<p>The production of Portland cement has a significant environmental impact due to high energy consumption, intensive raw material extraction, and substantial CO₂ emissions. Geopolymers represent a more sustainable alternative, particularly when produced from industrial by-products, as they exhibit a reduced environmental footprint. This study investigates the compressive strength and microstructural behavior of sustainable geopolymers produced with ceramic brick waste (BW) and ground granulated blast furnace slag (GGBFS) under different exposure conditions, including air curing, water immersion, and both partial and full immersion in seawater. Three mixtures were prepared by partially replacing BW with GGBFS at replacement levels of 0%, 25%, and 50% by mass. Compressive strength was assessed at curing ages of 7, 28, and 91&#xa0;days. In addition, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM/EDS), and thermogravimetry (TG) analyses were performed to characterize the microstructure. Replacing BW with GGBFS significantly enhanced the mechanical properties and eco-efficiency of the mixtures. At 91&#xa0;days, the compressive strength improved by up to 87% compared to the reference mixture, reaching 56&#xa0;MPa. Moreover, CO₂ emissions and normalized embodied energy decreased by up to 45%. Full immersion in seawater resulted in a 32.5% increase in compressive strength, which was attributed to the formation of a brucite (Mg(OH)₂) layer that reduced leaching and promoted geopolymerization. TG analysis quantified the brucite content at 4.50%, 5.57%, and 7.35% with increasing GGBFS content. The coexistence of C-(A)-S-H and N-A-S-H gels was confirmed, indicating matrix densification. These results contribute to advancing the understanding of durability in sustainable geopolymers for marine infrastructure applications.</p>