Romans Used Volcanic Ash ‘Hot-Mix’ As Concrete

Roman concrete is often hailed as a major engineering feat. It has allowed the Empire’s monumental structures to remain standing for two thousand years.

Many of the buildings, bridges, and aqueducts built by Roman architects are still operational.

Now, a new discovery at a perfectly preserved Pompeii construction site has revealed the long-held secret behind this legendary longevity.

"We were blessed to be able to open this time capsule of a construction site and find piles of material ready to be used for the wall. With this paper, we wanted to clearly define a technology and associate it with the Roman period in the year 79 C.E.," said MIT Associate Professor Admir Masic, who led the research.

The team uncovered the Romans' "hot-mixing" technique.

In hot-mixing, lime fragments, volcanic ash, and other dry ingredients were mixed before water was added, generating heat.

The intense heat generated during the mixing trapped highly reactive lime as tiny, gravel-like "clasts" within the concrete. When cracks inevitably formed over thousands of years, these lime clasts dissolved, actively filling and repairing the damage.

In this new research, Masic’s team analyzed an exquisitely preserved ancient construction site in Pompeii, unearthed from the 79 C.E. eruption of Mount Vesuvius.

The study involved analyzing samples from various construction stages: pre-mixed raw materials, a wall being built, completed walls (buttress and structural), and mortar used for repairs.

The Pompeii construction site provided the most definitive proof that the Romans employed hot-mixing for concrete.

Concrete samples from the site contained the characteristic self-healing lime clasts. The team also discovered intact quicklime fragments, pre-mixed with other dry ingredients, in a raw material pile, confirming the vital first step of the hot-mixing process.

"These results revealed that the Romans prepared their binding material by taking calcined limestone (quicklime), grinding them to a certain size, mixing it dry with volcanic ash, and then eventually adding water to create a cementing matrix," Masic noted.

The site also yielded a treasure trove of information about the volcanic ash itself.

Pumice particles, reacting with the concrete’s internal environment over time, created new mineral deposits, further strengthening the material.

This process significantly enhances the concrete’s long-term strength and its capacity for self-repair years after the monumental Roman structures were initially built.

"This material can heal itself over thousands of years, it is reactive, and it is highly dynamic. It has survived earthquakes and volcanoes. It has endured under the sea and survived degradation from the elements," said Masic.