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Roman structures made of unreinforced concrete are incredibly strong. Many of them have survived two millennia almost completely intact. Researchers have spent decades trying to figure out what makes this ancient building material so durable. Thanks to new analysis, they have finally pierced this manufacturing secret of this ancient concrete, endowed with self-repairing abilities.
The famous Pantheon in Rome, built in the 1st century BC. J.-C., is surmounted by the largest unreinforced concrete dome in the world. Having remained practically intact, it still has no structural weaknesses today. Likewise, some Roman-era aqueducts still supply water to Rome. The robustness of these buildings and structures in a variety of climates, seismic zones, and even in direct contact with seawater has never ceased to amaze scientists. Modern mortars and concretes do not show such durability.
Experts have long assumed that ancient concrete owes its extreme durability to a special ingredient: pozzolana – named after the Italian town of Pozzuoli – a natural rock made from volcanic ash, from which lime and water form cement hydrates. Architects and historians of the time also describe pozzolanic materials as key components of concrete. Looking more closely at samples of a Roman-era concrete wall, researchers at MIT observed other amazing features.
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A process based on quicklime, more reactive
Roman concrete has some shiny white mineral fragments called “lime clasts” that are not found in modern concrete. Until now, experts believed that these lime aggregates were the result of poor mixing or substandard raw materials. A theory that never really convinced Admir Masic, Professor of Civil and Environmental Engineering at MIT: “If the Romans put so much effort into creating an extraordinary building material, after all the detailed recipes tweaked over many centuries, why did they put so little effort into ensuring the production of a well blended end product? ‘ he emphasized in a press release.
Using high-resolution imaging and chemical mapping techniques, Masic and his team found another potential functionality of these calcified clasts. Previously, it was thought that lime — which is calcium oxide, with the formula CaO — was incorporated into Roman concrete in the form of slaked lime: the limestone is heated at a high temperature to produce a highly reactive caustic powder called quicklime, which is mixed with water mixed to make slaked lime (or calcium hydroxide).
Element map (calcium: red, silicon: blue, aluminum: green) of a 2 cm fragment of ancient Roman concrete collected from the archaeological site of Privernum, Italy (left). In the lower part of the fragment, a calcium-rich limestone fracture is clearly visible, which is responsible for the self-repairing properties of this ancient material. © L. Seymour et al.
But this process alone could not explain the presence of crushed limestone. These fragments were composed of various forms of calcium carbonate, and spectroscopic analysis indicated that these must have been formed at extreme temperatures. This has led researchers to speculate that the Romans may have used lime in its most reactive form, quicklime – a process called “hot mixing”.
As Masic explains, this method has several advantages. First of all, the high temperatures caused by the use of quicklime are necessary for certain chemical reactions; they also significantly reduce curing and setting times as all reactions are accelerated, allowing for a much faster build up.
Self-healing cracks
But that’s not all ! The hot mixing allows the limestone building blocks to adopt a relatively fragile nanoparticle architecture, which is therefore easy to break and provides an almost permanent reserve of calcium: this is key to this concrete’s self-repairing ability.
Through the hot mixing process, the calcium-rich limestone clasts are encapsulated within the cementitious matrix, which eventually undergoes carbonization. During cracking, water can enter and carry a calcium-enriched solution into the pore network to repair damage (Process 1) or serve as reactive calcium for post-pozzolanic reactions that further strengthen the material (Process 2). © L. Seymour et al.
When cracks begin to form, they preferentially move through calcareous clasts, which have a relatively larger surface area than other components. When water then enters these cracks, it reacts with the lime, creating a calcium-saturated solution that can recrystallize as calcium carbonate, thereby quickly filling the crack, or react with pozzolanic materials to further strengthen the composite. These reactions are spontaneous and thus automatically repair the cracks before they propagate further.
In fact, analysis of various concrete samples taken from archaeological sites has shown that they revealed ancient cracks that were filled with calcite.
The researchers tested this hypothesis by making their own different concrete samples, all based on quicklime: some following the Roman recipe, others the modern recipe. Then they intentionally broke up those pieces of concrete before introducing water into those spaces. They then found that after two weeks the cracks were completely blocked, the water could no longer flow through them. This was not the case with a control concrete sample made without quicklime.
The team is now working towards the commercialization of this modified cementitious material. Improving the durability of concrete by incorporating such self-healing properties would reduce cement’s carbon footprint, which today accounts for up to 8% of total global greenhouse gas emissions.
Source: L. Seymour et al., Science Advances