Building a new house from an old one - this phrase vividly describes sustainable circular economy. Building materials that have already been used should be returned to the production process of equal or equivalent materials at the end of their first use. What may sound a simple task in theory is Xella’s daily challenge in practice. For example, old autoclaved aerated concrete (AAC) must be made available, recipes and production processes must be adapted, and quality must be ensured by new technologies. "Our goal is to use old material as sensibly as possible and at the same time to create new solutions in places where this is no longer possible. In the best case, no more usable AAC ends up in landfills," says Oliver Kreft, head of international projects and responsible for circularity at Xella Technologie- und Forschungsgesellschaft mbH (Xella T&F). As one of the few manufacturers of building materials, Xella has its own research center and has thus for years had an additional driver for new products and their optimization alongside its own innovation teams in the national companies.

This is because it is important to pursue a wide variety of approaches when it comes to circularity.

Basic research: replacing cement as a binder

Even though AAC is made from natural raw materials, Xella uses cement and quicklime as binders. The use of binders accounts for the largest share of CO2 in the production of AAC. That's why Xella T&F has been researching together with the Karlsruhe Institute of Technology (KIT) since 2018 to produce a new binder from old AAC using the so-called “Resynergy” process. The first studies on this will be completed by the end of 2022.

Basic research: improved formulations

When an old house is to be turned into a new one, the question arises as to how much of it can be put to subsequent use or recycling. In the case of recycling, the quality of the products must not change. This is where Xella's many years of experience throughout Europe payoff: In almost all AAC plants, AAC-production-cracks and -residues and also leftovers from construction sites are processed to so-called AAC powder and returned to the production cycle - without any effect on the quality of our products. 

In 2021, the AAC plants of Xella processed more than 165.000 tons of AAC powder in their production of new AAC products. "All our efforts are currently focused on increasing this proportion even further. This saves CO2 and at the same time ensures more efficiency in the use of primary raw materials, production, and costs," says Oliver Kreft.

Basic research: tapping old material

But to increase the proportion of used AAC in the products, a further analysis is needed: In which regions of Europe will there be sufficient waste AAC for closed-loop recycling in the coming decades?

"The framework conditions for the landfilling of AAC vary greatly from country to country. We started the evaluation in Germany because the AAC production has been very well documented here since 1950," Oliver Kreft explains. However, he adds, there are already initial findings on other European locations. "We already have a good initial insight into which sites we can establish this form of circular economy first."

New approaches: Search for partnerships and new products from used AAC

Xella is currently conducting research in this area as part of a wide variety of projects. One of them is currently being prepared in Germany. A client is going to demolish a factory building there - as it turned out during planning, built at the time from reinforced AAC panels (Hebel AAC elements). The material will be deconstructed, sorted and crushed by a building material recycling company, and will then be taken directly to the nearby Xella plant and recirculated into the production processes.

In the coming years, the federal states will accumulate different amounts of waste AAC, but the total amount will be enormous. Much more than can be processed into new AAC blocks or elements. For this reason. Xella is currently also researching other ways to use it.

The path to circularity is ambitious, but Xella is already on it. Our current ambitious goal: to be able to use 100 percent of our leftovers in the production of AAC products or for new innovative building materials, by the end of 2024.