Civil Engineering Reference
In-Depth Information
Fig. 3-11. Metakaolin, a calcined clay. (69803)
Fig. 3-12. Scanning electron microscope micrograph of
calcined clay particles at 2000X. (69544)
Calcined clays are used in general purpose concrete
construction much the same as other pozzolans (Fig. 3-4).
They can be used as a partial replacement for the cement,
typically in the range of 15% to 35%, and to enhance resist-
ance to sulfate attack, control alkali-silica reactivity, and
reduce permeability. Calcined clays have a relative density
of between 2.40 and 2.61 with Blaine fineness ranging from
650 m
2
/kg to 1350 m
2
/kg. Calcined shale may contain on
the order of 5% to 10% calcium, which results in the mate-
rial having some cementing or hydraulic properties on its
own. Because of the amount of residual calcite that is not
fully calcined, and the bound water molecules in the clay
minerals, calcined shale will have a loss on ignition (LOI) of
perhaps 1% to 5%. The LOI value for calcined shale is not a
measure or indication of carbon content as would be the
case in fly ash.
Metakaolin, a special calcined clay, is produced by low-
temperature calcination of high purity kaolin clay. The prod-
uct is ground to an average particle size of about 1 to 2
micrometers. Metakaolin is used in special applications
where very low permeability or very high strength is re-
quired. In these applications, metakaolin is used more as an
additive to the concrete rather than a replacement of cement;
typical additions are around 10% of the cement mass.
Natural pozzolans are classified by ASTM C 618
(AASHTO M 295) as Class N pozzolans (Table 3-1).
ACI 232
(2000)
provides a review of natural pozzolans. Table 3-2
illustrates typical chemical analysis and selected properties
of pozzolans.
EFFECTS ON FRESHLY MIXED CONCRETE
This section provides a brief understanding of the freshly
mixed concrete properties that supplementary cementi-
tious materials affect and their degree of influence. First it
should be noted that these materials vary considerably in
their effect on concrete mixtures. The attributes of these
materials when added separately to a concrete mixture
can also be found in
blended cements using
supplementary cementi-
tious materials.
Table 3-2. Chemical Analysis and Selected Properties of Typical Fly Ash, Slag,
Silica Fume, Calcined Clay, Calcined Shale, and Metakaolin
Class F
Class C
Ground
Silica
Calcined
Calcined
fly ash
fly ash
slag
fume
clay
shale
Metakaolin
Water
Requirements
SiO
2
, %
52
35
35
90
58
50
53
Al
2
O
3
, %
23
18
12
0.4
29
20
43
Fe
2
O
3
, %
11
6
1
0.4
4
8
0.5
Concrete mixtures con-
taining fly ash generally
require less water (about
1% to 10% less at normal
dosages) for a given
slump than concrete
containing only port-
land cement. Higher
dosages can result in
greater water reduction
(Table 3-3). However,
some fly ashes can in-
CaO, %
5
21
40
1.6
1
8
0.1
SO
3
, %
0.8
4.1
9
0.4
0.5
0.4
0.1
Na
2
O, %
1.0
5.8
0.3
0.5
0.2
—
0.05
K
2
O, %
2.0
0.7
0.4
2.2
2
—
0.4
Total Na
eq. alk, %
2.2
6.3
0.6
1.9
1.5
—
0.3
Loss on
ignition, %
2.8
0.5
1.0
3.0
1.5
3.0
0.7
Blaine fineness,
m
2
/kg
420
420
400
20,000
990
730
19,000
Relative
density
2.38
2.65
2.94
2.40
2.50
2.63
2.50
