https://doi.org/10.14311/APP.2022.33.0226 Acta Polytechnica CTU Proceedings 33:226–231, 2022 © 2022 The Author(s). Licensed under a CC-BY 4.0 licence Published by the Czech Technical University in Prague UTILIZATION OF RECYCLED AND SECONDARY MATERIALS IN CONCRETE PRODUCTION - LCA Anna Horáková∗, Alena Kohoutková, Iva Broukalová Czech Technical University in Prague, Department of Concrete and Masonry Structures, Faculty of Civil Engineering, Thákurova 2077/7, 166 29 Praha 6 - Dejvice, Czech Republic ∗ corresponding author: anna.horakova@fsv.cvut.cz Abstract. The paper describes an assessment of concrete in terms of environmental impacts in relation to the utilization of recycled materials. The article includes a short summary of the literature search on evaluation methods for environmental impacts and on recycled and secondary materials. The environmental impacts of several concrete mixtures were calculated. The reference concrete mixture containing only cement as a binder and only natural aggregate was compared with other mixtures containing recycled and secondary materials as a binder or aggregates. Some of these concrete mixtures were then compared within the whole structure. Variant design of a simple structure was performed, whereas the variants differed in the concrete mixture. These variants were then evaluated in terms of environmental impacts. Keywords: Concrete, environmental impacts, life cycle assessment, recycled and secondary materials, variant design. 1. Introduction In past years, the issue of sustainable development and the impact of construction activities on the en- vironment are gaining importance. It is desirable to minimize negative environmental impact by suitable design, optimal manufacturing process and material selection. The environmental aspects of sustainable development in the construction industry consist also in the utilization of secondary raw materials in the design and construction of new structures. 1.1. State of the art Most often, studies deal with comparison conven- tional concrete and concrete containing waste or re- cycled materials such as fly ash, slag or recycled ag- gregate. The comparison of unit volumes of different concrete mixtures almost always shows, that lower cement dosages make the concrete more environmen- tally friendly. Cement production is responsible for a significant amount of released harmful emissions and its energy consumption is very high. Hence, it plays a crucial role in the overall environmental impact of a concrete structure and the cement content in the concrete mixture is a key factor for its environmental assessment. For example, a study [1] reported that a 30% reduction of the cement content leads to a 26.6% reduction in CO2 emissions. Similar results observed study [2]. The decrease of cement dosage leads also to an energy and raw materials savings. A study [2] reported a 21 % decrease in energy consumption and a 4,3% decrease in raw materials consumption for a 35% reduction of cement content in concrete. The fact, that the environmental impacts of the concrete structure are largely dependent on the ce- ment consumption, led to many research projects dealing with a utilization of latent hydraulic materials as a partial replacement of cement. These supplemen- tary cementitious materials (SCM) are capable of hy- drating when used together with cement. Typical ex- amples are fly ash, blast furnace slag and silica fume. The use of these materials as a partial cement replace- ment reduces the environmental impacts of concrete production. Moreover, the SCM are usually waste products, which are generated during the production of other materials or during the energy production. For example, fly ash is a by-product of thermal power plant electricity production and blast furnace slag is a by-product of steel production. Thus, there is an- other environmental advantage, which lies in a uti- lization of waste products. It should be mentioned that the replacement of cement by SCM generally does not leads to a decrease in strength of the con- crete. The strength of the concrete containing these materials can be comparable or even higher. Ce- ment production harms the environment especially in terms of global warming and climate change and in terms of energy consumption. Hence, most studies deal with these environmental impacts and the envi- ronmental benefits of using cement replacement ma- terials. The study [3] compares several concrete mix- tures with different fly ash replacement levels (25%, 30% and 40%). With the increasing cement replacing level, carbon dioxide emissions decrease. For replac- ing level of 25%, 30% and 40%, the study reported a decrease in CO2 emissions by 6%, 11% and 23%. Deterioration of mechanical properties occurred un- til the replacement level reached 40 %. On the con- trary, these mechanical properties improved for lower 226 https://doi.org/10.14311/APP.2022.33.0226 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 33/2022 Analysis of Concrete and Cement EPD REF FA BFS WPA WTR RCA PC Cement [kg/m3] 380 280 324 380 410 380 0 Water [kg/m3] 190 236 180 200 200 190 0 Fine aggregate [kg/m3] 705 1142 681 572 840 705 1580 Coarse aggregate [kg/m3] 1100 493 1160 1020 960 0 0 Fly ash [kg/m3] 0 95 0 0 0 0 0 Blast furnace slag [kg/m3] 0 0 36 0 0 0 0 Waste plastic aggregate [kg/m3] 0 0 0 143 0 0 0 Waste tyre rubber [kg/m3] 0 0 0 0 40 0 0 Recycled concrete aggregate [kg/m3] 0 0 0 0 0 1100 0 Waste PET (as a binder) [kg/m3] 0 0 0 0 0 0 472 Superplasticizer [kg/m3] 2 1.8 0 0 0.8 2 0 Table 1. The composition of the concrete mixtures. cement replacement levels. For example, at 30 % re- placement level, the compressive strength increased by 10 %. If the study was carried out for a real struc- ture, the difference in environmental impact would be more significant than when comparing unit quantities of concrete. This is due to possible reduction of the dimensions of supporting structural elements. Nev- ertheless, it should be mentioned, that the difference would not be significant in case of this study, because there is not a significant increase in strength. Some studies deal with the utilization of blast furnace slag as a partial replacement of cement. These studies re- ported, a reduction of environmental burden, when cement is partially replaced. For example, study [4] reported, that for 50 % replacement level, CO2 emis- sions reduced by 39 %. The study [2] reported similar results. The reduction of CO2 emissions, and thus the influence on global warming is the most significant environmental benefit. On the other hand, replacing cement by blast furnace slag has only little effect on raw material consumption. Many studies deal with comparison conventional concrete and concrete containing recycled aggregate, which is produced by crushing waste concrete or bricks. According to most of these studies, use of this recycled aggregate leads to reduction of released harmful emissions and consumed raw materials and energy. Furthermore, utilization of these waste ma- terials is another benefit. For example, the study [5] compares the environmental impacts of the produc- tion of conventional concrete containing only natu- ral aggregate and concrete containing recycled aggre- gate. The natural aggregate was partially or fully replaced by crushed waste concrete. According to this study, the use of recycled aggregate was advan- tageous for most environmental impact categories. This was the most significant for the consumption of raw materials, which decreased by 47 % when nat- ural aggregate was fully replaced by recycled aggre- gate. The reduction of energy consumption was less significant, about 30 % for fully replaced natural ag- gregate. Study [6] reported, that the benefit of using recycled aggregate strongly depend on the transport distance of recycled aggregate. If recycled aggregate is transported over a long distance, the environmen- tal burden caused by the transport may outweigh the environmental benefits of using waste material. 2. Methods This paper deals with the utilization of recycled and waste materials in concrete production and its ad- vantages in terms of environmental impacts. At first, the assessment in terms of environmental impacts was performed for concrete, which contains only cement as a binder and only natural aggregate (REF) [7]. Then, the assessment was performed for other con- crete mixtures, which contain secondary or waste ma- terials as a binder or aggregate. In two of these mate- rial variants, there was cement partially replaced by fly ash (FA) [8] or by blast furnace slag (BFS) [9]. In some other variants, there was natural aggregate partially or fully replaced by waste plastics aggregate (WPA) [10], crushed waste tyre rubber (WTR) [11] or recycled concrete aggregate, obtained from demol- ished buildings (RCA) [7]. One more material variant was included in this evaluation - innovative material called polymerconcrete, which is composed of aggre- gate and plastic waste, which replaces cement as a binder (PC) [12]. This material is made from fine aggregate (aggregate size 0 − 4 mm) and waste PET, which is cut into small pieces. The production con- sists in homogenizing the mixture of plastic waste and small aggregate at high temperature. According to the experiments performed in [13], average compres- sive strength of this material is 22,7 MPa and average flexural strength is 7,91 MPa. All these mixtures were compared with each other. In the next step, a sim- ple reinforced concrete structure - reinforced concrete frame - was designed from selected concrete mixtures. Then, these structures were compared in terms of en- vironmental impacts. The composition of the con- crete mixtures evaluated in this paper is shown in the Table 1. 227 A. Horáková, A. Kohoutková, I. Broukalová Acta Polytechnica CTU Proceedings Environmental impact Explanation Global Warming Global warming is a long-term increase in global average temperaturecaused by excessive production of greenhouse gases. Acidification Acidification is the ongoing decrease in the pH of the environment. This phenomenon is caused by the presence of acid-forming substances in the atmosphere, which react with water to form acids. Eutrophication Eutrophication of the environment leads to ecosystem disturbance due toexcessive nutrients in water and soil due to excessive fertilization. Photochemical Oxidant Creation Photochemical oxidants are air pollutants that are formed under the influence of sunlight by complex photochemical reactions in air that contain nitrogen oxides and reactive hydrocarbons and cause damage to organisms. Abiotic Depletion Abiotic depletion refers to the depletion of abiotic resources such asfossil fuels, minerals, and clay. Table 2. Environmental impacts. 2.1. Life cycle analysis The assessment of materials and structures in terms of environmental impacts was performed using Life- cycle assessment (LCA) according to relevant stan- dards [14]. The LCA approach is usually based on the whole life-cycle of the investigated product or at least its significant part. So, the assessment includes obtaining raw materials, their transport to the place of processing, manufacturing of the final product, use of the product and further maintenance or repairs if necessary, and final disposal of the product. How- ever, the prediction of the course of the phase of use is sometimes not possible. In these cases, the evalua- tion includes only a part of the life cycle, it includes for example only obtaining raw materials, their trans- port to the place of processing and manufacturing of the final product ("cradle to site" evaluation). In this paper, the assessment was performed for concrete as a material for further use, and for a concrete structure too. The evaluation of a unit quantity of different concrete types included the phase of obtaining of raw materials, their transport and processing and manu- facturing of the final product. The evaluation of a concrete structure included these phases too, and it includes in addition the phases of transport of con- crete and steel and manufacturing of final structure. At first, the consumption of raw materials and emissions released into the environment were defined for each interproduct such as cement, aggregate, wa- ter, fly ash, blast furnace slag and other materials. Within the assessment, the most significant environ- mental impacts were considered: consumption of raw materials, global warming and climate change, acid- ification and eutrophication of the environment and photooxidant formation. In LCA, these environmen- tal impacts are called impact categories. Principles of these environmental impacts are explained in Table 2. The environmental impacts of each interproduct were calculated. Impacts on the environment were quantified by so-called impact category indicators - measurable variables that can be used to observe changes in the environment. The values indicate the extent of environmental damage caused by hu- man activities. Usually, the impact category is influ- enced by various of substances where some substances are very harmful, and some less. Thus, all the sub- stances are converted to an equivalent amount of the reference substance (for example carbon dioxide for global warming and climate change or sulfur dioxide for acidification of the environment). The effect of a specific substance to each impact category was determined by so-called characteriza- tion models. A characterization model for a spe- cific impact category is a set of values that reflect the ability of various substances damage the environ- ment within the impact category. All of the issued substances are converted to the equivalent amount of a reference substance by using these values (char- acterization factors - CF). This paper used a char- acterization model which is recommended in Prod- uct category rules (PCR) for concrete products [1]. The resultant impact category indicator was calcu- lated according to the following relationship: V XY = CF 1, XY · ! m1i + CF 2, XY · ! m2i + ... + CF n, XY · ! mni (1) where V XY is a result of the impact category in- dicator (XY indicates the impact category), CF is a characterization factor and m is an amount of a released substance. When evaluating the impact categories, emissions of following substances were considered: carbon diox- ide CO2, sulfur dioxide SO2, nitrogen oxides NOx, carbon monoxide CO, methane CH4, Non-methane volatile organic compound NMVOC, nitrous oxide N2O, hydrochloric acid HCl, hydrofluoric acid HF, hydrogen sulfide H2S, ammonia NH3. In this paper, seven material variants in the amount of 1 m3 were compared (Table 1). In ad- dition, several of the above-mentioned material vari- 228 vol. 33/2022 Analysis of Concrete and Cement EPD Figure 1. Comparison of 1 m3 of concrete mixtures regarding sustainability. ants were compared in designed structure: conven- tional concrete, concrete with fly ash, concrete with blast furnace slag, concrete with plastic aggregate, concrete with waste tyre rubber and concrete with recycled concrete rubber - the structure design was performed for all material variants except polymer- concrete. Because of the lack of experience in rein- forcing of this material, the load-bearing structure designed from this material was not included in the assessment. A simple reinforced concrete frame was designed from different types of concrete. Because of different strength of the concrete types, the vol- ume of concrete needed for the construction depends on the material variant. The amount of steel is the same for all variants. The assessment of the material variants in terms of environmental impacts depends strongly on the volume of concrete, which is used for the structure. Therefore, the frame was designed with utilization of the highest load-bearing capacity of the structural elements to make this assessment relevant. The compressive strength of the mixtures and volume of concrete for the variants are shown in Table 3. Variant The compressive strength[MPa] The volume [m3] REF 32.3 740.00 FA 36.2 728.40 BFS 36.2 728.40 WPA 29.5 749.86 WTR 33.8 735.36 RCA 29.2 751.60 Table 3. The compressive strength of the concrete and volume ofăconcrete for designed variants. 3. Results 3.1. Results for 1m3 of concrete The results of the sustainability assessment are re- lated to the specific environmental impact. The fol- lowing figure shows the comparison of the concrete mixtures for considered impact categories. Polymerconcrete was evaluated for all most impact categories as the best material variant. This is prob- ably because this material does not contain any ce- ment, whose production is very burdening for the en- vironment. On the other hand, the energy consump- tion (the impact category Abiotic Depletion Poten- tial ADP fossil) is relatively high, higher consump- tion was calculated only for conventional concrete. The obvious reason is the high heat consumption for melting the waste polymer. Moreover, the use of this material for structures is limited and there is a lack of previous experience with its production. Very favourable results have also been obtained for con- crete with fly ash. The obvious reason is the reduc- tion of cement consumption due to its partial replace- ment by fly ash. In the case of concrete with blast furnace slag, cement is also partially replaced, but the replacement level is lower. Hence, the results are less favourable for this material variant. Material vari- ants, in which recycled materials replaced aggregates, were evaluated as less favourable in terms of environ- mental impacts. This is because the production of aggregates does not cause such a high environmental burden as the production of cement. Another reason is that the waste material (concrete, plastic, tyre rub- ber) for production of aggregates replacement must be mechanically processed (crushed). This process is energy intensive and reduces the environmental ben- efits of using waste material. Therefore, for reducing the environmental impacts it is preferable to replace cement. 3.2. Results for concrete structures For comparison of the whole structures, a simple re- inforced concrete frame was designed from different types of concrete. Regarding the environmental im- pacts of concrete frames designed from different ma- terial variants, the results did not differ significantly from the previous comparative study (comparison of unit quantities of concrete mixtures). All investi- gated concrete mixtures have the similar compressive strength. Therefore, the dimensions of the support- ing elements of the frame do not differ significantly and consumption of concrete is very similar for all de- signed variants. It is obvious, that a partial replace- ment of cement by SCM is the most advantageous in terms of environmental impacts. The Figure 3 shows the comparison of the designed variants for consid- ered impact categories and Figure 2 shows the sketch 229 A. Horáková, A. Kohoutková, I. Broukalová Acta Polytechnica CTU Proceedings of the designed frame. Figure 2. Designed concrete frame. Figure 3. Comparison of the concrete frames regard- ing sustainability. 4. Conclusion According to this study, in terms of environmental impacts, it is most advantageous to replace a part of cement by supplementary cementitious materials (SCM), such as fly ash or blast furnace slag. Cement production causes a large environmental burden and many environmental impacts are significantly depen- dent on cement consumption. It could be reduced by partial replacement of cement by SCM. Furthermore, supplementary cementitious materials are often waste or secondary materials and their utilization is another environmental benefit. Polymerconcrete is also very favourable from the environmental point of view, especially in terms of consumption of raw materials. However, the produc- tion of this material is energy intensive, due to the melting of waste plastic. Moreover, so far there is no experience with its use in structures. Therefore, it is desirable to find a suitable use for this material, for example non-bearing part of structures, such as floors or pavements. When choosing a material for a specific structure, it is necessary to investigate the influence of the trans- port of recycled materials to the structure site. The transport distance of recycled materials, especially re- cycled concrete aggregate or waste plastics aggregate, is probably greater than the transport distance of nat- ural aggregates. 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