Many listed buildings are made of natural limestone. It is very nice to work with this porous material and it looks beautiful, but the buildings do need a lot of maintenance. Nanoparticles offer the solution.

By injecting nanoparticles, the material becomes much more stable and stronger. Researchers from the Technical University of Vienna and the University of Oslo have investigated exactly how the crystallization of nanoparticles in the limestone works.

Before that, they looked at St. Stephen’s Cathedral, which is located in the middle of Vienna. It is one of the most famous buildings in the Austrian capital. The church is made of limestone. The natural material slowly crumbles due to wind and weather. The calcite minerals from which the rock is built are relatively weakly connected. The building is therefore often under scaffolding for restoration. This is a lengthy and expensive process.

Nano Injection

Fortunately, nanotechnology offers a solution here. By injecting a solution of a special silicate of nanoparticles into the limestone, the material becomes much stronger.

This method has been used for some time, but it was not yet entirely clear why it works so well. That has now changed with the findings of Professor Markus Valtiner of the Faculty of Applied Physics at TU Wien and his colleagues, who their studies in professional magazine Langmuir published.

Tight in the ranks

This artificial hardening process has been examined in a laboratory of the DESY synchrotron in Hamburg, Germany. Or just say under a very expensive microscope. For example, the best nanoparticles for strengthening this type of limestone have been identified. “We use a suspension, a liquid, in which the nanoparticles can float freely,” says Valtiner. “When this suspension ends up in the limestone, the water evaporates and the nanoparticles form stable molecular bridges between the minerals. This gives the stone extra stability.”

Colloidal Crystal

A special kind of crystallization occurs when the water evaporates. Normally, a crystal consists of an even arrangement of individual atoms. But in this case, entire nanoparticles arrange themselves in a tight crystal structure. This is called a ‘colloidal crystal’.

The artificial silicate molecules form colloidal crystals with each other when the liquid in the solution evaporates. These new compounds in the limestone make the natural material much stronger. It no longer crumbles due to weather influences or other causes.

Restoration work Stephansdom in Vienna. Photo: Archiv der Dombauhütte St. Stephan

X-rays

The team wanted to find out exactly how this crystallization process works. In Hamburg, therefore, an extremely high dose of X-rays was shot through the prepared material, just as the colloidal crystals were formed. “We really wanted to know which factors play a role in the strength of the nano-compounds. It was a search for the strongest combination for this material,” explains researcher and lead author of the study Joanna Dziadkowiec.

“We have tested all kinds of situations, different sizes of nanoparticles and differences in concentrations. Under the X-ray machine, we were able to analyze all these scenarios.” It became clear that the size of the nanoparticles played a key role in the search for the strongest solution.

Small is beautiful

“In the end we found out that the limestone gains the most stability when you inject the smallest possible nanoparticles. The cohesion between the minerals in the natural stone type benefits the most,” says Dziadkowiec. “If you use smaller particles, more connections are created in the colloidal crystal between the minerals. The more particles participate in the crystallization process, the more powerful the material becomes.”

more is better

So many small nanoparticles, that works best. “The concentration of the active substance in the suspension slightly alters the colloidal crystallization process,” said Valtiner.

These new insights can be used in the future to more effectively carry out restoration projects of historic buildings. So that they don’t have to rebuild after a few years.