Geoscience Reference
In-Depth Information
and their respective alkali-reactivity classification is
given in
Table 15
.
In ASR, there are two classes of minerals that are
known to be highly reactive in concrete. These are
metastable and disordered forms of quartz such as opal,
chalcedony (
143
), tridymite (see
355
), and cristobalite
(
129
), and alumino-silicate glasses in the matrix of
intermediate to acid volcanic rocks. These minerals may
be present in rocks that would otherwise be classified as
low or normal reactivity and care must be taken to
ensure that these are detected if present. Crushed
greywacke, a type of poorly sorted sandstone with >15%
fine-grained matrix, has also proved to be highly alkali-
silica reactive when used as concrete aggregate (Huenger
& Weidmueller, 2007) (
144
, and see
219
).
Limestones are usually of low alkali-silica reactivity
but care must be taken with silicified limestones as these
may be reactive due to the presence of chert (
145
).
As mentioned previously, natural aggregates exhibit
considerable variability with a wide range of different
lithologies potentially being present. As a result, an
alkali-reactive minor constituent may be present where
the main lithology is regarded as highly satisfactory and
inert. Certain lithologies may contain disordered or
submicroscopic forms of quartz that are reactive. Taking
the example of our granite quarry from p. 66, the typical
granite would be of low reactivity (
146
), but a
microgranite textural variant may contain alkali-reactive
disordered silica (
147
). Different shear zones within the
quarry have dynamically metamorphosed the granite
Table 15
Alkali-silica reactivity of rock, mineral, and artificial concrete aggregate constituents (compiled
from EN 7943 and BRE Digest 330)
Low reactivity
Normal reactivity
High reactivity
Arkose
4
Air-cooled blastfurnace slag
Chalcedony or chalcedonic silica
Amphibolite
1
Breccia
Cristobalite
Andesite
1
Chert
Greywacke (crushed)
Basalt
1
Conglomerate
Opal or opaline silica
Chalk
Flint
Tridymite
Diorite and microdiorite
1
Granulite
1
Recycled demolition waste
Dolerite
1
Greywacke (uncrushed)
Dolomite
2
Gritstone
4
Hornfels
1
Expanded clay/shale/slate
Quartzite
4
Feldspar
Gabbro
1
Rhyolite
1
Gneiss
1
Sandstone and siltstone
4
Granite and microgranite
1
Tuff
1
Limestone
2
Volcanic glass
5
Marble
2
Quartz
3
Schist
1
Sintered pulverized-fuel ash
Slate
Syenite and microsyenite
1
Trachyte
1
1
Rock type generally not alkali-silica reactive but may occasionally contain reactive forms of silica (possibly including opal,
chalcedony, tridymite, devitrified glass, microcrystalline or cyptocrystalline quartz, submicroscopic or disordered quartz).
2
Silicified limestones, dolomites, and marbles may be reactive due to the presence of disseminated microcrystalline or
cyptocrystalline quartz.
3
Not quartzite, nor microcrystalline or cryptocrystalline quartz. In addition, highly disordered quartz and submicroscopic silica or is
potentially reactive.
4
Sandstones may occasionally contain reactive forms of silica such as microcrystalline or cyptocrystalline quartz, strained or
disordered quartz, opal, chalcedony. Greywacke can be highly alkali reactive (when crushed) and is considered separately from
other sandstones.
5
Volcanic glass may be reactive if the noncrystalline glass has devitrified to become very finely crystalline.