If modern concrete structures are left alone for decades, they begin to crumble, but structures built by the ancient Romans are still standing strong 2,000 years later. Now, engineers have discovered inclusions in ancient concrete that help it self-repair, showing how to recreate a recipe that makes new buildings last longer.
Concrete is the most commonly used building material in the world, but it is not susceptible to damage. Weather and stress can cause small cracks to grow into larger cracks that ultimately threaten the integrity of the entire structure. Costly maintenance or replacement may be required to prevent catastrophic failure.
In contrast, ancient Roman structures have stood the test of time for over 2000 years. To figure out how, scientists have long examined samples of materials under a microscope, studied their composition, and uncovered the ingredients that give them such strength.
It stands out for its pozzolanic material, which is made from volcanic ash from certain regions of Italy. So does lime, and previous studies have found that this helps concrete actually get stronger over time in marine environments like bridge piers. Millimeter-sized white mineral clumps, called so-called, are usually ignored as a by-product, but in a new study, researchers have found that they may be there for some reason.
“The idea that the presence of these limestone deposits was simply due to poor quality control has always bothered me,” said Admir Masic, lead author of the study. “If the Romans put so much effort into making superior building materials, following all the detailed recipes that have been optimized over the centuries, why did they produce a well-mixed final product?” Little effort was put into ensuring production?? There must be more to this story.”
When
The team used several imaging and chemical mapping techniques to take a closer look at the limestone mass and found it to be made of a type of calcium carbonate that appears to have formed at high temperatures. It suggests that it was made by directly adding (or “hot-mixing”) a more reactive form of quicklime than the form presumed to have been used by man.
“There are two advantages to hot mixing,” says Masic. “First, when the entire concrete is heated to high temperatures, chemical reactions occur that are not possible when using only slaked lime, creating high-temperature-related compounds that would otherwise not form. Second, this elevated The temperature accelerates all the reactions, allowing for much quicker construction, so curing and hardening times are greatly reduced.”
But more importantly, these limestone masses play an active role in the self-healing of concrete. The hot mixing process makes the inclusions brittle, so when small cracks appear in the concrete, they migrate more easily through the limescale than the surrounding material. When water enters cracks, it reacts with lime to form a solution that hardens into calcium carbonate and seals the cracks.It can also react with pozzolanic materials to further strengthen the concrete itself.
So rather than being an unwanted by-product, these lime crusts are there for a reason, the team says. .
To test the hypothesis, the researchers created hot-mixed samples of ancient and modern concrete, cracked them, and ran water through the cracks for an extended period of time. Two weeks later, the ancient concrete sample had repaired cracks blocking water flow. Modern materials, on the other hand, did not cure at all.
The team says the discovery not only helps us understand the secrets of ancient engineering, but could also help improve modern concrete recipes. To that end, researchers are taking steps to commercialize the material.
A study was published in a journal scientific progress.
Source: MIT
