The role of the gasification forest fly ash (GFF) in geopolymerization

Abstract

The utilization of gasification biomass for renewable energy generation is increasing, nevertheless the ash produced from the process seldom repurposed and discarded in landfills, which imposes a potential risk of environmental contamination. Therefore, the objective of this research is to develop a novel type of geopolymer utilizing gasification forest fly ash (GFF) as a precursor. This GFF-based geopolymers could be used as alternative of ordinary Portland cement (OPC) to reduce both the global CO₂ emissions, and the depletion of natural resources. The geopolymers were synthesized using a powder component from Metakaolin (MK) (control mix), a hybrid geopolymer (MK and GFF), and a solo GFF, along with an alkaline activator. The resultant specimens were subjected to various characterization techniques: setting time, solubility, mechanical properties, X-ray diffraction analysis, Scanning Electron Microscope, Energy-dispersive X-ray spectroscopy, and thermogravimetric analysis. As the results demonstrate that the introduction of GFF as a precursor significantly reduced the setting time of the geopolymer. The high content of alkali metals in the GFF plays a major role in accelerating both the setting and hardening processes of the geopolymer paste. Geopolymers with maximum mechanical performance were produced by replacing 20 % of MK with GFF as precursor (GFF-20). Compared with MK-based geopolymers (GFF-0), the compressive strength of GFF-20 increased from 38 MPa and 37.3 MPa to 44.2 MPa and 41.7 MPa under sealed and immersion conditions respectively at 28 days. In addition, the flexural strength of GFF-20 increased from 6.2 MPa and 5.5 MPa up to 7.2 MPa and 6.3 MPa at same conditions and curing age. The microstructure and phase composition were completely altered by replacing the MK with GFF. The geopolymers with only GFF (GFF-100) demonstrate a compressive strength of 16.7 MPa under sealed conditions and 12.5 MPa under immersion conditions at 28 days, confirming that GFF can serve as a primary precursor for geopolymerization. This research shows the importance of utilizing GFF as precursor of geopolymer, to improve mechanical properties, as well as the obvious effect on microstructure and the formation of strong geopolymeric binders. © 2025 Elsevier Ltd