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Uncovering the Roman recipe for self-repairing concrete

The magnificent Pantheon in Rome stands defiant after 2,000 years of being built and is one of the best-preserved of all Ancient Roman buildings, in large part because of its continuous use throughout history. Its marble floors are sheltered beneath the world’s largest unreinforced concrete dome. Researchers have probed samples from Roman concrete structures for decades to find out why these ancient buildings endure when modern concrete may crumble after only a few decades.

In a recent study, scientists have moved much closer to the answer - Not only is Roman concrete exponentially more durable than modern concrete, but it can also repair itself. Creating a modern equivalent that lasts longer than existing materials could dramatically reduce the carbon footprint, and become a key component of resilient infrastructure. A little know fact is that concrete is second only to water as the world’s most consumed material, and producing it accounts for approximately 7 percent of global emissions.

Until now, efforts to explain the longevity of Roman concrete have pointed to its use of volcanic tephra - the fragments of rock emitted in an eruption. However prominent researchers from Harvard and laboratories in Italy and Switzerland, suggest another simpler reason: heat. Using a number of different scanning techniques, they examined a sample from a city wall in Privernum, a 2,000-year-old archaeological site near Rome, focusing on micro scale white chunks running through the sample called lime clasts, which are not found in modern concrete.

Previously the lime clasts were thought to be a product of not mixing the concrete properly, but the team’s scanning revealed that the clasts were formed at extremely high temperatures, and are made from various forms of calcium carbonate. They contain a kind of calcium that the team theorized could heal cracks by reacting with water, creating a solution that recrystallizes in fissures to fill them in. That calcium could be the 'missing link' explaining the material’s durability.

The question, then, was where the necessary heat came from to make those clasts. It had been thought that Roman concrete was created by combining water with a calcium compound called slaked lime. But what if the Romans used lime in a more reactive form, called quicklime. When mixed with water, quicklime reacts and produces heat.

To test the theory, the research team created concrete both with and without quicklime. They then cracked the blocks they had created and ran water through the fissures. Only the cracks in the concrete made with quicklime closed up - meaning they had found a recipe for the self-repairing material and proved a hypothesis that the Romans used quicklime in their concrete recipe (or hot mixing). The cracks are healed in two to three weeks using ingredients that are readily available and, most importantly, cheap.

Patents have now been secured by MIT and it is likely that a company will begin producing 'Roman-inspired' concrete by the end of the year.

[Above – The Pantheon south elevation]

So has this team actually solved the mystery of how Roman concrete was made? This recipe and process were lost over a millennium ago. No similar concrete existed until Joseph Aspdin of Great Britain took out a patent in 1824 for a material produced from a mixture of limestone and clay - known as Portland cement because it resembled Portland stone, a limestone used for building in England.

Modern concrete is made from fragments of rock combined with Portland cement—a mixture of limestone, clay or shale, and other ingredients ground and burned at 1,450 degrees Celsius. That process creates an enormous amount of greenhouse gas and produces concrete that’s not durable, degrading sometimes in as little as 50 years, especially in marine environments. Roman concrete, in comparison, is strong, requiring no steel reinforcing it, and is relatively cheap to manufacture.

Concrete infrastructure today, such as roads, can cost 6 to 10 times their initial price when factoring in repairs over their lifespan. Extending the life of concrete made today would dramatically reduce demand and the carbon footprint. When a new road is laid, a pothole shows up every three years on average. If we only had to fill potholes every 10 or 20 years that would be of huge benefit to all. - 01934 311017


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