Compatibility, effectiveness and susceptibility to degradation of alkoxysilane-based consolidation treatments on a carbonate stone

Alkoxysilane-based consolidation treatment are commonly employed in restoration and conservation works of Cultural Heritage buildings. However, their poor affinity to carbonate stones is well-known. In this paper, we evaluate three consolidant treatments (two of them modified to incorporate hydrophobic or photocatalytic properties), formulated to circumvent this drawback and their cracking during drying, through a surfactant-assisted synthesis. The consolidants were applied on Noto stone, a biocalcarenite (~95% CaCO3) employed in numerous Cultural Heritage buildings of Southern Italy. A comprehensive validation of the three consolidants, compared to a commercial TEOS based consolidant, was performed regarding their compatibility, effectiveness and susceptibility to degradation. The obtained results show that all consolidants improved the mechanical resistance of the stone, up to 6 mm for the commercial consolidant and up to 9 mm in depth for the surfactant-synthetized ones, though over-consolidation on the surface is a factor to watch for on the latter. Moreover, the treated stones exhibited a greater resistance to salts ingress respect to untreated ones related to the reduction in the smaller pores (0.1–2 μm range). In addition, all tested consolidants can be considered within the low and moderate incompatibility degree, though this factor is slightly higher than for the commercial one. Regarding the susceptibility to degradation, the surfactant-synthetized products experienced lower loss of consolidating effect, compared to the commercial one, after prolonged immersion in water and salts (sulphate) crystallization tests. However, the modification of the consolidant to obtain photocatalytic properties has a significant impact on its susceptibility against the latter.