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Scrap iron would make terrible ballast. Scrap implies it is shaped in whatever random form you find. This is not efficient for ballast. It would need to be cast into custom or at least regular shapes to be good ballast, and even then it is subject to corrosion and flaking away. Rust is many times the volume of the original iron, and this expansion due to corrosion is the primary cause of ferrocement or reinforced concrete spalling.

Lead is toxic, as are other heavy metals like depleted uranium.

Although these materials are much denser than concrete or sand, they require intense heat to melt and equipment to shape and install. Concrete or sand can be poured in. Sand has the additional benefit of being easily removable, not an environmental pollutant once removed, and essentially free.

Water is another free ballast- although it can be considered an environmental risk due to invasive species that may live in it. Although it is not heavier than the water you're displacing, it can match density while the container de-couples it from wave action and in combination with the container appear heavier than ambient water. Water has even better ability to be removed than sand as it can simply be pumped in and out or around the structure to adjust ballast and trim automatically. Water can be pumped to above the outside water line and provide semi-submersible functions to a buoyant structure.