Civil Engineering Reference
In-Depth Information
The Portland clinker used should contain a high amount of tricalcium silicate,
preferably more than 45%. This is necessary as the hydration of this phase produces the
calcium hydroxide needed for a pozzolanic reaction of the ash. The hydration of the
clinker minerals is mainly responsible for the setting and initial strength development of
the cement, as the reaction rate of the fly ash is rather slow. The hydration of the ash
contributes to strength only at longer hydration times, but also affects other properties of
the hardened material. The calcium sulfate added in the form of gypsum or anhydrite
serves to control the setting of the fresh paste in a similar way as in plain Portland
cement.
Alternatively, fly ash may be introduced directly into the fresh concrete mix as a
separate component, rather than as a constituent of the cement. To do this, Portland
cement, fly ash, and mixing water, together with the aggregate, are mixed in the concrete
mixer to produce fly ash concrete.
The chemical and physical requirements for fly ashes to be used as cement/concrete
constituents, as required by the specifications of various countries, are summarized in
Tables 9.2 and 9.3. The following physical parameters are commonly specified:
Table 9.2
Chemical requirements for fly ashes to be used as constituents of concrete
mixes.
Country/
specification
SiO
2
(
wt%)
SiO
2
+Al2O
3
+Fe
2
O
3
(
wt%)
MgO (
wt%)
SO
3
(
wt%)
Na
2
O
equiv (
wt%)
L.O.I (
wt%)
H
2
O (
wt%)
Australia/AS 1129
-
-
-
2.5
-
8.0
1.5
Canada
C
-
-
-
5.0
-
6.0
3.0
CAN3-A23.5M82 F
-
-
-
3.0
-
5.0
1.0
China/GB 1596-79
-
-
-
3.0
-
5.0
1.0
Germany/DIN 1045
-
-
-
4.0
4.0
5.0
-
India/IS 3812
35.0
70.0
5.0
3.0
1.5
12.0
-
Japan/JIS A6201
45.0
70.0
5.0
5.0
-
10.0
3.0
Former USSR/GOST
6269
40.0
-
-
3.0
-
10.0
-
UK/BS 3892
-
-
4.0
2.5
-
5.0
0.5
USA/ASTMC618 C
-
50.0
5.0
5.0
1.5
6.0
3.0
F
-
70.0
5.0
5.0
1.5
12.0
3.0
Germany: Cl 0.1 wt%; CaO total 8.0 wt%; CaO free 1.5 wt%
Japan: CaO-total 6.0 wt%
