In this investigation, OPC 43-grade, FYA Class-F, NFA (river sand of 4.75 mm), BA (4.75 mm), NCA (maximum 12.5 mm size) and RCA (maximum 12.5 mm) were used for the making of HVFYA-SCC. In this study, OPC and FYA agreed to IS 8112:2013 (40) and ASTM C618 (41), respectively. The physical characteristics of OPC are presented in Table 1. The specific gravity of BA and FYA are 2.09 and 2.10 respectively. The chemical composition of OPC, BA and FYA are presented in Table 2. The SEM image of the particles OPC and FYA are shown in Figures 1 (a) and 1 (b), likewise indicating that the OPC particles are angular in shape, and the FYA are smooth and spherical. OPC was procured from the ACC cement manufacturing plant at Ropar, India. FYA was collected from the Nabha power plant in Rajpura, India. NFA and NCA were collected from Trehti quarry at Pathankot, India. NFA was used in the present study as per code IS 383-2016 (42), and the NFA was found to be retained in Zone II by carrying out sieve analysis as per the specification of IS 383-2016 (42). BA replaced the NFA at 0%, 10%, 20%, and 30% collected from a Ropar thermal power plant in India. Figure 2 shows the particle size gradation of different ingredients. Figure 3 (a-d) illustrates the pictorial view and the SEM image of NFA and BA, respectively. Moreover, BA ranged in Zone II as per IS 383-2016 (42).

The NCA were used in the proportion of 25%, 42% and 33%, respectively, for 6.36 mm, 10 mm and 12.5 mm in saturated surface dry (SSD) conditions. The NCA was replaced with RCA at 0%, 25% and 50%, and to get RCA, the tested concrete specimens were crushed and sieved as per IRC:121-2017 (43) in Structures Testing Laboratory of the authors Institute. The physical properties of NFA, BA, NCA and RCA are presented in Table 3. Figure 3 (e-f) represents the pictorial view of the NCA and RCA used in this investigation respectively. However, it is worthwhile to mention that in the current investigation the authors have used weigh batching approach instead of equivalent volume batching approach for the mix design. Similar approach has been successfully implemented in significant number of investigations as available in literature (23, 44, 45). Moreover, the authors have incorporated only selected amount of RCA (maximum 50%) and BA (maximum 30%) in comparison to the investigations wherein the effect of density has been considered for the higher amount of RCA (>50%) and BA (>30%) (18, 20, 46-49). Since the amount of altered ingredients was less and the source of procurement was not changed during entire investigation therefore for the same reason the authors have not considered the effect of variation of density of BA and RCA in the current study. A high-range water reducer superplasticizer (SP) of MasterGlenium 51 (polycarboxylic ether-based) was utilized in different doses by weight of the binder to attain the desired HVFYA-SCC, confirming IS 9103 (50).

For the research, ten HVFYA-SCC mixes with a consistent binder content of 688 kg/m³ and a water to binder proportion of 0.25 were made. NFA and NCA additions of 887 kg/m³ and 711 kg/m³, correspondingly, were made. Out of ten HVFYA-SCC mixes, one control mix (CF70B0-R0) was prepared with 30% OPC and 70% FYA combining 100% NFA and 100% NCA. The remaining mixes were made using various substitutions, such as replacing NCA with 25% and 50% RCA and NFA with 10%, 20%, and 30% BA. The mix proportions for all the HVFYA-SCC mixes are revealed in Table 4. It is worthwhile to mention here that, the water absorption of both RCA and BA has higher values in comparison to that of NCA and NFA. The absorption factor was duly considered while preparation of all SCC mixes. The RCA were incorporated in surface saturated dry condition while additional water was added at the time of mixing of all HVFA-SCC mixes due to higher absorption of BA particles. The HVFYA-SCC mixes were prepared according to European federation of national associations representing concrete (EFNARC; 2005) (51) guidelines, and SP was incorporated in different proportions ranging from 0.65% to 1.38% by weight of the binder.

Figure 5(a) shows the difference in slump flow and T₅₀₀ time, where the estimated value of slump flow diameter was between 710-745 mm. This value is categorized under the SF2 class ranging from 660-750 mm as per EFNARC 2005 (51). Adding BA and RCA decreased the slump flow value, but adding an equivalent amount of SP dosage can prevent it. The results indicate that adding 30% BA negatively affected the slump values, while the mix with 50% RCA had the highest reduction (almost 4%) with reference to control HVFYA-SCC mix. Filling ability of HVFYA-SCC mixes was evaluated using T₅₀₀ time, which ranged from 3.3 to 4.8 seconds. Figure 5(b) indicates that the T₅₀₀ time increased with an increase in the amount of BA and RCA. Singh et al. (55) also observed a decrease in flow ability at substitution levels of 10% BA and 50% RCA for NFA and NCA, correspondingly. The control HVFYA-SCC mix (CF70B0-R0) had the minimum T₅₀₀ time, while the highest value was observed for the CF70B30-R50 mix. (15, 20).

Figure 5(b) indicates the difference in V-funnel time value and SP dosages, with RCA and BA containing HVFYA-SCC mixes having a better V-funnel time value as compared to control mix (CF70B0-R0). Alike results were described by Singh et al. (55) when studying the use of BA and RCA as a substitution for NFA and NCA, respectively. The V-funnel value for all HVFYA-SCC mixes ranged from 6-13.5 seconds. The mixes CF70B0-R0, CF70B10-R0, and CF70B10-R25 were categorized under the VF1 (0-8 seconds) viscosity class, while the VF2 (9-25 seconds) viscosity class included the CF70B10-R50, CF70B20-R0, CF70B20-R25, CF70B20-R50, CF70B30-R0, CF70B30-R25, and CF70B30-R50 mixes. Also, the results revealed that the variation in J ring spread, where the estimated value of flow diameter was between 700-740 mm (Figure 5(c)). All the mixes met the EFNARC 2005 (51) limit with blocking ratios between 0.92 and 0.98 (Figure 5(d)). The irregularly shaped porous particles of BA and the rough texture adhering mortar on RCA may have combined effects that reduce the passage ability of the mixes by increasing inter-particle friction and obstructing concrete flowability (56-58).

Figure 6 (a) indicates that higher BA content led to lower compressive strength at 28 days. The reduction was highest for the mix with the lowest BA content (CF70B10-R0), showing a decrease of 18%. However, for mixes with larger amounts of BA (20% and 30%), the decrease in compressive strength was only 5% with reference to control HVFYA-SCC mix (Figure 6 (b)). Similarly, Figure 6 (b) illustrates those mixes containing 10% BA exhibited an increase in compressive strength after 120 days. Previous studies (32, 58) suggest that HVFYA and BA in the mixes induce pozzolanic action at longer curing ages, which may explain the improved behavior.

After 28 days, the CF70B10-R25 and CF70B10-R50 mixes, which contained 10% BA and 25% and 50% RCA, exhibited lower compressive strength with reference to control HVFYA-SCC mix, with reductions of 24% and 29%, respectively At 120 days, the mix with 25% RCA demonstrated a slight improvement in compressive strength with reference to control HVFYA-SCC mix, which may be attributed to the presence of BA and FYA, resulting in additional pozzolanic reactions (15). The primary pozzolanic activity due to presence of FYA and BA has been explained in context to their chemical properties in which action due to pozzolanicity of CaO and SiO₂ mainly has been happened. It is worthwhile to mention here that BA has been incorporated in place of NFA with constricted amount up to 30% in preparation of HVFA-SCC mixes. Herein, as mentioned earlier the pozzolanic action of BA comes into picture at later ages and that too with less impact in comparison to that of FYA. Further it was inferred that the reactive SiO₂ is the chief parameter which strongly influences the Na₂O content as present in coal fly ashes (59, 60). The stronger influence of SiO₂ due to higher amount (60.57%in FYA) and presence of Na₂O leads to development of more C-A-S-H gels, consequently improve the strength properties of the HVFA-SCC mixes (60-62). Similar influence of K₂O has been noted wherein pozzolanic action has supplemented the strength aspects in concrete (59, 63, 64).

Conversely, the compressive strength of the CF70B10-R50 mix, containing 50% RCA, declined with increasing curing time, likely due to the presence of aged, adhered mortar in the RCA that deteriorated its structural properties (65). In contrast, the CF70B20-R25 and CF70B30-R25 mixes, containing 20% and 30% BA and 25% RCA, exhibited compressive strengths 7% and 4% higher than the control mix after 120 days, as shown in Figure 6(b). The enhancement in compressive strength can be attributed to the additional development of C-S-H during later stages of curing, which resulted from the pozzolanic reaction of BA and FYA (32, 58). Nonetheless, the frail interface transition zone (ITZ) between the adhering mortar and the porous aggregates might lead to a slight reduction in compressive strength when replacing 50% of RCA in comparison to 25% in all formulated mixes (58, 65).

In the current investigation the chemical analysis indicates higher amount of CaO and SiO₂ for FYA (1.43% and 60.57%) compared to that of BA 0.78% and 56.4%) respectively. Also, the particle size of FYA is much finer than that of BA particles consequent in higher reactivity of the former than reactivity of the later. In fact, the higher amount of aforesaid compounds generally results in improvement of the HVFA-SCC mixes due to additional pozzolanic reactions thereby increasing the heat of hydration. Moreover, the amount of CaO and SiO₂ for FYA and BA mostly depends on the source and type of coal or the parent material (15, 46). In India, due to bituminous type of coal (66), Class F type FYA is present along with lower grade of BA. Hence due to this reason the pozzolanic action of FYA is more in comparison to BA used in this investigation.

Figure 7(a) displays the direct shear strength results for all the HVFYA-SCC mixes after 7-days periods of curing. The data showed a general decline in direct shear strength as the BA concentration increased. Figure 7(b) further revealed a decrement of 5%, 1%, and 3% for HVFYA-SCC mixes CF70B10-R0, CF70B20-R0, and CF70B30-R0, correspondingly, reference to the control HVFYA-SCC mix. This pattern was too observed for shear strength at 28 and 56 days. The decrease in HVFYA-SCC mixes during the early stages of curing was due to the decline in heat of hydration of cement and pozzolanic activity of FYA and BA (24, 67). However, after 90 days, the shear strength of the HVFYA-SCC mixes was observed to be comparable to the HVFYA-control mix. The shear strength of HVFYA-SCC mixes with varying amounts of BA and curing time was analyzed in Figure 7(b). The results showed that CF70B10-R0, CF70B20-R0, and CF70B30-R0 mixes had a slight increase of up to 2% in shear strength than the control mix. The same trend was observed for mixes with 10% to 30% BA, where shear strength increased with curing time up to 120 days. On the other hand, when 25% and 50% RCA were added to the 10% BA mixes, a decrease in shear strength was observed with reference to control HVFYA-SCC mix. Specifically, the shear strength at 7 days dropped from 3.57 MPa for the CF70B0-R0 mix to 3.23 MPa and 3.11 MPa, respectively, when 25% and 50% of the RCA were replaced. HVFYA-SCC mixes with 20% and 30% BA showed a decrease in shear strength as curing increased up to 56 days.

Furthermore, after 90 days curing age, the shear strength of the control mix (CF70B0-R0) improved slightly from 6.45 MPa to 6.54 MPa with 25% replacement of RCA. BA and FYA may increase direct shear strength in HVFYA-SCC with lower RCA concentrations due to additional pozzolanic reactivity (15, 32). However, 50% RCA had the contrary effect, the shear strength of CF70B10-R50 decreased up to 1% after 90 days of curing. The weak adhered mortar in RCA leads to a reduction in over-all shear strength, which could be attributed to the maximum RCA content and poor structural properties (65, 68). Mixes with 20% and 30% BA showed similar trends, where extended curing time resulted in higher direct shear strength when 25% RCA was used as a replacement for NCA, while 50% RCA replacement resulted in reduced shear strength. The shear strength of the CF70B20-R25 mix improved from 3.49 MPa at 7 days of curing to 7.21 MPa at 120 days. As per the results, the CF70B20-R25 mix had higher strength than the control mix at 120 days periods of curing (7.00 MPa). Similarly, mix CF70B30-R25 exhibited 2% higher compressive strength after 120 days periods of curing with reference to control mix as depicted in Figure 7 (b).

Overall, the study revealed that the shear strength of 20% BA-based mix (CF70B20-R25) was significantly increased with the use of 25% RCA. This was in contrast to mixes with 10% and 30% replacement levels of BA with NFA. The increase in direct shear strength was more evident during later curing periods, where the mix CF70B20-R25 showed 2% lower shear strength than the control mix after 7 days, but significantly increased to 7.21 MPa after 120 days than the control mix (7.00 MPa). This rise in strength at later curing period was attributed to the pozzolanic effect of BA and FYA, which produced more C-S-H. Moreover, the interlocking nature of the aggregates was improved by the physical interface of fine BA with an irregular shape and sharp edge, making them more resistant to shear stress transferred during deformation (32, 58, 69).

Figure 8 (a), observed that the plane of failure is vertical i.e., just under the application of loading bar which represents the failure under pure shear failure along a single plane. However, Figure 8 (b), showed the failed specimens most of specimens failed due to breakage/splitting of coarse aggregates instead of failure of aggregate-matrix interface and even some of the RCA failed in the same manner which further increase the shear capacity due to denseness of matrix (25, 26). In contrast, the addition of 50% RCA as a NCA leads to a significant reduction in direct shear strength for all BA mixes, including CF70B10-R50, CF70B20-R50, and C750B30-R50, at different curing ages, which is much lesser than the control mix (CF70B0-R0). After 120 days curing, the direct shear strengths of HVFYA-SCC mixes that contained 50% RCA, such as CF70B10-R50, CF70B20-R50, and CF70B30-R50, were measured at 6.84 MPa, 6.90 MPa, and 6.86 MPa correspondingly, then the control HVFYA-SCC mix (7.00 MPa). The reduced direct shear strength can be attributed to the presence of old, residual adhering mortar in RCA, which contributes to the poor structural characteristics of materials at higher RCA content levels (65, 68).

Linear regression analysis has established a correlation between mechanical characteristics and durability characteristics (92). Previous studies have reported R2 values of 0.7, indicating a strong correlation among these characteristics (93, 94). Additionally, earlier research has suggested that fresh characteristics such as slump flow, T500, V-funnel and L-box values exhibit a linear relationship with variation of BA in HVFYA-SCC mixes (58, 67, 95, 96). Figure 17(a) displays the outcomes of the present study, demonstrating a strong correlation amongst BA versus slump flow, with R² values of 0.9967, 0.9988 and 0.9979 with RCA variation. Similarly, T₅₀₀ strongly correlates with BA with R² values above 0.9 (Figure 17(b)). Figures 17(c) & 17(d) showed a significant correlation between BA and V-funnel and L-box values respectively, with R² values higher than 0.9 (58, 67, 93). Figures 18 showed a significant correlation between compressive strength versus direct shear strength with R² values 0.9901 (93, 96, 97).

It is worthwhile to mention that different authors have tested a wide range of parameters in their investigations. In this investigation, some previous studies have been selected and the considered parameters such as w/b and w/c ratio, curing period, range of compressive strength and shear strength are listed in Table 5 along with the parameters of the current investigation for comparison.

The shear strength reported previously and observed in the current study are also listed for reference. The authors are of the opinion that in such a situation, it is rather difficult to arrive at meaningful comparison.

There are several levers available in the literature to achieve the carbon neutrality such as carbon dioxide capture, utilization and storage (CCUS) (98), carbon capture and sequestration (CCS) (98), global warming potential (GWP) (93) etc. Out of these techniques, herein, the influence of RCA, CBA in HVFA-SCC mixes has been measured using the concept of GWP. GWP is a measure of how much energy the emissions of 1 ton of a gas will absorb over a given period of time, relative to the emissions of 1 ton of carbon dioxide (CO₂). The sustainability of concrete is usually determined through CO₂ emissions involved in producing the materials used for construction.

In the current study, GWP was analysed to assess the ecological influence of using different materials in HVFYA-SCC. GWP measures the greenhouse gases released in terms of CO₂ equivalent during the entire lifecycle of raw materials, from extraction to disposal (99). Various reliable sources were consulted to determine GWP, as presented in Table 6 (93, 99-103). The present investigate to compare the effects of using various materials in HVFYA-SCC mixes: NFA, BA, NCA, and RCA. The results (Figure 19) show that the HVFYA-SCC mix CF70B0-R0, which contained NFA (100%) and 100%, had the maximum carbon emission at 201.96 kgCO₂e/m³. However, as the percentage of BA and RCA in other mixes increased, the carbon emission value declined. The HVFYA-SCC mix CF70B30-R50, with 30% BA and 50% RCA, had the lowest carbon emission value at 199.33 kgCO₂e/m³. These findings suggest that incorporating BA up to 30% and RCA up to 50% with NFA and NCA in HVFYA-SCC mixes is more environmentally sustainable as these mixes release less CO₂ compared to control HVFYA-SCC mix (CF70B0-R0).