Transportation of bulk dry bulk cargoes is carried out by specialized ships. The main bulk dry bulk goods transported by waterways are iron ore, coal and grain, followed by bauxite and phosphate rock. They account for about a quarter of the total volume of water transport. Ships used to carry such bulk cargoes include general-purpose bulk carriers, specialized mining ships, specialized grain ships, part-time ships and a small number of ships specialized in carrying cement, etc., which are billed on a volume basis.