Preparation and performance optimization of eco-friendly geopolymers prepared from coarser lignite-based waste fly ash

Fly ash (FA) is generated in massive quantities by burning coal in power plants. The quality of the FA depends on the origin and type of coal. In developing countries, FA produced is relatively low quality and coarser. It does not meet the criteria to be used as a construction material and requires modifications. The waste lignite-based FA generated from a power plant was used in this investigation to produce geopolymer. The raw FA-based geopolymer was tested for normal consistency, initial setting time, fluidity, and compressive strength at different molarities of sodium hydroxide (SH, 8 to 14 M). The effect of different sodium silicate to NaOH (S/N) ratios (1 to 2.5) on the properties of geopolymer was also determined. However, maximum compressive strength of 14.1 MPa was achieved using S/N ratio of 2.5 and 12 M SH solution. Then different techniques were applied to increase the compressive strength of geopolymer, including high-temperature treatment, adding polymers and chemical admixtures. Heat curing the plain geopolymers in an oven at elevated temperatures, from 60 to 80 °C at 10 °C intervals for 2 h, yielded better compressive strengths. At 80 °C, the samples achieved a 71.2% increase in strength. Superplasticizers, i.e., polynaphthalene sulfonate and polycarboxylate ether-based, remained slightly effective for geopolymers. It was also corroborated that styrene-butadiene rubber latex (SBR) latex and polyvinyl alcohol (PVA) enhance fluidity and compressive strength of geopolymer. SBR and PVA can be used as superplasticizers and strength enhancers in geopolymer. Samples with SBR and PVA as polymeric admixtures provided 72.7% and 72.8% higher compressive strength than the control sample. Furthermore, grinding FA by ball and disc milling increased the fineness of FA. The ground FA also provided a 71.3% increase in compressive strength as compared sieved control geopolymer. This study provides a new approach for the high-value utilization of coarser lignite-based waste FA, and develops a new strategy for the performance improvement of low-quality FA-based geopolymers.