https://doi.org/10.14311/APP.2022.33.0193 Acta Polytechnica CTU Proceedings 33:193–199, 2022 © 2022 The Author(s). Licensed under a CC-BY 4.0 licence Published by the Czech Technical University in Prague ENERGY FLOWS ALONG THE PRODUCTION AND USE OF SECONDARY MATERIALS WITH A SPECIAL FOCUS ON CONCRETE Karin Gruhler∗, Georg Schiller Leibniz Institute of Ecological Urban and Regional Development, Weberplatz 1, 01217 Dresden, Germany ∗ corresponding author: K.Gruhler@ioer.de Abstract. Urban mining in the existing building stock can contribute to securing raw materials and conserving natural resources if the potential of recycling construction waste is consistently exploited. From an ecological point of view, it is on the one hand interesting what amounts of primary materials can potentially be substituted and on the other how much energy need to be invested for this. At present, the recycling of construction waste usually is considered from a material perspective. There is lack of an approach, extending material-oriented considerations by energetic aspects. The aim is to develop a uniform research approach by which energy expenditure during recycling of important construction products can be determined. Besides concrete seven further construction products are investigated. Recycling paths are described and analysed along waste management processing steps taking into account the quality of the demolition materials and the quality requirements of the possi- ble new application variants in the construction sector. The result is a clear plea for more consistent recycling. The analyses of concrete indicate that "high-quality" recycling only results in energy ad- vantages when "high-grade" demolition material is used. However, so-called "down-cycling" solutions allow resource conservation to be combined with energy savings, even with lower-quality demolition materials. The single-minded focus on "high-quality" recycling according to the general understanding should therefore be questioned. Instead, preference should be given to solutions that take resource conservation into account in a more holistic way especially with regard to resource conservation and climate protection. Keywords: Circular economy, energy consumption, recycling, resource efficiency. 1. Introduction The recycling of construction waste can make an im- portant contribution to climate and nature protec- tion, but above all to the conservation of resources. In Germany, the German Resource Efficiency Pro- gramme formulated the goals of decoupling resource consumption from economic growth, doubling raw material productivity by 2020, reducing the environ- mental impacts associated with the use of natural resources as far as possible and further developing and expanding the circular economy [1]. In order to enable almost circular economic activity in the con- struction sector, the corresponding demolition mate- rials must be recyclable and be available with a cer- tain consistency in specific quantities. Resource conservation potentials using secondary materials expressed in tonnes of material are known for some selected construction products [2, 3]. How- ever, an assessment of the resource conservation po- tentials only on the basis of these material figures is too one-sided, as energy aspects are not taken into ac- count. There are individual studies that focus on spe- cific construction products with regard to the energy used in recycling [4]. However, there is no comprehen- sive overview for all important construction products. There is a lack of a common approach by which im- portant construction products can be presented syn- optically and compared to each other in terms of their energy expenditures during recycling. The aim of the present study was to extend the material-oriented studies on resource conservation potentials to include energy considerations and to de- velop a uniform approach by which important con- struction products can be presented and compared in a synoptic way with regard to their energy expen- diture during recycling [5]. The following questions should be clarified: Which energy expenditures are associated with recycling? What is the energy expen- diture of recycling compared to the energy expendi- ture of standard production without recycling? Are there energetic differences between "high-quality" ap- plication variants (in building construction) and less "high-quality" application variants (in civil engineer- ing and landscaping)? The investigations focused on eight construction products: concrete, bricks, sand-lime bricks, gypsum, flat glass, mineral (stone) wool, PVC profiles, and PVC floor covering. For all of these, we energeti- cally investigated the paths from demolition material to a new application variant in building construction, civil engineering or landscaping. This we did taking into account the quality of the demolition material and the quality requirements of the new application 193 https://doi.org/10.14311/APP.2022.33.0193 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en Karin Gruhler, Georg Schiller Acta Polytechnica CTU Proceedings Process chain demolition material secondary material application variant Concrete pc 1 concrete without recycled aggregatescoarse adhesions 2/16 type 1 concerete foundation pc 2 recycled aggregates C20/25broken concrete mixture 2/22 type 2 pc 3 (with brick, limestone, plaster) recycled aggregates ballast base layer0/32 for road construction for roads Table 1. Model-like pc for concrete. variant in the construction sector. 2. Methodology In order to carry out these investigations, we have de- veloped an analytical approach which generally regu- lates the methodological procedure, following the idea of a continuous Material Flow Analysis as described in [6]. 2.1. Balance framework and terms & definitions Starting point of the considerations are demolition materials. Demolition materials are quantities that are available in a certain quality after the demolition of a building (e.g. broken concrete without coarse adhesion). These materials come exclusively from building construction. Construction site waste is not included in the analysis. In order to be able to sup- ply demolition materials as secondary materials and substitute materials for a new application variant, they must be prepared accordingly. A new applica- tion variant is the use of demolition materials for a new purpose and location (e.g. broken concrete as aggregates for foundation concrete). All necessary preparation and further processing steps depend on the quality of the demolition mate- rial itself as well as the quality requirements of the new application variant. In order to take these de- pendencies into account, there is a need to consider the entire process from the demolition material to the new application variant. This is applied in the form of characteristic model-like process chains (pc). Due to the large variety of possible process chains (large variety of possible deconstruction qualities, types of processing, further processing methods, ap- plication variants) two to three demolition materials and two to three typical application variants were combined into model-like pc for each construction product. The pc are defined in consultation with experts from the relevant construction product as- sociations in Germany. Table 1 shows the pc for the example concrete. On the way from the demolition material to the new application variant, a secondary material is first created and in a further step a substitute. The terms "secondary material" and "substitute" are described more in detail in the standard DIN EN 15804:2014- 07 [7] (Table 2). With regard to DIN EN 15804:2014-07, the se- lected balance framework starts at the end of the waste treatment step of the preceding product sys- tem "building". Processes before the end of the waste property stage (such as collection and transport) are part of the waste treatment of the above product sys- tem "building". In contrast, processes that are re- quired after the end of waste property stage to allow primary materials to be replaced in another product system are considered to be outside the above system "building" (see [7], para. 6.3.4.5 Note 3). These reg- ulations are complied with here. The analyses start with the demolition material at the recycling com- pany and integrate all preparation steps for the pro- duction of the secondary material as well as its further processing up to the substitute (preceding demolition technology, collection processes, sorting processes and transports on the site are not considered). 2.2. 3-Step analysis approach of process chains The energetic analyses of the model-like pc of the different construction products follow a uniform me- thodical approach. It is divided into three steps: 1. Determination of the energy required to process a demolition material into a secondary material. 2. Determination of the energy expenditure for the further processing of the secondary material into a substitute that can replace a primary material in a functionally equivalent way, using energy add- ons/deductions. 3. Comparison of the energy expenditure for the sub- stitute with that for the primary material to be replaced (Figure 1). The steps of the methodical procedure are ex- plained more in detail in the following paragraphs using concrete as example (pc 1: from pure crushed concrete to recycled aggregate for foundation con- crete C20/25, Table 1). 2.2.1. Energy expenditure for processing secondary material Here the energy required to process the demoli- tion material into secondary material is calculated 194 vol. 33/2022 Energy Flows of Secondary Materials Term description of important terms Secondary material According to DIN EN 15804:2014-07 [7], para 3.29, a secondary material is - any form of material recovered from a previous use or from waste and replacing a primary material, - recorded at the point at which the secondary material enters the system from another system, - recovered from a previous use or from waste from a product system and used as input to another product system. Substitute Substitutes are "secondary materials having left the system for primary material production [and are declared] in Module D if they have functional equivalence to the replaced primary material" ([7], para 6.3.4.5, note 3). Table 2. Description of the terms "secondary material" and "substitute" according to DIN EN 15804:2014-07. Figure 1. Methodical procedure. in MJ/kg. The following list shows which analysis and calculation steps are required to determine the average energy expenditure. • Identification and definition of characteristic main processing steps to produce the secondary mate- rial (e.g. pre-screening, crushing, metal deposition etc.). • Definition of technical components/machinery for the identified processing steps (e.g. sieve, jaw crasher, magnetic separator etc.). • Determination of average energy expenditures for the defined components/machinery (e.g. sieve 0.0021 MJ/kg, jaw crasher 0.0022 MJ/kg, magnetic separator 0.0147 MJ/kg etc.). • Calculation of the energy required to produce the secondary material by adding the energy values of the identified process steps and defined machines (e.g. 0.036 MJ/kg for all steps). • Analysis of the material flows and calculation of the proportions for target product, co-products and waste. For pc 1 of concrete this is 48% target product (recycled aggregate 2/16 type 1), 43% co- products (crushed concrete 16/22, 22/32, 32/56, 56/x and Sand 0/1, 0/2) and 9% waste (pre-screen material 0/8 and impurities/foreign substances). For co-products there is the possibility of further use. They receive an energy allocation. There is no further use for waste and therefore no energy allocation. • Calculation of the energy allocation for the co- products based on the material flow proportions (e.g. 0.013 MJ/kg for crushed concrete > 16 mm and sand ≤ 2 mm). • Calculation of the energy required to produce the secondary material considering the energy allocations for co-products (e.g. 0.036 MJ/kg − 0.013 MJ/kg = 0.024 MJ/kg) In sum step 1 ends with the energy expenditure for processing the secondary material. 195 Karin Gruhler, Georg Schiller Acta Polytechnica CTU Proceedings Figure 2. Energy expenditures of the selected construction products - a comparison. 2.2.2. Energy expenditures for further processing up to a substitute Here it is analysed which energy expenditure is con- nected with the further processing of the secondary material to the substitute. Methodologically, all nec- essary measures and processes are analysed and the differences between the production process with re- cycled material and the standard production process without recycling are worked out. The differences can be associated with both additional and reduced energy expenditure. This means that the previous energy expenditure for the secondary material (0.024 MJ/kg) receives an energy add-on or an energy de- duction. Additional and reduced energy expenditure result mainly from modified recipes and modified pro- cess steps1. Modified recipes are, for example, differ- ent mixing ratios or additional admixtures of auxil- iary materials. Modified process steps can be addi- tional, shortened or unnecessary process steps. In the case of concrete, it is above all necessary to increase the cement content in the recipe (+ 1%). Modified process steps are not necessary (± 0%). This results in a total energy add-on of + 0.052 MJ/kg. In total, this results in the energy expenditure for the substi- tute (0.076 MJ/kg). 2.2.3. Comparison of the energy expenditure for the substitute with that of the primary material Finally, the production of construction products without recycled material is compared in terms of energy to that with recycled material. This is done 1Modified transport efforts also lead to additional or re- duced energy expenditure. However, they could not be calcu- lated due to the poor availability of data. There is a need for research. at the level of the material to be replaced or the material to be substituted. In this way, the energy expenditure of the substitute is compared with that of the primary material. This comparison allows as- sessments to be made as to whether recycling makes sense from an energy perspective. The energy charac- teristics of the primary materials are taken from the source ökobaudat [8] and DIN EN 15804 compliant secondary sources [9], [10], [11]. 3. Results Following the methodological procedure presented, energy expenditure for all pc of the selected construc- tion products was calculated for the production of secondary materials and further processing up to the substitutes and compared with the energy expendi- tures of primary materials (to be replaced) (Figure 2). In general, the results show that under the given framework there is almost nothing to prevent recy- cling. The energy expenditure for the secondary ma- terial and the substitute is usually lower than the energy expenditure for the primary material. Excep- tions are mineral wool and one pc of concrete (pc 2) and one of gypsum (pc 1). For mineral materials, recycling is not associated with excessive energy expenditures and makes sense above all with regard to the mass aspect, as recycling reduces the extraction of raw materials and protects the natural environment and landscape. For plastics the energy aspect is more significant. A specially look at concrete shows that the qual- ity of the demolition material as well as the ma- terial requirements of the new application variant have a significant influence on the energy expendi- tures for recycling (Table 3). As a rule, the prin- 196 vol. 33/2022 Energy Flows of Secondary Materials Construction product concrete pc 1 pc 2 pc 3 Demolition material concrete without broken concrete mixture coarse adhesions (with brick, limestone, plaster) ee of all processing steps 0.036 MJ/kg 0.031 MJ/kg 0.025 MJ/kg discharge co-products 43% 34% 10% energy allocation (ea) co-products 0.012 MJ/kg 0.009 MJ/kg 0.002 MJ/kg Secondary material recycled recycled ra 0/32 aggregates (ra) aggregates (ra) for road 2/16 type 1 2/22 type 2 construction ee of all processing steps with ea 0.024 MJ/kg 0.022 MJ/kg 0.023 MJ/kg Substitute modified recipes + 0.052 MJ/kg + 0.092 MJ/kg ± 0 MJ/kg modified process steps ± 0 MJ/kg ± 0 MJ/kg ± 0 MJ/kg ee with add-ons/deductions 0.076 MJ/kg 0.114 MJ/kg 0.023 MJ/kg for modification (77%) (118%) (14%) Primary material aggregate of aggregate of aggregate of gravel (2/32; 80%) gravel (2/32; 80%) gravel round (4/x; and chippings and chippings 50%) and gravel (2/15; 20%) (2/15; 20%) broken (4/x; 50%) ee for processing primary 0.0972 MJ/kg 0.0972 MJ/kg 0.168 MJ/kg material1 (100%) (100%) (100%) Application variant concrete crushed stone foundation C20/25 sub-base (roads) 1 Source: ökobaudat [8]. 2 Using ecoinvent [9] results in a different figure: 0.057 MJ/kg. This also influences the figure of the substitute due to the recipe. Table 3. Energy expenditure (ee) calculations of the different pc of concrete. ciple applies that "high-quality" demolition material (clean/without adhesions) is usually associated with a "high-quality" application variant (building con- struction) and a "low-quality" (unclean/ with adhe- sions) one with a "low-quality" application variant (civil engineering/landscaping). However, one and the same demolition material can also be used in different application variants (pc 2 and 3). For example, the demolition material crushed concrete mix can be used both as aggregates for the production of foundation concrete (pc 2) and as a bal- last base layer in road construction (pc 3). The first variant has higher quality requirements and is also associated with higher energy expenditure. These re- sult from a different secondary material: recycled ag- gregates 2/16 type 1 for concrete foundation (pc 2) and recycled aggregates 0/32 for road construction (pc 3). In addition, energy-relevant recipe adjust- ments (e.g. modified ratios for superplasticizer, water or cement) are required (pc 2) to produce a substitute equivalent to the primary material. A higher cement ratio content in particular has an impact on energy expenditure. Compared to the two pc, the crushed stone sub-base for road is thus the energetically bet- ter application variant. Likewise, two different demolition materials can have one and the same use (pc 1 and 2). In this way, both broken concrete without adhesions (pc 1) and crushed concrete mix (pc 2) can be used for the pro- duction of foundation concrete. Here, however, the "low-quality" demolition material (crushed concrete mix) is the less efficient variant in terms of energy. It is true that the production of aggregates 2/22 type 2 requires less energy than that of aggregates 2/16 type 1. However, changes in the recipe finally lead to a higher energy expenditure. In comparison of the two process chains, the broken concrete without ad- hesions is therefore the more energy-efficient starting demolition material. The examples for concrete show that the use of re- cycling material in building construction can lead to different energy results with different types of demo- lition material (pure concrete or concrete mix). For example, the energy expenditure for the substitute in pc 1 is approx. 20% lower and in pc 2 approx. 20% higher than the energy expenditure for the primary material to be replaced (aggregates for concrete foun- dation). In contrast, energy savings of approx. 85% 197 Karin Gruhler, Georg Schiller Acta Polytechnica CTU Proceedings are possible in road construction if recycled aggre- gates are used instead of primary aggregates (pc 3). If these results are taken into account, both "high- quality" and "lower-quality" recycling should be given attention in the interests of resource and climate pro- tection. 4. Summary and conclusions In line with the objectives we developed a standard uniform balance approach to assess energy expendi- ture. Using this approach we calculated two to three pc for each of the selected construction products und provided a first set of energetic figures for recycling. The analyses show that recycling is worthwhile. Compared to the use of primary materials, it is gen- erally not associated with an excessive use of energy. However, there are some exceptions. Every building product has its own specific quality requirements and must be considered individually. For mineral mate- rials, recycling makes sense above all with regard to the mass aspect, as recycling reduces the extraction of raw materials and protects the natural environment and landscape. For plastics the energy aspect is more significant. It also becomes clear that the pc of the construction products have to be considered from the demolition material until the new application variant is reached. This is because each new application variant is asso- ciated with certain quality requirements for the sec- ondary material up to the substitute, which finally affect the energy expenditure. In summary, the analyses of concrete show that "high-quality" recycling (building construction appli- cation) only brings energetic advantages if "high- quality" demolition material (pure concrete breakage) is used. "Low-quality" recycling (road work applica- tion) on the other hand allow resource conservation to be combined with energy savings, even with lower- quality demolition materials. The single-minded fo- cus on "high-quality recycling" according to the gen- eral understanding should therefore be questioned. Instead, preference should be given to solutions that take resource conservation into account in a more holistic way especially with regard to resource conser- vation and climate protection. In order to be able to assess this comprehensively, it is always necessary to look at the entire process chain, starting with the con- struction waste and its quality, the intended recycling product and the intermediate treatment and process- ing steps. This should not stop at today’s common technologies. Rather, it is important to keep an eye on innovative technical developments. For example, the use of geopolymers as cement substitutes can lead to a significant reduction in greenhouse gas emissions [12] and thus have an impact on the advantages of the recycling processes under consideration. Acknowledgements The authors would like to thank the Federal Institute for Research on Building, Urban Affairs and Spatial Devel- opment (BBSR) for funding the research as well as the entire project team. 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