The primitive technology guy made lime mortar from the shells of rainforest snails

Lime mortar was used for millennia to adhere stone blocks together. It was the primary mortar until Portland cement was introduced in the 19th century. In this video, the Primitive Technology guy made lime mortar from the shells of rainforest snails. It's quite an involved process, and is fascinating to watch. How did people ever come up with making lime mortar in the first place?