Xella Technology and Research, in a pilot study with the Hamburg‐based Otto Dörner GmbH waste management company and the Ytong plant in Wedel, has been testing since 2013 how and in what amounts autoclaved aerated concrete (AAC) remains from demolished buildings or from waste disposal sites can be reused for new AAC production. The processing of the salvaged AAC‐material should conform to the current technical standard: return of mixed demolition rubble, separation of contaminants (by metal separation, air density separation, swim‐sink separation, manual resorting), pre‐treatment in the crusher and sieves for predetermined grain size ranges. This is where grain sizes or moisture content, heavy metals, bitumen, sulphate or other impurities are analysed in detail. The sorting accuracy as performed by Otto Dörner has shown to be sufficient for reuse through reintroduction into AAC‐production. Up to 15 % salvaged AAC prepared in such a manner can be effortlessly reused. A sample production of AAC quality grade P4‐0.55 with granulated salvaged AAC in the Ytong plant in Wedel was successful.

Abstract. The awareness of the environmental impact of the building sector is increasing. Steel reinforced concrete is the most commonly used construction material, though with a high-embodied energy and carbon footprint. Large environmental gains may arise if an alternative to steel reinforced concrete is developed. In this context, ultra-high performance concrete (UHPC) materials are shown to be promising alternatives with advantages such as lower embodied energy and reduced environmental impact. Predictions suggest that UHPC composite elements for building envelopes could have other benefits such as an increased service life, optimised use of building area due to thinner elements and minimised maintenance due to the absence of reinforcement or use of non-corrosive reinforcing materials such as carbon fibres. In the framework of the H-HOUSE project funded by the European Commission, composite elements are developed. The aim is to create facade panels combining an autoclaved aerated concrete or cellular lightweight concrete insulation layer with an external UHPC supporting layer. To enhance occupant comfort and health, hygroscopic materials that are capable to buffer indoor air humidity shall be applied to the inside of such elements. Indoor air humidity levels are expected to be more stable, which shall subsequently improve the indoor climate and minimise potential decay to the construction.

In the present paper, a reuse strategy for autoclaved aerated concrete (AAC) demolition waste into the ongoing production of AAC is described. The challenge is huge: The decisive factor for successful recycling is primarily the quality of the AAC recyclate. This must be carefully analyzed before the material is reintroduced into the production cycle. Since 2010, Xella has been investigating how and in what quantities residual AAC from demolished buildings or landfills can be recycled for producing new AAC. Around 10%-and even up to 15%, depending upon its quality-of the pure AAC from landfills can be recycled for new production. In this pilot study, we investigate the chemical and mineralogical composition of the supplied building waste. Among others, the study focuses on the analysis of heavy metals, polycyclic aromatic hydrocarbon (PAH), sulfate, and total organic carbon (TOC) contents. This activity is Xella's preparation for the European Waste Framework Directive, pursuant to which at least 70% of all construction and demolition waste (CDW) must be recycled after 2020. It must furthermore be expected that building material manufacturers will be faced with a take back obligation for building materials once this directive enters into effect. K E Y W O R D Sautoclaved aerated concrete (AAC), construction and demolition waste (CDW), landfill, recycling

Autoclaved aerated concrete (AAC) contains a small quantity of sulphate. For example, a modern quality class PP2‐0,35 AAC (λ = 0.09 W/(mK)) from Xella contains about five per cent by mass of sulphate in the form of gypsum or anhydrite. The addition of sulphate reduces shrinkage and enhances compressive strength and durability. AAC thus has an almost unrestricted lifetime. Regarding the environmental acceptability of sulphate, dogmatic discussions have been held for years. What is certain: sulphate is not a hazardous substance. Calcium sulphate (gypsum) has been categorised according to the Directive (EC) No. 1272/2008 (CLP) as not hazardous. Xella's voluntary environmental declarations for AAC confirm not only the excellent ecological balance of this product but also the absence of hazardous substances. For construction and demolition (C&D) waste from AAC, disposal is ensured in Germany with landfill class I (“Non‐hazardous waste, domestic waste”). In order to save disposal costs, Xella offers to take back unmixed cutting waste, which arises in the course of new building or refurbishment, without charge at the Ytong‐factories. Xellas long‐term aim is a closed recycling loop for AAC. A collaborative pilot project between Xella and the Otto Dörner Entsorgung GmbH has shown that from the point of view of process and materials technology, production of high‐quality AAC is even possible under utilization of crushed AAC from demolition.

Der Gebäude‐ und Bausektor ist für einen erheblichen Teil der europäischen Treibhausgasemissionen verantwortlich. Diese Roadmap beschreibt, wie die europäische Porenbetonindustrie bis zum Jahr 2050 klimaneutral werden kann. Mit seiner Eigenschaft, als CO2‐Senke zu fungieren, hat Porenbeton – über Klimaneutralität hinaus – das Potenzial, CO2‐negativ zu werden. Das bedeutet, dass dieser Baustoff in seinem Lebenszyklus der Atmosphäre mehr Kohlenstoff entzieht, als bei Herstellung, Transport, Nutzung und Nachnutzung freigesetzt wurde. Die Szenarioentwicklung für das Jahr 2050 erfolgte auf Basis technischer Maßnahmen zur Effizienzoptimierung, der Umstellung von fossilen auf erneuerbare Energieträger sowie einer aktuellen Lebenszyklusanalyse für ein repräsentatives Porenbetonprodukt. Der überwiegende Anteil der CO2‐Emissionen im Lebenszyklus von Porenbeton entsteht nicht in der Produktion von Porenbeton selbst, sondern bei der Herstellung der Ausgangsstoffe Zement und Kalk. Aus diesem Grund orientiert sich die vorliegende Roadmap eng an den Dekarbonisierungspfaden europäischer Herstellerverbände der Zement‐ und Kalkindustrie. Mit dieser Roadmap verpflichten sich der Europäische Porenbetonverband (European Autoclaved Aerated Concrete Association, EAACA) und seine Mitglieder zur Erreichung der CO2‐Neutralität ihrer Produkte und zur Unterstützung bei der Verwirklichung eines klimaneutralen Europas.

Porenbeton ist aufgrund seiner hervorragenden Dämmeigenschaften ein häufig verwendetes Baumaterial für Mauersteine sowie vorgefertigte bewehrte Bauteile und Mineraldämmplatten – mit wachsender Beliebtheit. Altporenbeton aus dem Abbruch und Rückbau von Gebäuden wird derzeit hauptsächlich deponiert. Deponiekapazitäten nehmen jedoch ab und der rechtliche Rahmen verlangt feste Recyclingquoten. Um ein hochwertiges Recyclingnetzwerk von Altporenbeton zu etablieren, werden Informationen über rezyklierbare Mengen, ihr zeitliches Aufkommen und ihre regionale Verteilung benötigt. Da diese bislang nicht vorhanden sind, wurde ein neues Modell zur Quantifizierung von Altporenbeton unter Nutzung historischer Porenbetonproduktion, Bautätigkeit, regionaler Marktanteile von Porenbeton und Gebäudelebensdauern entwickelt. Das Modell wurde für Deutschland im Zeitraum 1950–2050 (jahresgenau) mit geografischer Unterteilung in 401 Regionen angewendet. In den nächsten Jahrzehnten ist den Ergebnissen zufolge mit stark steigenden Altporenbetonaufkommen zu rechnen. Das Aufkommen in Deutschland könnte von 160.000 m3 (2000) über 1.200.000 m3 (2020) auf mehr als 4.000.000m3 (2050) ansteigen. Es werden signifikante Mengen v. a. in großen deutschen Städten wie Berlin, Hamburg, München, Bremen, Hannover, Köln, Frankfurt und Stuttgart erwartet. Diese Ergebnisse bieten eine Entscheidungshilfe für die Kreislaufführung von Altporenbeton in Bezug auf Standort‐ und Kapazitätsplanung sowie Logistik.

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Establishing a circular economy for Xella's building materials – among them Autoclaved Aerated Concrete (AAC) – is and will be one of Xella's key ESG projects for the coming years. In Germany, Xella has made it its goal to duplicate the powder‐input in the main AAC‐products from annually 60,000 t to 120,000 t by 2030. AAC‐powder used today emerges from the processing of AAC production leftovers, cutting residues and leftovers returned from job sites. In order to achieve the 2030 goal, Xella Germany will (1) generate new sinks, and (2) leverage new sources for AAC‐powder. Current production recipes are designed to take up emerging powder‐quantities, but nothing beyond. The present study describes the inspection and evaluation of current powder input shares per product type and options for their full exploitation by recipe amendments. In 2021, prototypes for Ytong PP2‐0,35 and PP4‐0,50 containing 20 m.‐% powder, and Ytong PP4‐0,55 containing 30 m.‐% powder were developed by the Xella Technologie‐ und Forschungsgesellschaft mbH. In order to prospectively supply sufficient quantities of AAC‐powder, Xella Germany aims at leveraging waste‐AAC from building demolition, so called “post‐demolition AAC” or “pd‐AAC”. We combined the “zero burden” and the “avoided burden methodology” to expand the life cycle assessment for the investigated circularity scenarios over the edge of waste generation. Our assessment revealed substitution credits of 0.55 kg CO2e/kg pd‐AAC for Ytong PP2‐0,35, and 0.43 respectively 0.33 kg CO2e/kg pd‐AAC for Ytong PP4‐050 and PP4‐0,55. The present study illustrates that our ambition to duplicate Xella Germany's powder consumption is achievable, when the new formulations are fully and consequently implemented. The recipe amendments have the potential to reduce expenditures for sand, quicklime, cement and gypsum/anhydrite, thereby reducing Xella Germany's annual (Scope 3) AAC carbon emissions up to 36 kt CO2e.