Scientists have discovered the key to making Roman concrete survive forever.

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• concrete
• calcium
• romans
• mineral
• ancient
• pliny

The durability of Roman concrete is undeniable. Even after millennia, certain historic structures have remained intact. The secret of their longevity has eluded scientists for decades despite their best efforts. Scientists have done some sleuthing and discovered the secret to their durability.

Cement, gravel, sand, and water come together to form concrete. Scientist and chemist Admir Masic works at MIT in nearby Cambridge. He was working with a group that was investigating the Romans' method of blending these substances.

Hot mixing was thought to be the key by the researchers. Calcium oxide, often known as quicklime, is the mineral used in this process. This quicklime is combined with volcanic ash to create cement. Finally, water is added.

They reasoned that when cement was mixed at high temperatures, it wouldn't turn out perfectly smooth. Instead, there would be tiny pebbles that are rich in calcium. The concrete walls of ancient Roman structures were riddled with tiny pebbles. These might shed light on how such buildings have survived the test of time.

Masic's group had studied works by Pliny the Elder and the Roman architect Vitruvius. Their essays provided a few hints. The basic components were specified in great detail in these books. For instance, quicklime requires extremely pure limestone in its production. According to the writings, a potent amount of heat may be generated by combining quicklime and hot ash, followed by the addition of water. There was no mention of rocks. The group knew they were special nevertheless. Inclusions, in the form of tiny white boulders, were present in every sample of Roman concrete they had examined.

For a long time, Masic recalls, nobody knew where the inclusions originated from. Others speculated that perhaps the cement hadn't been properly blended. Yet the Romans had a highly ordered government. When asked how possible it is that "every operator [was] not mixing properly and every single [structure] had a defect," Masic replies, "not very likely."

His team speculated that maybe the inclusions weren't a flaw after all, but rather, a property of cement. The embedded bits at a single ancient Roman site were analyzed by the researchers. Analyses of these inclusions revealed a high calcium content.

And it brought up an intriguing possibility: perhaps the pebbles are assisting the structures in mending themselves. They might perhaps repair damage from weathering or even an earthquake. They might be able to provide the calcium for the fix. When it dissolves, some of the calcium can leak into the fissures and then re-crystallize there. Then, ta-da! The wound closed up.

A prayer that nothing goes boom

Modern cement production does not involve hot mixing. As a result, they chose to watch the procedure unfold. Extreme heat and perhaps an explosion can result from combining quicklime with water. Masic recalls that his team went ahead and accomplished it despite widespread advice to the contrary.

The initial process was recreating the rocks. Hot mixing was utilized as they watched. There was not a big bang. Heat, water vapor, and a cement mixture including tiny white pebbles rich in calcium were all that came of the process.

The following step was to put this cement to the test. The group made concrete using the hot-mixing method and without it, and then compared the results. Each concrete block was shattered in two. Separation between the parts was kept to a minimum. Then, water was slowly poured into the crevice to see how long it would take for the leak to stop.

"The findings were shocking," Masic exclaims. Within two to three weeks, the hot-mixed cement blocks were completely repaired. When we didn't use hot-mixed cement, our concrete cracked and never hardened. On January 6, 2019, the group published their results in Science Advances.