Researchers discover natural, self-healing ?concrete?

Researchers from California’s Stanford University found the fibre-reinforced, concrete-like rock inside the dormant Campi Flegrei volcano in Italy.

Campi Flegrei is situated within a large volcanic depression known as a caldera, co-located with the city of Pozzuoli. From 1982, the ground beneath Pozzuoli began to swell, rising by about 2m within two years. This was accompanied by a number of micro-earthquakes and later on, a magnitude four earthquake, which resulted in the evacuation of the city.

Tiziana Vanorio, assistant professor of geophysics at Stanford’s School of Earth, Energy and Environmental Sciences, said similar ground swelling had occurred at other calderas around the world but “never to this degree”.

“[It] usually requires far less [ground] uplift to trigger earthquakes at other places,” she commented. “At Campi Flegrei, the micro-earthquakes were delayed by months despite really large ground deformations.”

In order to understand how the surface of the caldera had been able to withstand so much pressure prior to cracking, Vanorio and another researcher began studying rock cores from the region. The pair found that the ground contained a volcanic ash called “pozzolana” – believed to have been used by the ancient Romans to construct long-lasting concrete structures such as the Pantheon – as well as tobermorite and ettringite, fibrous materials found in man-made concrete.

Chemical reactions

The researchers deduced that a chemical reaction, decarbonation, was responsible for the natural formation of calcium hydroxide – a compound also known as portlandite or hydrated lime, which is a key ingredient in man-made concrete.

“Circulating geothermal fluids transport this naturally occurring lime up to shallower depths, where it combines with the pozzolana ash in the caprock [a hard rock layer within the caldera’s surface] to form an impenetrable, concrete-like rock capable of withstanding very strong forces,” a Stanford University statement explained.

“This is the same chemical reaction that the ancient Romans unwittingly exploited to create their famous concrete, but in Campi Flegrei it happens naturally,” Vanorio added.

It was suggested that the pressure resulting from the decarbonation process was also what had caused Pozzuoli’s ground to rise in the 1980s, although, interestingly, the researchers believed the cracked caprock closed itself up after venting the pent-up gases and fluids.

“[As] more calcium hydroxide was produced at depth and transported to the surface, the damaged caprock was slowly repaired, its cracks ‘healed’ as more natural cement was produced,” the statement explained.

Vanorio said there was a need for “eco-friendly materials and concretes that can accommodate stresses more easily”, and indicated that further research into the conditions and processes behind the formation of Campi Flegrei’s caprock could potentially help scientists engineer a more durable and resilient concrete formula that could heal itself after damage.



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