Concrete is a ubiquitous building material consisting of aggregate bonded together by cement. The most common aggregates are sand and gravel, and the most common cements are based on calcium oxides formed by heating ground limestone. It's an inexpensive, durable and waterproof building material strong in compression with strength in flexion commonly added through internal steel mesh.
However, simpler concretes have been crafted throughout post-agricultural human history. Tabby concrete is a unique construct probably brought to the Americas by Spanish explorers from North Africa in the 1600’s. It was used extensively in the southeastern American colonies, taking advantage of huge ancient Native American oyster middensfound along the seacoast. The calcium carbonate (CaCO3) shells were ground and heated, releasing carbon dioxide(CO2) and leaving behind calcium oxide (CaO) or quicklime which forms a strong cement when water is added. Before hardening, this cement was mixed with unheated oyster shells as aggregate and layered within wooden forms, creating durable Tabby concrete structures. With a background of bald cypress and Spanish moss, this photo highlights the walls of a long abandoned sugar mill near Kings Bay in coastal southeast Georgia -- its Tabby concrete walls standing tall for nearly 200 years.