Role of pH value on electrophoretic deposition of nano-silica onto carbon fibers for a tailored bond behavior with cementitious matrices

Abstract

Electrophoretic deposition (EPD) of nano-silica (NS) onto carbon fiber (CF) surfaces may considerably improve the bond properties toward cementitious matrices. In the article at hand, varied pH conditions (neutral, acidic, and alkaline environments) are studied during EPD for a tailorable interfacial bond. Zeta potential measurements and cyclic voltammetry (CV) confirmed the importance of pH with regard to the EPD process. Scanning electron microscopy (SEM) and X- ray photoelectron spectroscopy (XPS) revealed that an acidic environment enables the most pronounced and homogeneous NS coating on the CF surfaces. X-ray diffraction (XRD) analysis delivered deeper insights into the microstructures of modified CFs, indicating a decreased graphite interlayer spacing d002 and an increased crystallite size Lc after treatment. This results in a changed temperature-stability as well as mechanical properties, as revealed by thermogravimetric analysis (TGA), single-fiber tensile tests, and Weibull analysis.To assess the interaction of the modified CFs with cementitious matrices, single-fiber pullout tests were performed, showing the highest bond strength and total pullout work of CFs treated in an acidic environment. This is mainly explained by a reaction of the high number of NS particles with calcium hydroxide to form calcium silicate hydrate (C-S-H) gel and, thus, intensifying the interfacial transition zone, subsequently increasing the bond properties of CF to a matrix. As well, abundant oxygen-related functional groups were introduced on the CF surfaces from anodic oxidation, leading to an enhanced bond by promoting precipitation of cement hydration products onto the CF surfaces.