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What Aggregates Are ...
Source: http://wrgis.wr.usgs.gov/docs/geologic/northwest_region/ofr95-681.html
Natural Aggregate Resources
Natural aggregate consists of rock fragments that are used in their natural state,
or are used after mechanical processing such as crushing, washing, and sizing.
Crushed stone, and sand and gravel are the two primary sources of natural aggregate.
Suitable aggregate consists of clean, uncoated
particles of proper size range, shape, physical soundness, hardness and strength, and
chemical properties. Crushed stone can substitute for
sand and gravel in most applications. Angular particles of crushed stone are
desirable in asphaltic mixes, because intergranular contact between the angular
particles provides strength. However, the rounded particles of sand and gravel are
preferable in cement concrete, because rounded particles improve the workability of
the wet concrete.
Aggregate Uses, Demand, and Value
Natural aggregate is used primarily in construction and repair of physical-societal
infrastructure (buildings, roads, bridges, runways, railways, dams, canals, and sewer
systems, for example). Urban areas and highways are major consumers of natural
aggregate. Sources are needed near markets, because the product is bulky, low in
unit value, and expensive to transport.
Crushed Stone
Competent, homogeneous, fine-grained rocks that are not too abrasive and have
not been decomposed by weathering generally are preferred as sources of crushed
stone for aggregate. For example, limestone, dolomite, and trap rock (basalt or
diabase) are good sources of crushed-stone for aggregate. Where these are not locally
available, other competent igneous and metamorphic rocks are used. However
some volcanic and volcano-sedimentary rocks are soft and friable, and some silicic
volcanic rocks tend to react deleteriously when used as aggregate in cement concrete.
Sand and Gravel
Sand and gravel deposits consist of rock or mineral fragments in loose, non-cohesive
bodies that result from sedimentary processes, including fluvial,
lacustrine, marine, eolian and glacial. Sand and gravel aggregate
is a mixture (aggregation) of sand and gravel in which gravel constitutes about 25
percent or more of the mixture. Gravel typically
occurs in layers or lenses with sand. Sand and gravel deposits are best when they
contain little silt. Minor replenishment of sand and gravel
deposits may occur, especially during flooding.
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