Eco-Efficient Alkali-Activated Slag–Fly Ash Mixtures for Enhanced Early Strength and Restoration of Degraded Sites

This study explores the early-age performance of eco-efficient alkali-activated slag–fly ash (AASF) mixtures using high-calcium fly ash (HCFA) and low-calcium fly ash (LCFA) at varying alkali activator-to-slag cement (AL/SC) ratios (15%, 20%, and 25%) under steam, water, and ambient curing conditions. Mix designs were developed with a fixed water-to-slag cement ratio of 50%, while fly ash partially replaced fine aggregate at a 20% substitution level. Fresh and hardened properties were investigated. The results revealed that increasing the AL/SC ratio led to reduced workability and increased flow loss, especially in HCFA mixtures, due to their higher calcium content and finer particle size, which promoted early stiffening. In contrast, LCFA mixtures exhibited greater slump flow and better workability retention owing to their slower dissolution rate. Regarding compressive strength, steam curing produced the highest performance. At 25% AL/SC, HCFA mixtures achieved 70 MPa at 28 days, while LCFA mixtures reached 68 MPa. Water curing showed moderate strength development, whereas ambient curing resulted in slower gains. These findings emphasize the influence of fly ash type, AL/SC ratio, and various curing conditions in enhancing the performance of eco-efficient AASF mixtures.