You're wasting your time with shell grit, it's entirely too large to be of ANY use in a worm bin. Same as crunched up eggshells. I've tried both and both were still chunks years later, not broken down in the least.
Google *mesh size* + lime for a better explanation. In short, for minerals, rock dust, lime, oyster shell flour (NOT "flower"), and stuff of that nature needs to be the consistancy of flour to actually become available.
What DOE'S work and works well is chicken laying mash/crumbles. It has the calcium, ground small enough to actually become available and worms flat out love it. Best of all, it's as cheap as "chicken feed", ~$15 for a 50lb bag. Guess that would translate to roughly $20AU/22kg bag.
Just, NEVER bury it (it will sour). Sprinkle lightly on the surface, mist lightly and it will be gone in a few days time, then feed again. The worms get fat and the population explodes, a real win-win.