We may have discovered the secret reinforcing ingredient in Roman concrete

A new Massachusetts Institute of Technology (MIT) study of a 2050-year-old Roman tomb has revealed some interesting data on the nature of Concrete from the Roman era. Although it is a very ancient technology, the durability of this material has long hampered experts, until now.

By studying the concrete of some of the existing Roman concrete structures, the team has discovered some interesting discoveries that may explain why Roman concrete lasted so long. The study was recently published in Journal of the American Ceramic Society.

In stark contrast, modern concrete tends to crack and disintegrate after only a few short decades through a process called swinging. But Roman concrete, invented before modern material science, seems to be able to last for centuries without the same problem. Often many existing Roman concrete structures are in fairly good condition given their age – a feat that cannot be reproduced with the help of modern concrete recipes.

The study focuses on the remarkably well-preserved tomb of a Roman noblewoman, The tomb of Cecilia Metella and Kastrum Caetani ruins in Rome. After analyzing the concrete used in the smallest detail, it seems that the secret may be the use of a volcanic aggregate and the interesting way in which concrete has reacted to rain and groundwater over the centuries.

“Understanding the formation and processes of ancient materials can inform researchers of new ways to create durable, sustainable building materials for the future,” he said. Admir Masic, Associate Professor of Civil and Environmental Engineering at MIT. “The tomb of Caecilia Metella is one of the oldest structures that still exists, offering insights that can inspire modern construction. “

The tomb is interesting in itself and is located at Apievski road in Rome. Landmark of Through Appia Antica, the tomb is a rotunda-shaped tower that sits on a square base, a total of about 70 feet (21 meters) high and 100 feet (29 meters) in diameter.

Source: Livioandronico2013 / Wikimedia Commons

It is believed to have been built around 30 BC, just at the time when the Roman Republic was transformed into an empire by Emperor Augustus. The size and quality of the tomb show that its eternal inhabitant, Kesilia, must have been a very important person.

And that she was. A member of an aristocratic family, she married the family of Marcus Crassus, who formed a famous alliance with Julius Caesar and Pompey.

“The construction of this very innovative and robust Via Appia Antica monument and landmark shows that it has been respected,” said Ari Jackson, a research professor of geology and geophysics at the University of Utah and co-author of the study, “Concrete Fabric 2050 Years Later. late reflects a strong and resilient presence. “

The tomb was built using a very complex method of construction (for the period) and a technique described by a famous Roman architect. Poisonous.

Thick walls of rough brick or volcanic rock aggregate are laid, each bound with mortar made of lime and volcanic tephra (porous fragments of glass and crystals of explosive eruptions). This would lead to structures that “do not fall into ruins for a long time”, Poisonous tell us.

Given the number of well-preserved Roman concrete structures today, it seems he has hit the money.

What is the secret behind the longevity of Roman concrete?

The study found that the secret of the long-lasting properties of Roman concrete it may be a mineral called leucite. The mineral is rich in potassium, which over time dissolves and effectively remodels and reorganizes the interface between volcanic aggregates and cementitious bonding matrix. This improves the adhesion of the concrete, making it stronger over time.

“Focusing on the design of modern concretes with constantly reinforcing interfacial zones can provide us with another strategy to improve durability of modern building materials“Masic explains.

“Doing so by integrating time-tested ‘Roman wisdom’ provides a sustainable strategy that could improve the longevity of our modern solutions by orders of magnitude,” he added.

Another member of the study, Linda Seymour, studied the microstructure of concrete with scientific tools. “Each of the tools we used added clues to the mortar processes,” Seymour explained.

Scanning electron microscopy (SEM) and energy-dispersion X-ray spectrometry are among the most exciting. The first is able to show the microstructures of the mortar on a micron scale. The latter allows researchers to determine the elementary composition.

“This information allows us to quickly explore different areas of mortar, and we could select building blocks related to our issues,” Seymour said. The trick, she adds, is to hit exactly the same building target with each tool when that target is only about the width of the hair.

Modern science revealing ancient secrets

Careful examination of the concrete walls of the tomb shows that the mortar contains volcanic tephra, which connects large pieces of brick and lava. Like the mortar used in the Trajan Markets, built 120 years later, this “glue” consists of a building block called the CASH bonding phase (calcium-aluminum-silicate-hydrate), along with crystals of a mineral called strätlingite.

Roman concrete coliseum
Source: Diliff / Wikimedia Commons

However, the tephra used for the tomb of Kecilia is much richer in leucite. Over the centuries, when the walls of the tomb were exposed to rain and groundwater, leucite dissolved, releasing more and more potassium into the mixture. For modern concrete, this would be very problematic, as the abundance of potassium would create expansive gels that would cause microcracking and possibly deterioration of the structure.

But in the walls of the tomb potassium dissolves and reconfigures the binding phase of CASH.

“X-ray diffraction and Raman spectroscopy techniques have allowed us to study how the solution has changed,” Seymour said.

“We have seen CASH domains that have been intact after 2050 and some that are split, thin or otherwise different in morphology. In particular, X-ray diffraction allows analysis of thin domains to their atomic structure. We see that thin domains acquire this nanocrystalline nature, ”she explains.

These reworked domains “obviously create strong cohesion components in concrete,” Jackson said. In these structures, unlike the Markets of Trajan, a small streetling formed.

“The interface between the units and mortar “Every concrete is essential for the strength of the structure,” says Masic. In modern concrete, the reactions of alkali-silica, which form expansive gels, can compromise the interfaces of even the hardest concrete. “

“It turns out that the interphase zones in the ancient Roman concrete The tomb of Caecilia Metella is constantly evolving through long-term remodeling, “Masic added.” These remodeling processes reinforce the interfacial zones and potentially contribute to improving the mechanical properties and resistance to destruction of the ancient material. “

You can read the original research paper at Journal of the American Ceramic Society.

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