Slag Crushed is a semi-processed loose material which used solely as a filler material in the production of Concrete Slabs. It is created by processing Slag in a Crusher and is used in both the Concrete Mixer and Concrete Mixer II.
Slag Crushed can be dumped and used as landfill, to include filling in the ocean, just as it's un-crushed Slag variant. Farms cannot be built, even partially, directly on top of Slag Crushed 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.