Effect of Activator Modulus on the Performance, Reaction Products, and Microstructure of Biomass Power Plant Ash-Based Alkali-Activated Binders

Abstract Biomass power plant ash (BPPA) is an emerging solid waste with considerable potential for resource utilization, whereas its low reactivity limits its direct application as a cementitious precursor. In this study, BPPA was blended with ground granulated blast furnace slag (GGBFS) to develop an alkali-activated binder for potential mine backfilling applications. The effects of activator modulus (n(SiO2)/n(Na2O)) on the fresh and hardened properties were systematically investigated through slump, setting time, and compressive strength tests. The phase evolution, reaction products, thermal characteristics, and microstructural development were analyzed by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermogravimetry-differential scanning calorimetry (TG-DSC), and scanning electron microscopy (SEM). The results show that increasing the activator modulus decreased the slump and prolonged the setting time. The compressive strength exhibited a nonmonotonic dependence on activator modulus, with a modulus of 1.4 achieving the highest 28-day compressive strength value of approximately 5.0 MPa under the investigated conditions. Microstructural analyses revealed that an appropriate activator modulus enhanced precursor dissolution and promoted the formation of calcium/sodium aluminosilicate hydrate-type reaction products, resulting in a more compact binder matrix. In contrast, excessive activator modulus reduced the availability of free OH– and increased the viscosity, which hindered precursor dissolution and weakened reaction product development. The strength development was therefore governed by the balance between precursor dissolution and gel precipitation. This study provides insights into activation chemistry regulation for the high-value utilization of BPPA and the design of sustainable alkali-activated binders for use in potential mine backfilling applications.