The amazing secrets of Roman concrete from 2,000 years ago. The discoveries in Pompeii surpass modern technology

Researchers are looking for greener and more sustainable options, exploring the secrets of concrete from 2,000 years ago. It has proven to be stronger and more durable than modern concrete, having the ability to self-repair and withstand extreme conditions.

In June 2024, Italy’s Ministry of Culture announced the discovery of a new room in the ruins of Pompeii, on which archaeologists gathered to marvel at the walls covered in a brilliant blue paint – an expensive pigment reserved for special rooms – and of the detailed frescoes with agricultural images, remarkably well preserved after nearly 2,000 years.

In a corner of the room, an ordinary pile of sandy soil was found, which this time amazed the specialists from MIT – the prestigious research university in Cambridge, Massachusetts. The material, light beige in color and grainy, was in fact an essential component of the Roman Empire, as experts would find. It is about the precursor of concrete, a cornerstone of the imperial infrastructure, used including in the construction of aqueducts through which drinking water reached cities such as Pompeii.

The discovery therefore opened up new opportunities for research on building materials from Ancient Rome, underlining their importance in finding sustainable modern solutions. They have the potential to transform the way structures, new and old, are built and preserved. Thus, researchers began to reinvent concrete by taking inspiration from the techniques used by Roman builders two millennia ago, according to The New York Times.

The material that amazed the researchers

“They managed to bring water to the city, and with water came hygiene. This progress technological it allowed them, first of all, to build Rome as we know it, but also to reproduce this model wherever they went in the wider world”noted Admira Masic, a chemist at MIT, referring to the grandiose Roman constructions that remain standing to this day.

Modern concrete, based on a material known as Portland cement, was developed in England in the 19th century and is arguably the most popular building material in the world. It’s cheap, strong and standardized, giving engineers around the world an easy-to-use material for building houses, dams, skyscrapers and more.

However, it was no small surprise to the specialists to find that it is much less resistant than the concrete used in ancient times in the Roman Empire. Over time, it has been discovered that today’s concrete develops cracks that can eventually lead to the destruction of the material if water gets inside.

Modern concrete production is extremely polluting

In addition, today’s concrete manufacturing is a major contributor to climate change, generating 8% of global carbon dioxide emissions.

By unlocking the secrets of ancient Roman concrete, researchers like Dr. Masic are trying to develop modern, greener, and more sustainable options.

“Even marine concrete (a special material used by the Romans in underwater constructions such as dikes, harbors and other structures exposed to sea water – ed.) survived in one of the most aggressive environments on Earth, without no maintenance”noted Dr. Marie Jackson, a geologist at the University of Utah, referring to the danger of saltwater corrosion.

The surprising discoveries of experts about ancient Roman concrete

During the research of the ancient concrete used by the Romans in construction, it was discovered that much of its strength comes from a hydrated mixture of calcium and aluminum silicates, known as CASH, with varied chemical formulas. However, the exact way in which the Romans produced this material remains unclear.

According to traditional belief, the Romans heated limestone, consisting mainly of calcium carbonate, to obtain a highly reactive material called quicklime or calcium oxide. Then they added water, forming calcium hydroxide, known as slaked lime. Finally, this material was combined with a bulk ingredient, most often volcanic ash, which provided the aluminum and silicon needed for concrete – the A and S of CASH.

The MIT specialist challenges this explanation, noting that many ancient concrete specimens contain visible white chunks known as clasts: “You see them everywhere – in Rome, in Africain Israel”.

The self-healing substance

Until now, these pieces were thought to be the unintended results of poor workmanship, but Dr. Admira Masic believes that in fact Roman engineers were too ingenious to consistently produce defective concrete: “People say that these lime clasts are the result of bad mixing of slaked lime. Our hypothesis is that we cannot talk about a faulty process, but about an integral part of the technology”.

According to MIT research, these lime clasts were actually calcium reservoirs that helped fill cracks, turning concrete into a self-healing material. As cracks formed, water seeped in and dissolved the calcium in the lime, which then formed solid calcium carbonate, essentially creating a new rock that filled the crack.

The MIT specialist’s theory is that the lime fragments did not come from slaked lime, but from quicklime, which the Romans added directly, in a process called hot mixing. As quicklime is highly reactive, it generates heat when combined with volcanic ash, heating the material to over 77 degrees Celsius, which causes the concrete to harden much faster. This technique also created hot zones of nearly 200 degrees Celsius, causing some of the quicklime to remain in small, intact pieces—the clasts visible in preserved concrete to this day, which give it its self-healing properties.

The glue that seals concrete cracks, created by a specialist from MIT

It seemed mission impossible to prove that the Romans intentionally left pieces of quicklime in the self-repairing concrete, given that these fragments have changed chemically over the centuries, but by examining the clasts with special microscopes, Dr. Admira Masic and his colleagues demonstrated that they really did start as quicklime.

The research led the MIT specialist to found a company, called DMAT, which aimed to integrate the principles of ancient concrete chemistry into modern versions. It sells an additive that it claims seals cracks in concrete, which in theory could reduce reliance on Portland cement, known for its carbon footprint.

Volcanic reactions

But not all researchers share the theory of hot mixing, not convinced that this is the key to the self-healing concrete of the Romans. Dr. Marie Jackson, for example, believes the secret actually lies in the bulky materials that have been mixed with the lime, often a type of volcanic ash called pozzolana. It has binding properties and reacts with lime (calcium) in the presence of water, contributing to the durability and strength of concrete.

Named after the Italian coastal town of Pozzuoli, where it was largely excavated, pozzolana activated special chemical reactions that gave ancient concrete its unmatched durability, the Utah geologist believes.

According to his research, the initial reaction between lime and pozzolana would actually have generated CASH compounds, which acted as an adhesive in ancient Roman concrete.

These materials continued to react over time, forming rare minerals such as strätlingite, Dr. Marie Jackson also discovered. Crystals of this mineral, with flake and needle-like shapes, helped bind the rough pieces to the concrete material and prevented the development of cracks.

Arches made of volcanic ash – pozzalana – which hardens in water

“Toughening of concrete appears to be essential for long-term strength”noted the specialist in geology, adding that this process “helped strengthen the bond and integrity of the material over the centuries.”

Dr. Jackson and his collaborators tested their hypotheses about ancient concrete by creating modern analogs of it. In one experiment, researchers built concrete arches, submerged them in salt water for 50 days, and then applied pressure to the top of them until the concrete began to warp and crack.

Then the arches were submerged again for almost a year and tested. The researchers found how the CASH compounds filled the small cracks/cracks and the arches withstood two to three times more force than before, depending on the specific test.

“The Roman builders were the true masters”

The team submerged the arches again, and at the end of October 2024, they plan to test them again after nearly three years of exposure to seawater.

“The way ROMANS they chose the materials actually prevented the propagation of cracks”said Dr. Marie Jackson.

The geology specialist and her collaborators think they know exactly when the Romans achieved this mastery of construction: the 1st century BC. Marcello’s Theater and Trajan’s Squares – two of the Roman sites studied by the Utah team – come in support of this theory.

Clay enriched with kaolinite, the key innovation inspired by Roman techniques

And an engineer at the University of Texas, Arlington, Warda Ashraf, has developed a type of concrete inspired by Roman techniques intended for underwater use to build more durable bridges, levees and artificial reefs while providing strength comparable to of ordinary modern concrete.

The key innovation was the use of clay enriched with the mineral kaolinite, a cheap and accessible material, to replace the volcanic ash in the ancient recipe. “We use exactly the same proportions that the ancient Roman engineers used”, said the engineer.

To chemically activate the clay, he heated it to over 700 degrees Celsius. But unlike regular Portland cement, which has to be fired in kilns at around 1,400 degrees Celsius, this process represents a significant energy saving, leading to a 70% reduction in carbon footprint, according to Dr Ashraf.

The Texas researchers tested their creation in the waters of the Gulf of Mexico, making dozens of concrete objects – cylinders, cubes and discs – that they placed in yards on the seabed with the help of divers. After a year, they found that the strength of the concrete had increased considerably.

These innovative approaches not only reduce energy costs, but also contribute to the development of greener construction solutions.