Civil Engineering Reference
In-Depth Information
et al.,
1992).
With increasing water/cement ratio the rate of the hydration process increases
significantly, but the hydration products remain the same.
In the presence of calcium sulfate and calcium hydroxide, ettringite rather than calcium
aluminates is formed in the hydration of both C
12
A
7
and C
11
A
7
CaF
2
. Small amounts of
monosulfate may also be formed. In pastes made of C
2
S and C
12
A
7
hydrated for 16 years
gehlenite hydrate (strätlingite, C
2
ASH
18
) was also found among the products of hydration
(Hannawayya, 1992).
5.2
ALITE-FLUOROALUMINATE CEMENT
Alite-fluoroaluminate cement, also called regulated set cement (in the USA) or jet-
cement (in Japan), contains tricalcium silicate (50-75%) and calcium fluoroaluminate
(20-30%) as its main constituents. Other constituents may include the ferrite phase,
dicalcium silicate, and free CaO.
Alite-fluoroaluminate clinker can be produced from raw meals containing—in addition
to CaO and SiO
2
—appropriate amounts of alumina and fluorspar (CaF
2
), by burning
them to temperatures of about 1300°C (Chvatal, 1973; Odler and Abdul-Maula, 1980;
Knöfel and Wang, 1994). The added CaF
2
acts as a mineralizer, and allows the formation
of tricalcium silicate at temperatures lower than usual in the production of ordinary
Portland cement clinker. At temperatures above about 1000°C the added fluorspar reacts
with lime and alumina to yield calcium fluoroaluminate. At the same time it prevents the
formation of tricalcium aluminate, which would be readily formed in the absence of
fluorine ions. An appropriate maximum burning temperature is essential, because if the
temperature is too high, C
12
A
7
.CaF
2
loses its stability and converts to tricalcium
aluminate. However, it has also been observed that in the range 1250-1450°C the
reactivity of the C
12
A
7
.CaF
2
phase is dependent on the clinkering temperature, and
increases as this temperature increases. The burning of the clinker may be performed
conveniently in a rotary kiln. The cooling rate in the production process must not be too
high, as rapid cooling promotes the incorporation of SiO
2
into the structure of
C
11
A
7
.CaF
2
, which—in due course—reduces the reactivity of this phase (Kanaya
et al.,
1998).
Unlike ordinary Portland clinker, alite-fluoroaluminate clinker is brownish in color,
rather than dark gray. It typically possesses a very low porosity, and the alite and belite
crystals that are present are distinctly smaller than those in ordinary Portland clinker.
Only some of the fluorine is in the C
11
A
7
.CaF
2
phase; the rest is incorporated into alite
(Odler and Abdul-Maula, 1980).
To obtain alite-fluoroaluminate cement, the alite-fluoroaluminate clinker must be
interground with anhydrite. The amount of anhydrite must be adjusted to the amount of
fluoroaluminate present in the clinker to enable its hydration to ettringite, and is usually
appreciably higher than in ordinary Portland cement.
Upon mixing with water, alite-fluoroaluminate cement exhibits very rapid hydration.
In paste hydration at ambient temperature about 40-50% of the C
11
A
7
.CaF
2
is hydrated
