Slag is a byproduct loose material which is used solely as a filler material in the production of Concrete Slabs after being processed into Slag Crushed in a Crusher. It is a major byproduct of smelting operations and Toxic Slurry processing.
Slag Crushed can be dumped and used as landfill, to include filling in the ocean, just as it's Slag Crushed variant. Farms cannot be built, even partially, directly on top of Slag and it will prevent its placement. As of Beta 0.3.11 (2022-01-23), if Slag Crushed is dumped then subsequently mined back up, it will be converted back to Slag. This is a consequence of the game limiting the number of textures required on the island and it does not have any known benefits to the player.