (OPC)
In ordinary portland cement (OPC), C2S accounts for 20-60 per
cent of the total composition; C3S 20-60 per cent; C3S,
0-16 per cent; and C4AF, 1-16 per cent.
Although OPC is suitable for all types of civil engineering works, it
cannot be used for mass concrete work like multi-storeyed buildings.
Moderate heat portland cement (MHPC)
OPC, when mixed with water and allowed to hydrate, generates lot of
heat, which is not suitable for mass concrete work. However, heat generated
during hydration can be lowered by altering the chemical composition of the
cement; such cement is called moderate heat portland cement. This cement is
more resistant to sulphate, as compared to OPC.
Rapid hardening cement (RHC)
Rapid hardening cement is a special purpose cement used for urgent
repairs (such as airport runway repairs). RHC or high early strength cement
develops compressive strength within 24 hours, as compared to 28 days in the
case of OPC. The average particle size is smaller in these cements and they
gain strength more quickly than ordinary cement. They generate more heat
in early stages and can be useful in cold weather concreting. However,
their principal use is in manufacturing precast concrete units where the
high early strength of the concrete permits quick re-use of moulds and
formwork.
Sulphate-resistant cement (SRC)
The compressive strength of concrete, which is made using OPC, MHPC and
RHC, deteriorates on account of continued contact with soil and water, which
are rich in sulphates.
SRC is a type of portland cement, which contains less than 5 per cent
tricalcium aluminate (C3A). SRC is used for marine construction or in places,
which are rich in sulphates.
Oil well cement (OWC)
Oil wells are drilled at depths of 500 metres or more below the ground
surface. After the drilling operation, wells are lined with an annulus made of
cement concrete. Since the temperature at these depths is over 1,000 C, if
Portland cement grout is pumped into the well, it would set instantly, and
obstruct the 'cementation' or setting process. Hence, cement used for lining
oil wells, should be able to withstand setting time by up to 40-120 minutes,
and thereafter, set within 24 hours. In addition, it should have a strength of
over 100 kg/cm2. In OWC, the percentage of tricalcium aluminate (C3A) is
reduced to less than 3 per cent in its total composition, in order to
control/modify the setting time. In India
White cement
White cement is a Portland cement made from specially selected raw
materials, usually pure chalk and white clay (kaolin) containing very small
quantities of iron oxides and manganese oxides. The chemical complexes formed
with iron oxide present in the cement raw meal give OPC cement its grey colour.
However, if the proportion of iron oxides is reduced to less than 0.4 per cent,
cement becomes white in colour. Iron oxide improves the burning of raw meal;
however, it is difficult to burn the raw meal for white cement, on account of
the low content of iron oxide. As white cement has all the physical properties
of OPC, it can be used in all types of construction where OPC is used; however,
its usage is limited, as it is more expensive than OPC.
Blended cement
In order to produce blended cement, certain natural or fabricated
compounds, such as pozzolona, slag and sandstone, are mixed with Portland
cement clinker and ground finely. Blended cement is more suitable for certain
applications, as compared to Portland cement.
Blended cement is also called low-heat cement, as it generates lesser
heat during hydration, compared with OPC. This cement is used for large
concrete works, such as dams and piers. Blended cement minimises the risk of
developing contraction cracks, on account of the lower heat of hydration of
these cements.
Portland
Blast furnace slag (a waste product of the pig iron furnace) can be used
to produce slag cement. However, blast furnace slag does not have cementitious
properties if it is cooled slowly and ground finely; hence, it is cooled
quickly or quenched and subsequently ground, to acquire cementitious
properties. The quenching process is called 'granulation', and the slag is
known as granulated blast furnace slag.
Granulated blast furnace slag is mixed with lime or OPC clinker and
ground to form slag cement. Portland
Super sulphate cement, another type of slag cement, is prepared by
grinding granulated slag, anhydrite and clinker (in the proportion of
70:15:15). This cement is more sulphate-resistant, than PBFSC or SRC.
Portland
Pozzolona is a clay matter (natural or synthetic), which when ground
with lime/clinker and mixed with water, produces cementitious compounds. Highly
reactive pozzolona or fly ash is mixed with Portland cement clinker and ground
with 5-6 per cent gypsum to form PPC. PPC contains up to 25 per cent pozzolona
or fly ash. PPC has a lower heat of hydration, as compared with OPC and is also
relatively more resistant to sulphates. As a standard, fly ash can be used to
the extent of 15-35 percent.
It has the physical properties of OPC, and hence, can be used for all
types of construction work for which OPC is used. However, in PPC, the
shrinkage is lesser, as compared with OPC.
Masonry cement
Most varieties of cement, when mixed with sand and water get converted
into mortar, which is coarse and not water retentive. Masonry cement is a more
plastic mortar and is used for masonry work, such as laying, binding and
plastering bricks. Portland
Speciality cement
Speciality cements have several special properties and are used in
specific applications.
Expansive cement/Shrinkage compensated cement
Concrete prepared from Portland cement or blended cement, shrinks on
setting and hardening. Cement should expand on setting and hardening when it is
used for pre-stressed, pre-fabricated concrete products and as a grout for
filling cracks. This cement is prepared by increasing the proportion of gypsum
and aluminous cement clinker to Portland cement clinker while grinding.
Super high strength cement
This type of cement is required for the urgent repairs of important
concrete structures, such as foundation pillars. This cement is prepared in jet
mills by finely grinding Portland cement clinker with a higher proportion of
tricalcium silicate. The tricalcium silicate content is around 60 per cent of
the clinker and its fineness should be at least 600 kg/cm2.
Alinite cement
A special low-energy cement process has been developed to manufacture
cement, in which, over 5 per cent calcium chloride is added to the raw meal
while grinding. As the burning point of raw meal is lowered significantly, less
fuel is required for burning. The calcium chloride is vapourised and condensed
in the kiln dust, which is re-circulated. A part of the chloride gets attached
to the clinker components, and increases its compressive strength.
This process is still in its development stage. However, this process
would be viable, if sufficient by-product waste, calcium chloride is available
at low cost.
