These chemists are the code to the long-lasting Roman concrete

MIT chemist Admir Masic really hoped his experiment wouldn’t explode.

Masic and his Roman colleagues attempted to recreate the ancient Roman art of concrete, a mixture of cement, gravel, sand and water. The researchers suspected that the key was a process called “hot mixing,” in which granules of calcium oxide, also known as quicklime, were mixed with mill ash to make cement. Then water is added.

Hot mixing, they thought, would eventually produce cement that was not completely light and mixed, but instead contained very little calcium. Those stones, ubiquitous in the concrete walls of Roman buildings, were the key to why they had stood the test of time.

Not only are modern cements made. The reaction of limestone with water is highly exothermic, which means it produces a lot of heat — and possibly an explosion.

“Everybody would say, ‘Crazy,'” Masicus said.

But no big bang happened. Instead, the reaction produces only heat, a moist breath of water vapor – and the Romans used it as a mixture of small white limestone rocks.

Researchers have tried for decades to trace the Roman recipe for concrete longevity — but with little success. The key to the hot mix idea was an educated guess.

Masic and his colleagues’ argument was popularized by the Roman architect Vitruvius and Pliny, who offered some clues as to how to proceed. These words were quoted, for example, as strict specifications of the raw materials, such that the limestone, which is the source of the quicklime, must be very pure, and the quicklime mixed with hot ash and water added could produce a lot of heat.

The rocks were not named, but the team’s feelings were important.

“In every specimen of Roman concrete we have seen, you can find these white inclusions,” plates of stone fixed in the walls. For many years, Masic says, the origin of those inclusions was obscure — researchers suspected they might have been incomplete mixing of the cement. But we are talking about the most learned Romans. How likely is it that the “all operator” [was] not mixing properly and individually [building] Does he have a disability? “

What if, the team suggested, these inclusions in the cement are actually a feature, not a bug? Researchers’ chemical analyzes of such rocks embedded in the walls at the archaeological site of Priverni in Italy indicated that the inclusions were very calcium-rich.

That suggested the tantalizing possibility that these rock buildings could help heal themselves from cracks due to weather or even earthquakes. A ready supply of calcium was already at hand: to dissolve it, to pour it into the cracks, and to re-crystallize it. Voila! Scar, healed.

But could the team actually see this through? Step one was to re-create the rocks by hot mixing and hope nothing exploded. Step two: Test the Roman-inspired cement. The team created the concrete with a hot mix process, and tested them next to it. Each block of concrete was broken in half, the pieces being placed at a not small distance. Then the water flowed down through the crack to see how long it took before the shot was released.

“The results are stunning,” Masic says. The logs incorporating the hot mix cement cured within two to three weeks. The team delivers the concrete product without hot mix cement at all on January 6th Journal of Sciences.

The craft recipe could be a planetary function. The Pantheon and its supporting concrete domes have stood for nearly two thousand years, for example, whereas modern concrete structures have a life span of perhaps 150 years, and have at best a chance of collapse (SN: 2/10/12). Nor did the Romans support their structures with iron bars.

More frequent additions of concrete structures mean more greenhouse gas emissions. Concrete manufacturing is a huge source of carbon dioxide to the atmosphere, so long-term versions could reduce that carbon footprint. “We make 4 gigatons a year of this stuff,” Masic says. This device produces as much as 1 metric ton of CO₂ per metric ton of concrete produced, already about 8 percent of global annual CO .₂ emissions

However, says Masic, the concrete industry resists change. For one, there are concerns about introducing new chemistry into a tried and true mixture with known mechanical properties. But “the key bottleneck in the industry is cost,” he said. Concrete is cheap, and companies don’t want to price themselves out of the competition.

The researchers hope that by restoring this technique, which has stood the test of time, and which involves little additional cost to manufacture, they could answer both of these concerns. In fact, they are banking on it: Masic and several of his colleagues have started what they call DMAT, who are now looking for seed money to start producing commercial Roman hot mix concrete. “This very thing appeals only to the fact that it is the stuff of a thousand years.”