In 2023, we reported on MIT scientists’ conclusion that the ancient Romans used “hot mixing” with quicklime, among other strategies, to make their famous concrete, giving the material a self-healing functionality. The only problem was that it didn’t match the recipe described in historical texts. Now, the same team is back with a new analysis of samples collected from a newly discovered site that confirms that the Romans did indeed use hot mixing, according to a new paper published in the journal Nature Communications.
As we previously reported, like today’s Portland cement (a basic ingredient in modern concrete), ancient Roman concrete was essentially a mixture of semi-liquid mortar and aggregate. Portland cement is typically 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 just a touch of gypsum added to obtain a smooth, flat surface. But the aggregate used to make Roman concrete was fist-sized pieces of stone or brick.
In his treatise On architecture (c. 30 CE), the Roman architect and engineer Vitruvius wrote about how to build concrete walls for burial structures that could last a long time without falling into disrepair. He recommended that walls be at least two feet thick and made either of “square red stone, or bricks or lava laid in courses.” The brick or volcanic rock aggregate must be bonded with a mortar composed of hydrated lime and porous fragments of glass and crystals from volcanic eruptions (called volcanic tephra).
Admir Masic, an environmental engineer at MIT, has been studying ancient Roman concrete for several years. For example, in 2019, Masic helped launch a new set of tools to analyze samples of Roman concrete from Privernum at multiple length scales, including Raman spectroscopy for chemical profiling and multi-detector energy dispersive spectroscopy (EDS) for phase mapping of the material. Masic also co-authored a 2021 study analyzing samples of ancient concrete used to build a 2,000-year-old mausoleum along the Appian Way in Rome, known as the tomb of Caecilia Metella, a noblewoman who lived in the first century CE.
And in 2023, Masic’s group analyzed samples taken from the concrete walls of the Privernum, focusing on strange chunks of white minerals known as “lime clasts”, which others had largely dismissed as resulting from inferior raw materials or poor mixing. Masic et al. concluded that this was not the case. Rather, the Romans deliberately used “hot mixing” with quicklime which gave the material self-healing functionality. When cracks begin to form in concrete, they are more likely to propagate through the lime fragments. The clasts can then react with water, producing a saturated calcium solution. This solution can either recrystallize in the form of calcium carbonate to fill the cracks, or react with the pozzolanic components to strengthen the composite material.
