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Figure 3. (a) Composition of CDW (processed data from [22]). (b) Typical composition of 8-16 mm RA from a recycling plant (processed data from [24)]).

Figure 3 (a) Composition of CDW (processed data from [22]). (b) Typical composition of 8-16 mm RA from a recycling plant (processed data from [24)]).

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Wang et al. [4] about worldwide CDW generation are illustrated in Figure 1. Globally, approximately 10 Bt of CDW are generated each year [8]. Kabirifar et al. [9] state that CDW occupies 35-65% of global landfill space. These numbers raise serious concerns. Since the usage of traditional materials is unsustainable, various waste materials have been repurposed in the manufacturing of concrete. In various studies, waste materials such as ground granulated blast-furnace slag (GGBS) [10,11]; fly ash [12]; glass [13,14], but also alternative materials such as rice husk ash, animal bones and human hair [15,16] are incorporated in concrete. CDW, due to the vast market for re-use, has a high potential for recycling. This combined with the aforementioned soaring growth and the immersed CDW generation, has rendered CDW a fast-growing commodity. Figure 1. Illustration of the countries with the highest CDW generation around the world in 2018 (processed data from Wang et al. [4]).
Wang et al. [4] about worldwide CDW generation are illustrated in Figure 1. Globally, approximately 10 Bt of CDW are generated each year [8]. Kabirifar et al. [9] state that CDW occupies 35-65% of global landfill space. These numbers raise serious concerns. Since the usage of traditional materials is unsustainable, various waste materials have been repurposed in the manufacturing of concrete. In various studies, waste materials such as ground granulated blast-furnace slag (GGBS) [10,11]; fly ash [12]; glass [13,14], but also alternative materials such as rice husk ash, animal bones and human hair [15,16] are incorporated in concrete. CDW, due to the vast market for re-use, has a high potential for recycling. This combined with the aforementioned soaring growth and the immersed CDW generation, has rendered CDW a fast-growing commodity. Figure 1. Illustration of the countries with the highest CDW generation around the world in 2018 (processed data from Wang et al. [4]).
Figure 2. Cradle-to-grave life-cycle assessment and input and output analysis of aggregate production.
Figure 2. Cradle-to-grave life-cycle assessment and input and output analysis of aggregate production.
Table 1. Physical properties of RAs and NA, according to Makul et al. [27]. Water Absorption (WA) and Workability: High WA due to high porosity directly affects the concrete’s properties. Additional water is required in the concrete in order to match the desired workability. According to research [27-30], the increase in WA recycled aggregate concrete (RAC) in correlation to natural aggregate concrete (NAC) is up to 40% for a replacement ratio (RR) of 50%. Wang et al. [4] states that according to research [31-34], the consistency of fresh RAC had a lower slump value than NAC, due to RAs’ higher WA and the rougher surfaces and irregular shapes.
Table 1. Physical properties of RAs and NA, according to Makul et al. [27]. Water Absorption (WA) and Workability: High WA due to high porosity directly affects the concrete’s properties. Additional water is required in the concrete in order to match the desired workability. According to research [27-30], the increase in WA recycled aggregate concrete (RAC) in correlation to natural aggregate concrete (NAC) is up to 40% for a replacement ratio (RR) of 50%. Wang et al. [4] states that according to research [31-34], the consistency of fresh RAC had a lower slump value than NAC, due to RAs’ higher WA and the rougher surfaces and irregular shapes.
Mechanical Properties versus Replacement Ratio Correlation Figure 4. Plot of mechanical properties change with different RCA RR, and the sample size at each RR for (a) compression strength, (b) E-modulus, (c) flexural strength, and (d) splitting tensile strength (processed data from [4]). Compressive strength: Compressive strength of concrete decreases, as the RR of NA increases [4,23,26,27,29,31,32,46-57]. Figure 4. represents the findings of Wang et al. [4] in which the correlation between the mechanical properties and the RR of RAs is depicted. Up to a degree of 50% RR, RAs have no significant impact on the compressive strength of concrete [4,26,27,29,46-52,54,56].
Mechanical Properties versus Replacement Ratio Correlation Figure 4. Plot of mechanical properties change with different RCA RR, and the sample size at each RR for (a) compression strength, (b) E-modulus, (c) flexural strength, and (d) splitting tensile strength (processed data from [4]). Compressive strength: Compressive strength of concrete decreases, as the RR of NA increases [4,23,26,27,29,31,32,46-57]. Figure 4. represents the findings of Wang et al. [4] in which the correlation between the mechanical properties and the RR of RAs is depicted. Up to a degree of 50% RR, RAs have no significant impact on the compressive strength of concrete [4,26,27,29,46-52,54,56].
Table 3. Calculation of Recycled Aggregates CO> footprint. 3.3. Combined Approach
Table 3. Calculation of Recycled Aggregates CO> footprint. 3.3. Combined Approach
Table 4. Comparison of RAC properties and replacement ratio.
Table 4. Comparison of RAC properties and replacement ratio.
Table 5. Summarized data on NA and RA production emissions. One tone of NA and RA produces 3.067 and 1.854 Kg of CO2 emissions, respectively, which stands for a 39.57% reduction in CO? total emissions (Table 5). Implementing the RR of 50% would result in a 19.78% reduction. To quantify the calculated values, i.e., the emissions already mentioned [58] could be at 11.7 MMT CO, e per year, hence a reduction of 2.9 MMT CO> emitted per year.
Table 5. Summarized data on NA and RA production emissions. One tone of NA and RA produces 3.067 and 1.854 Kg of CO2 emissions, respectively, which stands for a 39.57% reduction in CO? total emissions (Table 5). Implementing the RR of 50% would result in a 19.78% reduction. To quantify the calculated values, i.e., the emissions already mentioned [58] could be at 11.7 MMT CO, e per year, hence a reduction of 2.9 MMT CO> emitted per year.
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