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

TL;DR

This study compares two methods for making porous geopolymers, finding that microwave curing is faster and more sustainable while producing highly porous materials useful for environmental cleanup.

Contribution

The novel contribution is demonstrating that microwave-assisted curing improves porosity and sustainability in metakaolin-based geopolymers.

Findings

Microwave curing achieved up to 67% porosity with 3 wt% H2O2 and reduced synthesis time by 80%.

Microwave-cured geopolymers showed a more interconnected macroporous structure and better water retention.

Microwave curing had lower environmental impact according to green chemistry metrics.

Abstract

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 DOZNTM 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 DOZNTM 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.

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