Science may have solved the mystery of super-strong concrete

Science may have solved the mystery of super-strong concrete

Science may have solved the mystery of super-strong concrete

  • Ancient Roman concrete is incredibly durable, even more so than modern concrete.
  • Scientists have long wondered what gives it its incredible strength.
  • A team may have cracked the mystery by focusing on tiny white spots once thought to be blemishes.

It’s one of the great mysteries of archaeology: how did the Romans create concrete so strong that their buildings still stand 2,000 years later?

The question has long intrigued scientists, not only because concrete has incredible strength, but also because it appears to be able to self-repair, meaning cracks inexplicably disappear over time.

The Pantheon in Rome is a perfect example. The structure, built around the year 126, has been in constant use since then. But its intricate dome looks new even today.

So what makes this material so special? A group of scientists from MIT and Harvard, who published their findings in the peer-reviewed journal Science Advances on January 6, may have cracked it.

These scientists suggest that the secret lies in the white flecks found in concrete called “lime clasts.”

The dome of the Pantheon is depicted with light radiating through its center.  This complex concrete structure is made up of bevelled squares that line the entire surface of the dome.

This concrete dome inside the Pantheon has stood the test of time.

Photograph by Stephen Knowles/Getty

White imperfections that make concrete perfect

Although these stains are present in almost all Roman structures, they have generally been dismissed as imperfections in building materials.

All concrete is made up of a few basic elements: a liquid binder, called mortar, and aggregates (usually loose gravel, sand or small rocks). Roman mortar was made from lime, a chemical created by heating limestone.

The general belief was that the lime was first mixed with water, before being added to the aggregates. Stains were considered a sign that the mortar was not mixed well by the builders.

But Admir Masic, author of the research and professor of civil and environmental engineering at MIT, said he was never really convinced.

“If the Romans put so much effort into making an exceptional building material, following all the detailed recipes that had been optimized over many centuries, why would they put so little effort into ensuring the production of a well-blended end product? he said in a press release

After studying the spots more closely, Masic and the team of scientists concluded that they were likely to have been placed there on purpose. This, they found, was crucial to the self-healing property of concrete.

Extremely important spots for self-healing

A 3D model shows a concrete block lit in red blue and green.  A large red ball is present at the front of the bump.  Red represents calcium

Scientists have analyzed the elements of this concrete block. A false color image shows a large chunk of calcium, in red, in this image.

Courtesy of the researchers

The lime clasts – the white spots – are quite brittle, and that’s a good thing.

When cracks appear in the concrete, the clasts crumble, releasing calcium which can cross the fracture. When water seeps into the cracks, the calcium, seen above in red, reacts with the water, creating new crystals.

These crystals automatically fill the crack and fix the structure.

An aerial view of the pantheon shows the perfectly formed round dome.

Aerial view of Piazza della Rotonda with the Pantheon and city buildings, Rome.

Nico De Pasquale Photography/Getty Images

An explosive mixture

Scientists believe this could only happen if lime was added to concrete in its heated, powdered form.

Immediately after being heated in the kiln, lime is very reactive and can be dangerous.

He is extremely dehydrated at this point. So, as soon as this chemical comes into contact with water, it incorporates it into its chemical structure to make it a more stable molecule. This reaction releases a lot of energy, which releases intense heat.

When people make concrete, they usually add water to the lime powder first, let it cool, and then add the aggregates.

But by mixing lime with the aggregates, then adding water, the Romans created a controlled explosion, increasing the heat in the mixture just enough to change the chemical composition of the concrete, which led to the incorporation of lime clasts, Masic said.

The scientists put their theory to the test. They made concrete blocks, one using powdered lime and the other using a more modern version of concrete that had none. Then they deliberately broke the blocks to create cracks.

They found that the Roman formulation was repaired within weeks under a trickle of water. The modern formulation did not.

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