Cement 1.pptx

Materials for making concrete -
CEMENT PART 1

Manufacture of Portland cement
Raw materials required:
• Calcareous materials like limestone or chalk
• Argillaceous materials like shale or clay
The process of manufacture of cement consists of grinding the raw materials, mixing them intimately in certain
proportions depending upon their purity and composition and burning them in a kiln at a temperature of about
1300 to 1500 C at which temperature the material sinters and partially fuses to form nodular shaped clinker.
The clinker is cooled and ground to fine powder with addition of about 3 to 5 % of gypsum resulting in Portland
cement.
Two types of processes for manufacture depending upon whether the mixing and grinding of raw materials is
done in wet or dry conditions
Earlier, wet process was more popular because of the possibility of more accurate control in the mixing of raw
materials as techniques of intimate mixing of raw materials in the powder form was not available then.
Dry process gained momentum with the modern development of the technique of dry mixing of powdered
materials using compressed air.
Wet process requires more fuel compared to dry process as the slurry contains about 35 to 50 percent water
which has to be removed.

Wet process
• Limestone from quarries is first crushed to smaller fragments.
• Then it is taken to a ball or tube mill where it is mixed with
clay or shale and ground to a fine consistency of slurry with
the addition of water.
• The slurry is a liquid of creamy consistency with water
content of about 35 to 50% wherein particles crushed to the
fineness of IS sieve no 9 are held in suspension.
• The slurry is pumped to slurry tanks or basins where it is kept
in agitated condition by means of rotating arms with chains or
blowing compressed air from the bottom to prevent settling of
limestone and clay particles.
• The composition of the slurry is tested to give the required
chemical composition and corrected periodically in the tube
mill and also in the slurry tank by blending slurry from
different storage tanks.
• Finally, the corrected slurry is stored in the final storage tanks
and kept in a homogeneous condition by the agitation of
slurry.
• The corrected slurry is sprayed on to the upper end of a rotary
kiln against hot heavy hanging chains.

• The rotary kiln is a thick steel cylinder (diameter 3m to 8m) lined with refractory
materials, mounted on roller bearings and capable of rotating about its own axis
at specified speeds.
• The slurry on being sprayed against a hot surface of flexible chain loses moisture
and becomes flakes which peel off and fall on the floor.
• The rotation of the kiln causes the flakes to move from the upper end towards
the lower end of the kiln subjecting itself to higher and higher temperature.
• The kiln is fired from the lower end using powdered coal, oil or natural gas.
• As the material rolls down the kiln, it undergoes a series of chemical changes and
at temperature of about 1500 C in the hottest part of the kiln, about 20 to 30% of
the material gets fused
• The fused mass turns into nodular form of size 3mm to 20mm known as clinker
• The clinker drops into a rotary cooler where it is cooled under controlled
conditions.
• The clinker is stored in silos or bins
• The cooled clinker is ground in a ball mill with the addition of 3 to 5 % of gypsum
in order to prevent flash setting of cement.
• A ball mill consists of several compartments charged with progressively smaller
hardened steel balls.
• The particles crushed to the required fineness are separated by currents of air
and taken to storage silos from where the cement is bagged.

Dry process
• In the dry and semi-dry process the raw materials are
crushed dry and fed in correct proportions into a
grinding mill where they are dried and reduced to a very
fine powder.
• The dried powder called raw meal is then further
blended and corrected for its right composition and
mixed by means of compressed air.
• The aerated powder tends to behave almost like liquid
and in about one hour of aeration a uniform mixture is
obtained.
• The blended meal is further sieved and fed into a
rotating disc called granulator. 12% by weight of water
is added to make the blended meal into pellets.
• This is done in order to permit air flow for exchange of
heat for further chemical reactions and conversion into
clinker in rotary kiln
• Dry process is economical compared to wet process

Chemical Composition of Cement
The raw materials used for the manufacture of cement consist mainly of lime, silica, alumina and iron oxide.
These oxides interact with one another in the kiln at high temperature to form more complex compounds.
The relative proportions of these oxide compositions are responsible for influencing the various properties
of cement; in addition to rate of cooling and fineness of grinding. The identification of the major compounds
of cement is largely based on Bogue’s equations and hence called “Bogue’s Compounds”
Compound Symbol Chemical Formula Nick name Percentage
Tricalcium silicate C3S 3CaO. SiO2 Alite 30 – 50%
Dicalcium silicate C2S 2CaO. SiO2 Belite 20 – 45%
Tricalcium aluminate C3A 3CaO.Al2O3 Celite 8 – 12%
Tetracalcium alumino
ferrite
C4AF 4CaO. Al2O3. Fe2O3 Ferrite 6 – 10%

• The most important minor components of cement are gypsum, MgO, and alkali sulfates.
• Gypsum (2CaSO4 · 2H2O) is added in the last procedure of grinding the clinker to produce Portland
cement. The reason for adding gypsum in cement is to avoid the flash setting caused by fast reaction of
C3A, because it can react with C3A and form a hydration product called ettringite on the surface of C3A to
prevent further reaction of C3A as a barrier. The normal percentage of gypsum added cement is about 4–
5%. Only when gypsum is more than 3% in a Portland cement, can the formation of ettringite be
guaranteed.
• Minor oxides like K2O and Na2O are also formed in the kiln. Alkalies (MgO, Na2O, and K2O) can increase
the pH value of concrete up to 13.5, which is good for reinforcing steel protection. However, a high alkaline
environment can also cause some durability problems, such as alkali aggregate reaction and leaching.
• An increase in lime CaO content beyond a certain value makes it difficult to combine with other
compounds and free lime will exist in the clinker which causes unsoundness in cement.
Minor compounds in cement

• The insoluble residue , determined by
dissolving cement in hydrochloric acid is a
measure of the adulteration of cement
caused by impurities in gypsum.
• The loss on ignition shows the extent of
carbonation and hydration of free lime and
free magnesia.
• The amount of Bogue compounds in
cement can be controlled by :
• Changing the raw material feed
• Changing the temperature in the kiln
• Adjusting the duration of reaction in
the kiln
• Adjusting the amount of gypsum

The reaction of cement when mixed with water is called hydration. Tricalcium
silicate and dicalcium silicate are the most important compounds responsible
for strength. Together they constitute 70 to 80 per cent of cement. The
hydration of these products is responsible for the setting and hardening of
cement. In the presence of water, the silicates and aluminates form products of
hydration which result in a hard mass known as over a period of time. This
hard mass is known as hydrated cement paste. The hydration surface reactions
starts immediately on contact of cement with water. It is an exothermic
reaction with the cement grains becoming smaller as the reaction proceeds.
Hydration of cement

The primary initial reaction of C3A with water in the presence of a plentiful supply of gypsum results in the
formation of calcium sulfoaluminate hydrate which is commonly called ettringite, which is the name of a
naturally occurring mineral of the same composition. The formation of ettringite is right on the surface of the
particles of C3A can slow down the hydration of C3A because it acts as a diffusion barrier around C3A. Thus, it
can avoid a C3A flash setting. Ettringite is a needle-shaped crystal with a large volume expansion. Moreover,
ettringite is very aggressive and will make space to grow if there is no free space left. The effect of ettringite
on concrete strength can be evaluated in two cases.
In case 1, ettringite is formed before the paste has hardened and gained strength due to hydration of C3S. It
will contribute to the early strength development of concrete since the needle-shaped crystals can work as
reinforcement for the surrounding C–S–H, and the expansion is not so significant.
In case 2, if ettringite is formed after the concrete has hardened and free space has been occupied by other
hydration products, it will make its space to grow by breaking the hardened hydration products and hence
create cracks and volume instability.
The hydration products of C4AF are similar to those of C3A. However, the reaction rate of C4AF is slower
than that of C3A
Hydration of Tricalcium aluminate and Ferrite

Hydration of Calcium Silicates (C3S & C2S)
The hydration of both Calcium Silicates is similar differing only in the amount of Calcium hydroxide produced,
heat released and the reaction rate. The principal hydration product is C3S2H8, calcium silicate hydrate, C–
S–H which occupies about 50% of the structural component in a cement paste and forms directly on the
surface of cement particles. It is amorphous, fibrous with a large surface area forming a continuous
binding matrix and is the major factor for development of strength of cement paste
CH (Calcium hydroxide) is formed in solution by crystallization and occupies about 25% of the structural
component of cement paste. CH can bring the pH value to over 12 and it is good for corrosion protection
of steel. From a durability of concrete point of view, CH may lead to leaching due to its solubility,
carbonation due its reaction with carbon dioxide, alkali aggregate reaction due to its high pH value, or
sulfate attack due to its reaction with sulfate.

10 minutes 10 hours
18 hours 1-3 days 2 weeks
Anhydrous
cement

The rate of hydration during the first few days is
C3A > C3S > C4AF > C2S
C3S has a high early strength and C2S develops its
strength slowly in the early age but very fast later
on.
C3S contributes the most to early strength and C2S
to long-term strength of Portland cement.
The contribution to the strength of Portland
cement from C3A and C4AF is not significant.

TYPES OF CEMENTS
There are various types of cements in use.
1. Ordinary Portland Cement OPC
OPC has been classified into three categories based on the compressive strength of cement mortar cube in
N/mm2 at 28 days.
1. Grade 33 conforming to IS 269 – 2013
2. Grade 43 conforming to IS 8112- 2013
3. Grade 53 conforming to IS 12269- 2013
Higher grades of cement enable making stronger concrete as they have faster rate of development of
strength. The manufacture of OPC is decreasing due to popular use of blended cement which has lower
energy consumption, less polluting and other economic, technical reasons.

2. Portland Pozzolona Cement (PPC)
PPC is manufactured either by grinding together Portland cement clinker, gypsum and puzzolona such as
fly ash or by uniformly blending Portland cement and fine pozzolona. Puzzolonic material is a siliceous or
aluminous material which in finely divided form in the presence of water reacts with calcium hydroxide
to form compound possessing cementitious properties. The puzzolonic materials used are calcined clay
and fly ash. IS 1489 (Part 1) -2015 gives provisions for fly ash based Portland Pozzolona Cement. The
proportion of fly ash used can vary between 15% and 35% by weight of cement. The use of PPC is
desirable for enhancing durability as it results in increased impermeability, lower heat of hydration,
lower plastic shrinkage, reduced alkali aggregate expansion and improved resistance to aggressive
chemical agents and corrosion. However, rate of strength gain is slower than OPC and hence PPC requires
longer curing period and longer setting and hardening time.

3. Blended Cement
Portland cement containing a mineral additive becomes blended or composite cement. The use of blended
cement improves the properties of both fresh and hardened concrete due to extended hydration of cement
puzzolona mixture, reduced water demand and improved cohesion of the paste. There is increased durability
as it has lower permeability and improved microstructure arising from the reduction of pore sizes.
4. Rapid Hardening Cement
As the name suggests, it has higher rate of development of strength. It attains the same strength at three days
as developed by OPC at 7 days. It is attributed to higher fineness of grinding and higher C3S and lower C2S
content. Due to higher C3S content, it undergoes quick hydration but generates greater heat of hydration. It
should not be used in mass construction. It is recommended for use in the following situations:
1. In pre fabricated concrete construction
2. Where formwork is required to be removed early for reuse elsewhere
3. Road repair works
4. Cold weather concreting to reduce the damage due to frost

5. Extra Rapid Hardening Cement
It is obtained by grinding Calcium Chloride with rapid hardening Portland cement. Since Extra rapid hardening
cement is very sensitive, concrete should be transported, placed, compacted, and finished within 20 minutes
after mixing. After the addition of water, a very huge amount of heat is evolved within a short period of time
along with hydration. So, this type of cement is perfect for concreting in cold weather. At the age of one or two
days strength of Extra rapid hardening cement is 25% more than rapid hardening cement, and only 0-20 %
higher at 7 days but at 90 days both cement have nearly the same strength.
6. Portland Slag Cement
This cement is prepared by grinding Portland cement clinker and ground granulated blast furnace (GGBF) slag
with the addition of gypsum and permitted additives. The proportion of slag should not be less than 25% and
not more than 65% of Portland slag cement. The slag contains oxides of lime, alumina, and silica and easily
replaces clay or shale used in the manufacture of ordinary Portland cement. Portland slag cement can be used
for all purposes for which ordinary Portland cement is used. However, it has lower heat evolution and is more
durable.

7. Hydrophobic cement
Hydrophobic cement is prepared from ordinary Portland cement clinker by adding certain water repellent
chemicals during the grinding process. A water repellent coating is formed over each particle of cement that
prevents water or moisture from the air being absorbed by the cement. This film is broken during the mixing
of concrete and the normal hydration process takes place in the same manner as with the ordinary Portland
cement. This cement is ideal for storage for longer periods in extremely wet climatic conditions. The
hydrophobic agents can be oleic acid, stearic acid, naphthenic acid, etc. This cement is different from
waterproofing cement.
8. Sulphate Resisting Cement:
Since ordinary Portland cement is susceptible to attack of sulfate, sulfate resisting cement is developed to use
where the soil is infected with sulfates. Due to the attack of sulphate in OPC cement, there are chances of
expansion within the framework of concrete causing cracking and subsequent disruption. Sulphates resisting
cement has a high silicate content that is with low C3A and low C4AF.

Under the following conditions sulphate resisting cement is used:
•When concreting is done for marine structure in the zone of tidal variations. Concrete surfaces subjected to
alternate wetting and drying such as bridge piers, concrete surface in the tidal zone, apron, Buildings near the
seacoast
•Construction in contact with soils or groundwater having more than 0.2% or 0.3 % g/l sulfate salts respectively.
•In marshy soil or sulphate bearing soil.
•Concrete construction used for sewerage treatment, etc.
9. Quick setting Cement:
Quick setting cement is the cement which sets in a very short time. The initial setting time is 5 minutes and the
final setting time is 30 minutes. The composition of Quick Setting Cement consists of Clinker grinded with
Aluminum sulfate (1% to 3% by weight of clinker). The aluminum sulfate increases the hydration rate of silicate.
Uses of Quick Setting Cement
•It is used in underwater construction.
•It is also used in rainy & cold weather conditions.
•It is used a higher temperature where water evaporates easily.
•Used for anchoring or rock bolt mining and tunneling

10. White Cement
White cement is quite similar to Ordinary Portland Cement except for the white color. Amounts of iron oxide and
manganese oxide are low in White Cement. It is expensive than OPC and hence not economical for ordinary work.
It is usually used in decorative work. It can also be used for traffic barriers, tile grouts, swimming pools, roof tiles
patching materials, and terrazzo surfaces.
11. Air Entraining Cement
It is seen that entrainment of air or formation of gas bubbles while applying cement increases resistance to frost
action, fire, scaling, and other similar defects. Air-entraining cement is a special type of cement which entrains tiny
air bubbles in concrete. It is produced by grinding minute air entertaining materials with clinker by adding some
resinous materials e.g. vinsol resin to ordinary portland cement. When the water in concrete gets frizzed due to
low temperature, it expands. In air-entraining cement, the air voids in concrete provides space for water to expand
without cracking concrete. But this type of cement does not provide high strength in concrete. Air-Entraining
Cement is used in areas where the temperature is very low. It can also resists the Sulfate attack. It is used where
the de-icing chemical is used.

12. High alumina cement
This cement is obtained by grinding high alumina clinker consisting of monocalcium aluminates. High alumina
cement clinker is obtained by complete or partial fusion of a predetermined mixture of materials mainly
containing alumina (Al203) and lime (CaO) with a smaller proportion of iron oxides, silica (Si02) and other
oxides. High early strength, the high heat of hydration and very high durability against chemical attack are the
characteristics of high alumina cement. It is black in colour. Its rapid hardening properties are due to a higher
percentage of calcium aluminate in place of calcium silicate as found in ordinary Portland cement. The rapid
development of heat of this cement is of great advantage when concreting is to be done in freezing weather. Its use
in hot weather, however, is very limited due to increased porosity, hence reduction in strength.
13. Supersulphated cement
It is a hydraulic cement having sulphuric anhydride (SO3) content less than 5% and made by inter grinding mixture
of at least 7% granulated blast furnace slag, calcium sulfate and a little amount of lime or Portland clinker. This
cement is used in severe conditions such as marine works, mass concrete jobs to resist the attack of aggressive
waters, reinforced concrete pipes in groundwaters, concrete construction in sulphate bearing soils, and in chemical
works exposed to the high concentration of sulphates of weak solutions of mineral acid. It can also be used for the
underside of bridges over railways and for sewer pipes.

14. Coloured cement:
Coloured cement is made by adding colour carrying pigment with a Portland cement clinker. The dose of
pigment is 5-10 percentage of Portland cement. For achieving various colors, either white cement or grey
Portland cement is used as a base material.
15. Expansive cement:
Expansive cement is a type of cement that shows no change in volume on drying. This type of cement also
does not shrink while hardening or after that. This type of cement has been developed by using an
expansive agent and stabilizer. Generally, sulphoaluminated clinker is mixed with Portland cement with
stabilizer. This cement is used for grouting anchor bolts or grouting machine foundations,
grouting the prestressed concrete ducts where volume change is very sensitive for stability. There are three
types of expansive cement:
1.K Type expansive cement
2.M Type expansive cement
3.S Type expansive cement

16. Low Heat Cement
It is a spatial type of cement which produces low heat of hydration during the setting. Some chemical
composition of Ordinary Portland Cement is modified to reduce the heat of hydration. The chemical
composition of low heat cement:
•A low percentage (5%) of tricalcium aluminate (C3A)
•A higher percentage (46%) of declaiming silicate (C2S).
Uses of Low Heat Cement
•It is used for the construction of dam’s large footing, large raft slabs, and wind turbine plinths.
•It is also used for the construction of chemical plants.

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