Concrete Technology 2/2
Aalto University
School of Engineering
Department of Civil and Structural Engineering
Building Materials Technology
Chemical composition of Portland cement
Component
Abbreviation
Content %
CaO
C
60 - 67
SiO2
S
17 - 25
Al2O3
A
3-8
Fe2O3
F
0.5 - 6
MgO
M
0.1 - 4
N+K
0.2 - 1.3
S
1-3
Na2O + K2O
SO3-
Concrete Technology 2
Mineralogical composition of Portland cement
Mineral
Formula
Abbreviation
Name
Concentration
Tricalcium
silicate
3 CaO
SiO2
C3S
Alite
65 – 45 %
Dicalcium
silicate
2 CaO
SiO2
C2S
Belite
30 – 10 %
Tricalcium
aluminate
3 CaO
Al2O3
C3A
Celite
15 – 5 %
Tetracalcium
aluminate
ferrite
4 CaO
Al2O3 Fe2O3
C4AF
Celite
12 – 5 %
Other components
Magnesium oxide
MgO
Gypsum
CaSO4
2 H2O
Alkali oxides
Na2O and K2O
Possible supplementary binders
Free lime
CaOfree
Concrete Technology 2
Composition and properties of clinker
The reaction rate of the cement depends on the ratio
C3S/C2S and the fineness of the cement
The burning process of the clinker is steered by modules
calculated from the oxide composition of the raw mix which
are presented in percent not in mole fractions
Module
Calculation formula
Abbreviation
Variation
range
Hydraulic
CaO/(SiO2+Al2O3+Fe2O3)
HM
2.0 – 2.4
Lime saturation 100 CaO/(2.8 SiO2+1.1 Al2O3+
factor
0.7 Fe2O3)
Kst
LSF
90 - 102
Silicate
SiO2/(Al2O3+Fe2O3+Mn2O3)
SM
1.8 – 3.4
Aluminate
Al2O3/Fe2O3
AM
1.5 – 2.5
Silicic
SiO2/(Al2O3 + Fe2O3)
KM
3-4
Concrete Technology 2
Module
Property
Kst
Compressive strength (Rapid cements 95 – 100)
KM and SM
Sintering temperature
AM
Dosage of gypsum and sulfate resistance of the cement
Calculation of the composition of cement
Chemical analysis of the clinker gives the composition of the
oxides CaO, SiO2, Al2O3, Fe2O3 and SO3 in percent by weight.
The clinker mineral composition is calculated by the Bogue
system of equations
[O] = [B] [K] in which clinker minerals are
Concrete Technology 2
[K] = [C3S C2S C3A C4AF C ]T
are linear combinations of oxides
[O] = [C S A F
]T
When the oxide composition is known clinker mineral
amounts can be solved by the Bogue matrix
[K] = [B]-1 [O]
The elements of matrix [B] are obtained from Bogue’s system
of equations
Concrete Technology 2
C3S = 4.07 CaO -7.60 SiO2 +6.72 Al2O3 -1.43 Fe2O3 -2.85 SO3 -4.07 CaOfree
C2S = 2.87 SiO2 -0.754 C3S
C3A = 2.65 Al2O3 -1.69 Fe2O3
C4AF = 3.04 Fe2O3
CaSO4 = 1.70 SO3
The amount of free lime is taken into consideration as a
correction factor.
If A/F > 0.64 the equations can be used as such to calculate
the clinker composition
Concrete Technology 2
If A/F < 0.64
If Fe2O3 amount is large and C3A amount small additional
C2F is forming. This must be taken into consideration
(C4AF + C2F) = 2.100 Al2O3 + 1.702 Fe2O3
C3A = 0
C3S = 4.071 CaO – (7.600 SiO2 + 4.479 Al2O3 + 2.859 Fe2O3 +
+2.852 SO3)
Concrete Technology 2
Properties of the clinker minerals
The properties to be evaluated
-strength evolution
-final compressive strength (28 d or 91 d)
-hydration heat evolution
-chemical durability
Tricalcium silicate C3S
-fast strength increase
-high final strength
-high hydration heat evolution (500 J/g)
-durable against sulfates
-moderate water consumption during hydration
Normally desirable properties
Rapid cements
Concrete Technology 2
Dicalcium silicate C2S
-slow strength evolution
-high final compressive strength
-low hydration heat evolution (250 J/g)
-durable against sulfates
Desirable properties for
-low heat cements
-thick and large structures
Tricalcium aluminate C3A
-hardening reaction is very fast
-high water consumption during hydration
-low final strength
-very high hydration heat evolution
-is not durable against sulfates
Concrete Technology 2
These are generally undesirable properties
-to hinder the fast setting and hardening of this clinker
component gypsum CaSO4 2 H2O is introduced into the cement
-onto the C3A patches on the surface of a binder particle a layer of
trisulfate (ettringate) 3CaO Al2O3 3 CaSO4 31 H2O is formed
-this ettringite (C3 3H31) layer retards the hydration reaction of
C 3A
Tetracalcium aluminate ferrite C4AF
-slow strength evolution
-low final strength
-high hydration heat evolution (420 J/g)
-durable against sulfates
Gypsum C H2
-necessary component due to the fast reaction of C3A
-used as a setting retarder
-has beneficial effect on concrete strength evolution
Concrete Technology 2
Too high temperature during or after clinker grinding can cause the
evaporation of crystal water in gypsum and a water soluble anhydrite
form of gypsum is formed.
-in the mixer anhydrite gypsum takes the missing crystal water
from the mix water
-this decreases the mix water amount
-concrete experiences a false set and the mixer halts
-after a while the mixing should be started again
-concrete quality is not diminished
Free lime CaOfree
-excess of CaO in the raw mix of the clinker
-the reason for this is usually incomplete sintering of the clinker
-Finnish cements have usually 0.5 – 2 % free lime
-if there is a large quantity of free lime concrete can experience
expansion CaO + H2O
Ca(OH)2
Magnesium oxide MgO
-a slow reaction MgO + H2O
Mg(OH)2
-if a large quantity local expansion of the hardened concrete can
take place
Concrete Technology 2
Alkali oxides Na2O and K2O
-enhances the rate of hydration reactions
-increase somewhat the compressive strength at early age
-decrease the final compressive strength level
-alkali content in cement is represented by Na2Oeq
-Na2Oeq = Na2O + 0.658 K2O
-they can increase pH-value of the pore water in concrete by 1,
that means 10-fold
Concrete Technology 2
Hydration
Hydration = combination of water directly as water molecule
or hydroxyl ion with any substance such as clinker mineral
or lime to form a substance which is nearly insoluble into
water. This can take place in a liquid phase or as a surface
chemical phenomena at the surface area of the mineral and
the liquid.
Concrete Technology 2
1. Hydration products form a semipermeable membrane
over the clinker mineral patch on the surface of the binder
particle. Ca2+-cations penetrate easier through the
membrane than SiO44-anions
2. The space between the membrane and cement particle
becomes oversaturated of Ca2+ and SiO44Concrete Technology 2
3. Water penetrates through the membrane and causes
tension into the membrane which eventually ruptures
4. Solution rich in silicate is extruded into the external
solution which contains Ca25. This causes an immediate reaction to form C-S-H gel in
the form of thin-walled tubes which grow away from the
grain surface. The tubes can have different forms depending
on the shape of the hole in the membrane.
6. Eventually the hole becomes blocked up by new C-S-H
7. Rupture of the membrane takes place in another part of
the membrane
8. Finally the space is so filled with tubes and platelets that
the diffusion is slowed down
Concrete Technology 2
9. In the next phase the reactions begin to take place only as
surface phenomenon
10. At the end of the setting phase only about 15% of the
cement has hydrated to C-S-H gel
The percentage of the clinker composition in the anhydrous
cement particle during hydration is nearly invariant
compared to the original composition. There does not exist
selective hydration if gypsum is used as a retarder to C3A
hydration.
The hydration model presented above suits only to the
silicate clinker minerals C3S and C2S. Hydration of
aluminate clinker mineral C3A together with gypsum is
different and it will be presented later in the course.
Concrete Technology 2
Initial reactions
• Hydration of C3S and C2S
C-S-H + Ca(OH)2
• pH
12.4 – 13 water becomes saturated
• Aluminates begin to react with gypsum and ettringite is
generated
• Reaction products of C-S-H begin to form and a
membrane is generated over C3S and C2S
Induction period
• The rate of reactions is slowing down as C-S-H forms an
semipermeable membrane
Setting period
• Rate increasing stage of the chemical reactions
• Membrane-osmosis theory
Concrete Technology 2
Requirements for a good model to describe
the hydration mechanism of cement
• Describes the structure of the hydration products
developing at different ages
• Describes the changes in the rate of hydration reactions
• Describes the differences in ionic concentration in the
water phase
Ca2+-cations
K+-cations
Na+-cations
OH--anions
SiO44--anions
SO42--anions
Concrete Technology 2
Reaction of C3S and water
• The concentration of Ca2+ and OH- increase immediately
• Maximum values are noticed after 3-4 hours when water
solution reaches the saturation level of Ca(OH)2
• Ca2+ and OH- concentration begin to diminish when the
Ca(OH)2 crystals begin to precipitate at the end of the
induction (dormant) period
• The concentration of SiO44- in the water phase is very
small at the end of the induction period
Concrete Technology 2