Development and characterization of underwater-cast ultra-high-performance geopolymer concrete

Geopolymer concrete has emerged as a promising eco-friendly alternative to conventional cementitious materials, particularly for sustainable underwater engineering applications. However, its inherent brittleness limits its broader use in such environments. To address this challenge, this study developed and evaluated an innovative underwater ultra-high-performance geopolymer concrete (U-UHPGC). The effects of varying water-to-precursor (w/p) ratios (0.33, 0.30, and 0.27) and two fly ash (FA) to ground granulated blast furnace slag (GGBFS) ratios (5:5 and 2:8) on the anti-washout resistance, rheological behavior, mechanical performance, and microstructural characteristics of U-UHPGC were systematically investigated. The results revealed that lower w/p ratios and higher GGBFS contents significantly enhanced the yield stress and plastic viscosity of the paste, thereby improving its resistance to washout in underwater conditions. Furthermore, the optimized mix design achieved compressive and flexural strengths of up to 81.84 MPa and 12.83 MPa, respectively, alongside excellent flexural deformation capacity. Microstructural analysis demonstrated that reduced w/p ratios and increased GGBFS content led to a denser matrix, promoting stronger interfacial bonding between polypropylene fibers and the matrix. This enhanced bonding allowed the fibers to effectively bridge and arrest cracks. As a key environmental advantage, the U-UHPGC formulation embodies a substantial reduction approximately 40 % in both energy demand and carbon footprint over traditional U-UHPC. These findings contribute valuable insights for the advancement of high-performance geopolymer concretes tailored for durable and sustainable marine infrastructure.