The famous Pantheon in Rome boasts the world's largest unreinforced concrete dome—an architectural marvel that has endured for millennia, thanks to the incredible durability of ancient Roman concrete. For decades, scientists have been trying to determine precisely what makes the material so durable. A new analysis of samples taken from the concrete walls of the Privernum archaeological site near Rome has yielded insights into those elusive manufacturing secrets. It seems the Romans employed "hot mixing" with quicklime, among other strategies, that gave the material self-healing functionality, according to a new paper published in the journal Science Advances. //

It was believed that the Romans combined water with lime to make a highly chemically reactive paste (slaking), but this wouldn't explain the lime clasts. Masic thought they might have used the even more reactive quicklime (possibly in combination with slaked lime), and his suspicion was born out by the lab's analysis with chemical mapping and multi-scale imaging tools. The clasts were different forms of calcium carbonate, and spectroscopic analysis showed those clasts had formed at extremely high temperatures—aka hot mixing.

“The benefits of hot mixing are twofold,” Masic said. “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 to impart self-healing capabilities. Per Masic, when cracks begin to form in the concrete, they are more likely to move through the lime clasts. The clasts can then react with water, producing a solution saturated with calcium. That solution can either recrystallize as calcium carbonate to fill the cracks or react with the pozzolanic components to strengthen the composite material.

Masic et al. found evidence of calcite-filled cracks in other samples of Roman concrete, supporting their hypothesis. They also created concrete samples in the lab with a hot mixing process, using ancient and modern recipes, then deliberately cracked the samples and ran water through them. They found that the cracks in the samples made with hot-mixed quicklime healed completely within two weeks, while the cracks never healed in the samples without quicklime. //

mgsouth Seniorius Lurkius

DJ Farkus said:
So many questions... Did they pour the hot-mix, is it required to be poured hot? How high of temperatures are we talking here? I wonder how they heated batches on-site (or did they transport it for pouring)?

Thank you. Now I have a mental image of a wagon pulled by a brace of oxen, with a huge oak barrel slowly rotating in the back. Meanwhile, the drover is flicking a whip about, cursing the throng of people in the street, screaming he has a *!@#! load setting up and get out of the !#@!! way. (In Latin, of course.)