Have We Finally Discovered the Secret to the Durability of Ancient Roman Concrete?

Since I have launched into every new year for the past 25+ years by attending the World of Concrete , it is fitting that my first newsletter of the year centers on what could be one of the most exciting #concrete discoveries made in decades (maybe centuries). I am not the only one writing about this. Articles have appeared everywhere from the original MIT News announcement to CNN. The discovery centers around the long-pondered question of why ancient Roman concrete lasts considerably longer than our modern equivalents.

Anyone who questions that last statement need look no further than ancient Roman structures including the Pantheon, which still stands today as the world's largest unreinforced concrete dome, over 1800 years after its construction. The ancient Romans were masters of engineering, constructing vast networks of roads, aqueducts, ports, and massive buildings, whose remains have survived for two millennia. Many of these structures were built with concrete, including the aforementioned Pantheon and ancient aqueducts that are still used to bring water into Rome today. The Romans put incredible effort into developing and producing their concrete, helping them to transform their infrastructure and architectural landscape.

Throughout the entire ancient Roman Empire, architectural elements, such as walls and foundations, and infrastructure systems, including aqueducts, roads, and bridges, were created from unreinforced concrete. This concrete was typically composed of volcanic tuff and other coarse aggregates, and bound by a mortar based on lime and pozzolanic materials such as volcanic ash. Volcanic ash can be compared to our modern pozzolanic equivalent fly ash, which is a byproduct of burning coal at power plants, however, the volcanic ash had properties that have not been able to be duplicated.

For many years, researchers have assumed that the key to the durability of ancient Roman concrete was the volcanic ash. The basis for the assumption was not unfounded as there is evidence and documentation showing that the Romans shipped this material throughout their Empire along with specific instructions on how use it to produce concrete for both earth bearing structures and marine structures (ancient mix designs). Now, a team of investigators from around the world, including Admir Masic from the MIT Concrete Sustainability Hub believes they have identified additional, and potentially reproducible, manufacturing techniques used in ancient concrete-manufacturing strategies that incorporated several key self-healing functionalities.

The?findings?are published today in the journal Science Magazine , in a paper by MIT professor of civil and environmental engineering Admir Masic , former graduate students Linda Seymour, Ph.D. and Janille Maragh, Ph.D., P.E. , along with Paolo Sabatini from Italy, Michel Di Tommaso from Switzerland, and James Weaver from the Wyss Institute at Harvard University .

The significance of these findings cannot be overstated. Improving the durability and longevity of modern concrete is one of the keys to improving sustainability. Concrete is the most widely used man made material on the planet and is incorporated into the construction of modern buildings, bridges, roads, dams, and many other structures. Increasing its longevity, especially through the incorporation of self-healing properties that can potentially remain for decades (or centuries) without the need for costly repairs or the need to tear down the structure and build a new one goes a long way towards improving sustainability.

A Practical Example

For example, there is a baseball stadium constructed with precast concrete that is showing so much deterioration after only 25 years that replacement of the entire facility is a frequent topic of discussion. The mechanism for deterioration is believed to be water entering cracks in the concrete causing further deterioration and corrosion of the reinforcing steel. Interestingly enough, the water appears to be “salt water” caused by pressure washing the structure after all the salty peanut shells are discarded on the floor. If the concrete had contained the self healing properties discovered by the researchers in this study, it is certainly plausible that deterioration being seen in this stadium could have been substantially slowed or even eliminated.

Think about this. Substantial money has been spent repairing this stadium, and in the end, it still may not be enough to save it. Think about the sustainability of replacing a stadium. This is actually a really interesting example, because its failure method has direct correlation to the research that was conducted. You can read more about the research findings below. One of the experiments done to verify the findings was to make concrete cylinders using the researchers newly formulated mix designs. The cylinders where cracked, and water was run through the cylinders. After only two weeks, the cracks had completely healed and would not permit water penetration, while the control cylinder made with typical concrete still allowed water flow. Further investigation revealed that the water combined with quicklime in the mix to form a calcite that sealed the crack. In my example, this means that the act of cleaning the stadium floor would have doubled as the “maintenance” to ensure the continued durability of the structure.

The Durability of Roman Concrete May be Reproducible

The problem with the assumption that the durability of Roman concrete was derived from specific properties of the volcanic ash found only in the area of Pozzuoli more than a thousand years ago, is that we can’t duplicate the material today and ship it all over the world. This seems to have made the durability of the concrete made centuries ago, out of our reach today. Fortunately, researchers believe they have found a different mechanism responsible for much of the ancient concrete’s durability.

From MIT News:

Under closer examination, these ancient samples also contain small, distinctive, millimeter-scale bright white mineral features, which have been long recognized as a ubiquitous component of Roman concretes. These white chunks, often referred to as “lime clasts,” originate from lime, another key component of the ancient concrete mix. “Ever since I first began working with ancient Roman concrete, I’ve always been fascinated by these features,” says Masic. “These are not found in modern concrete formulations, so why are they present in these ancient materials?”

Previously disregarded as merely evidence of sloppy mixing practices, or poor-quality raw materials, the new study suggests that these tiny lime clasts gave the concrete a previously unrecognized self-healing capability. “The idea that the presence of these lime clasts was simply attributed to low quality control always bothered me,” says Masic. “If the Romans put so much effort into making an outstanding construction material, following all of the detailed recipes that had been optimized over the course of many centuries, why would they put so little effort into ensuring the production of a well-mixed final product? There has to be more to this story.”

Upon further characterization of these lime clasts, using high-resolution multiscale imaging and chemical mapping techniques pioneered in Masic’s research lab, the researchers gained new insights into the potential functionality of these lime clasts.

What Does It All Mean?

It turns out, these “lime clasts” are the mechanism for sequestering the compounds that can later combine with water to heal cracks in the concrete. All that was needed to produce a concrete mix that formed identical lime clasts was to directly introduce lime into the concrete mix design in the form of quicklime, a highly reactive form of lime, which produces a highly exothermic reaction. Concrete researchers call this hot-mixing.

When the researchers produced concrete based on their hypothesis, the result was hardened concrete that showed very similar microarchitecture to the ancient Roman samples. This supported the theory that the lime clasts were not the result of “sloppy” manufacturing processes, but rather an intentionally introduced element of the manufacturing process.

To further prove out their theory, the researchers cast concrete cylinders made from their new concrete, and obtained the results described above. After they were intentionally cracked and subjected to water, calcite did indeed form to seal up the cracks, just like what has been seen in samples from ancient Roman structures.

The research shows incredible potential, and it's exciting to see results of this scale coming out of the industry funded MIT Concrete Sustainability Hub (funded by the Portland Cement Association and the CONCRETE ADVANCEMENT FOUNDATION )

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Additional Resources

• Visit MIT's Concrete Sustainability Hub at https://cshub.mit.edu/
• Download the entire research report on the ancient Roman concrete findings at Science.org here - Hot mixing: Mechanistic insights into the durability of ancient Roman concrete | Science Advances
• Learn more about #concrete in #construction at https://www.dhirubhai.net/learning/construction-management-concrete-construction/welcome