The strength and microstructure of two geopolymers derived from metakaolin and red mud-fly ash admixture: A comparative study

The effects of source materials on the microstructure and mechanical properties were studied by comparing two types of geopolymers synthesized from metakaolin, a non-waste material, and the admixture of two wastes, red mud and fly ash. Unconfined compression testing was conducted to assess their curing time and mechanical properties, while X-ray diffraction and scanning electron microscopy employed to examine geopolymerization reactions and the composition and microstructure of the end products. For a given Si/Al ratio, the metakaolin-derived geopolymer exhibits higher compressive strength than the waste-based one. Both geopolymers contain a significant amount of voids and unreacted phases as inactive fillers within the geopolymer binder, resulting in complexity and variability in their mechanical behavior. The difference in strength and microstructure between the two geopolymers is attributed to the different reactivity of source materials, percentage of nonreactive fillers, and alkalinity for geopolymerization reactions.

► Two geopolymers derived from metakaolin and red mud were comparatively studied.
► Strength is affected by nonreactive fillers, unreacted reactive and neoformed phases.
► Calcined metakaolin enhances rate of geopolymerization and improves strength.
► Metakaolin-based geopolymer is less porous than the red mud-based one.
► The actual Si/Al ratio in pure geopolymer binder is less than the designed value.