Hi,
I am a novice in telescope making but I have some advice regarding the dental stone as I made my grinding tool using it, it seems a good stuff which cured fast and becomes hard without the need for epoxy.
If you heat the dental stone after it cured (1 hour max), it will become harder in shorter period of time 90 degrees C is enough I think, as it will take a little bet time to cool down.
No small pieces will scattered from it while grinding, but it absorbed a lot of water after it gets dry, pure a cup of water on it before you use it each time you start grinding with it after it gets dry.
I wish that my info helps you.
Happy ATMing