Systematic evaluation of the effectiveness, compatibility and durability of four typical protective agents for sandstones in Nankan Grottoes, China

The weathering issues in Nankan Grottoes has significantly impaired the appreciation of its historic and artistic values. Yet, there is a lack of appropriate protective materials. This work aimed to evaluate the effectiveness, compatibility and durability of four protective products (MTES, OT-40, Acrylic, and nano-SiO₂) for three sandstones (yellow, cyan, and gray) presented in Nankan Grottoes. The hydrophobic effect, mechanical property, microscopic observation, chromatic alteration, and weathering resistance of the treated samples were systematically evaluated. MTES and OT-40 were the most effective in hydrophobic effect. Nano-SiO₂ contributed least to hardness improvement due to its lowest application amounts. Overall chromatic changes in the treated samples were below the detection limit of the naked human eye (ΔE^*<3), except for the yellow sandstone treated with OT-40 and cyan sandstone treated with MTES. Microscopic observations revealed that the four agents provided better protection for grey sandstone, due to the smallest grain size and porosity of the gray sandstone. The four agents improved the salt weathering resistance of the three sandstones, except for the yellow sandstone treated with nano-SiO₂, whose salt weathering resistance was reduced. Therefore, hydrophobic nano-SiO₂ may severely damage porous sandstone during salt weathering, due to the huge interfacial stress produced at the interaction between the hydrophobic superficial layer and the hydrophilic rock substrate. The anti-UV ageing stability relied mainly on the porosity and grain size of the substrates. Our research has advanced the fundamental understanding of the performance of novel protective materials, also paved the way for the conservation of sandstone heritages.