4. INTRODUCTION:-
Definition: “Cement is a crystalline compound of calcium silicates
and other calcium compounds having hydraulic properties”
TYPES OF CEMENT:-
Natural cements:- Misleadingly called Roman. It is made from
argillaceous limestone or interbedded limestone and clay or shale, with
few raw materials. Because they were found to be inferior to Portland,
most plants switched.
Properties:- Natural cement possesses hydraulic qualities, but is quick-
setting and of relatively low strength.
Portland cement:- An extremely finely ground product obtained by
calcining together, at about 1500C,an intimate and properly
proportioned mixture of argillaceous and calcareous raw materials,
without the addition of anything subsequent to calcinations, excepting
the retarder gypsum.
5. Properties:- Portland cement is most important and reliable cementing
material, used for constructional works.
Pozzolan-lime cements: Original Roman cements. Only a small
quantity is manufactured in the U.S. Mix of pozzolans with lime.
Properties:- Pozzolan cement possess hydraulic properties. they are
seldom used as such at present time, but are mixed with Portland
cement.
SLAG CEMENT:- Made from blast furnace slag and hydrated lime.
The blast furnace slag is granulated by pouring it into a stream of cold
water.
Properties :- slag cement set more slowly than Portland cement. They
have lower strength and are in abrasion-resistance.
7. Portland cement is made by mixing substances containing CaCo3
with substances containing SiO2, Al2O3, Fe2O3 and heating them to a
clinker which is subsequently ground to powder and mixed with 2-6%
gypsum
CLINKER GYPSUM
8. COMPOUND COMPOSITION OF PORTLAND CEMENT
Oxides interact with eachother in the kiln to form more complex
products (compounds). Basically, the major compounds of P.C. can be
listed as:
Name Chemical Formula Abbreviations
Tri Calcium Silicate 3CaO.SiO2 C3S
Di Calcium Silicate 2CaO.SiO2 C2S
Tri Calcium Aluminate 3CaO.Al2O3 C3A
Tetra Calcium
Alumino Ferrite
4CaO.Al2O3.Fe2O3 C4AF
9. The degree to which the potential reactions can proceed to
“equilibrium” depends on:
1) Fineness of raw materials & their intermixing.
2) The temperature & time that mix is held in the critical zone of the
kiln.
3) The grade of cooling of clinker may also be effective on the
internal structure of major compounds.
There are also some minor compounds which constitute few %, so
they are usually negligible. Moreover, portland cement compounds are
rarely pure.
For example in C3S, MgO & Al2O3 replaces CaO randomly.
C3S→ALITE & C2S→BELITE
Ferrite Phase: C4AF is not a true compound. The ferrite phase
ranges from C2AF to C6AF. *C4AF represents an average.
10. MANUFACTURE OF PORTLAND CEMENT
1) Mixing of raw material:-
(i) Dry process: The raw materials [Limestone (or chalk) and clay (or
shake)] are crushed (in gyratory crushers) into roughly 2-5 cm size
pieces. Then, these are ground to fine powder (in ball mills/tube mills).
Each separate powdered ingredient is stored in a separate hopper.
Then, the powdered materials are mixed in the required proportions to
get dry ‘raw mix’, which is stored in storage bins (called silos) and kept
ready to be fed in a rotary kiln.
Raw materials are mixed in calculated proportions so that the average
composition of the final product is as follows:
11. Component Percentage range by mass
Lime (CaO) 60-69
Silica (SiO2) 17-25
Alumina (Al2O3) 3-8
Iron Oxide (Fe2O3) 2-1
Magnesium Oxide (MgO) 1-5
Sulphur Trioxide (SO3) 1-3
Alkali Oxides (Na2O + K2O) 0.3-1.5
12. (ii) Wet process: The calcareous raw materials are crushed, powdered
and stored in big storage tanks (called silos). The argilaceous materials
(say clay) is thoroughly mixed with water in wash mills, to remove any
adhering organic matter, etc. The basin-washed clay is also stored.
Powdered limestone (from silos) and washed wet clay (from basins)
are allowed to flow in channel, in the right proportions. From the
channel, the two raw materials are led to ‘grinding mills’, where they
are mixed intimately to from a paste, called slurry [grinding operation is
carried out in either tube mill or ball mill or both].
The slurry is led to a ‘correcting basin’, where its chemical
composition may be adjusted, if necessary. This slurry contains about
38 to 40 percent water.
The slurry is finally stored tanks and kept ready for feeding to a
rotary kiln.
13. 2) Burning:- Burning is usually done in rotary kiln, which is a steel
tube, about 2.5 to 3.0m In diameter and 90 to 120m in length, lined
inside with refractory bricks. The kiln is laid in slightly inclined
position at a gradient of 1 in 25 to 1 in 30.
3) Grinding:- The cooled clinkers are ground to a fine powder in a ball
mills or tube mills. During final grinding, a small quantity of powder
gypsum is added. so that the resulting cement does not set very quickly,
when it comes in contact with water. Gypsum, thus, acts as a retarding
agent for early setting of cement.
4) Packing:- The ground cement is stored in silos , from which it is fed
to automatic packing machines. Each bag, usually, contain 50kg of
cement.
14. SETTING AND HARDENING OF PORTLAND CEMENT
Setting:- Is defined as stiffening of the original plastic mass. Due to
initial gel formation.
Hardening:- Is development of strength. Due to crystallization.
15.
16. PLAIN CEMENT CONCRETE (PCC)
The intimate mixture of cement, sand, coarse aggregate (jelly) and water
is known as plain cement concrete. A small quantities of admixtures like
air entraining agents, water proofing agents, workability agents may also
be added to impart special properties to the plain cement concrete.
Uses of plain cement concrete is listed below:
1. As bed concrete below the wall footings, column footings and on walls
below beams.
2. As sill concrete to get a hard and even surface at window and
ventilator sills.
3. As coping concrete over the parapet and compound walls.
4. For flagging the area around the buildings.
5. For making pavements.
6. For making tennis courts, basket ball courts etc.
17. REINFORCED CEMENT CONCRETE (RCC)
Concrete is good in resisting compressive stress but is very weak
in resiting tensile stresses. Hence reinforcement is provided in the
Concrete wherever tensile stress is expected.
The best reinforcing material is steel, since its tensile strength Is
high and bond between steel and concrete is good. Since elastic
modulus of steel is quite high compared to concrete, the force
developed in steel is high. A cage of reinforcements is prepared as per
the design requirements, kept in the form work and then green
concrete is poured.
After the concrete hardens, the form work is removed. The
composite material of steel and concrete, now called R.C.C. acts as a
structural member and can resist tensile as well as compressive forces
efficiently
18. USES:-
Main use is in the fabrication of concrete and mortars
Modern uses:
Building (floors, beams, columns, roofing, piles, bricks, mortar,
panels, plaster)
Transport (roads, pathways, crossings, bridges, viaducts, tunnels,
parking, etc.)
Water (pipes, drains, canals, dams, tanks, pools, etc.)
Civil (piers, docks, retaining walls, silos, warehousing, poles, pylons,
fencing)
Agriculture (buildings, processing, housing, irrigation)
