Researchers at Vienna University of Technology and the University of Oslo have for the first time examined in detail how silicate nanoparticles could help save historic buildings from weathering. Many of our important historical buildings were made from porous rock such as limestone, which tends to weather badly over the years as it mostly consists of calcite minerals that are relatively weakly bound to each other. This causes parts of the stone to crumble away over the years, requiring expensive restoration and conservation.
Although the resistance of the stone can be improved through treatment with special silicate nanoparticles, understanding what happens in this process and which nanoparticles are best for the purpose has remained unknown. However, this new study, reported in the journal Langmuir [Dziadkowiec et al. Langmuir (2022) DOI: 10.1021/acs.langmuir.2c00486], demonstrated how this artificial hardening process takes place and which nanoparticles are best suited.
The team used a liquid suspension where the nanoparticles can initially float around freely. When the suspension penetrates the rock, the aqueous part evaporates, with the nanoparticles then forming durable bridges between the minerals and providing extra stability. As the water evaporates, an extremely unique type of crystallization happens. The silicate nanoparticles combine to form the “colloidal crystals” when they dry in the rock, jointly producing new connections between the individual mineral surfaces, enhancing the strength of the stone.
The very strong X-rays generated by the DESY synchrotron analyzed the crystallization during the drying process, key to understanding what the strength of the bonds is based on. Various sizes and concentrations of nanoparticles were used to examine the crystallization process, demonstrating the size of the particles is important to achieving the optimal strength gain. The adhesive force created by the colloidal crystals was also measured using a special interference microscope to investigate the tiny forces between two surfaces.
This showed that the smaller the nanoparticles, the more they can strengthen the cohesion between the grains of minerals. With smaller particles, more binding sites are produced in the colloidal crystal between two grains, and due to the amount of particles involved, the force they use to hold the minerals together also increases. The number of particles present in the emulsion is also key.
As researcher Markus Valtiner said: “Depending on the particle concentration, the crystallization process proceeds slightly differently, and this has an influence on how the colloidal crystals form in detail”. The results of the study will help in making old stone restoration work more durable and targeted.
“Depending on the particle concentration, the crystallization process proceeds slightly differently, and this has an influence on how the colloidal crystals form in detail”Markus Valtiner
Restoration works at St. Stephen's Cathedral in Vienna. credit: Archiv der Dombauhütte St. Stephan