– Researchers have revealed the secret of the legendary ancient Roman concrete
The ancient building materials are amazingly durable. Experiments with the recipes now show: some of the additives gave him self-healing powers.
The durability of ancient Roman concrete is legendary—scientists have been trying to figure out why for decades. Now US researchers want to uncover the secret: Small lumps of lime may have given concrete and mortar stabilizing properties, the group led by Admir Masic of the Massachusetts Institute of Technology (MIT) writes in Science Advances magazine. Experiments with a mixture of building materials have revealed some kind of self-healing powers. If cracks form in the concrete through which water passes, calcareous minerals are formed that can fill the cavities again.
The concrete used by the Romans is very durable: some aqueducts from Roman times are still used today to supply water to the Italian capital, Rome. The 1,900-year-old Pantheon is still standing thanks to its “Opus caementitium,” Roman concrete, and is used as a Catholic church.
In past decades, scholars have examined Roman concrete and mortar again and again. Among others, they identified volcanic ash and rock as well as seawater as ingredients that increase shelf life through various chemical reactions.
Meaning of limestone nuggets
The previously neglected component are millimeter-sized lumps of limestone—once discarded as inclusions. “Since I started working with ancient Roman concrete, I have always been fascinated by these components,” Musk was quoted as saying in the MIT issue. The idea that lime lumps were the result of poor quality control had always bothered him. So Musk and his team tested the hypothesis that this uncrushed lime has a specific function in the concrete mix.
To do this, they first examined the mortar found on the walls of the ancient city of Prefernum near the present-day city of Priferno, southeast of Rome. Thus, Roman mortar and concrete consist of sand, volcanic ash, pieces of volcanic rock, water, lumps of quicklime, and sometimes slaked lime as well. The reaction of water with the surface of unhydrated lime (calcium oxide) produces slaked lime (calcium hydroxide), which is accompanied by a thermal evolution of up to 60°C.
These formulations can also improve the durability of 3D printed concrete formwork.
Slaked lime, in turn, reacts with water and sand to form a cement-like binder, which releases more heat. Scientists talk about “hot mixing”. On the other hand, these interactions bind the rock fragments more firmly into the cement matrix. On the other hand, processing and setup times are significantly reduced, Musk explains. With modern concrete, on the other hand, the aggregate should not interact with the cement if possible.
Based on these results, Masic and colleagues created concrete mixes with and without lime blocks. They split the hardened concrete and put the parts together so that there was a 0.5mm gap left. Then they let the water pass through this gap. While water still flowed through the gap almost unimpeded after 30 days in the concrete without lime lumps, there was almost no water through the concrete with lime lumps. Because the fracture in the concrete has also cracked the lime blocks, and the uncrushed lime has reacted with water to form slaked lime, which leads to mineralization and filling of the gap.
These fittings could also improve the durability of 3D-printed concrete structures, says Masic. In addition, these mixtures are much more environmentally friendly than cement production today, which is responsible for eight percent of global carbon dioxide emissions. Masic is also making concrete that can absorb the greenhouse gas carbon dioxide from the air.