An understanding of the processing of aluminosilicate mineral wastes is important for the development of large-scale waste-based geopolymerisation, especially for the encapsulation of heavy metals. This paper examines the effect of alkali metal silicate precursor solutions on geopolymerisation. Specifically, it addresses the effect of the degree of polycondensation of silicate solutions on the microstructure of waste-based geopolymers containing heavy metals (Pb and Cu). The microstructural mechanisms of heavy metal immobilisation were assessed through a variety of techniques such as X-ray diffraction (XRD), Infrared (IR) spectroscopy and Scanning Electron Microscopy (SEM) and compared to alternative polysilicate remediation methods. Nuclear Magnetic Resonance (NMR) spectroscopy was used to investigate the effects of pH and alkali metal on the speciation and equilibria of silicate solutions. The role of variable calcium sources on the physical and chemical characteristics of the final product was also investigated. Calcium silica hydrate (CSH), although present at pH 12, was largely unobserved at high pH (pH = 14), which is the optimal condition for geopolymerisation. (C) 2002 Elsevier Science B.V. All rights reserved.