New synthesis method for the production of coal fly ash-based foamed geopolymers

Foamed geopolymers were synthesised using a South African Class F coal fly ash (FA), sodium hydroxide (NaOH), and the novel foaming agent sodium hypochlorite (NaOCl) at slightly elevated temperatures (90 °C). The synthetic method has the advantage of maintaining control over foaming; the mixed and prepared cementitious slurry containing NaOCl is stable for at least 1 h at room temperature, thus, avoiding premature foaming of the slurry before moulding. In this study, the effect of NaOH/FA on the formation of foamed geopolymers was investigated at predetermined NaOCl/FA ratios (by weight). The NaOH/FA ratio was changed over the range of 0.16-0.24 at a constant NaOCl/FA composition of 0.50. Increasing NaOH/FA improved geopolymerisation up to a ratio of 0.20 and porosity up to a ratio of 0.22, after which geopolymerisation and porosity declined. These results were substantiated through XRD, SEM, FTIR, Raman analyses, mercury intrusion porosimetry, nitrogen adsorption/desorption, and helium density measurements. The formulations developed in this study may be used to fabricate materials suitable for application in construction as light weight fireproof insulation, internal walls or ceiling tiles.