We finally know why ancient Roman concrete was so strong

(ORDO NEWS) — The ancient Romans were masters of construction and engineering, perhaps the most famous of which were aqueducts.

And these still-functional marvels are based on a unique building material: pozzolanic concrete, the incredibly strong concrete that gave Roman buildings their incredible strength.

Even today, one of their structures, the Pantheon, is still intact and almost 2,000 years old. years – holds the record for the largest unreinforced concrete dome in the world.

The properties of this concrete are usually attributed to its ingredients: pozzolana, a mixture of volcanic ash named after the Italian city of Pozzuoli, where you can find significant reserves of it – and lime. When mixed with water, these two materials can react to form strong concrete.

But, as it turns out, that’s not all. An international team of researchers led by the Massachusetts Institute of Technology (MIT) found that not only were the materials slightly different from what we might have thought, but the methods used to mix them were also different.

the smoking cannons were small white pieces of lime found in what otherwise appears to be well-mixed concrete.

The presence of these chunks has previously been attributed to poor mixing or materials, but it didn’t make sense to materials scientist Admir Masik at MIT.

“The idea that the presence of these lime chips was simply attributed to me has always been a concern for poor quality control,” Masic says.

“If the Romans put so much effort into creating an outstanding building material, following all the detailed recipes that have been perfected over the centuries, why did they put so little effort into ensuring the production of a quality end product? There must be more to this story.”

Masik and his team, led by MIT civil engineer Linda Seymour, carefully examined 2,000-year-old Roman concrete samples from the Privernum archaeological site in Italy.

These samples were subjected to large area scanning electron microscopy and energy dispersive X-ray spectroscopy, X-ray powder diffraction and confocal Raman imaging to better understand lime clasts.

One of the issues in mind was the nature of the lime used. The standard understanding of pozzolanic concrete is that it uses slaked lime.

First, limestone is heated at high temperatures to produce a highly reactive, caustic powder called quicklime or calcium oxide.

Mixing quicklime with water produces slaked lime or calcium hydroxide: a slightly less reactive and less caustic paste.

According to the theory, it was this slaked lime that the ancient Romans mixed with pozzolana.

Based on the team’s analysis, the lime chips in their samples do not match this method. Most likely, Roman concrete was made by mixing quicklime directly with pozzolana and water at extremely high temperatures, alone or in addition to slaked lime. The team calls this process “hot mixing,” which creates lime chips.

“Hot mixing has two benefits,” Masik says.

“First, when all concrete is heated to high temperatures, chemicals can be used that would not be possible if you only used slaked lime, producing heat-related compounds that would not otherwise form.

Secondly, this elevated temperature greatly reduces curing and setting times, as all reactions are accelerated, allowing for much faster construction.”

And one more Advantage: fragments of lime give the concrete a remarkable ability to self-heal.

When cracks form in concrete, they preferentially migrate to lime chips, which have a higher surface area than other particles in the matrix.

When water enters a crack, it reacts with the lime to form a calcium-rich solution that dries and hardens as calcium carbonate, sealing the crack and preventing it from spreading further.

This has been observed. in concrete from another 2,000-year-old site, the tomb of Caecilia Metella, where cracks in the concrete were filled with calcite.

It could also explain why Roman concrete from seawalls built 2,000 years ago has survived intact for millennia despite constant ocean impacts.

So the team tested their findings by making pozzolanic concrete from ancient and modern recipes using quicklime.

They also made test concrete without quicklime and conducted cracking tests. Of course, the cracked quicklime concrete healed completely within two weeks, but the control concrete remained cracked.

The team is now working on commercializing their concrete as a more environmentally friendly alternative to existing concretes.

“It’s interesting to think about how these stronger concrete mixes can increase not only the life of these materials, but also increase the durability of 3D printed concrete mixes,” Masik says.


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