Roman aqueducts built more than 2,000 years ago are still in operation. The Pantheon in Rome remains the largest dome made of unreinforced concrete in the world, with a diameter of 43.3 meters. At the same time, it is not unusual to see structures less than a decade old collapsing. Understanding why Roman structures remain standing has been the subject of studies by many researchers around the world. Why, even in hostile environments such as coastlines or seismic zones, do these structures remain intact? Is there some miraculous material or method that has been lost in history? An international group of researchers led by the Massachusetts Institute of Technology (MIT) has shed light on these questions, discovering that these structures had a previously overlooked capacity for self-healing, and the potentially huge environmental impact it can have, to create more durable concrete structures in the future.
The conclusions were reached through microscopic analysis, using X-ray technologies, of a sample of Roman concrete on the wall of the ancient city of Privernum, near Rome. This highlighted what we already knew about the composition of Roman concrete: volcanic tuff and other coarse aggregates, bound together by a mortar based on lime and pozzolana (a material found in volcanic ash, named after the town of Pozzuoli, in the vicinity of Vesuvius). The analysis also highlighted tiny white minerals, called "lime clasts," which had been noted previously but were attributed to a sloppy mixing process or low-quality raw materials. What this new study suggests is that it is these harmless white lumps that give concrete a previously unrecognized capacity for self-healing.
The analysis revealed different forms of calcium carbonate, which was not present as a raw material in the initial concrete mix. What the research results suggested is that these lime clasts are actually a source of easily breakable and reactive calcium within the concrete mix. When the structure cracks and water penetrates (and this usually occurs where there are more lime clasts), there is a chemical reaction that creates a saturated calcium solution, which crystallizes as calcium carbonate and quickly fills the cracks, reacting with the pozzolan and further strengthening the material. In other words, there is a self-healing reaction of the cracks inside the pieces, which occurs spontaneously and indefinitely.
But this chemical process only occurs, according to the researchers, because of the way these structures are manufactured. They point out that the quicklime was not mixed with water before being added to the other ingredients, as we currently understand the process of concrete production (also called slacking). Instead, it is likely that so-called "hot mixing" method was used, which means that the quicklime is mixed with the ash and aggregates first, before the water is added.
With these findings, the scientists intend to use these same methods in the creation of modern concretes that have the same self-healing characteristics. According to them, this a "method to reduce cement’s carbon footprint (which accounts for up to 8% of total global greenhouse gas emissions), to improve the longevity of concrete through the incorporation of self-healing functionalities. The resulting extended use life, combined with a reduction in the need for extensive repair, could thus reduce the environmental impact and improve the economic life cycle of modern cementitious constructs."
To learn more details about the scientific study and follow the authors for new findings, access the full paper.
