Sustainable Self-Compacting Concrete with Recycled Aggregates, Ground Granulated Blast Slag, and Limestone Filler: A Technical and Environmental Assessment

Abstract

This study investigates the use of demolition waste as recycled coarse aggregates (RCAs) to replace natural coarse aggregates (NCAs), and the use of ground granulated blast slag (GGBS) and limestone filler (LF) as a supplementary cementitious material, in self-compacting concrete (SCC), with proportions of 150 kg/m³ for GGBS and 180 kg/m³ for LF. Various SCC mixtures were prepared with RCA proportions of 0, 25, 50, 75, and 100%, while maintaining fixed dosages of LF and GGBS. Initially, RCA was incorporated in a dry state, followed by a water dosage adjustment for mixtures containing 100% RCA, equivalent to 20 min of RCA absorption. The experimental investigation evaluated the evolution of flow properties through tests such as slump flow, flow time (T500), L-box, sieve stability, fresh density, and air content. The static yield stress and plastic viscosity were also calculated using mathematical models. Additionally, hardened properties, including short-term and long-term compressive strength and capillary water absorption, were assessed. An environmental impact analysis of using demolition waste was conducted, revealing that a total NCA replacement with RCA is viable for both fresh and hardened states, provided that the RCA water absorption is managed and a reactive mineral additive is incorporated. For a 50% replacement of natural aggregates with recycled aggregates, this approach significantly reduces environmental impacts, lowering fossil fuel consumption by up to 35% and greenhouse gas emissions by up to 32%.

1. Introduction

Reducing greenhouse gas emissions is a global priority to ensure a sustainable future, necessitating immediate action to mitigate the environmental impact of human activities. The construction sector is a major contributor to pollution, accounting for approximately 10% of global greenhouse gas emissions [1,2]. Concrete, the second most widely used material after water, has an annual global consumption of 14 billion cubic meters and represents a significant source of emissions in construction. The production of one ton of concrete emits approximately 235 kg of CO₂ equivalent, primarily due to raw material extraction, transportation, and cement production, with the latter contributing 7–9% of global emissions—three times that of air transport [1,2].

The activities associated with the building and construction sector have resulted in increased energy consumption and emissions, with the industry accounting for 36% of global final energy consumption and 39% of CO₂ emissions in 2018. Of this total, 11% originated from the manufacturing of construction materials, including steel, aggregates, cement, and glass [3,4]. Projections by researchers at MIT indicate that without intervention, the sector could emit up to 62 gigatons of CO₂ equivalent between 2016 and 2050, equivalent to emissions generated by 156 trillion kilometers driven by passenger vehicles [3].

The substantial energy consumption and reliance on non-renewable resources in the construction industry can be mitigated through a circular economy approach, which prioritizes the reuse and valorization of waste, particularly construction and demolition waste (CDW), as secondary raw materials. According to recent statistics, the construction sector generates over three billion tons of CDW annually on a global scale [5]. The implementation of a circular economy framework presents a viable solution to both environmental and economic challenges by enhancing waste management practices, reducing the over-exploitation of natural resources, and subsequently lowering CO₂ emissions [6]. Furthermore, recycling CDW offers economic benefits, as it is more cost-effective than conventional waste management methods, such as sorting, transportation, and landfilling [7].

In this context, the valorization of CDW as secondary raw materials represents a relevant strategy for improving the environmental sustainability of the construction sector. Among the most widely used materials in construction is concrete, particularly self-compacting concrete (SCC). SCC is characterized by its high fluidity and stability, allowing it to flow and fill molds without the need for mechanical vibration, which is particularly advantageous for densely reinforced structures. Compared to conventional vibrated concrete, SCC contains a higher volume of cementitious paste, achieved either by increasing the cement content or incorporating supplementary cementitious materials (SCMs), such as limestone fillers (LF), ground granulated blast-furnace slag (GGBS), fly ash (FA), and waste glass powder (WGP) [8,9]. This results in improved flowability, stability, and enhanced mechanical properties and durability.

The high water absorption capacity of recycled concrete aggregates (RCAs) influences the fresh properties of SCC, often leading to reduced workability [10,11]. Research indicates that RCA substitution rates of 50% and 100% by weight adversely impact SCC workability [12]. However, RCA can be successfully incorporated into SCC when the natural coarse aggregate (NCA) substitution rate remains below 50% [13,14]. In its hardened state, the mechanical properties and durability of SCC incorporating RCA have been extensively investigated. Studies suggest that replacing NCA with RCA by up to 30% does not significantly affect compressive, flexural, or tensile strengths [15,16]. Additionally, pre-saturated RCA has been shown to enhance SCC compressive strength at 28 and 90 days [17,18].

Djelloul et al. [14] demonstrated that incorporating up to 30% GGBS improves the performance of SCC containing RCA. Similarly, Guo et al. [19] found that combining RCA with SCMs, such as FA, GGBS, and silica fume (SF), helps to maintain SCC’s workability and mechanical performance. Khodair [20] further reported that incorporating GGBS and FA increases SCC fluidity, though it extends the flow time, making the mixture more viscous.

The utilization of construction and demolition waste, such as recycled aggregates (RAs), requires a comprehensive understanding of their chemical and physical properties to optimize their use as secondary raw materials. Recycled aggregates are heterogeneous materials that often contain residual mortar adhering to the aggregate surface, significantly influencing their performance characteristics. The old mortar content in RAs can range from 20% to 60%, depending on particle size, leading to high porosity, increased water absorption, and reduced density [1]. These properties affect the behavior of recycled aggregate concrete, impacting its rheology, mechanical performance, and durability (e.g., creep, elasticity, and shrinkage) [21,22].

Moreover, the interfacial transition zone (ITZ) between the RA and the cement matrix plays a critical role in determining the mechanical properties and durability of concrete [23,24]. Unlike conventional concrete, recycled aggregate concrete contains multiple interfaces: the old ITZ between parent aggregates and bonding mortar, and the new ITZ between new and old mortar [25]. The old ITZ, which often contains voids and microcracks resulting from deconstruction and processing, adversely affects the overall properties of the concrete [26].

From an environmental perspective, SCMs, as industrial byproducts, contribute to reducing carbon emissions in the construction sector [27]. In Algeria, the steel industry produces approximately one million tons of slag annually, a figure projected to increase due to rising steel demand, particularly with the planned exploitation of the Gara Djebilet iron ore deposit. At present, only 20% of this slag is utilized in cement production (CEM II 42.5/A). While several studies have explored the incorporation of GGBS in concrete mixtures, its application in SCC remains relatively limited [28].

This study aims to investigate the effects of the complete substitution of LF with GGBS on the properties of SCC formulations incorporating RCA sourced from the demolition of a building constructed in the 2000s in eastern Algeria. SCC mixtures were developed with varying RA content (0%, 25%, 50%, 75%, and 100% by volume), while maintaining constant quantities of SCMs.

The fresh-state properties of SCC were evaluated using standard characterization tests, including the slump test with the Abrams cone to measure fluidity, the T500 test to assess flow time and viscosity, the L-BOX test to simulate flow capacity in complex structures, the sieve stability test to evaluate segregation resistance, and tests for density and entrained air content to verify the composition and trapped air within the concrete.

In the concrete’s hardened state, compressive strength and water capillary absorption tests were conducted to assess mechanical properties and durability. Furthermore, correlations between various properties were analyzed. Scanning Electron Microscopy (SEM) was employed to investigate the microstructure, particularly the ITZ, to gain deeper insights into the impact of recycled aggregates on the overall performance of SCC.

Following the experimental study, an environmental impact assessment was conducted to evaluate the potential of RCA valorization in reducing energy consumption and greenhouse gas emissions. Specifically, the assessment compared two scenarios: the first involving the production of natural aggregates, and the second incorporating 50% recycled aggregates. The findings indicate that the use of RCA significantly reduces energy consumption and, consequently, greenhouse gas emissions.

This study highlights the benefits of incorporating recycled aggregates and SCMs, particularly GGBS, in SCC formulations to enhance sustainability in the construction sector. By adopting a circular economy approach, the industry can reduce its reliance on natural resources, lower CO₂ emissions, and improve waste management practices.

2. Experimental Program

2.1. Materials

2.1.1. Cement

Ordinary Portland Cement (CEM I 42.5 R), commonly referred to as OPC, produced by the Biskra Cement Plant (Biskra, Algeria) in compliance with the Algerian standard NA 442 (2013) [29], was used for the preparation of SCC mixtures. The physico-chemical and mineralogical composition of the OPC, determined using Bogue formulas, is presented in

Table 1. Physical properties and oxide composition of cement.

Physical Properties
	Bulk Density	True Density	Blaine Specific Surface	Initial Setting Time	Final Setting Time	Normal Consistency	Residue on 40 μm Sieve
	(kg/m3)	(kg/m3)	(cm2/g)	(min)	(min)	(%)	(%)
OPC	980	3100	3728	220	280	26	12
Oxide Composition Estimation via Bogue Equations
Oxides	C3S		C2S	C3A		C4AF	Gypsum
(%)	68.8		8.6	5.6		11.0	5

and

Table 2. Physical properties and chemical compositions of SCMs and OPC.

Physical Characteristics
	Bulk Density	True Density		Blaine Specific Surface	40 μm Sieve Residue		80 μm Sieve Residue	D50	Water Demand
	(kg/m3)	(kg/m3)		(cm2/g)	(%)		(%)	(µm)	(%)
LF	1090	2680		5108	12.9		2.5	6.9	31
GGBS	940	2900		5797	12.2		2.3	3.4	33
Chemical compositions
Oxides (%)	CaO	Al2O3	Fe2O3	SiO2	MgO	Na2O	SO3	K2O	LOI
LF	53.49	0.02	0.27	0.05	0.22	0.07	0.00	0.01	43.29
GGBS	44.73	6.80	1.60	36.37	3.21	0.24	0.35	0.68	-
OPC	64.67	4.41	3.62	21.0	2.37	-	2.70	0.67	2.92

.

2.1.2. Mineral Admixtures

This study utilized two types of mineral admixtures. The limestone filler (LF), marketed as Bexcarb 10N by SNC Bexcarb Benbrahim and Associates, is a whitish powder derived from grinding limestone rock. X-ray diffraction (XRD) analysis (Figure 1a) indicated that the LF primarily consists of calcite (CaCO₃). The GGBS used was sourced from the El Hadjar steel complex in eastern Algeria. Approximately 60% of the slag produced is rapidly cooled upon exiting the furnace, resulting in a particle size range of 0 to 5 mm. The GGBS was dried at 105 °C for 24 h, and subsequently ground in a ball mill for 16 h. XRD analysis (Figure 1b) revealed that the GGBS mainly comprises an amorphous glassy phase. The X-ray diffraction was performed using a Siemens D5000 diffractometer, equipped with a cobalt anticathode (Kα Co, λ = 1.789 Ǻ), over a range of 10 to 70° (2θ), with a step size of 0.02° and a counting time of 12 s.

The physical properties and chemical compositions of the supplementary cementitious materials (SCMs) are presented in Table 2.

Figure 2 presents the particle size distribution of the cement and the Supplementary Materials used, from which the D₅₀ of these materials was determined by identifying the 50% cumulative volume line.

2.1.3. Aggregates

In this study, the granular skeleton of the SCC mixtures consisted of two natural sands: fine siliceous sand (S1) with a particle size of 0/2 mm from Oum Ali (Tébessa, Algeria), and crushed sand (S2) with a particle size of 0/4 mm, derived from limestone rock crushing by the National Aggregates Company (ENG) in El Kherroub (Constantine, Algeria). Additionally, two types of gravel were used: natural coarse aggregate (NCA) with a particle size of 4/10 mm from the Ain Abid quarry, located 40 km from Constantine (Figure 3a), and recycled concrete aggregate (RCA) with a particle size of 4/10 mm (Figure 3b), obtained from crushed demolition concrete blocks (Figure 4). These blocks were cleaned to remove contaminants such as wood debris and steel, before being crushed and screened to the desired size. The physico-mechanical properties of the aggregates are detailed in

Table 3. Physical and mechanical properties of aggregates.

Characteristics	Unit	S1	S2	NCA	RCA
Apparent density	kg/m3	1480	1500	1490	1180
Absolute density	kg/m3	2620	2620	2670	2420
d/D	-	0/2	0/4	4/10	4/10
Fineness modulus	-	2.10	3.20	-	-
Sand equivalent	%	79	77.2	-	-
Methylene blue	%	0.75	0.75	-	-
Absorption coefficients at 24 h	%	1.72	2.15	1.5	5
Flattening coefficient	%	-	-	14.7	10.8
Los Angeles testing *	%	-	-	24	40
Micro Deval testing *	%	-	-	19	38.6

* Values determined for the 6.3/10 fraction.

. These RCA characterization results are similar to the results obtained in the literature, particularly concerning the apparent density of recycled aggregates [26,27,28,29,30,31,32]. This is mainly due to the presence of the old cement matrix adhering to the recycled aggregates. This matrix, being more porous and therefore less dense, directly influences the apparent density of recycled aggregates compared to natural aggregates. Concerning the mechanical characterization of aggregates, the NF EN 12620 standard requires that the aggregates used in concrete formulations must have a fragmentation resistance of at least Class LA50 (Los Angeles value < 50%). The recycled aggregates used in our study meet this criterion (LA value ≤ 40).

The water absorption kinetics of RCA were determined using the pycnometer method, in accordance with (NF EN 1097-6, 2022) [33]. Three RCA samples were tested at intervals of 5, 10, 20, and 30 min, and at 1, 2, 4, 8, 24, and 48 h of water saturation. Unlike the standard method, which requires wiping the surface water from the RCA before each weighing, potentially causing detachment of fine particles after multiple attempts [34,35] and thereby affecting the accuracy of the water absorption values, the pycnometer method eliminates the need for wiping, preserving the RCA’s integrity for more reliable results. Figure 5 displays the findings.

The absorption coefficients demonstrated good repeatability, as indicated by the relatively small or even absent error bars at certain points. The RCA absorption coefficients increased proportionally with time, with a rapid absorption phase occurring within the first 20 min. Beyond this period, the increase in absorption was minimal and began to stabilize, with an estimated increase of only 0.5% between 20 min and 24 h. These findings align with the existing literature, where several studies [31,32,33,34] describe RCA absorption kinetics in two distinct phases: an initial rapid absorption phase followed by a slower phase. According to Liang et al. [34], the higher capillary pressure in the small pores and the large contact surface area of the RCA facilitate rapid absorption in the first phase. In the second phase, lower capillary pressure in larger pores (previously acting as escape routes) and bubbles adhering to the RCA surfaces slow down further absorption [34]. The literature suggests that the transition between these two phases typically occurs around 10 min, with RCA absorbing between 50% and 80% of the 24 h absorption value within this period [36]. It is thus necessary to account for RCA absorption during concrete mixing by adjusting the mixing water accordingly [32,33,34,35,36,37]. As shown in Figure 5, the absorption at 20 min corresponds to 89% of the total absorption at 24 h, which was used to adjust the water content for SCC mixtures incorporating 100% RCA that did not initially meet self-compacting criteria.

Figure 6 presents the particle size distribution of the aggregates used in this study. The continuous curves, without any discontinuities, confirm their suitability for use in SCC.

2.1.4. Chemical Admixture

A high-range water-reducing superplasticizer (SP), commercially known as POLYFLOW LSR 8800, provided by Solu Est (Annaba, Algeria) was used. It is a new-generation product based on non-chlorinated polycarboxylate.

Table 4. Superplasticizer technical characteristics.

	Absolute Density	pH	Solid Content	Cl Ion Content	Na2O eq Content	Use Range
LSR 8800	1.07 ± 0.02	5.0 to 5.5	29%	≤0.1%	≤1.0%	0.3 à 3.0%

summarizes its technical characteristics.

2.2. Mixture Design

Twelve SCC mixtures were developed for this study using an empirical formulation approach based on AFGC recommendations [38] and the European standard (NF EN 206-9, 2010) [39]. To achieve sufficient paste volume for self-compaction while maintaining a reasonable cement content, LF and GGBS were incorporated into the SCC formulations. All mixtures contained a cement dosage of 350 kg/m³. The target flow class was SF1, as specified in (NF EN 206-9, 2010) [39], with a target strength class of C30/37 for structural concrete and an exposure class of XC1–XC4, according to (NF EN 206+A2, 2021) [40]. The SCC mixture proportions are summarized in

Table 5. SCC mixture proportions.

Group	Mix	OPC kg/m3	SCM kg/m3	Water kg/m3	SP kg/m3	S1 kg/m3	S2 kg/m3	NCA kg/m3	RCA kg/m3	Weff/B
LF	SCC0	350	180	189	5.83	525.5	310	784.4	-	0.36
	SCC25	350	180	189	5.83	525.5	310	588.3	177.8	0.36
	SCC50	350	180	189	5.83	525.5	310	392.2	355.8	0.36
	SCC75	350	180	189	5.83	525.5	310	196.1	533.7	0.36
	SCC100	350	180	189	5.83	525.5	310	-	711.6	0.36
	SCC100C *	350	180	196	5.83	525.5	310	-	711.6	0.37
GGBS	SCC0	350	150	189	5.50	525.5	310	784.4	-	0.38
	SCC25	350	150	189	5.50	525.5	310	588.3	177.8	0.38
	SCC50	350	150	189	5.50	525.5	310	392.2	355.8	0.38
	SCC75	350	150	189	5.50	525.5	310	196.1	533.7	0.38
	SCC100	350	150	189	5.50	525.5	310	-	711.6	0.38
	SCC100C *	350	150	196	5.50	525.5	310	-	711.6	0.39

* SCC corrected by adjusting the mixing water dosage.

.

Various volumetric substitution rates of NCA with RCA were selected for this study: 0%, 25%, 50%, 75%, and 100%. Initially, RCA was incorporated into the mixtures in its dry state. For mixtures containing 100% RCA, a water dosage adjustment was made. Given RCA’s higher water absorption capacity compared to NCA (refer to Table 3), and based on prior studies [22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41], achieving a saturated surface dry (SSD) condition minimizes water absorption and ensures workability comparable to that of NCA-based concrete. In this study, an additional amount of water, equivalent to RCA’s absorption within 20 min, was incorporated during mixing. Beyond 20 min, absorption stabilized at a low rate (absorption kinetics will be detailed in the Results Section). Concrete specimens were demolded 24 h after casting, and then cured in water at 20 °C ± 2 °C until the designated testing periods in the hardened state.

2.3. Test Methods

2.3.1. SCC Fresh-State Properties

The Abrams cone slump test assesses the fluidity of mixtures in an unconfined state. The slump value is determined as the average of two perpendicular diameters measured when the flow ceases, following the standard (NF EN 12350-8, 2019) [42]. This value typically ranges between 550 and 850 mm, as specified by (NF EN 206-9, 2010) [39]. Visual inspection of the spread provides indications of segregation, such as the presence of aggregate accumulation in the center without sufficient paste, or the formation of a bleed water halo around the perimeter. The slump value can be correlated with the yield stress of the concrete using the Sedran correlation, as outlined in Equation (1) [43].

$${\mathsf{\tau }}_0={\displaystyle \frac{\mathsf{\rho }}{1174}}\times (808-\mathrm{S}\mathrm{f})$$

(1)

where ρ = the concrete density in kg/m³ and Sf = the spread in mm.

The T500 spread time measures the duration required for the concrete to reach a diameter of 500 mm after the cone is lifted. For the same final spread diameter, a higher paste viscosity results in a longer spread time, leading to a higher T500 value. This value can be correlated with the concrete’s viscosity using the Sedran correlation, as expressed in Equation (2) [43].

$$\mathsf{\mu }={\displaystyle \frac{\mathsf{\rho }}{1000}}\times \left(0.026\times \mathrm{S}\mathrm{f}-2.39\right)\times {\mathrm{T}}_{500}$$

(2)

where µ = the material plastic viscosity in Pa, ρ = the concrete density in kg/m³, Sf = the spread in mm, and T₅₀₀ = the time required for SCC to reach a diameter of 500 mm (in seconds). The L-box test is used to assess the workability of concrete in confined spaces and to ensure that its placement will not be hindered by blocking phenomena, according to the standard (NF EN 12350-10, 2010) [44]. The vertical part is filled with concrete, and when the gate separating the vertical part from the horizontal part is lifted, the concrete passes through three reinforcing bars. When the flow stops, the concrete heights in the vertical part and at the end of the horizontal part are measured (denoted H₁ and H₂, respectively). The filling rate corresponds to the H₂/H₁ ratio, which must be between 0.80 and 1 (NF EN 206-9, 2010) [39].

The sieve stability test evaluates the resistance of SCC to static segregation of coarse aggregates larger than 5 mm, in accordance with (NF EN 12350-11 2010) [4,45]. The procedure involves resting a 10 L concrete sample for 15 min before pouring 4.8 kg ± 0.2 kg of the concrete onto a 5 mm sieve. After a 2 min interval, the passing laitance is collected and weighed, with the weight expressed as a percentage of the initial concrete mass. To meet stability criteria, the value must be ≤20% for the SR1 class or ≤15% for the SR2 class, as per (NF EN 206-9, 2010) [39].

2.3.2. Hardened-State Properties of SCC

Compressive strength was assessed on 10 cm cubic specimens at 2, 7, 28, and 90 days, utilizing a 2000 kN hydraulic press, in accordance with (NF EN 12390-3, 2019) [46]. The compressive strength value represents the average crushing stress of three specimens tested at each specified age. Additionally, the tensile strength was determined using the splitting tensile test, performed on 11 × 22 cm cylindrical specimens at the same ages of 2, 7, 28, and 90 days, in accordance with (NF P 18-408) [47].

The water absorption was assessed by a capillarity test conducted on cylindrical specimens with dimensions of 110 × 50 mm at 60 days of maturity, following AFPC-AFREM guidelines (AFREM, 1997) [48]. After curing, the specimens were dried at 80 °C until a constant mass was achieved. Water absorption was measured at intervals of 15 and 30 min, and 1, 2, 4, 8, and 24 h. The capillary absorption coefficient was calculated using Equation (3).

$${\mathrm{Ca}}_{\mathrm{t}}={\displaystyle \frac{{\mathrm{M}}_{\mathrm{t}}-{\mathrm{M}}_0}{\mathrm{A}}}$$

(3)

where Ca_t = the absorption coefficient at time t in kg/m², M_t = the mass of the specimen at a given time in kg, M₀ = the initial mass of the specimen in kg, and A = the area of the specimen in contact with water in (m²).

3. Results

3.1. Fresh Properties of SCC

3.1.1. Slump Flow Test and Slump Flow Time

Figure 7 illustrates the evolution of the spread and T500 spread time as a function of the RCA substitution rate and the type of mineral additives used. All SCC mixtures display spread values and T500 times within the standard ranges for SCC, as defined by NF (EN 206-9, 2010) [39], which specifies spread values between 550 and 850 mm and T500 times greater than 2 s, except for mixtures containing 100% dry RCA.

Increasing the RCA content results in a gradual reduction in the spread of SCC, regardless of the mineral additive used, until a critical fluidity threshold is reached (spread < 550 mm). This reduction is attributed to the zero moisture content and high water absorption capacity of RCA, as shown in Table 3, which causes the mixing water to be consumed and decreases fluidity. The decrease is proportional to the RCA substitution rate. Adjusting the water dosage for mixtures with 100% RCA improves SCC fluidity; adding 4.4% water (equivalent to 20 min of RCA absorption, see Section 3.1) enhances workability by 27% and 42% for the LF and GGBS mixtures, respectively. The fluidity of RCA-based concretes is influenced by the shape, moisture content, and replacement rate of the RCA [31]. De Andrade Salgado [49] noted that the shape of coarse aggregates influences flow: rounded aggregates improve flow but reduce adhesion, while angular ones decrease flow but enhance bond strength. Studies show that RCA tends to be flatter and more elongated than NCA [36]. Sun et al. [22] observed that increasing aggregate moisture under SSD conditions significantly enhances concrete workability, while Zhang et al. [50] found that slump values increase with RCA saturation. Higher RCA substitution rates consistently reduce workability [11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51]. RCA’s higher water absorption is mainly due to residual mortar, which contains microcracks and pores, absorbing water from the paste and reducing workability.

Figure 7 shows that SCC mixtures with LF achieve higher slump values compared to those with GGBS, primarily due to the different water demands of the components. GGBS has a higher water demand than LF, associated with its finer particle size (see Section 2.1.2.) and specific surface area, as supported by studies [52,53]. Additionally, the average particle diameter (D50) of LF is significantly larger (about 51% greater) than that of GGBS (see Table 2). Research by Younsi et al. [54] suggests that larger D50 values correlate with lower water demand.

The T500 slump time, defined as the time required for SCC to reach a 500 mm diameter after lifting the Abrams cone, is an indicator of concrete viscosity. Figure 7 (red markers) shows that all RCA-based mixtures exhibit T500 values greater than 2 s, as per (NF EN 206-9, 2010) [39]. Increasing the RCA content proportionally extends flow time, with mixtures containing 100% dry RCA recording 7.8 s (LF) and 11.5 s (GGBS). However, adjusting the water dosage for RCA absorption significantly reduces the T500 by 64% and 71% for LF and GGBS mixtures, respectively. Martínez-García et al. [55] noted similar trends in flow time as RCA replacement rates increase, which they attributed to the low mobility of RCA due to reduced paste content [12] and the angular, elongated shape of RCA, which limits deformability. Excess fines in RCA also contribute to higher water demand, further reducing workability [50].

The use of GGBS increases SCC viscosity, resulting in longer flow times. Replacing LF with GGBS increased the T500 by 13–47% across all RCA replacement rates. Benjeddou et al. [56] established a relationship between LF-based grout flow times and Blaine fineness, linking higher particle counts with reduced interparticle distances and increased friction. Given that GGBS is finer than LF, the increased particle count reduces interparticle distances in the cement matrix, increasing friction and extending T500 times. Additionally, the higher water demand of GGBS further reduces workability and slows deformability.

3.1.2. L-Box Test

Figure 8 presents the L-box test results, following (NF EN 12350,10) [44]. The passing ability index (PL) measures SCC mobility in confined conditions with obstacles, represented in this case by three reinforcing bars. All SCC mixtures achieved PL values equal to or exceeding the standard limit of 0.8 (NF EN 206-9, 2010) [39], except those containing 100% dry RCA. The PL index decreased as the RCA replacement rate increased, while maintaining a constant SP dosage and w/c ratio, regardless of the mineral admixture used. For mixtures with 100% dry RCA, flow was poor in the horizontal section of the L-box, with some mixtures becoming obstructed at the reinforcement bars, resulting in critical PL indices of 0.72 (LF) and 0.51 (GGBS). This decrease is attributed to the higher water absorption of RCA compared to NCA, due to the presence of old mortar, which increases mixture viscosity and reduces SCC mobility. As demonstrated earlier (see Section 3.2.1.), adjusting the water dosage to account for RCA absorption improved the passing ability of 100% RCA SCCs by 36% (LF) and 76% (GGBS), resulting in PL values well above the critical threshold set by the standard.

Kebaïli et al. [12] reported a reduction in the passing ability (PL) index with increasing incorporation rates of recycled concrete aggregate (RCA). For substitution levels of 40%, 60%, and 100%, the PL values failed to meet the required standards, even though the mixing water content was increased to account for RCA’s higher absorption. The primary reason for this outcome was attributed to the increased particle content within the RCA volume. Similar findings were reported by Barroqueiro et al. [57], who also documented a decline in the PL index with greater RCA incorporation. This decrease is attributed to RCA’s higher water absorption compared to natural coarse aggregate (NCA), while maintaining a consistent water-to-cement (w/c) ratio and superplasticizer (SP) dosage.

A review of RCA-based self-consolidating concretes (SCCs) conducted by Santo et al. [58] identified an interesting trend: some studies noted an increase in the PL index with increasing RCA content. However, this improvement in passing ability was achieved by increasing both the w/c ratio and SP dosage to offset the water absorption of RCA.

The substitution of LF with GGBS was found to reduce the passing ability of SCCs in the L-box test. Specifically, PL index values for GGBS series SCCs were consistently lower than those containing LF at all RCA levels. This effect is attributed to the increased viscosity of fresh SCCs containing GGBS, resulting in mixtures that were more viscous and less flowable, especially in confined environments, such as those with reinforcement (e.g., 100% dry RCA SCC). Han et al. [59] further confirmed that adding GGBS at dosages up to 50% increased the apparent viscosity of cement pastes, primarily due to the particle shape and surface texture of the slag.

3.1.3. Sieve Segregation Test

The static segregation of SCCs was evaluated using the sieve stability test, in accordance with the (NF EN 12350, 11) [44,45] standard. The segregation index (SR), expressed as a percentage, quantifies the amount of bleed water (segregated component) passing through the sieve relative to the total concrete initially poured onto it. Figure 9 presents the sieve stability results as a function of RCA incorporation rates. The SR values for all dry RCA-based SCCs remained below 15% (SR Class 2), while corrected 100% RCA SCCs exhibited SR values below 20% (SR Class 1), in compliance with the NF EN 206-9 [39] standard.

The results demonstrate a decreasing trend in SR values with higher RCA content, with reductions of 64.7% and 82.6% observed between 0% and 100% substitution for LF and GGBS, respectively. This trend can be explained by the progressive increase in water absorption associated with RCA. Moreover, increasing the w/c ratio to compensate for RCA absorption in 100% RCA SCCs resulted in a slight increase in the SR index, reflecting reduced segregation resistance. However, this reduction was not detrimental, as the water absorbed by RCA did not reach full saturation, allowing excess water to contribute to a higher SR index.

Similar findings have been reported in the literature. For instance, Safiuddin et al. [13] and Rizwan et al. [60] observed that increasing RCA content led to a reduction in the SR index. They attributed this outcome to the angular shape and rough surface of RCA particles, which enhance the cohesion of SCCs. While increased cohesion may reduce segregation resistance, the overall stability of the mixtures remained within acceptable limits [13].

The incorporation of GGBS in RCA-based self-consolidating concretes (SCCs) enhanced the cohesion of the mixtures compared to those containing LF. As illustrated in Figure 9, the SR index of GGBS-based SCCs decreased, indicating improved segregation resistance. The cohesion of concrete is closely linked to its viscosity; higher viscosity results in a more cohesive and “sticky” mixture, as particle movement within the matrix becomes more restricted (as seen in the case of 100% dry RCA SCC). This increased cohesion reduces the risk of segregation and ensures a more uniform distribution of components throughout the concrete.

As discussed earlier, the addition of GGBS to the cement matrix increases the viscosity of the mixture, with the extent of this effect depending on the GGBS dosage and the water-to-cement (w/c) ratio. The improved viscosity and cohesion achieved with GGBS help to maintain the stability of the mixture, particularly in applications requiring high segregation resistance.

3.1.4. Fresh Density and Air Content

Figure 10 illustrates the variation in the fresh density and air content of SCC mixtures for both series as a function of the RCA substitution rate. The results indicate a decline in fresh density with increasing RCA incorporation, accompanied by a corresponding rise in air content, irrespective of the type of mineral addition used. These trends are consistent with findings reported in previous studies [3,19,52,53] which attribute the decrease in density to the lower specific gravity of RCA, and the increase in air content to the introduction of more porous aggregate particles.

The reduction in the fresh density of SCC can primarily be attributed to the density difference between RCA and natural coarse aggregate (NCA). RCA exhibits lower density due to its composition and porous structure, which result from the presence of adhered mortar and the inherently low density of these aggregates. As shown in Table 3, the density of RCA is approximately 9.4% lower than that of NCA. Consequently, for the same concrete volume, mixtures containing RCA exhibit reduced density corresponding to the proportion of RCA used.

In addition to density differences, other factors such as porosity, aggregate surface texture, shape, and size influence the fresh density of SCC [61]. Pores within the cement paste can be filled with water or air, and the presence of these voids decreases the fresh concrete’s density while increasing its air content. Lavado et al. [62] reported that RCA’s rough surface texture and angular shape promote greater air entrapment within the mix, contributing to increased air content.

As illustrated in Figure 11, the fresh density of SCC mixtures is highly sensitive to fluctuations in air content, with a clear inverse relationship between the two parameters. An increase in air content consistently leads to a decrease in fresh density across all SCC mixtures. This relationship further explains why higher RCA content results in higher air content within the concrete.

The influence of mineral additions on these parameters was not significant, as no notable differences in fresh density or air content were observed between mixtures containing LF and those with GGBS.

3.1.5. Rheological Correlations

Figure 12 presents the results for static yield stress and plastic viscosity, obtained through correlations and the application of Equations (1) and (2). The findings indicate that both rheological parameters increased with higher RCA content, irrespective of the type of mineral addition used. However, increasing the water-to-cement (w/c) ratio in corrected 100% RCA SCCs substantially reduced the yield stress, by 70% and 68% for SCCs with LF and GGBS, respectively, compared to their dry 100% RCA counterparts.

A similar reduction was observed in plastic viscosity, with decreases of 47% and 56% for LF and GGBS, respectively. As discussed in earlier sections, these changes are primarily attributed to the high absorption capacity of RCA, which results from the presence of old mortar adhered to the aggregate surface. The increased absorption affects the rheological behavior by enhancing the yield stress and viscosity in dry mixtures.

These trends align with observations reported in previous studies, which similarly highlight the influence of RCA’s high absorption capacity on the rheological parameters of SCC mixtures. Mandal et al. [63] recently investigated the rheological properties of SCCs with varying RCA incorporation rates of 0%, 25%, 50%, 75%, and 100%, while maintaining a constant water-to-cement (w/c) ratio and superplasticizer (SP) dosage. The study revealed that increasing the RCA content led to higher static yield stress and plastic viscosity values. This increase in rheological parameters was attributed to the heightened internal friction between aggregates, which becomes more pronounced as the RCA content in the concrete rises [63,64].

The highest values for yield stress and plastic viscosity were recorded in SCCs containing GGBS. Specifically, the yield stress was found to increase by 55.7% in mixtures with 0% RCA and by 9.8% in mixtures with 100% dry RCA, while plastic viscosity increased by 20.3% and 38.4% under the same respective conditions. These increases are linked to the greater fineness of GGBS, expressed in specific surface area (SSA), compared to LF.

Jiao et al. [65], in a comprehensive literature review on the influence of supplementary cementitious materials (SCMs) on concrete rheology, report similar findings. The review highlights the significant impact of SCM fineness and composition on rheological behavior, further supporting the observations made by Mandal et al. [63].

Research in the literature has established correlations between the rheological parameters and the fresh properties of SCCs [63,64,65,66,67,68,69,70]. In this context, Figure 13 presents correlations between rheological properties and selected conventional fresh properties of SCCs. Specifically, static yield stress is correlated with L-box filling capacity (Figure 13a), while plastic viscosity is correlated with both filling capacity (Figure 13b) and sieve stability (Figure 13c).

The relationship between yield stress and L-box filling capacity shows strong linear correlations, with coefficients of determination (R²) of 0.93 and 0.96 for SCCs containing LF and GGBS, respectively. These findings align with those reported by González-Taboada et al. [66]. Similarly, a robust linear correlation can be observed between plastic viscosity and filling capacity, with R² values of 0.95 for LF and 0.99 for GGBS.

An additional correlation was identified between plastic viscosity and sieve stability, following a power-law trend. This relationship yields R² coefficients of 0.84 and 0.90 for LF and GGBS, respectively. These correlations indicate that rheological parameters, such as yield stress and plastic viscosity, can be used to predict and establish the fresh properties of SCCs [63].

3.2. Hardned Properties of SCC

3.2.1. Compressive Strength

Figure 14 illustrates the evolution of compressive strength across different SCC mixtures as a function of RCA content at curing intervals of 2, 7, 28, and 90 days. Similarly to conventional SCCs containing natural coarse aggregate (NCA), the compressive strength of RCA-based SCC increases with the age of the concrete. However, an increase in the RCA incorporation rate results in a reduction in compressive strength at each curing age, irrespective of the type of mineral admixture employed. These findings align with those reported in the existing literature [71,72,73] confirming the inverse relationship between RCA content and compressive strength.

It is well known that the mechanical strength of concrete partly depends on the strength of its constituent aggregates. In particular, the quality of recycled concrete aggregate (RCA) has a significant influence on the performance of hardened concrete. As noted by Poon et al. [71] and Tabsh and Abdelfatah [72], the mechanical properties of RCA are closely related to the strength of the parent concrete. RCA derived from high-performance concrete tends to possess a relatively dense interfacial transition zone (ITZ), while RCA from normal-strength concrete exhibits a more porous ITZ. In this study, the RCA originated from demolition concrete obtained from an old residential building, suggesting that the parent concrete was of average quality.

Post-failure observations of specimens broken under compression indicated that fracture lines generally formed around the aggregates. Similarly, Butler et al. [73] found that in C30 strength class concretes, fracture planes tended to develop around RCA particles, demonstrating that the ITZ can impair mechanical strength. This reduction in strength is primarily attributed to the poor quality of the ITZ, which arises from the old mortar adhering to the surface of the RCA. The old mortar is characterized by high porosity, as described by Rashid et al. [67]. The quantity of adhered mortar depends on the aggregate size: larger aggregates tend to retain more mortar. Given that the maximum aggregate size used in this study was 10 mm, the amount of adhered mortar is considered moderate.

The 28-day compressive strengths of mixtures with 50% and 100% RCA substitution were reduced by approximately 14.7% to 15.8% and 28.9% to 34.4%, respectively, for both series. These reductions are lower than those reported in the literature review by Piccinali et al. [68]. Another factor contributing to the reduction in compressive strength is the high water absorption of RCA, which necessitates an increased water-to-cement (w/c) ratio to achieve adequate workability. Consequently, a reduction in mechanical strength is expected, as observed in the 100% corrected SCC mixtures, where the higher w/c ratio resulted in diminished strength. Nevertheless, the compressive strengths obtained for all mixtures exceeded the targeted strength class of C30/37.

The type of mineral admixture also influenced the mechanical performance of the concrete. Early-age strengths (at 2 and 7 days) were higher in mixtures containing LF, while long-term strengths (at 28 and 90 days) were superior in mixtures incorporating GGBS. These findings align with those reported by Ali Boucetta et al. [30]. The enhanced early-age performance of LF is attributed to its physical effect, as the finer particle size of LF compared to cement promotes cement hydration by providing multiple nucleation sites. This phenomenon was also noted by Ye et al. [69], who explained that these nucleation sites facilitate the formation of new hydrated crystals. However, the influence of LF diminishes over time, reducing the rate of strength development beyond 7 days [70], as shown in Figure 14a.

In contrast, the strength gain observed beyond 7 days in GGBS-containing mixtures is due to the formation of additional calcium silicate hydrate (C-S-H) through a latent hydraulic reaction [11]. This reaction requires an alkaline activator, which is supplied by the calcium hydroxide (CH) generated during cement hydration, thus explaining the gradual improvement in strength over time. Mindess [74] reported that the formation of secondary C-S-H contributes to reducing porosity and densifying the microstructure by increasing the solid volume of hydrates in the paste.

Microscopic observation with SEM-SE micrographs of the SCC100C concrete at 50 µm and 20 µm scales is shown in Figure 15 and Figure 16.

Figure 15 illustrates the formation of the interfacial transition zones (ITZs) between the old cement paste and the new one. In concretes containing RCA, the presence of two ITZs is a key factor influencing the reduction in mechanical strength. RCA inherently retains residual mortar from the original concrete, leading the formation of an additional ITZ between the old mortar and the original aggregate. This double interface, which is often more porous and weaker, creates a zone of weakness where cracks can propagate more easily under load, thereby contributing to a reduction in compressive strength.

Figure 16 shows the formation C-S-H gels, portlandite, and ettringite in both the mixes based on LF and GGBS. For all the samples observed, it is clear that the cementitious matrices are homogeneous, and there are no major defects or excessive porosity.

3.2.2. Tensile Strength

In addition to the compressive strength results, Figure 17 shows the evolution of the tensile strengths of different SCC mixes, formulated with natural and recycled aggregates, at various substitution rates and curing ages of 2, 7, 28, and 90 days. The results show that the reference SCC formulated with 100% natural aggregate (NA) has the best splitting tensile strengths, irrespective of the age of the concrete and the type of mineral addition used, whether limestone filler or granulated slag. On the other hand, the gradual replacement of NA with recycled aggregate (RCA) leads to a marked reduction in tensile strength, which becomes more pronounced as the substitution rate increases. For example, in the case of total replacement of NA at 28 days, the reduction in splitting tensile strength is around 30.2% for SCC containing LF, and around 29.2% for SCC incorporating GGBS. This trend is similar to that observed previously for compressive strength, suggesting overall consistency in the mechanical behavior of RCA-based SCC. This reduction in mechanical performance is strongly linked to the properties of RCA, notably its higher porosity and water absorption capacity. These aspects have been discussed in detail in the Section on compressive strength. Similar observations were reported by Kou and Poon [10,24]. Similarly, to what was observed for compressive strength, the latent hydraulic power of blast furnace slag improved tensile strength from 28 days, compared with the values obtained with limestone filler. These obtained results are in accordance with those reported in the literature [62,63,71,72,73].

3.2.3. Capillary Water Absorption

The capillary water absorption coefficient reflects concrete’s capacity to absorb water through its pore network: the more extensive the capillary network, the higher the absorption and, consequently, the greater the absorption coefficient. Figure 18 illustrates the capillary absorption coefficients as a function of the square root of time for the SCC mixtures in the two series.

The water absorption coefficient (Cat) was calculated using Equation (3), with testing performed after 60 days of specimen curing. The reported Cat values represent the average of three measurements. Due to the high consistency among these values, the error bars are minimal and not visible at many data points. Capillary water absorption increased significantly with higher RCA content. At 24 h, the increase in absorption ranged between 0% and 100% RCA, with respective increases of 139% and 212% for the LF and GGBS series. The substantial water absorption of RCA, attributed to the presence of old mortar adhering to the aggregate surface, particularly within the interfacial transition zone (ITZ), plays a key role in this increase.

Additionally, raising the w/c ratio, as in the case of the corrected 100% RCA SCC mixtures, to meet RCA’s water demands and ensure proper self-compaction contributes to the development of additional pores and capillaries, further enhancing the concrete’s capillary absorption. Beyond 50% substitution of NCA by RCA, the slopes of the absorption curves between 0 and 8 h increase and change in pattern, especially in the LF series. According to Badreddine-Bessa [75], this change suggests the presence of larger pores.

The use of GGBS mitigated capillary water absorption in SCC compared to the use of LF across all RCA replacement levels. For instance, at 24 h, the capillary absorption of the 100% RCA mixture was reduced by 37.6% with GGBS. This improvement is attributed to a reduction in both the diameter and number of pores and capillaries, resulting from the formation of new hydrates. As noted in previous research [30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76], GGBS positively affects capillary absorption through the formation of secondary calcium silicate hydrate (C-S-H) via hydraulic reactions. These C-S-H phases reorganize the pore structure by lining the inner surfaces of pores and capillaries, refining the pore network, and enhancing the microstructure.

3.3. Environmental Assessments

In addition to the depletion of natural resources, the production of natural aggregates generates considerable CO₂ emissions, estimated at approximately 32 kg CO₂-eq per ton [77]. Furthermore, the transportation of these aggregates to concrete plants contributes about 5% to the global warming potential (greenhouse effect) [4]. Implementing sustainable practices, such as recycling demolition waste, utilizing alternative materials, and improving production efficiency, can significantly mitigate these environmental impacts. These measures not only protect natural resources, but also foster more sustainable and responsible development.

A preliminary life cycle analysis (LCA) was conducted, and is discussed in this Section, to assess the environmental benefits, greenhouse gas (GHG) emissions contributing to global warming, and energy consumption associated with the production and transportation of both natural and recycled aggregates derived from demolition waste for concrete production. Two scenarios were proposed, as illustrated in Figure 19. The analysis was based on data from a national aggregate production company located in El-Kherroub, Constantine, in eastern Algeria, which supplied the natural coarse aggregate (NCA) used in this research.

According to the company’s technical data, the annual production of 3/8 class gravel—comparable to the 4/10 class used in this study—amounts to approximately 84,000 tons. This quarry was chosen not only for the relevance of its aggregate for concrete production, but also due to the depletion of natural resources in the Annaba region, underscoring the need to explore more sustainable material sourcing options.

The first scenario in this study focuses on the environmental impact associated with the annual production of 84,000 tons of natural coarse aggregate for concrete production (NCA). The second scenario examines the use of recycled coarse aggregate (RCA) as a 50% replacement of NCA for concrete production. This substitution rate was determined based on the mechanical characterization of the SCC findings presented earlier in this study.

Table 6. Global warming potential (GWP) and energy consumption values of natural and recycled gravel production.

	Energy, Fossil Fuels (MJ/t)	GWP (kg eq. CO2/t)	References
Natural gravel production	30	7.9	Calculated
Coarse recycled concrete aggregate	4	2.5	[77,78]
Transportation (truck transport 40 tons)	0.39	0.07	Calculated

,

Table 7. Energy consumption for the production of natural and recycled aggregate.

Production Process Activities	Gravel Quantity	Energy Consumption
		Natural Gravel Production	Recycled Gravel
		Extraction, Primary Crushing, Secondary Crushing, Screening	Primary Crushing, Separation, Secondary Crushing, Screening
Production (MJ)	1 ton	30	4
	42,000 tons	1260	168
Transportation (MJ)	1 ton	54.6 (140 Km)	21.6 (55 km)
	42,000	2293.2	907.2
Total energy consumed (MJ)	1 ton	84.6	25.6
	42,000 tons	3553.2	1075.2
Total energy consumption prevented (MJ)	42,000 tons	2478

and

Table 8. Determination of the global warming potential of natural and recycled aggregates.

Production Process Activities	Gravel Quantity Production	Greenhouse Gas Emissions
		Natural Gravel Production	Recycled Gravel
		Extraction, Primary Crushing, Secondary Crushing, Screening	Primary Crushing, Separation, Secondary Crushing, Screening
Production (kg eq. CO2)	1 ton	7.9	2.5
	42,000 tons	331.8	105
Transportation (kg eq. CO2)	1 ton	10 (140 Km)	3.9 (55 km)
	42,000	420	163.8
Total energy consumed (kg eq. CO2)	1 ton	17.9	6.4
	42,000 tons	751.8	268.8
Total energy consumption prevented (kg eq. CO2)	42,000 tons	483

summarize the energy consumption and CO₂ emissions generated during the production processes of NCA and RCA, as well as the transportation of aggregates from the quarry and demolition waste from disposal sites to the concrete production plant. The plant is located approximately 140 km from the NCA production quarry and 55 km from the demolition waste disposal site. Upon arrival at the plant, the transported demolition waste undergoes production treatments, as depicted in the second scenario (Figure 19).

To estimate the energy consumption and global warming potential of NCA production, the national aggregate production company provided data on electricity and fuel usage. However, due to the limited availability of data on RCA production, particularly within Algeria, energy consumption and CO₂ emissions for RCA were assessed based on findings from the literature [77,78]. Transportation-related energy consumption was calculated based on the fuel consumption of a truck with a load capacity of 40 tons.

The results of the estimates indicate that replacing 50% of natural gravel with recycled gravel can achieve a positive energy balance. This partial substitution reduces energy consumption by 35%, corresponding to 483 kg CO₂-eq. In addition to these energy savings, the integration of 50% RCA in concrete production results in a substantial reduction in greenhouse gas emissions, estimated at approximately 32%, equivalent to 2478 MJ. Furthermore, the use of 50% RCA in concrete production extends the lifespan of the raw material extraction quarry by 25 years.

This strategy contributes to the sustainable management of construction and demolition waste by repurposing it for concrete production. It aligns with the global objectives of sustainable development by reducing the environmental footprint of the construction industry and promoting the responsible use of natural resources.

4. Conclusions

This study investigates the feasibility of using recycled concrete aggregate (RCA) in the production of self-compacting concrete (SCC). The primary challenge associated with RCA lies in its high water absorption capacity and low strength, which result from the presence of adhering old mortar. The highly fluid nature of SCC further complicates its use with RCA. At first glance, these characteristics appear contradictory; however, they needed to be integrated into a single material that maintains both self-compacting properties and adequate mechanical properties.

Twelve SCC mixtures were evaluated, incorporating varying RCA replacement levels (0%, 25%, 50%, 75%, and 100%) and two types of mineral additives: LF and GGBS. The key findings are summarized as follows:

• Fresh-State Properties: Increasing the RCA content reduced the free flow of SCC (as observed in the slump flow test) and horizontal flow (L-box test), while increasing the flow time (T500 test) and resistance to static segregation (sieve stability test). For mixtures containing 100% RCA, specific adjustments to the water-to-cement (W/C) ratio were made to compensate for the water absorption of recycled aggregate, which is particularly pronounced within the first 20 min. These adjustments successfully improved the SCC’s properties, maintaining its flowability and stability, despite the challenges posed by RCA’s characteristics. The addition of GGBS, compared to LF, exhibited a declining trend in fresh-state properties.
• Rheological Parameters: Higher RCA content increased both the static yield stress and plastic viscosity, effects that were further amplified with the use of GGBS. Rheological parameters, derived using the model proposed by Sedran and de Larrard (1999), correlated well with the L-box and stability test results.
• Mechanical Strength: The significant porosity within RCA, primarily in the old mortar, led to a proportional reduction in mechanical strength as the RCA content increased. This reduction, estimated at 28 days, was 8.2%, 15.8%, 21.6%, and 28.2% for mixtures containing LF, and 5.6%, 14.7%, 22.9%, and 27% for those incorporating GGBS, depending on the progressive replacement of NA with RCA from 0 to 100%. However, the inclusion of GGBS had a positive impact on long-term strength, partially offsetting the reduction observed in the recycled SCC mixtures. Notably, at 90 days, the mixtures containing 100% RCA with an adjusted water dosage and incorporating GGBS exhibited 27.7% higher strength compared to SCC mixtures containing LF.
  A similar trend was observed for splitting tensile strength. SCC formulated with 100% NA showed the best performance, while the gradual replacement of NA with recycled aggregate resulted in a marked reduction. At 28 days, this reduction reached around 30.2% for BAPs containing LF, and 29.2% for those incorporating GGBS.
• Capillary Water Absorption: An increase in capillary water absorption was observed with higher RCA content, due to the material’s high porosity, especially in the interfacial transition zone (ITZ). However, replacing LF with GGBS resulted in lower capillary absorption coefficients.
• Environmental Impact: For the Annaba region in eastern Algeria, the annual application of a 50% RCA replacement rate in concrete production could potentially reduce fossil fuel consumption by up to 35% and greenhouse gas emissions by as much as 32%. These results demonstrate that using recycled aggregates significantly limits the environmental impact of concrete production, while reducing dependence on the extraction of natural aggregates. By integrating RCA into the production of SCC, this study highlights the role of recycled aggregates in reducing the carbon footprint of the concrete industry by valorizing construction wastes. This finding emphasizes the importance of developing practical solutions to promote the use of recycled materials in construction practices.

In conclusion, the use of RCA as a complete substitute for natural coarse aggregate (NCA) in SCC production is feasible, provided that RCA’s water absorption is accounted for in the mix design and reactive mineral additives are incorporated. This study underscores the importance of using sustainable materials to mitigate environmental impacts and promote environmentally responsible construction practices.

From this perspective, future research should focus on optimizing RCA treatment methods to enhance their properties and exploring additional SCM combinations to improve the performance of SCC in structural applications.