ballast
noun [ U ] uk/ˈbæl.əst/ us/ˈbæl.əst/
(船的)压舱物;(气球的)镇重物;(用作路基的)道砟,石碴
heavy matter such as sand or stone that is used at the bottom of a ship or a hot-air balloon to make it heavier, or the small stones on which railways and roads are made