Investigation of Mechanical Properties of Recycled Aggregate Concrete Incorporating Basalt Fiber, Copper Slag, and Ground Granulated Blast Furnace Slag

Abstract

Facing sand and gravel shortages, construction waste accumulation, and the “double carbon” goals, improving the performance of recycled aggregate concrete (RAC) and utilizing mineral waste slag are key to the development of green, low-carbon building materials. To enhance the mechanical performance of RAC and facilitate the sustainable utilization of mineral waste, this study innovatively incorporated copper slag (CS), ground granulated blast furnace slag (GGBS), and basalt fiber (BF) into RAC. The modified RAC’s compressive, split tensile, and flexural strengths were systematically investigated. Experimental results indicated that incorporating appropriate amounts of CS or GGBS as single admixtures could effectively enhance the mechanical properties of RAC, with 20% (w) GGBS showing the most pronounced improvement. Compared with RAC, its 28 d compressive strength, split tensile strength and flexural strength were improved by 21.3%, 9.7% and 8.1%, respectively. As opposed to single admixture, 10% CS + 10% GGBS admixture can further improve the mechanical properties of recycled concrete. Compared with RAC, its 28 d compressive strength, split tensile strength, and flexural strength were improved by 25.6%, 29.7%, and 16.6%. The study also showed that 0.2% BF admixed on top of 10% CS + 10% GGBS could still significantly improve the mechanical properties of recycled concrete, and its 28 d compressive strength, split tensile strength, and flexural strength were improved by 31.3%, 35.9%, and 31.2%, compared with RAC, respectively. By XRF, SEM, and EDS techniques, the underlying mechanisms governing the mechanical behavior of RAC were elucidated from the microscale perspective of basalt fiber and industrial waste residues. These findings provide a solid theoretical foundation and a viable technical pathway for the widespread application of recycled aggregate concrete in civil engineering projects.

1. Introduction

The UN Environment Programme reported in 2022 that the global building industry creates over 3 billion tonnes of construction and demolition waste (CDW), with concrete debris accounting for over 50% [1]. Rapid urban expansion and infrastructure upgrades are highlighting this challenge. Traditional concrete production’s use of natural aggregates and high-carbon emissions strain the environment. China creates over 2 billion tonnes of building rubbish annually, while wealthy nations in the US and Europe produce 1.5–3.0 tonnes per person [2,3]. Thus, sustainable development research has focused on creative building waste usage. Recycling concrete is popular in the building sector due to its economic and environmental benefits. Studies show that recycled coarse aggregate (RCA) can totally or partially replace natural particles in fresh concrete. According to resource recycling development, this method reduces building waste in landfills and the requirement to collect natural sand and gravel [4,5,6].

Some wealthy countries have made advances in reusing construction waste. In Western Europe, the Netherlands and Germany are leaders in RCA technology, and their construction waste recycling rate exceeds 70%. In East Asia, Japan has explicitly mandated that public construction projects use more than 30% recycled materials [7,8]. Significant technological barriers exist to using RCA technology, particularly the performance difference between RAC and traditional materials. These constraints limit its application. These technical issues stem from recycled aggregate’s high porosity and water absorption. The structural weakness in the interfacial bonding region of the old and new mortar and the insufficient strength of the interfacial transition zone (ITZ) will affect the concrete’s strength [7,8,9], which will affect the construction performance, mechanical properties, and long-term durability of fresh concrete.

Recent research has improved RCA’s performance through physical strengthening, chemical modification, and composite mineral admixture [7,8,9,10]. Unlike the first two procedures, composite mineral admixtures are easy to process, low-carbon, and environmentally friendly. Additionally, its pozzolanic reactions improve recycled concrete’s interfacial qualities and mechanical properties. Some mineral admixtures can also be used as partial cement replacements due to their excellent physical and mechanical properties (e.g., low water absorption, high density, high hardness, strong abrasion resistance, good compressive resistance, etc.), which can improve concrete strength [11,12,13,14,15]. Silva, Y F. [16], Lye, C. Q [17], Najimi, M [11], and others found that replacing cement with copper slag powder improves concrete’s mechanical and durability. On the basis of microstructural investigations, academics found that CS creates an excellent particle gradation with cement and that its volcanic ash qualities increase concrete’s properties [18,19,20,21,22].

Along with CS, GGBS is another excellent mineral additive that boosts the efficiency of recycled aggregate concrete. The primary component is high-purity silica aluminate, prepared using precise grinding and a rigorous screening procedure that ensures consistent particle gradation. Its high silica (SiO₂) concentration is its defining characteristic; its trace element content, including calcium and iron, is relatively low; it has outstanding physical and chemical properties; and it is very stable [23,24,25,26,27,28]. Researchers such as Majhi R. K. [29], Saranya P. [30], and others have discovered that GGBS can react with Ca(OH)₂ (CH) in the cement slurry to produce calcium silicate hydrate(C-S-H) cementitious materials. These materials may improve the structure of concrete on a microscopic level, which is the primary way that concrete’s mechanical properties are enhanced [31].

The natural silicate fiber known as basalt has remarkable engineering qualities, including resilience to high and low temperatures, high fracture strength, and corrosion [32,33,34,35,36]. The utilization of basalt fibers to enhance the mechanical properties of concrete represents a relatively innovative approach, which has demonstrated substantial potential for improvement and promising application prospects [37,38,39,40]. Researchers like Del Bosque, I. S. [32] and Liu Q. [33] have shown experimentally that BF can replace traditional reinforcing materials. Basalt fibers are stronger and more heat-resistant than glass fibers, and more economically viable than carbon fibers [35].

In pursuit of sustainable construction, RAC has garnered significant attention; however, its inherent mechanical deficiencies necessitate enhancement strategies. While mineral admixtures (e.g., GGBS, CS) and fibers (e.g., BF) have been widely studied for conventional concrete, their synergistic application in RAC remains underexplored, particularly the combined use of CS, GGBS, and BF. To address this gap, our study pioneered a systematic investigation into optimizing RAC’s performance through a hierarchical blending strategy.

(1)

Single/composite admixtures: We first evaluated how single and hybrid incorporation of GGBS and CS (key industrial by-products enhancing the cementitious matrix’s density and pozzolanic reactivity) mitigates RAC’s strength limitations. This phase identified optimal cementitious blends to establish a performance baseline.

(2)

Fiber reinforcement: Building on optimal GGBS/CS formulations, we then introduced BF—selected for its high tensile strength and corrosion resistance—to counteract RAC’s brittleness. Multiple BF volume fractions were tested to determine the dosage that maximizes mechanical synergy.

(3)

Microstructural validation: EDS and scanning electron microscopy (SEM) analysis directly correlates microscale mechanisms (e.g., interfacial transition zone refinement, pore structure densification, and fiber bridging) with macroscale property improvements, elucidating the triple-composite (CS-GGBS-BF) enhancement mechanism unique to RAC systems.

This work provides the first comprehensive framework for concurrently leveraging cementitious by-products and fiber reinforcement in RAC, offering actionable pathways to advance solid waste valorization and decarbonization in the built environment.

2. Materials and Experimental Program

2.1. Materials

2.1.1. Aggregate

Xinyu Cement Products Co., Ltd, Qiqihar, China. offers natural and recycled coarse aggregates. The natural coarse aggregate is granite, while the recycled coarse aggregate is derived from the crushing, cleaning, and sieving of construction and demolition waste. Both aggregates were selected from particle size ranges of 5–10 mm and 10–20 mm and were combined in a 1:1 ratio for use as coarse aggregates. Sand was sourced from local rivers, with a particle size of ≤5 mm and a fineness modulus ranging from 2.2 to 2.4; water was obtained from local tap sources. The principal physico-mechanical characteristics of coarse aggregate (CA) are presented in

Table 1. Physical and mechanical properties of coarse aggregate.

Coarse Aggregate	Apparent Density (g/cm3)	Packing Density (g/cm3)	Crushing Value	Void Ratio	Moisture Content	Water Absorption
NCA	2545.43	1425	17.8%	47.9%	1.80%	0.62%
RCA	2240.38	1260	23.2%	50.49%	1.95%	4.79%
Coarse aggregate	Content of needle and flake particles	Angularity index	Los Angeles abrasion loss rate	Impact toughness (kJ/m2)	Böhme wear value (cm3/50 cm2)	Mass loss rate (rotational wear)
NCA	5.6%	4.0	20.4%	15.7	7.0	3.1%
RCA	14.2%	10.0	36.2%	7.3	15.0	7.5%

, while the particle size distribution of fine aggregate (FA) and coarse aggregate is illustrated in Figure 1. Both coarse and fine aggregates comply with the specifications of Chinese standards GB/T14685-2022 [41] and GB/T14684-2022 [42].

2.1.2. Binding Material

Ordinary P-C42.5 silicate cement was chosen as the cement type. All indices conformed to the stipulations of the Chinese standard GB175-2023 [43] and the testing criteria. The mineral admixtures utilized were CS generated during the copper refining process by Heilongjiang Zijin Copper Co., Ltd, Qiqihar, China, and S95-GGBS created during the iron refining process by Hebei Jingye Iron & Steel Co, Shijiazhuang, China.

Table 2. Composition of cementitious materials (wt.%).

Binding Material	SiO2	Al2O3	Fe2O3	CaO	K2O	MgO	Na2O	Mn2O3	SO3	CuO	ZnO	TiO2	Others
Cement	31.60	10.95	3.85	44.23	1.19	-	0.72	-	3.67	-	-	0.61	3.18
CS	24.87	3.09	64.27	1.05	0.56	1.89	1.4	-	1.45	0.04	-		1.38
GGBS	34.20	17.60	1.01	34.00	-	6.21	-	-	1.62	-	-		5.38

displays the chemical composition of the cementitious materials identified by X-ray fluorescence (XRF).

Table 3. The particle size distribution of CS.

Size (mm)	(wt.%)
≥0.15	0.47
0.074~0.15	0.47
0.045~0.074	7.73
0.03~0.045	19.80
≤0.03	71.53

and

Table 4. The particle size distribution of GGBS.

Size (mm)	(wt.%)
≥0.8	24.7
0.045~0.8	47.1
≤0.045	28.2

display the particle size distributions of CS and GGBS, respectively.

2.1.3. Basalt Fiber

The fundamental physical characteristics of BF are presented in

Table 5. Physical properties of basalt fiber.

Length (mm)	Breadth (mm)	Relative Density (kg·cm−3)	Tensile Strength (MPa)	Ultimate Tensile Ratio (%)	Alkali-Resistant Strength Retention Rate (%)	Modulus of Elasticity (MPa)
12	1	2.62	1550	3.6	35.8	35.8

.

2.2. Concrete Proportioning

According to the Chinese standard JGJ55-2011 [44], this study designed regular concrete with a C30 strength grade (which was chosen due to its prevalence in mainstream construction projects, where the 30 MPa compressive strength optimally balances structural requirements with cost-effectiveness). To create RAC, 30% (w) of natural crushed stone was replaced with RCA. The specimens were classified as follows: NAC denotes pure natural aggregate concrete; RAC denotes recycled aggregate concrete without mineral admixtures. C5, C10, C15, C20, and C30 denote RAC mixtures in which CS was used to replace 5%, 10%, 15%, 20%, and 30% of the cement mass, respectively. G10, G20, and G30 denote RAC mixtures in which GGBS was used to replace 10%, 20%, and 30% of the cement mass, respectively. G10, G20, and G30 denote RAC mixtures with GGBS replacing 10%, 20%, and 30% of the cement mass. In addition, 0.1 BF, 0.2 BF, and 0.3 BF denote the incorporation of 0.1%, 0.2%, and 0.3% by volume of BF, respectively.

In this study, in accordance with the Chinese standard JGJ55-2011, C30-grade ordinary concrete was designed, and 30% (by weight) of natural aggregates was replaced with RCA to fabricate recycled concrete. The selection of this RCA replacement ratio was based on comprehensive considerations. Prior research on RAC specimens with varying RCA substitution rates has demonstrated that a 30% replacement rate strikes an optimal balance between resource recycling efficiency and the preservation of concrete’s performance [10,13,17]. A higher substitution rate, exceeding 40%, typically leads to a notable decline in both the mechanical properties and durability of concrete. This deterioration is primarily attributed to the presence of aged mortar adhering to the RCA’s surface and the inherently porous nature of the aggregates, which disrupts the matrix–aggregate interface and compromises structural integrity. Conversely, a substitution rate below 30% fails to fully capitalize on the environmental and economic advantages of waste concrete recycling, limiting the sustainability potential of the material.

For the mineral admixtures, GGBS and CS were selected with different cement mass substitution rates. The substitution rates of 10%, 20%, and 30% for GGBS were set on the basis of the consideration of its hydration characteristics and mechanism of action [31]. GGBS acts mainly through the reaction of volcanic ash, which is a slow but long-lasting reaction process. Studies have shown that when the substitution rate is lower than 10%, it is difficult to significantly improve the microstructure and long-term performance of concrete; after 30%, not only will the early strength development be inhibited, but it also may interfere with the hydration of other components due to the change in the alkaline environment of the system.

The cement mass substitution rate of CS was set at 5%, 10%, 15%, 20%, and 30%, and the gradient design refers to the optimization of the performance of industrial solid waste. CS in concrete manifests a dual functionality of “low-dosage filling and high-dosage activation.” At substitution levels ≤ 10%, CS predominantly serves as a physical filler, refining particle gradation and reducing porosity by occupying interstitial spaces. As the dosage increases, the active SiO₂ and Al₂O₃ components within CS trigger pozzolanic reactions. This transition from physical packing to chemical reactivity enriches the C-S-H gel network, thereby enhancing the concrete’s mechanical properties and microstructural stability [11]. However, when the dosage exceeds 30%, the impurities (e.g., FeO, MnO) in CS may interfere with the normal hydration process of cement, change the composition and structure of hydration products, and lead to the deterioration of concrete properties. In this study, the mechanism of CS dosage on the mechanical properties of recycled concrete were systematically analyzed through the design of multi-gradient dosage, and the optimal dosage interval could be precisely determined.

The BF volume mixing amounts of 0.1%, 0.2%, and 0.3% were set with reference to the data of Zheng Y [45], which can effectively be used to study the changing law of fiber crack-resisting and toughening effect. A dosage lower than 0.1% makes it difficult to form an effective crack-resistant network, while a dosage higher than 0.3% is likely to lead to fiber agglomeration and reduce the homogeneity of concrete. In addition, the water-to-binder ratio was fixed at 0.5, and the details of the test ratios are shown in

Table 6. Trial mix proportions.

Mixture	Sand (kg/m3)	Cement (kg/m3)	CS (kg/m3)	GGBS (kg/m3)	NCA (kg/m3)	RCA (kg/m3)	Water (kg/m3)	BF (%)
NAC	650	300	0	0	1000	0	150	-
RAC	650	300	0	0	700	300	150	-
C5	650	285	15	0	700	300	150	-
C10	650	270	30	0	700	300	150	-
C15	650	255	45	0	700	300	150	-
C20	650	240	60	0	700	300	150	-
C30	650	210	90	0	700	300	150	-
G10	650	270	0	30	700	300	150	-
G20	650	240	0	60	700	300	150	-
G30	650	210	0	90	700	300	150	-
C5-G10	650	255	15	30	700	300	150	-
C5-G20	650	225	15	60	700	300	150	-
C5-G30	650	195	15	90	700	300	150	-
C10-G10	650	240	30	30	700	300	150	-
C10-G20	650	210	30	60	700	300	150	-
C10-G30	650	180	30	90	700	300	150	-
C15-G10	650	225	45	30	700	300	150	-
C15-G20	650	195	45	60	700	300	150	-
C15-G30	650	165	45	90	700	300	150	-
C20-G10	650	210	60	30	700	300	150	-
C20-G20	650	180	60	60	700	300	150	-
C20-G30	650	150	60	90	700	300	150	-
C30-G10	650	180	90	30	700	300	150	-
C30-G20	650	150	90	60	700	300	150	-
C30-G30	650	120	90	90	700	300	150	-
C10-G10-0.1BF	650	240	30	30	700	300	150	0.1
C10-G10-0.2BF	650	240	30	30	700	300	150	0.2
C10-G10-0.3BF	650	240	30	30	700	300	150	0.3

.

2.3. Pilot Program Design

According to the Chinese specification GB/T50081-2019 [46], a total of 28 groups of specimens were made, with 21 specimens in each group. Among them, 15 specimens in each group were cubes with dimensions of 100 mm × 100 mm × 100 mm, and the remaining 6 specimens were prisms with dimensions of 100 mm × 100 mm × 400 mm. In this study, the workability of concrete was systematically evaluated via the slump test, which was meticulously conducted in strict compliance with the Chinese standard GB/T 50080-2016 [47]. The specimens were demolded after 24 h of casting and then transferred to a standard conservation room at a temperature of 20 ± 2 °C and a relative humidity ≥ 95%. The specimens were cured for 3, 7, and 28 d and then subjected to 3, 7, and 28 d compressive tests, 7 and 28 d split tensile tests, and 7 and 28 d flexural strength tests.

For the mechanical property tests, the loading rate and mode were strictly controlled in accordance with the standard. The compressive strength test for cubic specimens was conducted at a loading rate of 1.0 ± 0.1 MPa/s, applying a uniaxial compression load until failure. The split tensile strength test used a loading rate of 0.05 ± 0.01 MPa/s, with a splitting force applied diametrically to the cubic specimens. For the flexural strength test of prismatic specimens, a three-point bending loading mode was adopted, and the loading rate was set at 0.02 ± 0.002 MPa/s until the specimens fractured. Throughout the testing process, the laboratory ambient temperature was maintained at 20 ± 5 °C, and the relative humidity was kept at 60–80% to ensure the consistency of the test conditions.

In addition to mechanical property testing, advanced techniques were used for microstructural characterization. XRF was used to analyze the mineral chemistry of the specimens. SEM combined with EDS was used to observe the hydration products and micro-morphology of the specimens. Figure 2 illustrates the specimen preparation and testing procedures, and Figure 3 presents the specimens before and after loading.

2.4. Calculation Method

The Chinese national standard GB/T50081-2019 was used to determine the concrete specimens’ compressive strength, split tensile strength, and flexural strength. The mechanical performance parameters of the test group were compared and analyzed with those of the control group RAC after the experiment yielded data for the test group. The corresponding performance increment ratio was then calculated. The following is the precise calculation formula

$$r={\displaystyle \frac{F_i-F_1}{F_1}}\times 100\%$$

(1)

where $r$ is the growth rate (%), $F_i$ is the mechanical property data of the experimental group, and $F_1$ is the mechanical property data for RAC.

3. Results and Discussion

3.1. Compression Test Results and Analyses

3.1.1. Effect of the Single Mineral Admixture on the Compressive Strength of Recycled Concrete

Concrete’s early compressive strength is decreased by copper slag, according to research by A-Jabri K [13] and Afshoon, I [48]. However, over time, the negative impact of copper slag on compressive strength gradually lessens. The strength of concrete after a particular age can be significantly higher than that of the control group without copper slag when copper slag is added in amounts below 15%. It is generally agreed that using copper slag correctly can increase the concrete’s long-term strength. The compressive strength rises and falls as the CS dosage increases, as seen in Figure 4a. Of these, the RAC compressive strength of C10 specimens increases most noticeably, with compressive strengths of 18.7 MPa, 26.8 MPa, and 41.1 MPa at 3, 7, and 28 d, for example. Compared with the baseline RAC, the specimens’ compressive strengths at 3, 7, and 28 d are significantly higher than those of the control group without copper dross (Figure 4b). The compressive strength rose by 11.3%, 14.0%, and 18.4% after 28 d, respectively, suggesting that CS enhanced RAC’s compressive strength. This experimental outcome is consistent with Al-Jabri’s study [13]. After 28 d, he discovered that the compressive strength of concrete cubes rose by 16% when 10% of the cement was substituted with copper slag.

The enhancement of RAC by CS is attributed to the synergistic interplay of physical filling and chemical activation. The micro-particles of CS, with a size distribution complementary to that of cement, effectively fill the voids between recycled aggregates and the cement paste, optimizing the particle gradation and reducing the overall porosity of the system. Meanwhile, the active components present in the copper slag powder, such as SiO₂ and Al₂O₃, undergo pozzolanic reactions in the alkaline environment generated by cement hydration. This generates secondary hydration products, including C-S-H and C-A-H gels, which not only densify the inherently weak ITZ of recycled aggregates but also reinforce the cement paste matrix. By refining the microstructure, enhancing the ITZ’s bonding strength and improving matrix continuity, this dual mechanism action significantly enhances the mechanical properties of recycled concrete. The SEM results presented in Section 4 further validate the aforementioned macroscopic phenomena, providing microscopic evidence for structural optimization and performance improvement.

Figure 4a also shows the effect of single GGBS doping on the compressive strength of RAC. With the increase in GGBS doping, the compressive strength of RAC showed an increasing and then decreasing trend; the most significant increase in the compressive strength of RAC was observed for G20, with compressive strengths of 19.7 MPa, 31.1 MPa, and 42.1 MPa for 3 d, 7 d, and 28 d, respectively. According to the data in Figure 4b, the compressive strengths at 3 d, 7 d, and 28 d are 17.3%, 32.3%, and 21.3% higher, respectively, compared with the RAC. This agrees with the findings of Sara B [49] that GGBS is more effective than CS in increasing the compressive strength of RAC. Specifically, the 28 d compressive strength of G20 sample was 42.1 MPa, which was only 0.04% lower than the 28 d compressive strength of NAC. This is due to the fact that GGBS contains many reactive minerals, such as SiO₂ and CaO, which can react with the CH in the cementite at the RAC interface to form C-S-H cement materials. This can greatly enhance the interfacial structure of the RAC. In addition, through the physical filling effect, the GGBS used in the test formed a good particle gradation with the cement particles, which reduced the porosity of the internal structure of the concrete and improved the early compressive strength of the RAC. According to the interfacial optimization, this is in line with the findings of Saranya P [30], Sara B [49], and other researchers.

3.1.2. Effect of Complex Mineral Admixtures on the Compressive Strength of RAC

Figure 5a–c display the compressive test results of RAC made by combining CS and GGBS in varying amounts to substitute for cement. This RAC with two different kinds of additive largely satisfies the C30 grade criterion for compressive strength. Mixing too much admixture when compounding is not advisable because the RAC’s compressive strength tended to increase and decrease with the CS admixture’s rise when the amount of the GGBS admixture was fixed. The RAC compressive strength gradually reduced with the increase in the CS admixture when the amount of the GGBS admixture was fixed [45]. The C10-G10 specimens’ 3 d, 7 d, and 28 d compressive strengths were raised by 48.2%, 37.7%, and 25.6%, respectively, in comparison with RAC and by 12.7%, 10.1%, and 5.3%, in contrast to NAC, as indicated by the data in Figure 5d. This is because GGBS has a higher activity than CS, which is primarily composed of CaO and SiO₂. When cement hydration produces CH, it reacts with the active SiO₂ and Al₂O₃ in GGBS to produce more C-S-H gel (hydrated calcium silicate) and C-A-S-H gel (hydrated calcium aluminosilicate), which fills the pores and strengthens the cementation. Powdered copper tailing slag has little early action. The cement particles may be covered by precipitates, with hydration products produced by its heavy metal constituents. The heat of hydration is inhibited [31,33,49]. Nevertheless, 71.53% of the total CS employed in this investigation had a particle size of less than 0.03 mm. By physically filling the pores and strengthening the binding, the early stage can lower the porosity, increase the density of the mortar, create a good gradation between the cement particles and GGBS, and fill the spaces between the cement particles. Excellent grading enhances the overall strength of RAC and optimizes the microstructure of the interfacial transition zone (ITZ) by the physical filling effect. Later, SiO₂ and CaO in the CS hydrate with CH are produced by cement hydration to create more C-S-H gel, filling the ITZ as best as possible and increasing the RAC’s overall strength [50,51]. These macroscopic findings will be further corroborated and elucidated through subsequent microstructural analyses.

3.1.3. BF Improves the Compressive Strength of C10-G10 Compound RAC

The compressive strength comparison of C10-G10 specimens externally doped with BF at 3, 7, and 28 d is illustrated in Figure 6a, indicating an enhancement in strength relative to the reference group RAC. The compressive strength of C10-G10 specimens exhibited an initial increase followed by a decline with the augmentation of BF volumetric doping (0.1%, 0.2%, 0.3%), suggesting that excessive BF doping was unsuccessful in enhancing strength [52,53,54]. According to the data presented in Figure 6a,b, the compressive strength of C10-G10-0.2BF specimens was optimal, exhibiting strengths of 26.3 MPa, 37.8 MPa, and 48.2 MPa at 3 d, 7 d, and 28 d, respectively. These values were 56.5%, 60.9%, and 38.9% superior to those of RAC and 18.6%, 14.2%, and 10.1% greater than those of NAC, respectively, and surpassed other BF dosage groups. A 0.1% BF dispersion demonstrates superior performance; nonetheless, the improvement is marginal, with a 28-day strength of 45.6 MPa, which is merely 4.1% greater than that of NAC at 43.8 MPa. The 0.2% BF is uniformly dispersed, successfully bridging microcracks and enhancing both early (3 d) and late (28 d) strength, with the 28 d strength being 5.7% greater than that of the 0.1% BF group. The 0.3% BF agglomeration phenomenon was apparent, leading to heightened porosity, and the 28-day strength (41.2 MPa) was inferior to that of the 0.2% BF group, indicating that excessive mixing adversely affected strength development [54,55]. The 0.2% BF markedly improved the compressive strength of the CS10-GS10 specimens, considering the fiber-reinforcing effect and workability. Zheng [53] examined the incorporation of 12 mm of BF into concrete examples and determined that a 0.2% BF mixing ratio is best.

3.2. Split Tensile Test Results and Analysis

3.2.1. Effect of Single Mineral Admixture on Splitting Tensile Strength of Recycled Concrete

Figure 7a depicts the effect of single doping of CS on the RAC splitting strength. As CS doping increases, the RAC splitting strength initially increases and then eventually decreases. The C10 specimen exhibits the maximum splitting strength, with 7-day and 28-day splitting values of 2.24 MPa and 3.11 MPa, respectively. With the exception of the C10 specimen, the 7-day and 28-day splitting strengths of single-mixed CS recycled concrete specimens were reduced in comparison with the RAC specimens under varying mixing circumstances. Figure 7b demonstrates that the split strength of C10 specimens increased by 3.23% and 2.6% at 7 and 28 d, respectively, in comparison with the RAC specimens with a 10% admixture. Singh, J [50] and Silva, Y. F. [16] found that incorporation of 10% CS enhanced the split tensile strength of concrete.

According to the experimental findings in Figure 7a on the impact of a single addition of GGBS on the splitting strength of RAC, the splitting strength of GS recycled concrete specimens is less than that of regular RAC specimens at both 7 and 28 d, except for the G20 specimen. The variations in the splitting tensile strength of RAC specimens combined with CS align with this pattern. As the GGBS doping increased, the splitting strength of the RAC exhibited a pattern of initially rising and subsequently declining. Figure 7b illustrates that the splitting strength of G20 specimens at 7 and 28 d is improved by 9.2% and 10.2% compared with RAC, respectively; in contrast to NAC, the top-performing G20 specimen in the experimental group achieved a 28-day splitting strength of 3.34 MPa, which is merely 0.09 MPa less than NAC’s 3.43 MPa, indicating commendable performance.

3.2.2. Effect of Complex Mineral Admixtures on the Splitting Tensile Strength of RAC

Data presented in Figure 8a,b indicate that the C10-G10 specimen has the most favorable impact on splitting tensile strength, with values of 2.71 MPa and 3.53 MPa at 7 and 28 d, respectively. Figure 8c indicates that the C10-G10 specimen exhibits a 24.9% enhancement in 7-day splitting strength relative to RAC. The 28-day splitting strength exhibits a 16.5% enhancement, whereas the 7-day splitting strength shows a 5.4% increase compared with the NAC. The splitting strength at 28 d was augmented by 2.9%. The microaggregate action of CS synergizes with the volcanic ash activity of GGBS to optimize the pore structure and promote the densification of the interfacial transition zone. The optimal dosage (10%) mitigates the detrimental inertia impact of CS overdosage (>15%) and addresses the issue of inadequate early strength associated with single GGBS dosage (>20%). The incorporation of 20% solid waste (10% CS + 10% GGBS) markedly decreases the quantity of cement, rendering it both ecologically sustainable and cost-effective. Consequently, a ratio of 10% CS and 10% GGBS is more effective in enhancing the splitting tensile capabilities of recycled concrete.

3.2.3. BF Improves the Splitting Tensile Strength of C10-G10 Compound RAC

Figure 9a,b illustrate the impact of basalt fiber on the splitting tensile strength of C10-G10 specimens, revealing that specimens infused with 0.2% basalt fiber demonstrated optimal performance in terms of splitting tensile strength. The split tensile strength of the specimens in this group achieved 2.95 MPa and 3.86 MPa at 7 and 28 d of maintenance, respectively, indicating substantial strength improvement. The split tensile strength of the 0.2% BF-doped group rose by 35.9% and 27.3% at 7 and 28 d, respectively, compared with the reference group RAC. The strength enhancement achieved 14.8% and 12.5% relative to the alternative benchmark group, NAC. The data indicate the beneficial impact of basalt fibers on concrete’s strength and underscore the need to choose a suitable additive to improve concrete’s qualities. Significantly, within all BF groups with varying doses, the 0.2% dosage group exhibited the most exceptional performance, hence underscoring the application value of BF in augmenting the mechanical properties of concrete.

3.3. Flexural Test Results and Analysis

3.3.1. Effect of Single Mineral Admixtures on the Flexural Strength of Recycled Concrete

Flexural tests on specimens of crushed copper slag concrete with a 10% cement replacement rate revealed a 14.53% improvement in flexural strength after 28 d, according to Lye, C. Q. et al. [17]. The flexural strength trend of single-mixed CS recycled concrete at 7 and 28 d is identical to the compressive strength and split tensile strength trends, as shown in Figure 10. Furthermore, in the CS and GBBS recycled concrete specimens, respectively, the C10 and G20 groups had the best flexural strength performance. At 7 and 28 d, the C10 specimens’ flexural strength rose by 4.5% and 3.7%, respectively, compared with the baseline group RAC.

Figure 10a illustrates how mono-doped GGBS affects the flexural strength of RAC. The flexural strength of G20 specimens was 4.66 MPa at 7 d and 5.87 MPa at 28 d, according to a comparison of the data in the figure. The data in Figure 7b show that compared with RAC, the flexural strength of G20 specimens rose by 10.2% and 8.1% at 7 and 28 d, respectively. The enhanced effect of CS and GGBS on RAC’s splitting tensile strength and flexural strength was comparatively mild compared with the rise in compressive strength of RAC at 7 and 28 d.

3.3.2. Effect of Complex Mineral Admixtures on the Flexural Strength of RAC

The data analysis presented in Figure 11 indicates that the 7-day and 28-day flexural strengths of CS10-GS10 specimens achieved 5.13 MPa and 6.34 MPa, respectively, surpassing those of RAC (4.23 MPa and 5.43 MPa) by 21.3% and 16.6%, and exceeding NAC (4.97 MPa and 6.28 MPa) by 3.2% and 0.9%, demonstrating superior performance compared with other doping combinations. The flexural strength of the CS10-GS10 specimen in the double-doped combination is exceptional, surpassing that of the CS10-GS20 specimen (4.68 MPa, 5.87 MPa) and the CS20-GS10 specimen (4.25 MPa, 5.83 MPa), indicating that 10% doping of CS and GGBS yields the optimal synergistic effect. The experimental data validate that the optimal double doping ratio can refine the cementitious system and improve the material’s flexural capabilities. Excessive doping, on the other hand, may result in a reduction in material strength due to incomplete reactions or dilution effects.

3.3.3. BF Improves the Flexural Strength of C10-G10 Compound RAC

The data analysis in Figure 12a,b indicates that the C10-G10-0.2BF doping group exhibits the highest performance in the flexural strength test. The specimens in this group exhibited 7-day and 28-day strength values of 5.55 MPa and 6.78 MPa, respectively, reflecting enhancements of 31.2% and 24.9% relative to the RAC, and 11.7% and 8.0% relative to the NAC, respectively. The 7-day and 28-day strengths of the 0.1% BF doping group were 5.11 MPa and 6.32 MPa, respectively, representing enhancements of 20.8% and 16.4% compared with the RAC. The 0.3% BF group exhibited worse flexural strength (4.87 MPa at 7d and 5.96 MPa at 28 d), attributable to inadequate fiber dispersion and homogeneity, which was inferior to the performance of the 0.2% BF group. The study findings indicated that the C10-G10-0.2BF ratio demonstrated appropriate reinforcement during both the 7-day and 28-day maintenance periods, primarily due to the improvement in the mechanical characteristics facilitated by the practical bridging effect of the fibrous materials [55].

3.4. Workability

Figure 13 illustrates the slump values of NAC, RAC, C10, G10, C10-G10, and C10-G10-0.2BF. Experimental results demonstrate that incorporating CS and BF decreases RAC slump, while adding GGBS has the opposite effect. The angular shape and moderate fineness of CS particles increase internal friction within the concrete matrix, impeding workability; conversely, the spherical form of GGBS promotes particle movement, enhancing the slump of G10 specimens. Significantly, despite these variations, all measured slump values fall within the acceptable range specified by China’s GB 50164-2011 [56] standard, confirming adequate workability and validating the practical applicability of these mixtures in construction projects. This finding validates the effectiveness of the mixture’s design, which successfully reconciles constructability with mechanical performance. It is important to note that due to the study’s emphasis on mechanical properties, water-reducing agents were not utilized; however, for applications demanding higher slump values, incorporating such admixtures provides an efficient approach to adjusting workability.

4. Microstructure Analysis

4.1. SEM

The SEM method was used to examine the micro-morphology of the four concrete specimens (NAC, RAC, C10-G10, and C10-G10-0.2BF) and the findings are displayed in Figure 14 and Figure 15. The natural aggregate (crushed stone) in the NAC specimen was created by long-term geological action and has a dense structure, low porosity, and low water absorption, as seen in the image of Figure 14a. There are fewer pores and microcracks inside the NAC because of its interface with the freshly mixed cement paste, the thin thickness of the transition zone, and the high interfacial bond strength. More pores and larger-sized microcracks were seen in the RAC specimens on the paste’s surface, at the interface between the paste and the aggregate, and where the old and new pastes met, as shown in the image in Figure 14b. Uneven vibration during the RAC preparation process and the aggregate displacement phenomenon under the molding pressure could be the source of these structural flaws. Large volumes of blocky calcium hydroxide and needle-like ettringite (AFt) crystals can be detected inside the pores by using higher magnification SEM pictures. On the other hand, there is not enough densification of the pore filling because of the relatively weak link between the crystals and the flocculent hydrated calcium silicate. The mechanical qualities of the RAC binding system are negatively impacted by these tiny structural characteristics, which also compromise the system’s integrity.

The microscopic image analysis in Figure 15a indicates that the C10-G10 specimens exhibited favorable microstructural properties with no discernible pores or cracks. The AFt crystals were found to interact with a substantial quantity of flocculent C-S-H gel, successfully filling the microcracks in RAC. A minor amount of bulk CH was observed to be evenly dispersed in the slurry, with certain pore areas filled with hydration products and CS particles; these characteristics markedly improved the binding capabilities of the material. The microanalysis of Figure 15b indicates that the C10-G10 specimens with 0.2% BF have a more compact microstructure. The BF material demonstrates a notable bridging effect under matrix stress, attributable to its superior tensile characteristics. The synergistic effect of AFt was more evident with flocculated C-S-H, whereas CH existed in an irregular bulk form. The relevant literature [48,49,50] indicates that this phenomenon arises from the transformative effect of volcanic ash’s reactive materials on CH. Consequently, the CH content in the microscopic pictures is relatively minimal. The experimental findings indicate that combining the two materials significantly enhances the hydration reaction process and produces more C-S-H gel networks with robust connections. These hydration products are interlinked to create a more cohesive bond, improving the matrix’s overall efficacy. The subsequent hydration reaction enhanced the product’s composition and refined the interfacial transition zone structure, yielding superior mechanical qualities.

4.2. EDS

The point scan examinations of the RAC, NAC, and C10-G10 specimens cured for 28 d were conducted using the EDS technique, with the findings displayed in Figure 16a,c,e. The test data indicate that the concrete specimens predominantly comprise O, Mg, Al, Si, Ca, and Fe. The Ca/Si ratios for the RAC, NAC, and C10-G10 specimens were determined to be 1.91, 1.37, and 1.25, respectively, while the (Al + Fe)/Ca ratios were 0.151, 0.146, and 0.141, respectively. Taylor’s study [57] revealed that when the Ca/Si ratio was elevated within the range of 0.8 to 2.5, while the (Al + Fe)/Ca ratio remained below 0.2, the formation of C-S-H gels progressively increases as the Ca/Si ratio diminishes. Conversely, the concentrations of CH and AFt diminish correspondingly. The experimental results indicate that the Ca/Si ratios of NAC and C10-G10 diminished by 28.3% and 34.6%, respectively, compared with RAC, confirming that incorporating CS and GGBS positively influences the development of C-S-H gels.

5. Discussion

This study created a schematic diagram of the microscopic action mechanism to systematically elucidate the strengthening pathway of CS, GGBS, and BF in synergistically modified recycled concrete. Figure 17a illustrates that the unloaded RAC specimens exhibited multi-crack propagation under external stress, with the deteriorating process related to the porous structural characteristics and elevated water absorption qualities of the recycled coarse aggregate. The internal porosity of the aggregate results in heightened stress concentration, while the compromised interfacial bonding between the old and new mortar, along with the degradation of mechanical properties in the ITZ, substantially diminishes the load transfer efficiency of the composite system, ultimately leading to a decline in macroscopic mechanical properties.

Figure 17b illustrates that, in contrast, the C10-G10-0.2%BF modified specimens exhibited a pronounced fracture inhibition effect during the loading procedure. The enhancement mechanism is ascribed to several synergistic processes: the microaggregate filling effect of CS and GGBS, which creates an optimal particle gradation with cement particles in the initial phase, fills the voids among cement particles, elevates the initial densities of the slurry, diminishes the primary porosity, offers additional nucleation sites for cement hydration products (e.g., C-S-H), and promotes the uniform spatial distribution of early hydration products. The metal ions in CS may also engage in the hydration reaction to create stable molecules, mitigating the adverse effects on strength [58,59,60]. Furthermore, CS may include a minor quantity of sulfate, enabling the aluminum phase in GGBS to react and produce AFt, enhancing the early strength of RAC. Both CS and GGBS effectively diminish the heat of hydration, alleviate early temperature stress, inhibit microcrack formation, and indirectly improve the mechanical characteristics of RAC. CS and GGBS comprise CaO, SiO₂, Al₂O₃, and various other constituents found in a particular proportion within the RAC. A distinct volcanic ash activity occurs during the late cement hydration and calcium hydroxide reaction stage, forming extra calcium–silicate–hydrate gel and enhancing concrete’s strength in its later stages [61,62] BF operates by bridging and crack-blocking mechanisms, establishing a three-dimensional network inside the concrete that inhibits microcrack propagation, distributes the load, and postpones the stress concentration at the crack tips, thus improving the mechanical properties of RAC specimens [63].

6. Conclusions

In this study, a portion of the cement was substituted with CS and GGBS as cementitious materials, and 30% (w) of the NCA was replaced with RCA to create RAC. The study concentrated on how the mechanical characteristics of RAC were affected by the independent and combination integration of CS and GGBS. It further investigated the enhancement effects of adding varying volume contents of BF on the mechanical properties of RAC, based on the identification of the ideal mix proportion of mineral admixtures. Using SEM and EDS technologies, it methodically examined the microstructural development of changed RAC. The study came to the following conclusions:

• The ternary system formed by the combined addition of CS, GGBS, and BF significantly enhances the mechanical properties of RAC. When singly doped, the optimal replacement rates for CS and GGBS to maximize RAC’s performance are 10% and 20%, respectively, with GGBS exhibiting a more pronounced strengthening effect (21.3%, 10.2%, and 8.1% improvements in 28-day compressive, splitting tensile, and flexural strengths vs. 18.4%, 2.6%, and 3.7% for CS). When co-doped, CS and GGBS synergistically surpass the single-doping effects, with the 10% CS + 10% GGBS configuration yielding the most substantial enhancements (32.9%, 16.5%, and 16.8% increases vs. RAC; 5.3%, 2.9%, and 1.0% vs. NAC. Building upon this binary system, incorporating 0.2% BF further elevates performance: compared with the baseline RAC, the ternary blend achieves 38.9%, 27.4%, and 24.9% improvements in compressive, splitting tensile, and flexural strengths, respectively, while outperforming NAC by 11.4%, 12.5%, and 8.0%.
• The mechanism by which CS, GGBS, and BF reinforce RAC can be summarized as follows. CS and GGBS form a gradation filling effect with cement particles to enhance concrete compactness; GGBS accelerates early hydration while CS strengthens late-stage hydration, generating C-S-H gels that collectively improve mechanical properties; additionally, BF forms a three-dimensional network to inhibit crack propagation. EDS analysis showed that the Ca/Si ratios of NAC and C10-G10 samples decreased by 36.4% and 28.3%, respectively, compared with RAC, verifying that incorporation of CS and GGBS promoted hydration. SEM results indicated that incorporation of CS and GGBS significantly refined the RAC microstructure: the microstructure density of the C10-G10 and C10-G10-0.2BF samples was higher than that of RAC, with some pores filled by denser C-S-H gels. The EDS analysis showed that the Ca/Si ratios of the NAC and C10-G10 samples decreased by 36.4% and 28.3%, respectively, compared with the RAC. Thus, it was verified that the incorporation of CS and GGBS promoted the hydration reaction. The SEM results showed that the incorporation of CS and GGBS greatly enhanced the microstructure of the RAC samples, and the microstructural density of the C10-G10 and C10-G10-0.2BF samples was increased compared with that of the RAC samples, and some of the pores were filled with C-S-H of higher density.
• Using eco-friendly materials like CS, GGBS, and BF can advance sustainable building practices. Engineers can conduct additional research on how solid waste impacts RAC’s performance.