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How are aggregates produced?

There are two main ways of making natural aggregates: by crushing hard rock or by processing sand and gravel. These methods are different and are explained below.

Hard Rock Quarrying
Before hard rock can be removed, the overlaying topsoil and 'overburden' (weak or weathered rock) must first be put to one side to expose the fresh, hard rock underneath. This first stage of quarrying is called 'stripping'.

The fresh rock can then be quarried in a 'step' pattern that we call benching. Over many years a hard rock quarry, with its series of benches, can become very deep - sometimes hundreds of metres deep. A busy quarry can remove over 1 million tonnes of rock per year.

A modern hard rock quarry with benches
To make aggregates from hard rock we must break the rock up into smaller pieces. In the early days of European settlement of Australia the rock was broken by hand - men (often convicts) used hammers to break the rock into blocks and also into chunks called 'spalls'. It was incredibly hard work. These days we use technology to make the job very much easier and very much quicker.

To break the rock, explosives are used to blast a certain amount of rock in a careful and controlled manner. The explosives are placed in holes that have been carefully and accurately drilled down into the bench. When everyone is safely away from the blast area, the explosives are detonated (or exploded) by a person known as a 'shotfirer'. Because most quarries have neighbours, the blasts are designed so that noise and vibration are minimised.

It can take several days to drill the rock and a day to place the explosives. But the blast is over in just a few seconds!

After the rock has been broken by blasting, it can be loaded by excavators or face shovels into dump trucks. It is then hauled by dump truck to a crushing plant. Occasionally a conveyor is used to take the broken rock to the crusher.

An excavator loading a dump truck with broken rock
At the crushing plant, the rock is broken and shaped into smaller and smaller pieces by crushing machines called rock crushers. Usually, the rock is sorted into different sizes by passing it over special screens. The screens are like sieves that let smaller rocks through but not the larger ones. The finished rock products are known as aggregates.

Modern crushing and screening plants are fully automated and operated by a controller with the help of computers and closed circuit TV.

A modern crushing/screening plant
In some quarries the aggregates are washed before being carefully stored in large heaps called stockpiles or in large bins.

The aggregates are now ready to be used. Larger quarries may produce over 20 different types of crushed aggregates according to size, shape, strength or even colour.

Stockpiles fed by conveyor belt from the screening plant
A customer will order a certain type of aggregate. The aggregates are then loaded into road trucks usually by front-end loader. The truck load of aggregates is then weighed, the load made secure and then delivered to wherever it is needed.

Aggregates being loaded into a truck
Aggregates are needed in large quantities and are bulky. They are expensive to transport long distances. Quarries therefore need to be close to where the aggregates are eventually used.

Sand and Gravel
Sand and gravel are another equally important source of natural aggregates. They are formed from the natural erosion of the earth's crust. Sun, wind, rain and ice cause rocks to slowly break down over millions of years. These products of erosion and weathering are transported by the action of water.

The main component of sand is called quartz or silica. While most minerals eventually break down and are carried away by the water or the sea, silica is hard and chemically stable. It therefore concentrates in our rivers and around our coastlines forming deposits of sand. Because it is hard and durable it is ideal for making aggregates.

Sand and gravel quarries on land are often referred to as pits. They are not as deep as most hard rock quarries. A typical depth range is between 5 and 10 metres. Some deposits consist almost entirely of sand. Mud and clay often occurs with the sand, either mixed together or as layers.

Some sand deposits are formed from the weathering of sandstone. The sandstone rock breaks down into the original sand grains.

Sand and gravel deposits do not require blasting . The sand and gravel just has to be dug out and sent to the processing plant. Deposits can be mined as either dry or wet workings. In wet workings the sand and/or gravel is removed from below water level.

The materials that overlay the sand and gravel must first be removed. This soil overburden can be valuable and is stored for use later for rehabilitation and restoration of the site. Sometimes some of the soils or loams can be sold and used for landscaping purposes.

In dry workings bulldozers, scrapers, excavators and front end loaders are used to scrape and dig the sand and gravel. It is then loaded into dump trucks and trucked to the sand processing plant.

Extraction of friable weathered sandstone
In wet workings on land, equipment such as excavators, draglines and dredges can be used. A dragline looks something like a crane and has a large bucket which is cast into the deposit to scoop up the sand and gravel and bring it ashore.

Dredges float on pontoons on the water. They have a large pump on deck and a steel structure called a 'ladder' which is lowered down below water to the sand and gravel. At the end of the ladder is a cutter or set of water sprays. The dredge operates by the cutter loosening the sand and gravel and the pump sucking the sand and gravel to the surface and then pumping it to a processing plant.

A sand-dredge in operation
The processing of sand and gravel involves washing and screening. Some sand plants produce several different washed sand products with different sized particles or sand grains. If large gravel is present, rock crushers can be used to crush the gravel to make coarse aggregates. Sometimes the gravel is not crushed but just washed and separated into different sizes using screens.

A most important feature of sand processing for use in concrete is to make sure that mud and clay and any organic materials are removed from the sand. These unwanted materials are often called 'slimes' and can be harmful in concrete because they can make the concrete weak. Sand processing plants use water and special pieces of equipment to separate the slimes. The slimes are then piped to specially built dams called tailings dams where the 'slimes' settle to the bottom.

Once the sand has been separated from the slimes, it can be pumped as a mixture of sand and water (a sand slurry) to a 'dewatering' device such as a 'cyclone'.

A cyclone
A cyclone is a clever and commonly used device for helping process minerals. Though it has no moving parts, sand and water is pumped inside and separated by a type of 'cyclone' (or vortex) action. In sand processing a cyclone is used to dewater the sand before the sand is placed in a stockpile.

Once in stockpile, the sand can drain and is then ready to be loaded for delivery.

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