Geoscience Reference
In-Depth Information
Natural sands and gravels are superficial deposits of
unconsolidated glacial, fluvial, or marine sediments.
They are easily excavated by mechanical diggers and
sieved or screened into different sizes. An important
aggregates source, they are particularly valuable as
concrete aggregate (see
154-156
) and mortar aggregate
(see
279
and
280
). They are composed of the more
durable rock fragments that have been released from
their parent rock and abraded by physical weathering
composition. For example, Figure
122
shows a beach
sand composed of minerals weathered out from basalt
and andesite lava flows on a Caribbean island. The
properties of gravel, and to a lesser extent sand, largely
depend on the rocks from which they are derived.
Aggregates may be 'monomictic' in that they contain
only one type of rock (e.g.
132
) or 'polymictic' with a
number of different rock constituents (e.g.
131
). Certain
natural deposits (e.g. wadi gravels) can be highly
polymictic and may contain in excess of twenty
different constituents (
123
).
Natural marine deposits are dredged from the sea floor
and may be recognized by the presence of marine shell
fragments (
124
) or remnants of encrusting marine
organisms (
125
). Figure
126
shows a rare case of coral
being used as coarse aggregate for concrete. On occasion,
organic contaminants such as sand eels, may be found to
be dredged-in with marine aggregate (French, 2005).
Crushed rock aggregates are produced from a variety
of consolidated rocks that are at the surface, or near
enough to the surface, for extraction to be economic. The
rocks are quarried by blasting, before being crushed and
screened into different particle sizes. Hard, compact types
of limestone are major source of crushed rock aggregate
(see
158
). Sandstones are a comparatively minor source
because they tend to be friable and porous, except those
that have been strongly cemented by silica to form
sedimentary quartzites (see
155
). Igneous and meta-
morphic rocks provide a fair proportion of crushed rock
aggregate, especially for roadstone (see
364
).
Artificial (or manufactured) aggregates are often
derived from industrial waste. These include power station
waste (pulverised fuel ash [PFA] and furnace bottom ash),
blastfurnace slag, colliery spoil, china clay waste, slate
waste, spent oil shale, steel slag, and incinerated refuse.
They are increasingly being used as alternatives to natural
aggregates as a way of conserving natural resources and
disposing of waste arisings (Sherwood, 2001). Figure
127
shows sintered PFA which is used as lightweight concrete
aggregate. Figure
128
shows boiler clinker aggregate,
which is found in the concrete of historic 'filler-joist' floor
construction. Figure
129
shows flint aggregate that has
been heat-treated (calcined) to give it a white colour.
Calcined flint is used as a decorative aggregate for white
concrete cladding panels.
122
123
122
Mortar containing beach sand fine aggregate
(Mustique, Saint Vincent, and the Grenadines)
composed of feldspars (grey/white) and pyroxenes
(brightly coloured); XPT, ×35.
123
Polymictic marine sand fine aggregate (Oman)
with eight constituents. Consisting chiefly of
serpentinized peridotite (brightly coloured) and shell,
with minor peridotite, pyroxene, quartz, chert,
sandstone, and calcrete; XPT, ×35.
