Hydration of coked MgO-C-Al refractories

Magnesia-carbon bricks collected from steel plants kept in open atmosphere were found to crack and disintegrate on prolonged exposure. To understand the mechanism of this behaviour, MgO-C-Al and Al-C samples were prepared, coked, and kept in open atmosphere. The samples crumbled to dust after few days. XRD showed the reduction of phases such as Al4C3, Al4O4C, Al2O3, and Al caused the disintegration with formation of amorphous phases. SEM showed the presence of hexagonal stack-like structures which converted to dust-like phases which EPMA identified as combination of aluminium and oxygen without the presence of carbon. Thermodynamic stability analysis for Al-O-C-H quaternary and Al-O-H ternary phase diagrams was done to explain the mechanism of hydration and consequent disintegration of coked MgO-C-Al bricks as well as Al-C samples on exposure to moist atmosphere at ambient temperature. On hydration, phases present in the coked samples convert to Al(OH)3 which causes the disintegration.