Roman concrete piers built two thousand years ago along the Mediterranean coast are stronger now than the day they were poured, because seawater seeping into the volcanic ash mix triggers the slow growth of a rare crystal called aluminous tobermorite that welds new cracks shut

Roman piers along the Mediterranean have stood for two thousand years while modern reinforced concrete fails in fifty. Geologist Marie Jackson and colleagues have identified the crystal doing the slow repair work: aluminous tobermorite, seeded by seawater reacting with volcanic ash.