Photoactive and water-repellent g-C3N4/poly(alkyl siloxane) coatings on porous sandstone

Abstract Graphitic carbon nitride (g-C 3 N 4 ) was synthesised by thermal polycondensation of melamine and subsequently exfoliated by calcination at 600 °C for 30, 45, and 60 min. The resulting nanoparticles—differing in morphology, size, and specific surface area—were dispersed in a poly(alkyl siloxane) hydrophobising agent at concentrations of 2 g·L⁻¹ and 4 g·L⁻¹ and applied to the surface of porous quartz sandstone. The effect of the degree of g-C 3 N 4 exfoliation on the self-cleaning performance of the resulting coatings and their interaction with water was evaluated. Photodegradation tests performed on sandstone samples artificially stained with Rhodamine B confirmed a self-cleaning effect for coatings containing particles exfoliated for 45 and 60 min, at a concentration of 4 g of g-C 3 N 4 per litre of poly(alkyl siloxane). The addition of nano-g-C 3 N 4 enhanced the hydrophobicity of the sandstone surface. The coatings substantially reduced water uptake into the stone’s pore system while still allowing water vapour to evaporate freely into the surrounding environment. g-C 3 N 4 thus appears to be a promising photocatalytically active nanomaterial for self-cleaning and water-repellent surface protection of building and sculptural stone—a field in which it has not yet been applied.