An international study overseen by the Massachusetts Institute of Technology has revealed the mystery ingredient that has prolonged the life of Roman concrete – quicklime.

Recognised as being more durable than its modern counterpart, Roman concrete has allowed for the Pantheon and Colosseum to have remained standing for millennia. The team, consisting of researchers from the US, Italy and Switzerland found white chunks in the concrete (lime clasts) which enable the concrete to heal cracks over time.

While Roman texts indicate the use of slaked lime, further study indicated lime clasts were created through the utilisation of quicklime, or calcium oxide. Analysis also found the concrete was mixed at high temperatures, which makes for increased durability.

“The benefits of hot mixing are twofold,” says MiT Associate Professor Admir Masic.

“First, when the overall concrete is heated to high temperatures, it allows chemistries that are not possible if you only used slaked lime, producing high-temperature-associated compounds that would not otherwise form. Second, this increased temperature significantly reduces curing and setting times since all the reactions are accelerated, allowing for much faster construction.”

The research team made two samples of concrete in order to test the durability of lime clast-strengthened compound. One was made to Roman standards, and the other to modern. The concrete was then cracked. After a fortnight, water was unable to flow through the Roman concrete, but moved freely through the modern chunk of concrete, which did not contain quicklime.

Lime clasts can dissolve into cracks and recrystallise following exposure to water and are able to heal cracks created by weathering. As a result, using quicklime could result in durable and therefore more sustainable concrete.

"Concrete allowed the Romans to have an architectural revolution," Masic says.

"Romans were able to create and turn the cities into something that is extraordinary and beautiful to live in. And that revolution basically changed completely the way humans live.

For more information on the study, click here.

Jarrod Reedie

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