Effect of BPPA/GGBFS Ratio on the Mechanical Performance, Reaction Evolution and Microstructural Development of Alkali-Activated Binders

This study developed alkali-activated binders based on biomass power plant ash (BPPA) and ground granulated blast furnace slag (GGBFS) for potential application in coal-mine goaf backfilling. Five precursor proportions, ranging from 100% BPPA to 100% GGBFS, were investigated to clarify the influence of precursor composition on fresh properties, mechanical performance, reaction-product evolution and microstructural development. Increasing the GGBFS content reduced slump and shortened both initial and final setting times, indicating accelerated precursor dissolution and early structural build-up. Compressive strength increased nonlinearly with GGBFS incorporation. Multiscale characterization consistently demonstrated that GGBFS promoted the transformation of the initially quartz-rich and weakly reactive BPPA system into a calcium-rich aluminosilicate binding matrix. This transformation was accompanied by changes in the Si-O-T bonding environment, increased formation of hydrated reaction products and progressive filling and bridging of the spaces between residual precursor particles. Consequently, the hardened matrix evolved from a porous particle-supported structure containing isolated reaction regions into a compact gel-supported network that contributed to more effective stress transfer within hardened matrix. Among the investigated mixtures, the formulation containing 25% BPPA and 75% GGBFS exhibited a favorable combination of BPPA utilization, processability, and mechanical performance, indicating its potential for further evaluation in coal-mine goaf backfilling applications.