St. Stephen’s Cathedral in Vienna, among others, was constructed with limestone. Limestone is simple to deal with, but it does not weather well. It is composed primarily of calcite minerals that are extremely loosely connected to one another, which explains why sections of the stone continue to crumble with time, frequently necessitating expensive repair and conservation procedures.

However, it is possible to boost the stone’s resilience by treating it with nanoparticles of silicate. The technology is now in use, but the precise nature of the procedure and the nanoparticles most suited for this use were unknown until recently. Using intricate trials at the DESY synchrotron in Hamburg and microscopic investigations in Vienna, a team of researchers from TU Wien and the University of Oslo have now been able to determine precisely how this artificial hardening process occurs. Thus, the researchers would be able to establish which nanoparticles are ideal for this purpose.

Prof. Markus Valtiner from the Institute of Applied Physics at TU Wien explains, “We utilize a suspension, a liquid, in which the nanoparticles first float freely.” When this solution enters the rock, the water component evaporates, and the nanoparticles build stable bridges between the minerals, therefore enhancing the rock’s stability.

This technique is already employed in restoration technology, but its physical processes were unknown until recently. When water evaporates, a highly particular form of crystallisation takes place: Typically, a crystal consists of a consistent arrangement of atoms. However, not only atoms but even whole nanoparticles can form a regular structure; this is known as a “colloidal crystal.”

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Colloidal crystals are formed as the silicate nanoparticles harden in the rock, creating new connections between the surfaces of the individual minerals. This improves the stone’s longevity.

Measurements conducted at the DESY research center and in Vienna. The TU Vienna research team utilized the DESY synchrotron facility to study this crystallisation process in detail. There, extremely powerful X-rays may be produced, which can be utilized to analyze crystallisation throughout the drying process.

Joanna Dziadkowiec (University of Oslo and TU Wien), the lead author of the journal in which the study results have now been revealed, explains, “It was crucial to establish precisely what determines the strength of the connections that develop.” “Using nanoparticles of various sizes and concentrations, we analyzed the crystallisation process using X-rays.” It has been demonstrated that particle size is crucial for optimum strength increase.

The Vienna Technical University also tested the adhesive force generated by the colloidal crystals for this purpose. For this objective, a specialized interference microscope that is ideal for detecting minute stresses between two surfaces was utilized.

Joanna Dziadkowiec explains, “We were able to demonstrate that the smaller the nanoparticles, the more they may increase the cohesiveness between mineral grains.” “Using smaller particles creates more binding sites in the colloidal crystal between two grains of minerals, and as the number of particles grows, so does the force by which they keep the minerals together.” The number of particles present in the emulsion is also crucial. Markus Valtiner explains, “The crystallisation process progresses somewhat differently depending on the particle concentration, and this affects the formation of colloidal crystals in fine detail.” Now, the additional information will be utilized to make restoration efforts more lasting and focused.

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