The famous Pantheon in Rome boasts the world’s largest unreinforced concrete dome – an architectural marvel that has endured for thousands of years thanks to the incredible durability of ancient Roman concrete. For decades scientists have been trying to find out what exactly makes the material so durable. A new analysis of samples taken from the concrete walls at the Privernum archaeological site near Rome has provided insight into these elusive manufacturing secrets. It seems that the Romans used “hot mixing” with quicklime, among other strategies, which according to a gave the material a self-healing function new paper published in the journal Science Advances.
As we have reported before, like today Portland cement (a basic ingredient of modern concrete), ancient Roman concrete was basically a mixture of a semi-liquid mortar and aggregate. Portland cement is typically made by heating limestone and clay (as well as sandstone, ash, chalk, and iron) in a kiln. The resulting clinker is then ground into a fine powder, with just a touch of gypsum added to give a smooth, even surface. But the aggregate used to make Roman concrete was fist-sized chunks of stone or brick.
In his treatise Architecture (c. AD 30), Roman architect and engineer vitruvius wrote about how to build concrete walls for burial sites that will last a long time without decaying. He recommended that the walls be at least two feet thick and be of either “square red stone, or of brick or lava laid in layers”. The brick or volcanic rock aggregate should be bound with mortar made of hydrated lime and porous glass fragments and crystals from volcanic eruptions (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 colleagues (MIT’s Janille Maragh and Harvard’s James Weaver) pioneered a new set of tools for analyzing Roman concrete samples from Privernum at multiple length scales – specifically, Raman spectroscopy for chemical profiling and energy-dispersive spectroscopy with multiple detectors (EDS) for phase imaging of the material.
Masic also co-authored a Study 2021 Analyzing samples of the ancient concrete used to construct a 2,000-year-old mausoleum along Rome’s Via Appia known as Tomb of Cecilia Metella, a noblewoman who lived in the first century AD. It is widely considered to be one of the best preserved monuments on the Via Appia. They used the Extended light source to identify the many different minerals contained in the samples and their orientation, and scanning electron microscopy.
They discovered that the tomb’s mortar was similar to the tomb’s walls Markets of Trajan: volcanic tephra from the Pozzolane Rosse pyroclastic flow, connecting large boulders of brick and lava aggregates. However, the tephra used in the tomb’s mortar contained much more potassium-rich leucite. The potassium in the mortar, in turn, dissolved and effectively reconfigured the binding phase. Some parts remained intact after more than 2,000 years, while other areas looked paler and showed some signs of fission. In fact, the structure somewhat resembled nanocrystals. Thus, the interfacial zones are constantly evolving through long-term remodeling and reinforcing these interfacial zones.
For this latest study, Masic wanted to take a closer look at strange white chunks of minerals known as “calcareous clasts,” which others have largely dismissed as the result of inferior raw materials or poor mixing. “The idea that the presence of these limestone fractures is simply due to poor quality control has always bothered me,” said Masic. “If the Romans took so much trouble to produce an excellent building material, after all the detailed recipes tweaked over many centuries, why should they put so little effort into ensuring the production of a well-mixed end product? There has to be more to this story.”
It was thought that the Romans combined water with lime to produce a highly chemically reactive paste (slaking), but this would not explain the lime quarries. Masic thought they might have used the even more reactive quicklime (possibly in combination with slaked lime), and his suspicion was confirmed by the lab’s analysis using chemical mapping and multiscale imaging tools. The clasts were various forms of calcium carbonate, and spectroscopic analysis showed that these clasts had formed at extremely high temperatures – also known as hot mixing.
“The benefits of hot mixing are twofold,” said Masik. “First, having all the concrete heated to high temperatures enables chemistry that would not be possible using just slaked lime, creating high-temperature-associated compounds that would not otherwise form. Second, this increased temperature shortens curing and setting times significantly because all reactions are accelerated, allowing for a much faster design.”
It also appears to confer self-healing powers. According to Masic, when cracks start to form in the concrete, they are more likely to move through the layers of lime. The clasts can then react with water, creating a solution saturated with calcium. This solution can either recrystallize as calcium carbonate to fill the cracks or react with the pozzolanic components to strengthen the composite.
mask et al. found evidence of calcite-filled cracks in other samples of Roman concrete, supporting their hypothesis. They also prepared concrete samples in the lab using a hot-mix process using ancient and modern recipes, then intentionally broke the samples and ran water through them. They found that the cracks in the samples made with hot-mixed quicklime healed completely within two weeks, while the cracks in the samples without quicklime never healed.
DOI: Advances in Science, 2022. 10.1126/sciadv.add1602 (About DOIs).
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