Sustainable lightweight gypsum from recycled waste: Microstructure and performance

This study explores a sustainable method for producing lightweight gypsum components by reusing gypsum waste (GW) as a secondary raw material. A physical foaming technique, based on a whipped natural foaming agent, was employed and the possibility to use pure grade citric acid as setting retarder has been investigated. The recycling process involved grinding and thermal treatment, then three recycling temperatures have been investigated with the aim to preserve the chemical and mineralogical characteristics of the recycled hemihydrate gypsum. After choosing the best recycling temperature, two full recycling cycles were performed to evaluate the progressive impact of the recycling on material properties. Lightweight samples produced after each cycle were analyzed for physical, mechanical and microstructural performance, including 3D internal structure via X-ray micro-computed tomography (Micro-CT). While the chemical composition remained unchanged, the recycling process significantly affected the microstructure, leading to reduced workability, shorter setting times and lower porosity. Mechanical tests revealed a clear degradation in performance with successive recycling cycles, particularly in flexural and compressive strength. Micro-CT analyses showed a reduction in pores volume, size and connectivity across the cycles. Anyway, recycling gypsum waste in the production of lightweight gypsum- based materials results to be suitable, especially for non-structural elements, promoting a circular approach in building materials.