Coupled effect of SiO2 content and hydrogen-enriched atmospheres on the reduction behavior and microstructural evolution of fired hematite pellets

• Higher hydrogen proportions accelerate the reduction rate of fired hematite pellets, but the effect strongly depends on SiO2 content. • For 1–2wt% SiO2, increasing hydrogen worsens post-reduction quality (swelling rises from 26.14% to 34.26%; strength drops from 110 to 78 N), while for 3–4wt% SiO2 it improves quality (swelling falls from 15.26% to 9.23%; strength rises from 179 to 271 N). • Under 100% H2, 1wt% SiO2 pellets follow a mixed gas-diffusion and uniform reaction model, whereas 4wt% SiO2 pellets follow an unreacted core model. • High-SiO2 pellets form Al-bearing calcium silicate slags that strengthen intergranular bonding, buffer phase-transformation stress, and promote granular or layered metallic iron, mitigating catastrophic swelling.