Civil Engineering Reference
In-Depth Information
In portland cement, the hydration of the C
3
A phase is controlled by
the addition of gypsum. The flash set is thus avoided. The C
3
A phase reacts
with gypsum in a few minutes to form ettringite as follows:
C
3
A + 3C
-
H
2
+ 26H
C
3
A•3C
-
H
32
→
After all gypsum is converted to ettringite, the excess C
3
A will
react with ettringite to form the low sulfoaluminate hydrate. If calcium
hydroxide is present the compound C
4
AH
13
will also form.
C
3
A•3C
-
H
32
+ 2C
3
A + 4H
3[C
3
A•C
-
H
12
]
→
The effect of KOH or C
3
S on the formation of ettringite at different
temperatures up to 80
o
C has been reported.
[3]
Elevated temperatures do not
prevent the formation of ettringite but the amounts depend on the concen-
tration of other additives. Potassium hydroxide retards ettringite formation.
At concentrations above 1 mol/l syngenite is also formed. In the presence
of C
3
S ettringite is formed at all temperatures. Only small amounts of
ettringite are formed in the presence of 0.5 mol/l of KOH and C
3
S.
3.4
The Ferrite Phase
The ferrite phase constitutes about 8-13% of an average portland
cement. In portland cement the ferrite phase may have a variable composi-
tion that can be expressed as C
2
(A
n
F
1-
n
) where O <
n
< 0.7.
Of the cement minerals, the ferrite phase has received much less
attention than others with regard to its hydration and physico-mechanical
characteristics. This may partly be ascribed to the assumption that the
ferrite phase and the C
3
A phase behave in a similar manner. There is
evidence, however, that significant differences exist.
The C
4
AF phase is known to yield the same sequence of products
as C
3
A. The reactions are slower, however. In the presence of water C
4
AF
reacts as follows:
C
4
AF + 16H
→
2C
2
(A,F)H
8
C
4
AF + 16H
→
C
4
(A,F)H
13
+ (A,F)H
3
