Making your own rock is an experimental process, everybody has their own formula that probably began as the original
GARF recipe. Like cooking, homemade rock is as individual as the 'chef'. Only through trial and error will you come up with a recipe that you'll really like. Considering that you can get the ingredients to make over 600lbs of oystercrete (one 94lb bag of portland, ten 50lb bags of crushed oyster shell) for about $60-$75... that's a lot of cooking you can do, until you find
your secret recipe. :lmao:
I use a mixture of 5Ã"šÃ‚½ parts oyster shell / 1 part portland, mixing in just enough water to get a doughy effect... almost like making oatmeal cookies. I use dampened play sand (silica based) in a long rubbermaid container (with lid) as a base mold to form my pieces.
While scooping in the oystercrete, I also drop or lay in some pasta (on the bottom, inside, outer surfaces), forming tunnels, spaces and textures in or on the rock.
After I mold my pieces, I close the lid to keep moisture in... keeping concrete moist during initial setup helps it cure better... letting them cure for about a week. When I take them out of the mold, I rinse them with the hose to get rid of any loose sand/shell/pasta, then soak them in another container of FW for about 8-10 weeks with a powerhead for circulation, changing the water every 2-3 days.
After the FW soak, I empty the old FW and begin the SW soak for the next 4-6 weeks (actually longer in my case, I'm preparing for a 125g in the near future) using regular SW mix (RO/DI & IO salt) to begin with. During this time, I also add in water from my weekly water changes. This begins a seeding cycle of bacteria, coraline spores, free floating pods, etc. By the time I'll be ready to use my rocks, they will be 'live'... some even have a decent coating of coraline now.

BTW, because of the long curing process I use, I don't bother with pH measurements... I figure that a bunch of happy pods are a good sign.