The secret of the longevity of Roman concrete

BarcelonaThe concrete used by the Romans two thousand years ago has always fascinated science, as it allowed an architectural revolution with which buildings, bridges, and aqueducts were erected, which still survive. The scientific journal Science Advances from the American Association for the Advancement of Science (AAAS) has prepared a report highlighting the role of the long and progressive carbonation of minerals. The study suggests that for years it has been believed that the resistance of Roman material was caused by the mixture of volcanic ash and lime they used. The opus caementicium did contain this mixture. However, the research published in the North American journal concludes that the differentiating factor is calcium carbonate. This chemical compound, otherwise called calcite, is the predominant binding component in the remains analyzed in the report.

As the research points out, calcite has a regenerative capacity that seals cracks that may appear in the concrete. To demonstrate the presence of the compound, the team has analyzed remains of buildings such as Villa Adriana (Tivoli), from the 2nd century AD, using advanced techniques of microtomography, 3D X-rays, and electronic microscopy.

A method with more than two thousand years of backing

“One of the challenges facing the concrete industry is reducing its carbon footprint,” explains Paulo Monteiro, professor of Civil and Environmental Engineering at the University of California and one of the authors of the article. According to Monteiro, one of the possible strategies is to replace part of the clinker, one of the main components of Portland cement (the most widely used hydraulic binder in modern construction), with small amounts of calcium carbonate and other materials. The professor explains that this inclusion is effective over time. “Roman concrete offers something that no laboratory can provide, and that is a functional carbonation experiment with more than two thousand years of evidence,” he says.

As Monteiro indicates, the uniqueness of the finding leads to a rethinking of the view of Roman remains: “Our work invites us to rethink these buildings as materials that are still chemically alive,” claims the professor from the University of California. The scientist explores the importance of the process that converts lime-derived components into an interconnected network of calcite that seals small cracks. He calls it a “silent process”. Monteiro also states that the procedure allows us to understand the remains of a Roman building “as a structure shaped by the atmosphere itself”.

Adaptation to the environment

The carbonation process in concrete occurs when there is hydration of calcium oxide particles. The reaction is due to the contact of external carbon dioxide, i.e., air and humidity, with the ions of quicklime. This chemical procedure, which may seem instantaneous, is the result of a very gradual, millennia-long evolution that forms calcite foundations that reinforce the existing base. Therefore, calcium carbonate fills the voids that are generated, limiting the entry of fluids or chemicals.

The article also highlights the Romans' capacity for environmental adaptation. Builders made different strategic uses for areas closer to the sea and for areas further away, with specific materials in the opus caementicium. The analysis has made it possible to illustrate that they used techniques such as "hot mixing," which generated high temperatures during lime hydration in certain areas, and how they used seawater with volcanic ash to mix a marine concrete with properties different from those of terrestrial material.

The new-old model

The production of concrete worldwide is estimated to be around 33,000 million tons per year, according to the Global Cement and Concrete Association. The carbon footprint of this production represents a real problem within the sector. Despite the pollution it produces, Paulo Monteiro explains that concrete also performs the task of absorbing CO₂ from the air: “Before, everyone did everything to avoid the carbonation of concrete. Now, with environmental control, we are aware that it also eliminates part of the CO₂ from the atmosphere,” warns the researcher.

The study allows for proposing a new way of designing concrete today. "The objective of the scientific article is not to accelerate the carbonation process that has taken centuries and centuries, but to assess the potential of calcite in the creation of new, more durable foundations," concludes the scientist.