Quicklime, pozzolanic material ‘hot-mixed’ to make concrete can endure for millenia; It can also help reduce environment impact of cement production
New research from Massachusetts Institute of Technology, Harvard University, and laboratories in Italy and Switzerland have discovered the secret behind Roman construction abilities, whose remains are still an enigma after millennia. A paper published in journal Science Advances January 6, 2023 shows the Romans used a ‘hot-mixing’ method to create their durable structures.
The unexpected ancient manufacturing strategy might hold the key to the architectural creations by the Romans, said Hot mixing: Mechanistic insights into the durability of ancient Roman concrete. The method gave the concrete self-healing properties so that cracks would fill before they could spread.
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The ancient Romans were masters of engineering, constructing vast networks of roads, aqueducts, ports and massive buildings, whose remains have survived for two millennia, said a press note on the MIT website.
Many of these structures were built with concrete: Rome’s famed Pantheon, which has the world’s largest unreinforced concrete dome and was dedicated in 128 CE, is still intact.
Some ancient Roman aqueducts still deliver water to Rome today. Meanwhile, many modern concrete structures have crumbled after a few decades.
For many years, researchers assumed the key to the ancient concrete’s durability was based on one ingredient: Pozzolanic material, such as volcanic ash from the area of Pozzuoli on the Bay of Naples.
Under closer examination, the researchers found the ancient samples also contain small, distinctive, millimetre-scale bright white mineral features, which have been long recognised as a ubiquitous component of Roman concretes. These white chunks often called “lime clasts,” originate from lime, another critical part of the ancient concrete mix.
The relict lime casts were earlier thought of as evidence of sloppy mixing practices or poor-quality raw materials. On close examination, they found the Romans employed a hot mixing method and used quicklime in conjunction with, or instead of, slaked lime for producing concrete.
Hot mixing, the team has now concluded, was actually the key to the super-durable nature.
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During the hot mixing process, the lime clasts created a brittle structure with a reactive calcium source. The calcium would travel through the lime clasts, react with water and quickly fill any cracks.
A schematic of the proposed mechanism for self-healing within ancient Roman mortars. Source: Science Advances
The calcium-saturated solution could also react with pozzolanic materials to further strengthen the composite material. These reactions take place spontaneously and automatically heal the cracks before they spread.
The team is now working to commercialise this modified cement material. The extended functional lifespan and the development of lighter-weight concrete forms could help reduce the environmental impact of cement production. It currently accounts for about 8 per cent of global greenhouse gas emissions.
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