Swiss researchers solve the mystery of Roman concrete

Some of the buildings built 2,000 years ago in ancient Rome still defy decay to this day. A new study with Swiss participation has now revealed the secret behind the longevity of the ancient Romans’ concrete: Contrary to previous assumptions, they heated their concrete while mixing it.

The resulting chemical composition of the Roman concrete gave it self-healing ability, the materials scientists wrote in a study published Friday in the journal Material Science. If water penetrates the concrete through a crack, chemical processes take place that strengthen the material and fill any cracks.

For the study, the researchers took samples from a 2,000-year-old Roman concrete wall in Priverno (I) and examined them with special X-ray equipment. The analyzes showed that the Romans had mixed the concrete hot and used so-called burnt lime. This created lime lumps with a large surface area in the concrete.

According to the researchers, these lime lumps serve as a source of calcium, which fills the voids created when it comes into contact with water. In addition, penetrating water causes the lime to react with another special component of Roman concrete: the pozzolana. In previous studies, the scientists found that this volcanic rock was mixed with concrete in ancient Rome.

As the limestone reacts with the pozzolana, crystalline structures are formed that become progressively harder over time. This strengthens the concrete. Because the calcium in the lime lumps is stored until it comes into contact with water through cracks, the self-healing ability of Roman concrete lasts for thousands of years.

Inspired by the concrete of ancient Rome, the researchers developed a new cement mixture. “Their self-healing potential paves the way for more durable, resilient and sustainable concrete formulations,” the authors wrote in the study.

(aeg/sda)