Rocks come in just about any size, shape and colour. Sand, gravel and clay are all types of rock.
Rocks are often classified according to how they formed in the earth’s crust. There are three groups of rocks : igneous, sedimentary and metamorphic.

Igneous rocks are formed when lava or molten rock cools and hardens. The word 'igneous' comes from a word meaning 'fire'. Basalt is an igneous rock 'born' from a volcano. Granite is another type of igneous rock but it is formed inside the earth rather than from a volcano.

Sedimentary rocks are made from other rocks. They form over millions of years from rock particles (or organic remains) that have settled in layers, often at the bottom of ancient seas. Sandstone (from sand), limestone (from the skeletons of billions of microscopic sea animals) and conglomerate (made from rocks and pebbles of different sizes) are typical examples. Sometimes you can see the layers in sedimentary rocks and find fossils in them.

Metamorphic rocks are the third type of rock. They are formed when intense heat or tremendous pressure changes existing rocks.  The existing rocks are 'cooked' and change their form and appearance. The word 'metamorphic' means to 'change form'. Quartzite is a metamorphic rock formed when sandstone is 'cooked'. Similarly, the rock called marble is formed from limestone.

What is a quarry?

A Quarry ... is a place where rock, gravel or sand is dug from the earth to build or make things that we need, such as materials for our house, or for a road or sports field. A quarry is therefore a type of rock and sand 'supermarket'!

Quarrying refers to the method of digging and processing rock or sand and gravel. Mining is similar in some ways to quarrying, but mining usually refers to digging for metal minerals or coal. Quarries are also usually smaller than mines and much nearer to our towns and cities where we use most of the rock materials they provide. When quarrying has finished, the site is rehabilitated. Many old quarries are used for entirely different purposes such as landfill and recycling centres, nature conservation and residential development.

Quarries have existed in South Africa for as long as people have been here. Our South African ancestors did not use metals and relied on rock or stone to make many of the weapons and tools they needed to live. So they established the first quarries and showed great skill in working with rock and stone.

Today, we also use rocks to make the very important things on which we depend. Just about everything we make or build can be traced back to rocks. Rock is one of our most abundant natural resources and quarries provide rock products including crushed rock, sand ,gravel, clay and building (or dimension) stone.

There are more than 1000 hard rock and sand and gravel quarries in South Africa today. They range in size from very large 'metropolitan' quarries near big cities making hundreds of thousands of tonnes of quarry products a year, to very small quarries that supply rock products for use in remote areas when needed.

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:

1. FIND
2. DIG
3. PROCESS
4. MAKE
5. USE
6. 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.

What are aggregates used for?

For tens of thousands of years, humans have used rocks in the most ingenious ways to provide shelter, to make tools and to build important structures. This legacy continues today. We use aggregates in an amazing variety of ways.

Aggregates are used to help build our houses, schools, roads, bridges, commercial and industrial buildings, airports, railways, sport fields and to solve environmental problems. It is hard to imagine what life would be like without aggregates. Aggregates are needed in large quantities. On average we use about 100,000 tonnes of aggregates every day in South Africa!

Aggregates can be used on their own or combined with other materials to form 'man-made' rocks like concrete, bricks or asphalt pavement.

Some main uses of aggregates are:

• Making concrete for houses, buildings, roads, bridges, airports etc
• Foundations for roads, driveways, carparks and footpaths
• Making concrete products such as pipes, blocks and roof tiles
• Surfacing and repairing roads
• Making asphalt paving
• Railway ballast
• Landscaping

A typical brick veneer house with a tile roof needs about 60 tonnes of crushed rock, sand and clay aggregates for its construction. If we include the driveway and landscaping the amount is closer to 100 tonnes.

Concrete is the most widely used construction material in the world. Concrete is made by mixing with crushed rock and sand aggregates, cement and water. The cement and water form a paste that hardens to glue or 'bind' the aggregates together to make the concrete.

Cement is different from concrete. Cement is made by grinding limestone and clay into a powder. The mixture is then melted in a kiln at very high temperatures. A 'new' rock called 'clinker is formed. A mineral called gypsum is then added and the clinker ground into the fine grey powder we call Portland cement.

Most of the aggregates we produce in South Africa each year are used for building and construction purposes. But not all. Aggregates are also used to make agricultural products that improve soil quality and to help solve environmental problems. Aggregates are even used in medicines and in the manufacture of plastics, paint and cosmetics.

Some people say we are entering the 'second stone-age'. Silica sand is used to help make silicon chips that run computers. More and more buildings are being made from 'tilt up' concrete panels. Rock products are being used in increasing quantities for environmental protection ... from the protection of land, rivers, beaches and harbours against erosion, through to the use of certain finely ground rocks to replenish topsoils with minerals, and so increase crop yields.

Aggregates are even used in ostrich farming - the birds swallow the aggregates to help with the digestion of their food!

Examples of uses of aggregates

• ROADS AND HIGHWAYS use about 35% of all aggregates produced;
• OTHER PUBLIC WORKS AND BUILDINGS (bridges, airports, schools, sports grounds etc) use about 20%;
• HOUSING (including single houses and apartment blocks) use about 20% of all aggregates that are produced and;
• COMMERCE AND INDUSTRY (supermarkets, factories, business premises) use about 20%

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.

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.  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.  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 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 Quarrying

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.

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 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.

How safe are quarries?

Safety is extremely important in quarries.

Quarrying companies and governments go to great lengths to make sure that quarries are as safe as possible. There are very important laws in place requiring people to minimise the risks (or dangers) associated with quarrying. These laws don’t just apply to the quarry company, but to anyone that visits a quarry. You must NEVER go into a quarry unless you have the permission of the quarry manager and are accompanied by a quarry representative.

There are obvious but also hidden dangers for anyone unfamiliar or uninvited who goes into a quarry. Quarrying involves the removal of earth and the creation of large holes in the ground. Sometimes these are very deep with steep ‘cliffs’. The rock can be very loose. Hard rock quarrying involves the blasting of rock. Large and heavy earthmoving equipment like excavators, dump trucks and front end loaders operate in the quarry. And there can be lots of trucks.

In Australia, over the past 10 years several young people have tragically been killed by accident when they went into quarries without permission. In some cases they were killed by collapsing stockpiles and in other cases they fell over the edge of benches. Several young men have been killed whilst riding motorbikes in quarries.

PLEASE DO NOT ENTER A QUARRY UNLESS YOU HAVE THE PERMISSION OF THE QUARRY MANAGER AND YOU ARE WITH A REPRESENTATIVE OF THE QUARRY.

IF YOU ENTER A QUARRY ILLEGALLY YOU ARE PUTTING YOURSELF AT GREAT RISK. YOU ARE ALSO BREAKING THE LAW AND COULD BE PROSECUTED.

The construction materials used to build a typical house (brick veneer with a tiled roof) contain a surprising amount of aggregates. A typical house contains about 80 tonnes of aggregates.

Can you identify which materials are used to make the house that you live in?

Careers in the Quarrying Industry

There are many different types of careers available in the quarrying industry. The types of career opportunities are similar to those in the mining industry but it should be understood that employment levels in the South African quarrying industry are smaller than the Australian mining industry.  Indirectly, the industry supports many thousands more jobs through support services and support industries.

The quarrying industry supports and employs the following types of workers: