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What are aggregates?

Natural aggregate is the name given to the processed rock and sand products from quarries. Natural aggregates come from two main sources within the earth's crust:

Hard rock (either igneous, sedimentary or metamorphic rock) or;

Sand and gravel deposits.

The major steps involved in the production of aggregates are:

FIND

DIG

PROCESS

MAKE

USE

RE-CYCLE

People who study rocks are called geologists. Geologists help find hard rock and sand and gravel deposits that are used as sources for natural aggregates. Not all rock types are suitable to process into aggregates. In order to be useful, aggregates must have certain properties such as good strength, the right shape, resistance to wear and resistance to chemical erosion.

Aggregates are often classified according to their size. Coarse aggregates are larger than a match-head (5 mm) in size and can be as large a tennis ball, or sometimes even larger. Coarse aggregates can be made from any of the three rock groups. Common coarse aggregates include those made from basalt, granite, sandstone, limestone, dolomite, quartzite and greywacke.

Fine aggregates are those that are smaller than the size of a match-head. Sand is the most common fine aggregate. It consists largely of quartz (the mineral is silica whose chemical formula is SiO2). Silica is the most common mineral in the earth's crust. It is very durable meaning that it is resistant to both chemical and physical erosion. The size of the sand particles are very important and largely decide what the sand can be used for. Sand is therefore frequently referred to as being fine, medium or coarse in size.

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