Civil Engineering Reference
In-Depth Information
2.0
CALCIUM ALUMINATE CEMENTS
2.1
Basic Reactions
Calcium aluminate cements (CAC) have a wide range of alumina
content (38 to 90%). The chemistry and principle cement-water reactions
for high alumina cement—a widely used non-portland cement—are de-
scribed in detail in Ch. 9. The primary binding phase is calcium
monoaluminate (CaAl
2
O
4
or CA). Refractory cements contain higher
alumina contents (70 to 90%).
In CAC, the CA reacts with water to form a series of calcium
aluminate hydrates. These include CAH
10
, C
2
AH
8
, C
3
AH
6
, and AH
3
(an
amorphous phase). The metastable hydrates, CAH
10
and C
2
AH
8
, convert to
C
3
AH
6
. The following scheme summarizes the conversion reactions.
The conversion that occurs as a consequence of the transformation of the
hexagonal phases, CAH
10
or C
2
AH
8
, into the cubic phase, C
3
AH
6
, is known
to be accompanied by a loss of strength of the hardened alumina cement.
The conversion reactions themselves are described according to
the following equations:
Eq. (1)
3CAH
10
→
C
3
AH
6
+ 2AH
3
+ 18H
Eq. (2)
3C
2
AH
8
→
2C
3
AH
6
+ AH
3
+ 9H
The conversion of CAH
10
to C
3
AH
6
results in a volume decrease to
about 50% whereas that of C
2
AH
8
to the cubic phase results in a decrease
of about 65% of the original volume of the reactants. It is apparent that
methods to identify and determine the amounts of the aluminate hydrates in
CAC concretes are useful for a meaningful diagnosis of potential problems.
