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Why Does Ancient Roman Concrete Last So Long?

Ancient Romans built concrete sea walls that have withstood pounding ocean waves for more than 2,000 years. Now, an international team has discovered a clue to the concrete’s longevity: a rare mineral forms during chemical reactions between the concrete and seawater that strengthen the material.

Structural engineers might be able to use these insights to make stronger, more-sustainable concrete, says team leader Marie Jackson, a geologist at the University of Utah in Salt Lake City. She and her colleagues report their findings on 3 July in American Mineralogist

Modern concrete uses a paste of water and Portland cement, a fine powder made mostly of limestone and clay, to hold together small rocks. But it degrades within decades, especially in harsh marine environments. Instead of Portland cement, the Roman concrete used a mix of volcanic ash and lime to bind rock fragments. The Roman scholar Pliny the Elder described underwater concrete structures that become “a single stone mass, impregnable to the waves and every day stronger.” This piqued Jackson’s interest. “For me the question was, how does this material become a rock?” she says..