Surface Modification of Historical Sille Stones with Silane-Based Coatings

Abstract This study aimed to enhance the hydrophobic properties of Sille stone, a material commonly employed in historic buildings in the Konya province and its surrounding areas, through a silane-based surface modification. For this purpose, using tetraethoxysilane (TEOS), hexadecyltrimethoxysilane (HDTMS), and chlorotrimethylsilane (TMCS) silanes, the effects of the TEOS/HDTMS ratio, a single modifier silane (HDTMS), and a combination of two modifier silanes (HDTMS + TMCS) on the surface properties of Sille stone were investigated. Contact angle (CA) measurement, Fourier Transform Infrared Spectroscopy (FT-IR), Atomic Force Microscopy (AFM), and Scanning Electron Microscopy (SEM) analyses were performed on pristine and surface-modified stone samples to evaluate the effectiveness and compatibility of the coatings. The natural hydrophilic property of Sille stone (CA: 7.81°) was transformed into a highly hydrophobic surface (CA up to 126°) through surface modification using low concentrations of TEOS/HDTMS and a combination of two modifier silanes (HDTMS + TMCS). FT-IR spectra confirmed that the hydrolyzable groups of the silanes reacted with the OH groups on the Sille stone surface, forming -Si-O-Si- bonds, and the presence of C-H bonds in the spectra indicated the presence of silane modifiers on the stone surface. SEM micrographs indicated that the silane-based coatings masked the microstructural elements (the large pores and needle-like fibrous structures) of the Sille stone, indicating potential compatibility between the coating and the natural stone substrate. TMCS silane as a secondary modifier resulted in a denser, more homogeneous coating with less crack defects. Consequently, when HDTMS + TMCS modifier silanes were employed in conjunction, a more protective layer was formed on the surface of the Sille stone, resulting in a markedly hydrophobic surface.