Synergistic mechanism and mechanical performance of fly ash–ground granulated blast furnace slag cementitious composites

Partial replacement of ordinary Portland cement (OPC) with fly ash (FA) and ground granulated blast furnace slag (GGBS) is a critical approach for low-carbon cementitious materials. However, the quantitative multi-scale synergistic mechanism of 1:1 FA–GGBS systems under wide replacement rates remains unclear. This work characterized the macroscopic performance (fluidity, 7 d and 28 d compressive strength) and microstructural characteristics (hydration products, pore structure, products morphology) of FA–GGBS composite cement mortars using TG-DSC, MIP, SEM-EDS and particle size analysis. A distinct positive synergistic effect between FA and GGBS was verified. This positive synergistic effect is reflected by the optimal balance between workability and mechanical performance at 20% replacement. With increasing OPC replacement, fluidity increased initially then stabilized, while 28 d compressive strength increased first and then decreased, peaked at 20% replacement, reaching 51.16 MPa (19.17% higher than the control). This optimal dosage yielded the closest particle packing to the Fuller curve and the highest packing density. The CH content was reduced by 20.65%, harmful pores (>50 nm) were notably diminished, and a denser, more uniform C-(A)-S-H gel interlocking matrix was formed. The synergy originates from the morphological, filling, hydration-coupling and interlocking effects. These findings systematically evaluate the performance of a fixed 1:1 FA-GGBS blend under the tested conditions and identify its optimal replacement dosage, providing a theoretical basis for industrial solid waste valorization and low-carbon cementitious material design.