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SCIENCE & INNOVATION28 SCIENCE & INNOVATION AAC WORLDWIDE %u2022 2.2026AAC RecyclingUtilization of AAC powder and granulate for gypsum based building materialsThe utilization of autoclaved aerated concrete (AAC) powder and granulate as lightweight aggregate for gypsum based products offer new opportunities for AAC recycling. One application involves using fine AAC powder to produce lightweight gypsum plaster by replacing the perlite components previously used. Another application involves a flowable and self-leveling mortar from AAC granulate and gypsum binder, used as the infill for steel framed wall elements. To ensure good compatibility between AAC and the gypsum binder, the AAC must be dried at temperatures ranging from 120%u00b0C to 140%u00b0C in order to calcine all calcium sulfate dihydrate into hemihydrate. Otherwise, the small amount of calcium sulfate dihydrate in AAC would significantly accelerate the setting process of the gypsum mortar. The required particle size depends on the desired product and respectively different methods are suitable for size-reduction. Recently produced AAC with a density class below 350 kg/m%u00b3 yields in a lightweight aggregate with bulk densities of approximately 500 kg/m%u00b3. AAC demolition waste of higher density class yields a still suf,ficiently lightweight aggregate with bulk densities of approximately 600 kg/m%u00b3. The mix ratio depends on the desired product properties. For both applications 40-70 M-% AAC are feasible. Prospectively, AAC could become a sustainable and cost-effective alternative to other lightweight aggregate for gypsum based products.AAC waste is generated during production, construction and post-demolition. Especially, the volume of post-demolition AAC is expected to increases over the next decades [1]. Closed-Loop recycling for the production of new AAC has been limited to a certain amount depending on the unit density class [2, 3]. Addition of up to 15 M-% AAC powder (0-1 mm) is reported to be feasible for the production of AAC density class 550 kg/m%u00b3 [2]. Other recycling options include the production of lightweight aggregate concrete, light mortar, floor screed or gypsum-based building materials [4, 5] as well as recyclable foam stone blocks made from coarse AAC-granules [6]. The utilization of AAC powder and granulate for gypsum-based building materials is a relatively new field of research and so far, a good compatibility between AAC particles and the gypsum matrix has been reported for substitution ratios of up to 30 M-% [5]. Recently, the research projects %u201cGipsgebundene Bauplatten aus feinem Rezyklat-Porenbeton-Brechsand%u201d and %u201cUntersuchungen zum Einsatz von Porenbeton-Brechsanden in Gipsbauprodukten - Ausfachf%u00fcllung bei Stahlleichtbaurahmen und Gips-Putz%u201d have been funded by the German Federal Institute for Research on Building, Urban Affairs and Spatial Development and the Federal Ministry of Research, Technology and Space. These two projects outline the processing of AAC construction and demolition waste into a lightweight aggregate, which can be used for lightweight gypsum based building plaster [7] gypsum wall panels or gypsum-based self-levelling mortar for steel-framed wall systems. In conclusion, these studies demonstrate that AAC can be utilized as a sustainable and cost-effective lightweight aggregate for gypsum-based building materials.Ph.D. Karsten Mesecke and Dipl.-Ing. Frank Hlawatsch; MPA Bremen | Leibniz-Institute for Materials Engineering %u2013 IWT, Germany Prof. Dr.-Ing. Robert-B. Wudtke and Dipl.-Ing. (FH) Katrin Schmidt; ThIWert | Th%u00fcringer Innovationszentrum f%u00fcr Wertstoffe, Germany Prof. Dr.-Ing. Daniel Ufermann-Wallmeier; Fachhochschule Dortmund, Germany