Developing low-alkalinity geopolymer concrete for enhanced durability of BFRP bars toward sustainable marine engineering

Geopolymer concrete reinforced with basalt fiber-reinforced polymer (BFRP) is a promising alternative to conventional steel-reinforced concrete in marine environment. However, despite BFRP's longer lifespan than steel, the durability remains constrained by its degradation in high-alkalinity concrete, hindering the application of BFRP-reinforced geopolymer concrete. This study aims to develop low-alkalinity geopolymer concrete to address this challenge. To achieve this goal, the effect of initial Na/Al, Si/Al and water/binder ratios on geopolymer pore solution pH and compressive strength were investigated using response surface methodology. The developed low-alkalinity formulation was evaluated based on the performance of embedded BFRP bars in the accelerated aging test. Results showed that the most effective way to reduce geopolymer pore solution pH is increasing the initial Si/Al ratio, which is attributed to the increased Si concentration in the pore solution and the transformation of dominant anion from OH⁻ to [SiO(OH)₃]⁻. Decreasing the initial Na/Al ratio also reduces the pH by reducing the Na concentration in pore solution, but this approach is less effective. These compositional adjustments reduce N-A-S-H formation but concurrently increase the Si/Al ratio of N-A-S-H, thus maintaining enough strength. The developed low-alkalinity formulation, with an initial Si/Al ratio of 2.5 and a Na/Al ratio of 0.75, achieves a pore solution pH of 12.5. The water/binder ratio has minimal impact on pH, and higher strength can be achieved by lowering the water/binder ratio. The embedded BFRP bars exhibit 17 % higher tensile strength retention compared to those in normal geopolymer concrete after 180 days at 55°C and are estimated to maintain over 70 % strength retention consistently according to the prediction equation. Enhanced BFRP durability with low-alkalinity geopolymer concrete offers a novel solution for sustainable marine infrastructure.