Civil Engineering Reference
In-Depth Information
Figure 3 compares the rates of hydration of C
3
S and C
2
S.
[31]
The
absolute rates differ from one sample to the other. It is general knowledge
C
3
S is much more reactive than
β
-C
2
S.
Figure 3.
The relative rates of hydration of 3 CaO•SiO
2
and 2 CaO•SiO
2
.
(With permission,
Noyes Publications,
Concrete Admixtures Handbook
, V. S. Ramachandran, ed., 2
nd
Ed.,
1995.)
3.3
Tricalcium Aluminate
In portland cement, although the average C
3
A content is
about 4-11%, it influences significantly the early reactions. The phenom-
enon of
flash set
, the formation of various calcium aluminate hydrates, and
calcium carbo-and sulfoaluminates involve the reactions of C
3
A. Higher
amounts of C
3
A in portland cement may pose durability problems. For
example, a cement which is exposed to sulfate solutions should not contain
more than 5% C
3
A.
Tricalcium aluminate reacts with water to form C
2
AH
8
and C
4
AH
13
(hexagonal phases). These products are thermodynamically unstable so
that without stabilizers or admixtures they convert to the C
3
AH
6
phase
(cubic phase). In a paste, hydration is slightly retarded in the presence of
CH. In dilute suspensions the first hydrate formed is C
4
AH
19
.
