Civil Engineering Reference
In-Depth Information
structure based on a framework of AlO
4
tetrahedra; some oxygen atoms are shared
between two and others between three tetrahedra.
M
onocalcium aluminate
(CaO.Al
2
O
3
; abbreviation CA) is the main constituent of all
types of calcium aluminate cement, and is mainly responsible for the properties of this
binder. In nearly pure form it may be synthesized by burning an equimolar blend of CaO
and Al
2
O
3
above about 800 °C. In the course of the process other calcium aluminate
phases are also formed as intermediates, most notably C
12
A
7
(
Fig. 10.1). CA is
monoclinic and pseudohexagonal (
a
=0.8700 nm,
b
=0.8092 nm,
c
= 1.5191 nm, =90.3°,
d
=2945 kg/m
3
). It exhibits distinct hydraulic properties and reacts rapidly with water. Its
crystal structure consists of a three-dimensional framework of AlO
4
tetrahedra sharing
corners with Ca
2+
ions in between. An analogous barium compound, monobarium
aluminate (BaO.Al
2
O
3
) is also known, and exhibits similar hydraulic properties as the
calcium analog (Borovkova
et al.,
1992; Raina
et al.,
1992).
P
entacalcium trialuminate
(5CaO.3Al
2
O
3
; abbreviation C
5
A
3
) is probably an
equilibrium phase in the strictly binary system CaO-Al
2
O
3
. It may be produced in a solid-
state reaction from CA and C
3
A under dry conditions.
Figure 10.1
Development of Al
2
O
3
-bearing phases (in mol.%) in the burning
of an equimolar blend of CaO and Al
2
O
3
at 1200 °C.
In normal moisture air it decomposes rapidly to C
12
A
7
and CA. As C
5
A
3
is stable only in
the absence of any humidity and oxygen, it has no practical importance, even though it
may react readily with water.
D
odeca-calcium heptaaluminate
(12CaO.7Al
2
O
3
; abbreviation C
12
A
7
) is a common,
but in most instances only a minor, constituent of calcium aluminate cements. It is cubic
(
a
=1.1983 nm,
D
=2680 kg/m
3
). The ideal structure of C
12
A
7
consists of an incomplete
framework of corner-sharing AlO
4
tetrahedra with a composition of A1
7
O
16
11−
and Ca
2
ions; one additional O
2
− ion is statistically distributed between 12 sites. C
12
A
7
produced
