Civil Engineering Reference
In-Depth Information
within the first 2 hours, and about 70% within 7 days (Uchikawa and Tsukiyma, 1973;
Knöfel and Wang, 1994). The hydration of alite is significantly slower: only about 10%
of it hydrates within 2 hours, and about 40-65% within one day.
The main product of hydration at shorter hydration times is ettringite (AFt phase),
which is formed in a reaction between calcium fluoroaluminate and anhydrite. At the
same time aluminum hydroxide is also formed. The fluorine in the fluoroaluminate phase
enters the structure of aluminum hydroxide, yielding aluminum hydroxide fluoride. The
process can be expressed by the following equation:
(5.1)
Tricalcium silicate hydrates to yield the C-S-H phase and calcium hydroxide, which in
due course may participate in the hydration of the fluoroaluminate phase and the
formation of ettringite:
(5.2)
The formed C-S-H phase varies in its composition, and also contains distinct amounts of
Al
2
O
3
and SO
3
(Uchikawa
et al.,
1978).
In the later stage of hydration, after the supply of calcium sulfate has been exhausted,
some AFm may also be formed at the expense of AFt:
(5.3)
Compared with ordinary Portland cement pastes made with the same water/cement ratio,
the porosity of mature alite-fluoroaluminate cement pastes is distinctly lower, mainly
because of the high combined water content of the resulting ettringite phase.
At ambient temperature, pastes made from alite-fluoroaluminate cement exhibit a
setting time between about 5 and 30 minutes, depending on their C
12
A
7
content, and a
measurable strength within about 2 hours. If desired, the setting time may be extended by
the use of suitable retarders, such as citric acid, oxalic acid, boric acid, calcium sulfate
hemihydrate, or some polyphosphates (Chvatal, 1973; Uchikawa and Uchida, 1973;
Knöfel and Wang, 1994). The short-term strength depends on the SO
3
/Al
2
O
3
ratio of the
cement, and may be adjusted by the amount of gypsum interground with the clinker.
Within the range 0.26-0.51 the strength increases with this ratio; however, further
increase of the ratio results in a drastic strength reduction and uncontrolled expansion
(Knöfel and Wang, 1993).
After a very rapid initial strength development, associated with the formation of the
ettringite phase, the strength increases rather slowly between about 3 hours and one day,
as the formation of additional ettringite is slowed down, and the amount of formed C-S-H
is still low. Subsequently, the strength development accelerates again as more C-S-H is
