i have played a fair bit with rock making... aragocrete can be expensive...oyster shell leads to a more porous rock in my experience, and a hell of a lot cheaper....40lbs of oyster shell from a livestock place set me back $7... a 50/50 split also works

the thicker and more solid a piece the longer it takes to cure... the garf stuff is a bit out of date in terms of aquarium rock making...i can post some more info if ur interested