CO2 sequestration experiments by producing geopolymer specimens from lignite fly ash

The mineral-based sequestration of CO₂ offers a permanent and safe method to store CO₂ captured from flue gas. In this process, CO₂ reacts with calcium and/or magnesium-bearing materials to form stable, environmentally benign carbonate minerals. In a recent research project, CO₂ sequestration was performed on deposited fly ash from Visonta, Northern Hungary, coupled with mechanical activation. The resultant carbonate phases were analyzed for their suitability as geopolymer-based construction materials. CO₂ sequestration and mechanical activation occurred in a Fritsch Pulverisette 5 planetary ball mill under wet conditions. The samples were dried post-sequestration, and 20 × 20 × 20 mm geopolymer specimens were prepared and tested for compressive strength after seven days. It was found that bassanite, the primary calcium source in fly ash, reacts with CO₂ only in the presence of NaOH. Additionally, low NaOH concentrations (0.5 M) during grinding positively impacted the compressive strength of the geopolymer, while higher concentrations reduced it. At 2 M NaOH, the specimen failed to set fully, deforming plastically under pressure. The experiment yielded a maximum compressive strength of 16.1 MPa for the geopolymer using 0.5 M NaOH.