Once you get it wet(such as in a gecko's stomach) it turns hard, like cement and blocks their intestines. It can cause intestinal perforations(holes in intestinal wall) that can cause a full body bacterial infection(sepsis) and they can die.

And it doesn't hold humidity for beans. Calcium is dry, and when you add water, it's just wet, and doesn't dry out(think quicksand, or clay dirt when it gets wet, it's gross).

It's definitely not a substrate to use if you want your gecko to live for longer than a year. They don't live on sand in the wild anyway, and wouldn't come into contact with calcium sand at all. They live on hard, compact clay and rocky outcroppings in Pakistan, etc. The rough slate tiles would be more natural for them, and you could easily make ledges and different levels/ramps by gluing them together with silicone.