Evaluation of the Effect of Microsilica and Carbon Fiber-Reinforced Polymer (CFRP) on the Corrosion Resistance and Bond Strength of Reinforcing Rebars in Geopolymer Concrete

Corrosion of steel rebars remains one of the major challenges affecting the durability and long-term performance of reinforced concrete structures, particularly in chloride-rich and marine environments where the passive protective layer surrounding steel gradually deteriorates. This process results in premature deterioration, increased maintenance costs, and reduced service life. Geopolymer concrete has emerged as a sustainable alternative to ordinary Portland cement because of its use of industrial by-products such as ground granulated blast furnace slag (GGBS), lower carbon emissions, and excellent mechanical and chemical properties. However, despite these advantages, geopolymer concrete is still susceptible to drying shrinkage, brittleness, and the formation of interconnected pores and microcracks that facilitate chloride penetration and initiate reinforcement corrosion. To overcome these limitations, supplementary materials have been widely investigated to improve the microstructure and durability of geopolymer systems. Microsilica is known for refining the pore structure, densifying the matrix, and reducing permeability, while carbon fiber-reinforced polymer (CFRP) has attracted attention because of its ability to improve toughness and limit crack propagation. Although the individual benefits of these materials have been extensively reported, their combined influence on the corrosion behavior and bond performance of reinforcing steel in slag-based geopolymer concrete has not been sufficiently studied. Therefore, this study evaluated the individual and combined effects of microsilica and CFRP on reinforcement corrosion and steel–concrete bond performance under accelerated chloride exposure. Four geopolymer mixtures were prepared: a control mixture containing only GGBS, a mixture with 5% microsilica, a mixture reinforced with 1.5% CFRP, and a hybrid mixture incorporating both 5% microsilica and 1.5% CFRP. Steel rebars were centrally embedded in the specimens, which were cured for 28 days. Accelerated corrosion was induced using the impressed current technique in a 3.5 wt.% sodium chloride solution for exposure periods of 1, 7, and 15 days to simulate severe marine conditions. Corrosion performance was assessed through potentiodynamic polarization measurements, while bond behavior was evaluated using pull-out tests. Field emission scanning electron microscopy (FESEM) was also employed to examine the microstructure and correlate microscopic characteristics with the electrochemical and mechanical responses. The results showed that microsilica significantly enhanced the durability of geopolymer concrete by reducing corrosion current density, refining the pore structure, and improving the bond strength between reinforcing steel and the surrounding matrix. The denser microstructure effectively restricted chloride penetration and strengthened the interfacial transition zone, leading to improved corrosion resistance. In contrast, specimens containing only CFRP exhibited higher corrosion rates and lower bond strength than the control mixture. This behavior was mainly attributed to the electrical conductivity of the carbon fibers, which likely promoted localized galvanic effects and accelerated electrochemical reactions at the steel surface. The hybrid mixture containing both microsilica and CFRP performed considerably better than the CFRP-only mixture, indicating that the microstructural refinement provided by microsilica mitigated much of the adverse electrochemical effects associated with carbon fibers. FESEM observations confirmed these findings by revealing a denser and more homogeneous matrix with fewer interconnected pores and reduced cracking in microsilica-containing specimens. Overall, the results demonstrate that microsilica is highly effective in improving both corrosion resistance and bond performance in GGBS-based geopolymer concrete. Moreover, combining microsilica with CFRP offers a balanced strategy that preserves the mechanical advantages of fiber reinforcement while minimizing durability-related drawbacks, providing valuable guidance for the development of sustainable and corrosion-resistant geopolymer concrete structures for aggressive chloride-rich environments.