Carbothermal Reduction and Nitridation of Geopolymer-Carbon Composites: A Study of Reaction Mechanisms

The mechanism of carbothermal reduction and nitridation of potassium-graphite and potassium-, sodium- and ammonium-carbon black geopolymer composite systems was studied by varying the reaction conditions of temperature, time and reaction atmosphere. The effects of different carbon and alkali types were examined and it was determined that the formation of β-sialon occurs by a series of complex reaction pathways with conventional and new pathways observed.

Potassium-graphite and ammonium-carbon black composites reacted mostly through the conventional mechanism understood for clay based systems while significant kinetic hindrance to the crystallisation or melting of potassium- and sodium-carbon black composites resulted in the observation of non-conventional pathways. These new reaction pathways were determined to be a consequence of eutectic formation in the samples resulting in silica-rich and alumina-rich phases. The silica-rich phases formed silicon oxynitride type materials, suggested to be K-O1-sialon and Na-O-sialon, which subsequently formed a low z-value β-sialon phase with further carbothermal reduction and nitridation. The alumina-rich phase reacted via X-phase sialon and through direct conversion to produce a high z-value phase resulting in overall diphasic β-sialon materials.

To explain these results, the reaction pathways were discussed in context of previously determined mechanisms and a schematic representation for the formation of β-sialon presented, having some implications for the design of reactive systems.