Page 62 - Handbook of Thermal Analysis of Construction Materials

Section 3.0 - Individual Cement Compounds 45

In portland cement, the hydration of the C A phase is controlled by
3
the addition of gypsum. The flash set is thus avoided. The C A phase reacts
3
with gypsum in a few minutes to form ettringite as follows:
– –
C A + 3CSH + 26H → C A•3CSH 32
3
2
3
After all gypsum is converted to ettringite, the excess C A will
3
react with ettringite to form the low sulfoaluminate hydrate. If calcium
hydroxide is present the compound C AH will also form.
4
13
– –
C A•3CSH + 2C A + 4H → 3[C A•CSH ]
3
12
32
3
3
The effect of KOH or C S on the formation of ettringite at different
3
o
[3]
temperatures up to 80 C has been reported. 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 S ettringite is formed at all temperatures. Only small amounts of
3
ettringite are formed in the presence of 0.5 mol/l of KOH and C S.
3
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 (A F ) where O < n < 0.7.
2
n 1- n
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 A phase behave in a similar manner. There is
3
evidence, however, that significant differences exist.
The C AF phase is known to yield the same sequence of products
4
as C A. The reactions are slower, however. In the presence of water C AF
4
3
reacts as follows:
C AF + 16H → 2C (A,F)H 8
4
2
C AF + 16H → C (A,F)H + (A,F)H 3
4
13
4