Back in 2023 we reported on MIT scientists' finding that the ancient Romans, among other methods, used “hot mixing” with quicklime to make their famous concrete, giving the material its self-healing function. The only catch was that it did not follow the recipe described in historical texts. Now the same team is back with a new analysis of samples collected from the newly discovered site that confirms that the Romans did indeed use hot mixing, according to a new paper published in the journal Nature Communications.
How do we previously reportedlike today Portland cement (main ingredient of modern concrete), ancient Roman concrete It was essentially a mixture of semi-liquid mortar and aggregate. Portland cement is usually made by heating limestone and clay (as well as sandstone, ash, chalk and iron) in a kiln. The resulting clinker is then ground into a fine powder with a small amount of gypsum added to produce a smooth and flat surface. But the aggregate used to make Roman concrete consisted of fist-sized pieces of stone or brick.
In his treatise About architecture (c. 30 AD), Roman architect and engineer. Vitruvius wrote about how to build concrete walls for burial structures that could stand for a long time without collapsing. He recommended that the walls be at least two feet thick and made of either “rectangular red stone or brick or lava laid in courses.” The volcanic rock brick or aggregate must be bonded with a mortar consisting of slaked lime and porous glass fragments and volcanic eruption crystals (known as volcanic tephra).
Admir Masic, an environmental engineer at the Massachusetts Institute of Technology, has been studying ancient Roman concrete for several years. For example, in 2019, Masic helped develop a new set of tools to analyze Roman concrete samples from Privernum at multiple scales, particularly Raman spectroscopy for chemical profiling and multi-detector energy dispersive spectroscopy (EDS) for phase mapping of the material. Masic was also co-author 2021 study analysis of samples of ancient concrete used to build the 2,000-year-old mausoleum on the Appian Way in Rome, known as Cecilia Metella's gravenoblewoman who lived in the first century AD.
And in 2023, Masick's team analyzed samples taken from the concrete walls of Privernum, focusing on strange white mineral chunks known as “lime flakes” that others had largely dismissed as the result of poor quality raw materials or poor mixing. Masic et al. came to the conclusion that this was not the case. Rather, the Romans deliberately used “hot mixing” with quicklime, which gave the material its self-healing properties. When cracks begin to form in concrete, they are more likely to propagate through limestone debris. The debris can then react with water to form a calcium-rich solution. This solution can either recrystallize as calcium carbonate, filling cracks, or react with pozzolanic components to strengthen the composite material.
