Historical stone surfaces are magnificent but undergo damage with time. Their aging and decay are inevitable when exposed to the outside environment. This is because the often porous architectural stone surfaces are prone to water-induced weathering, which is the process of being worn by long exposure to the atmosphere. Rainwater that percolates within the stone can cause chemical weathering, mainly mineral dissolution and adverse reprecipitation, as well as mechanical weathering when the pore water undergoes freezing and thawing cycles. These processes are slow but in the long term, they induce severe and irreversible changes that ultimately lead to grain disintegration and stone crumbling. These changes are especially evident on porous and malleable sedimentary stones such as limestone (Figure 1), which was the common building material in the past but hardly withstands weathering due to relatively loose bonding between the constituent calcite grains. In order to stop further decay, the loosened stone structure needs consolidating treatments. However, keeping the historic stone buildings preserved requires major financial resources and state-of-the-art treatment strategies. How can nanoparticles help in their protection and restoration?
It is possible to increase the resistance of the stone by treating it with certain types of silica nanoparticles. The idea sounds simple: inject a liquid, water-based suspension of dispersed nanoparticles into the damaged stone material, let the aqueous part evaporate, and wait until nanoparticles come together to form stable bridges between loosened grains. Ultimately, the silica nanoparticles act locally as cement between the original mineral grains. Such a nanosilica-based consolidation method has been already occasionally applied for stone restoration on-site (Ban et al., 2020); however, it still remains in the testing phase. This is because it is unclear what exactly happens after the nanoparticles are injected into damaged stone materials. Thus, it remains difficult to design a universal treatment that would be compatible with many different types of stones with varying amount of empty pore spaces, textures, and mineral compositions.
Our research team from TU Wien and the University of Oslo designed an experimental method to clarify how this artificial hardening process takes place by combining X-ray scattering experiments at a synchrotron facility (an exteremely powerful source of X-rays) DESY (Deutsches Elektronen-Synchrotron) in Hamburg with microscopic force examinations using a special force measuring technique called surface forces apparatus (SFA) at the Vienna University of Technology (Dziadkowiec et al., 2022). That way, we got nanoscale insight into each stage of the consolidating process in a single pore.
We first investigated what happens with the water-dispersed silica nanoparticles during the evaporation of the liquid phase in a single pore using a high-energy X-ray scattering probe at the synchrotron facility. The synchrotron-generated X-rays allowed us to analyze how these particles aggregate together when the liquid phase dries off. We found out that as the small silica nanoparticles dry and come together they also act as glue, which binds the two opposing pore walls strongly together. Thus, dried silica particles form a very stable bridge connections inside pore spaces. These silica bridges can serve as a local cement between mineral grains in damaged building stone materials. Similar bridges form also in building stone materials saturated with a liquid suspension of silica nanoparticles (Figure 2).
We then designed a method to quantify the exact strength of the silica bridges formed in a single pore using a force measuring technique, called the surface forces apparatus (SFA). SFA is a perfect tool here, as the force can be measured between two macroscopic surfaces connected by silica nanoparticles. In detail, we let silica nanoparticles aggregate and form bridges between the two opposing mineral walls in a special single pore geometry. We then quantified how strong are these bonds by stretching the two mineral walls apart at a controlled separation velocity. As such, we tested the tensile mechanical strength of the silica bridges on stretching.
These measurements have shown that the size of the particles is decisive for the optimal strength of the bridges. We were able to show that the smaller the nanoparticles, the more can they improve the cohesion between silica-bridge bonded mineral grains. When smaller particles are used, there are more binding sites within the bridge made of colloidal silica crystal, which strengthens the bond made by a nanosilica bridge. Overall bridge strength depends also on the concentration of silica nanoparticles. Depending on the particle concentration, the crystallization process proceeds slightly differently, and this has an influence on how the colloidal silica crystals form in detail. In general, at higher concentrations of silica nanoparticles, there is a higher number of particles involved, and the force with which they hold the opposing mineral walls together is also higher. However, at the highest concentration of particles that we probed, the strength of the bridges decreased again. This is caused by the way that the nanosilica suspension dries inside the pore and how fast the particle can migrate into the centre of the pore.
The method we designed helps us to understand the nanoscale mechanism of nanosilica-induced grain consolidation in a simplified single pore geometry. It allows us to study the consolidation process systematically by varying a number of system properties: such as the type and size of nanoparticles, surface properties of the mineral grains, their roughness, and mineral reactivity in contact with water-based nanoparticle suspensions, and so on. This knowledge can be used to develop more universal consolidating treatments that can be used in various on-site settings. Although we cannot predict the long-term effects of the applied treatments on-site yet, we can now measure which nanoparticles will be the most effective by probing different nanoparticle-mineral grains combinations and checking their compatibility. And, we know that size matters: just by decreasing the silica nanoparticles size from 70 to 10 nm, we can get colloidal silica bridges an order of magnitude stronger.
Ban, M., Aliotta, L., Gigante, V., Mascha, E., Sola, A., & Lazzeri, A. (2020). Distribution depth of stone consolidants applied on-site: Analytical modelling with field and lab cross-validation. Construction and Building Materials, 259, 120394.
Dziadkowiec, J., Cheng, H. W., Ludwig, M., Ban, M., Tausendpfund, T. P., von Klitzing, R., Mezger M., & Valtiner, M. (2022). Cohesion Gain Induced by Nanosilica Consolidants for Monumental Stone Restoration. Langmuir.
Joanna Dziadkowiec is a postdoctoral researcher in the Njord Centre, at the University of Oslo, where she also completed her PhD. In 2019-2022, she was a visiting scientist at the Vienna University of Technology (Austria) where she worked on her individual FRIPRO research project granted by The Research Council of Norway. Her research focuses on surface interactions applied in geophysics and material science. She specializes in surface forces measurements in confinement using the Surface Forces Apparatus.
Markus Valtiner is a professor of applied interface physics at the Vienna University of Technology. He was the recipient of the Otto-Hahn medal of the Max-Planck-Society for his Ph.D. work at the Max-Planck-Institut für Eisenforschung in Düsseldorf, Germany (2008), and of the Peter Mark Award of the American Vacuum society (2017). His research interests focus on solid/liquid interfaces, single-molecule interactions, and adhesion. He also studies corrosion in confined spaces. He is an expert in force probe experiments; in particular, the Atomic Force Microscopy and the Surface forces Apparatus.
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