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Aggregates are all around us, in our roads, houses, schools and hospitals. They are also used to filter our water and cleanse emissions from power stations. Aggregate comes in a variety of forms, each with its own characteristics and properties, and these determine its many uses.
Essentially aggregates can refer to any granular material formed from a natural rock substance. It is usually further defined either:
Hard rock deposits are quarried from a fresh face and broken by mechanical means into aggregate. Geologically they can be igneous rocks such as granite or basalt; sedimentary rocks such as limestone or sandstone; or metamorphic rocks such as quartzite. As aggregates, they are often referred to as crushed rock or by their individual geological names. Explosives will need to be used to break the rock face into pieces and often the rock will pass through several stages of crushing and screening to create the final product.
Sand and gravel can be of any geological origin, but it has already been broken into pieces by the natural processes of weathering, transported by water or ice, and then deposited in a loose form. Gravel can still be in large pieces, which will need crushing, and usually all the material will need to be washed to remove fine clay particles.
Marine aggregates are sand and gravel dredged from the sea floor in permitted areas of the UK continental shelf. Further processing is again used to crush, screen and wash the material to provide the required products. Under the European Standards for Aggregates, primary aggregates are also known as 'natural aggregates'.
In many parts of the aggregate industry this term is used interchangeably with 'recycled' aggregates. However, secondary aggregates can be more correctly defined as aggregates produced as a by-product of other mining or quarrying activities such as china clay waste, slate waste and colliery spoil, or as a by-product of other industrial processes, e.g. blast furnace slag, incinerator ash, or the ash from coal-fired power stations. Under the European Standards, mineral wastes are included in the definition of 'natural aggregates', whereas the aggregates derived from industrial processes are defined as 'manufactured aggregates'.
Recycled aggregates are materials produced by the recycling of construction and demolition waste. They can be crushed concrete, bricks or glass, asphalt planings (ie the surface layers of roads removed during roadworks) or spent rail ballast. Processing includes crushing and screening, as with primary aggregates, but also the removal of metal, plastic or wood waste. In some locations top soil is produced as a by-product of this processing.
A variety of rocks are, when crushed, suitable for use as aggregates. Their technical suitability for different aggregate applications depends on their physical characteristics, such as crushing strength, porosity and resistance to impact, abrasion and polishing. Lower quality aggregates may be acceptable for other applications, such as constructional fill. Higher quality aggregates are required for demanding applications, such as in road pavements and in concrete. This is commonly derived from hard, dense and cemented sedimentary rock (most limestones and certain sandstones) and the tougher, crystalline, igneous rocks.
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Limestones are sedimentary rocks composed mainly of calcium carbonate (CaCO3). With an increase in magnesium carbonate (MgCO3) content they grade into dolomite. Most limestones and dolomites are hard and durable and useful for aggregate. They are common rock types and consequently are widely extracted for aggregate materials. Chalk is a form of fine-grained limestone but is soft and porous and generally unsuitable for aggregate use.
In Great Britain, limestone (including dolomite) provides 54% of the crushed rock aggregate produced. Limestones of Carboniferous age are the major source of limestone aggregate and they represent the largest resources of good-quality aggregate in Britain. These limestones are commonly thickly bedded, consistent deposits, which are structurally simple and can be quarried extensively and economically. They typically produce aggregates that are strong and durable, with low water absorption suitable for roadstone (sub-base and road-base layers) and concreting aggregate.
Igneous rocks produce strong aggregates with a degree of skid resistance and are hence suitable for many road surfacing applications.
Igneous rocks are particularly important in areas where other aggregate materials such as crushed limestone or natural gravels are not widespread. Roadstone is the largest consumer of igneous rock in Britain. Igneous rocks tend to produce strong aggregates with a degree of skid resistance and are hence suitable for many road surfacing applications as well as for use in the lower parts of the road pavement.
Sandstones are sedimentary rocks consisting of sand-sized particles composed predominantly of quartz, but with variable amounts of feldspar and rock fragments set in a fine-grained matrix or mineral cement. Compositional differences, both of the sand grains and the matrix, give rise to different rock names under the general heading of 'sandstone.' The suitability of a sandstone for aggregate use mainly depends on its strength, porosity and durability. These qualities are related to mode of formation and geological history. Thus the mineralogical composition, grain size, degree of grain sorting, nature and degree of cementation, degree of compaction and weathering state are fundamental rock properties which directly affect the end-use performance of the sandstone and its economic potential. Individual sandstone units also vary in thickness and lateral extent.
Many types of sandstone are too porous and weak to be used other than as sources of constructional fill. In general, older more indurated sandstones (subjected to tectonic compression), exhibit higher strengths and are suitable for more demanding aggregate uses, such as in concrete and particularly as roadstone. Some sandstones also have a high polishing and abrasion resistance and are particularly valued for road surfacing where they provide resistance to skidding.
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