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