Just an update as to how each LR mix worked out...
Batch One was 3:1 salt to cement and my first attempt, I made it too wet and removed it from the mold too early. Even so the flat pieces of this hardened well, the only piece that still is yet to harden fully is the large pyramid shaped piece which i think was too thick. This attempt is still listed as a failure, as even though it hardened, the stuff is so porous it floats, kinda not what I was after. They've been left in a tub of water outside in the hope at some point they'll become saturated enough to sink...
The largest floating pieces are Batch One
Overall I can definitely see some uses for floating rocks, and if glued to other rocks the floating component isn't a problem. The rocks are extremely porous and after a month outside they've hardened well. They aren't what i was after though.
Batch Two was 2.5:1 salt to cement the mix was dryer then Batch One and left in the mold for a longer period, the pieces were all like flat pancakes. This worked out well, the pieces are still extremely porous but thankfully don't have the propensity to float. I'm fairly happy with this mix, but not sure how well it'd work for thick pieces, all the pieces made were only about a cm thick.
Batch Two are the flat rounded pieces
Batch Three the salt to cement ratio was 1.5:1, the mix was still fairly wet. These rocks worked out well and salt released very quickly for such thick rocks. I'm quite happy with these and these ones also don't float. The three largest rocks are Batch Three
Batch Four was my rubble, a mixture of 1:1 was used and the mix was made extremely dry. This led to extremely porous rocks, but had the downside that about half of the rubble pieces floated, which is a tad irritating. This mixture would have worked better with larger pieces....
The smallest pieces in both pics are batch Four
The pH still rises quickly so at least another week of 100% water changes 2 or 3 times a day, but I'm getting there!