I got myself a SCA cube and wanted the rock to look a certain way.
2 part # 2 potland cement (low alcalinity from HD)
3 parts corse crushed coral
2 part fine crushed coral
1 part argonite sand
Add little water at the time and mixed together until it feels like the perfect oat meal consistency.
Then add 2 parts corse water softener salt mix and put in mold
Plexi glass support structure that got coated in the aggregate. I used templates to make flat spots on the shelf rock and made holes so 1/2” PVC pipe can fit in and connect the two top pieces to the shelf rock. I used the same templates for the top rocks so they fir perfectly onto the shelve rock.