Hydrogen's influence on microstructure and Young's modulus evolution in a phosphate bonded alumina refractory

The effects of hydrogen exposure on a burnt phosphate bonded high alumina refractory, a material potentially used in Direct Reduction Reactors, are investigated in the present study. Refractory samples were continuously purged with hydrogen at 900 °C. The results reveal a decrease in mullite content, accompanied by a shift toward aluminum-rich mullite. The proposed process is driven by enrichment of reduced Na, K, and P oxides in the glassy phase. This change alters the activity difference of SiO2between mullite and the glassy phase, promoting mullite's instability and causing some Si to migrate toward the glassy phase. Microstructural changes, including the formation of microcracks, were observed after hydrogen exposure. These changes significantly impacted the material's mechanical properties, as evidenced by a reduction in the dynamic Young's modulus from approximately 30 GPa to 22 GPa at room temperature. Interestingly, a partial recovery of the Young's modulus was observed after two subsequent thermal cycles under nitrogen purging. High-temperature mechanical performance was further assessed through compression tests at 900 °C, which demonstrated a decline in compressive strength with prolonged hydrogen exposure.