Alkylsiloxane/alkoxysilane sols as hydrophobic treatments for concrete: A comparative study of bulk vs surface application

Water and waterborne decay agents (e.g. salts, microorganisms) are commonly associated with undesired alterations and damages on concrete elements. A strategy to mitigate their impact is to decrease water retention in the material either by surface treatments or admixtures. In this work, hydrophobic concretes were developed by the addition of a hydrophobic sol containing TEOS and PDMS oligomers, synthetized by a surfactant-assisted sol-gel route, either as an admixture or as a surface treatment. The hydrophobic performance was similar for both application modes (> 70% capillary absorption reduction) and higher than concrete containing a commercial (calcium stearate) admixture or a nanosilica-based hydrophobic coating. Addition as an admixture led to a higher durability in the rain and abrasion tests.The hydrophobic sol as an admixture promotes an increase of surface roughness and porosity, as well as the formation of C-S-H like reaction products with the cement matrix components, as evidenced by AFM, MIP, SEM and FTIR. Despite the higher porosity, impact resistance and material cohesion were not negatively affected respect to the plain concrete. Application as a surface treatment decreased porosity and led to a higher amorphous SiO2 content. The material cohesion and impact resistance was increased by this application, although penetration was limited to the first 4 mm and the hydrophobic properties were more susceptible to mechanical damages to the surface.