Comparative Assessment of Conventional and Microwave Curing Synthesis Routes for Metakaolin-Based Porous Geopolymers: Characterization and Environmental Metrics

Geopolymers have gained relevance in environmental applications, and in recent years they have been studied as sustainable adsorbent materials. Increasing their porosity remains one of the main challenges. Various methodologies have been applied for the synthesis of porous geopolymers; however, energy efficiency and environmental considerations associated with the synthesis process must be considered. This study compares two synthesis routes for porous metakaolin-based geopolymers using hydrogen peroxide as a foaming agent and two curing methods: conventional oven curing and microwave-assisted curing. Structural, physical, and chemical properties were evaluated using XRD, FT-IR, SEM/EDS, TGA, and density-porosity analyses. Additionally, a quantitative environmental assessment based on the 12 principles of green chemistry was conducted using the DOZN^TM software version 2.0. The results confirmed that the addition of H₂O₂ did not alter the geopolymeric structure, as evidenced by FT-IR and XRD, regardless of curing method. Porosity increased significantly with the foaming agent, reaching up to ~65% for conventionally cured samples and a maximum of 67% for microwave-cured geopolymers at 3 wt% H₂O₂, with a minimum bulk density of 0.79 g/cm³. High-power microwave-assisted curing reduced the synthesis time to 5 min (≈80% reduction) while promoting a more developed and interconnected macroporous structure, as observed by SEM and supported by enhanced water retention behavior in TGA analyses. The green chemistry assessment demonstrated that microwave curing presents a lower overall impact within the DOZN^TM framework, primarily associated with improved energy efficiency (GCP-6), while acknowledging that this assessment does not constitute a full life cycle analysis. Overall, microwave-assisted synthesis emerges as a more sustainable and efficient route for producing highly porous, hydrophilic geopolymers with strong potential for the adsorption of aqueous pollutants in environmental applications.