Roman concrete is an amazing thing. This is one of the main reasons why we know so much about Roman architecture today. Many buildings built by the Romans still survive in one form or another thanks to their ingenious concrete and construction technologies.
However, there is still a lot we don't understand about exactly how the Romans made such strong concrete or built all those impressive buildings, houses, public baths, bridges and roads.
Now, new research The study, conducted by researchers at the Massachusetts Institute of Technology (MIT) and published in the journal Nature Communications, sheds new light on Roman concrete and construction technology.
This was thanks to parts recovered from partially built premises at Pompeii, a work site abandoned by workers after the eruption of Vesuvius in 79 AD.
New information about concrete production
Discovery of this special construction site made the news early last year.
Builders were literally renovating a house in the city center when Vesuvius exploded in the first century AD.
This unique find included tiles sorted for recycling and wine containers known as amphorae, which were reused to transport building materials.
But most importantly, it is also evidence that the dry material was prepared before being mixed to make concrete.
It is this dry material that is the focus of the new study. Having access to actual materials before mixing provides a unique opportunity to understand the concrete making process and how these materials reacted to the addition of water.
It rewrote our understanding of Roman concrete production.
Self-healing concrete
The researchers behind this new paper examined the chemical composition of materials found at the site and identified some key elements: incredibly tiny pieces of quicklime that change our understanding of how concrete was made.
Quicklime is calcium oxide that is created by heating high purity limestone (calcium carbonate).
The process of mixing concrete, as the authors of the study explain, took place in the atrium of this house. Workers mixed dry lime (crushed lime) with pozzolana (volcanic ash).
When water was added, the chemical reaction produced heat. In other words, it was exothermic reaction. This is known as “hot mixing,” and the result is a completely different type of concrete than what you buy at the hardware store.
Adding water to quicklime produces what is called slaked lime and also generates heat. In the slaked lime, the researchers found tiny, undissolved “lime clasts” that retained the reactive properties of the quicklime. If cracks form in this concrete, the lime pieces react with water to heal the crack.
In other words, this form of Roman concrete can literally heal yourself.
Techniques old and new
However, it is difficult to say how widespread this method was in ancient Rome.
Much of our understanding of Roman concrete comes from the writings of the ancient Roman architect Vitruvius.
He advised use pozzolana mixed with limebut this text was not intended to refer to hot mixing.
However, if we look at another Roman author, Pliny the Elder, we find a clear description of the reaction of quicklime with water, which is the basis exothermic reaction engaged in the preparation of hot concrete.
So the ancients knew about hot mixing, but we know less about how widespread this technique was.
Perhaps more important are the details in the texts of experiments with different mixture of sand, pozzolana and limewhich led to the mixture used by builders in Pompeii.
The MIT research team had previously discovered lime fragments (these tiny pieces of quicklime) in the Roman ruins at Privernum, about 43 kilometers north of Pompeii.
Also worth noting healing of cracks was seen in the concrete of the tomb of the noblewoman Caecilia Metella near Rome on the Via Appia (a famous Roman road).
Now this new Pompeii study established that hot mixing took place and how it helped improve Roman concrete, scientists can find cases where cracks in concrete were healed in this way.
Questions remain
Overall, this new research is exciting, but we should not assume that all Roman buildings were built to a high standard.
The ancient Romans were able to make exceptional concrete solutions, but, as Pliny the Elder notes, bad solution caused the collapse of buildings in Rome. Just because they could make a good mortar doesn't mean they always did.
Questions, of course, remain.
Can we generalize from a single example of this new study from Pompeii 79 AD? e., to interpret all forms of Roman concrete?
Does this show progress compared to Vitruvius, who wrote shortly before?
Was the use of quicklime to make stronger concrete in this house in Pompeii, built in 79 AD, a response to the presence of earthquakes in the region and the expectation of future cracking?
To answer any of these questions, further research is needed to see how common limestone debris is in Roman concrete in general, and to determine where Roman concrete has healed itself.
This edited article is republished from Talk under Creative Commons license. Read original article.
