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Article

Valorization of Recycled Aggregate and Copper Slag for Sustainable Concrete Mixtures: Mechanical, Physical, and Environmental Performance

by
Pamela Wendy Caballero Arredondo
1,
Yimmy Fernando Silva
1,2,3,*,
Gerardo Araya-Letelier
1,2 and
Héctor Hernández
1
1
School of Civil Construction, Faculty of Engineering, Pontificia Universidad Católica de Chile, Santiago 7820680, Chile
2
Concrete Innovation Hub UC (CIHUC), Faculty of Engineering, Pontificia Universidad Católica de Chile, Santiago 7820680, Chile
3
Composite Materials Group (CENM), Universidad del Valle, Cali 760042, Colombia
*
Author to whom correspondence should be addressed.
Sustainability 2024, 16(24), 11239; https://doi.org/10.3390/su162411239
Submission received: 19 November 2024 / Revised: 10 December 2024 / Accepted: 17 December 2024 / Published: 21 December 2024
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Abstract

:
The increasing environmental impacts caused by the high demand for concrete production have underscored the need for sustainable alternatives in the design of eco-concrete mixtures. Additionally, important industries, such as construction and mining, generate massive amounts of waste/by-products that could be repurposed towards sustainability. Consequently, this study investigates the valorization of copper slag (CS), a by-product of the mining industry as a supplementary cementitious material (SCM), and concrete as recycled coarse aggregate (RCA), derived from construction and demolition waste, as partial substitutes for Ordinary Portland Cement (OPC) and natural coarse aggregate (NCA), respectively. Eco-concrete mixtures were designed with varying replacement levels: 15% for CS, and 0%, 20%, 50%, and 100% for RCA. The mechanical properties (compressive, indirect tensile, and flexural strengths), permeability characteristics (porosity and capillary suction), and environmental impacts (carbon footprint) of these mixtures were evaluated. The results showed that the use of CS and of increasing proportions of RCA led to a monotonic loss in each of the concretes’ mechanical strength properties at 7, 28 and 90 days of curing. However, at extended ages (180 days of curing), the concrete mixtures with CS and only NCA presented an average compressive strength 1.2% higher than that of the reference concrete (mixture with only OPC and natural aggregate). Additionally, the concrete mixture with CS and 20% RCA achieved 3.2% and 5.8% higher average values than the reference concrete in terms of its indirect tensile strength and flexural strength, respectively. Finally, a cradle-to-gate life cycle assessment (LCA) analysis was implemented, whose results showed that the greatest effect on reducing the carbon emission impacts occurred due to the substitution of OPC with CS, which confirmed that the adequate technical performances of some of the concrete mixtures developed in this study are positively complemented with reduced environmental impacts. Moreover, this study presents a viable approach to minimizing resource consumption and waste generation, contributing to the advancement of eco-friendly construction materials, which aligns with the sustainable development goals.

1. Introduction

Concrete is the most widely used manufactured material in the construction of housing and infrastructure [1,2] due to its advantages, including durability, strength, cost effectiveness, local availability, and constructability [3,4,5]. However, its high embodied energy, as well as its high demand, have caused severe environmental problems, due to the generation of carbon dioxide (CO2), large-scale mining of natural aggregate, and accumulation of construction and demolition waste, among other issues [6,7]. For example, the construction industry was responsible for the emission of 11.7 gigatonnes of CO2, equivalent to 37% of the global total, in 2020 [8]; of this, approximately 8% was generated by cement production [9], due to the high consumption of this binder, which reaches approximately 4.2 billion tons annually. Additionally, nearly 40 billion tons of aggregates (both fine and coarse) are used annually for concrete production [10,11]. To mitigate the environmental impacts generated by the construction industry, researchers and industry practitioners have proposed different strategies to reduce the negative effects linked to concrete manufacturing [12,13,14]. In the field of concrete technology, various promising alternatives have arisen, including, but not limited to, the following: (i) the use of recycled aggregate as a replacement for natural aggregate [15,16]; and (ii) the use of supplementary cementitious materials (SCMs) as a partial replacement for cement, such as coconut shell ash, calcined clay, masonry residue, copper slag (CS), sugarcane bagasse ash, ground granulated blast furnace slag (GGBFS), and fly ash (FA), which have been shown to improve compressive, flexural, and durability characteristics [17,18,19,20,21,22,23,24].
The use of SCMs such as FA [25], GGBFS [1], silica fume [26], and metakaolin (MK) [27] in cement-based materials (i.e., cement pastes, mortar, and concrete mixtures) as partial replacements offers several advantages, such as improved mechanical performance and long-term durability. At the same time, in the search for sustainable alternatives to traditional SCMs, and in response to the high volume of CS generated by copper industries, the valorization of this waste/by-product is becoming increasingly attractive [19,28,29]. CS is a waste/by-product of pure copper refining and smelting [30]. Worldwide, about 68.8 million tons of CS are generated annually [31], and this high amount is due to the fact that for each ton of copper metal produced, 2.2 to 3 tons of CS are generated [3,32]. In detail, CS consists of several oxides, such as iron oxide (Fe2O3), silica (SiO2), alumina (Al2O3), limestone (CaO), magnesium oxide (MgO), and other metals in smaller quantities [33,34]. The composition of CS depends on the smelting method. The use of CS as an alternative SCM to those previously mentioned has shown initial promising benefits in the production of new concrete mixtures, such as a decrease in the release of heat accumulated during hydration, the improvement of flowability, and a reduction in absorption and permeability [3,35]. For instance, the lower accumulated heat release during hydration (when compared to other SCMs, such as silica fume) is evidence of a reduction in the mechanical performance at short ages exhibited by concrete mixtures with CS. However, this effect could be mitigated by further reducing its particle size to increase its surface area (enhancing the activation of CS) and performing rapid cooling upon generation. CS specifically delays the setting of the paste by maintaining the same water content in the mixture (this was determined in this same CS in a parallel study carried out by isothermal calorimetry).
An interesting example of the use of CS in cement-based materials is a study by Gopalakrishnan and Nithiyanantham [36], which evaluated the use of CS as SCM in mortar mixtures. The authors stated that the incorporation of CS (30 wt% replacing of ordinary Portland cement (OPC)) improved compressive strength and significantly reduced porosity after 28 days of curing, with an 11% increase in mechanical strength and a 50% decrease in porosity compared to a reference mixture (100% OPC). However, some studies have reported another behavior in relation to mechanical performance, with a negative effect observed when incorporating CS into concrete mixtures at an early age. For example, Mirhosseini et al. [37] used different percentages of CS (from 20 to 40 wt%) in concrete mixtures, finding a loss of strength in all the mixtures with CS. The losses exceeded 25% when compared to a reference mixture without CS.
On a related note, the use of recycled aggregate from concrete construction and demolition waste (CDW) as a partial or full replacement for natural aggregates provides a viable solution for reducing the demand for virgin materials and carbon emissions while also minimizing the need to dispose of CDW in landfills [38,39,40]. Since aggregates constitute between 60% and 70% of concrete mixtures, this creates a demand of several billion tons of aggregates annually. For instance, in China, 17.89 billion tons of aggregates were used in the production of concrete in 2021 [41]. Nevertheless, recycled coarse aggregate (RCA) from CDW, unlike natural coarse aggregate (NCA), is characterized by adhered mortar, which leads to higher absorption, lower density, greater porosity, an unsatisfactory crushing index, and the presence of an interfacial transition zone (ITZ) [7,15,42]. The quality of RCA determines its application; high-quality RCA is suitable for structural concrete, whereas low-quality RCA is limited to non-structural concrete or use in roadbed fill. Among the factors influencing RCA quality, Xia and Zhao [43] investigated the effect of the ITZ and the old mortar adhered to RCA by preparing concrete mixtures with different water-to-cement ratios (0.4, 0.5, and 0.6) and mortar contents. It was found that the recycled aggregates from concrete with a lower water-to-cement ratio presented enhanced compressive strength performance. In response to the current need to develop more sustainable concrete mixtures and explore alternative materials with reduced environmental impacts, this research analyzed the dual effect of using CS as SCM and recycled aggregate from CDW as a coarse aggregate in the design of new concrete mixtures, an area that remains underexplored in both the short and long term. To the best of the authors’ knowledge, this is the first study evaluating the combined effect of CS and RCA in the mechanical/durability and environmental performance of concrete mixtures in both the short and long term. Moreover, this study also provides a comprehensive understanding of the benefits and disadvantages of the combined use of this by-product and CDW. Consequently, the objective of this work is to assess the combined effects of increasing the dosages of CS (0% and 15% vol. replacement of OPC) and RCA (0%, 20%, 50%, and 100% vol. replacement of NCA) in concrete mixtures (including a control mixture with only OPC and 100% NCA), evaluating the following features: (i) workability (slump); (ii) mechanical properties in the short and long term (i.e., compressive strength, flexural strength, and indirect tensile strength); (iii) permeability properties in the short and long term (i.e., absorption and capillary suction); and (iv) environmental impacts (i.e., amount of CO2 generated in each of the concrete mixtures). To achieve the above-mentioned objectives, this paper is organized as follows: Section 2 describes the concrete constituent materials, the design mixtures, the experimental methods, and the carbon footprint assessment methodology employed in this study. Detailed results and a discussion are provided in Section 3. Finally, Section 4 presents the main conclusions and future work recommendations of this study.

2. Materials and Methods

In this study, OPC type I, according to ASTM C150 [44], was utilized as a cementitious binder. The CS used as SCM was obtained from the Codelco company, located in Valparaíso, Chile the world’s largest copper producer [45]. The chemical composition, determined using an X-ray fluorescence spectrometer (XRF), and the morphology of the particles of OPC and CS are presented in Table 1 and Figure 1, respectively. It can be observed that the main oxides of the CS are Fe2O3, SiO2, and Al2O3, and the morphology of this waste/by-product after milling corresponds to irregular and angular particles. Similarly, the OPC exhibits comparable morphological characteristics, including an irregular shape and a range of particle sizes.
Figure 2 shows the particle size distribution of OPC and CS, as determined by laser diffraction. The median particle sizes of OPC and CS were 26.48 µm and 17.37 µm, respectively. This is further illustrated in Figure 2, where a significant percentage of larger particles can be observed in the granulometry of the CS (red line).
Another technique used to characterize the pozzolanic activity of CS through the quantification of portlandite (CH) was thermogravimetric analysis (TGA). The first derivatives of the thermogravimetric curves (DTG curves) were used to determine the decomposition temperature ranges of the reaction products formed. Powder samples (reference: paste with 100% OPC; and 20% CS: paste with 80% OPC–20% CS) were analyzed at a heating rate of 10 °C/min in a temperature range of 30 to 1000 °C and a nitrogen atmosphere (it is important to note that typically, the pozzolanic properties of SCMs are evaluated using a 20% replacement of cement [46,47]; however, other proportions can also be used). The percentage of fixed lime (CH (%)) was estimated according to Equation (1) [48].
F i x e d   C H   ( % ) = C H c × C % − C H p C H c × C %
where
CHc: amount of CH in the control paste for a given curing time,
CHp: amount of CH in the pozzolan paste at the same age,
C%: proportion of cement in the paste.
Figure 3 illustrates three distinct peaks: (i) the first peak, attributed to the evaporation of free water and the dehydration of C-S-H and C-A-S-H gels and ettringite (E); (ii) the second peak, corresponding to the dehydration of CH (the peak of interest for the calculation of fixed lime (%)); and (iii) the third peak, corresponding to the results from the decarbonation of CaCO3. The amount of CH at 28 days of curing for the reference and 20% CS mixtures was 12.47 and 9.61, respectively, which is equivalent to a CH fixation of 3.33%. This low fixation value indicates a limited pozzolanic performance of CS at early ages [49].
Local aggregates were utilized for the manufacture of the concrete mixtures. The natural fine aggregate (NFA) consisted of natural medium river sand with a fineness modulus of 2.58, a bulk density of 2590 kg/m3, and a water absorption rate of 1.62%. The NCA was crushed gravel with a maximum nominal size of 12.5 mm, and the RCA originated from the demolition of concrete structural elements of a 40-year-old building in Santiago, Chile (Figure 4). These elements exhibited a strength of 38 ± 2,1 MPa, as measured via concrete core tests. The particle gradations of the NFA, NCA, and RCA, and the physical properties of the coarse aggregate, are shown in Figure 5 and Table 2. It is worth mentioning that the ITZ of the RCA was not characterized, but the medium-to-low quality of the RCA could be observed by the result of high fragmentation resistance obtained using the Los Angeles abrasion test (27.98), which was approximately twice as high as the value obtained for the NCA (14.43). Furthermore, a superplasticizer (SP) of the polycarboxylate retarding type, with a water-reducing ratio of 30–40% and density of 1050 kg/m3, was incorporated in this study.

2.1. Concrete Mixture Designs

Five distinct concrete mixture designs were developed to investigate the effects of CS and RCA on the fresh and hardened properties of concrete. These mixtures incorporated varying levels of replacement for OPC with CS (0 and 15% by volume), and NCA with RCA (0, 20, 50, and 100% by volume). The 15% replacement level for OPC with CS was chosen based on prior research [3], which demonstrated that higher substitution rates resulted in significant compressive strength losses compared to a reference mixture containing 100% OPC. To ensure consistency in the mix design proportion, adjustments were made to the quantities of water and aggregates (natural and recycled), based on the actual moisture content of the aggregates. Table 3 presents the detailed proportions for each concrete mixture. It is important to note that the water content (196 kg/m3), SP dosage (2.4 kg/m3), and NFA quantity (889.8 kg/m3) were kept constant across all the mixtures. Additionally, corrections for aggregate (NFA, NCA, and RCA) moisture content were systematically performed.
The mixing process followed the following steps. (i) The NCA was pre-wetted by mixing it with water for 30 s, followed by a 300 s rest period to allow maximum water absorption by the NCA and/or RCA; (ii) sand was added and mixed for 60 s. (iii) The OPC was added and mixed for 120 s to homogenize all the particulate material. For the mixtures containing CS, OPC and CS were pre-mixed in a dry state before being introduced into the mixer. (iv) The water and the SP were added and mixed for 180 s (all the materials in the mix). (v) The mixer was stopped for two minutes, after which it was restarted for an additional 60 s. The homogenization of the mixture was checked at this stage.
For specimen preparation (cylindrical and prismatic specimens), molds were meticulously cleaned, and a release agent was applied a day prior to use. After filling the molds and compacting the mixtures, the specimens were leveled and left covered in the molds for 24 ± 2 h before demolding. Subsequently, they were submerged in lime-saturated water until the designated testing period.

2.2. Experimental Methods

A slump test of fresh mixture was performed in accordance with ASTM C 143 [53] to evaluate the influence of CS and RCA on the workability of the mixtures. In their hardened state, the compressive strength of the different mixtures was determined using cylindrical specimens (100 mm × 200 mm) tested with a universal testing machine, following the guidelines of ASTM C33 [54], at 7, 28, 120, and 180 days of curing. The splitting tensile strength of the concrete cylinders (100 mm × 200 mm) was assessed according to ASTM C496 specifications [55] at 28, 120, and 180 days of curing. The flexural strength of the different mixtures was evaluated using prismatic specimens (150 mm × 150 mm × 300 mm) in compliance with ASTM C293 [56] at 28 and 180 days of curing. Three concrete samples from each mixture and each curing age were used for the evaluation of each mechanical property.
In addition, the porosity and absorption, as well as the capillary suction, of the mixtures were evaluated based on ASTM C642 [57] and ASTM C1585 [58], respectively, for specimens cured for 28 and 180 days.

2.3. Carbon Footprint Analysis

Analysis of the carbon footprint of the designed concrete mixtures was performed by utilizing primary experimental data and secondary data extracted from the Inventory of Carbon and Energy (ICE) database [59]. The ICE provides a rigorous and dependable repository of data concerning embodied energy and carbon, offering extensive information on conventional construction materials within the cradle-to-gate boundary [60,61]. The analysis focused on estimating the emissions associated with the production of the raw materials used in the concrete mixtures. Figure 6 illustrates the cradle-to-gate life cycle assessment (LCA) (modules A1–A3) for the concrete mixtures incorporating CS and RCA.
It is worth mentioning that the cradle-to-gate LCAs developed for each mixture did not account for the trade-off between the reduction in carbon footprint and loss in mechanical properties (expressed as kg·CO2·eq and kg·CO2·eq/MPa), including compressive strength, flexural strength, and indirect tensile strength. This trade-off analysis would require defining specific structural applications for the mixtures, which is beyond the scope of this study, and is recommended for future investigations.

3. Results

3.1. Slump Test

Figure 7 shows the effect on slump of CS and RCA in the mixtures (where the bars represent the average values, and the error bars indicate one standard deviation above and below the mean). The same representation is used in Figure 7, Figure 8 and Figure 9, showing the results of the slump test performed on the different concrete mixtures. The substitution of OPC with CS (M2) resulted in an increase in slump when compared to the reference mixture with 100% OPC (M1). This behavior is attributed to the fact that CS exhibits poor water absorption due to its glassy texture. Additionally, the average particle size of CS (26.48 µm) was larger than that of OPC (17.37 µm), leading to the CS particles having a smaller specific surface area, and therefore requiring less water to wet their surface [62,63]. It is worth noting that further evaluations, such as particle packing density, could be conducted to assess the impact of CS reactivity and its contribution to the long-term strength of concrete. Such evaluations are recommended for future research.
On the other hand, for the concrete mixtures containing 50% and 100% RCA, there was a pronounced reduction in slump, indicating a loss in workability as the RCA content increased. The most significant reduction was observed in the mixture with 100% RCA (M5), with decreases of 17% and 24% compared to mixtures M1 and M2, respectively. The slump reduction observed in M4 and M5 is attributed to the angularity and surface roughness of the RCA, which are likely intensified by the crushing process [17,53].
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Figure 7. Effect on slump of the CS and RCA in concrete.
Figure 7. Effect on slump of the CS and RCA in concrete.
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3.2. Compressive Strength

The influence of CS and RCA on the compressive strength of the different concrete mixtures is presented in Figure 8. It can be seen that the average strength of all the mixtures improved with prolonged curing, due to the increased degree of cement hydration and the contribution of CS as a cement substitute. These factors collectively generated a more compact microstructure [3,64]. Furthermore, as seen in Figure 8, when comparing the mixture with 100% OPC (M1) and the M2 mixture (in which only the OPC was replaced by 15% CS), a compressive strength reduction of 17.8%, 8.51%, and 1.93% was observed at 7, 28, and 120 days of curing, respectively. This early-age strength reduction can be attributed to the low pozzolanic reactivity of CS during early hydration, resulting in a dilution effect. The dilution effect of CS led to reduced C-S-H formation due to the lower alite and belite content of the mixture, as well as an increase in the effective water–cement ratio because CS has a lower water demand [49,65]. However, at a longer age (180 days of curing), the average compressive strength of M2 exhibited a 1.2% increase compared to M1.
On the other hand, the mixtures containing RCA (M3, M4, and M5), compared to the mixtures without RCA (M1 and M2), demonstrated a progressive strength decrease as the RCA content increased, at all the curing ages evaluated. At 28 days of curing, comparing mixtures M3, M4, and M5 with M2 revealed strength losses of 2.93%, 10.5%, and 15.4%, respectively. By 180 days of curing, this strength loss was more pronounced, ranging from 2.7% to 25%, when comparing the RCA mixtures with M2. This behavior is attributed to the influence of the old ITZ in RCA, which governs the concrete failure. The influence of the RCA became more pronounced as the strength of the new concrete mixtures exceeded the strength of the original concrete from which the RCA was derived.
In general, the strength reduction was caused by the adhered mortar in the RCA and the ITZ, which lowered the mechanical and physical quality of this type of aggregate. RCA typically has a lower density and Los Angeles abrasion resistance, as well as a higher absorption capacity and porosity [66]. Additionally, concrete mixtures with RCA exhibited two different ITZs: (i) between the natural aggregate and the old mortar; and (ii) between the RCA and the new cementitious matrix. These dual ITZs resulted in reduced strength, as the ITZ is considered the weakest region in concrete, and is where cracking typically initiates [67].
It is important to note that countries with developed construction standards have national guidelines or limits for maximum RCA content. For example, Australia, Brazil, and Germany limit the RCA content to 30%, 20%, and 25% or 35%, respectively, to balance mechanical performance and durability with the environmental benefits of reduced natural aggregate usage [68]. These recommended limits align with the performance observed in mixture M3, the compressive strength of which remained largely unaffected by the use of 20% RCA.
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Figure 8. Compressive strength of concrete with CS and different proportions of RCA at different curing ages.
Figure 8. Compressive strength of concrete with CS and different proportions of RCA at different curing ages.
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3.3. Splitting Tensile Strength

The evaluation of tensile strength helps to determine the stress level at which cracking leads to concrete failure [69,70]. Figure 9 presents the indirect tensile strength results for the different concrete mixtures. At 28 days of curing, the mixture with 15% CS (M2) showed a higher average strength compared to the reference mixture (M1), reaching 4.32 MPa (an increase of 4.1%). This behavior was maintained regardless of the curing time, despite the slow reaction rate of CS. This improvement is attributed to the sharp angular edges of CS particles, which enhances strength [3,31,71]. On the other hand, the concretes with RCA at 28 days of curing showed a progressive reduction in tensile strength as the RCA content increased. This behavior can be attributed to the weaker microstructure of RCA, caused by the adhered mortar on the original aggregate, in comparison with NCA [41]. Therefore, the volume of ITZ increased in the concrete mixtures as the RCA content increased, leading to a reduction in indirect tensile strength. At 28 and 180 days of curing, the concrete mixtures with 50% RCA (M4) and 100% RCA (M5) exhibited decreases of 9.5% and 14.8, and 5.25% and 11.98%, respectively, compared to the M2 mixture. In addition, it is important to note that mixture M3 exhibited the best tensile strength performance at prolonged curing ages (120 and 180 days). This behavior is attributed to the microstructural modifications in RCA pores [72,73], as well as the angularity and rougher surface of RCA, which generates a better mechanical interlocking between the cement paste and this type of aggregate, minimizing the negative effects of adhered mortar [74,75]. Furthermore, mixture M3 had a lower overall ITZ volume compared to M4 and M5. A similar behavior was observed by Megharani et al. [76], in whose study concrete mixtures containing 10–30% RCA demonstrated higher indirect tensile strengths than the reference mixture.
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Figure 9. Splitting tensile strength of concrete with CS and different proportions of RCA at different curing ages.
Figure 9. Splitting tensile strength of concrete with CS and different proportions of RCA at different curing ages.
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3.4. Flexural Strength

The influence of CS and RCA on the flexural strength of the concrete mixtures at 28 and 180 days of curing is shown in Figure 10. At 28 days, a reduction in flexural strength was observed in the mixtures containing recycled materials (CS and RCA) compared to the reference mixture (M1). This reduction was more pronounced in the mixture with total replacement of NCA by RCA (M5), with a 15.2% decrease in flexural strength. However, this reduction in flexural strength was less severe than the compressive strength loss. This difference can be attributed to the greater angularity of the RCA, due to the adhered mortar, compared to the NCA. This angularity enhanced the bond between RCA and the new cementitious matrix, which is the primary factor affecting flexural strength [64,77].
At 180 days of curing, the M3 mixture exhibited the best flexural performance among all mixtures, achieving a 5.8% higher strength than the reference mixture (M1). This improvement can be attributed to the continued development of flexural strength over time, which enhanced the adhesion between the new cementitious matrix and the RCA surface due to its higher roughness and the internal curing effect provided by RCA’s higher water absorption capacity [78]. However, despite the improved performance of the new ITZ, when 100% RCA (M5) was used, the overall performance of the concrete mixture was largely governed by the quality of the RCA. This was due to the higher volume of interface in the mixture’s composition. As a result, the RCA content in concrete designs requires special consideration.

3.5. Density, Absorption, and Voids

Table 4 presents the density, absorption, and voids test results of the different mixtures at 28 and 180 days of curing. At 28 days, it is evident that the use of CS resulted in an increase in density, attributed to the higher density of this material compared to OPC. A similar effect was observed with the use of RCA, which has lower density than NCA. The concrete mixtures exhibited progressively lower density values as the RCA content increased. This can be explained by the correlation between the macro porosity of the RCA [79] and the reduced density in the mixtures with this recycled material (RCA). Furthermore, this is also related to the greater water loss in mixtures with RCA due to their higher absorption capacity [80], which led to a higher average porosity ranging from 12.7% to 34.6%, and 10.47% to 31.9%, in comparison to M1 and M2 mixtures (concretes without RCA), respectively. A similar behavior was reported by Kapoor et al. [81], who observed greater water absorption in self-compacting concrete (SCC) containing 50% and 100% of RCA compared to SCC without RCA. At 180 days of curing, all the mixtures exhibited higher densities and reduced water absorption and voids adsorption compared to their 28 days counterparts. This improvement can be attributed to the increased degree of hydration in all the mixtures, that generated a higher formation of hydration products, such as C-S-H, which filled the pores, consequently decreasing water absorption and porosity [82].

3.6. Capillary Water Absorption (Sorptivity)

Figure 11 presents the initial and secondary absorption rates of the different concrete mixtures at 28 and 180 days of curing. These rates are essential coefficients for estimating the service life of concrete, with a lower sorptivity value indicating an increase in service life [83]. In Figure 11a, it can be observed that the use of CS and RCA in the design of the concrete mixtures led to an increase in the rate of capillary water absorption at 28 days. When comparing mixture M1 to the other concrete mixtures, the initial water absorption rate increased from 1.8% to 52.6% with the use of CS and the total replacement of NCA by RCA, with the lowest rate being 0.0116 mm/s1/2 and the highest rate being 0.0177 mm/s1/2 for M1 and M5, respectively. The presence of high amounts of RCA significantly influenced the increase in the amount of water absorbed. The increased sorptivity in mixtures containing recycled materials can be attributed to their pore structure, higher effective porosity [71], and the dilution effect [84] caused at 28 days of curing by the partial replacement of OPC with CS.
At 180 days of curing (Figure 11b), a reduction in capillary water absorption was observed in all the mixtures. In detail, the capillary water absorption values at 180 days for M1 to M5 were 39.8%, 43.4%, 45.4%, 50.2%, and 45.4%, when compared to their counterparts cured for 28 days, respectively. These reductions were due to adequate curing, which promoted a higher degree of hydration of the OPC, resulting in greater formation of C-S-H gels, which reduced the capillary pores and improved the resistance to water access [85]. In addition, at 180 days of curing, the beneficial effect of CS as an SCM was seen, as it contributed to a more refined capillary pore structure, leading to reduced water absorption [3].

3.7. Carbon Footprint Results of the Concrete Mixtures

As outlined in the methodology in Section 2.3, the embodied carbon (EC) of each concrete mixture was estimated using a cradle-to-gate LCA approach encompassing modules A1 to A3, as illustrated in Figure 6. As previously mentioned, these mixtures incorporate 15% recycled CS as a replacement for OPC, and varying proportions of crushed recycled concrete as aggregate at 0%, 20%, 50%, and 100% substitution levels. Recognizing that transportation distances significantly contribute to environmental impacts [86], two production scenarios were analyzed. The first scenario assumed concrete production in Santiago (Chile), which had specific distances from the cement plant and the CS supplier. The second scenario considered concrete production in Viña del Mar (Chile), reflecting different transportation distances. These scenarios were chosen to represent the current locations of material suppliers and to assess how transportation distances affect overall environmental impacts. For RCA, a maximum acquisition radius of 5 km within each city was assumed. The locations analyzed are depicted in Figure 12, in which the CS supplier is marked with a red icon, the nearby cement plants are marked with yellow icons, and the cities are marked with blue icons.
Considering these two scenarios, Figure 13 illustrates the corresponding results for the EC assessment. The data show that producing concrete with recycled components in Santiago does not result in significant environmental benefits compared to production in Viña del Mar, which is located closer to material suppliers. In the optimal case for the Viña del Mar location, a reduction of approximately 6% in EC was observed when CS was added to the mixture and no natural aggregate was used. This highlights the significant impact of transport distances on the EC of construction materials. Furthermore, the analysis of RCA substitution for Viña del Mar revealed that substituting NCA with RCA did not significantly reduce the carbon footprint. Notably, as the substitution rate increased from 0% to 100%, emissions slightly increased. This finding aligns with previous research, which found that replacing NCA with RCA at proportions of 50% and 100% leads to higher carbon emissions [87]. However, some studies report contradictory results, showing that CO2 generation per m3 of concrete can decrease with the use of RCA [8,88].
Therefore, depending on transport distances, the most significant reductions in EC were achieved by substituting only the cement constituent, which accounts for nearly 95% of the carbon burden in concrete [89], with CS. The impact of the amount of OPC has been widely recognized in several studies [90,91], indicating that the EC is mainly influenced by the binder phase. Consequently, greater reductions in EC could be achieved with higher levels of OPC substitution. This highlights the advantages of using locally sourced by-products or waste materials with pozzolanic activity as partial cement substitutes, which offers both environmental and economic benefits [92]. On the other hand, the results indicate that incorporating RCA materials into concrete does not always yield environmental benefits. To achieve substantial reductions in concrete-related emissions, focus should be placed on replacing cement, minimizing transport distances, and reducing the carbon emission factors associated with energy use, both in transportation and manufacturing processes. For instance, utilizing clean energy in the crushing of recycled aggregate and the grinding of CS will result in lower associated carbon emissions. This is particularly important because the emissions associated with the grinding process in the study accounted for 6.7% to 7.3% of the total carbon emissions across the analyzed mixtures. These considerations should guide decision-making regarding the selection of raw materials and energy sources for construction products.
It is crucial to highlight that carbon emissions are strongly influenced by the manufacturing process and the type of energy used (e.g., transportation by truck and operational activities), leading to significant variations in carbon emissions related to materials [91], and recycling constituents for concrete production is not an exception to this. As shown in Figure 13, the resulting emission rates ranged from a minimum of 375.2 kgCO2e/m3 to a maximum of 400.2 kgCO2e/m3. According to previous research, these values fall within the expected range for concrete that incorporates recycled constituents [92].
In addition to the calculation of kgCO2·eq, the ratio of kg·CO2·eq/MPa was determined, considering the compressive strength at 28 and 180 days of curing, as illustrated in Figure 14. This behavior was evaluated in three concrete mixtures (M1, M2, and M5), which showed lower values of kgCO2·eq/MPa at 180 days due to the development of hydration products and pozzolanic reactions evident at longer curing ages. At 28 days of curing, the lowest kg·CO2·eq/MPa values were presented by mixture M1, since it had the highest compressive strength. However, after 180 days of curing, mixture M2 exhibited the lowest value, as it had the highest compressive strength at this age, while also generating the least kg·CO2·eq in Viña del Mar, with its application close to the CS source. In contrast, the M5 mixture (with CS and 100% RCA), at both curing ages, showed the highest kgCO2·eq/MPa ratio. This is because the use of RCA as a total replacement for NCA leads to a loss of compressive strength.

4. Conclusions

This study investigated the fresh and hardened performance (both short- and long-term) of concrete mixtures with the combined partial replacement of ordinary Portland cement (OPC) with copper slag (CS), and of natural coarse aggregate (NCA) with recycled coarse aggregate (RCA) from concrete construction and demolition waste (CDW), and analyzed the embodied carbon (EC) of each mixture. This research confirms that it is feasible to use CS as a supplementary cementitious material (SCM) partially replacing OPC, and to use RCA as replacement for NCA, in the same concrete mixture. Based on the experimental results and analysis of this research, the following conclusions can be made:
  • The replacement of 15% by vol. of OPC with CS is viable, despite the dilution effect that results in a loss of compressive strength at early ages (7 and 28 days of curing). However, at longer curing periods (120 and 180 days), the difference in compressive strength between the 100% OPC mixture (M1) and the 15% CS mixture (M2) became insignificant. Notably, at 180 days, mixture M2 achieved an average compressive strength that was slightly higher (1.2%) compared to M1.
  • The favorable performances of the M2 and M3 (15% CS and 20% RCA) mixtures in terms of indirect tensile strength and flexural strength suggest potential applications in engineering contexts, such as pavements, for which concrete flexural strength performance is critical. This improvement can be explained by the increased angularity and roughness of CS and RCA, compared to OPC and NCA, respectively.
  • The negative effect of RCA on the mechanical properties of concrete was most significant in terms of compressive strength performance, compared to indirect tensile and flexural strength performance. For example, after 180 days of curing, when comparing mixture M2 (15% CS and 0% RCA) with M5 (15% CS and 100% RCA), the compressive, indirect tensile, and flexural strengths decreased by 24.1%, 11.98%, and 8.05%, respectively.
  • Porosity, absorption, and sorptivity were negatively affected by high RCA contents (50% and 100%) in the concrete mixtures at both the evaluated ages (28 and 180 days of curing). However, mixture M2 (15% CS and 0% RCA) exhibited 2.87% less porosity at 180 days of curing compared to the reference mixture (M1), which can be attributed to the slow pozzolanic effect of CS.
  • From the EC analyses, major environmental benefits were obtained through partially replacing OPC with CS, while the substitution of NCA with RCA had a minor impact.
  • The development of more sustainable concrete mixtures can be achieved with higher substitution rates of OPC with CS. However, it is essential to address the low reactivity of CS. Striking a balance between CO2 emission reduction and potential impacts on mechanical performance and durability is crucial. Additionally, future research should focus on methods to enhance the reactivity of CS in cementitious matrices, optimizing its performance as an SCM.

Author Contributions

Conceptualization, P.W.C.A., Y.F.S., G.A.-L. and H.H.; methodology, P.W.C.A., Y.F.S. and G.A.-L.; Software, Y.F.S.; validation, P.W.C.A., Y.F.S., G.A.-L. and H.H.; formal analysis, P.W.C.A., Y.F.S., G.A.-L. and H.H.; investigation, P.W.C.A., Y.F.S., G.A.-L.; resources, Y.F.S.; data curation, P.W.C.A., Y.F.S. and G.A.-L.; writing—original draft preparation, P.W.C.A., Y.F.S., G.A.-L. and H.H.; writing—review and editing, Y.F.S. and G.A.-L. visualization, Y.F.S. and G.A.-L.; supervision, Y.F.S. and G.A.-L.; project administration, Y.F.S.; funding acquisition, Y.F.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Fondecyt de Iniciación N°11221114 and The APC was funded by Fondecyt de Iniciación.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this research are available on request from the corresponding author.

Acknowledgments

The authors would like to acknowledge the support of the Agencia Nacional de Investigación y Desarrollo (ANID) through grant FONDECYT iniciacion’ #11221114, the Academic Insertion Program of the Pontificia Universidad Católica de Chile (PIA UC), the Concrete Innovation Hub UC (CIHUC), Codelco, and Sika.

Conflicts of Interest

The authors declare no conflict of interest.

Nomenclature

Al2O3Alumina
CaOLimestone
CDWConstruction and demolition waste
CO2Carbon dioxide
CSCopper slag
ECEmbodied carbon
FAFly ash
Fe2O3Iron oxide
GGBFSGranulated blast-furnace slag
ICEInventory of Carbon and Energy
IDIdentification code of concrete mixtures
NCANatural coarse aggregate
NFANatural fine aggregate
M1Concrete mixture with 100% OPC
M2Concrete mixture with 15% CS
M3Concrete mixture with 15% CS and 20% RCA
M4Concrete mixture with 15% CS and 50% RCA
M5Concrete mixture with 15% CS and 100% RCA
MgOMagnesium oxide
ITZInterfacial transition zone
LOILoss on ignition
LCALife cycle assessment
OPCOrdinary Portland cement
SCMSupplementary cementitious material
SiO2Silica
RCARecycled coarse aggregate
SPSuper plasticizer
XRFX-ray fluorescence

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Figure 1. SEM micrographs at 1000× magnification of (a) CS and (b) OPC.
Figure 1. SEM micrographs at 1000× magnification of (a) CS and (b) OPC.
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Figure 2. Particle size distribution of OPC and CS.
Figure 2. Particle size distribution of OPC and CS.
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Figure 3. DTG curves of pastes 100% OPC (Ref.) and 80% OPC–20% CS (20% CS).
Figure 3. DTG curves of pastes 100% OPC (Ref.) and 80% OPC–20% CS (20% CS).
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Figure 4. Concrete wastes that were crushed to produce the RCA.
Figure 4. Concrete wastes that were crushed to produce the RCA.
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Figure 5. Percentage passing of NFA, NCA, and RCA.
Figure 5. Percentage passing of NFA, NCA, and RCA.
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Figure 6. Processes included in this cradle-to-gate LCA study within the framework of the entire LCA of a construction project.
Figure 6. Processes included in this cradle-to-gate LCA study within the framework of the entire LCA of a construction project.
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Figure 10. Flexural strength of concrete with CS and different proportions of RCA at different curing ages.
Figure 10. Flexural strength of concrete with CS and different proportions of RCA at different curing ages.
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Figure 11. Water absorption of different concrete mixtures, (a) 28 days and (b) 180 days of curing.
Figure 11. Water absorption of different concrete mixtures, (a) 28 days and (b) 180 days of curing.
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Figure 12. Locations of the CS supplier (red mark) and cement plant (yellow mark), and the cities in which concrete mixtures are assumed to be produced (blue mark). Obtained from Google Maps.
Figure 12. Locations of the CS supplier (red mark) and cement plant (yellow mark), and the cities in which concrete mixtures are assumed to be produced (blue mark). Obtained from Google Maps.
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Figure 13. Comparison of the embodied carbon in the mixtures with CA–RCA and the reference mixture.
Figure 13. Comparison of the embodied carbon in the mixtures with CA–RCA and the reference mixture.
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Figure 14. kg·CO2·eq/MPa ratio calculated for concretes M1, M2, and M5.
Figure 14. kg·CO2·eq/MPa ratio calculated for concretes M1, M2, and M5.
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Table 1. Chemical composition of CS and OPC.
Table 1. Chemical composition of CS and OPC.
Chemical CompositionCSOPC
Fe2O359.153.42
SiO229.1525.08
CaO2.8459.15
Al2O35.845.12
CuO1.34-
SO31.152.73
K2O1.280.77
TiO20.480.28
Na2O1.020.13
PbO0.20-
LOI *−5.161.99
* LOI: lost on ignition.
Table 2. Physical and mechanical properties of NCA and RCA.
Table 2. Physical and mechanical properties of NCA and RCA.
CharacteristicStandardAggregate
NCARCA
Density (kg/m3)ASTM C127 [50]26202290
Water absorption (%)ASTM C127 [50]1.716.83
Nominal maximum size (mm)ASTM C125 [51]12.712.7
Abrasion resistanceASTM C131 [52]14.4327.98
Table 3. Mixture proportioning of concrete with CS and RCA, and identification code (ID).
Table 3. Mixture proportioning of concrete with CS and RCA, and identification code (ID).
MixturesMIX
ID		Vol. Replacement Level (%) of NCA by RCAOPC (kg/m3)SC
(kg/m3)		Water
(kg/m3)		NFA (kg/m3)NCA
(kg/m3)		RCA
(kg/m3)		SP
(kg/m3)
ReferenceM104000196889.8830.902.4
15% CSM2034071.4196889.8830.902.4
15% CS–20% RCAM32034071.4196889.8664.7145.22.4
15% CS–50% RCAM45034071.4196889.8415.4363.12.4
15% CS–100% RCAM510034071.4196889.80726.22.4
Table 4. Water absorption, bulk density, and voids of concrete with CS and RCA.
Table 4. Water absorption, bulk density, and voids of concrete with CS and RCA.
Mixture (ID)Water Absorption (%)Bulk Density (kg/m3)Voids
(%)
281802818028180
M17.775.222590261617.0511.28
M27.935.072610263917.5911.17
M38.765.162578261918.6511.43
M49.636.822510254319.3414.16
M510.467.592463248019.6615.46
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MDPI and ACS Style

Arredondo, P.W.C.; Silva, Y.F.; Araya-Letelier, G.; Hernández, H. Valorization of Recycled Aggregate and Copper Slag for Sustainable Concrete Mixtures: Mechanical, Physical, and Environmental Performance. Sustainability 2024, 16, 11239. https://doi.org/10.3390/su162411239

AMA Style

Arredondo PWC, Silva YF, Araya-Letelier G, Hernández H. Valorization of Recycled Aggregate and Copper Slag for Sustainable Concrete Mixtures: Mechanical, Physical, and Environmental Performance. Sustainability. 2024; 16(24):11239. https://doi.org/10.3390/su162411239

Chicago/Turabian Style

Arredondo, Pamela Wendy Caballero, Yimmy Fernando Silva, Gerardo Araya-Letelier, and Héctor Hernández. 2024. "Valorization of Recycled Aggregate and Copper Slag for Sustainable Concrete Mixtures: Mechanical, Physical, and Environmental Performance" Sustainability 16, no. 24: 11239. https://doi.org/10.3390/su162411239

APA Style

Arredondo, P. W. C., Silva, Y. F., Araya-Letelier, G., & Hernández, H. (2024). Valorization of Recycled Aggregate and Copper Slag for Sustainable Concrete Mixtures: Mechanical, Physical, and Environmental Performance. Sustainability, 16(24), 11239. https://doi.org/10.3390/su162411239

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