Layer double hydroxide (LDH): Synthesis and incorporation in alkoxysilanes for stone conservation

Abstract

Layer double hydroxides (LDH) carry potential as additives for modifying alkoxysilanes in building materials conservation due to their unique ability to exchange ions with the surrounding environment. This paper reported, for the first time, the potential benefits of CaAl-LDH (LDH-S) prepared by the co-precipitation method as additives in alkoxysilanes for stone consolidation. The LDH prepared in this work were compared with commercial CaAl-LDH (LDH-C). Both additives revealed high crystallinity and purity; however, LDH-S had larger dimensions (approximately 85 vs 55 nm) and formed agglomerates upon drying. While consolidants containing LDH additions above 0.5 % (w/v) showed increased viscosity and susceptibility to cracking, concentrations up to 2 % did not compromise the penetration of the alkoxysilane oligomers into stone pores or significantly jeopardized their capacity to strengthen the stone in depth. At higher concentrations, both LDH tended to accumulate on the surface of the stone investigated (limestone with pores of approximately 0.8 μm). Contrarily, the smaller LDH-C, at lower concentrations (0.125 % to 0.5 %), demonstrated a viable incorporation with adequate distribution over the surface. The incorporation of LDH into alkoxysilanes mitigated the formation of hard crust; prevented siloxanes accumulation near the surface and minimized adverse effects on the appearance of light-coloured stones. These findings indicate that the incorporation of small amounts of LDH into alkoxysilanes consolidants is viable and paves the way for introducing new functionalities into diverse conservation treatments since LDH can carry antifouling or biocide agents.