The Closed-Loop System in Focus
According to the German Federal Office of Statistics, of the some 386 million total tons of waste that accrued in Germany during 2011, construction and demolition waste accounted for almost 200 million tons. This can be classified into the material groups mineral construction waste, road construction waste and building site waste. These include aerated concrete and gypsumbased construction waste. A large proportion of these waste materials can be re-used as aggregates in recycled building materials, e.g. for road building, in the manufacture of asphalt and concrete or in landfill site construction. However, some building site waste still remains to be disposed of.
With this inconceivable volume of waste it quickly becomes clear that, in terms of a resource-conserving approach, the issue of recycling is particularly significant to the building materials industry. We are not so much concerned here with so-called downcycling – in which the treated rubble is re-used for a different purpose, e.g. road building – but with recycling the waste as autoclaved aerated concrete. Xella has decided to set itself the objective of closing the material loop for autoclaved aerated concrete.
70 percent recycling from 2020
This is our response to the European Waste Framework Directive, pursuant to which at least 70 percent of all construction and demolition waste must be recycled after 2020. It must furthermore be expected that building material manufacturers will be faced with a takeback obligation for building materials once this directive enters into effect. The challenge is huge: In order to be able to re-use or recycle building materials they must first meet the corresponding requirements relating to their grade of purity. In particular, they may not contain contaminants and should where possible be free of foreign substances. Bitumen residues from old roofing sealants, for example, would cause unattractive discoloration in new autoclaved aerated concrete. Old nails, screws, anchors etc., are even worse – because they can cause manufacturing defects.
In line with the “Disposing and recycling of autoclaved aerated concrete” project run by the Xella Research and Development Center in Brück near Berlin, we have been investigating since 2011 how and in what quantities residual autoclaved aerated concrete from demolished buildings or landfills can be recycled for producing new autoclaved aerated concrete. Around 10 percent – and even up to 15 percent, depending upon its quality – of the pure autoclaved aerated concrete from landfills can be recycled for new production. Today, unsaleable production waste from Xella’s autoclaved aerated concrete plants is already being treated and fully reintegrated into the production cycle.
The quality of the waste is the decisive factor
The decisive factor for successful recycling is therefore primarily the quality of the autoclaved aerated concrete recyclate. This is carefully analyzed before the material is reintroduced into the production cycle. The Research and Development Center is therefore undertaking a pilot study to investigate the chemical and mineralogical composition of the supplied building waste. Among others, this study focuses on the analysis of grain sizes, the heavy metal, polycyclic aromatic hydrocarbon (PAH), bitumen, sulfate and total organic carbon (TOC) content. Even if valid results will only become available in several years’ time, we can already conclude that autoclaved aerated concrete waste is fundamentally suitable for recycling.