1 Introduction

The depletion of natural aggregates and raw materials in modern construction poses a significant challenge. Addressing this issue involves recognizing the imperative problem of construction and demolition waste present in aging concrete structures. Mitigating the exploitation of natural aggregates and incorporating construction and demolition waste into concrete production emerge as viable solutions. Given that a substantial portion of global infrastructure relies on concrete, the deterioration of concrete structures due to environmental factors and a lack of timely maintenance are widespread.

When concrete structures undergo demolition or renovation, concrete recycling has become a prevalent method for repurposing the resulting rubble. Establishing the suitability of recycled aggregates has gained paramount importance, particularly amid the increasing emphasis on sustainable construction practices. The utilization of construction and demolition wastes is emerging as a pivotal and forward-looking approach for ensuring the sustainability of construction projects. Against this backdrop, numerous studies have delved into the utilization of recycled concrete aggregates for producing concrete intended for both structural and non-structural applications. There is limited literature addressing the work, as outlined below. Marija Nedeljkovi et al. [1] Concentrating on the physical and chemical attributes, engineering features, and durability aspects of concretes incorporating fine recycled concrete aggregates (fRCA) in contrast to the characteristics of fine natural aggregates, the primary restrictive factors of fRCA are pinpointed, including its elevated water absorption, moisture state, particle agglomeration, and adhered mortar. Omar Kouider Djelloul, et al. [2] presents findings on the impact of incorporating coarse and fine recycled concrete aggregates (RCA) in self-compacting concrete (SCC) with ground granulated blast-furnace slag (GGBFS) as a substitute for cement. Sayed Ahmed et al. [3]. This study delved into the substitution of natural coarse aggregate (NCA) with recycled concrete aggregates (RCA) at various ratios (0%, 50%, 75%, and 100%) for the production of self-compacting concrete (SCC). Various supplementary cementations materials (SCMs) such as Nano-silica (NS), fly ash (FA), and metakaolin (MK), along with PVA fibres, were integrated into the SCC mixtures. Pawan Kumar & Navdeep Singh [4] evaluated the performance of Recycled Concrete Aggregates (RCA) based Self-compacting Concrete (RCASCC) comprising coal ashes and Silica Fume (SF). Coal ashes Fly Ash (FA) and Coal Bottom Ash (CBA)] were incorporated as partial replacement for Ordinary Portland Cement (OPC) and Fine Natural Aggregates (FNA) respectively, while RCA were incorporated in place of Coarse Natural Aggregates (CNA). Author found that the successful contribution of SF, coal ashes (CBA + FA), and RCA leading towards the sustainable development of nonconventional SCC with identical performance.

Dr. Sumit Gandhi et al. [5] substituting recycled concrete aggregates (RCA) in various ratios (0%, 50%, 75%, and 100%). The SCC combinations included PVA fibres as well as several supplementary cementations materials (SCMs) ingredients, including Nano-silica (NS), fly ash (FA), and metakaolin (MK). The optimal combination has met EFNARC requirements and had good fresh qualities was SCC with 100% RCA replacement, 20% MK, and 22% FA. Author evaluated that the Compressive strength decreased by 8.20% after 100% RCA substitution, whereas maximum load and flexural stiffness by 3.20 and 16.25%, respectively.

Víctor Revilla-Cuesta et al. [6] examines the deformational behavior under compression and bending of SCC containing 100% coarse and 0%, 50%, and 100% fine Recycled Aggregate (RA), limestone and RA green aggregate powders sized 0/0.5 mm, and Ground Granulated Blast-furnace Slag (GGBS) cement. From the result it is found that the deformational behavior, which was successfully modeled with maximum deviations of ± 10%, fine RA may be used in combination with GGBS and limestone powder, although he recommended that fine RA should not exceed proportions of 50%.

Víctor Revilla-Cuesta et al. [6] analyzed the effect of the content of RA and their maturity (time elapsed between casting and crushing of the parent concrete from which RA are obtained) on the properties of High performance concrete (HPC). Adopted five mixes manufactured with 0%, 25%, and 100% of coarse and fine RA of different maturities, 7 days (early-age RA) and 6 months (matured RA).It is evidenced that the lower stiffness of early-age RA and their shrinkage amplified to all types of shrinkage of HPC around 10–20%.

Osama Zaid et al. [7] explores the mechanical performance of sustainable concrete integrating waste materials as aggregates in a three-stage study. Ran Bir Singh et al. [8] utilized the flow-curve test protocol conducted on a coaxial cylinder rheometer. For each silica fume dosage, the volumetric replacement levels of coarse recycled concrete aggregate were set at 0%, 50%, and 100%. Yunyang Wang, et al. [9] study delved into various aspects of fresh and hardened C40 Self-Compacting Concrete (SCC), examining parameters such as slump, T500 (time for SCC to reach a 500 mm nominal diameter round configuration), slump flow, flow time, compressive strength, and modulus of elasticity. Tang et al. [10] incorporation of recycled concrete aggregate (RCA) in Self-Compacting Concrete (SCC) holds promise for diminishing both the environmental footprint and financial expenses linked to the increasingly favored concrete type. K C Pandaa, P K Balb [11] explores the impact of varying quantities of recycled coarse aggregate (RCA) sourced from a 25-year-old demolished Town Club building in the Banki region of Cuttack, N.A.C, on the properties of self-compacting concrete (SCC). The findings are juxtaposed with those of normal vibrated concrete (NVC) containing 100% natural coarse aggregate (NCA). The investigation encompasses crucial aspects such as the physical and mechanical properties of both natural and recycled aggregates. Boudali et al. [12] investigates the performance of both self-compacting concrete (SCC) and self-compacting sand concrete (SCSC) by integrating recycled concrete fines and aggregate in diverse sulphate environments. Rebeca Martínez-García, et al. [13] introduces a novel investigation into the application of coarse recycled concrete aggregate (CRA) for the development of self-compacting concrete (SCC). Various concrete formulations involving substitution rates of 20%, 50%, and 100% are systematically compared with a control concrete. The primary objective of this comparative analysis is to assess the impact of CRA on the fresh SCC and its ensuing physical and mechanical properties. The parameters under scrutiny encompass material characterization, self-compactability, compressive strength, and the tensile and flexural strength of the resulting concrete. The outcomes suggest that integrating CRA in SCC production is viable, resulting in minimal losses in key characteristics. Lilia Señas, et al. [14] to investigate how recycled aggregates influence self-consolidating concrete. Concrete mixes were formulated with a 50% replacement of coarse aggregate using recycled aggregates (specifically, Patagonia gravel) and a 20% replacement of fine aggregate (natural sand) with crushed concrete powders. Kanish Kapoor M.E, et al. [15] study examines the durability performance of Self-Compacting Concrete (SCC) incorporating Recycled Concrete Aggregates (RCA) as partial or full replacements for Natural Coarse Aggregates (NCA) and selected mineral admixtures as partial substitutes for Portland Cement (PC). Kiran Mansingrao Mane [16] study utilizes MATLAB software and the neural network toolbox to construct a model predicting the flexural strength of concrete incorporating pozzolanic materials and partially replacing NFA with Manufactured Sand (MS). Santosh Kumar Karri, et al. [17] focuses on assessing the performance of AASC incorporating quartz sand as a fine aggregate under different exposure conditions. Asad S. Albostami, et al. [18] data-driven methodologies were employed to forecast the compressive strength (CS) of self-compacting concrete (SCC) incorporating recycled plastic aggregates (RPA). A comprehensive database comprising 400 experimental datasets served as the basis for evaluating the performance of multi-objective genetic algorithm evolutionary polynomial regression (MOGA-EPR) and gene expression programming (GEP). Sevket Can Bostanci, et al. [19] the engineering and durability properties of concretes with equal 28-day design strength (40 and 50 N/mm2). Babar Ali et al. [20] delves into the strength and economic aspects of concrete through the individual and combined incorporation of Fly Ash (FA) and Recycled Concrete Aggregate (RCA). Nine distinct concrete mixtures were prepared, altering the levels of RCA and FA incorporation. Khaldoun Rahal [21] explores the mechanical properties of Recycled Aggregate Concrete (RAC) in comparison to conventional Normal Aggregate Concrete. Ten concrete mixes were formulated with target compressive cube strength ranging from 20 to 50 MPa, utilizing both normal and recycled coarse aggregates. Compressive cube strength, indirect shear strength at various ages (1, 3, 7, 14, 28, and 56 days), strains at maximum compressive stress, and modulus of elasticity (tested using concrete cylinders at 28 days) were examined. Sherif Yehia et al. [22] explores the feasibility of utilizing 100% recycled aggregate in concrete to meet the durability and strength requirements for diverse applications. Muhammad Asad Nawaz et al. [23] explores the benefits of incorporating fly ash with an alkali-silicate activator to enhance the properties of recycled aggregate mortar (RAM). The durability parameters show a notable correlation with compressive strength, indicating that improvements in microstructural development leading to higher mechanical strength also contribute to enhanced durability.

The literature review reveals that the utilization of recycled aggregate in concrete yields suboptimal results in terms of mechanical strength without proper treatment processes. Additionally, challenges arise in terms of cost-effectiveness during treatment processes, especially when incorporating various mixtures and admixtures to enhance strength properties. To address this issue, we introduced an optimal percentage of Novel additive in the mix.

2 Novel additive proportion

To ascertain the ideal proportion of the novel additive for achieving peak strength, we are examining six percentage variations, ranging from 0 to 10%, with increments of 2%. Detailed breakdown of the mix combinations are shown in Table 1. The mix design is formulated to determine the optimum compressive strength in six variant mixes with novel additive. These mixes are considering with 100% natural coarse aggregate and manufactured sand (Natural fine aggregate-NFA) excluding any recycled aggregate materials and it is illustrated in Table 2. The outcome at the 28-day age for the sixth mix reveals the highest compressive strength of 51.24 MPa, achieved with a 10% inclusion of the novel additive. Upon interpreting these findings, the decision has been made to include 10% of the novel additive in subsequent design mix studies to fulfill the outlined research objectives.

Table 1 Proportion of novel additive
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Table 2 Mix proportion of the concrete mix
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3 Fresh properties

Rheodynamic flowable concrete (Rd-FC) is designed to fill intricate and congested formwork without the need for vibration. Fresh property test helps to confirm that the concrete's flowability and filling capacity are suitable for the specific formwork geometry, ensuring proper compaction and consolidation. To evaluate the trail mix concrete's practical workability and verify its compliance with prescribed standards for flowability and ease of placement, tests such as slump flow (Fig. 1), V funnel (Fig. 1), and L box (Fig. 1), have been carried out. These tests are conducted for different trial mixes by varying the Recycled coarse and fine aggregate with natural aggregates by approximate method of mix design and is tabulated in Table 3.

Fig. 1
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Slump flow, L box, V funnel

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Table 3 Fresh properties of the Rd-FC
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The slump flow for various mixes is controlled within the range of 600 mm to 680 mm through adjustments in mix proportions and super plasticizer dosages, demonstrating favorable flowability. Notably, the trial mix containing natural coarse and fine aggregates exhibits higher slump flow compared to the mix incorporating recycled aggregates. This disparity can be attributed to the cementitious particles in the recycled aggregate mix, influencing the binding of aggregates. The manifestation of flowability is particularly evident in the V-funnel flow. The trial mix featuring a higher volume of natural aggregates displays elevated V-funnel flow and L box results compared to the mix utilizing recycled aggregate. This distinction is likely due to the greater volume of aggregates and the heightened gravitational impact in the former case. The attained fresh properties meet the specified standards for flowing ability in Rd-FC, ensuring satisfaction with the desired characteristics.

3.1 Mechanical properties

The mechanical properties of concrete encompass a range of characteristics that reflect its behavior under various loading conditions. Key mechanical properties of concrete include: Compressive Strength, split tensile strength and flexural strength. Table 4 presents various trial mixes that have been analyzed to assess the strength properties. These trials involve variations in the proportions of natural coarse aggregate and recycled coarse aggregate within the concrete mix, both with and without the inclusion of a novel additive.

  1. 1)

    Compressive Strength

Table 4 Mix proportion for compressive strength
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The compressive strength of concrete is a critical mechanical property that measures its ability to withstand axial loads or forces applied along its axis. It is one of the most important parameters used to assess the performance and durability of concrete in various structural applications. In this study cubes measuring 150 mm x 150 mm x 150 mm underwent compressive strength testing at intervals of 7, 14, 28, 56, and 90 days. To maintain the good compressive strength for the Rd-FC the new cementatious material called novel additive(NA) have been used in the mix. The progression of strength development over time is tabulated in Table 5 and it is illustrated in Fig. 2.

Table 5 Compressive strength results
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Fig. 2
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Compressive strength in Mpa

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The compressive strength analysis revealed that trial mixes from 3 to 15 exhibited lower compressive strength than trial mixes 1 and 2 (conventional control mix - CCM). This outcome was anticipated due to the inherent impurities present in recycled coarse and fine aggregate. Specifically, trial mixes 3 and 4 were conducted to assess compressive strength using 100% recycled fine aggregate (RFA) with 100% natural coarse aggregate (NCA), incorporating 10% natural aggregate (NA) and excluding NA. The results indicated a decrease of approximately 41.25 MPa (without NA) and 40.85 MPa (with NA) at 28 days compared to CCM mixes (trial mixes 1 and 2).

In trial mixes 5 to 15, the strength was evaluated by incorporating 100% RFA and varying the recycled coarse aggregate (RCA) from 0% to 100%, with 10% NA and without 10% NA. The objective of this study was to ascertain the outcomes using both RFA and RCA in concrete mixes with and without 10% NA. Notably, trial mix-9 demonstrated superior strength, reaching 39.51 MPa at 28 days across the various RCA variations. The concrete exhibits increased strength at 56 days and 90 days as the age progresses, attributed to the presence of Ground Granulated Blast Furnace Slag (GGBS) in the concrete mix. The compressive strength of concrete produced with recycled aggregate fell within the range of 38 to 41 MPa at the age of 28 days, warranting the classification of the concrete as M30 grade for further studies.

  1. 2)

    Split tensile strength

The split tensile strength test is performed on concrete to evaluate its tensile strength, which is a measure of the material's resistance to tensile forces. In this investigation, split tensile strength is evaluated using 100mm * 200mm cylinders at various ages, namely 7, 14, 28, 56, and 90 days. This testing protocol aligns with the procedures followed in the compressive strength test, as detailed in Table 6. The corresponding results are graphically represented in Fig. 3. Notably, trial mix-9 demonstrates a commendable strength of 5.32MPa at 28 days. The trend of increasing strength with age is evident, with the strength reaching 6.83MPa at 90 days.

  1. 3)

    Flexural strength

Table 6 Split tensile strength results
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Fig. 3
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Split tensile strength in MPa

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The flexural strength test on prismatic specimens of 50mmx50mmx100mm is essential for a comprehensive assessment of concrete's mechanical properties, particularly its behavior under bending or flexural loads. Third-point loading was employed on basic concrete prisms to ascertain the flexural strength for all trail mixes. The flexural strength test was conducted on all trial mix formulations at intervals of 7, 14, 28, 56, and 90 days, and the corresponding results have been recorded in Table 7. Once again, Trial Mix 9 exhibited a flexural strength of 4.4MPa, representing the highest strength among all other mixes at 28 days. This observation is visually depicted in Figure 4. Periodic strength increment at the age of 56 and 90 days can be observed.

Table 7 Flexural strength results
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Fig. 4
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Flexural strength in MPa

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Upon evaluating the results and analyzing the proportional mix for the mechanical properties of the concrete, it is evident that the mix design yields advantageous strength in resisting tensile forces, thereby enhancing performance under tensile and flexural loading conditions. The compressive strength of the concrete meets the anticipated requirements, affirming the ability of recycled aggregate concrete to produce structurally sound elements. Furthermore, the obtained flexural strength of the recycled aggregate mix concrete is notably higher, indicating successful integration of recycled aggregates into the mix and providing effective support against bending forces.

4 Microstructural analysis

Additional experimental investigations were conducted, involving the selection of four mix proportions for a comprehensive analysis of microstructural characteristics, such as SEM (Scanning Electron Microscopy).Within this research, the study will involve the analysis of particle morphology, agglomeration, clustering characteristics, and the interfacial transition zone between aggregates and cement in Recycled Aggregate-Based Rheodynamic Flowable Concrete (Rd-FC). This analysis, conducted through SEM technique, is anticipated to impact the strength and durability performance of the material.

Two control mixes of conventional concrete (CCM-1, CCM-2) and two recycled concrete mixes (RCM-1, RCM-2) were chosen for this study and are documented in Table 8. These same mixes were subjected to SEM analysis at the age of 7, 14, 28 days strength to delve into their microstructural attributes.

Table 8 Selected mix proportions for SEM
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SEM study are valuable for optimizing concrete mix designs, improving material properties, and ensuring the long-term performance of concrete structures. In Mix-1, the particles exhibit little spherical perturbations and some agglomeration of smaller particles at 7 days. Notably, well-formed interfacial transition zones are observed between the cement and aggregates. The transition zones and spherical agglomeration formations are highlighted as 1 and 2 in Fig. 5. In Mix-1, there is a notable increase in spherical perturbations and clustering morphology observed in the particles at 14 days, as depicted in Fig. 6. This phenomenon is attributed to the strengthening process aimed at achieving full strength by the 28th day. A well-bonded interaction between particles and aggregates is evident, accompanied by a well-formed morphology, showcasing the mix's attainment of full strength by 28 days, as illustrated in Fig. 7.

Fig. 5
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Mix-1 (100%FA+100NCA+0%NA)-7 DAYS

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Fig. 6
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Mix-1 (100%FA+100NCA+0%NA)-14 DAYS

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Fig. 7
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Mix-1 (100%FA+100NCA+0%NA)-28 DAYS

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In Mix-2, the incorporation of 10% NA and recycled aggregates results in particles displaying rock-like structures, minimal agglomeration, and fewer pores at 7 days. Notably, well-formed interfacial transition zones are evident between the cement and aggregates. The transition zones and rock-like formations are indicated as 1 and 2 in Fig. 8. At 14 days, there is a noticeable reduction in rock-like formations and clustering bonding morphology in Mix-2 particles, as shown in Fig. 9. This change is attributed to the strengthening process aimed at achieving full strength by the 28th day. A well-developed interfacial transition zone (ITZ) is characterized by reduced porosity and a seamless transition between the aggregate and cement matrix, minimizing pathways for potential deterioration agents. Furthermore, Mix-2 exhibits a well-bonded interaction between particles and aggregates, coupled with a well-formed morphology, indicating the mix's attainment of full strength by 28 days. This is illustrated in Fig. 10.

Fig. 8
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Mix-2 (100RFA+50%NCA+50%RCA+10%NA)-7 DAYS

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Fig. 9
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Mix-2 (100RFA+50%NCA+50%RCA+10%NA)-14 DAYS

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Fig. 10
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Mix-2 (100RFA+50%NCA+50%RCA+10%NA)-28 DAYS

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In Mix-3, recycled aggregates results in particles displaying skeletal like structures, minimal agglomeration, and less pores at 7 days. The uniformity of the skeletal morphology across the material provides information about the homogeneity of the microstructure. A consistent skeletal framework suggests a more uniform material composition is shown in Fig. 11. At 14 days, there is a noticeable reduction in skeletal and clustering bonding morphology in Mix-3 particles, as shown in Fig. 12. This change is attributed to the strengthening process aimed at achieving full strength by the 28th day. The degree of porosity within the skeletal structure can also be assessed in the structure. A more compact skeletal morphology with minimal voids indicates lower porosity, which is often desirable for materials like concrete. Furthermore, Mix-3 exhibits a well-bonded interaction between particles and aggregates, coupled with a well-formed morphology, indicating the mix's attainment of full strength by 28 days. This is illustrated in Fig. 13.

Fig. 11
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Mix-3 (100RFA+50%NCA+50%RCA+0%NA)-7 DAYS

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Fig. 12
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Mix-3 (100RFA+50%NCA+50%RCA+0%NA)-14 DAYS

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Fig. 13
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Mix-3 (100RFA+50%NCA+50%RCA+0%NA)-28 DAYS

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In Mix-4, particles display minor spherical perturbations and some agglomeration of smaller particles at 7 days, as depicted in Fig. 14. Importantly, well-defined interfacial transition zones are evident between the cement and aggregates. Subsequently, at 14 days, Mix-4 exhibits a noticeable increase in spherical perturbations and clustering morphology in the particles, as illustrated in Fig. 15. This development is attributed to the strengthening process aimed at achieving full strength by the 28th day. The microstructural analysis reveals a well-bonded interaction between particles and aggregates, characterized by a well-formed morphology, highlighting the mix's successful attainment of full strength by 28 days. This is depicted in Fig. 16.

Fig. 14
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Mix-4 (100RFA+50%NCA+50%RCA+0%NA)-7 DAYS

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Fig. 15
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Mix-4 (100RFA+50%NCA+50%RCA+0%NA)-14 DAYS

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Fig. 16
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Mix-4 (100RFA+50%NCA+50%RCA+0%NA)-28 DAYS

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5 Conclusions

The imperative challenge arises from the depletion of natural aggregates and raw materials in contemporary construction, leading to a critical issue of construction and demolition waste within aging concrete structures. Addressing this concern involves minimizing the reliance on natural aggregates and embracing the reuse of construction and demolition waste in the concrete industry. This study establishes the viability of substituting natural coarse aggregate (NCA) with recycled coarse aggregates (RCA) ranging from 0% to 100%, along with the complete replacement of natural fine aggregate (MS) with recycled fine aggregate, to formulate Rheodynamic flowable concrete (Rh-FC). To augment the strength of Rh-FC mixtures, a Novel additive has been incorporated.

The compressive strength of Trial Mix-9 exhibits a strength of 39.51MPa at 28 days, classifying it as M30 grade concrete. The utilization of recycled aggregate in M30 grade concrete, particularly in Trial Mix-9, demonstrates notable progress in strength achievement, attributed to the application of the novel additive. Trial Mix-9 displays commendable strength of 5.32MPa and 4.4MPa in both split tensile and flexural strength tests. Based on these findings, the incorporation of the novel additive in recycled aggregate concrete enhances its strength properties, aligning with structural design expectations.

SEM analysis illustrates a robust bond between cement and aggregates in CCM-1&2 and RCM-1&2, creating a well-developed interfacial transition zone characterized by certain clustering morphology and reduced porosity. This signifies the favorable binding properties of recycled aggregate in cementitious materials, fostering dense characteristics and maintaining compactness within the mix.