Recent study reveals secret of Roman magnificent buildings
Modern concrete has a lifespan of fewer than 100 years, so it is a mystery that Roman structures made from concrete are still holding up. A recent study may have solved this mystery.
Romans were master builders who left their mark all across the ancient world. Their influence is still evident today thanks to a material known as pozzolanic concrete. Thanks to the unique properties of this concrete, Romans were able to build larger, more complex-shaped architectural structures for purposes that were not previously possible, including constructions in the sea. And remarkably, many of these structures are still standing.
The material gets its name from its principal component; volcanic ash which is predominant in the Italian city of Pozzuoli. The ash is mixed with sand, quicklime (burned limestone), ceramic fragments (cocciopesto), or another pozzolana. An team of researchers from Harvard University, and laboratories in Italy and Switzerland and led by Prof. Admir Masic from the Massachusetts Institute of Technology (MIT) found that not only are the materials slightly different from what we may have thought, but the techniques used to mix them were also different. Water was added in a process called hot mixing that raised the temperature of the mixture to up to 200 degrees.
Roman concrete is less uniform than modern concrete, containing tiny granules of white calcium, called lime clasts, that don’t dissolve but remain trapped in the concrete.
“The idea that the presence of these lime clasts was simply attributed to low quality control always bothered me,” Masic told Eureka Alert. “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.”
Masic and his team collected samples of concrete from Privernum, an ancient settlement 100 kilometers south of Rome, and studied the mortar’s composition using electron microscopy and x-ray spectroscopy. They discovered that cracks in the concrete had been filled with calcium carbonate, the same material as the lime clasts.
They understood that if water runs through the fissures, it dissolves the calcium in the lime clasts. The calcium then precipitates and recrystallizes along the cracks, eventually sealing them.
This self-healing property allows Roman concrete to remain durable in a variety of environments including direct and extended contact with saltwater. By the middle of the first century CE, the principles of underwater construction in concrete were well-known to Roman builders. The Israeli Mediterranean port city of Caesarea was the earliest known example to have made use of underwater Roman concrete technology on such a large scale. One of the outstanding features of the city is the harbor which is protected by a massive breakwater that was formed using enormous rocks and concrete. At the time of its completion, it was the largest artificial harbor ever built in the open sea. It is regarded as one of the most remarkable examples of Roman engineering of the Augustan Age. Parts of the underwater concrete construction, which are visible today, show little erosion or damage 2000 years later.
The scientists tested their hypotheses by making cylinders of Roman concrete using the hot mixing technique. After the concrete set, they intentionally cracked it, separating the two halves by .5 millimeters, and running water over it. After a few weeks, the cracks had sealed themselves. Control sets of modern concrete cylinders remained separated.
The researchers hope that replacing current techniques with Roman concrete will positively affect the environment. Modern concrete, the most common building material in use today, is a mixture of sand, gravel, water and so-called Portland cement. Portland cement is made by burning limestone, clay and other materials in kilns that reach temperatures of more than 1,400 degrees centigrade. This process releases up to 1 metric ton of CO2e emissions per metric ton of produced material. The production of Portland cement accounts for up to 8% of total global greenhouse gas emissions. By reverting to Roman concrete, the resulting extended use life, combined with a reduction in the need for extensive repair, could thus reduce the environmental impact and improve the economic life cycle of modern cementitious constructs.
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