Originally Posted by Gobyfan
Oyster shells in place of sand would probably help the cement hold its form better, however the sand absorbs moisture which aids in the drying process. So instead of waiting 24 hours for my LR
mixture to settle I only waited 12 hours. Which is why I chose to go with sand.
With cement you aren't actually waiting for it to dry. You are waiting for it to cure which is a hydration process. They make cement by driving all of the water out of limestone. As it cures it recrystallizes into calcium carbonate. Cement can even be cured underwater. The reason for using the oyster shells in the mix is to give the porous structure to the rocks. The reason for using it as the casting mold is to avoid the silicates that come along with using sand, of course there are silicates in the cement, but they seem to be pretty tightly bound once the hydration is fully completed.
It has been over a decade since I made those rocks so I can't really remember how long I let them harden before I took them out. I suspect I formed them, and just left them until the next day for good measure.