Flexible-rigid hybrid siloxane network for stone heritage conservation

Siloxane oligomers produced through the hydrolysis of tri-alkoxysilanes or tetra-alkoxysilanes have been widely used as consolidants for stone heritage. However, the resulting xerogels tend to crack due to the volume shrinkage after curing. In this study, a novel flexible-rigid hybrid siloxane oligomer material is designed by the self-catalyzed copolymerization of di-alkoxysilane (3-Aminopropyl) dimethoxymethylsilane with tetra-alkoxysilane tetraethyl orthosilicate. The flexible linear SiOSi segments produced by di-alkoxysilane are inserted into the tetra-alkoxysilane derived rigid three-dimensional silicon-oxygen domains, as confirmed by spectroscopic characterizations. This unique structure alleviates the shrinkage stress during curing and enables the formation of crack-free xerogels. Therefore, the hybrid siloxane oligomer can invade into the interior of weathered stone to form one three-dimensional continuous network to strengthen its fragile structure. As a result, the flexible-rigid hybrid siloxane oligomer outperforms both as-made polysiloxane consolidant and commercial consolidant for the consolidation of the weathered Tianlong Mountain Grottoes stone samples. The consolidation treatment was evaluated in terms of effectiveness (Leeb hardness, ultrasonic velocity and compressive strength) and compatibility (pore size distribution, water vapor transmission rate and color variation). Following the treatment, the consolidated samples demonstrate remarkable improvements in mechanical properties, reaching a Leeb hardness of 410 HL and a compressive strength of 8.98 Mpa. Additionally, permeability and color variation of the stone after consolidation are all within acceptable ranges. The results confirm the positive consolidation performance of the flexible-rigid hybrid siloxane oligomer on weathered Tianlong Mountain Grottoes stone, indicating promising practical applications.