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What are aggregates?

Introduction
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.

Basic definition
Essentially aggregates can refer to any granular material formed from a natural rock substance. It is usually further defined either:
  • by its source: primary, secondary, recycled
  • by its geology: limestone, granite, sand and gravel, etc
  • by its grading: coarse, fine
  • by its end use: concrete aggregates, etc.
Primary aggregates
These are materials extracted directly from the ground in quarries or pits. They can be either sand and gravel, or hard rock.

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 quarry

Sand and gravel quarry.

  Hard rock quarry

Hard rock quarry.

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'.

Secondary 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.

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