The chemical composition of cement

This presentation covers the chemical constituents of Portland cement (PC) and the effects and properties of each of the main and minor compounds that make up the (PC). Their typical ranges in PC and in various types of PC. (edited)

1. •The constituents of cement
•Properties of the constituents
•Why is it essential to control or monitor
composition of cement?

2. Introduction
Cement, in general, obtains its strength from chemical reaction with water.
This process is better comprehended by a greater insight of the chemical composition of cement.
Cement is mainly composed of lime, silica, alumina and iron in their oxide forms, these compounds
go through interactions with one another in very high temperatures to form more complex
compounds which make up the cement. The relative proportions of the mentioned oxides have a
direct influence on the various properties of cement, furthermore different proportions of the
constituents can produce different types of cement along with specific additives or different
admixtures used for different purpose cements. It is worth mentioning, the precise control of the
chemical constituent proportions have also made it possible to build up strength of cement to a
relatively great extent.

3. The Material Constituents of Cement
•The raw materials of cement are mainly composed of
calcareous and argillaceous substances namely limestone
or chalky materials (CaCO
3) and silica, alumina occurring
in clay or shale materials among others such as iron
oxides. An average raw mix composition is ~80%
limestone and ~20% clay.
•Mineral analysis performed for the rocks to ensure correct
amounts of materials. For a factory to acquire a consistent
cement product, close control of the chemistry of the
product is necessary to ensure that the same mixture of
minerals is used every time, therefore the exact
composition of the raw materials is determined to see
whether other ingredients are needed.
•After they are fed into mill together, they are made to
undergo heating during which the raw material
compounds break down to oxides that with further heating
react with each other to form the main cement
compounds, releasing carbon dioxide in the process.
Hence the key raw materials of cement can be considered
as lime (CaO), silica (SiO
2), alumina (Al
2O
3) and iron
oxide (Fe
2O
3).
Limestone rock
Limestone powder
Shale rock
Boulderclay

4. Typical Chemical composition of Cement
(ASTM, UK & European standards)
Approximate limits of PC main constituents
Oxide Abbr.symbol* Percent content
CaO C 60-67
SiO
2 S 17-25
Al
2O
3 A 3-8
Fe
2O
3 F 0.5-6
Typicalpercentage ranges
of minor constituents
MgO 0.1-4
Alkalis 0.2-1.3
SO
3 1-3
TiO
2 0.1-0.4
P
2O
5 0.1-0.2
LOI Upto 3
Insoluble
residue
<= 0.75
❖The alkalis in general, refer to K
2O & Na
2O
content.
❖The main constituents (oxides) constitute
approximately 95% of clinker (but they’re found
in combined form as the clinker compounds.
*The shorthand notation is specifically used to describe cement
constituent oxides

5. The functions and effects of the Oxide
Constituents (Major Oxides + Minor Oxides)
Lime (CaO) :
•Major ingredient in cement
•Reacts with silica to form the main strength
imparting mineral compounds in cement (C
3S,
C
2S)
•In less than required quantities, cement will
decrease in strength and causes quick set with
actual development of strength
•If it’s present in excess quantities, can make
the cement unsound (causes expansion of
cement and hence disintegration may occur)
Silica (SiO
2) :
•Major ingredient giving strength to cement
•Responsible for formation of calcium silicates
(C
3S, C
2S)) which are the basic compounds
imparting strength to cement
•Excess quantities will cause slow set

6. Alumina (Al
2O
3) :
•Main ingredient in cement
•Provides quick setting time to cement
•Excess quantities will weaken the cement
•Presence of sufficient amount lowers the
clinkering temperature
•With CaO combines to form C
3A
compound
Iron Oxide (Fe
2O
3) :
•Also, a main ingredient in cement
•Provides color to cement
•Acts as a flux helping the fusion of raw
materials during burning phase
•At very high temperatures brings about
chemical reaction with alumina and lime
to form C
4AF compound
•Contributes to hardness and strength of
cement (by means of the above
compound)

7. Magnesia (MgO) :
•Minor constituent
•Percentage should be low. Studies have confirmed that presence of about 2% MgOcan improve combustion of raw
materials (accelerating clinkerization reactions), encourage absorption of free lime and formation of C
3S (lowering
formation temp. of C
3S), thus increasing the strength development and shortening setting time.
•The amount of MgO limited between 4-6 %
•Small quantities also contribute to the color of the cement
•Excess magnesia, (when the content ofMgOis more than 3.0%), can reduce the burnability of raw meal, increase the
content of free lime in clinker and reduce strength of cement and also extends the setting time
•One of the causes of unsoundness in cement due to its tendency to cause expansion after hydration
MgO + H
2O Mg(OH)
2+ Expansion
•Mg(OH)
2and Ca(OH)
2which is relatively soft material, large crystal, soluble in water and low surface area, and causing
expansion of the product, internal stress that lead to cracks and lead to unsound cement
•However, MgOpowder additive used in concrete (MgOconcrete) for its effect of shrinkage-compensation. Most of the
volume expansion of MgOin concrete occurs at late ages (after 7 days). . The expansion of MgOconcrete therefore closely
matches the shrinkage of (mass) concrete as it cools

8. Sulfur Trioxide (SO3) :
•Minor constituent
•Excess SO
3causes unsoundness of cement. Findings of a previous experiment to address the effects of increasing SO
3content
of cement on performance and durability of concrete indicated that increasing SO
3increases expansion in lime and sulfate
•Excess of SO
3
-
causes expansion and cement unsoundness. SO
3
-
reacts slowly with C
3A to form ettringite and expansion follows
SO
3
-
+ C
3A ettringite + Expansion
•SO
3has a close relationship with alkalis, forms a liquid phase with alkalis which immiscible with main clinker liquid (molten
C
3A & C
4AF). Upon cooling forms crystals of alkali sulfates
•Insufficient SO
3to combine with the alkali oxides can be associated with higher temperature needs for stabilizing C
2S (above
1250°C) thus also impeding formation of C
3S.
•Excess SO
3 over alkalis also hampers the formation of C
3S, increasing the temp. of formation of it, requiring hard burning to
lower free lime level
•Deficiency of SO
3 is associated with increased C
3A activity and difficulty in obtaining satisfactory early stage fluidity and
flowability of concrete
•ASTM C150-05 & BS EN 197-1 specify the amount of gypsum as the mass of SO
3 present
•SO
3content should not exceed ≤3% (in cement). The vulnerability of cement to SO3 attack can be reduced by the use of cement
low in C
3A.
(i.e. Sulphate resistance PC C
3A= 1-5 %).
•The sources of SO
3are:
» raw material (limestone & clay)
» firing (some SO
3come from fuel during cement manufacture
» gypsum added at the grinding stage.
•Iraqi specification (No. 5) & British Standard (No. 12) limited the amount of SO
3according the percentage of C
3A
SO
3≤ 2.5% C
3A<5% for S.R.P.C
SO
3≤ 2.8% C
3A > 5 for O.P.C

9. Alkalis :
•Mainly referring to Na
2O and K
2O
•Should be present in very small amounts (typically <=1%)
•Has a strong tendency to unite with SO
3to form alkali sulfates which can condense in lower
temp. regions forming build-ups and blockages
•Excess alkaline matter can later result in efflorescence and increases drying shrinkage of
hardened paste
•Have the tendency to increase early compressive strength but later CS is reduced
•Responsible for AAR (alkali-aggregate reaction) causing disintegration of concrete (substantial
expansion). Cement used under such circumstances often has their alkali content limited to
0.6%
•Content should not exceed 1.3%

10. Free lime :
•As the burning temperature of raw materials fed into kiln to produce clinker increases the lime
combination does also, but a percent of it that remains uncombined (preferably a tiny percentage)
hence called as free lime
•In cement factories, some incidents may lead to (uncombined lime) free lime such as inappropriate
dosage of the raw mixture, an instant shutdown of the oven, a reduction of burning of the clinker or a
slow cooling of the clinker.
•Experiments have shown that water demand increases according to the increase of the percentage of
the free lime
•Causes unsoundness (expansion) of cement. This is due to rise in formation of Ca(OH)
2.
CaO + H
2O Ca(OH)
2+ Expansion + heat
•Free lime content limited between (0-4)%
•The amount of free CaO used as a measure of cement manufacture efficiency (if CaO free = 0% this
mean good manufacture)
•Also the mechanical strength of 2, 7 and 28 days decreases according to the increase of the free lime.

11. Some other inclusions in cement composition
Loss on ignition (LOI) :
•Can be determined (according to ASTM C114) by heating
up a cement sample to 900 –1000°C until a constant
weight is obtained. The weight loss of the sample due to
heating is then determined.
•A high LOI can indicate prehydrationand carbonation,
which may be caused by exposure to atmosphere due to
improper and prolonged storage (e. g. clinker stored
outside in wet conditions) or adulteration during transport
of clinker
•It can be an indicator of the quality of the final product
Insoluble Residue :
•Commonly refers to free silica left in cement after
combustion
•It is not desirable oxide in cement but it is not harmful
such as CaO
•IR is a measure of contamination of cement arising from
impurities in gypsum or mainlyfrom silica that has not
reacted during the burning process in the kiln.
•It’s inert substance indicating the completeness of the
reactions inside kiln
•The amount of free SiO
2is a measure of cement
manufacture efficiency
•For (IR) ASTM C150 limit is 0.75, BS EN 197-1 limit is
1.5% of mass of cement and filler. Can be determined by
HCl. It’s the fraction of cement that is insoluble in HCl

12. CaSO
4:
•Calcium Sulfate (CaSO
4) present in cement in the form of
added gypsum.
•Intentionally added to retard the setting action of cement
(prevents flash set due to C
3A).
•The gypsum amount added to clinker is essential and
depends on C
3A content. For example increasing fineness
has the effect of increasing C
3A available and this
increases gypsum requirement. Gypsum can react with
both C
3A & C
4AF
•CaSO4 content varied between 2.1-4.6 % for various
types of PC
Chloride, Cl
-
:
•Chlorides can also be a commonly undesirable
presence.
•React with C
3A in a manner similar to that of
SO
3causing expansion, but to lower extent,
due to the formation of calcium chloro-
aluminate. Cl
-
also accelerates the reaction of
C
3A with gypsum
•Alkali chlorides are very volatile and may
cause build-ups & blockages.
•Chlorides are also behind the corrosion in steel
(reinforcement)
•Fast reaction with C
3A which is desirable to
minimize chlorides in cement leading to lesser
chance of occurrence of corrosion of steel

13. Water:
•The main source of H
2O is gypsum that contains 21% of water.
•During grinding stage when gypsum inter ground with too hot clinker some of this water is lost.
CaSO
4.2H
2O CaSO
4.1/2H
2O + 1.5H
2O
40 +32 +16x4 + 2x18= 172
2x18/172 = 21%
•When cement is mixed with water these hemi-hydrated gypsum is hydrated to form needle-shaped
crystal of gypsum causing false of set of cement
•Another source is combined water during storage of cement, water comes from atmosphere
combined with C
3S, C
2S, C
3A, C
4AF, free lime, MgO and this is not desirable, cement losses it's
binding properties after combined with water.
•The loss on ignition shows the extent of carbonation and hydration of free lime and free magnesia
due to the exposure of cement to the atmosphere. The max loss on ignition at 1000
o
C permitted by
BS and ASTM is 3% and 4% in Iraqi Specification

14. Gypsum for Retardation of Setting
•The clinker produced in rotary kiln makes cement which is naturally very quick setting. In order to retard
its set sufficiently to enable it to meet commercial requirements, gypsum is added to cement clinker.
•Gypsum and C3A react to form insoluble calcium sulphoaluminate
ettringite (3CaO.Al
2O
3.3CaSO
4.31H
2O)
•As more C3A comes into solution, the composition changes, the sulphate content decreasing continuously
monosulphate (3CaO.Al
2O
3.CaSO
4.12H
2O)
•From the structural point of view the formation of these products is useful during the early stage of
hydration, in consequence, the size of pores in hydrated cement paste is reduced and strength is increased.

15. Optimum Gypsum Content (OGC):
•Minor compounds in general, are harmful (causing
disruption/disintegration) to cement when their content
exceeds a certain amount. Gypsum has the potential to
react with many of them so it is imperative to control its
quantity.
•OGC is the quantity of gypsum that gives maximum
strength, minimum shrinkage and expansion
•Can be determined also by monitoring the generation of
heat of hydration. If the amount of gypsum added is
correct, there will be little C3A left to react after all the
gypsum has combined
•OPC depends on:
1.SO
3content (increase with increasing C
3A)
2.fineness of cement (increasing with increasing in
cement fineness)
3.alkali content (increase)
4.curing temperature (increase)
5.curing time (increase)
6.free lime and MgO (decrease)
7.chloride content (decrease)
SO
3%
OGC
Strength
SO
3%
OGC
Shrinkage in air
SO
3%
OGC
Expansion in water

17. Principal Compounds in PC
❖The mainconstituent oxides interact with each other at high temperatures to form more
complicated compounds referred to as clinker minerals denoted by the simplified nomenclature
C
3S, C
2S, C
3A and C
4AF.

18. Further illustration of the reactions forming the clinker compounds are as follows:

19. ❖As mentioned, the PC clinker contains four principal compounds. The composition of the
compounds and their typical ranges according to US, UK & European PC clinker is as follows;
Name of
compound
Chemical Formula
Abbreviated
Appellation
Mineral
Name
Typical
Range (% by
mass)
Typical
Quantity (%
by mass)
Tricalcium
silicate
3CaO.SiO2 C
3S Alite 46-65 59
Dicalcium
silicate
2CaO.SiO2 C
2S Belite 10-30 16
Tricalcium
aluminate
3CaO.Al2O3 C
3A Aluminate 5-12 9
Tetracalcium
aluminoferrite
4CaO.Al2O3.Fe2O3 C
4AF Ferrite 6-12 10

20. Basic Properties of Cement Compound
Compound C
3S C
2S C
3A C
4AF
Chemical composition 3CaO.SiO
2 2CaO.SiO
2 3CaO.Al
2O
3 4CaO.Al
2O
3.Fe
2O
3
Rate of hydration Rapid (hours) Slow (days) instantaneous Very rapid (min)
Strength development Rapid (days) Slow (weeks) Very rapid (one day)
Very rapid
(one day)
Ultimate strength
High
(ten of N/mm
2
)
Probably high (tens N/mm
2
) Low (few (N/mm
2
) Low (few N/mm
2
)
Heat of hydration
Medium
500 j/gm
Low
250 j/gm
Very high
850 j/gm
Medium
420 j/gm
Remarks
Characteristic constitute of
P.C
-
Unstable in water & sensitive to
sulphateattack
Imports to the cement its
characteristic gray colour
Compound limits in P.C
(wt%)
20 -70 % 8-50 % 0-15 % 5-15 %
Compound content limit in
O.P.C. wt %
35-55 % 15-35 % 7-15 % 5-10 %

21. Attributes of the clinker compounds
C
3S :
•Found in highest quantity in cement
•Has a major effect on strength of cement
especially early strength of cement as it
hardens quite rapidly. Increase in percentage
will result in higher early strength
•The higher the quantity of C
3S, the higher the
heat of hydration and gain of strength
•Occurs in small equi-dimensional colorless
grains
C
2S :
•Present less than C
3S in cement
•Major effect on long-term strength.
Responsible for strength development mainly
beyond one week.
•Reacts slowly with water
•Occurs in 3 different forms; αC
2S, βC
2S and
γC
2S. αC
2S occurs at higher temperatures and
at further temperature rise changes to βC
2S.
Then at cooling it undergoes change to γC
2S.
However at conventional cooling rate of OPC,
the βC
2S is preserved in the clinker. It forms
round grains

22. C
3A :
•Found in relatively small percentage
•Forms rectangular crystals
•Generally, the presence of C
3A in cement is undesirable
•Contributes little to the strength except at early stage due
to fast hydration
•Fast hydration when coming into contact with water
causing almost immediate stiffening of the paste called
flash-set. Gypsum added to prevent this
•Fast hydration of C
3A also causing higher heat of
hydration. High rate of heat development may lead to
cracks
•Gives the cement poor sulfate resistance. When cement
paste attacked by sulfates, sulfo-aluminate formed from
C
3A can cause disruptions in hardened paste such as
volumetric changes due to drying shrinkage
•Cement with low C
3A content generates less heat,
develops higher strength ultimately and has better
resistance to sulfate attacks
•However, acts as a flux, can reduce the clinkering
temperature
•Found in rectangular crystals
C
4AF :
•Found in relatively small percentage
•Acts as a flux also reducing clinkering temperature. If it
wasn’t present the reaction in the kiln would be much
more slower and would probably be incomplete
•Contributes very little to strength but its presence
accelerates hydration of silicates
•Responsible for the gray color of OPC
•Found as solid solution

23. Thus the formation of cement can be displayed schematically in the following
pattern ;
Component elements
Component oxide
Cement Compounds (on burning)
Portland cement
Ca Si O
2 Al Fe
CaO SiO
2 Al
2O
3 Fe
2O
3
Various kinds of Portland cement
C
3S C
2S C
3A C
4AF

24. The control ratios of clinker composition
The optimization of clinker composition in cement plants can be controlled by three ratios;
1. Lime Saturation Factor (LSF) 2. Silica Ratio (SR) 3. Alumina Ratio (AR)
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•LSF is the most critical ratio computed by ratio of lime to silica, alumina and iron oxide. It rules the proportions of
C
3S & C
2S. LSF greater than 100% leads to excess lime which cannot be combined and stays as free lime. As low level
of free lime (preferably less than 2%) must be achieved the LSF for a clinker would normally be in the range of 95-98%.
By studying the influence of LSF on the content of C
3S & C
2S, a change of 1% in LSF (at a given percent of free lime)
translates into ~2% change in C
3S meaning that variability of C
3S is approximately double that of LSF.
•SR is the ratio of silica to alumina and iron oxide. For a certain LSF, the higher the SR the higher the C
3S formation and
the lower the C
3A & C
4AF formation. It is of great significance to note that the higher the SR the less the flux (molten
liquid) will be which makes clinker combination more difficult unless LSF reduced to compensate.
•AR value can mostly define the quantity of liquid phase formed when partial melting first happens. High AR will have
high C
3A content which can be disadvantageous in some circumstances (e. g. when needed to minimize concrete
temperature rise). Luckily, AR ratio can be easily controlled by adding small amounts of iron oxides to the mix.
for optimal clinker, the standard ratios have been designated as;
LSF = 95-97%SR = 2.4-2.6 AR = 1.5-1.8

25. The percentage of clinker compounds can be calculated by means of a method proposed by Bogue in 1929
which involves the following assumptions;
•Entirety of Fe
2O
3is combined as C
4AF
•The remaining Al
2O
3(after deducting the portion combined in C
4AF) is combined as C
3A
The mathematical formulas expressed as follows
*
:
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2−6.718??????&#3627408473;
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3)
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2&#3627408454;=2.867&#3627408454;????????????
2−0.753??????
3&#3627408454;
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3??????=2.65??????&#3627408473;
2??????
3−1.692??????&#3627408466;
2??????
3
??????
4????????????=3.043??????&#3627408466;
2??????
3
* These calculated values may not be accurate. They may not be in total agreement with proportions of them determined
through quantitative X-ray diffraction.
Calculation of percentage of clinker compound
Note:Minor compounds such as
MgO, Mn2O3, K2O and Na2O
usually amount to not more than
few percent of wt. of cement.

26. Example
Calculate the percentage proportion of
main compounds in cement if the oxide
percentages are as shown in the table:
Solution:
C3S = 4.07 (CaO –free CaO) –(7.6 (SiO
2) + 6.72 (Al
2O
3) + 1.43 (Fe
2O
3) + 2.85 (SO
3))
= 4.07 x (64.4 –0.8) –7.6 x 20 –6.72 x 5.8 –1.43 x 3.2 –2.85 x 2.6)
= 56 %
C2S = 2.87 (SiO
2) -0.753 (C
3S)
= 2.87 x 20 –0.753 x 56
= 15.1 %
C3A = 2.65 (Al
2O
3) -1.69 (Fe
2O
3)
= 2.65 x 5.8 –1.69 x 3.2
= 10 %
C4AF = 3.04 (Fe
2O
3)
= 3.043 x 3.2
= 9.7 %
Oxide Composition (per cent)
CaO(lime)
SiO
2(Silica)
Al
2O
3(alumina)
Fe
2O
3(iron oxide)
SO
3(sulphurtrioxide)
MgO(Magnesia)
Na
2O (Soda)
K
2O (potash)
LOI (loss on ignition)
Free lime CaO
64.4
20
5.8
3.2
2.6
1.8
0.4
0.3
0.8
0.8
Note:
Total % of the main compound = 56+15.1+10+9.7 = 90.8%
Total % of secondary compounds = 9.2 %
The secondary compounds include
(LOI + Na
2O+K
2O+MgO+CaO free +CaSo
4)
CaSO4 = 1.7 SO
3

27. Impact of change in oxide composition on
compound composition of cement
Let’s observe the effect of changes in oxide percentages on cement compounds of type III PC;
❖Column (1) shows typical data for type III cement, column (2) shows changes when 3% lime decreased while others
increased
If the lime content is decreased by 3% and other oxides increased as shown in the above table in the
raw materials of rapid hardening (type III) cement, the percentage of C
3S decreases considerably and
that of C
2S sees a huge increase
Oxide
Percentagein cement
Compounds
Percentage of compounds
(1) (2) (1) (2)
CaO 66.0 63.0 C
3S 65.0 33.0
SiO
2 20.0 22.0 C
2S 8.0 38.0
Al
2O
3 7.0 7.7 C
3A 14.0 15.0
Fe
2O
3 3.0 3.3 C
4AF 9.0 10.0
Others 4.0 4.0

28. Similarly, if alumina is decreased from 7% to 5.5%, iron oxide increases from 3% to 4.5% while lime
and silica kept unchanged, the C3S content increases and that of C2S decreases while C3A & C4AF
contents also highly affected as shown below:
❖Column (3) shows a change of 1.5% in alumina and iron oxide contents compared with the values of typical
contents of a type III cement shown in column (1)
Oxide
Percentagein cement
Compounds
Percentage of compounds
(1) (3) (1) (3)
CaO 66.0 66.0 C
3S 65.0 73.0
SiO
2 20.0 20.0 C
2S 8.0 2.0
Al
2O
3 7.0 5.5 C
3A 14.0 7.0
Fe
2O
3 3.0 4.5 C
4AF 9.0 14.0
Others 4.0 4.0

29. Compound composition of different types of PC
The below tabulation shows average composition of various types of Portland cement;
Typeof
cement
Compoundcomposition %
C
3S C
2S C
3A C
4AFCaSO
4
Free
lime
MgO LOI
Type I
(OPC)
avg 49 25 12 8 2.9 0.8 2.4 1.2
Type II
(modified)
avg 46 29 6 12 2.8 0.6 3.0 1.0
Type III
(rapid
hardening
)
avg 56 15 12 8 3.9 1.3 2.6 1.9
Type IV
(lowheat)
avg 30 46 5 13 2.9 0.3 2.7 1.0
Type V
(sulfate
resisting)
avg 43 36 4 12 2.7 0.4 1.6 1.0

30. References
[1] Advanced Concrete Technology part I –Constituent Materials by John Newman & Ban Seng Choo
[2] Concrete Technology by A. M. Neville & J.J. Brooks (2
nd
edition 2010)
[3] Concrete Technology by B. L. Gupta & Amit Gupta
[4] AASHTO T 105 and ASTM C 114: Chemical Analysis of Hydraulic Cement
https://pavementinteractive.org/reference-desk/testing/cement-tests/portland-cement-loss-on-
ignition/#:~:text=Loss%20on%20ignition%20is%20calculated,to%20heating%20is%20then%20determined.
[5] Chemical Composition of Cement
https://constructionhow.com/chemical-composition-of-cement/
[6] https://www.slideshare.net/gauravhtandon1/cement-
72728888#:~:text=Chemical%20Composition%20of%20Portland%20Cement,to%20form%20more%20complex%20compou
nds.
[7] Functions of cement ingredients
https://civiltoday.com/civil-engineering-materials/cement/10-cement-ingredients-with-
functions#:~:text=Alumina%20imparts%20quick%20setting%20property,Excess%20alumina%20weakens%20the%20cement
.
[8] THE EFFECT OF MAGNESIUM OXIDE ON THE PROPERTIES OF PORTLAND CEMENT
https://trid.trb.org/view/95572
[9] Effect of SO3 and MgOon Portland cement clinker: Formation of clinker phases and alitepolymorphism by XuerunLi &
WenlongXu