LR alternative?

[digitalis]

Do you think it's safe to use some old concrete blocks in my sump for more "live rock"? They are the thin ones, not the really thick ones. Should I maybe put them in a bucket of water for a week, and check the pH to be sure it's not really high? I know that since cement is really just limestone, it has a really high pH. Once it's cured enough, it should be just plain rock, eh?

I need a place for my mangroves and macro to set on, and right now, the little bit of live rock I have stacked in there is just a little less straight up than a cliff. I'm afraid it's going to fall over one day. If I could stack a couple blocks next to it, I know it would be fine.

[grallster]

That should work, but I think you might have to cure them for a month or more--but I'm not 100% sure on that. I'd make sure that water circulation can get around & behind those blocks though.
Personally, I'd use a piece of plexiglass (perforated with holes for circulation) cut to size and wedged into the sump and pile the rocks against it. You could even cable tie some or all of the rocks to the holes in the plexi if you wanted it super stable.