Ok. So I have mixed up 2 batches of rock over the weekend. The first, I think, I am not so happy with. The second turned out much better...
The First Batch
First I dumped in 5 parts salt and 1 part cement. Then I screwed up by adding a bit of water BEFORE I mixed the dry materials... Crud...
So I fixed that and mixed it all up. We made about half the stuff into sand molds and the other half on flat sheets of plastic, mostly with some rock salt underneeth to give the underside a little tecture.
My real problems/concerns with the finished product? First, I am a pretty concerned that the rock is just not going to be very strong. I am attributing this to (a) too much salt in the mixture and (b) too little water added to the mixture. Of course, this is a best guess and I really have no freaking clue.
Anyhow, we took the first batch and put them in a tub to soak in water after about 24 hours (which was yesterday). I will probably go out today to take a look and see how solid the rocks feel after another 24 hours of curing.
The Second Batch
......went MUCH better, as things tend to do after a little practice.
First of all, my wife and I both thought that lowering the salt : cement ratio was a good idea. So for batch #2, we went with 3:1 salt:cement. And the end mix had overall more water, which I think was good.
I also discovered a nice method for mixing both the dry and the wet mix. It was better, I believe, both because it got a better mix and it was a lot less messy on the hands.
First, I added the salt and cement in layers. This helped to kick start the mix. And I mixed a reasonably small batch -- the salt and cement dry mix probably only filled about a fourth or third of the bucket. And then to actually mix both the dry and wet mixture, I tipped the bucket over at about a 45 degree angle and then quickly rotated the bucket back and forth. I pretty easily was able to figure out a pattern of motion which caused the mixture to flop over on itself and churn nicely.
Anyhow, I feel like it mixed more thoroughly then when I did it by hand. Or maybe that is just a fantasy idea just to keep my hands clean.
Our next big discovery was that instead of using sand molds, I bought an extra bag of salt and we used it to mold the rock. I started by putting about an inch or 2 of salt into the rubbermade tubs. Then I sort of three dimensionally sculpted in layers from the bottom down, alternating the adding of cement and then pouring in some more salt around that cement layer.
Also, when opening up one of the bags of salt, we saved aside some bigger chunks of salt that were stuck together. We then used these in our rock forming to create caves, holes, etc. This could have been done with loose salt. But the chunks held together better. And if you pick "hard" bags of salt from Lowes/HD, then you are guaranteed to get a few pieces in every bag.
Using the above techniques, I was EXTREMELY please with our ability to sculpt rock which, I believe, will be a LOT more interesting. Primarily, the resulting rock will be, I believe, a lot more three dimensionally sculpted and less two dimensional, like our first batch.
And by using rock salt for molding, any residual "mold material" will just wash/dissolve off in the curing process. I think this will create a more interesting tecture on outside surface of the rock.
And lastly, the mold mixture is reusable. I think a good system would be to use your the mold salt from your last batch as the rock salt for your next batch. If you kept using the same salt for your mold, I suspect you would eventually have to replace it. This way you'll waste a little less rock salt.
Anyhow, I will probably let batch two set for another day and then pull them out and see how they look. Or maybe I will get impatient and pull them out today. We'll see.
And I'll post some pics eventually...