Solidification and stabilisation of metal plating sludge with fly ash geopolymers

The disposal of metal plating sludge (PS) to landfills is an increasingly common practice in developed and developing countries. This paper presents a novel PS stabilisation technique using fly ash (FA)-based geopolymers as a waste management option. Strength and microstructural evaluations were carried out on PS-FA geopolymers at various values of factors, including liquid alkaline activator (L = sodium silicate (Na 2 SiO 3 )/sodium hydroxide (NaOH) ratio), L/FA ratios, FA/PS ratios and curing time. Increasing the amount of PS in PS-FA geopolymers reduced the total porosity, subsequently resulting in higher unconfined compressive strength (UCS) values. The main geopolymerisation product of the FA-based geopolymer was sodium aluminosilicate hydrate (N-A-S-H) gel. Since PS had a high calcium content, it would react with the high amount of silica and alumina present in FA to generate calcium aluminosilicate hydrate (C-A-S-H) gel in PS-FA geopolymers. This C-A-S-H gel coexisted with the N-A-S-H gel from FA-based geopolymers and hence significantly accelerated UCS development of PS-FA geopolymers over time. The UCS values of PS-FA geopolymers were found to meet the US Environmental Protection Agency requirement for disposal to landfills. The outcomes from this research can be translated into a regulatory framework for managing PS and diverting this industrial waste from landfills.