Environmental Engineering Reference
In-Depth Information
TABLE 4.3
Chemical Composition Ranges for High-Alumina
Cements
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Composition (%)
Grade
Al
2
O
3
CaO
SiO
2
Fe
2
O
3
+FeO
Standard/Low alumina
36-42
36-42
3-8
12-20
Low alumina/Low Iron
48-60
36-42
3-8
1-3
Medium alumina
65-75
25-35
< 0.5
< 0.5
High alumina
> 80
< 20
< 0.2
< 0.2
4.4
HIGH-ALUMINA CEMENTS
High-alumina cements are different from portland cements in that they are composed
of calcium aluminates rather than calcium silicates. There are a number of different
high-alumina cements with their alumina content from 36 to 80%. Scrivener and
Capmas
64
divide them into four types, whose composition is summarized in Table
4.3. The main mineral phase of high-alumina cements is CA. Some possible minor
phases are C
3
(A,F), CA
2
, C
12
A
7
, C
2
AS, and C
2
S. High-alumina cements are mainly
used in refractory concrete, since they can withstand high temperatures, but are often
used in combination with other minerals and admixtures in concrete for construction
to give a wide range of properties, including rapid setting and drying, and controlled
expansion or shrinkage compensation.
64
CA hydrates differently at low and high temperature. The hydration product is
CAH
10
at low temperature (< 20°C) and C
3
AH
6
and AH
3
at high temperature
(> 30°C). The hydration product of C
3
(A,F) is C
3
(A,F)H
6
at low and high tempera-
tures. The hydration reaction can be expressed as follows:
At low temperature:
CA + 10 H
2
O → CAH
10
(4.11)
C
3
(A,F) + 6 H
2
O → C
3
(A,F)H
6
(4.12)
At high temperature:
CA + 12 H
2
O → CAH
6
+ 2AH
3
(4.13)
CAH
10
will convert to CAH
6
and AH
3
at high temperature:
CAH
10
→ CAH
6
+ 2AH
3
+ 18H
2
O
(4.14)
The conversion is accompanied by the increase of porosity and the decrease of
strength of hardened cement paste and concrete. The hydration chemistry, as
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