Comparison of red mud and coal gangue blended geopolymers synthesized through thermal activation and mechanical grinding preactivation

Two binary geopolymers were prepared from the mixture of low-calcium bayer red mud and kaolinite-based coal gangue. The geopolymers were synthesized using two affordable and feasible methods mainly differed in preactivation and curing processes. The influence of the mix ratio of raw material and alkali activator on the compressive strength properties of two series geopolymers was investigated, and the optimal specimens of which were then determined. Furthermore, a comparative research on the mineral composition, crystal structure and microstructure of the superior geopolymers was performed through XRD, TG-DTA, and SEM-EDXS. Results revealed that GR8G2 was the optimum sample for the series geopolymer I and CR9G1 for geopolymer II. The compressive strength of all samples ranges from 15.05 to 30.25 MPa, the geopolymer synthesized from mechanical grinding preactivation displayed better developed strength. Mineral composition and microstructure studies indicated that both the final products are consist of amorphous silica-alumina-based geopolymer gels and some impurity fillers. This study showed that the two geopolymer synthesis methods for red mud and coal gangue were feasible, and the geopolymers may be applied as a new building material. Further studies are necessary to determine the optimal application parameters.