Quarries principally produce sand and gravel and crushed rock for construction and these materials are usually described as' aggregates'. In addition, quarries produce substantial quantities of coal, chemical grade limestone, gypsum, common clays, china clay or kaolin, ball clays and silica sand. Thus, quarries are often associated with process plants the most important of which are ready-mixed-concrete plants, coating plants to produce asphalt and bituminous road-making materials, cement and lime burning kilns, concrete block and pipe works, brick works, pottery works and plaster/plasterboard factories.