From my understanding, the bacteria that living in the liverock does the 'filtering'. The more pores the rock the more bacteria. If this concrete rock in pores, and is left in the ocean it grow. Then it should behavior the same way. I am chooses not to find out first hand however. lol
Leaching phosphates for ??? however long does not appeal to me.
I also think you will find Real Live rock of any type will have no life in it.