Innovative recycling approaches to enable new material loops for AAC

This paper shows different approaches to save mineral resources or energy for the production of autoclaved aerated concrete (AAC) using secondary materials. The goal is to implement new recycling routes in the building industry and to establish new material loops. It could be shown, that AAC can act as a sink for fine fractions from building rubble consisting of waste concrete and calcium silicate masonry units (CSU) by replacing the primary quartz sand. By choosing secondary raw materials like rice husk ash with a higher solubility than quartz, it is even possible to achieve AAC with the desired properties at lower autoclaving temperatures. After the end of life, AAC can be treated with the patented approach “ENSUBA,” a wet‐chemical washing procedure, which enables the complete removal of sulfate ions, which usually cause problems in the recycling of AAC. After this treatment waste AAC can easily be brought back to the material cycle and for example be used as a raw material for cement production. Most importantly, this ENSUBA approach can be combined sensibly with a carbon capture technique like the chilled ammonia process. This process captures CO₂ from the cement production in an ammonia solution, leading to the precipitation of ammonium carbonate, which is needed for the ENSUBA process.