Those white lumps in 2,000-year-old Roman concrete, long dismissed as sloppy mixing, are actually a self-healing feature — when water seeps into a crack, the lime clasts dissolve and recrystallize, sealing the gap.

MIT researchers found the Romans used "hot mixing" with quicklime, a trick now being revived to make modern concrete last far longer.

Source: Riddle solved: Why was Roman concrete so durable? — MIT Civil and Environmental Engineering