Critical threshold effect and microscopic mechanism of bottom ash replacement ratio in an alkali-activated slag-bottom ash system

To achieve efficient utilization of municipal solid waste incineration bottom ash (BA), this study combined it with ground granulated blast-furnace slag (GGBS) to prepare a novel cementitious material via alkali activation technology. The research focused on investigating the influence of Na₂SiO₃ dosage on material properties when BA replaced GGBS at different ratios (0 %, 25 %, 50 %, and 75 %). Experimental results revealed a critical threshold for BA content (approximately 50 %): below this threshold, compressive strength continuously increased with higher Na₂SiO₃ dosage; above this threshold, compressive strength initially increased and then decreased with increasing Na₂SiO₃ dosage, with a sharp decline observed when Na₂SiO₃ exceeded 20 %. Microstructural analyses based on X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), and mercury intrusion porosimetry (MIP) indicated that BA content and Na₂SiO₃ dosage jointly regulate the formation of C-(A)-S-H gel, while excessive Na₂SiO₃ in high-BA systems inhibits the gelation reaction. This study elucidates the critical threshold effect of BA in alkali-activated systems, providing a theoretical basis for the mix design of waste-based cementitious materials and a feasible pathway for the resource utilization of BA. • Bottom ash and ground granulated blast-furnace slag are co-activated to create a alkali-activated material. • The contents of bottom ash and Na₂SiO₃ play a critical regulatory role in the compressive strength. • Microscopy shows excess Na₂SiO₃ in high-BA mixes suppresses C-S-H, while low-BA mixes enhance gel formation. • The process turns bottom ash into a high-value concrete substitute, cutting landfill and CO₂.