# Case Study on Secondary Building Materials
## 4 Discussion  
The circularity of the global economy was at 7.2% in the year 2022. This is even lower than in the preceding years (2018: 9.1%, 2020: 8.6%) [35]. This is attributable to the continuously increasing world population, as well as rising material extraction and use. In Europe, the situation is better. The European Commission has decided on an action plan for measures focusing on sectors with a high potential for circularity, such as construction and building, in an attempt to generate less waste [36]. After long-lasting discussions, the Secondary Building Material Decree was finally penned in 2021 in Germany [21], and will be enforced from August 2023 onwards. For the first time, the requirements for the utilization of mineral waste will be uniformly defined for all 16 German federal states. However, that alone does not solve the problem of inferior quality of mineral waste, even after treatment, compared to natural materials. This is also the reason why different quality levels have been defined for recycling aggregates, soil-like materials, and incineration bottom ashes in the decree. The best quality levels can often only be achieved with sub-fractions of materials. Whereas the coarse fraction of **C&D waste** (>2 mm) can usually be used as an aggregate without limitations, the utilization of the sand fraction is problematic due to higher contaminant contents. The manual sorting of the coarse fraction (>16 mm) of treated C&D waste revealed that the best exploitable fraction \"stones\" had a contamination amount of only 14.2%. The 25.8%-share of bricks leads to the lower classification of recycled aggregate type 2 (brick content <30%), according to DAFStb [27]. The treatment was performed with a mobile jigging machine [24]. With further optimization, the separation of the brick fraction from the stone and mortar fraction seems applicable since the difference in density between brick and concrete is high enough (bulk density 1.63 g cm\({}^{-3}\) versus 2.42 g cm\({}^{-1}\), [37]). For the utilization of **soil-like materials**, requirements of various legal areas must be considered, the aim of which is to promote the circular economy for the conservation of natural resources and, at the same time, to ensure precautions against harmful soil changes. The utilization of soil-like material looks promising since it has been accepted that the release of contaminants such as PAHs is more relevant than the mere content in the solid. However, complex contamination with organic and inorganic pollutants may present a barrier to more widespread utilization. The utilization of soil-like materials with low PAH-release can still fail when certain limits for heavy metal leaching or sulfate are exceeded. Overall, it has been shown that the new requirements of the amended  
Figure 7: Content of carbonate in solid IBA samples as a function of the storage time. Experimental data are displayed as circles and fitted concentrations are displayed as crosses. German Federal Soil Protection and Contaminated Sites Decree [21] are appropriate to open additional possibilities for the high-quality utilization of soil-like materials, e.g., for backfilling, recultivation, renaturation, composting, or landscaping. Most **incineration bottom ash** in Europe undergoes a treatment process, at least for the separation of iron and other metals. The average energy demand for the treatment is 29.8 MJ/ton. However, among 17.65 million metric tons in the year 2018, around 50% ended up as unrecovered material in landfills, along with a considerable amount of elemental metals [38]. The recovery of elemental metals, such as Fe, Al, and Cu (and the respective alloys) from IBA exhibit the most prominent advantage in the management of this waste [19]. The utilization of the elemental metals in metallurgy leads to approximately 62.5 kg CO\({}_{2}\)-eq. per ton of incinerated waste, indicating the global warming potential of the waste incineration process [39]. For comparison, the incineration of municipal solid waste has an impact of approx.