Ancient Roman structures have been seen to be incredibly strong. This is thanks to the materials and techniques that they used.
Pozzolana and Lime
During ancient times, Romans were experts when it came to engineering and construction. This is probably famously expressed through their aqueducts. According to Science Alert, such marvels depend on a unique material known as pozzolanic concrete, which is a concrete seen to have extraordinary durability, that makes Roman structures remarkably strong.
Up to this point, the Roman structure Pantheon remains sturdily intract despite being close to 2,000 years old. Science Alert notes how the said structure is the record-holder for being the biggest dome in the world that has concrete that is not reinforced.
The Ancient Engineering Technologies of the University of Wisconsin-Madison reports how such concrete properties have been attributed to its materials: pozzolana and lime. Pozzolana is a combination of volcanic ash with its name taken after the city of Pozzuoli, where a deposit of significant amounts can be spotted. When these materials get combined with water, they end up with durable concrete.
While its strength can be attributed to these materials, the story does not end there. A global research team led by MIT discovered that the techniques for mixing also stood out.
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Unique Mixing Technique
Thinking that poor materials and mixing were not sufficient explanations of the presence of lime chunks in pozzolanic concrete, Masic and the research team looked into Roman concrete samples worth 2,000 years old. These samples were taken from Privernum, an archaeological site within Italy. Findings were then published in the Science Advances.
Such samples were subjected to various methodologies, including powder X-ray diffraction, in order to know more about the lime clasts.
One of the pressing questions pertained to the nature of the lime that was used. Science Alert notes how general understanding about pozzolanic concrete is that it makes use of lime that is slaked. At first, the limestone is exposed to extremely high temperatures in order to come up with calcium oxide or quicklime.
Combining water and quicklime leads to the production of calcium hydroxide or slaked lime, which is a paste that is less caustic and reactive. Based on such assumptions, slake lime was the material that got mixed with pozzolana.
According to their analysis, such clasts from the samples do not align with such a method. Rathen that this, the concrete was likely made through combining quickline with water and pozzolana directly at remarkably high temperature levels, on its own or as a slaked lime addition. This is a process that the team refers to as "hot mixing," which leads to lime clasts.
Masic notes how this leads to two-fold benefits. Firstly, heated concrete enables certain chemistries that would not take place if only slaked lime was used. Secondly, the temperature increase also decreases the setting and curing times because every reaction gets accelerated and enables speedier construction.
On top of this, Science Alert reports another benefit, which is that self-healing capacities are granted by the clasts to the concrete. When cracks start forming on the concrete, they move to the lime clasts. Such clasts have surface areas which are higher compared to other matrix particles. When the water enters the crack, it interacts with the line and comes up with a calcium-rich solution that dries up and hardens to become calcium carbonate.
This then glues the crack and stops the crack from further spreading.
The team evaluated their findings by coming up with pozzolanic concrete with quickline. They also had a control concrete that did not use quicklime. As part of their study, they did crack tests. They observed how the quicklime concrete healed fully within two weeks while the control concrete remained cracked.
The researchers are now working on commercializing the concrete as it is more eco friendly compared to present-day concrete.
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