The impressive structures of ancient Rome have survived for millennia. This is a testament to Roman engineers’ ingenuity, which made concrete possible.

What construction materials helped to keep these colossal buildings, such as the Pantheon (which boasts the largest unreinforced dome on the planet) and the Colosseum afloat for more than 2000 years?

The tale behind Italy’s most-visited cultural site

Roman concrete is often more durable than modern concrete. It can easily deteriorate in a matter of years. Researchers behind a new study claim to have found the mystery ingredient that enabled the Romans build intricate structures in difficult locations like docks and sewers.

The study team included researchers from Italy, the United States and Switzerland. They analyzed concrete samples dated to 2,000 years ago that were taken from a wall of Privernum in central Italy. The concrete samples are identical in composition to other concrete used throughout the Roman Empire.

Lime clasts are white chunks of concrete that can be used to repair cracks that have formed over time. The presence of white chunks in concrete was previously overlooked as evidence for poor mixing techniques or low-quality raw material.

Admir Masic is an associate professor of civil-environmental engineering at Massachusetts Institute of Technology. “It was hard for me to believe ancient Roman (engineers), would not do well because they really put in a lot of effort when choosing and processing the materials,” said Admir Masic.

Masic explained that scholars wrote down exact instructions and imposed them on construction sites across the Roman Empire.

This discovery could lead to concrete production today being more durable, possibly shaking up society in the same way that the Romans did.

Masic stated, “Concrete allowed Romans to have an architecture revolution.” “Romans were able turn cities into beautiful places to live in and create extraordinary architecture. That revolution fundamentally changed how humans live.

Tourists visit Rome’s Colosseum in June 2019. Credit to EyesWideOpen/Getty Images

Concrete’s strength and durability are assured by lime clasts

Concrete is an artificial stone or rock. Concrete is formed by mixing cement with a binding agent. Concrete typically contains limestone, water and coarseaggregate.

Roman texts recommended the use of slaked limestone (when lime is first mixed with water before it is mixed), in the binding agent. This was why scholars believed that this was the way Roman concrete was made, Masic explained.

The researchers further investigated and concluded that lime cements were formed by lime oxide (calcium oxide), which is the most reactive and dangerous form of limestone. This was used in concrete mixing, instead of or in addition to slaked.

Further analysis of concrete revealed that lime clasts were formed at extreme temperatures due to quicklime use. This was why “hot mixing” was crucial for concrete’s durability.

Masic stated that hot mixing offers two advantages. “First, when the overall concrete is heated to high temperatures, it allows chemistries that are not possible if you only used slaked lime, producing high-temperature-associated compounds that would not otherwise form. This increases the speed of construction by reducing curing time and setting times. 

An experiment was conducted to see if lime clasts could be responsible for Roman concrete’s apparent ability repair itself.

They prepared two concrete samples. One was made according to Roman formulas. The other was made according to modern standards. Both were then deliberately cracked. After two weeks, water was unable to flow through concrete made from a Roman formula, while it did so through concrete made without quicklime.

Researchers discovered that lime clasts can be broken down into cracks by exposure to water. This allows for the healing of cracks that have been weathered before they spread. Researchers believe this self-healing capability could open the door to longer-lasting and more sustainable modern concrete. This would help reduce concrete’s carbon footprint which, according to the research, accounts for around 8% of the world’s greenhouse gas emissions.

Researchers believed that volcanic ash from Pozzuoli, in the Bay of Naples was the key ingredient to the strength of Roman concrete for years. This kind of ash was transported across Roman Empire to be used for construction and was called a key ingredient in concrete by historians as well as architects.

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