Rome’s famous Pantheon boasts the world’s largest solid concrete dome. This is an architectural wonder that has endured for thousands of years thanks to the incredible durability of ancient Roman concrete. For decades, scientists have been trying to figure out exactly what makes this material so durable. A new analysis of samples taken from a concrete wall at the Privernum archaeological site near Rome provides insight into these elusive manufacturing secrets. According to a new paper published in the journal Science Advances, the Romans seemingly employed “hot mixing” with quicklime, among other strategies, to give the material self-healing capabilities.
As we have previously reported, like today’s Portland cement (the basic component of modern concrete), ancient Roman concrete was basically a mixture of semi-liquid mortar and aggregates. Portland cement is typically made by heating limestone and clay (as well as sandstone, ash, chalk and iron) in a kiln. Grind the resulting clinker into a fine powder and add a little gypsum to get a smooth, even surface. However, the aggregates used to make concrete in Roman times were made of fist-sized stones and bricks.
in his paper About architecture (c. 30 A.D.), the Roman architect and engineer Vitruvius wrote of how to construct a funerary concrete wall that could withstand long periods of time without falling into ruin. He recommended that the walls be at least two feet thick and made of “square red stone or bricks or lava laid on the course.” The brick or volcanic rock mass must be held together with a mortar composed of slaked lime and porous glass fragments and volcanic eruption crystals (known as volcanic tephra).
Admir Masic, an environmental engineer at MIT, has been studying ancient Roman concrete for several years. For example, in 2019, Masic and two of his colleagues (Janille Maragh at MIT and his James Weaver at Harvard) published a new tool for analyzing Roman concrete samples at multiple length scales from Privernum. developed a set of (EDS) for phase mapping of materials.
Masic is also part of a 2021 study that analyzed ancient concrete samples used to build a 2,000-year-old mausoleum along Rome’s Appian Way, known as the tomb of Caecilia Metella, a noblewoman who lived in the first century AD. was also a co-author. He is widely regarded as one of the best preserved monuments on the Appian Way. They used the Advanced Light Source to identify different minerals in the sample and their orientation, and used scanning electron microscopy.
They found that the tomb’s mortar resembled the walls of Trajan’s marketplace. This is volcanic tephra from the Pozzolan Rosse pyroclastic flows that combine large masses of brick and lava aggregates. However, the tephra used in the tomb mortar contained far more potassium-rich muscovite. Potassium in the mortar dissolved and effectively reconstituted the bonded phase. Some parts of him remained intact after more than 2,000 years, while others were faint and showed signs of splitting. In fact, their structure somewhat resembled nanocrystals. As such, interfacial zones constantly evolve through long-term remodeling, strengthening those interfacial zones.
For this latest study, Masic wanted to take a closer look at a strange white mineral mass known as “lime crust.” “The idea that the presence of these limescales was simply due to poor quality control has always haunted me,” Masic said. If so much effort has been put into making good building materials according to all the detailed recipes that have been made, why have they put so little effort into ensuring the production of a well-mixed final product? Was it?? There must be more to this story.”
The Romans were thought to have combined water and lime to create a chemically reactive paste (slaked lime), but this does not explain the limescale. Masic thinks he may have used even more reactive quicklime (perhaps in combination with slaked lime), and chemical mapping and multiscale his imaging lab analysis with his tools confirm his suspicion. was born. The clumps were different forms of calcium carbonate and spectroscopic analysis showed that they were formed at very high temperatures – aka hot mixing.
“There are two advantages to hot mixing,” says Masic. “First, when the entire concrete is heated to high temperatures, chemical reactions occur that are not possible when using only slaked lime, creating high-temperature-related compounds that would otherwise not form. Second, this elevated The temperature accelerates all the reactions, allowing for much quicker construction, so curing and hardening times are greatly reduced.”
It also seems to grant self-healing abilities. According to Masic, when concrete starts cracking, they are more likely to migrate through limestone. The mass then reacts with water to produce a solution saturated with calcium. The solution either recrystallizes as calcium carbonate to fill cracks or reacts with pozzolanic components to strengthen the composite.
mass and others. found evidence of calcite-filled cracks in other samples of Roman concrete, supporting their hypothesis. They also used ancient and modern recipes to create concrete samples in a laboratory with a high temperature mixing process, deliberately cracking the samples to allow water to flow. found that the cracks in the samples healed completely within two weeks, whereas the cracks never healed in samples without quicklime.
DOI: Science Advances, 2022. 10.1126/sciadv.add1602 (About DOI).
