Roman Empire’s pozzolanic concrete lasts 2,000 years
The Roman Empire was home to many knowledgeable artisans, and engineers who are famous for such structures as the Aqueducts. These structures are very much still around to this day.
And it seems they were constructed from pozzolanic concrete. It is this very material that gave Roman buildings and other constructions their incredible durability.
The Pantheon, which was built almost 2,000 years ago, is still standing and holds a world record for being the world’s largest dome made from non reinforced concrete.
But when the Roman EmpIre fell in 476 AD, the precedented Roman recipe for concrete was lost to the world. Just after the Dark age, an Italian friar named Giovanni Giocondo built the Pont Notre-Dame Bridge in Paris using remnant information from the ancient Roman cement recipe.
How is this concrete so durable?
Its amazingly strong properties seem to derive from its composition of ingredients, one of which is pozzolana, which is made form volcanic ash and lime mixed with water. The resulting building material produced is a very strong concrete compound.
Even so, it seems there’s more to this than meets the eye, as discovered by a group of international researchers led by the Massachusetts Institute of Technology.
They discovered that slightly different material and different techniques to mix the concrete were responsible for its properties. It seems that small white chunks of lime were found inside what otherwise seemed to be well mixed concrete.
It was thought that these chunks were the result of poor mixing, but Admit Masic of the research group seemed to think otherwise. “The idea that the presence of these lime clasts was simply attributed to low quality control always bothered me,” Admit says.
Linda Seymour, MIT civil engineer, headed up the team with Masic to study the 2,000 year old samples of concrete taken from an archaeological site, Privernum in Italy.
https://news.mit.edu/2023/roman-concrete-durability-lime-casts-0106
They were analysed with large-area scanning electron microscope, and energy-dispersive x-ray spectroscopy, powder x-ray diffraction and confocal Raman imaging.
As a result of this process of research the team was able to understand more about the presence of the lime clasts.
Roman high-temperature-associated compounds quicklime concrete is eco friendly
One of the major questions was, why was this particular type of lime used. It was previously thought that pozzolanic concrete normally contained slaked lime, where limestone is heated at a high temperature to produce quicklime, also known as calcium paste.
Previous knowledge had assumed that pozzolana was created using this method. The word is from Italian area Pozzuoli. Its volcanic ash had silicon glass.
Pozzolana(ash) is a natural siliceous or siliceous-aluminous material which reacts with calcium hydroxide in the presence of water at the room temperature.
However the team discovered information that did not support this as the lime clasts in their tests did not appear to be consistent with the aforementioned method.
They concluded that it was more likely that quicklime, pozzolana and water were mixed together directly at very high temperatures, in a process that the team has described as “hot mixing” creating such lime clasts.
The benefits of hot mixing are two folded. First, when the overall concrete is heated to high temperatures, it allows chemistries that are not possible if you only used slaked lime, producing high temperature associated compounds that would not otherwise form.
Second, this increased temperature significantly reduces curing and setting times since all the reactions are accelerated allowing for much faster construction.
It also seems that these lime clasts give the concrete the ability to maintain and heal itself, because when cracks form they will travel to the lime clasts as a matter of preference because they have a higher surface area in comparison to other particles in the matrix.
Once water enter this crack, it causes a reaction with the lime to form a calcium solution with dries and hardens inside the cracks essentially acting as a glue, and preventing further expansion of the crack in the material.
The same behaviour has been found in anther site from 2,000 year ago at the Tomb of Caecilia Mettella, where the cracks had been filled by calcite. This theory would also explain why Roman sea walls survived 2 millenia of battering by the sea’s waves.
So the research team compared 2 types of pozzolanic concrete, a modern control concrete and one made from the ancient Roman recipe. Crack tests were performed and the findings were that the control concrete did not heal and stayed cracked, while the quicklime concrete recipe from 2,000 years ago healed within 2 weeks.
The aim now is to commercialise this as a new more environmentally friendly concrete when compared with other modern options.
It’s exciting to think about how these more durable concrete formulations could expand not only the service life of these materials, but also how it could improve the durability of 3D printed concrete formulations.
Source: MIT News
