Civil Engineering Reference
In-Depth Information
Table 2.1
Composition of ordinary Portland cement
Short
notation
Weight
percentage
Chemical name
Chemical formula
Tricalcium silicate
3CaO.SiO
2
C
3
S
55
Dicalcium silicate
2CaO.SiO
2
C
2
S
18
Tricalcium aluminate
3CaO.Al
2
O
3
C
3
A
10
Tetracalcium aluminoferrite
3CaO.Al
2
O
3
.Fe
2
O
3
C
4
AF
8
Calcium sulfate dehydrate (gypsum)
CaSO
4
.2H
2
O
CSH
2
6
usually present. Calcium silicates are the main component of cement,
accounting for almost three-fourths of its composition. In fact, calcium
silicates are the components responsible for the main cementing proper-
ties. The presence of tricalcium silicate accounts for the main difference
between the early generation of cements and the OPC produced currently.
When mixed with water, the cement constituents undergo a series of
chemical reactions that lead to the hardening of the mix. The reaction of
cement constituents with water is referred to as hydration, and the prod-
ucts of this reaction are referred to as hydration products. Characteristics
of hydration of cement constituents, including the reaction rate, heat of
reaction, and contribution to the resulting strength development are sum-
marised in Table 2.2. As shown, C
3
A and C
3
S are the most reactive con-
stituents, whereas C
2
S has a considerably slower hydration rate. However,
the rate of hydration does not necessarily have a direct influence on the
strength development rate. In fact, as shown in Figure 2.2, the calcium
silicates are responsible for most of the strength development of Portland
cement. C
3
S is responsible for most of the early strength developing in the
first 4 weeks, and both C
3
S and C
2
S contribute almost equally to the long-
term strength of concrete. All Portland cement hydration reactions liberate
heat and thus are exothermic. As a result, concrete heats up continuously
internally during the hydration process. The heat liberation of a typical
Portland cement concrete is estimated to be about 265 J/g and 473 J/g after
3 days and 1 year, respectively.
Table 2.2
Hydration characteristics of OPC constituents
Reaction heat
liberation
Contribution to
cement strength
Cement constituent
Reaction rate
C
3
S
Moderate
Moderate
High
C
2
S
Slow
Low
Initially low but high later
C
3
A + CSH
2
Fast
Very high
Low
C
4
AF + CSH
2
Moderate
Moderate
Low
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