Sandstone monuments still standing after storms: Waterproofing and durability protection

Using the Surface waterproofing to solve moisture expansion which leads to the acceleration of surface weathering of sandstone relics is a hot topic. This study focuses on surface coating protection because of the special status of cultural relics. The fluorinated ZnO/SiO2 nano-coating was applied on the sandstone matrix in this work. The loading of the fluorinated ZnO/SiO2 in the nano-coating, the penetration depth of the nano-coating in the sandstone matrix, and the waterproofing effect of the sandstone surface were investigated within the limitation of the color difference systematically. The freeze-thaw cycles, light exposure, water absorption, and aging tests were carried out to evaluate the environmental durability of the nano-coating in a simulated humid environment of southwestern of China. All the tests meet the criteria for the heritage protection color difference. When the loading of fluorinated ZnO/SiO2 nano-coating is 0.4%, the water contact angle (WCA) and oil contact angle (OCA) are significantly improved, which are larger than 160° and 140°, respectively. The WCA and OCA have little changed after freeze-thaw cycle for three weeks, and the WCA only decreases only 7% after 300 h UV aging treatment. This significantly enhanced stability is due to the good penetration depth of the fluorinated ZnO/SiO2 nano-coating on the sandstone matrix. Moreover, the excellent air permeability is achieved as well to ensure substrate breathing. This work would provide a new idea for the long-term protection of sandstone heritage in practical environment.