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6 November 2025

Effects of Mineral Admixtures and Mixing Techniques on the Performance of Steel Fibre-Reinforced Recycled Aggregate Concrete

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Department of Civil Engineering, University of Engineering & Technology Taxila, Taxila 47050, Pakistan
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Buildings2025, 15(21), 4010;https://doi.org/10.3390/buildings15214010
This article belongs to the Section Building Materials, and Repair & Renovation
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Abstract

In this work, the synergistic effects of mineral admixtures and advanced mixing processes are systematically accounted for steel fibre-reinforced recycled aggregate concrete (SFR-RAC). It studies the improvement of performance optimization in SFR-RAC, inherently weak ITZ by adding 0.5% hooked steel fibres and replacing cement with ground granulated blast furnace slag (25–50%), fly ash (20–40%) and silica fume (7–14%). The efficiency of double-mixing (DM) and triple-mixing (TM) procedures were comprehensively evaluated. Results showed that mineral admixtures could improve mortar-aggregate interface bond, and the triple-mix technique contributed to such improvement. The maximum performance was observed for the combination of 7%SF with triple mixing (7%SF-TM), which presented increased compressive, tensile and flexural strengths by 7–18%, 12–29%, and 16–31% respectively. The durability was significantly improved, and the water resistance could increase by 53% with addition of 7%SF-TM, chloride penetration depth reduced by 86% when incorporated with 25%GGBS-TM, acid attack decreased by 84% with addition of 14%SF-TM. Microstructural analysis (SEM, XRD) confirmed that these enhancements stem from a denser matrix and refined ITZ due to increased C–S–H formation. This study confirms that the strategic integration of fibre reinforcement, pozzolanic admixtures and optimized mixing protocols presents a viable pathway for producing sustainable concrete from construction waste.

1. Introduction

Concrete is the most widely used construction material globally, valued for its affordability, high compressive strength and durability [1]. However, the extensive consumption of natural aggregates and the energy-intensive production of Portland cement pose significant environmental threats, including resource depletion and substantial greenhouse gas emissions [2]. In parallel, rapid urbanization and industrialization generate enormous volumes of building and demolition debris (BDD), with annual estimates of 1.85 billion tons in China and 320–380 million metric tons in Europe [3]. In developing regions like Pakistan, factors such as natural disasters and urban expansion further exacerbate BDD accumulation, leading to severe waste management challenges [4,5]. Recycling this debris into recycled concrete aggregate (RCA) presents a dual opportunity: conserving natural resources and mitigating waste disposal problems, thereby promoting a circular economy in construction [3].
Recycled Aggregate Concrete (RAC) is an environmentally friendly material which uses wastes in concrete but has weaker mechanical properties and lower durability than ordinary concrete. It is mainly due to the porous attached mortar of RCA which forms a poor ITZ with the new mortar matrix [6]. The earliest methods to enhance RCA quality were based on treatment such as mechanical crushing, acid wash and bio-techniques to detach this bound mortar [7,8,9,10]. However, these methods are typically highly energy intensive, expensive and environmentally unfriendly. Thus, attentions have been given to more applicable techniques from academia such as the application of mineral admixtures—industrial waste materials: fly ash (FA), silica fume (SF), ground granulated blast-furnace slag (GGFS), metakaolin, nanoparticles such as nano-silica and graphene oxide coating. These materials develop the microstructure of concrete through pozzolanic reactions and particle packing, leading to enhancement of the ITZ and reduction in permeability [11,12,13].
GGBS has been proven to improve ITZ as well as the long-term strength and durability especially against chloride ion attack. However, its effectiveness is dose-dependent and high addition levels (55%) may decrease early-age mechanical performance [14,15]. Fly ash (FA) increases workability and has been reported to promote strength gain at later ages but can retard early age strength development. Its pozzolanic activity is especially advantageous in the porous RAC field [11,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29]. SF, being a high pozzolic material is an effective additive in enhancing the mechanical strength and durability through the packing of binder matrix. Varying authors have shown that RAC with 5–10% SF can exceed the NAG in its performance [11,20,23,30,31,32,33,34,35,36,37,38].
To improve performance even more, novel mixing techniques have been proposed. The triple-mixing (TM) method refers to coating RCA with a cementitious or pozzolanic slurry prior to mixing the remaining ingredients. This forms a more, dense strong ITZ as in situ pozzolanic reaction is encouraged that depletes the calcium hydroxide and effectively fills the voids in the attached mortar [39,40,41]. Likewise, it has been demonstrated that the two-stage mixing method (TSMA) can improve ITZ and increase compressive strength dramatically [36,42]. These techniques are better than simple mixing methods, which concentrate mainly on the pre saturation.
At the same time, adding fibres also overcomes handicaps of low tensile strength and brittleness in concrete. Fibers provide a substantially increased toughness, flexural strength, and resistance to cracking [13,19,20,21,33,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60]. However, it could enhance permeability in some cases because they can leave voids after being eroded and form an interconnected network, which is a trade off between enhancement of mechanical properties and durability [13,19]. Accordingly, a combined approach of fibres with mineral admixtures is quite promising as the admixtures may offset possible deteriorating effects of the fibres on durability by promoting densification [47,48,49,50,51].
While the binary combinations including fibre and admixtures, and admixture mixing technique have been mainly discussed, synergistic impacts of a ternary combination of fibre, mineral admixtures and advanced mixing technique on 100% recycled concrete aggregate concrete (RCA) are relatively limited. The novelty of this study is to explore the synergistic influence of steel fibres (0.5%), various mineral admixtures (GGBS, FA, SF) and innovative mixing techniques (DM, TM) in case of 100% RCA mixes. This integrated approach can effectively compensate the multi-faceted deficiencies of RAC: fibers can bridge the macro-crack; admixtures are able to densify micro-pore structure and advanced mixing improve ITZ. The improvements in mechanical and durability properties are thoroughly investigated, aside from microstructural analyses (SEM, XRD) to determine the underlying processes. This integral process offers a route towards high-performing and sustainable concrete making full use of the potential of 100% recycled concrete aggregates.

2. Experimental Program

2.1. Materials

Ordinary Portland Cement (OPC) Grade 53 was used as the primary binder. It was partially replaced with three mineral admixtures: Ground Granulated Blast Furnace Slag (GGBS), Class F Fly Ash (FA), and Silica Fume (SF). Their chemical and physical properties are detailed in Table 1.
Table 1. Chemical composition and physical properties of binders.
The fine aggregate used was natural siliceous sand. The recycled concrete aggregate (RCA) was sourced from laboratory waste concrete (parent strength: 35–40 MPa), manually crushed, and sieved to a maximum size of 20 mm [61]. To ensure a valid comparison, the particle size distribution of the RCA was matched to that of a standard natural coarse aggregate, as shown in Figure 1a, while the fine aggregate gradation is presented in Figure 1b.
Figure 1. (a) Particle Size Distribution of recycled concrete aggregate (RCA) and Natural concrete aggregate (NCA). (b) Particle Size Distribution of Fine aggregates.
The physical properties of the fine aggregate and recycled concrete aggregate (RCA) are summarized in Table 2. The RCA exhibited a high 24 h. water absorption of 6.11%, primarily due to its porous adhered mortar content [11]. The adhered mortar content was quantified at approximately 45% by mass using the acid digestion method [62]. The Los Angeles abrasion value of 38% indicated a relatively good resistance to degradation [63,64,65,66]. Furthermore, the RCA was preconditioned to a saturated surface dry (SSD) state before batching to account for its high absorption and maintain free water-to-binder ratio of 0.38 across all mixtures. Potable water was used for mixing in all mixes.
Table 2. Physical properties of fine aggregate and RCA according to ASTM Standard.
Hooked-end steel fibres (HSF) with a length of 34 mm, diameter of 0.9 mm, tensile strength of 1200 MPa, and density of 7750 kg/m3 were used at a volume fraction of 0.5% [56,57] as shown Figure 2.
Figure 2. Overview of Hooked steel fibres (HSF).

2.2. Mix Proportions

Two series of Steel Fibre-Reinforced Recycled Aggregate Concrete (SFR-RAC) were designed: a Double-Mixing series (DFRAC) and a Triple-Mixing series (TFRAC). Both series were designed with a constant water-to-binder ratio of 0.38 and incorporated 0.5% by volume of steel fibres, targeting a compressive strength range of 35–40 MPa. To investigate the effect of supplementary cementitious materials, a portion of the cement was replaced with Ground Granulated Blast-furnace Slag (GGBS) at 25% and 50%, Fly Ash (FA) at 20% and 40%, and Silica Fume (SF) at 7% and 14% by weight. A polycarboxylate-based superplasticizer was used, with its dosage adjusted in each mix to maintain a consistent target slump of 75–100 mm. The complete mix proportions are provided in Table 3.
Table 3. Mix proportions of all concrete mixtures of Steel fiber reinforced RAC (SFR-RAC).

2.3. Mixing Procedure

Mechanical mixing was performed using two distinct methods: the double-mixing (DM) and triple-mixing (TM) techniques. The double-mixing (DM) procedure consisted of two primary stages: (1) pre-wetting the recycled concrete aggregate (RCA) with 50% of the total mixing water, followed by (2) the introduction of the cementitious materials, sand, remaining water, and chemical admixtures [36,42]. The triple-mixing (TM) technique involved a more complex sequence: (1) the RCA was initially coated with a slurry of admixtures, mixed at a water-to-binder ratio of 0.38 for 15 s, after which (2) the remaining ingredients were added to complete the mix [39]. The procedural sequences for both the DM and TM methods are illustrated schematically in Figure 3a,b.
Figure 3. (a) Detailed procedure of the Double-Mixing Technique (DM). (b) Detailed procedure of the Triple-Mixing Technique (TM).

2.4. Specimen Preparation and Testing

The workability of all freshly mixed concrete was evaluated immediately after mixing using the slump test in accordance with [67]. The mix proportions were designed to achieve a target slump of 75–100 mm. For RCA mixtures, any observed reduction in workability was compensated by adjusting the dosage of a high-range water-reducing admixture (HRWRA), as detailed in Table 3. A high-range water-reducing admixture of polycarboxylate-ether (PCE) type commercially referred to as Ultra Superplasticizer 470, with specific weight equal to 1.155 g/cm3 was adopted [21].
This superplasticizer was selected due to its high-dispersed properties and compatibility with the mineral additives employed. In the case of RAC, in which the mixing conditions are unfriendly and contribution from fibers to the total flexural capacity cannot be overemphasized, all additions were made to avoid fiber balling and to provide even distribution in as follows: (i) Fibres were discretely added to the running mixer using a duration of 60 s. (ii) The mixing time was increased by 2 min above that attained when everything else is complete. (iii) Visual assesment of the fresh concrete in each mix ensured there were not any visible clusters present within the fiber slurry, and since no clusters were observed a successful dispersion was achieved [43,47,57].
A total of 574 specimens were prepared for mechanical and durability testing, which include 100 mm cubes for compressive strength, Ø100 × 200 mm cylinders for split tensile strength and rapid chloride penetration test (CPT), and 100 × 100 × 500 mm prisms for flexural strength. Moulds filling was made in three layers by a vibrating table compaction per layer. After 24 h, specimens were demoulded and then cured (in potable water at a constant temperature of 20 ± 2 °C) to specified test age [67]. The casting, demoulding and testing processes are shown in Figure 4a–c respectively.
Figure 4. (a) Casting of all samples. (b) Demoulding of specimens. (c) Testing of a Cube.

2.5. Testing Methods

The mechanical and durability attributes of the concrete mixed were studied based on standard test methods. At the 7, 28 and 90 d age of curing the values for compressive strength [68], split tensile strength [69] and flexural strength [70] were recorded. The durability was evaluated with water absorption [71], rapid chloride penetration test [72] and acid resistance [73]. In addition, the composition of hydration products, type of pore network (C-S-H and CH) from selected samples was studied by X-ray diffraction (XRD) and scanning electron microscope (SEM), as well as the quality of ITZ [74,75,76,77,78,79,80,81,82].

2.6. Statistical Analysis

All experiments are shown as the mean SD. Statistical significance of the differences between mixtures was tested for each property at 7, 28 and 90 days by one-way analysis of variance (ANOVA) at a confidence level of 95% (α = 0.05). Following a significant overall effect of the ANOVA (p < 0.05), post hoc Tukey’s Honest Significance Difference (HSD) tests were used to determine specific differences between individual mixture groups [83].

3. Results and Discussion

3.1. Compressive Strength

The TM method provided superior improvement compared with the DM method, with an average percentage increase of 11% in compressive strength of SFRAC over that achieved by SFRAC obtained using DM (p < 0.05). Interestingly, the TFRAC-7%SF combination even outperformed all DFRAC compositions as illustrated in Figure 5. The error bars on the data variance reaffirm the integrity of these trends.
Figure 5. Compressive Strength of all DFRAC and TFRAC mixes ±1 standard deviation shown as Error bars.
The effectiveness of TM was particularly evident in combination with certain mineral admixtures. The maximum strength enhancement (17% and 23%) was obtained for the mix with 7% silica fume, which is explained by the denser ITZ promoted by more homogeneous and compact form of microstructure around aggregate [11,44]. Likewise, when mixed with 25% GGBS, TM gave uniform strength gains of 10–17%. It was observed that mixtures with 40% fly ash experienced a drop in early-age strength, however, they performed well in TFRAC, and the loss was only about 3% at 90 days when compared to its’ similar DFRAC mixes. All these results combined demonstrate that the triple-mixing approach successfully integrates with mineral admixture to densify ITZ and further improve compressive strength.

3.2. Splitting Tensile Strength

The use of the TM method improved the splitting tensile strength of TFRAC and TFRAC largely exhibited an increased splitting tensile strength by around 29% in comparison with mixtures added with DM-treated RA, as presented in Figure 6. Statistical analysis by one-way ANOVA also confirmed these differences were significant (p < 0.05).
Figure 6. DFRAC and TFRAC Mixes—Tensile Strength and the error bars show ±1 standard deviation.
The performance was strongly influenced by the nature and content of mineral admixtures. Maximum enhancements occurred with 7% SF, which resulted in the increases of 24% and 29% at 28 and 90 days, respectively. Similar gains were also made with 25% GGBS (18–27%) and 20%FA (16–25%). Even at higher dosages such as 14%SF, 50%GGBS and 40%FA, the effect was not as significant implying that maximum benefit of TM application lies within certain optimal replacement levels. The strength gained rate gradually decreased while the strength enhancement rates kept a similar trend of being in good agreement with low variation (error bars = ±1 SD), which is due to the synergistic effect of the activity of TM method and the addition of mineral admixtures. Thus, the TM process would lead to a better matrix whose interfacial transition zone (ITZ) was improved by an enhanced pozzolanic reaction. This denser micro-structure is considered to offer a better substrate for bonding between the steel fibre and matrix, resulting in higher stress transfer from the graphene nanosheets to the basin-like surface of the fibre, as well as more efficient bridging by short fibres on micro-cracks that are blocked.

3.3. Flexural Strength

The flexural strength of fibre-reinforced concrete is a critical design property that is primarily enhanced by the capacity of fibres for crack bridging and must be determined experimentally, as it cannot be reliably predicted from compressive strength. As illustrated in Figure 7, both mixing methods improved flexural strength over the control, with the triple-mixing technique (TFRAC) yielding statistically superior results to the double-mixing method (DFRAC) across all mixtures (p < 0.05). The most significant enhancement was achieved with the TFRAC method incorporating 7% silica fume (SF), which exhibited a 31% increase in flexural strength over its DFRAC counterpart at 90 days. Substantial gains were also confirmed for TFRAC mixtures with 25% GGBS (29%) and 20% fly ash (FA) (25%). While higher admixture contents (50% GGBS, 40% FA, 14% SF) still conferred a statistically significant improvement, the benefits were less pronounced, indicating that the efficacy of the TFRAC method is optimized at specific replacement levels.
Figure 7. TFRAC and DFRAC Mixes Flexural Strengths, and Error bars are shown as ±1 standard deviation.
The triple mixing technique and the mineral admixtures have a synergetic effect on the increase of the compressive strength. Comparatively, the TM method also generates denser and more homogeneous matrix by improving ITZ, which is beneficial to promote the bonding between steel fibre and matrix. This enhanced bonding provides for a more efficient stress transfer and crack bridging, directly leading to the superior flexural behaviour. Error bars showing variability of data indicate how small the variance is in this manufacturing enhancement [46]. Generally, it can be seen from the test results that the use of a triple-mixing method together with optimized mineral admixture ratios is an effective approach for improving the flexural performance of steel fibre-reinforced recycled aggregate concrete.

3.4. Water Absorption

The contribution of mineral admixtures and mixing method to water resistance is shown in Figure 8. Statistical analysis indicated that the use of mineral admixture indeed improved water absorption resistance in both DFRAC and TFRAC (p < 0.05), with a further and constant 19–22% improvement obtained using triple mixing technique (TFRAC) over what was observed in corresponding DFRAC mixes. The reduction in water absorption was in the range of 21–53% about control mix. The optimum admixture was found to be 7% silica fume (SF), with a 46–53% decrease obtained. Next 20% FA and 25% GGBS had percent reduction ranges as 38–45% and 36–41%, respectively. Significantly more limited responses were found with increased admixture dosage (14% SF, 40% FA, 50% GGBS) but improvements remained statistical although less notable; signifying a saturation in the applied admixture quantity for performance gain.
Figure 8. Water Absorption of DFRAC and TFRAC MixesResults Error bars indicate ±1 standard deviation.
The superior performance, especially of the TFRAC mixes is attributed to a synergistic effect. The pore-blocker/mineral admixtures are a class of fillers/matrix densifiers, that clog the inner voids and cover the surface of crushed aggregates. Meanwhile, the triple mixing technique also contributes to more uniform dispersion of these admixtures and steel fibers that consecutively affect the ITZ in a positive manner [47]. The structure in this overall response is also improved by having a more solid matrix itself.

3.5. Chloride Ion Penetration

Next, from the Figure 9 chloride ion penetration (CP) resistance statistical analysis, it can be concluded that mineral admixtures significantly improve the performance of steel fibre-reinforced recycled aggregate concrete (SFR-RAC), with reductions between 12% and 85% compared to control mixes (p < 0.05). Triple-mixing (TFRAC) demonstrated superior outcomes compared with double mixing method (DFRAC) in all tested formulations; the differences were statistically significant.
Figure 9. The Chloride Ion penetrability of DFRAC and TFRAC Mixes; and Error bars ±1 standard deviation.
The effectiveness was largely influenced by the type of admixture and dosage. One way ANOVA and post hoc comparisons demonstrated that the 25% GGBS mix TFRAC prepared offered the maximum improvement, decreasing chloride penetration by more than 85%. Fly ash at 20% dose was highly effective (achieving as high as 76% reduction in TFRAC); however, a higher dosage of 40% showed significantly less effectiveness. Silica fume (7 and 14%) gave less but still significant improvements with a maximum strength reduction of 55% in DFRAC mixes.
They proposed that these performance improvements would originate from combined effects. The reduction in the overall permeability of the admixed concrete is attributed to pozzolanic reactions and filler effects, whereas high alumina content in GGBS and fly ash contributes to improving the chloride binding capacity [14,15,23]. The TFRAC process as statistically more significant indicates the importance in establishing a homogenized network and a refined pore-network, for permeability to be further decreased. The observation that GGBS contributes towards the major improvement in chloride barrier agrees to its substantial efficacy as an admixture for improving chloride resistance [15].

3.6. Acid Attack Resistance

Statistical analysis of the data presented in Figure 10 confirms that both mineral admixtures and the triple-mixing technique (TFRAC) significantly enhance the acid attack resistance (AAR) of steel fiber-reinforced recycled aggregate concrete (SFR-RAC). A one-way ANOVA revealed significant main effects for both mixing technique and admixture type, as well as a significant interaction between them (p < 0.05). Post hoc tests confirmed that the TFRAC method consistently yielded statistically superior performance to the double-mixing (DFRAC) method across all mix designs.
Figure 10. Loss in Mass due to acid attack of DFRAC and TFRAC after 28 and 90 days of immersion in 5% H2SO4 solution and Error bars represent ±1 standard deviation.
The resistance to acid actions was directly related with the mineral admixture type and amount. Silica fume (SF) resulted in the highest degree of improvement, and AAR increased 81–84% when incorporated with 14% SF which was significantly better than FA and GGBS containing series (p < 0.05). Fly ash performance was also quite good with 40% FA in TFRAC increasing the resistance by 66–78%. Although GGBS was found to be statistically inferior to SF and FA, the magnitude of improvement in 50% GGBS -TFRAC mix was still impressive with strength increase between 57–66%.
These improved wear properties can be ascribed to several synergistic mechanisms. The steel fibres give a reinforcement framework that restricts the formation of cracks from acid generated expansion. Mineral admixtures, especially silica fume, chemically react with calcium hydroxide and form higher volume of stable impermeable C-S-H gels. From the physical point of view, such method for triple-mixing contribute to more uniform dispersion of these admixtures which are factors inducing refinement in microstructure and the decreasing permeability [23]. The excellent statistical behaviour of silica-based additions reflects their efficiency at densifying matrix and enhancing the chemical resistance.

3.7. Statistical Analysis of Results

Statistically significant differences between mechanical and durability properties (Figure 5, Figure 6, Figure 7, Figure 8, Figure 9 and Figure 10) were thoroughly confirmed. One-way ANOVA was conducted with post-hoc Tukey’s HSD tests for the data related to each property if a statistically significant overall effect was obtained (p < 0.05). Table 4 presents the results, which revealed that mix design was (extremely) significantly affected (p < 0.001) for all properties at 7, 28 and at 90 days.
Table 4. P-values of the one-way ANOVA analysis on mechanical and durability properties.
The post hoc analysis validates two main outcomes of the experimental results. First, triple-mixing (TM) method consistently produced significantly better strength results (p < 0.05) compared to the double-mixing (DM) for almost all of mix designs. Second, the precise TM combination with optimized mineral admixture doses was important. The TFRAC-7SF blend was a separate statistical homogeneous group that had higher compressive, tensile and flexural strength compared to all other mixes. Regarding the durability, TFRAC-25GS was statistically better for chloride resistance and the TFRAC-20FA gave the most balanced development of properties. On the other hand, higher replacement levels (such as 50% GGBS, 40% FA) demonstrated consistently significant reduction in efficiency of activation thereby indicating that both TM and mineral admixtures are most effective at certain optimal dosages.
The statistical results have quantitatively confirmed that the increased ITZ density and a finer microstructure by triple mixing technique are responsible for the better performance, which is further synergistically promoted by judicious use of mineral admixtures.

3.8. Micro Structural Analysis

The microstructural characterization offers experimental direct evidence of mechanical and durability behavior trends previewed in this work. As shown in Figure 11a–g, the porosity of ITZ decreases away from the aggregate surface, which is like that observed for normal concrete. However, the ITZ density was notably higher in SF mixes and TM mixtures. Visual evidence of a denser ITZ in the SEM images presented and fewer microcracks in these optimized mixes was confirmed, with the higher strengths and stabilities observed being directly proportional to densities.
Figure 11. (a) SEM images of ITZ for control mixes. (b) SEM-images indicating the ITZ for 25% GGBS. (c) SEM images of ITZ for 50% GGBS. (d) SEM images of the ITZ for 20% FA. (e) SEM of ITZ for 40%FA. (f) SEM images of ITZ for 7% SF. (g) SEM image of SF in concrete specimens. Micrographs of the ITZ for the 14% SF specimen, using SEM.
Quantitative validation for this observation was made using X-ray Diffraction (XRD) analysis. For optimized mixes, especially those that contain silica fume, the peaks of portlandite (Ca(OH)2) were evident and decreased substantially with much higher amorphous hump attributed to C-S-H gel evidenced as presented in Figure 12. This proves a full pozzolanic reaction that represents the main root of matrix densification. GGBS and fly ash, we note here also contribute to the pozzolanic reactions, but showed the nature of performing dose depended. ITZ porosity was effectively reduced by GGBS at 25%, while at the higher level (50%), partial Portland cement dilution led to increased porosity. In the same way, 20% of fly ash enhanced ITZ while 40% cause the formation of bigger pores and microcracks. Due to its small particle size and high reactivity, silica fume was the most ideal admixture for ITZ refining to optimize the structural contribution ability of composites. Moreover, the XRD results show that GGBS can result in more C-S-H phase which increases strength properties of material [75,76].
Figure 12. XRD analysis of SFR-RAC specimens.

4. Conclusions

This research provides a comprehensive examination of their combined influence on the performance of steel fibre-reinforced recycled aggregate concrete (SFR-RAC). The experimental findings, along with the comprehensive statistical comparison (ANOVA, p < 0.05), can derive the following main statements:
The triple-mixing (TM) approach exhibited statistically superior performance over the double-mixing (DM) technique in all mechanical and durability features. It is mainly due the ability of TM method to form a denser and more uniform microstructure, especially by producing a finer ITZ around the recycled aggregates.
The synergistic effect between TM and certain mineral admixtures at an optimum dosage: Mechanical Properties: The TFRAC- 7%SF mixture was the best mix that yielded highest compressive, tensile, and flexural strength increases (as high as 23%, 29% and 31%, respectively). Durability Performance: TFRAC-25%GGBS presented an excellent resistance to chloride ion penetration (maximum reduction of 86%), while TFRAC-40%FA and TFRAC-14%SF had the best performance with respect to acid attack (up to 78%, 84%, respectively).
A trend was consistently observed for admixture dosages: lower replacement levels (7% SF, 25% GGBS and 20% FA) being found optimum to improve mechanical properties while higher levels (14% SF, 50% GGBS and 40% FA) were beneficial to improve the long-term chemical resistance. It highlights the necessity of designing amount of admixture depending on the main purpose for performance.
SEM/XRD analysis validated the mechanism for these enhancements. The addition of polycarboxylate-based super-plasticisers and ultrafine slag promoted the generation of more C-S-H gel, consuming portlandite and resulted in a denser pore structure and stronger ITZ.
In conclusion, this study shows that the triple-mixing approach can be strategically utilized with optimized mineral admixture to overcome demerits of recycled aggregate concrete. For high mechanical strength, TFRAC-7%SF mix is preferred while application subjected to chloride environment will be successfully using the TFRAC-25%GGBS. Indeed, such approach offers a viable solution to making high-performance green concrete with industrial by-products to evolve an eco-efficient construction industry.
Limitations and Future Work: From an industry perspective, the current work is targeted towards mix optimization in an SFR-RAC system. Future research will have to include the direct comparison with its natural aggregate concrete equivalent and study the impact of different replacement percentages of recycled concrete aggregate (RCA) for enhancing the wider application.

Author Contributions

Conceptualization, M.Q. and M.Y.; methodology, M.Q. and M.Y.; validation, M.Q. and M.Y.; formal analysis, M.Q.; investigation, M.Q.; resources, M.Q.; writing—original draft preparation, M.Q.; writing—review and editing, M.Y.; visualization, M.Q.; supervision, M.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request, without undue reservation.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
XRDX-ray Diffraction
SEMScanning Electron Microscope
C–S–HCalcium Silicate Hydrate
DMDouble-Mixing Technique
TMTriple-Mixing Technique
SFR-RACSteel fiber reinforced recycled aggregate concrete
GGBS Ground granulated blast furnace slag
FA fly ash
SF Silica fume
RAC Recycled aggregate concrete
BDD Building and demolition debris
NAC Natural aggregate concrete
DFRAC Double Mixing Fibre-Recycled Aggregate Concrete
TFRAC Triple Mixing Fibre-Recycled Aggregate Concrete
OPC Ordinary Portland cement
ASTM American Society of Testing Materials
NCA Natural coarse aggregate
RCA Recycled Coarse aggregate
ACI American Concrete Institute
MPa Mega Pascal
HSF Hooked Steel Fibres
SP Super Plasticizers
ITZ Interfacial transition zone

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