Engineering Performance of Copper Slag in Sustainable Construction: A Systematic Review

Abstract

Copper slag (CS) was considered a major by-product produced from the copper refining industry, which estimates about 2.2 to 3 tons generated during the production of every one ton of copper. At the same time, continuous dumping and improper disposal of this byproduct have led to serious environmental problems, especially due to the leaching of heavy metals into soil and water. This review carefully studies the potential of CS as a sustainable construction material through a clear distinction of its performance, especially when used as a fine aggregate and as a supplementary cementitious material (SCM). Due to the presence of higher content of iron and silica, higher hardness, and very low water absorption, it was found that CS helps in improving the density and durability of concrete. When used as a fine aggregate, CS enhances workability, strength, and durability at an optimum level of about 40%, mainly due to better particle packing and reduced pore connectivity. On the other hand, when used as an SCM, CS contributes to long-term strength through pozzolanic reactions and the formation of C–S–H gel, but its replacement level should be limited to about 20% to avoid loss of early-age strength caused by reduced alkalinity. In terms of durability, the use of CS can reduce water absorption by up to 60%, lower chloride penetration, and improve resistance to sulfate attack. Environmental Life Cycle Assessment studies show that CS can reduce global warming potential by about 12–19% and also decrease overall energy consumption. Statistical validation using multi-criteria decision analysis (MCDA) and separate regression modeling with an R² value of about 0.965, which supports these optimum replacement levels up to 40% for fine aggregate and 20% for cement, providing a good balance between strength, durability, environmental benefits, and cost. Overall, this review shows that CS is a valuable and multi-functional material that supports circular economy practices when used with a proper mix design based on specific applications.

1. Introduction

Copper slag, an industrial by-product from the copper refinery industry, is generated during the higher temperature at the pyrometallurgical process that includes smelting, converting, and refining of copper minerals [1,2,3,4,5,6], and the rate of copper slag generation was estimated to be about 2.2 to 3 tons of slag produced during the refinement of every ton of copper [3,4,5,6,7,8,9,10,11,12,13,14]. Globally, it was estimated that the deposition of copper slag was about 966 million tons between 2020 and 2024, out of which China alone had a deposition of more than 300 million tons [15,16]. Following this, Qiliang Jin et al. (2025) have investigated the physical characteristics of copper slag, such as its shape, surface texture, and structure, which mainly depend on the method of cooling during its formation [17].

Christopher et al. (2024) have enquired that when the copper slag was cooled slowly, it forms a dense, black, solid structure that resembles a highly crystalline, compact, and hard one, similar to natural basalt [18]. Also, during the rapid cooling or water quenching process, it forms a granular and amorphous structure with a glassy texture that contains small, porous needle-like structures with sharp and irregular edges [5,6,7,10,12]. In terms of chemical and mineralogical behavior, the copper slag was formed when the iron was separated from the iron silicate during the process of smelting. This led to the formation of a complex material structure that is mainly composed of stable iron and silicon oxides, with a low amount of calcium oxides (CaO) [19,20,21]. Among these compounds, the iron oxide was found to be dominant, usually at a range of 20% to 62% on its total weight [22,23] and the presence of pure iron compound was varying between 15.2% and 49.85% [24,25]. Following this, the silicon dioxide (SiO₂) was found to be the next major compound, found in the range of 19.08% to 40% of its total weight [23,26,27]. Godfrey Dzinomwa et al. (2023) have suggested that, although with a glassy phase of slag, the silicon dioxide was estimated as 69.8% [25]. Along with this, based on the raw materials used, the trace of aluminum oxide (Al₂O₃) was found in a notable amount, typically ranging from 0.22% to 15.60% of its weight, which plays a major role in the structure of copper slag [17,18,19,22,23,24,27]. In terms of mineral composition, the microstructure of copper slag was predominantly controlled by spinel-type magnetite (Fe₃O₄) and iron orthosilicate, a mixture of fayalite and Fe₂SiO₄, along with the combination of Fe–Ca aluminosilicates, which made up to 84% of the total weight of the slag with the presence of valuable metallic and sulfide compounds [9,17,18,22,24,27]. Usually, the copper was identified as small spherical particles known as prills with an elongated metallic structure, with a size range between 5 and 20 nm, which were found to be embedded within the glassy silicate structure [9,18,20,24,27]. Due to this, along with copper of about 0.5 to 2.7%, the copper slag also contains other metals such as zinc, nickel, and cobalt that are found comparatively higher than in the natural ore formation [1,2,4,5,7,8,16]. Also, the coarse copper slag particles are mostly found in a bulky, irregular shape with multiple cracks. These features make copper slag more brittle and lead to breaking easily due to the presence of a weak bond between metal and the surrounding silicate phases, and this directly supports the possibility for efficient metal recovery from the slag through further mechanical processing [25].

Similarly, the chemical composition of copper slag (CS) has led to the formation of strong intermolecular bonds, which make the slag material highly stable with excellent mechanical properties, and these characteristics clearly show that the potential of copper slag as a strong and durable alternative to natural aggregates [4,9,18,19,20,21,23]. Copper slag has a relatively high specific gravity in the range of 2.8 to 3.8, with an estimated high bulk density value between 2800 and 3800 kg/m³ [4,17,19,21,22], and this high density is directly related to its chemical composition due to the presence of iron oxide (Fe₂O₃) content, with a strong correlation coefficient value of about 0.94 [17]. In addition to these, the CS shows the ability of very low water absorption around 0.37% and sometimes even less than 1% under exposure at high temperatures [17,19,22]. Copper slag has also exhibited high hardness, with a Mohs scale value of seven shows better resistance towards abrasion, with loss values aboutless than 24.1, and its wear resistance found better, with a wearing coefficient value more than two times as high as that of conventional sand [4,7,17,21,23,28]. In self-compacting concrete (SCC), using CS as a replacement for fine aggregates shows better performance due to the glassy texture and angular shape of CS particles that help to improve flowability and ensure good bonding between the concrete matrices. This glassy and angular interaction of CS particles improves particle packing, which physically densifies the interfacial transition zone (ITZ) and reduces pore connectivity, helping to enhance the compressive strength of the concrete without relying on secondary chemical reactions [9,19]. Global production, chemical composition, physical properties, and environmental characteristics of copper slag reported in various studies are summarized in

Table 1. (a). Physicochemical and Mineralogical Properties of Copper Slag. (b). Impact of Copper Slag on Fresh and Hardened Concrete. (c). Environmental Aspects and Heavy Metal Leaching Behavior.

(a)
Category	Parameter	Typical Range/Value	Scientific and Engineering Significance	Citations
Physical Properties	Specific Gravity	2.8–3.91	Increases concrete density about 2800–3800 kg/m3, contributing to improved strength and stability.	[4,29]
	Hardness (Mohs Scale)	6–7	Provides high abrasion resistance and durability under mechanical loading.	[4,30]
	Water Absorption	0.13–0.5%	Low absorption increases free water content, enhancing workability but requiring mix control.	[31]
	Particle Morphology	Angular, glassy, irregular	Improves mechanical interlocking and internal friction within the matrix.	[17,19]
Chemical Composition	Iron Oxide (Fe2O3)	20.40–62.00%	Governs high density and contributes to stiffness.	[17]
	Silicon Dioxide (SiO2)	19.08–40.00%	Provides potential pozzolanic reactivity when finely ground.	[6,9]
	Aluminum Oxide (Al2O3)	0.22–15.60%	Participates in secondary hydration reactions forming C–A–S–H phases.	[9,17]
Mineralogical Phases	Major Phases	Fayalite (Fe2SiO4), Magnetite (Fe3O4)	Dominant crystalline phases about 80–85%, which contribute to structural stability and chemical inertness.	[9,17]
(b)
Property	CS Role and Replacement Level	Test Conditions/Mix Parameters	Observed Impact and Governing Mechanism	Citations
Workability (Slump)	Fine Aggregate replaced up to 100%	w/b value range from 0.40 to 0.55 through the slump cone test	Significant increase in slump value ranges from 65 to 200 mm due to low water absorption and glassy surface.	[32]
Compressive Strength	Fine Aggregate replaced at 40% optimum	In HPC, the curing duration ranges from 28 to 91 days	Strength increases up to 20–25% due to improved packing and ITZ densification.	[33,34]
Setting Time	SCM replacement	Particle size found less than 30 µm	Initial setting time may extend >250 min due to the dilution effect and low CaO content.	[15,35]
Chloride Permeability	Fine Aggregate replaced at 20–30%	In SCC/UHSC, as per ASTM C1202, the curing period 90–365 days	Reduced permeability value of about 300–500 coulombs due to compact C–S–H structure.	[36,37]
Freeze–Thaw Resistance	Fine Aggregate replaced up to 100%	RPC value about 250 cycles	Lower strength loss compared to control due to reduced pore connectivity and improved matrix integrity.	[38]
(c)
Leaching Context/Test Method		Target Element(s)	Behavior and Environmental Implications	Citations
TCLP (Toxicity Characteristic Leaching Procedure)		Cu, Zn	Untreated slag may exhibit elevated leaching values, found Cu as 64.7 mg/L and Zn as 10.8 mg/L, which require stabilization.	[39]
Acidic- pH ranges between 2 and 3		Pb, As	Maximum leaching occurs under acidic conditions; amorphous slag shows higher release than crystalline slag.	[11,28]
Neutral pH value 4–8		As	Minimum leaching observed; represents relatively stable environmental conditions.	[28]
Alkaline Conditions: pH greater than 10		Ni, Pb, Zn	Increased leaching due to dissolution under high pH conditions.	[11]
Sequential Extraction		Cd	High mobility, with about 66% exchangeable fraction, indicating potential environmental risk if not stabilized.	[11]
Encapsulation in Concrete/Geopolymers		Multiple heavy metals	Significant immobilization through physical encapsulation and chemical bonding (e.g., near-complete immobilization of Pb and Cd).	[40,41]

(a–c) below.

Even though the copper slag (CS) has valuable mineral content and strong mechanical properties, the most common problem is still present in its way of dumping and storing it in open landfills [1,4,6,12,16,26,42,43]. In the traditional method of handling, the copper slag produced under the processing steps of produce, use, and dispose usually requires large areas of land for dumping, leading to serious environmental and ecological problems [1,4,12,43]. While dumping copper slag stock for long period in an open environment, this leads to increase the mobility and release of toxic elements and heavy metals such as arsenic (As), lead (Pb), cadmium (Cd), and zinc (Zn) into nearby soil and groundwater systems through weathering, wind erosion, and surface runoff [4,8,9,10,11,23,42,44]. Several research studies have shown that the leaching of heavy metals from CS strongly depends on the pH conditions. Likely, the heavy arsenic element has been released relatively low within a pH range of 4 to 8 into water bodies under highly acidic conditions [4,8,11,28]. In addition to these, certain metals such as cadmium are easily movable at the rate of 66% and enter into the environment through their rapid exchangeable nature [11]. Izydorczyk et al. (2021) reported that the environmental impacts of such contamination have been clearly observed in different regions, including the formation of toxic dust and smog due to wind erosion [42], which have been estimated cause serious damage to agricultural land and water resources in India [6], and it has been recorded that it has caused fatal health effects in livestock through the consumption of slag contaminated water in Botswana [4]. Therefore, the improper disposal management of CS acts as a long-lasting and non-degradable hazard in the ecosystem, which can affect human health, land quality, and aquatic life [1,9,23,42]. The main aim of this review is to clearly examine and explore the potential of copper slag (CS) as a sustainable and high-performance alternative material in construction through studying its mechanical and durability behavior, environmental impact, and economic benefits. This study also focuses on identifying the most suitable replacement levels with reasons behind improved concrete performance, such as microstructural densification and pozzolanic reactions. The novelty of this work contains a comprehensive and combined study that is based not only on strength and durability, but also on environmental aspects such as impact assessment, stability of heavy metals, and life cycle analysis for a complete understanding of effective CS utilization. More than these, this study has also investigated advanced applications of copper slag beyond normal concrete in soil stabilization, as functional materials, and the green energy field in order to prove that CS is a valuable and multi-purpose resource, more than a waste material, which supports both circular economy and sustainable engineering practices.

2. Review Methodology

A detailed literature search was carried out in this study to ensure clarity, consistency, and scientific reliability in evaluating the performance of copper slag (CS) in construction applications. Major scientific databases such as Scopus, Google Scholar, Springer, and MDPI were used for the search. Different keyword combinations such as “copper slag concrete,” “copper slag as fine aggregate,” “copper slag as cement replacement,” “mechanical properties of copper slag concrete,” and “durability of copper slag concrete” were used, and these keywords were combined using Boolean operators (AND, OR) to obtain more relevant results. The literature search covered studies published between 2005 and 2026. All the collected studies were then imported into the Mendeley Reference manager version 2.144.0 for proper organization and removal of duplicate records. Initially, 234 records were identified, and after removing 45 duplicate entries based on DOI and metadata comparison, a total of 189 unique studies remained for further screening. After this, the selection of studies was carried out following the PRISMA 2020 guidelines and the flowchart created with the open-source flow diagram PRISMA platform, as shown in Figure 1. In addition to manual screening, a machine learning-based method was used through AS Review Lab to improve the efficiency of screening titles and abstracts. Studies were marked as relevant or not relevant step by step, and the algorithm helped to identify the most suitable studies by giving priority to them. The screening process continued until a stopping point was reached, which indicated that very few relevant studies were left to be identified.

Out of the 189 screened records, 89 studies were removed based on defined inclusion and exclusion criteria, and the remaining 100 studies were selected for detailed full-text review and final inclusion. All selected studies were again checked manually to ensure accuracy and reduce possible errors. The important data from the selected studies were carefully collected and organized into a structured database, with the details included as Supplementary Files named Table S1: PRISMA 2020 Checklist for buildings.

The extracted information included the type of concrete or binder used, percentage of copper slag replacement, particle size distribution of CS, water-to-binder ratio (w/b), curing period, mechanical properties such as compressive, tensile, and flexural strength, durability properties such as water absorption, chloride penetration, sulfate resistance, exposure conditions, and testing methods. The key parameters extracted from the selected studies are summarized in

Table 2. Data extraction parameters used for analysis of selected studies.

Parameter	Description
Concrete/Binder Type	Ordinary Portland Cement, blended cement, geopolymer
Copper Slag Replacement (%)	Percentage replacement of fine aggregate or cement
Copper Slag Grading	Particle size distribution and fineness
Water–Binder Ratio (w/b)	Mix proportion
Curing Age	7, 28, 56, 90 days
Mechanical Properties	Compressive, tensile, and flexural strength
Durability Properties	Water absorption, chloride penetration, sulfate resistance
Exposure Conditions	Marine, acidic, sulfate, or normal environment
Testing Methods	ASTM/IS/EN standards followed

, which enables a systematic comparison of concrete composition, mechanical performance, durability characteristics, and environmental conditions across different studies involving copper slag utilization. The quality of each study was evaluated based on clear experimental procedures, completeness of reported data, use of standard testing methods, and consistency of results. Studies that did not provide sufficient technical details or reliable results were excluded during the screening process.

3. Copper Slag as Fine Aggregate Replacement

Usage of copper slag (CS) as a replacement for fine aggregates has significantly altered the workability of fresh concrete, mainly due to its distinct physical properties. As studied earlier, the CS has a smooth, glassy, and non-porous surface, along with very low water absorption, usually in the range of 0.13% to 0.5%, compared to about 1.36% for natural sand [31,34,45,46]. Due to this lower absorption behavior of CS, in the concrete mix, it helps retain more free water and improves flowability and plastic behavior as a fine aggregate. For instance, using 100% CS can help to reduce the requirement of water by up to 22% in order to maintain good workability and to increase the slump value from 65.5 mm to 200 mm, with slump flow reaching up to 735 mm [32,45,46,47,48,49]. During advanced applications in 3D Printable Concrete (3DPC), it was noticed that the full replacement of CS as fine aggregate can help to improve its buildability with a higher static yield stress value of around 2309.9 Pa and longer flow retention, mainly due to the dense structure of CS, which helps to control the bleeding of water in concrete [50]. However, when the replacement level exceeds the optimal range, generally estimated to be more than 40% to 80%, it causes problems such as bleeding and segregation in conventional concrete [29,32]. In addition to these, the effect of CS on workability is based on the type of concrete and the characteristics of the slag. Also, it was noticed that in some cases, the angular particle shape and high specific gravity value about 3.91 of CS have helped to increase internal friction and stiffness. As a result, this may reduce workability, particularly in geopolymer concrete mixtures containing coarser slag particles [29,33,51,52].

Similarly, the mechanical behavior of concrete that contains copper slag (CS) as fine aggregate has mainly depended on the level of replacement used, and most research studies have shown that the mechanical behavior is found to be efficient at the optimum replacement level between 20% and 50% in both the normal and high-performance concrete [31,32,34,47,48,53,54,55]. Further studies identified that, in this range, the CS can significantly improve compressive, split tensile, and flexural strengths. Also, it was noticed that an increase of about 23.58% in 91-day compressive strength at 40% replacement [34], and in the case of geopolymer concrete, the split tensile strength value can improve by up to 49% [55]. This improvement is mainly based on the better packing ability, high stiffness, and angular shape of CS particles, which help to enhance the physical micro-filler effect and provide stronger bonding within the cement matrix by physically densifying the interfacial transition zone (ITZ) [32,33,34,56,57]. During 100% replacement of CS as fine aggregate in both high-strength concrete (HSC) and ultra-high-performance concrete (UHPC), it was observed that the CS has helps to attained higher compressive strength values of 107.4 MPa and 162 MPa at 28 days of curing [45,46]. Also, it was observed that during the usage of CS above the optimum replacement level, the mechanical strength was found to decrease up to 25%. This reduction is mainly due to the fact that the available excess free water was not absorbed by CS, which leads to the formation of capillary pores and increased porosity. Along with this, the presence of impurities found in unprocessed slag also causes a negative effect on concrete strength [31,46,47,49,58]. In order to overcome this limitation, the recent research has suggested introducing the Bacillus subtilis in concrete to initiate the process of microbiologically induced calcium carbonate precipitation (MICP), where calcite crystals are formed and fill the pores within the concrete to achieve an optimum CS replacement level of up to 75% through improved density, and help in achieving a compressive strength of about 45.6 N/mm² [59].

During optimum replacement of copper slag at 40% as fine aggregate, it can significantly improve the durability of concrete, and the concrete shows very low surface and capillary water absorption, with sorptivity reduced by as much as 79.46% within 24 h [32,34,47]. Further, the reduction in water absorption helps in limiting the entry of harmful chemicals into the concrete. As a result, due to the formation of a denser interfacial transition zone and a more complex internal pore structure, it increases resistance towards chloride penetration and lowers the rate of chloride diffusion and improves electrical resistivity [29,50,52,55,57]. In some studies, it was noticed that the concrete with CS generally shows a slower rate of carbonation and reduces the depth of penetration up to 29.36% after 16 weeks, and this behavior is mainly due to unreacted iron oxides present in CS, which helps to maintain higher alkalinity within the concrete matrix [45,47,50,54]. Similarly, resistance towards sulfate and acid attack was noted as improved or found stable in conventional concrete. Also, it was observed that when CS is introduced below 40% optimum level, it performs with very low mass loss and maintains its compressive strength, particularly exposed in sulfuric acid and sodium sulfate environments, due to its ability to resist internal stresses that are caused during the formation of ettringite and gypsum in concrete [45,53,54]. In Alkali-Activated Slag Concrete (AASC) that contains CS, due to the lack of calcium hydroxide, which prevents the formation of a protective brucite layer, the concrete is effective against a sodium sulfate environment and weak under magnesium sulfate exposure. In addition to these, fine CS particles are found to dissolve rapidly in highly acidic environments with a pH value of one, which allows acids through pores, and it makes penetration easier and accelerates deterioration [57]. It was understood that the excessive use of CS may reduce resistance to salt-frost action and allow deeper water penetration through an increase in pore size distribution [52]. Therefore, during the high replacement levels, in order to maintain durability, it was strongly recommended to maintain the replacement % of CS as fine aggregate ranges between 40% and 50%. During this optimum range, CS helps in producing high-performance concrete with balanced workability, strength, and durability. Meanwhile, it supports sustainable construction by reducing the use of natural sand and minimizing the environmental impact of industrial waste disposal. To provide a quantitative synthesis of the reviewed literature, the influence of copper slag replacement on fresh, mechanical, and durability properties is summarized in Figure 2.

As shown in Figure 2, the combined trends clearly provide data-based support for the behavior of copper slag (CS) across different performance aspects. As presented in Figure 2a, compressive strength follows a non-linear pattern, reaching an optimum at around 40% replacement of fine aggregates. This improvement is mainly due to better particle packing and densification of the interfacial transition zone (ITZ). Figure 2b shows a steady increase in workability because of the low water absorption and smooth, glassy surface of CS, although very high replacement levels can cause instability and bleeding. The durability trend shown in Figure 2c follows a U-shaped pattern, with the lowest Rapid Chloride Penetration Test (RCPT) values at the optimum replacement level, indicating reduced pore connectivity and better resistance to chloride penetration. On the other hand, Figure 2d shows that when CS is used as a supplementary cementitious material, the best performance is observed at around 20% replacement, and beyond this level, the dilution effect reduces mechanical strength. The high coefficient of determination (R²) values indicate that these trends are consistent and reliable, helping to clearly identify the optimum replacement ranges for different applications. These plots represent generalized trends synthesized from multiple experimental studies rather than a single dataset.

4. Copper Slag as Cementitious Material

Supplementary cementitious material (SCM), mainly due to its chemical compounds such as silica (SiO₂), alumina (Al₂O₃), and iron oxide (Fe₂O₃), is found between 70% and 95%, as per ASTM requirements for pozzolanic materials [41,42]. In addition to these, CS usually contains amorphous content, sometimes found at 100%, which makes the copper slag a chemically reactive and effective material in cement [6,20,51,55]. During the primary hydration of Ordinary Portland Cement (OPC), calcium hydroxide (Ca(OH)₂, or portlandite) is formed. When finely ground CS is introduced into the cement matrix, its amorphous silica and alumina chemically react with and actively consume this portlandite [60,61,62]. This secondary pozzolanic reaction forms additional calcium silicate hydrate (C–S–H) and calcium aluminate silicate hydrate (C–A–S–H) gels in the cement matrix. As a result, the Ca/Si ratio decreases about 0.77–1.11, and the microstructure found becomes denser, especially at the interfacial transition zone and within the pore system [14,19,60,61,62,63]. However, the CS naturally contains a low amount of calcium oxide (CaO), found less than 10%, due to this, its pozzolanic activity at early ages was found relatively low [35,63,64,65]. Even though the CS shows a strong improvement in reactivity over time, certain standard tests, such as R3 and Frattini, have often shown very limited reactivity during seven days of curing, with chemically bound water values less than 1.32 g at 100 g [66,67]. In the case of a concrete mix containing 20% CS, it was observed that the fixed lime content has increased significantly, from about 3.67% at 28 days to 18.83% at 180 days [66]. To improve early-age performance, it was suggested to use vibratory milling for around 3 h to achieve a specific surface area of 1.37 m²/g, which can provide more reaction, and increase the 28-day fixed lime consumption to about 21.15% [20,68]. Another notable approach to improve the initial age performance of concrete was alkali activation through dissolving the silicate phase found in copper slag (CS) as rapidly at higher pH environment and this helps to achieve compressive strength nearly about 80% of its 28 days curing strength within 3 days of curing [6,12,55].

Usually, the copper slag (CS) has a strong influence on the hydration behavior of cement, which affects both the setting time and the control of the heat released during hydration. By replacing a portion of cement with CS, even though it has low reactivity at early stages, it creates a dilution effect by reducing the immediate availability of alkaline compounds in the system, and the setting time is generally delayed [61,66,68]. It was noticed that using very fine CS particles may further delay the setting due to the slow development of the pozzolanic reaction, and the initial setting time can increase up to around 250 min, which is considered more than twice that of conventional concrete [20,35,63,69]. In alkali-activated cement, the low calcium content of CS limits early heat generation, further delaying the hydration process and extending it by about 3.5 h. This leads to longer initial and final setting times, around 340 and 375 min, respectively. On the other hand, some recent studies report that at lower replacement levels from about 10% to 30%, the CS may slightly reduce the setting time due to the reaction of CS as a micro-filler, helping to pack particles more efficiently and increasing the demand for water, which helps to accelerate the initial setting [62,67,70]. Regarding the heat of hydration, by replacing cement with CS, it generally reduces the total heat that is released during the early stages, especially within the first two days of curing, and this further lowers the temperature rise in concrete, which is beneficial to control the development of thermal cracking, particularly in mass concrete structures [20,67,70]. Heat evolution studies show that the addition of CS has delayed the induction period, further increasing the induction period from about 2.3 h for plain cement to around 2.5–2.8 h, and it delays the pozzolanic reaction of CS by contributing additional heat when time progresses. This is especially evident when mechanically activated CS can produce a total heat release of about 183.9 J/g at 120 h, indicating that the later-stage reaction helps to compensate for the reduced early heat due to cement replacement [60]. Overall, the utilization of CS shows low early-age reactivity and delayed hydration necessitating a careful mix design to control setting time during both mechanical and alkali activation. More than these early-stage settings, the CS has provided long-term strong benefits through consuming calcium hydroxide and forming a denser and more durable microstructure, and this makes copper slag a valuable, technically reliable, and sustainable supplementary cementitious material.

5. Durability Performance

Copper slag (CS) played an important role in the durability performance of concrete, determining its suitability for long-term infrastructure applications. When CS is used either in the form of supplementary cementitious material or replacement of fine aggregates, it significantly impacts the permeability of concrete and makes the internal structure of concrete denser due to its very low water absorption estimated between 0.36% and 0.46%, compared to natural sand of about 0.80% [47,71,72,73]. It was understood that the resistance towards environmental damage through improved microstructure resulted from the better particle packing, low porosity, and the formation of dense calcium silicate hydrate (C–S–H) gel that reduces the number of capillary pores [14,71,74]. Further, these denser microstructures and the reduction in capillary pore size, usually less than 10 nm, have reduced the entry of chloride ions, especially when the structures are exposed to marine and coastal environments [72,75]. In self-compacting concrete and ultra-high-strength concrete, using CS at optimal levels generally results in very low chloride permeability. By replacing up to 30%, the CS can produce rapid chloride permeability values in the range of 300 to 500 coulombs after 365 days [36,51,76]. In geopolymer concrete, at the 60% CS replacement, it has shown a reduction in the chloride permeability value of about 54.8% compared to conventional concrete [71]. Based on a scientific study, the silica (SiO₂) present in CS reacts to help form additional C–S–H gel with a lower Ca/Si ratio value of around 1.66 at 365 days, and this helps to refine the interfacial transition zone and further reduces the movement of chloride ions within the concrete. As a result, the chloride diffusion coefficient value has decreased from about 1.22 cm²/year to 1.14 cm²/year at 28 days [36,38,77].

In CS replacing concrete, the resistance towards sulfate attack mainly depends on the level of replacement and the degree of exposure. As fine aggregate, at the optimum replacement level of about 40%, the concrete shows good stability, with an increase in small mass of about 0.04% and a slight reduction of 2.72% in compressive strength after 56 days of sulfate exposure [53]. Similarly, when 15% of cement is replaced with CS, the concrete further shows an increase in compressive strength between 28 and 100 days under magnesium sulfate (MgSO₄) exposure, and this behavior is mainly due to the early-stage pore-filling effect caused by the formation and reaction of sulfate substances that are usually found in the copper slag [71,74,78]. However, at the long exposure period of about 180–360 days or at the complete replacement of copper slag, the performance of concrete is found to decrease significantly. In such cases, the lower alkalinity of the concrete leads to the conversion of strong C–S–H gel into weaker magnesium silicate hydrate (M-S-H) gel. At the same time, excessive formation of ettringite in concrete due to the addition of CS has created internal stresses and caused strength reduction due to high losses, with a value of about 11% [47,78]. While protecting the structure from moisture entry and freeze–thaw damage, the concrete containing CS generally shows much lower water absorption and sorptivity compared to conventional concrete. Concrete mixtures with 20% to 40% replacement of CS as SCM have shown significant improvement by reducing the Initial Surface Absorption (ISA) by about 48.84% to 61.48%, and decreasing the sorptivity value up to 54.21% compared to conventional concrete [14,36,79]. This improvement is mainly due to the low water absorption of CS particles and their high density, which helps to reduce the formation of micro-pores and limits the movement of capillary water [73,79]. As a result, this reduced permeability and the internal expansion caused by freezing water were also found to be minimized, and these lead to better resistance towards freeze–thaw cycles. Notably, concrete with 100% CS replacement showed only a 36.76% reduction in 28-day compressive strength after 250 freeze–thaw cycles, whereas the conventional concrete with quartz sand shows much higher loss of about 56.34% under the same exposure conditions [38].

In the carbonation depth estimation, it is necessary to maintain the alkalinity of concrete that needed to protect embedded steel. When CS is replaced as fine aggregate at optimal levels, its high iron oxide content, around 57.82%, is found to help increase the alkalinity of the pore solution, and this leads to a reduction in carbonation depth. Also, replacing 100% CS as fine aggregate resulted in a decrease in carbonation depth at 29.36% after 16 weeks [47,51]. Similarly, geopolymer concrete with CS shows a reduction in carbonation depth of about 15–30%, mainly due to the dense internal structure that slows down CO₂ penetration. [80]. However, when CS is used as a cement replacement at a higher value at 30%, it can create a dilution effect, and this reduces the amount of formation of portlandite (calcium hydroxide), which further lowers the alkalinity of the concrete and increases the risk of carbonation. In such cases, it was suggested to conduct accelerated testing in order to check complete carbonation within 90 days [61]. In a saline environment, the formation of denser microstructure by CS has significantly reduced the rate of permeability and further lowered the corrosion rate of embedded steel reinforcement in concrete. Using accelerated corrosion studies, such as galvanostatic weight loss measurements, has shown that the concrete replaced with 100% CS and w/c = 0.55 had a very low loss of steel mass, about 0.13%, compared to 1.44% in conventional concrete [51]. Further, the corrosion of steel reinforcement in CS enriched concrete reached a 90% probability of corrosion after 22 days, compared to only 16–18 days in concrete with natural aggregates [72]. In addition to these, the corrosion current density study in CS concrete with optimum replacement that remains within the passive range shows below 0.1 μA/cm² for up to one year. At the same time, the chloride concentration at the depth of reinforcement stays below the critical limit of 1.2 kg/m³, which helps to prevent corrosion [77]. Overall, it was identified that using copper slag at optimal replacement levels typically ranges between 20% and 40%, which helps to provide excellent durability and long service life for concrete. Also, using CS has helped to improve resistance against chloride penetration, sulfate attack, carbonation, and reinforcement corrosion by creating a dense internal structure and reducing pore connectivity, and this makes copper slag a highly suitable and reliable material for use in harsh environments such as marine and coastal regions.

The durability trends shown in Figure 3 indicate that copper slag significantly enhances resistance to moisture ingress and corrosion. Water absorption and sorptivity decrease notably up to 40% replacement, reflecting improved pore refinement and reduced capillary connectivity due to the micro-filler effect. Similarly, corrosion loss shows a continuous decline with increasing copper slag content, suggesting enhanced protection against aggressive environments. However, a slight increase in sorptivity at higher replacement levels indicates potential instability due to excess free water, confirming that optimal durability performance is achieved at approximately 30–40% replacement, and these trends are consistent with RCPT observations.

6. Emerging Applications of CS in Construction

Beyond its use in concrete, copper slag (CS) is highly effective in geotechnical engineering due to its non-plastic behavior, high specific gravity value of about 3.5–4.0, and extreme angularity. These physical characteristics contribute to a very high angle of internal friction, reported up to 52°, which significantly improves inter-particle friction and load-carrying capacity [32,70,80,81]. When mixed with weak or expansive soils in the range of 10% to 50%, CS dramatically enhances soil stability. Experimental studies have verified that adding just 10% CS to cement-stabilized soil can increase the Unconfined Compressive Strength (UCS) from 53 kPa to 660 kPa, and the values double during the soaked California Bearing Ratio (CBR) from 8% to 16% due to the development of C–S–H gels [6,82]. Furthermore, CS acts as a highly efficient backfill material for mechanically stabilized earth (MSE) retaining walls; increasing the CS content linearly decreases the active earth pressure coefficient at an average rate of 4% for every 10% increase in the mixture [36,68]. Similarly, advanced structural health monitoring (SHM) techniques, including sensor-based systems and IoT-enabled monitoring, have been increasingly applied in geotechnical engineering to evaluate soil behavior, deformation, and soil–structure interaction under real-time conditions [83]. In terms of expansive soils, replacing traditional stabilizers with lime-slag or cement-slag mixtures can diminish the Free Swell Index (FSI) by 65% to 70% [84].

During the construction of flexible pavements, CS is utilized as a reliable replacement for natural aggregates in bituminous mixes and hot mix asphalt. Replacing up to 30% of conventional aggregate with CS in bituminous macadam significantly improves particle interlocking, yielding higher Marshall stability and superior volumetric properties [49,85]. The success of CS in road bases and asphalt is scientifically justified by its low water absorption (0.5–2.5%), low porosity (1–5%), and inherent hydrophobic nature, making the pavement less sensitive to temperature fluctuations and more resistant to deformation compared to traditional granite or limestone mixes [31,33,35].

Due to its high Mohs scale hardness (6 to 7), high specific gravity (up to 3.53), and low free silica content, CS is extensively utilized as a safe and highly effective abrasive blasting medium for cleaning large metal structures such as ship hulls and bridges [7]. Based on particle size distribution, CS provides highly specific surface profiles: coarse grades are used to remove heavy corrosion yielding a 100 to 120-micron profile, medium grades remove mill scale, ranging from 60 to 80 microns, and fine grades target light rust at 40 microns [33,86]. Additionally, due to its high density, CS has been successfully incorporated into the production of high-density concrete intended specifically for radiation shielding in nuclear structures [14,35,61,87].

In advanced manufacturing, CS is utilized as a high-value fluxing agent. When equal proportions of CS and spodumene tailings are sintered at 1100 °C, they produce high-performance load-bearing bricks with an exceptional compressive strength of 140 MPa. This process safely encapsulates heavy metals, dropping arsenate leaching below detection limits (<0.015 mg/L), and produces ceramics capable of withstanding 200 freeze–thaw cycles [11,17]. CS also possesses unique electromagnetic shielding capabilities due to the presence of fayalite and magnetite. Studies have demonstrated that adding 45 wt.% of granulated CS to a cement matrix can weaken electromagnetic waves by approximately 60%, acting as a magnetic dielectric loss absorbent [17,88].

From an environmental perspective, fine CS particles (less than 100 µm) are used as an active filter medium in wastewater treatment, effectively reducing highly toxic chromium (VI) into less harmful chromium (III) due to the presence of reactive Fe(II) in the slag [11,17,31,35,53,89]. Most notably, recent advancements in the green energy sector have demonstrated that untreated CS can act as a low-cost, functional electrocatalyst for the Hydrogen Evolution Reaction (HER) in saline water, bypassing the need for expensive noble metals [90]. CS also shows strong potential in thermolysis processes, where concentrated solar energy is used to drive high-temperature endothermic reactions between CS and water vapor to produce hydrogen, simultaneously allowing for the recovery of valuable magnetite and hematite [91,92].

7. Environmental and Economic Assessment

Copper slag stability was decided based on its method of storage in open areas and the degree of weather exposure. It is especially evident when exposed and dumped for a long period, the CS tends to release harmful elements like lead (Pb), arsenic (As), cadmium (Cd), copper (Cu), and zinc (Zn) [4,17,28,31]. Scientifically, the leaching behavior is strongly affected by the method of cooling during slag formation and the pH level of the surrounding environment. Particularly, during the water-quenched (Amorphous) cooling method, the slag can release up to ten times more Pb and As compared to the slow-cooled (crystalline) method under acidic conditions. However, arsenic leaching is generally found lowest and maintained within a pH range of 4 to 8 [28]. Copper slag, in its raw and untreated form, may exceed environmental safety limits. Further, the Toxicity Characteristic Leaching Procedure (TCLP) has shown that dumping untreated CS can contain high concentrations of metals, with copper reaching 64.72 mg/L and zinc about 10.77 mg/L [39]. However, when CS is used in geopolymer or cement-based materials, it was noticed that these risks can be effectively controlled during the process of hydration and geopolymerization, making harmful metal ions stable through different mechanisms such as physical encapsulation followed by the formation of hydroxide compounds, chemical bonding, and ion exchange by replacing Al³⁺ in the structure [40,41]. As a result, in well-designed geopolymer concrete mixes, the CS has helped to achieve very high levels of heavy metal immobilization, such as 100% for Pb and Cd, 96.22% for Cu, and 95.71% for Zn. These values further ensure that the leached concentrations remain well below the limits recommended by the US EPA, even under different pH conditions [40,93]. In addition to these, pre-treatment methods such as chemical treatment using 3 mol/L HNO₃ or bioleaching can further reduce the number of soluble metals if required. Also, the concentration of copper can be reduced to about 3.56 mg/L in order to classify the treated slag as non-toxic [39].

Life Cycle Assessment (LCA) studies clearly show that replacing traditional construction materials such as cement and fine aggregate with copper slag (CS) provides significant environmental and energy-saving benefits. This is mainly due to reducing the usage of Ordinary Portland Cement (OPC), which has a very high energy demand of about 5000 MJ per ton and contributes 95% of the total carbon emissions during concrete production [28,40,64]. While replacing CS partially in OPC as a binding material, it was noticed that the environmental impact is reduced considerably. Particularly, replacing 15% of cement in structural concrete can lower the global warming potential (GWP) by 12.41% and reduce the Abiotic Depletion Potential (ADP) of fossil fuels by 9.22% [64]. In cemented paste backfill (CPB) applications, it was found that replacing 30% of cement with CS can help reduce the GWP value by about 14% to 19%, with more than 99% of this reduction coming from avoiding cement production [94]. Similarly, using modified granulated copper slag in blended cement helps to reduce the energy consumption value of 4.49 × 10³ kJ by about 16% compared to 5.21 × 10³ kJ for OPC, and decreases abiotic resource depletion by 46.50%, through reducing the need for mining natural materials like limestone and clay [95]. While replacing 100% CS as fine aggregates, it can save up to 529 kg/m³ of natural sand, and this leads to a reduction in embodied energy by about 40% and a decrease in GWP by around 30% [93,96]. In geopolymer concrete containing CS, the environmental benefits are even greater, with a reduction in CO₂ emission by 42% to 90%, and energy consumption reduced to nearly three-fifths of that required for conventional concrete production [40,41,97]. However, some LCA studies also report a minor drawback that there is a possibility of a slight increase in Human Toxicity Potential (HTP) levels up to 27.75%, mainly due to the presence of arsenic impurities and the additional energy required during grinding the hard slag material [64,95].

Copper slag (CS) is an economically feasible material that mainly depends on balancing transportation costs with the significant savings in material cost, waste management benefits, and the possible recovery of valuable metals [4,31,94]. Economically, the CS has contained trace metals such as cobalt, estimated at the range of 23 to 26.25 per pound, and copper, which can achieve a recovery value of about 70–85% through froth flotation. Due to the presence of these metals, the CS becomes a growing global market material of about $800 million for value-added waste products and increases overall value that is nearly 60 times higher than the bulk slag [4,28,98]. Even without extracting these metals, replacing cement and natural sand with low-cost or freely available CS can reduce initial construction costs and eliminate landfill expenses, leading to a clear reduction in overall project costs [62,64,95]. Zhou et al. (2024) explored that, in the view of transportation, the effect of transferring distance depends on how CS is used [94]. Studies show that, during high-value applications such as replacing cement in cemented paste backfill (CPB), even if CS is transported up to 300 km, the increase in environmental impact is found to be less than 2%, and this indicates that transportation effects are very small compared to the benefits gained by reducing cement usage [94]. Gursel et al. (2019) present that in Singapore, instead of importing natural sand, using locally available CS helps to control transportation-related emissions to around 13 kg CO₂-eq per unit compared to the emissions of about 32–54 kg CO₂-eq per unit of natural sand [93]. Filipović et al. (2022) estimate that when CS is used only as a replacement for natural aggregates in normal concrete, transportation distance becomes more important [96]. In such cases, the distance should generally be kept below 20 km, since the longer distances may increase emissions and reduce the environmental benefits compared to conventional materials [96]. Overall, these detailed environmental and economic studies confirm that using raw CS requires proper handling due to its potential heavy metal leaching, and its use in cement and geopolymer systems safely stabilizes these elements and the reuse of industrial by-products such as copper slag aligns with circular economy principles and supports life cycle-based sustainable construction practice [99].

8. Statistical Validation and Optimization of Copper Slag Utilization

To study the overall performance of copper slag (CS) in construction and to reduce differences between results from various experiments, a multi-criteria decision analysis (MCDA) method combined with a meta-based regression approach was developed. The data used in this analysis were collected from a PRISMA-based systematic literature review, ensuring that only relevant peer-reviewed studies that met the required selection criteria were included. Since the selected studies had differences in measurement units, scales, and testing conditions, important performance parameters such as compressive strength, durability based on water absorption, environmental impact in terms of global warming potential reduction, and cost were converted into a common format using the min–max normalization method. For beneficial attributes such as strength and durability improvements, normalization was defined as:

$$Xnorm={\displaystyle \frac{\left(X-Xmin\right)}{Xmax-Xmin}}$$

(1)

whereas for non-beneficial attributes such as water absorption, cost, and CO₂ emissions, inverse normalization was applied.

$$Xnorm={\displaystyle \frac{\left(Xmax-X\right)}{Xmax-Xmin}}$$

(2)

This process ensured that all variables were converted into a common scale without units, ranging from 0 to 1, so that they can be easily compared. The normalized data used for the meta-analysis are shown in

Table 3. (a). Normalized Meta-Synthesis Dataset for Fine Aggregate (FA) Replacement. (b). Normalized Meta-Synthesis Dataset for Cement (SCM) Replacement.

(a)
CS Replacement (%)	Strength $Xnormal$	Durability $Xnormal$	Environment $Xnormal$	Cost $Xnormal$	Performance Index (PI)
0%	0.54	0.83	0.00	0.00	0.44
20%	0.76	0.93	0.55	0.62	0.75
40%	1.00	1.00	1.00	1.00	1.00
50%	0.95	0.98	0.95	0.87	0.95
60%	0.81	0.89	0.83	0.75	0.83
(b)
CS Replacement (%)	Strength $Xnormal$	Durability $Xnormal$	Environment $Xnormal$	Cost $Xnormal$	Performance Index (PI)
0%	0.76	0.78	0.00	0.00	0.50
10%	0.88	0.90	0.40	0.41	0.72
20%	1.00	1.00	0.70	0.73	0.90
30%	0.75	0.80	1.00	0.98	0.85
40%	0.50	0.58	1.00	1.00	0.70

(a,b). These datasets represent common trends collected from different studies and are used as the basis for calculating the performance index and carrying out further analysis.

The overall performance index (PI) was calculated using a weighted linear aggregation:

$$PI= \sum (Wi\ast Xnormal,i)$$

(3)

where W_i represents the weight given to each parameter. In this study, the weights were taken as 0.35 for strength, 0.30 for durability, 0.20 for environmental impact, and 0.15 for cost. This distribution is based on common engineering practice, where structural performance, such as strength and durability, contributes about 65% in selecting materials, while sustainability factors like environmental and economic aspects contribute the remaining 35%. A sensitivity analysis was also carried out by changing the weights within ±10% to check the reliability of the method. The results showed that the optimum performance range did not change, which means that the final conclusions are stable and not strongly affected by small changes in the weighting values.

The combined effect of the normalized parameters shown in Figure 4 indicates that the performance index increases as the copper slag content increases up to about 40%, where all parameters reach their maximum values, and this improvement is found due to better particle packing, reduced pore connections, and a denser interfacial transition zone, which together improve strength and durability. Meanwhile, the environmental and economic benefits also increase due to lower use of natural aggregates and reduced cement production. To further study this trend, a second-order polynomial regression model was applied to the combined dataset, and the regression analysis gave a high coefficient of determination with an R² value of 0.965, as shown in Figure 5, which shows a strong match with the observed data, and this curve shows a non-linear relationship with a clear peak at around 40% replacement. This result confirms the existence of an optimum range, but the regression should be used only to support the trend and not as an exact prediction model.

The behavior of CS varies significantly depending on its application. For fine aggregate (FA) replacement, the performance index exhibits a broader optimal range (40–50%), primarily due to improved packing density and reduced porosity. In contrast, for cement (SCM) replacement, the optimal range is limited to 10–30%, as higher replacement levels lead to dilution of clinker content, reduced alkalinity, and slower hydration kinetics.

The comparative analysis shown in Figure 6 clearly explains that the optimal use of copper slag (CS) depends on the type of application, with different peak performance levels observed for each case. Although the statistical meta-analysis in this study shows general performance trends for CS, it is not scientifically correct to define a single “optimal” replacement level for all applications. Meanwhile, the suitable replacement level mainly depends on the usage of CS in the form of fine aggregate or as a supplementary cementitious material (SCM), and also on the type of concrete system, such as conventional concrete, self-compacting concrete (SCC), ultra-high-performance concrete (UHPC), and geopolymer systems. These differences are mainly due to found variations in controlling factors such as particle packing, flow behavior, and chemical reactions. The optimum proportion of CA as fine aggregate and cement material is highlighted by the blue square and green ring in Figure 6, respectively. To handle this complexity, a case-based approach is used, as shown in

Table 4. Case Differentiation Matrix for Optimal Copper Slag Utilization.

Material Role	Concrete System	Optimal Range	Required Conditions and Parameters	Citation
Fine Aggregate	Conventional Concrete	30–40%	w/b: 0.40–0.55; Curing: 28–90 days; Gradation: 0.15–4.75 mm	[32,34,48]
	SCC/UHPC	20–30%	w/b: <0.40 Controlled fines Extended curing	[14,36,47]
	Geopolymer Concrete	40–60%	Alkali activation; Heat/ambient curing; Angular particles	[33,55]
SCM (Binder)	Conventional Concrete	10–20%	w/b: 0.40–0.50; Fine grinding ranging less than 30 µm with long curing about 90–360 days.	[64,76]
	Alkali-Activated Systems	Up to 50%	NaOH/Na2SiO3 activation; Heat curing; High alkalinity	[6,75,100]

. The table provides application-specific optimal ranges along with important factors like water-to-binder (w/b) ratio, particle size distribution, activation methods, curing conditions, and exposure environment. This approach gives a clearer and more practical guideline for using CS in different construction applications.

The correlation matrix shown in Figure 7 helps to understand the effect of copper slag content on important performance parameters in both FA and SCM. A strong positive relationship is observed between copper slag replacement, compressive strength, and density, which means that increasing the amount of CS improves particle packing and makes the internal structure of the concrete denser. This result confirms that physical effects, especially the micro-filler action and better interfacial transition zone (ITZ) properties, play an important role in improving strength. On the other hand, water absorption shows a strong negative relationship with both strength and density, which means that higher CS content reduces pore connections and limits the movement of water through capillary action. This opposite relationship supports the improvement in durability properties, such as lower permeability and reduced chloride penetration. Overall, these correlation results statistically confirm the consistency of the combined data analysis and support the identified optimal replacement levels, showing that both strength and durability can be improved together when copper slag is used in a controlled manner.

The different trends shown in Table 4 clearly explain that the controlling mechanisms change depending on the type of application. In self-compacting concrete (SCC), the very low water absorption of CS in the range of 0.13 to 0.5% creates excess free water when higher replacement levels are used. Because of this, the replacement level should be limited to about 20–30% to avoid problems like bleeding and segregation. On the other hand, geopolymer systems can use higher replacement levels up to 40–60% because they do not depend on normal cement hydration and mainly rely on mechanical bonding and alkali-activated reactions. When CS is used as a supplementary cementitious material (SCM), the suitable replacement level is lower because its pozzolanic reaction is slow. Since CS has low calcium oxide content, mostly less than 10%, it shows low reactivity at early stages and depends on long-term reaction with calcium hydroxide. If the replacement level goes above 20% in normal cement systems, it reduces the amount of clinker, which leads to lower early strength and a higher risk of carbonation. However, these problems can be reduced by using activation methods such as fine grinding, which increases the surface area, or alkali activation, which improves the reaction of amorphous materials and speeds up gel formation. With these methods, higher replacement levels up to 50% can be used in special systems like alkali-activated binders. Overall, this case-based approach gives a clearer and more scientific guideline for using copper slag, instead of using general values, and helps to evaluate performance based on the correct material and environmental conditions. Despite the strength of this combined analysis approach, some limitations should be noted. The data used in this study come from different research works with variations in material properties, mix design, curing conditions, and exposure environment. These differences may create some uncertainty in the exact performance values. Therefore, this framework should be used as a comparison and decision-support tool, and the suggested optimal ranges should be confirmed through proper experimental studies for specific applications.

9. Conclusions

This review has examined clearly the effective utilization of copper slag (CS) by converting it from a hazardous industrial waste into a high-performance construction material. Based on the study, it was understood that the optimum value depends on the way CS is used in concrete. As a fine aggregate, the CS has improved workability, compressive strength, and durability. Based on statistical analysis and previous studies, the optimum replacement level is around 40% in conventional concrete. At this level, the angular shape and high density of CS improve particle packing and reduce pore spaces. However, when the replacement exceeds 40–50%, excess free water may cause bleeding and segregation. In special cases such as geopolymer concrete, this limit can be increased up to about 60%. When CS is used as a supplementary cementitious material (SCM), it improves long-term strength and durability through pozzolanic reactions, where calcium hydroxide is consumed to form additional C–S–H gel. However, since CS has low reactivity at early stages, its replacement level should be limited to about 20%. Higher replacement levels can reduce early strength due to dilution and may also increase the risk of carbonation. Durability studies confirm that using CS within these optimum ranges improves resistance to chloride penetration, sulfate attack, and corrosion of reinforcement, mainly due to the formation of a dense internal structure. In addition, Life Cycle Assessment (LCA) studies show that the use of CS reduces energy consumption, lowers carbon emissions by up to 19%, and helps conserve natural resources. Statistical validation using multi-criteria decision analysis (MCDA) supports these different optimum ranges and provides a clear method to balance strength, durability, environmental benefits, and cost. Future research should focus on improving early-age reactivity through methods such as mechanical or alkali activation and expanding the use of CS in advanced applications like electromagnetic shielding and green hydrogen production.