Optimizing Strength and Workability in Ggbs/Waste Foundry Sand Alkali-Activated Concrete

This study investigates the early-age performance of alkali-activated concrete (AAC) using ground granulated blast furnace slag (GGBS) and waste foundry sand (WFS) as a sustainable binder system. Two GGBS/WFS ratios—70/30 and 40/60—were designed to target high-strength (50–60 MPa) and typical structural-grade strength (30–40 MPa) applications, respectively. The influence of water-to-solid (W/S) ratio and two chemical admixtures—Borax decahydrate (B8) and a zinc nitrate–sodium gluconate blend (ZN1SG2)—was assessed. Compressive strength improved with decreasing W/S ratio, with B8-modified 70/30 mixes reaching ∼53 MPa at 7 days. ZN1SG2 substantially enhanced workability (slump up to 210 mm) but reduced early strength, indicating a trade-off between flowability and mechanical performance. Partial GGBS replacement with up to 20% WFS had minimal impact on strength, while higher WFS contents led to reductions, likely due to reduced reactivity. B8 consistently offered a favourable balance of strength and workability across both binder ratios. These findings demonstrate promising early-age performance of WFS–GGBS AAC under ambient curing, with further work underway to evaluate 28-day strength, durability and microstructural evolution.