Civil Engineering Reference
In-Depth Information
In a hydration of the cement at ambient temperature calcium aluminate ferrite hydrate
[C
2
(A,F)H
8
or CA
2
[(Al, Fe)(OH)
5
]
2.3
H
2
O] is formed as the main product of hydration,
together with some C
3
(A,F)H
6
[or Ca
3
[(Al,Fe)(OH)
6
]
2
]. These are analogues of the
C
2
AH
8
and C
3
AH
6
phases (also formed in the hydration of CA or C
3
A), in which some
of the Al
3+
is substituted by Fe
3+
. With increasing temperature the amount of C
3
(A,F)H
6
formed will increase at the expense of C
2
(A, F)H
8
. Some iron hydroxide gel may also be
formed in the hydration. The hydration progresses rather slowly, attaining a degree of
hydration of about 50% after 28 days. This rate may be increased by adding suitable
additives to the mix, such as triethanolamin or sodium carbonate.
Ferrite cement is used only to a very limited extent. It currently finds use as a binder in
the production of iron ore pellets.
15.6
SULFOFERRITE AND SULFOALUMINOFERRITE CEMENTS
In the system CaO-Fe
2
O
3
-CaSO
4
the sulfate-bearing phases 4CaO.3Fe
2
O
3
.SO
3
(abbreviation C
4
F
3
) and 3CaO.Fe
2
O
3
.SO
3
(abbreviation C
3
F ) exist, as well as
monocalcium ferrite (CaO.Fe
2
O
3
, abbreviation CF) and dicalcium ferrite (2CaO.Fe
2
O
3
,
abbreviation C
2
F).
I
f a starting mix that contains the oxides CaO, Fe
2
O
3
, and SO
3
is burnt to high
temperatures, initially CF is formed in a reaction between CaO and Fe
2
O
3
at temperatures
between 800 and 1100°C. In the presence of CaSO
4
the primary formed CF reacts
further, to yield C
4
F
3
at 950-1200 °C. In mixes with high CaO contents—that is, with
C/F > 2—the CF reacts further with additional CaO at 1100-1200 °C to yield C
2
F, which
in turn may react with CaSO
4
to yield C
3
F.
O
ut of the two calcium ferrite phases, C
2
F hydrates appreciably faster than CF. In
paste hydration about 65% of the former and 10% of the latter hydrate within 28 days at
ambient temperature. Initially, low-basicity hydroferrites are formed as products of
hydration, which contribute to a moderate strength of the hardened paste. However, after
a few days of hydration these convert into cubic C
3
FH
6
and amorphous Fe(OH)
3
, and this
conversion is associated with an almost complete loss of bonding properties.
I
n the hydration of C
3
FS the products of hydration are a ferric AFt phase (ferric
ettringite, C
3
F.3C .31H), C
4
FH
13
, and iron hydroxide [Fe(OH)
3
, abbreviation FH
3
]. In
the hydration of the low-basicity sulfoferrite C
4
F
3
S, initially a ferric AFm phase (ferric
monosulfate, C
3
F.C .12H) is formed, together with some ferric ettringite. However, the
latter phase also subsequently converts to ferric monosulfate. Conversion of the formed
hexagonal calcium hydroferrite into C
3
FH
6
does not occur.
The hydration of both calcium sulfoferrites is much faster than that of calcium ferrites.
Here the C
3
F phase hydrates faster than C
4
F
3
. In paste hydration both of them exhibit a
favorable strength development. The hardening process is accompanied by a distinct
expansion, which is greater in the hydration of C
3
FS (15 mm/m compared with 7 mm/m
for C
4
F
3
).
