Concrete

A construction material made by mixing cement and water with aggregates such as sand and gravel. Once cured it is strong in compression but weak in tension, so it is usually reinforced with steel.

Ancient Roman concrete lasts 2,000 years by healing itself. In 2023 MIT found the lime specks in it were deliberate, not careless. When water enters a crack, the lime dissolves into calcium that reacts with CO2, sealing it as Calcium carbonate.

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In 2011, France had to destroy an armored vehicle in a Libyan city. Their fix: a 300 kg concrete bomb — a casing of concrete, not explosives. No blast, no shrapnel; the GPS-guided block smashed the vehicle on impact. The USA had used the trick in Iraqi no-fly zones since the 1990s.

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