Synthesis of alkali-activated mortar using phosphogypsum-neutralised bauxite residue

Abstract

The present study was aimed at perspective utilisation of two wastes, bauxite residue (BR), an extremely alkaline material, and phosphogypsum (PG), a highly acidic substance, of entirely different natures. Development of alkali-activated mortar targeting pavement applications was explored. As BR and PG alone proved to be ineffectual in developing geopolymer/alkali-activated mortar because of undesirably low compressive strength, the challenge lies in synthesis of alkali-activated mortar with high strength using a combination of these wastes. PG proportions of 10, 20, 30, 40 and 50%; sodium hydroxide (NaOH) molarities of 8, 10, 12 and 14 M; and sodium silicate (Na2SiO3)/sodium hydroxide ratios of 0.5, 1.0, 1.5, 2.0 and 2.5 were chosen as variable parameters to develop the mortar. From the comprehensive experimental results, 30% PG, 12 M sodium hydroxide and sodium silicate/sodium hydroxide ratio of 1.5 were found as the optimum parameters for synthesising the mortar. It was demonstrated that the mortar made at a 70:30 combination of BR and PG had superior compressive strength of 31.24 MPa, minimum abrasion loss of 1.52 mm and water absorption of <7%, apart from constraining leaching of potentially toxic elements. On account of mechanical, durability and environmental performance, the present study recommends the aforementioned combination as ideally suitable material in pavement applications.