Strage,confused arguments. Don't confuse silica ,which your tank is made from, with silicate minerals that are common components of many rocks, and not very stable in seawater. If silica dissolved easily, the market for glass tanks would be pretty small...... Clean beach sands can often be nearly 100% silica, and are perfectly fine for aquaria, they aren't going to dissolve.
Seeing as portland cement doesn't contain silica, and neither does aragonite sand, by definition, I doubt concrete made of such will contian any either. Even if it did, it wouldn't dissolve anyway.
Back to Joes question, I would think you might be safe with most concretes, though you can never be sure, and they are certainly porous and permeable enough (more than most live rock I think). But do you want to go there? You're goign to have to soak them for a few weeks to make sure they don't drive up pH too high, plus they'll look ugly, plus they come with no living matter, and a principle reason for using liverock is the biodiversity it brings. I's not single species of bacteria that do any single part of the nitrogen cycle, plus the species are generally not well known (just guessed), plus you don't get hitchikers on concrete. So you have to seed the rock anyway, and end up with at best second hand, second rate biodiversity. I wouldn't - for such a small tank jsut get the best LR you can.
