Ancient Romans built arched bridges, waterproof port infrastructure and aqueducts that enabled the rise of their empire and that are still standing—and often still used. They did so with a type of cement that is far sturdier than what is used today, but exactly how Roman cement was made was something of a mystery. Now researchers have found proof of an explanation they had proposed in 2023 that could offer insights into how to build longer-lasting concrete today.
In his first-century B.C.E. work De Architectura, Vitruvius, one of the most famous architects of the Roman Empire, described Roman cement as being made with what we today call slaked lime, or hydrated, heated limestone. But based on the discovery of the makeup of chunks called “lime clasts” found at a previous excavation in Pompeii, Massachusetts Institute of Technology environmental engineer Admir Masic and his colleagues proposed in a 2023 paper that ancient builders instead used a process called “hot mixing.” In this method, highly reactive quicklime (dry heated limestone) is mixed with volcanic ash and water, setting off a chemical reaction that produces heat and gives the material self-healing capabilities.
To reaffirm his discovery, Masic and his team returned to Pompeii in 2024 and visited a house that was under renovation when Mount Vesuvius erupted, freezing the place in time. “I literally felt like I was a worker in 79 C.E.,” Masic says.
Inside one of the rooms, among stones, roof tiles and tools, the researchers found large piles of dry, premixed mortar ingredients—a blend of volcanic ash and granules of quicklime—waiting to be hydrated and applied to walls, Masic says.
Rows of ceramic roof tiles and a stack of yellow tuff blocks in Atrium 2 of the newly excavated Regio IX (Archaeological Park of Pompeii), showing materials prepared for the ongoing reconstruction works.
Archaeological Park of Pompeii
The team also showed that the lime clasts—previous thought to be impurities from incomplete mixing—had the distinctive physical and chemical characteristics that could only be produced by a technique in which water was added to the quicklime and ash mixture, not by the method Vitruvius wrote about. The researchers published their results on Tuesday in Nature Communications.
The hot mixing method creates the fragmented, highly porous lime clasts within the mortar that allow calcium to easily travel through the material and recrystallize to fill cracks. Understanding and mastering this “self-healing” technology will allow engineers to use the technique in modern construction. Modern cement is made by heating limestone and clay in giant kilns to form a material called clinker, which is ground into a powder and mixed with water on-site to make concrete. It is strong but short-lived, often cracking and degrading within decades.
The findings will also enable restorers to repair the remains of the Roman Empire with a more compatible approach. “We will definitely have new recipes for restoration that will come out of this,” Masic says.
“Few topics in Roman archaeology are more worth our attention than the development of concrete,” says archaeologist Tom Brughmans of Aarhus University in Denmark, who was not involved in the study. The new research is “simply a beautiful observation, an archaeologist’s dream.”