The durability of ancient Roman concrete is legendary. Some ancient aqueducts are still used today to supply Rome with water. Thanks to the “opus caementitium”, the Roman concrete, the 1900 year old Pantheon still exists and is used as a Catholic church. For decades, scientists have tried to figure out what makes Roman concrete so durable. US scientists now report that they have solved the mystery: small pieces of lime could have given stabilizing properties to concrete and mortar, writes the group led by Admir Masic of the Massachusetts Institute of Technology (MIT) in the journal Science Advances. Experiments with this mixture of building material revealed a kind of self-healing power. If cracks form in the concrete through which water flows, limestone minerals form that can re-fill the cavities.
In recent decades, scientists have repeatedly studied Roman concrete and mortar. Among other things, they identified volcanic ash and rock, as well as seawater, as ingredients that increase shelf life through various chemical reactions. However, millimeter-sized chunks of limestone were initially discarded as impurities. “Ever since I started working with ancient Roman concrete, I’ve always been fascinated by these components,” Masic said in an MIT statement. The idea that the presence of lumps of lime is just due to poor quality control has always bothered him. Masic and his team therefore tested the assumption that this un-slaked lime has a specific function in the concrete mix.
Ancient Roman concrete production was more environmentally friendly than today
To do this, they examined the mortar walls of the ancient city of Privernum, near modern-day Privernum, southeast of Rome. Thus Roman mortar and concrete consisted of sand, volcanic ash, pieces of volcanic rock, water, and pieces of quicklime—and sometimes slaked lime as well. The reaction of water with the surface of unslaked lime (calcium oxide) produces slaked lime (calcium hydroxide), which is accompanied by the development of heat of up to 60 degrees Celsius.
Slaked lime, in turn, reacts with water and sand to form a cement-like binder, releasing even more heat. Scientists speak of “hot mixing”. On the one hand, these reactions bond the rock fragments more firmly to the cement matrix. On the other hand, curing and setting times are significantly reduced, explains Masic. With modern concrete, on the other hand, the aggregates should not react with the cement if possible.
Based on these findings, Masic and his colleagues created concrete mixes with and without lumps of lime. They split the hardened concrete and put the pieces together so that a gap of 0.5 millimeters remained. Then they let the water run through this gap. While water still flowed through the gap almost unimpeded after 30 days in lime-free concrete, almost no water passed through the lime-laden concrete. Because the crack in the concrete also split the clods of lime, and the unslaked lime reacted with the water to form the slaked lime, mineralizing and filling the gap.
These formulations can also improve the durability of 3D-printed concrete formulations, says Masic. Furthermore, these mixes are significantly more environmentally friendly than current cement production, which is responsible for 8% of global carbon dioxide emissions. Masic is also working on a concrete that can absorb the greenhouse gas carbon dioxide from the air.