Comparative Assessment of Compressive and Splitting Tensile Strengths of Alkali-Activated Slag–Fly Ash Mixtures Cured in Steam Conditions by Incorporating High- and Low-Calcium Fly Ash

This study examines the fresh and early-age mechanical properties of alkali-activated slag–fly ash (AASF) mixtures incorporating two types of fly ash as high-calcium (HCFA) and low-calcium (LCFA), that used at a fixed substitution level of 20% by mass of fine aggregate. The experimental program evaluated three water-to-slag cement (W/SC) ratios (45%, 50%, and 55%), while maintaining a constant alkali activator-to-slag cement (AL/SC) ratio of 20%. All mixtures were steam-cured to replicate accelerated construction conditions. Fresh properties were assessed using slump flow and flow loss tests, whereas compressive and splitting tensile strengths were measured at 1, 3, 7, and 28 days. Results revealed that HCFA mixtures exhibited reduced slump flow and higher flow loss compared with LCFA mixtures, particularly at lower W/SC ratios. Mechanically, HCFA achieved superior early compressive and tensile strengths, with significant gains even at low W/SC ratios, while LCFA offered enhanced workability and more gradual strength development. These findings highlight the combined influence of fly ash type and W/SC ratio on balancing workability and strength in AASF systems under steam curing, offering guidance for optimizing eco-efficient mixtures.