Actually Canna, getting rid of the membranes and associated proteins is part of it. By roasting the shells you denature the proteins and make them easily dismantled by soil biota. As I mentioned, toasting or charring them also makes the phosphorous more available, and the calcium to a lesser extent. To really make the calcium quickly available you would char the shells, grind them up, and put them into a container with an appropriate amount of vinegar. You might be worried about the pH of using vinegar, but the calcium compound in the shell will react with vinegar like baking soda does (just not as quickly.) As the slow fizz happens the gas that is thrown off will contain the extra hydrogen ions which make the solution acidic.
Tums are high in the same calcium compound as egg shells. When you have acid indigestion you eat a Tums. It fizzes in your tummy with the excess stomach acid. Usually you burp or fart after a while. The acid indigestion is gone.
Egg shells are not just calcium. Like bone there is also a lot of phosphorous in them. They also have about the same calcium to magnesium ratio as bottled Cal-Mag products - 5:1.
One good way to char the egg shells is to do it outside on the BBQ.