1.

2 Cement
• is pulverized material that develops binding force
due to a reaction with water
• It is binding material
• Function of cement is, first to bind the aggregates
together, and second to fill the voids between
aggregate particles to form a compact mass
• It is only about 10% by vol. of concrete mix, but is
active portion of binding medium and only
scientifically controlled ingredient of concrete
• The inventor, Joseph Aspdin, called it Portland
cement because when it hardened, it produced a
material similar to stone from the quarries near
Portland in England
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1.2.1 Oxide & Compound composition
• the raw material is always in the form of oxides in nature
• The range of % of various oxides which are used in cement
are:
Oxide % Content(Avg.)
1. Lime, CaO 60 – 65 (63)
2. Silica, SiO2 17 – 25 (20)
3. Alumina, Al2O3 3 – 8 (6.3)
4. Iron Oxide, Fe2O3 0.5 – 6 (3.6)
5. Magnesia, MgO 0.5 – 4 (2.4)
6. Sulphur Trioxide, SO3 1.3 – 3 (1.5)
7. Alkalis, Na2O, K2O……… 0.5 – 1.3 (1.0)
• After heating raw materials in kiln, the oxides get converted
into Silicates and Aluminates in addition to some other
compounds
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 1.2.1 Oxide & Compound composition
Cement Compounds Chemical formula %
Tri–Calcium Silicate(C3S) 3CaO.SiO2 – 39-50%
Di-Calcium Silicate(C2S) 2CaO.SiO2 – 20-45%
Tri-Calcium Aluminate(C3A) 3CaO.Al2O3 – 8-12%
Tetra-Calcium Alumino Ferrite(C4AF)-4CaO.Al2O3.Fe2O3 – 6-10%
• Out of these compounds, strength of cement is
contributed mainly by Silicates
• It is common practice in Cement Industry to compute
the compound composition of Portland cement from
oxide analysis by using a set of equation, which is
developed by R. H. Bogue

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1.2.1 Oxide & Compound composition
• Bogue equation for estimating the theoretical or the
potential compound composition of Portland cement
are:
• %C3S = 4.071C – 7.60S – 6.718A – 1.430F – 2.850 𝑆
• %C2S = 2.867S – 0.7544C3S
• %C3A = 2.650A – 1.692F
• %C4AF = 3.043F
Where,
C = CaO, S = SiO2, A = Al2O3, F = Fe2O3 & 𝑆 = SO3
• Silicates react with water to produce a gel called
Calcium Silicate Hydrate or C-S-H gel

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1.2.1 Oxide & Compound composition
Q. Find cement compounds or Bogue composition, if
oxides are used in % as follows:
i) CaO = 63, SiO2 = 20, Al2O3 = 6.3, Fe2O3 = 3.6, MgO
=2.4, SO3 = 1.5
[Ans: C3S = 52.73%, C2S = 17.58%, C3A = 10.60%, C4AF = 10.95%]

ii) SiO2 = 20.6, Al2O3 = 5.07, Fe2O3 = 2.9, MgO =1.53,

SO3 = 2.53, K2O = 0.73, Na2O = 0.15,CaO = 63.9
[Ans: C3S = 58.1%, C2S = 15.6%, C3A = 8.5%, C4AF = 8.8%]

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 1.2.2 Hydration of cement
• The chemical reaction of cement with water is
termed as hydration of cement which changes the
cement into the binding material with strong
adhesive property
• Hydration of cement is irreversible process
• various compounds present in cement reacts
individually with water to produce various hydration
products. Major are C-S-H (C3S2H3) gel and CH
(Ca(OH)2)
Understanding the basics of hydration is important to
• Ensure the strength & durability of concrete
• Recognize and mitigate the stresses to prevent
cracking
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 1.2.2 Hydration of cement
1) Hydration of Silicates
2C3S + 6H -> C-S-H gel + 3CH + 114 KJ/mole
100% 24% 75% 49%
(61%) (39%)
2C2S + 4H -> C-S-H gel + 3CH + 43 KJ/mole
100% 21% 99% 22%
(82%) (18%)
Where, H – H2O, C-S-H =C3S2H3, CH = Ca(OH)2,
C = CaO, S = SiO2
• These reactions are exothermic reactions with lot of
heat generation
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 1.2.2 Hydration of cement
1) Hydration of Silicates
C3S:
• Mainly contribute the early strength of cement
• Produce high heat of hydration
• Quality/density of C-S-H gel produce is slightly inferior
than from C2S
• Produce more Ca(OH)2 than C2S
C2S:
• Mainly contributes the later strength of cement
• Produce low heat of hydration
• C-S-H gel produce is dense and with high specific surface
• Produce less Ca(OH)2 than C3S
• More durable than C3S in acidic & sulphur medium
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 1.2.2 Hydration of cement
2) Hydration of C3A
2C3A + 6H -> 2C3AH6
• The product C3AH6, hydrate calcium aluminate is
responsible for flash setting of cement & does not
contribute in strength
with presence of Gypsum
C3A + 32H + 3CaSO4 -> C6A 𝑆3H32
C3A + 18H + CaSO4 -> C4A 𝑆H18
Where, C = CaO, A = Al2O3, C6A 𝑆3H32= tri-sulphate
hydrate (ettringite), C4A 𝑆H18 = monosulphate hydrate
(ettringite)
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 1.2.2 Hydration of cement
2) Hydration of C3A
• Strength contribution of this compound is negligible
• High heat of hydration
• Very fast reaction leading flash set of concrete, to
control its fast setting character gypsum is added to
cement
• Produce C3AH6 harmful for durability as is likely to
attack by Sulphur

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 1.2.2 Hydration of cement
3) Hydration of C4AF
C4AF + 2CH + 10H -> C3AH6 + C3FH6
-> C6AFH12
• C4AF is a minor constituents of cement
• Does not contribute to strength or negligible
• Produce high heat of hydration
• Show more resistance to sulphate attack than C3A
• Control the color of the cement
• C3AH6 - hydrate calcium aluminate
• C3FH6 - hydrate calcium ferrite
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 1.2.2 Hydration of cement
• Comparision of the characteristics of the four
compounds of cement
C3S C2S C3A C4AF

Rate of Hydration moderate slow fast moderate

Early strength high low low low

Ultimate strength high high low low

Heat of hydration Medium Low High Medium

Typical(Cal/g) 120 60 320 100

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 1.2.2 Hydration of cement
C-S-H gel
• Is the most important & major product obtained
from hydrate
• It covers nearly 50-60% solid volume in completely
hydrated paste
• It shows poorly crystalline fibrous mass
• Exhibits the strength & binding property to concrete

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 1.2.2 Hydration of cement
Ca(OH)2
• It is unwanted or undesirable product in concrete,
but it covers nearly 20-25% volume & it shows
distinctive hexagonal prism morphology
• Reacts with sulphate present in water/ environment
to form calcium sulphate, which reacts with C3A
causing deterioration in concrete called sulphate
attack
• It is alkaline in nature, ph nearly 13 & resist corrision
of reinforcement

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 1.2.2 Hydration of cement
Sulphur Attack Chemistry
• Ca(OH)2 + sulphur compound = CaSO4
• C3A + 32H + CaSO4 = C6A 𝑆3H32 (ettringite)
• Ettringite cause large volume changes and
deterioration in hardened concrete

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 1.2.2 Hydration of cement
Mechanism of Hydration
Two approach of mechanism of hydration of
cement:
1) Through solution hydration
• Anhydrous cement compounds dissolves into
their ionic form and reacts with water to form
hydrated compounds having low solubility
• The hydrated products get precipitated from
the super saturated solution

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 1.2.2 Hydration of cement
 Mechanism of Hydration
2) Solid state hydration
• The water reacts with cement compounds at its surface in
solid state
• So, reaction takes place on compounds without the
compounds going into solution
• The reaction proceeds to the inner part of compounds with
time producing the hydrated compounds
 In hydration of cement, first mechanism is dominant in the
early stages due to large quantity of water available.
 At later stages, second mechanism is proceeds as ionic
mobility in the solution becomes restricted
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 1.2.2 Hydration of cement
 Water requirement for hydration
• C3S requires 24% & C2S requires 21% of water by
weight of cement, for their hydration
• In average 23% of water required for the chemical
reaction with cement is called Bound Water
• Certain quantity of water also can be within the gel
pore called Gel Water & about 15% of water is requires
to fill up gel pores
• It has been found that if water require to fill up the gel
pores is inadequate, formation of gel itself will stop,
bond water & gel water are also called Complementary
• Therefore, total 38% water required for full hydration
• Additional water will cause unnecessary capillary pores
in cement paste
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 1.2.2 Hydration of cement
Heat of hydration

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 1.2.2 Hydration of cement
Heat of hydration
• Hydration of cement is exothermic
• The rate of hydration of various compound in
cement is different, thus producing different
amount of heat during the way of reaction
• Heat liberation at peak A, is due to rapid
hydration of C3A/C3S
• The reduction in heat liberation occurs due to
retardation of C3A caused by gypsum
• Peak B is due to the reaction of C3S/formation of
ettringite and this rate of heat evolution also
decreases with time

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 1.2.2 Hydration of cement
Rate of hydration

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 1.2.2 Hydration of cement
 Rate of hydration
• C3A reacts with water very fastly in flash setting i.e.
stiffening without strength development.
• C-A-H phase prevents the hydration of C3S & C2S. But some
gypsum dissolves immediately in water & sulphate ions in
the solution reacts with C3A to form a protective membrane
& thus retards the hydration.
• When all sulphate is consumed hydration accelerates by
reaction of water with silicates
• Therefore, C3S becomes almost solely responsible for the
strength gaining upto 7 days by C-S-H gel
• The later age increase in strength is due to the hydration of
C2S
• Therefore, the rate of strength development can be
modified by change in the relative quantities of compounds
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