Mechanical properties of seawater and sea-sand geopolymer concrete using response surface methodology

As sustainable construction gains prominence, seawater and sea sand geopolymer concrete (SWSSGC) is emerging as a promising eco-friendly alternative. This study evaluated the effects of fly ash ratio, aggregate-to-binder ratio, and water-to-binder ratio on slump, splitting tensile strength, axial compressive strength, and splitting compressive strength using response surface methodology (RSM) and Box-Behnken design (BBD). The results show that increasing the fly ash ratio and water-to-binder ratio enhances workability, but it compromises mechanical properties. Conversely, reducing the aggregate-to-binder ratio lowers slump yet slightly improves mechanical performance. Excessively low water-to-binder ratios and fly ash ratios are detrimental to early-age strength, whereas a higher fly ash ratio contributes to greater long-term strength gain. Digital image correlation (DIC) technology was used to observe the formation and evolution of failure modes under compression. The optimized mix (50% fly ash, A/B = 3, W/B = 0.4) achieved a compressive strength of 61.48 MPa and a slump of 208 mm. Microscopic analysis revealed that Ca²⁺ from slag promotes C-(A)-S-H gel formation, while high fly ash ratio and water-to-binder ratios lead to incomplete reactions and microvoids.