Influence of fly ash content on pore structure regulation in alkali-activated slag under alkaline conditions

This study investigates the influence of fly ash on the pore structure regulation of alkali-activated slag materials under varying alkali activator concentrations. The binders were activated using sodium silicate (water glass) and sodium hydroxide (NaOH), with a fixed silicate modulus of 1.2 and a constant water-to-precursor mass ratio of 0.40. Fly ash-to-slag ratios of 0/4, 1/3, 2/2, and 3/1 were tested with NaOH concentrations of 5 %, 7.5 %, and 10 %. Characterization techniques including MIP, XRD, FTIR, and SEM-EDS were used to evaluate changes in pore structure, phase composition, and microstructure. Results show that at low alkali concentrations, higher fly ash content significantly enhances pore size control, with notable porosity changes observed during early curing. At 5 % NaOH, the porosity after 1 day increased from 8.65 % for a 0/4 fly ash-to-slag ratio binder to 42.52 % for a 3/1 ratio. With prolonged curing, the structure became denser, reducing the maximum porosity by 43.8 %–23.89 % at 28 days. The pore size distribution changed markedly: the volume of pores smaller than 20 nm increased by 2.19 times, while those between 20–50 nm, 50–200 nm, and > 200 nm decreased by 93.5 %, 97.2 %, and 32.3 %, respectively, indicating mesopore refinement and macrostructural compaction. Although higher fly ash content preserves baseline porosity, it induces significant temporal variations in structural stability. To account for this, a density-normalized volume characterization method was proposed. Microstructural observations revealed that sulfate phases formation plays a critical role in volume stability and pore structure refinement. These findings clarify fly ash’s role in regulating alkali-activated systems and provide practical insights for designing durable, sustainable cementitious materials. • Fly ash significantly influences pore structure regulation in alkali-activated slag. • The optimal alkali content enhances gel phase formation, reducing porosity. • Volume stability is strongly dependent on alkali content and fly ash dosage. • Sulfate crystallization affects volume stability in high-alkali environments. • The proposed normalized volume method reliably assesses internal volume stability.