EFFECTIVENESS OF NANOSILICA DISPERSIONS AS CONSOLIDANTS FOR POROUS ARCHITECTURAL SURFACES

The problem of chemical and physical decay of Cultural Heritage materials and, in particular, the loss of cohesion strength in the porous materials have been faced in many different ways during the last decades. Recently nanodispersion consolidant systems seem to play an increasing role reaching a growing importance thanks to their promising results. Alcohol-based nanodispersions of calcium and magnesium hydroxides and carbonates have been successfully employed for the consolidation and restoration of frescoes and mural paintings. Beside nanolime dispersions, aqueous dispersions of nanosilica particles could represent a valuable, compatible and ecological alternative to the use of the traditional solvent-based products. However, inorganic and organic dispersions seems to have low ability to penetrate really in depth in the support, leading to a reduced efficiency of the treatment. The chemical- physical interactions of the nanosilica dispersions and the substrate and the relationship between the porosity of supports and the dimension of the colloidal particles are to be taken into account to evaluate and comprehend the ability of such products to deeply penetrate. Therefore, this knowledge is essential for the choice of the most suitable materials for intervention, but unfortunately has not been well defined yet. The aim of this research is to study the chemical interaction between nanosilica dispersions and carbonatic and silicatic matrixes, and to understand the mechanism by which silica nanodispersions (particles size from 10nm to 50nm) penetrate into stone supports having different porosity and different pore radius distribution. This study takes into consideration the behaviour of some commercial water-based silica dispersions applied on Lecce stone and on brick substrates, focusing in particular on particle dimensions, physical-chemical characteristics and penetration depth of the colloidal dispersion in relation to the substrate. FT-IR analysis, XRD and 29Si NMR-MAS spectroscopy were used to characterise the nanodispersions and the interaction between them and the selected substrates. SEM-EDX analysis on brick and Lecce stone samples, on which equal volumes of silica dispersions were applied, allowed to study the distribution and penetration depth of the nanosilica dispersions. The 29Si NMR spectroscopy gave significant information for the variation of the chemical environment of silicon atoms of the different silica dispersions, but no reactivity between silica and calcium carbonate were detected. The most plausible hypothesis is that the substrate acts as a filter. In this way, there’s an initial passage of particles across the substrate. The increase of particle concentration may lead to a slowing down of the flow towards the internal part of the substrate and thus it may lead to the formation of a silica layer on the surface, as show the SEM observations.