# Case Study on Secondary Building Materials
## 1 Introduction  
The world's material consumption increased from 71,000 million metric tons in the year 1900 to 101,500 million metric tons in 2021 [1, 2]. Responsible for half of this tonnage are mineral materials, mainly used in the building and construction industry, making them the largest resource extracted globally [3]. Their main use is as aggregates in infrastructure projects and the production of concrete [4]. The natural resources for sand and gravel are not scarce, and the specific energy consumption for the production of aggregates is dependent on the processing route (e.g., higher for the crushing of stone and lower for the extraction of river aggregates) in the range of 11-38 M]/ton only [5, 6, 7, 8, 9, 10, 11, 12]. For comparison, the specific energy consumption figures for the production of steel and cement are 13,400 and 3040 M]/ton, respectively [9]. Although the specific energy consumption for aggregate production is low, the environmental impact is still high just due to the extremely large material flows. It has been found that the energy demand for the production of recycled aggregates is even slightly higher (20-25%) than that of natural material [8, 9]. However, the substitution of natural aggregates by recycled material at least saves natural resources and supports a circular economy. Construction and demolition waste (C&D waste, European waste catalogue code EWC 17) is the most important source for secondary building materials, with 221 million metric tons being consumed in Germany in 2020 [13]. The amount of concrete, bricks, tiles, and ceramics (EWC 17 01) was 61 million metric tons in the year 2020 [13]. The steel recovered from recycled aggregate concrete structures is advantageous under all circumstances [5]. The supply of recycled mineral material cannot cover the demand for aggregates completely. For example, the demand for fine and coarse aggregates in Germany was 585 million metric tons in the year 2020;however, only 13.2% were produced by recycling measures [14]. The applied technologies for the treatment of C&D waste were reviewed recently [15]. A further sub-group within C&D waste is soil-like material such as waste with EWC code 17 05 04 (soil and stones) or 17 05 07 (dredging spoil). In Germany the amount of this category was 129 million metric tons (year 2020, [14]). Most of this waste was recycled and utilized, while only 14.3% was disposed of to landfills. Compared to natural soil, soil-like materials exhibit secondary raw materials and comprise (a) excavated and/or anthropogenically influenced soils, (b) dredged material from earthworks and waterway construction measures, (c) soils treated to remove pollutants, and (d) soils produced from excavated soil and other material such as compost. Soil-like materials can be contaminated with pollutants depending on their previous use or origin. The potential applications of soil-like materials depend not only on the content of pollutants in the solid matter, but also on their potential release into the environment. Another source for secondary building materials is the bottom ash of municipal solid waste incineration (MSW incineration bottom ash, herein abbreviated as IBA, EWC code 19 01 12). Throughout Europe, the annual production of IBA is 19 million metric tons. In Germany, the figure is 6.05 million metric tons [16]. At around 25%, IBA represents the largest residue fraction in thermal waste treatment. It consists of solid phases already contained in municipal waste, such as pieces of glass, ceramics, ash, and metals (ferrous and non-ferrous metals), as well as new phases formed during the combustion process [17]. The five main chemical elements in bottom ash are Si, Ca, Fe, Al, and Na. While Si and Ca are bound as oxides and silicates, Al and Fe also occur in their elemental form. Na is additionally present as chloride, while Ca is as sulfate [18]. The treatment of IBA is focused on the recovery of mineral material (app. 90% of the IBA) and elemental metals (ferrous and non-ferrous, app. 10%).