A
Anonymous
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I'm not sure that I know what cement live rock is, but I suspect this refers to some sort of amalgam of aquarium-friendly substances cemented together, in which case I think you can find more information on the GARF web page, which I am afraid is painful to navagate ( http://www.garf.org ). They use it to make little plugs for their coral cuttings.
In essence, any rock will work that is rather porous. What you want is a lot of surface area for the aerobic bacteria to grow on, and a lot of slowly-exchanging rather deep channels in which the anerobic bacteria that turn nitrates or nitrites into nitrogen gas can grow. Any material that satisfies those criteria and does not leach toxins into the aquarium should work, although some materials I suspect are better than others. For example, pacific live rock is more porous (a better material) that florida aquicultured live rock, which is limestone from quaries. The limestone still works, but is only about half as efficient by mass. (I use it anyway because it is more ecologically sound.)
In essence, any rock will work that is rather porous. What you want is a lot of surface area for the aerobic bacteria to grow on, and a lot of slowly-exchanging rather deep channels in which the anerobic bacteria that turn nitrates or nitrites into nitrogen gas can grow. Any material that satisfies those criteria and does not leach toxins into the aquarium should work, although some materials I suspect are better than others. For example, pacific live rock is more porous (a better material) that florida aquicultured live rock, which is limestone from quaries. The limestone still works, but is only about half as efficient by mass. (I use it anyway because it is more ecologically sound.)