Microstructural evolution and acoustic performance of heat-consolidated lightweight geopolymer granular panels

Achieving broadband sound absorption in lightweight materials requires understanding how pore morphology and tortuosity govern energy dissipation within porous networks. This study investigates the microstructural evolution and acoustic performance of heat-consolidated metakaolin-based geopolymer panels fabricated from foamed granules using hydrogen-peroxide to generate porosity. Panels are consolidated without external binders, resulting in lightweight structures with sufficient mechanical integrity for acoustic applications. Increasing foaming content promotes interconnected pore formation and tortuosity, transforming the structure from discrete intergranular voids into a continuous porous network. Microstructural analysis confirms the coexistence of intergranular and intragranular porosity, with a hierarchical pore structure spanning nanometer to millimeter scales and open porosity ranging 25-40%. Across the investigated compositions, these microstructural variations are associated with a transition from localized resonance-dominated absorption toward broadband dissipation, as observed in normal-incidence acoustic measurements over 200-4300 Hz, with absorption coefficients reaching 0.8-0.9 for panels prepared with foaming levels 4-5 wt% hydrogen-peroxide. The geopolymer panels exhibit acoustic performance comparable to conventional polyurethane and melamine foams while providing mechanical strength at the MPa scale (up to similar to 4 MPa), compared to the kPa-level strength of polymer-based absorbers. Unlike the highly elastic nature of polymeric materials, the geopolymer panels possess a rigid structural response, making them suitable for applications requiring both sound absorption and mechanical stability. Overall, the results show that granular geopolymer systems consolidated without any external binders provide a viable pathway for achieving broadband acoustic performance while maintaining mechanical integrity and providing a proof-of-concept for manufacturing methodology.