Civil Engineering Reference
In-Depth Information
in air of normal humidity always contains small amounts of water incorporated into its
crystal lattice in the form of two OH
−
ions replacing the O
2−
ion. This small amount of
water is essential for the structure to be stable. Upon heating, the amount of water bound
in this way gradually increases up to about 950°C but declines upon further heating, until
the material melts almost anhydrous at about 1400°C. The maximum amount of water
that may be bound within the structure of C
12
A
7
is about 1.3%, which corresponds to the
formula C
12
A
7
H. Alternatively, the O
2−
ion may also be replaced by a pair of halide ions.
The fluoride analog C
11
A
7
.CaF
2
is a constituent of some special cements, and may form
a continuous range of solid solutions with C
12
A
7
. In dry oxygen a C
12
A
7
analog that
contains excessive oxygen may also be formed. Here the excessive amount of oxygen is
present in the form of peroxide (O
2
2−
) ions, substituting for O
2−
. C
12
A
7
reacts rapidly
with water, and exhibits distinct hydraulic properties.
T
etracalcium trialuminate
(4CaO.3Al
2
O
3
; abbreviation C
4
A
3
) may be synthesized by
thermally dehydrating the compound C
4
A
3
H produced by a hydrothermal process. It is
not a constituent of calcium aluminate cements.
D
icalcium aluminate
(2CaO.Al
2
O
3
; abbreviation C
2
A) can be synthesized only under
high-pressure conditions, and is of no practical importance for cement chemistry.
T
ricalcium aluminate
(3CaO.Al
2
O
3
; abbreviation C
3
A) is a regular constituent of
Portland cements. It is not present in calcium aluminate cement. Its structure and
hydration are discussed in section 2.1.3. It is the most reactive of all calcium aluminates.
10.2
CALCIUM ALUMINATE HYDRATE PHASES
The following calcium aluminate hydrate phases may be formed in the hydration of
calcium aluminates.
M
onocalcium aluminate hydrate
(CaO.Al
2
O
3
.10H
2
O; abbreviation CAH
10
) is a
hexagonal phase with a probable ionic structure Ca
3
[Al(OH)
4
]
6
.18H
2
O (
a
=1.644 nm,
c
=0.831 nm,
D
=1730 kg/m
3
). Most of the water in the structure is only loosely bound,
and a loss of water may begin at 80% RH. The compound also loses water upon heat-ing,
and this loss is accompanied by a deterioration of crystallinity. By 200-350°C the
material becomes amorphous, and by 1000 °C it converts to monocalcium aluminate
(CA). DTA curves are characterized by an exotherm at 130-150 °C, associated with the
loss of molecular water, and another one at 290-310°C due to dehydroxylation. If the
water cannot escape easily, the thermal decomposition of CAH
10
progresses differently,
and C
3
AH
6
—together with gibbsite (AH
3
)—is formed instead. The latter is the typical
case, if hardened calcium aluminate cement is heated. CAH
10
is thermodynamically
unstable with respect to C
3
AH
6
.
D
icalcium aluminate hydrate
(2CaO.Al
2
O
3
.8H
2
O; abbreviation C
2
AH
8
) is a
hexagonal phase belonging among the AFm phases. It has a layered structure, which may
be described by the formula [Ca
2
Al(OH)
6
].[Al(OH)
3
(H
2
O)
3
]OH. The Al is octahedrally
coordinated, and the layer thickness is
c
=1.07 nm (
a
=0.574 nm,
D
=1950 kg/m
3
). Upon
drying or heating the material converts to hydrates with lower water content, including
C
2
AH
5
. C
2
AH
8
is also thermodynamically unstable with respect to C
3
AH
6
.
