Well, here is an answer to Moses' question, and a fuller report.
I didn't specifically keep it moist, but it was covered in sand and the sand was slightly damp to the touch, so I think it had sufficient moisture as it cured. My initial report was based on two pieces that I had in a styrafoam box that had been made with the second batch (in which the initial mix was a little more liquid than the first batch). The rest of the pieces were covered in sand in a 5 gallon bucket. I didn't empty the bucket until three or four days after the initial report. Those pieces still tended to crumble around the edges, but were much stronger than the first two I looked at. Here are my (untested) conclusions:
--The drier mix was more difficult to work with, but made stronger pieces in the long run.
--The pieces needed to cure a good week before handling, I probably disturbed the pieces in the box too early.
--The pieces in the 5 gallon bucket probably also benefited because they dried more slowly. It might have helped if I had misted the sand in the styrafoam box since it had more surface area to dry out.
When I finally get some time (work has been nutty lately), I plan to make a second batch without the sanded grout, make sure they are slightly damp during cure time, and curing for at least a week. I'll let you know how that turns out. In the meantime, about 5 of the pieces from the first batch are out soaking in a stock tank before putting them into the display tank.
