Ancient Roman concrete could self-heal thanks to “hot mixing” with quicklime

An unexpected ancient manufacturing strategy may hold the key to designing concrete that lasts for millennia.

During the hot mixing process, the lime clasts develop a characteristically brittle nanoparticulate architecture, creating an easily fractured and reactive calcium source, which, as the team proposed, could provide critical self-healing functionality.

As tiny cracks form within the concrete, they can preferentially travel through the high-surface-area lime clasts.

This material can then react with water, creating a calcium-saturated solution, which can recrystallize as calcium carbonate and quickly fill the crack or react with pozzolanic materials to strengthen the composite material further. These reactions take place spontaneously and, therefore, automatically heal the cracks before they spread. Previous support for this hypothesis was found through the examination of other Roman concrete samples that exhibited calcite-filled cracks.

To prove that this was indeed the mechanism responsible for the durability of the Roman concrete, the team produced samples of hot-mixed concrete that incorporated both ancient and modern formulations, deliberately cracked them, and then ran water through the cracks. Sure enough: Within two weeks the cracks had completely healed and the water could no longer flow. An identical chunk of concrete made without quicklime never healed, and the water just kept flowing through the sample. As a result of these successful tests, the team is working to commercialize this modified cement material.

“It’s exciting to think about how these more durable concrete formulations could expand not only the service life of these materials, but also how it could improve the durability of 3D-printed concrete formulations,” says Masic.

Through the extended functional lifespan and the development of lighter-weight concrete forms, he hopes that these efforts could help reduce the environmental impact of cement production, which currently accounts for about 8 percent of global greenhouse gas emissions. Along with other new formulations, such as concrete that can actually absorb carbon dioxide from the air, another current research focus of the Masic lab, these improvements could help to reduce concrete’s global climate impact.

The research team included Janille Maragh at MIT, Paolo Sabatini at DMAT in Italy, Michel Di Tommaso at the Instituto Meccanica dei Materiali in Switzerland, and James Weaver at the Wyss Institute for Biologically Inspired Engineering at Harvard University.

The work was carried out with the assistance of the Archeological Museum of Priverno in Italy.