Potentials and Design of a Circular Economy for Autoclaved Aerated Concrete

Limiting anthropogenic climate change and transforming to a more sustainable lifestyle are among the current generation’s most vital challenges. The built environment plays a crucial role in this context due to high resource consumption and greenhouse gas (GHG) emissions. Therefore, construction and demolition waste recycling is gaining importance but is difficult to realise for some building materials, including autoclaved aerated concrete (AAC). AAC has a low density and excellent thermal insulation properties due to its porous structure. Hence, AAC is a frequently used building material. However, recycling post-demolition AAC (pd-AAC) from the demolition and deconstruction of buildings is complicated as it has low compressive strength and contains sulphate. Therefore, pd-AAC is mainly landfilled. While there are some new pd-AAC recycling approaches, the quantitative, ecological and economic potential of pd-AAC recycling is unknown. Furthermore, no research compares different recycling approaches or examines recycling network structures to identify a circular economy design for AAC. This dissertation addresses these research gaps and answers the following research question: How can a circular economy for autoclaved aerated concrete be designed, and what quantitative, ecological, and economic potential does it have in Germany and Europe?
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Quantification shows that pd-AAC volumes reach 1.2 Mm³ in Germany in 2020 and are expected to rise significantly to over 4 Mm³ by 2050 (Study A). At the European level (Study B), ten times the German volumes can be expected. A life cycle assessment is conducted to identify the ecological potential of different pd-AAC recycling options (Study C). Using pd-AAC to partly substitute inputs of the lightweight aggregate concrete, light mortar, shuttering block, and AAC production is most promising. The pd-AAC processing only causes little impact, and significant environmental savings can be achieved due to the avoided production of primary materials, reaching total GHG savings of pd-AAC recycling of around 280,000 t CO2 Eq/a in Germany and more than 8 Mt CO2 Eq/a in Europe in the future. Additionally, a new recycling option, the production of recycled belite cement clinker (RC-BCC), proves to be ecologically beneficial despite energy-intensive processing (Study D). RC-BCC can replace emission-intensive primary Portland cement. Moreover, pd-AAC recycling has significant economic potential (Study E). Even smaller recycling plants can process pd-AAC cheaper than the average landfilling costs. However, RC-BCC production is not economically viable with current technologies. Mathematical modelling and optimisation methods are used to determine the best design of a pd-AAC recycling network (Study F). According to the computation results, large recycling plants should be preferred, and landfilling should be avoided. Overall, savings of around 4,600 M€ can be achieved until 2050 compared to the status quo.
This dissertation shows that pd-AAC recycling has a significant quantitative and ecological potential. Establishing high-quality recycling options to deal with the increasing future pd-AAC volumes is urgent. An optimally designed pd-AAC recycling network reaches high economic savings and supports the change towards a circular economy of AAC.