2. What is Portland cement?
It is a fine powder produced by heating materials in a kiln
to form what is called clinker, grinding the clinker, and
adding small amounts of other materials. Several types of
Portland cement are available, with the most common being
called ordinary Portland cement (OPC) which is grey in
color, but a white Portland cement is also available.
3. Portland Cement
Portland cement gets its strength from chemical reactions
between the cement and water. The process is known
as hydration. This is a complex process that is best
understood by first understanding the chemical composition
of cement.
Portland cement is manufactured by crushing, milling and
proportioning the following materials:
Lime or calcium oxide, CaO: from limestone, chalk,
shells, shale or calcareous rock
Silica, SiO2: from sand, old bottles, clay or argillaceous
rock
Alumina, Al2O3: from bauxite, recycled aluminum, clay
Iron, Fe2O3: from clay, iron ore, scrap iron and fly ash
Gypsum, CaSO4.2H20: found together with limestone
4. Chemical Composition of cement
Compound Formula Shorthand form
Calcium oxide
(lime)
Ca0 C
Silicon dioxide
(silica)
SiO2 S
Aluminum oxide
(alumina)
Al2O3 A
Iron oxide Fe2O3 F
Water H2O H
Sulfate SO3 S
5. Chemical composition
Approximate Oxide Composition Limits of Ordinary
Portland Cement
Oxide Per cent content
CaO 60–67
SiO2 17–25
Al2O3 3.0–8.0
Fe2O3 0.5–6.0
MgO 0.1–4.0
Alkalies ( K2O, Na2O) 0.4–1.3
SO3 1.3–3.0
6. Major compunds of cement
Name of
Compound
Formula Abbreviated
Formula
Tricalcium
silicate
3CaO.SiO2 C3S
Dicalcium silicate 2 CaO.SiO2 C2S
Tricalcium
aluminate
3 CaO.Al2O3 C3A
Tetracalcium
aluminoferrite
4CaO.Al2O3.Fe2O3 C4AF
7. Properties of cement compound
These compounds contribute to the properties of cement in different
ways
Tricalciumaluminate,C3A:-
It liberates a lot of heat during the early stages of hydration, but has
little strength contribution. Gypsum slows down the hydration rate of
C3A. Cement low in C3A is sulfate resistant.
Tricalciumsilicate,C3S:-
This compound hydrates and hardens rapidly. It is largely responsible
for portland cement’s initial set and early strength gain.
Dicalciumsilicate,C2S:
C2S hydrates and hardens slowly. It is largely responsible for strength
gain after one week.
Ferrite,C4AF:
This is a fluxing agent which reduces the melting temperature of the
raw materials in the kiln (from 3,000o F to 2,600o F). It hydrates
rapidly, but does not contribute much to strength of the cement paste.
By mixing these compounds appropriately, manufacturers can produce
different types of cement to suit several construction environments.
8. Percentages of major compunds
C3S = 4.07 (CaO) – 7.60 (SiO2) – 6.72 (Al2O3) – 1.43
(Fe2O3) – 2.85 (SO3)
C2S = 2.87 (SiO2) – 0.754 (3CaO.SiO2)
C3A = 2.65 (Al2O3) – 1.69 (Fe2O3)
C4AF= 3.04 (Fe2O3)
The identification of the major compounds of cement is
largely based on Bogue’s equations and hence it is called
“Bogue’s Compounds”
9. HYDRATION OF CEMENT
It is the reaction (series of chemical reactions) of cement
with water to form the binding material. In other words, in
the presence of water, the silicates (C3S and C2S) and
aluminates (C3A and C4AF) form products of hydration
which in time produce a firm and hard mass - the hydrated
cement paste.
There are two ways in which compounds of the type
present in cement can react with water: In the first, a direct
addition of some molecules of water takes place, this being
a true reaction of hydration.
The second type of reaction with water is hydrolysis,
10. Setting of cement
When water is mixed with cement to form a paste, reaction
starts. In its pure form, the finely ground cement is
extremely sensitive to water. Out of the three main
compounds, viz. C3A, C3S and C2S, reacts quickly with
water to produce a jelly-like compound which starts
solidifying . The action of changing from a fluid state to a
solid state is called ‘setting’ and should not be confused
with ‘hardening’.
During the next stage of hydration, cement paste starts
hardening owing to the reaction of C3S and C2S with water
and the paste gains strength. The first few minutes, the
setting action is more predominant and later on the
hardening action becomes dominant.
11. Structure of hydrated cement
At any stage of hydration, the hardened paste consists of:
1. crystallized hydrates of the various compounds (calcium silicates
hydrate,tricalcium aluminates hydrate and calcium ferrite) , referred to
collectively as gel.
2. crystals of Ca(OH)2 produced from the hydration of the silicates.
3.some minor components.
4.unhydrated cement.
5.Voids Capillary pores
12. a- Capillary pores: the capillary pores at any stage of
hydration represent that part of the
gross volume which has not been filled by the products of
hydration. The volume of the
capillary system is reduced with the progress of hydration.
Thus the capillary porosity of the paste depends on:
? The water/cement ratio of the mix. When w/c ratio > 0.38
– The gel volume is not
enough to fill all the available voids.
? The degree of hydration, which influence by the type of
cement.
b- Gel pores: The gel pores are interconnected interstitial
spaces between the gel particles.
The gel pores are much smaller than the capillary pores. It
could be contain large amount
of evaporable water.
13. Water held in hydrated cement paste
We can see that water in hydrated cement is held with
varying degrees of firmness and can be
classified as:
1. Free water: present in the capillary pores – with weak
firmness, and evaporate quickly
leaving the paste.
2. Chemically combined water: form a definite part of the
hydrated compounds.
3. Gel water: which found a) Gel pores
b) Part of it is held by the surface force of the gel
particles - It is known as the adsorbed water.
14. Test on physcial properties
Fineness. Fineness, or particle size of portland cement
affects rate of hydration, which is responsible for the
rate of strength gain. ...
Soundness. ...
Consistency. ...
Setting Time. ...
Compressive Strength. ...
Heat of hydration. ...
Loss on Ignition. ...
Specific gravity (relative density)
15. Different grades of cement
The grade 43 and 53 in cement mainly corresponds to the
average compressive strength attained after 28 days ( 6724
hours) in mega pascals (Mpa) of at least three mortar cubes (
area of face 50 cm squared) composed of one part cement, 3
parts of standard s and ( conforming to IS 650:1966) by mass
and P/4 ( P is the percentage of water required to produce a
paste of standard consistency as per IS standard) + 3
percentage ( of combined mass of cement plus sand) of water ,
prepared, stored and tested in the manner described in
methods of physical test for hydraulic cement.
721 hr not less than 23 MPa for 43 grade, 27 MPa for 53
grade
1682 hrs not less than 33MPa for 43 grade, 37MPa for 53
grade
6724 hrs not less than 43MPa for 43 grade, 53 MPa for
53grade.
