Tabby construction in DC?

We were on a jobsite last month where there was a section of concrete paving. The concrete paving was old and largely broken up. I wasn’t expecting this but I found old mollusk shells in the concrete, similiar to the old tabby walls common on the shores of the Carolina’s.

Tabby concrete construction is a method of building using fired mollusk shells to create a lime cement. Together with the resulting lime binder the shells themselves also were kept in the concrete as an aggregate.