The Pantheon in Rome still stands defiantly 2,000 years after it was erected, its marble floors protected by the world’s biggest unreinforced concrete dome. Researchers have been probing samples from Roman concrete constructions—tombs, breakwaters, aqueducts, and wharves—for decades to learn why these old structures survive whereas modern concrete crumbles after only a few decades.
According to a recent study, Roman concrete is not only tenfold more durable than modern concrete, but it can also repair itself. The explanation for this repair, according to Admir Masic and his MIT colleagues, as well as experts from Harvard and facilities in Italy and Switzerland, is Heat.
They investigated a sample from a city wall in Roman Privernum, a 2,000-year-old archaeological site near Rome, using a variety of scanning techniques, focused on millimeter-scale white pieces running through the sample, known as lime clasts. These clasts do not exist in contemporary concrete.
The test samples came from the archaeological site of Privernum, near Rome, Italy (A), and shown as a photogrammetry-based three-dimensional reconstruction (B). The architectural mortar samples were collected from the bordering concrete city wall (C). Large-area EDS mapping of a fracture surface (D) reveals the calcium-rich (red), aluminum-rich (blue), silicon-rich (green), and sulfur-rich (yellow) regions of the mortar. Further imaging of polished cross-sections (E) shows aggregate-scale relict lime clasts within the mortar (the large red features denoted by asterisks). The colored arrows in (E) denote the pore-bordering rings visible in the EDS data that are rich in calcium (red) or sulfur (yellow), which are enlarged at right to show additional detail. Photo credits (B and C): Roberto Scalesse and Gianfranco Quaranta, Associazione AREA3, Italy.
Historically, lime clasts were assumed to be a result of improper concrete mixing. Nevertheless, scanning by the researchers revealed that the clasts in Roman concrete were created at extraordinarily high temperatures and are composed of diverse kinds of calcium carbonate.
They contain a kind of calcium that Masic’s team theorized could heal cracks by reacting with water, creating a solution that recrystallizes in fissures to fill them in. That calcium, he says, could be the “missing link” explaining the material’s durability.
Patents have now been secured by MIT. Masic says a company will begin producing what he calls Roman-inspired concrete by year’s end. Concrete infrastructure today, such as roads, cost six to 10 times their initial price when factoring in repairs over their lifespan.
So developing a modern counterpart that outlasts existing materials might minimize climate emissions and become an important component of robust infrastructure, such as seawalls. Currently, concrete is second only to water as the world’s most consumed material, and making it accounts for about 7 percent of global emissions.
Reference- MIT Climate Portal, Science Advances, Wired, The Verge, Interesting Engineering, Popular Science