What special property may have helped Roman concrete last so long?

Answer

A 2023 MIT study suggests hot mixing created the clasts. Researchers are still testing the idea.

Context

Roman builders mixed lime, volcanic ash and rock aggregate, and structures such as harbors and the Pantheon dome have survived for roughly two thousand years. For a long time, researchers credited mainly the volcanic ash, which reacts chemically with lime in a pozzolanic reaction. The MIT study led to a more nuanced picture. Millimeter-sized white chunks of lime, called lime clasts, were once dismissed as signs of poor mixing, but the team argued that they are a feature that matters.

The researchers propose that the Romans added quicklime directly to the mix rather than only slaked lime. Quicklime reacts with water and gives off heat, so the mixture got hot, a process the authors call hot mixing. The high temperature would have produced the clasts and sped up curing. When a crack later forms, it tends to run through these brittle, reactive lumps. Water entering the crack dissolves calcium from the clasts, and that calcium then recrystallizes as calcium carbonate, filling the gap and sealing it.

The team tested this by cracking samples made with and without quicklime and running water through them. According to MIT, the hot-mixed sample healed within about two weeks and stopped leaking, while the control sample did not heal. The phrase may have is important: the work comes from modern lab replicas and analysis of ancient samples, and other researchers have noted that Roman recipes varied and that seawater chemistry also played a role in harbor concrete. The finding is a leading hypothesis, not a settled account of every Roman structure.