Surface modification of fumed silica nanoparticles to make a super- hydrophobic diffusion layer as a protection for the building facade

In this article, the goal is to build a super-hydrophobic diffusion layer to preserve the facade of the building. In this research, first, the surface of fumed silica nanoparticles was modified by polydimethylsiloxane polymer, and then a suspension of modified nanoparticles was made based on water/ethanol and the stabilizing agent sodium dodecyl sulfate. The super- hydrophobic diffusion layer was applied on the brick surface of the buildings using brushing and spraying process. In order t o evaluate the properties of the super-hydrophobic diffusion layer on the brick surface, Rilem test, breathability, efflorescence, contact angle and water drop sliding angle were used. The results showed that the zeta potential for the suspension containing hydrophilic fumed silica nanoparticles and the suspension containing hydrophobic fumed silica nanoparticles is -25.4 mV and -55.9 mV, respectively. Based on the Fourier transform infrared spectrometry test, since no significant shift of the spectra was observed, therefore, the mechanism of connection of modifier molecules with the surface of nanoparticles was reported as physical. The minimum and maximum amount of polydimethylsiloxane polymer for hydrophobization the surface of fumed silica nanoparticles was 5 and 15% by weight. The contact angle of the water drop on the surface of the brick, which is coated with a hydrophobic diffusion layer, is more than 150◦ and the sliding angle is 4◦, which shows the super-hydrophobic property of this layer. This super-hydrophobic property was achieved according to the Cassie-Baxter mechanism due to the creation of a surface with micro/nano scale roughness and very low surface energy. According to Rilem test, after 20 minutes, water did not penetrate into the hydrophobic brick. Also, according to the efflorescence test, after 15 days, no salt deposits were observed on the surface of the brick. In addition, the super-hydrophobic diffusion layer created a favorable breathability on the brick surface of the building facade.