[whatcaneyedo]

I don't think its critical. As long as what you're using is inert so that it wont leach anything harmful into your system it should be fine. However if you don't already have the sand I personally wouldn't bother. The larger disks on average probably take up a minute of my time altogether to make each... I've made hundreds which means I've spent hours creating them. A much faster thing to do which would cost about the same or maybe just a little bit more would be to buy some marble tile and cut it into small squares on a wet tile saw. Instantly you'll have little calcium carbonate frag mounts that only take seconds to make and don't need to cure for weeks. If you plan on getting into fragging a lot you'll want a tile saw anyways... Every other month Princess Auto has a great deal on one. When I run out of sand this is what I intend to do.

This is unrelated but I've been mostly using a 4:1 sand to cement mix and decided to stick with it. The disks are a little bit weaker at first but once they've cured a bit longer they are quite strong. I also try to only add as much water as I need. Using too much water so that the mixture is runny again reduces their strength.