Process optimization for the synthesis of ceramsites in terms of mechanical strength and phosphate adsorption capacity

Red mud (RM), an industrial waste of bauxite refinery, shows great potential in adsorptive phosphate immobilization but granulation of RM enables the ease for field application. Red-mud-based ceramsites with 12 compositions that blended Korean red mud, American red mud, ocher, and bentonite were synthesized through firing process (600–1000 °C). The porosity, bulk density, mechanical strength, mineralogical composition, and phosphate adsorption capacity of granulated RM were characterized and analyzed. The crystallization of plagioclases, nepheline and gehlenite was observed in the ceramsites with high alkali flux content, which enhanced both porosity and phosphate adsorption capacity. The characteristics of the ceramsites without phase transition were highly correlated with porosity. The mechanical strength of ceramsites was governed by crack population, describable by the Weibull distribution model, and thus the maximal tensile stress correlated negatively with porosity. Results showed that 32 wt % of KRREM and USREM treated at 1000 and 900 °C, respectively, yielded the best performing ceramites in terms of mechanical strength and phosphate adsorption capacity. Ultimately, the phosphate adsorption capacity, as affected by initial phosphate concentration, contact time, and temperature, of the optimized ceramsites was studied.