Aggregate is the name for sand, gravel, crushed stone and similar items used as construction materials. Aggregate is used to make asphalt, cement and concrete, and is also used in erosion control, water purification and emission control systems for electric power plants. Sand, gravel and crushed stone products are also required to make paint, glass, plastics and paper. Aggregate production usually requires that natural materials be dredged from the ground or the water, crushed, sometimes crushed again, and transported for sale.
Aggregate production and the use of recycled aggregates were on the rise in the early 21st century. Recycled aggregate production mainly uses materials salvaged from old buildings and roads that have been replaced. Recycling aggregate can be cost effective, in many cases allowing the recycled material to compete with newly produced material on a cost basis.
Natural gravel and sand are normally dug or dredged from special quarries, pits, or river, lake or sea beds for aggregate production. Stone or rock aggregate usually comes from special surface-mined quarries where the rock is blasted into large boulders or chunks, then crushed into smaller pieces by compression crushers. Some operations use an impact rock crusher for the first crushing because it can upgrade poor quality materials and facilitate rebar removal from recycled materials.
After the first crushing, the material is moved by truck or rock conveyor to a facility to be processed into final sizes by a cone crusher or an impact crusher. Once the material is sized, it may be sent through an aggregate washer to remove clay and other unwanted materials. Machines used to remove unwanted materials include a log washer, heavy media separator and attrition mill. These procedures are used to improve the quality of the aggregate batch so it will meet required quality standards.
Screening is used to remove unwanted materials, and it is also used to grade batches. In screening, the aggregate batch passes over screens with specific sized holes, and may or may not be washed. Washing removes clay and other unwanted materials, and also removes particles that are not the correct size.
Aggregate is graded according to particle size, and different sizes are required for different uses. Aggregate batches are stored separately during and after processing so the final product is not contaminated by other batches of different size grades or from different types of rock. Contamination is a serious quality problem in the aggregate industry, especially when the finished product is used to make cement, concrete and asphalt.
Statistics show that most aggregate is used within 40 to 50 miles (64 to 80.5 kilometers) of where it is produced. More than 90 percent is transported by dump truck, and some is transported by railroad. It is a heavy material that is produced at a low cost per ton, so the price of aggregate is heavily influenced by the cost of transportation.