Restoration works at St. Stephen's cathedral in Vienna. Image: Archiv der Dombauhütte St. Stephan.
Restoration works at St. Stephen's cathedral in Vienna. Image: Archiv der Dombauhütte St. Stephan.

Many historic buildings, such as St. Stephen's Cathedral in Vienna, Austria, are built of limestone. But while limestone is easy to work with, it does not withstand weathering well. It consists mainly of calcite minerals that are relatively weakly bound to each other, which is why parts of the stone crumble away over the years, requiring costly restoration and conservation treatments.

It is possible, however, to increase the resistance of the stone by treating it with special silicate nanoparticles. The method is already being used, but what exactly happens in the process and which nanoparticles are best suited for this purpose has been unclear until now.

A research team from the Vienna University of Technology (TU Wien) and the University of Oslo in Norway has now been able to clarify exactly how this artificial hardening process takes place. The researchers did this by conducting elaborate experiments at the DESY synchrotron in Hamburg, Germany, and with microscopic examinations in Vienna. In this way, they were able to determine which nanoparticles are best suited for the hardening purpose.

"We use a suspension, a liquid, in which the nanoparticles initially float around freely," says Markus Valtiner from the Institute of Applied Physics at TU Wien. "When this suspension gets into the rock, then the aqueous part evaporates, the nanoparticles form stable bridges between the minerals and give the rock additional stability."

This method is already used in restoration technology, but until now it was not known exactly what physical processes take place in the limestone. What was known was that, when the water evaporates, a very special kind of crystallization occurs.

Normally, a crystal is a regular arrangement of individual atoms. But entire nanoparticles can also arrange themselves in a regular structure – this is referred to as a ‘colloidal crystal’. The silicate nanoparticles come together to form such colloidal crystals when they dry in the rock and thus jointly create new connections between the individual mineral surfaces. This increases the strength of the natural stone.

To observe this crystallization process in detail, the TU Wien research team used the DESY synchrotron facility in Hamburg. Extremely strong X-rays can be generated there, and these X-rays are used to analyze the crystallization during the drying process.

"This was very important to understand exactly what the strength of the bonds that form depends on," says Joanna Dziadkowiec from the University of Oslo and TU Wien, who is first author of a paper on this work in Langmuir. "We used nanoparticles of different sizes and concentrations and studied the crystallization process with X-ray analyses." The researchers found that the size of the particles is decisive for optimal strength gain.

They also measured the adhesive force created by the colloidal crystals. For this purpose, they used a special interference microscope, which is perfectly suited for measuring tiny forces between two surfaces.

"We were able to show: the smaller the nanoparticles, the more can they strengthen the cohesion between the grains of minerals," says Dziadkowiec. "If you use smaller particles, more binding sites are created in the colloidal crystal between two grains of minerals, and with the number of particles involved, the force with which they hold the minerals together thus also increases."

The number of particles present in the emulsion is also important. "Depending on the particle concentration, the crystallization process proceeds slightly differently, and this has an influence on how the colloidal crystals form in detail," says Valtiner. These new findings will now be used to make restoration work more durable and more targeted.

This story is adapted from material from TU Wien, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.