Thanks KowKilla... That's about what I did (followed the GARF recipe). The oyster shell stuff I made came out very porous as well. I was pretty excited about filling a reef tank w/ this stuff, until the long branch I made broke off in my hand... I mixed the portland cement 1:5 w/ oyster shells, but mixed it wet. Thought I didn't have enough concrete to get it to stick, so I added a little more, mixed, and then let it setup. It's odd that too much cement would make the rock brittle (if anything, you'd think it would be the opposite way!), but maybe that's it. I think I'll give your method a go, and see how that batch comes out. mix it all dry, and then add the water... The other tip I got so far, was to make it a dry mixture, almost like day-old oatmeal. Last time I was much runnier than that, so perhaps my problem was too much water.
Thanks.
- Mac