The Colosseum, the Roman Theater of Mérida, Caesarea Maritima in present-day Israel … these are just a few examples Roman architecture and engineering which has reached our days; Many of them were built on a concrete base, the Roman ‘opus caementicium’. As an exemplar of this technique, the famous Pantheon in Rome, which has the world’s largest unreinforced concrete dome and was built between 118 and 125 AD and is still intact. It still stands nineteen centuries after its construction, while its structures concrete The modern ones have collapsed after a few decades. So what is that ‘secret’ ingredient that made Roman building materials so durable?
This is a question to which scientists have been searching for an answer for a long time. Even more so considering that they have been used for elements such as docks, sewers or dikes that tolerate extreme conditions. The buildings have remained standing even during earthquakes.
Now, a team of researchers from MIT, Harvard University, and laboratories in Italy and Switzerland says they’ve solved the mystery: quicklime. This material will not only cause the material to dry earlier, but also ‘self repair‘ for longer. The findings have just been published in the journal ‘Science Advances’.
beyond volcanic ash
For many years it was thought that the Roman concrete was volcanic ash from the area of the key. Pozzuoliin the bay Naples, This gave the material pozzolanic properties when it reacted with calcium hydroxide to form hydraulic compounds similar to those generated during hydration of cement clinker. The element is believed to have been shipped throughout the vast Roman Empire for use in manufacturing as a key ingredient.
However, this was not enough. Analyzing the concrete, the researchers also found that the samples contained millimetre-sized, bright white grains called lime clasts – one of the building blocks of concrete – which was attributed to an ‘oversight’ ‘finish off’ was considered. Construction material.
“Ever since I started working with ancient Roman concrete, I’ve always been fascinated by these features,” he explains. i admire messik, MIT professor of civil and environmental engineering and first author of the paper. “These components are not found in modern concrete formulations; So why are they present in these ancient materials?
Messick always resisted the idea that these lime blasts were a construction fault. “If the Romans put so much effort into creating an excellent building material, following all the elaborate recipes adapted over many centuries, why did they put so little care into the final production? There must have been more to the story,” says the researcher.
By further analyzing these eruptions using high-resolution multiscale imaging and laboratory chemical mapping techniques, the team gained new insights into the potential functionality of these limestone eruptions.
cooking concrete two millennia ago
Historically, it was believed that when lime was incorporated into concrete, it first combined with water to form a highly reactive pasty material, a process known as liming. off, But this process alone could not explain the presence of lime flakes. Messick then wondered whether it was possible that the Romans used lime, but in its most reactive form: slaked lime.
By studying samples of this ancient material, the team determined that the white inclusions were composed of different forms of calcium carbonate. And spectroscopic examination provided clues that these granules were formed at extreme temperatures, as would be expected from an exothermic reaction resulting from the use of quicklime rather than slaked lime. or other than that. The hot mix, the researchers concluded, was actually the key to its super-durable nature.
“The benefits of hot blending are twofold,” says Messick. On the one hand, when the mixture is heated to high temperatures, processes occur that are not possible if only slaked lime is used. On the other hand, this increase in temperature significantly reduces curing and setting times, as all reactions are accelerated, allowing for much faster fabrication.
The benefits do not end here. During the hot mixing process, the lime flakes develop an architecture nanoparticles The particularly brittle, forming an easily brittle and reactive calcium source could, as the team proposed, confer the material’s ‘self-healing’ power. In particular, it helps when cracks begin to form within the concrete: these veins are ‘guided’ through these cracks. But this material then reacts with water, creating a saturated calcium solution that can recrystallize as calcium carbonate and quickly fill the crack. or react with pozzolanic materials to further strengthen the composite material. These reactions occur spontaneously and therefore the cracks regenerate spontaneously before they propagate.
Making concrete ‘the Roman way’
To prove their theory, the team made samples of hot-mix concrete incorporating ancient and modern formulations, deliberately cracked them, and then ran water through the cracks. Sure enough: In two weeks, the cracks had fully regenerated and water could no longer flow through. A similar piece of concrete made without slaked lime never cures on its own, and water continues to flow through the sample. The team is so confident of their discovery that they are working to produce a commercial version of this Roman building material.
“It is exciting to think about how these more sustainable formulations may not only extend the lifetime of these materials, but also how they may improve the durability of 3D printing formulations,” says Messick.
Through the development of extended service lives and lightweight concrete forms, they hope these efforts can help reduce the environmental impact of cement production, which currently accounts for about 8 percent of global greenhouse gas emissions.