The man behind the Primitive Technology video series demonstrates how to make lime mortar from the shells of rainforest snails. He was able to do this by heating them up in his primitive kiln, slaking them in water, and then mixing them into a lime putty.
Lime is basically calcium carbonate (CaCo3). The general source of lime is limestone and various other calcareous minerals, though shells, egg shells and coral are other sources of lime. When heated above 840 degrees Celsius, the lime decomposes into calcium oxide (CaO) or Quicklime and releases carbon dioxide (CO2). When water is added to the quicklime it becomes calcium hydroxide Ca (HCO3)2 or lime putty. From here the calcium hydroxide can then be shaped into a form and allowed to set. Carbon dioxide enters the lime putty as it dries causing it to turn back into calcium carbonate. The new calcium carbonate has then set, remaining solid and water resistant. (read more)