One and Two-dimensional NMR Studies to evaluate the performance of consolidants in porous media with a wide range of pore sizes: Applications to cultural heritage

The conservation of historical buildings and outdoor cultural assets is of great value for the humankind. In this context, consolidation treatments represent a fundamental procedure to assure the survival of deteriorated cultural artefacts. For porous materials characterized by a wide distribution of pore sizes, adequate consolidation treatments require a product with a sufficient penetration depth and homogeneous distribution inside the unconsolidated substrate. Since water is one of the main deterioration agents of stone, the wettability of rock should also be minimized. A non-invasive and non-destructive technique, able to localize the consolidation agent and the water uptake, is needed to evaluate the efficiency of the treatments. NMR techniques are good candidates to satisfy this requirement. The present study aims at investigating the efficiency of two commercial consolidants, i.e. ethyl silicate and nano-silica, and a new formulated mixture of a fluoroelastomer and nano-silica by NMR relaxometry and MRI. The tested compounds showed adequate characteristics to be used as consolidating agents for highly porous carbonate rocks like limestone. All the products, in fact, penetrated uniformly and homogeneously into the porous structure, and preserved the connectivity of the stone matrix, as demonstrated by T2-T2 relaxation exchange experiments. Moreover, the hydrophilic behavior of commercial consolidants could be reduced, when the newly formulated agent was used. This study shows how the combination of different NMR techniques, applied in the time domain and with spatial resolution, is a powerful approach for evaluating the consolidation efficiency.