Those white chunks in ancient Roman concrete were not sloppy mixing. Researchers found the lime clasts let the concrete heal itself: water seeping into a crack dissolves them and new calcium carbonate crystals seal the gap.

It helps explain why Roman harbor walls still stand in the surf while modern concrete can crumble within decades, and it could inspire longer-lasting, lower-carbon cement today.

Source: Scientific American, Ancient Roman Concrete Has 'Self-Healing' Capabilities