Co-solidification of bauxite residue and coal ash into indurated monolith via ambient geopolymerisation for in situ environmental application

Bauxite residues generated from alumina refineries worldwide have accumulated to more than 4 billion tons, at an annual increment of ~ 0.15 billion tons. It is imperative and urgent for the alumina sector to develop field-operable disposal solutions for rapid and cost-effective stabilisation of alkaline bauxite residues (BR) in the storage facility to minimise/prevent potential environmental risks. Taking advantage of the availability of coal ash (CA) on site, we studied a feasible way to synthesise geopolymer from active (amorphous) aluminosilicate components of BR and CA via the alkaline hydrolysis under ambient conditions. The new geopolymeric binder effectively solidifies BR-CA mixtures into indurated monoliths whose unconstrained compressive strength (UCS) can reach as high as ~ 20 MPa after 8 weeks. The Full Factorial Experimental Design was used to study relative influences of BR:CA ratio, modulus of activating solution, and H2O/Na2O ratio on UCS. Micro-spectroscopic structural analyses using electron-dispersive X-ray spectroscopy and X-ray Photoelectron Spectroscopy suggested a co-occurrence of cement-like calcium aluminosilicate hydrate (C-A-S-H) and Na-rich aluminosilicate 3D-extended network (geopolymer) within the binder phase. The advantage of this ambient geopolymerisation, without resorting to elevated temperature curing, renders a feasible way of valorising BR and CA for environmental management of alkaline wastes at alumina refineries. [Display omitted] •Bauxite residue and coal ash are co-solidified into indurated structures via the ambient geopolymerisation.•Unconstrained compressive strength of the monolith geopolymeric block reaches ~ 20 MPa after 8 weeks.•The geopolymer binder was synthesised from amorphous aluminosilicate components via the alkaline hydrolysis.•Co-occurrence of cement-like calcium aluminosilicate hydrate (C-A-S-H) and geopolymer network within the binder phase.•Full Factorial 2k design was adopted to study main and interaction effects of operational factors on geopolymer production.