Analysis of the Role of Fly Ash and Calcium Hydroxide in Synergistically Regulating the Fluidity, Setting, and Strength of Alkali-Activated Slag

To address the issue of excessively rapid setting of alkali-activated slag (AAS), which significantly restricted its engineering application, this study systematically investigated the effects of incorporating fly ash (FA) and calcium hydroxide (CH) on fluidity, setting behavior, and compressive strength development. The reaction mechanisms and synergistic enhancement effects among GBFS, FA, and CH were analyzed using XRD and FT-IR techniques. Results demonstrated that the combined incorporation of FA and CH enabled effective and synergistic regulation of AAS. The calcium-supplying capability of CH counteracted the compressive strength loss induced by FA, thereby creating favorable conditions for FA to exert its effects in both delaying hydration and improving fluidity. With increasing FA content, the setting time was further extended, fluidity continued to improve, and the reduction in compressive strength was mitigated. The optimal comprehensive performance was achieved at a GBFS:FA:CH mass ratio of 6.5:3:0.5, yielding a 28 d compressive strength of 51.6 MPa, a fluidity of 188 mm, and initial and final setting times of 60 min and 81 min, respectively, representing improvements of 20% to 50% compared to the reference mixture. Microstructural analysis indicated that the addition of FA and CH supplemented calcium, silicon, and aluminum components in AAS, promoted the formation of silicate and aluminosilicate crystalline phases, enhanced the diversity of polycondensation products, and significantly increased the crosslinking degree of silicate and aluminate tetrahedra as well as the polymerization degree of the gel phase. These findings revealed the intrinsic mechanism behind the performance optimization of the composite system.