I haven't used sand for my bearded dragons yet for a couple different reasons. One of it is that as babies and juveniles I mist twice a day and when wet the sand tends to clump. I use the tray liners and I'm loving it.
As far as calcium, for years, reefers have thought that using calcium carbonate sand will help stabilize the water's pH. This has pretty much been proven ineffectively, because in order for the calcium to dissolve like that, the aquarium must have a very low pH. By that time, most inhabitants would have long died.
After chemical reactions, very little calcium is usable. Most of it becomes precipatate.
I personally don't feel sand is digestible. Very very little is digested and the overwhelming majority has to pass out through the system somehow or become impacted. There are certain places in the US with natural calcium carbonate sands, but most are available in the tropics.
This is my guess, but if the grain size is large or too much sand is consumed, a small amount, much less than 1% can dissolve, but the rest will become impacted unless it can pass through the system.