Sustainable Gypsum Composites with Recycled Plasterboard Aggregates and Metakaolin Waste: Hydration, Microstructure and Acoustic Performance

This study examines gypsum-based matrix composites reinforced with recycled gypsum plasterboard aggregates (RGPAs) and metakaolin waste (MKW) from expanded glass granule production. The study addresses the current lack of research on the combined use of RGPAs and MKW within a single gypsum matrix, providing new insight into their interaction and the potential of MKW to counterbalance RGPA-induced heterogeneity. The results show that RGPAs accelerate dihydrate nucleation, while MKW prolongs hydration through water adsorption, producing a more controlled thermal profile. MKW also enhances composite durability, allowing for the materials to retain a greater proportion of their strength after water exposure. XRD and SEM analyses confirm that MKW refines crystal morphology and enhances microstructural stability, whereas RGPAs increase porosity and heterogeneity. MKW further modifies the hydration pathway by slowing hemihydrate dissolution and delaying dihydrate precipitation, while RGPAs contribute inert mineral phases without participating in hydration. Gypsum dihydrate remains the dominant phase in all composites, but MKW-containing mixtures exhibit partial retention of bassanite, confirming its influence on hydration kinetics. Acoustic tests demonstrate that all composites reduce noise by 8.5–10.5 dBA, with 90BGMK achieving the lowest sound pressure level (82 dBA) and strongest high-frequency attenuation. Overall, MKW effectively compensates for RGPA-induced heterogeneity, enabling the development of sustainable, mechanically robust, and acoustically efficient gypsum composites aligned with circular economy principles.