Shear strength and microstructural investigation on high-volume fly ash self-compacting concrete containing recycled concrete aggregates and coal bottom ash

2.1. Materials

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 (4040. IS: 8112. 2013. Ordinary portland cement, 43 grade- specification. Bur. Indian Stand. New Delhi, India. 1-17.
) and ASTM C618 (4141. ASTM C 618. 2014. Standard specification for coal fly ash and raw or calcined natural pozzolan for use in concrete. 1–5.
), 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 (4242. IS:383. 2016. Indian Standard coarse and fine aggregate for concrete- specification. Bur. Indian Stand. New Delhi, India. 1–21.
), and the NFA was found to be retained in Zone II by carrying out sieve analysis as per the specification of IS 383-2016 (4242. IS:383. 2016. Indian Standard coarse and fine aggregate for concrete- specification. Bur. Indian Stand. New Delhi, India. 1–21.
). 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 (4242. IS:383. 2016. Indian Standard coarse and fine aggregate for concrete- specification. Bur. Indian Stand. New Delhi, India. 1–21.
).

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 (4343. IRC:121. 2017. Guidelines for use of construction and demolition waste in road sector. Indian Road Congress. 1:1-28.
) 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 (2323. Siddique R, Aggarwal P, Aggarwal Y. 2012. Influence of water/powder ratio on strength properties of self-compacting concrete containing coal fly ash and bottom ash. Constr. Build. Mater. 29:73–81. https://doi.org/10.1016/j.conbuildmat.2011.10.035.
, 4444. Jamaluddin N, Hamzah AF, Wan Ibrahim MH, Jaya RP, Arshad MF, Abidin NEZ, et al. 2016. Fresh properties and flexural strength of self-compacting concrete integrating coal bottom ash. in. MATEC Web Conf. 47:1–6.
, 4545. Siddique R, Aggarwal P, Aggarwal Y. 2012. Mechanical and durability properties of self-compacting concrete containing fly ash and bottom ash. J. Sustain. Cem. Mater. 1(3):67–82. https://doi.org/10.1080/21650373.2012.726820.
). 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%) (1818. Kapoor K, Singh SP, Singh B. 2017. Permeability of self-compacting concrete made with recycled concrete aggregates and metakaolin. J. Sustain. Cem. Mater. 6(5):293–313. https://doi.org/10.1080/21650373.2017.1280426.
, 2020. Kumar P, Singh N. 2020. Influence of recycled concrete aggregates and coal bottom ash on various properties of high volume fly ash-self compacting concrete. J. Build. Eng. 32:101491. https://doi.org/10.1016/j.jobe.2020.101491.
, 46-4946. Singh N, Nassar RUD, Shehnazdeep K, Anjani B. 2022. Microstructural characteristics and carbonation resistance of coal bottom ash based concrete mixtures. Mag. Concr. Res. 74:364–378. https://doi.org/10.1680/jmacr.20.00125.
47. Kapoor K, Singh SP, Singh B. 2016. Durability of self-compacting concrete made with recycled concrete aggregates and mineral admixtures. Constr. Build. Mater. 128:67–76. https://doi.org/10.1016/j.conbuildmat.2016.10.026.
48. Chiranjiakumari Devi S, Ahmad Khan R. 2020. Influence of graphene oxide on sulfate attack and carbonation of concrete containing recycled concrete aggregate. Constr. Build. Mater. 250:118883. https://doi.org/10.1016/j.conbuildmat.2020.118883.
49. Singh M, Siddique R. 2014. Compressive strength, drying shrinkage and chemical resistance of concrete incorporating coal bottom ash as partial or total replacement of sand. Constr. Build. Mater. 68:39–48. https://doi.org/10.1016/j.conbuildmat.2014.06.034.
). 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 (5050. IS 9103. 1999. Specification for concrete admixtures. Bur. Indian Stand. New Delhi, India. 1–22.
).

2.2. Mixtures

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) (5151. EFNARC. 2005. The european guidelines for self-compacting concrete specification, production and use. 1-68.
) guidelines, and SP was incorporated in different proportions ranging from 0.65% to 1.38% by weight of the binder.

2.3. Experimental methodology

2.3.2. Direct shear strength

 ⌅

Direct shear strength of HVFYA-SCC mixes was tested as per method recommended by Bairagi and Modhera (53)53. Bairagi NK, Modhera CD. 2001. Shear strength of fibre reinforced concrete. Indian Concr. Inst. J. 1(4):47–52.
. Two plates of light steel (150 x 85 x 10 mm and 150 x 110 x 10 mm) and two bars (12 mm and 22 mm in diameter) were utilized to carry out the test on a CTM. Plates were positioned on top of the L-shaped specimen, and a 22 mm dia. rod was placed together to the plate as presented in Figure 4 (a). The specimen was tested for accuracy and reliability at 7, 28, 56, 90, and 120 days using a 2000 kN CTM at a 2.1 kN/sec loading rate (Figure 4).

FIGURE 4.  (a) Design of specimen for shear strength, (b) testing arrangement, (c) tested specimens.

3.1. Workability and compressive strength

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 (5151. EFNARC. 2005. The european guidelines for self-compacting concrete specification, production and use. 1-68.
). 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. (5555. Singh N, Arya S, Mithul Raj M. 2019. Assessing the performance of self-compacting concrete made with recycled concrete aggregates and coal bottom ash using ultrasonic pulse velocity. In: Agnihotri, A., Reddy, K., Bansal, A. (eds) Recycled waste materials. Lecture notes in civil engineering, 32. Springer, Singapore. https://doi.org/10.1007/978-981-13-7017-5_19.
) 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. (1515. Singh N, Mithulraj M, Arya S. 2019. Utilization of coal bottom ash in recycled concrete aggregates based self compacting concrete blended with metakaolin. Resour. Conserv. Recycl. 144:240–251. https://doi.org/10.1016/j.resconrec.2019.01.044.
, 2020. Kumar P, Singh N. 2020. Influence of recycled concrete aggregates and coal bottom ash on various properties of high volume fly ash-self compacting concrete. J. Build. Eng. 32:101491. https://doi.org/10.1016/j.jobe.2020.101491.
).

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. (5555. Singh N, Arya S, Mithul Raj M. 2019. Assessing the performance of self-compacting concrete made with recycled concrete aggregates and coal bottom ash using ultrasonic pulse velocity. In: Agnihotri, A., Reddy, K., Bansal, A. (eds) Recycled waste materials. Lecture notes in civil engineering, 32. Springer, Singapore. https://doi.org/10.1007/978-981-13-7017-5_19.
) 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 (5151. EFNARC. 2005. The european guidelines for self-compacting concrete specification, production and use. 1-68.
) 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 (56-5856. Martínez-García R, Guerra-Romero IM, Morán-del Pozo JM, De Brito J, Juan-Valdés A. 2020. Recycling aggregates for self-compacting concrete production: A feasible option. Materials 13(4):868. https://doi.org/10.3390/ma13040868.
57. Tuyan M, Mardani-Aghabaglou A, Ramyar K. 2014. Freeze-thaw resistance, mechanical and transport properties of self-consolidating concrete incorporating coarse recycled concrete aggregate. Mater. Des. 53:983–991. https://doi.org/10.1016/j.matdes.2013.07.100.
58. Rafieizonooz M, Mirza J, Salim MR, Hussin MW, Khankhaje E. 2016. Investigation of coal bottom ash and fly ash in concrete as replacement for sand and cement. Constr. Build Mater. 116:15–24. https://doi.org/10.1016/j.conbuildmat.2016.04.080.
).

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 (3232. Meena A, Singh N, Singh SP. 2023. High-volume fly ash Self consolidating concrete with coal bottom ash and recycled concrete aggregates: fresh, mechanical and microstructural properties. J. Build. Eng. 63:105447. https://doi.org/10.1016/j.jobe.2022.105447.
, 5858. Rafieizonooz M, Mirza J, Salim MR, Hussin MW, Khankhaje E. 2016. Investigation of coal bottom ash and fly ash in concrete as replacement for sand and cement. Constr. Build Mater. 116:15–24. https://doi.org/10.1016/j.conbuildmat.2016.04.080.
) 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 (1515. Singh N, Mithulraj M, Arya S. 2019. Utilization of coal bottom ash in recycled concrete aggregates based self compacting concrete blended with metakaolin. Resour. Conserv. Recycl. 144:240–251. https://doi.org/10.1016/j.resconrec.2019.01.044.
). 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 (5959. Sanjuán MA, Argiz C. 2016. Coal fly ash alkalis content characterization by means of a full factorial design. Mater. Lett. 164:528–531. https://doi.org/10.1016/j.matlet.2015.11.034.
, 6060. Allahverdi A, Shaverdi B, Kani EN. 2010. Influence of sodium oxide on properties of fresh and hardened paste of alkali-activated blast-furnace slag. Int. J. Civ. Eng. 8(4):304–14.
). 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 (60-6260. Allahverdi A, Shaverdi B, Kani EN. 2010. Influence of sodium oxide on properties of fresh and hardened paste of alkali-activated blast-furnace slag. Int. J. Civ. Eng. 8(4):304–14.
61. Shukla A. 2020. Effect of sodium oxide on physical and mechanical properties of fly-ash based geopolymer composites. Indian J. Sci. Technol. 13(38):3994–4002. https://doi.org/10.17485/IJST/v13i38.1663.
62. Zhang J, Shi C, Zhang Z. 2021. Effect of Na₂O concentration and water/binder ratio on carbonation of alkali-activated slag/fly ash cements. Constr. Build. Mater. 269:121258. https://doi.org/10.1016/j.conbuildmat.2020.121258.
). Similar influence of K₂O has been noted wherein pozzolanic action has supplemented the strength aspects in concrete (5959. Sanjuán MA, Argiz C. 2016. Coal fly ash alkalis content characterization by means of a full factorial design. Mater. Lett. 164:528–531. https://doi.org/10.1016/j.matlet.2015.11.034.
, 6363. Santos TA, Neto JSA, Cilla MS, Ribeiro DV. 2022. Influence of the content of alkalis (Na₂O and K₂O), MgO, and SO₃ present in the granite rock fine in the production of portland clinker. J. Mater. Civ. Eng. 34(3):1–12. https://doi.org/10.1061/(ASCE)MT.1943-5533.0004201.
, 6464. De Pádua PGL, Cordeiro GC. 2021. Effect of K₂O content on properties of sugar cane bagasse ash-cement-based systems. Adv. Cem. Res. 34(2):57–66. https://doi.org/10.1680/jadcr.20.00082.
).

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 (6565. Kurda R, de Brito J, Silvestre JD. 2017. Influence of recycled aggregates and high contents of fly ash on concrete fresh properties. Cem. Concr. Compos. J. 84:198–213. https://doi.org/10.1016/j.cemconcomp.2017.09.009.
). 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 (3232. Meena A, Singh N, Singh SP. 2023. High-volume fly ash Self consolidating concrete with coal bottom ash and recycled concrete aggregates: fresh, mechanical and microstructural properties. J. Build. Eng. 63:105447. https://doi.org/10.1016/j.jobe.2022.105447.
, 5858. Rafieizonooz M, Mirza J, Salim MR, Hussin MW, Khankhaje E. 2016. Investigation of coal bottom ash and fly ash in concrete as replacement for sand and cement. Constr. Build Mater. 116:15–24. https://doi.org/10.1016/j.conbuildmat.2016.04.080.
). 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 (5858. Rafieizonooz M, Mirza J, Salim MR, Hussin MW, Khankhaje E. 2016. Investigation of coal bottom ash and fly ash in concrete as replacement for sand and cement. Constr. Build Mater. 116:15–24. https://doi.org/10.1016/j.conbuildmat.2016.04.080.
, 6565. Kurda R, de Brito J, Silvestre JD. 2017. Influence of recycled aggregates and high contents of fly ash on concrete fresh properties. Cem. Concr. Compos. J. 84:198–213. https://doi.org/10.1016/j.cemconcomp.2017.09.009.
).

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 (1515. Singh N, Mithulraj M, Arya S. 2019. Utilization of coal bottom ash in recycled concrete aggregates based self compacting concrete blended with metakaolin. Resour. Conserv. Recycl. 144:240–251. https://doi.org/10.1016/j.resconrec.2019.01.044.
, 4646. Singh N, Nassar RUD, Shehnazdeep K, Anjani B. 2022. Microstructural characteristics and carbonation resistance of coal bottom ash based concrete mixtures. Mag. Concr. Res. 74:364–378. https://doi.org/10.1680/jmacr.20.00125.
). In India, due to bituminous type of coal (6666. India Today. 2018. Indian coal reserves: classification of coal and where it is found in the country. India Today Web Desk, New Delhi, India. Retrieved from https://www.indiatoday.in/education-today/gk-current-affairs/story/indian-coal-reserves-classification-of-coal-and-where-it-is-found-in-the-country-1338928-2018-09-13.
), 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.

3.2. Direct shear strength

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 (2424. Zainal Abidin NE, Wan Ibrahim MH, Jamaluddin N, Kamaruddin K, Hamzah AF. 2014. The effect of bottom ash on fresh characteristic, compressive strength and water absorption of self-compacting concrete. Appl. Mech. Mater. 660:145–151. https://doi.org/10.4028/www.scientific.net/AMM.660.145.
, 6767. Majhi RK, Nayak AN. 2019. Properties of concrete incorporating coal fly ash and coal bottom ash. J. Inst. Eng. India Ser. A. 100:459–469. https://doi.org/10.1007/s40030-019-00374-y.
). 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 (1515. Singh N, Mithulraj M, Arya S. 2019. Utilization of coal bottom ash in recycled concrete aggregates based self compacting concrete blended with metakaolin. Resour. Conserv. Recycl. 144:240–251. https://doi.org/10.1016/j.resconrec.2019.01.044.
, 3232. Meena A, Singh N, Singh SP. 2023. High-volume fly ash Self consolidating concrete with coal bottom ash and recycled concrete aggregates: fresh, mechanical and microstructural properties. J. Build. Eng. 63:105447. https://doi.org/10.1016/j.jobe.2022.105447.
). 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 (6565. Kurda R, de Brito J, Silvestre JD. 2017. Influence of recycled aggregates and high contents of fly ash on concrete fresh properties. Cem. Concr. Compos. J. 84:198–213. https://doi.org/10.1016/j.cemconcomp.2017.09.009.
, 6868. Singh N, Singh SP. 2016. Carbonation and electrical resistance of self compacting concrete made with recycled concrete aggregates and metakaolin. Constr. Build. Mater. 121:400–409. https://doi.org/10.1016/j.conbuildmat.2016.06.009.
). 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 (3232. Meena A, Singh N, Singh SP. 2023. High-volume fly ash Self consolidating concrete with coal bottom ash and recycled concrete aggregates: fresh, mechanical and microstructural properties. J. Build. Eng. 63:105447. https://doi.org/10.1016/j.jobe.2022.105447.
, 5858. Rafieizonooz M, Mirza J, Salim MR, Hussin MW, Khankhaje E. 2016. Investigation of coal bottom ash and fly ash in concrete as replacement for sand and cement. Constr. Build Mater. 116:15–24. https://doi.org/10.1016/j.conbuildmat.2016.04.080.
, 6969. Singh M, Siddique R, Ait-Mokhtar K, Belarbi R. 2015. Durability properties of concrete made with high volumes of low-calcium coal bottom ash as a replacement of two types of sand. J. Mater. Civ. Eng. 28(4):04015175. https://doi.org/10.1061/(ASCE)MT.1943-5533.0001464.
).

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 (2525. Xiao J, Xie H, Yang Z. 2012. Shear transfer across a crack in recycled aggregate concrete. Cem. Concr. Res. 42(5):700–709. https://doi.org/10.1016/j.cemconres.2012.02.006.
, 2626. Waseem SA, Singh B. 2016. Shear transfer strength of normal and high-strength recycled aggregate concrete – An experimental investigation. Constr. Build. Mater. 125:29–40. https://doi.org/10.1016/j.conbuildmat.2016.08.022.
). 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 (6565. Kurda R, de Brito J, Silvestre JD. 2017. Influence of recycled aggregates and high contents of fly ash on concrete fresh properties. Cem. Concr. Compos. J. 84:198–213. https://doi.org/10.1016/j.cemconcomp.2017.09.009.
, 6868. Singh N, Singh SP. 2016. Carbonation and electrical resistance of self compacting concrete made with recycled concrete aggregates and metakaolin. Constr. Build. Mater. 121:400–409. https://doi.org/10.1016/j.conbuildmat.2016.06.009.
).

3.3. Microstructural analysis

3.3.1. X-ray diffraction analysis

 ⌅

Herein the effect of addition of BA primarily in HVFYA-SCC mixes has been discussed. It has also been observed that with addition of BA in HVFYA-SCC mixes an ample variation in the counts of peaks of different compounds [such as Portlandite (P), calcium silicate hydrate (C-S-H), quartz (Q), calcium silicate (CS) and, gismondine (CASH)] has been noticed. In fact, all the HVFYA-SCC mixes have been compared with the control HVFYA-SCC (CF70B0-R0) mix. The additional peaks which are not participating in enhancing the performance of HVFYA- SCC mixes have not been discussed. XRD analysis results after 28 and 120 curing days, are presented in Figures 9-10. The following mineral phases were identified by XRD in all HVFYA-SCC mixes: Portlandite (P), calcium silicate hydrate (C-S-H), quartz (Q), calcium silicate (CS) and, gismondine (CASH) after 28 and 120 days of curing. Also these mineral phases are prominent in later ages, the pozzolanic reactions occur due to presence FYA and BA primarily in mortar phases (4646. Singh N, Nassar RUD, Shehnazdeep K, Anjani B. 2022. Microstructural characteristics and carbonation resistance of coal bottom ash based concrete mixtures. Mag. Concr. Res. 74:364–378. https://doi.org/10.1680/jmacr.20.00125.
, 5454. Jain A, Chaudhary S, Gupta R. 2022. Mechanical and microstructural characterization of fly ash blended self-compacting concrete containing granite waste. Constr. Build. Mater. 314:125480. https://doi.org/10.1016/j.conbuildmat.2021.125480.
, 70-7270. Wilben S, Supit M, Uddin F, Shaikh A. 2015. Durability properties of high volume fly ash concrete containing nano-silica. Mater. Struct. 48:2431–2445. https://doi.org/10.1617/s11527-014-0329-0.
71. Sun J, Shen X, Tan G, Tanner JE. 2019. Modification effects of Nano-SiO₂ on early compressive strength and hydration characteristics of high-volume fly ash concrete. J. Mater. Civ. Eng. 31(6):1–12. https://doi.org/10.1061/(ASCE)MT.1943-5533.0002665.
72. Jindal BB. 2018. Feasibility study of ambient cured geopolymer concrete - a review. Adv. Concr. Constr. 6(4):387–405. https://doi.org/10.12989/acc.2018.6.4.387.
), hence for the same reason the investigation have been made for the current study.

The prominent peaks for P were located at around 29.37° and 50.11° (4646. Singh N, Nassar RUD, Shehnazdeep K, Anjani B. 2022. Microstructural characteristics and carbonation resistance of coal bottom ash based concrete mixtures. Mag. Concr. Res. 74:364–378. https://doi.org/10.1680/jmacr.20.00125.
, 72-7472. Jindal BB. 2018. Feasibility study of ambient cured geopolymer concrete - a review. Adv. Concr. Constr. 6(4):387–405. https://doi.org/10.12989/acc.2018.6.4.387.
73. Devi SC, Khan RA. 2020. Effect of sulfate attack and carbonation in graphene oxide–reinforced concrete containing recycled concrete aggregate. J. Mater. Civ. Eng. 32(11):04020339. https://doi.org/10.1061/(ASCE)MT.1943-5533.0003415.
74. Timakul P, Rattanaprasit W, Aungkavattana P. 2016. Improving compressive strength of fly ash-based geopolymer composites by basalt fibers addition. Ceram. Int. 42(5):6288–6295. https://doi.org/10.1016/j.ceramint.2016.01.014.
). For Q, the intensities were observed at 20.84°, 26.66°, 39.52°, 44.64°, 50.16°, 59.94° and 68.13°. Additionally, C-S intensity was located at 33.15°, the greatest intensity peaks of C-S-H were detected at 20.84°, 29.37°, 33.15°, 44.64° and 50.11° and for CASH shows 2θ at 20.84°. After 28 days curing stage (Figure 9(a-c)), the BA-based HVFYA-SCC mixes exhibited lower peaks of C-S-H than the HVFYA-SCC mix CF70B0-R0, which might be the decrease in the heat of hydration of cement and slowdown of FYA and BA pozzolanic action throughout the initial periods of curing are the reasons for observed incline in HVFYA-SCC mixes (4646. Singh N, Nassar RUD, Shehnazdeep K, Anjani B. 2022. Microstructural characteristics and carbonation resistance of coal bottom ash based concrete mixtures. Mag. Concr. Res. 74:364–378. https://doi.org/10.1680/jmacr.20.00125.
).

FIGURE 9.  XRD patterns of the HVFYA-SCC mixes with 10-30% BA at 28 days of curing: (a) 0% RCA, (b) 25% RCA and (c) 50% RCA.

The mix CF70B20-R0, which predominates the mechanical properties of the concrete matrix, and the most important intensity peaks of C-S-H were found at 29.09° after 120 days of curing. Figure 10(a) revealed that the intensity of C-S-H was more for mix CF70B20-R0 than in other HVFYA-SCC mixes. The trends of C-S-H intensities in the HVFYA-SCC mixes are as follows CF70B20-R0> CF70B30-R0> CF70B30-R0 > CF70B0-R0. Overall, the peaks of P were found to be lower for mix CF70B20-R0, which may be owing to the pozzolanic interaction between the FYA and BA at120 days curing age (7575. Jain A, Gupta R, Chaudhary S. 2019. Performance of self-compacting concrete comprising granite cutting waste as fine aggregate. Constr. Build. Mater. 221:539–552. https://doi.org/10.1016/j.conbuildmat.2019.06.104.
).

FIGURE 10.  XRD patterns of the HVFYA-SCC mixes with 10-30% BA at 120 days of curing: (a) 0% RCA, (b) 25% RCA and (c) 50% RCA.

Figure 10(b) depicted that the intensity of C-S-H was more for CF70B20-R25. In contrast, the intensity of the hydrated product was lower in other HVFYA-SCC mixes, including the HVFYA-SCC mix CF70B0-R0. The trends of hydrated products were as follows CF70B20-R25> CF70B30-R25> CF70B10-R25 > CF70B0-R0.

Figure 10(c) shows that after 120 days curing periods, the HVFYA-SCC mix CF70B20-R50 has higher hydrated products than the other mixture. Overall, when varying the concentration of RCA, the CF70B20-R25 mix had superior hydrated product intensity supported by mechanical strength. It can be observed that the intensity of peaks P was decreased or gone entirely, particularly in CF70B20-R25 combinations. Moreover, BA pozzolanic response and filler effect in the HVFYA-SCC mix increased strength properties at the optimal dose of 20% BA and 25% RCA content (5858. Rafieizonooz M, Mirza J, Salim MR, Hussin MW, Khankhaje E. 2016. Investigation of coal bottom ash and fly ash in concrete as replacement for sand and cement. Constr. Build Mater. 116:15–24. https://doi.org/10.1016/j.conbuildmat.2016.04.080.
, 6969. Singh M, Siddique R, Ait-Mokhtar K, Belarbi R. 2015. Durability properties of concrete made with high volumes of low-calcium coal bottom ash as a replacement of two types of sand. J. Mater. Civ. Eng. 28(4):04015175. https://doi.org/10.1061/(ASCE)MT.1943-5533.0001464.
). The presence of Pozzolanic materials benefited the process by producing enough hydration products in the shape of C-S-H (5454. Jain A, Chaudhary S, Gupta R. 2022. Mechanical and microstructural characterization of fly ash blended self-compacting concrete containing granite waste. Constr. Build. Mater. 314:125480. https://doi.org/10.1016/j.conbuildmat.2021.125480.
, 7070. Wilben S, Supit M, Uddin F, Shaikh A. 2015. Durability properties of high volume fly ash concrete containing nano-silica. Mater. Struct. 48:2431–2445. https://doi.org/10.1617/s11527-014-0329-0.
, 7171. Sun J, Shen X, Tan G, Tanner JE. 2019. Modification effects of Nano-SiO₂ on early compressive strength and hydration characteristics of high-volume fly ash concrete. J. Mater. Civ. Eng. 31(6):1–12. https://doi.org/10.1061/(ASCE)MT.1943-5533.0002665.
, 7676. Cheng R, Gen M, Tsujimura Y. 1996. A tutorial survey of job-shop scheduling problems using genetic algorithms - I. Representation. Comput. Ind. Eng. 30(4):983–997. https://doi.org/10.1016/0360-8352(96)00047-2.
, 7777. Bhatrola K, Kothiyal NC. 2023. Comparative study of physico-mechanical performance of PPC mortar incorporated 1D/2D functionalized nanomaterials. Int. J. Appl. Ceram. Technol. 20(4):2478–2498. https://ceramics.onlinelibrary.wiley.com/doi/abs/10.1111/ijac.14372.
).

3.3.2. Fourier transform infrared spectroscopy

 ⌅

FTIR was used to test concrete mixes for distinctive bands at different wavenumbers. The HVFYA-SCC mix was found to contain diverse molecular groups after 28- and 120-days periods of curing, as shown in Figures 11 (a-c) and 12 (a-c). The 4000 to 500 cm^-1 wavenumber FTIR spectra revealed the chemical interaction between the cementitious matrix, BA, and FYA.

FIGURE 11.  FTIR patterns of the HVFYA-SCC mixes with 10-30% BA at 28 days of curing: (a) 0% RCA, (b) 25% RCA and (c) 50% RCA.

FIGURE 12.  FTIR patterns of the HVFYA-SCC mixes with 10-30% BA at 120 days of curing: (a) 0% RCA, (b) 25% RCA and (c) 50% RCA.

The transmittance bands at 3753 cm^-1 represent vibration of the C-H groups produced by silicates in hydrated cement, while the bands at 3357 cm^-1 represent Ca(OH)₂ bonding frequency. As extra C-S-H forms, more Ca(OH)₂ consumed (7878. Biricik H, Sarier N. 2014. Comparative study of the characteristics of nano silica-, silica fume- and fly ash-incorporated cement mortars. Mater. Res. 17(3):570–582. https://doi.org/10.1590/S1516-14392014005000054.
, 7979. Bhatrola K, Kothiyal NC. 2023. Influence of (1D/2D) hybrid nanomaterials on the mechanical and durability properties of pozzolana portland cementitious mortar. J. Adhes. Sci. Technol. 1–25. https://doi.org/10.1080/01694243.2023.2226287.
).

Among the HVFYA-SCC mixes tested, the CF70B20-R25 mix demonstrated the most significant decrease in the O-H group band after 120 days, while CF70B0-R0, CF70B10-R0, and CF70B30-R0 exhibited the least considerable decrease. The BA and FYA compounds in the HVFYA-SCC matrix exhibited higher pozzolanic activity than the HVFYA-SCC mix CF70B0-R0, resulting in formation of more C-S-H at a later stage. In the CF70B10-R25 and CF70B30-R25 mixes, the intensity of O-H groups decreased significantly than HVFYA-SCC mix CF70B0-R0 after 120 days, likely due to Ca(OH)₂ consumption by FYA, as shown by XRD results.

Adding an optimal amount of BA in the HVFYA-SCC mixes improved the content of C-S-H, resultant in less free water availability and less Ca(OH)₂ than HVFYA-SCC mix CF70B0-R0. The prominent peaks of 2911 cm^-1 and 1447 cm^-1 represent the widening manners of C-O bands of CaCO₃ and hydration products (CH), respectively, representing the reaction among Ca(OH)₂ and CO₂. The absorbance of the H-O-H band at 1567 cm^-1 is recognized as a water bond (5454. Jain A, Chaudhary S, Gupta R. 2022. Mechanical and microstructural characterization of fly ash blended self-compacting concrete containing granite waste. Constr. Build. Mater. 314:125480. https://doi.org/10.1016/j.conbuildmat.2021.125480.
, 8080. Lo FC, Lee MG, Lo SL. 2021. Effect of coal ash and rice husk ash partial replacement in ordinary Portland cement on pervious concrete. Constr. Build. Mater. 286:122947. https://doi.org/10.1016/j.conbuildmat.2021.122947.
), and the widening absorbance of the S-O band at 1020 cm-1 is identified as Ettringite (8181. Ylmén R, Jäglid U, Steenari BM, Panas I. 2009. Early hydration and setting of Portland cement monitored by IR, SEM and Vicat techniques. Cem. Concr. Res. 39(5):433–439. https://doi.org/10.1016/j.cemconres.2009.01.017.
).

The intensity of Si-O bonds in C₂S and C₃S reduces as C-S-H forms over time, and the absorbance bands observed at 823 cm^-1 represent the vibrations of these bonds. Adding 20% BA to the HVFYA-SCC mix (CF70B20-R0) showed a significant decrement in the intensity of Si-O bonds as compared to HVFYA-SCC mix CF70B0-R0, indicating that BA and FYA increased the content of C-S-H after higher hydration age. The absorbance bands observed at 1257 cm^-1 correspond to T-O-Si (T = tetrahedral Al and Si), which related to main hydration product of C-S-H (8080. Lo FC, Lee MG, Lo SL. 2021. Effect of coal ash and rice husk ash partial replacement in ordinary Portland cement on pervious concrete. Constr. Build. Mater. 286:122947. https://doi.org/10.1016/j.conbuildmat.2021.122947.
, 8282. Lee TC, Rao MK. 2009. Recycling municipal incinerator fly- and scrubber-ash into fused slag for the substantial replacement of cement in cement-mortars. Waste Manag. 29(6):1952–1959. https://doi.org/10.1016/j.wasman.2009.01.002.
, 8383. Maurya SK, Kothiyal NC. 2023. Effect of graphene oxide and functionalized carbon nanotubes on mechanical and durability properties of high volume fly-ash cement nanocomposite. Eur. J. Environ. Civ. Eng. 1–17. https://doi.org/10.1080/19648189.2023.2229401.
). After a higher curing age, the BA and FYA compounds demonstrated higher pozzolanic activity and reacted with Ca(OH)₂, resulting in greater intensity in the T-O-Si band and important to developing more C-S-H. These results, visible in FTIR spectra, confirm the outcomes for compressive and shear strength.

3.3.3. Scanning electron microscopy

 ⌅

Various investigations have demonstrated that the mechanical characteristics of concrete are significantly influenced by the microstructure of the material (8484. Sharma R, Khan RA. 2017. Sustainable use of copper slag in self compacting concrete containing supplementary cementitious materials. J. Clean. Prod. 151:179–192. https://doi.org/10.1016/j.jclepro.2017.03.031.
, 8585. Singh N, Singh SP. 2016. Carbonation resistance and microstructural analysis of low and high volume fly ash self compacting concrete containing recycled concrete aggregates. Constr. Build. Mater. 127:828–842. https://doi.org/10.1016/j.conbuildmat.2016.10.067.
). This microstructure is affected by factors such as hydration time, cement type, water-cement ratio, and mineral admixtures (3232. Meena A, Singh N, Singh SP. 2023. High-volume fly ash Self consolidating concrete with coal bottom ash and recycled concrete aggregates: fresh, mechanical and microstructural properties. J. Build. Eng. 63:105447. https://doi.org/10.1016/j.jobe.2022.105447.
, 86-8986. Siddique R. 2013. Compressive strength, water absorption, sorptivity, abrasion resistance and permeability of self-compacting concrete containing coal bottom ash. Constr. Build. Mater. 47:1444–1450. https://doi.org/10.1016/j.conbuildmat.2013.06.081.
87. Gooi S, Mousa AA, Kong D. 2020. A critical review and gap analysis on the use of coal bottom ash as a substitute constituent in concrete. J. Clean. Prod. 268:121752. https://doi.org/10.1016/j.jclepro.2020.121752.
88. Singh M, Siddique R. 2014. Strength properties and micro-structural properties of concrete containing coal bottom ash as partial replacement of fine aggregate. Constr. Build. Mater. 50:246–256. https://doi.org/10.1016/j.conbuildmat.2013.09.026.
89. Akhil, Singh N. 2024. Microstructural characteristics of iron-steel slag concrete: A brief review. Mater. Today Proc. https://doi.org/10.1016/j.matpr.2023.03.548.
). The microstructure of concrete is comprised of the aggregate, interfacial transition zone (ITZ), and hydrated cement paste. In this study, SEM images of broken HVFYA-SCC specimens were taken in secondary electrons mode to investigate the microstructure.

The strength of concrete generally depends on the strength of the aggregates, porosity, source of collection of concrete debris, cement matrix, and ITZ between the matrix and the aggregates (1919. Singh N, Singh SP. 2018. Validation of carbonation behavior of self compacting concrete made with recycled aggregates using microstructural and crystallization investigations. Eur. J. Environ. Civ. Eng. 24(13):2187-2210. https://doi.org/10.1080/19648189.2018.1500312.
). Therefore, HVFA-SCC mixes made with higher replacement levels of RCA resulted in lower strength. Figure 13 and Figure 14 present the SEM images showing weak characteristics of RCA such as micro and macro pores, residual mortar/adhered mortar, cracks/fissures etc. Herein, the observed characteristics are certainly responsible for the overall poor structural behaviour of HVFA -SCC mixes (1818. Kapoor K, Singh SP, Singh B. 2017. Permeability of self-compacting concrete made with recycled concrete aggregates and metakaolin. J. Sustain. Cem. Mater. 6(5):293–313. https://doi.org/10.1080/21650373.2017.1280426.
, 1919. Singh N, Singh SP. 2018. Validation of carbonation behavior of self compacting concrete made with recycled aggregates using microstructural and crystallization investigations. Eur. J. Environ. Civ. Eng. 24(13):2187-2210. https://doi.org/10.1080/19648189.2018.1500312.
).

FIGURE 13.  SEM image showing NCA and RCA [1919. Singh N, Singh SP. 2018. Validation of carbonation behavior of self compacting concrete made with recycled aggregates using microstructural and crystallization investigations. Eur. J. Environ. Civ. Eng. 24(13):2187-2210. https://doi.org/10.1080/19648189.2018.1500312.
].

FIGURE 14.  SEM image showing NCA and RCA based concrte [1919. Singh N, Singh SP. 2018. Validation of carbonation behavior of self compacting concrete made with recycled aggregates using microstructural and crystallization investigations. Eur. J. Environ. Civ. Eng. 24(13):2187-2210. https://doi.org/10.1080/19648189.2018.1500312.
].

The SEM images revealed that HVFYA-SCC mixes containing a higher content of BA had more microscopic air gaps, indicating their higher water-absorbing nature (2424. Zainal Abidin NE, Wan Ibrahim MH, Jamaluddin N, Kamaruddin K, Hamzah AF. 2014. The effect of bottom ash on fresh characteristic, compressive strength and water absorption of self-compacting concrete. Appl. Mech. Mater. 660:145–151. https://doi.org/10.4028/www.scientific.net/AMM.660.145.
). Figure 15 (h-j) shows that voids grew and multiplied with 30% BA. Microstructure of HVFYA-SCC specimens with BA (30%) differed from that of the control HVFYA-SCC mix because of the slow pozzolanic activity of BA, which caused a slight reduction in compressive and shear strength behavior during the initial curing ages.

FIGURE 15.  Microscopic images of HVFYA-SCC mixes at 120 days: (a) CF70B0-R0, (b) CF70B10-R0, (c) CF70B10-R25, (d) CF70B10-R50, (e) CF70B20-R0, (f) CF70B20-R25, (g) CF70B20-R50, (h-i) CF70B30-R0, (j) CF70B30-R25, (k) CF70B30-R50.

Simultaneously, the presence of BA further contributed in production of lower amount of C-S-H, consequently improving the microstructure of HVFYA-SCC mix. At initial ages, various irregular blocks and sheets were noticed for C-S-H, as the curing age continued for 120 days the irregular blocks were converted into dense crystals of C-S-H with rocky presence (Figure 15 (f)). As a result of dense and perfect development of C-S-H as identified in the images the resultant concrete matrix flaws were filled more efficiently leading towards better performance in term of shear and compressive strength (CF70B20-R25 mix) (Figure 15 (f)). Specific additional SEM images for the selected HVFYA-SCC mixes which are performing best and second best in comparison to the control HVFYA-SCC (CF70B0-R0) mix have also been incorporated to supplement the behaviour of HVFYA-SCC mixes (CF70B20-R25 & CF70B20-R0). Further, it is important here to mention that the presence of C-S-H gel / crystals that are generally present in platy or hexagonal or in layered cloud form (s) are noticed at low magnification / scale. The clear differentiation of C-S-H crystals / gel can be noticed in Figure 16 (a-d).

FIGURE 16.  Microscopic images of HVFYA-SCC mixes at 120 days: (a) and (b) CF70B20-R25, (c) and (d) CF70B20-R0.

Furthermore, Figure 15 (c-d), Figure 15 (g) and Figure 15 (j-k) for the HVFYA-SCC mixes CF70B10-R25, CF70B10-R50, CF70B20-R50, CF70B30-R25 and CF70B30-R50 present the concerns of presence of RCA (25% and 50%). The SEM images define weak bonding behavior due to presence of new and old mortar interfaces with clear boundaries. Since, the boundaries/ITZs’ between the old and new mortar were weak in nature, resulting in significant drop in compressive and shear strength for the HVFYA-SCC mix CF70B10-R50, CF70B20-R50 and CF70B30-R50 in Figure 15 (d), Figure15 (g) and Figure 15 (k) respectively. Further, in Figure 15 (i) the unburnt coal particles were also spotted in SEM images for the HVFYA-SCC mixes with higher content of BA (30%). It has been confirmed from the literature that presence of unburnt carbon particles is a vital character of BA making it unsuitable for incorporation as an alternative of conventional aggregates in construction. Moreover, the larger quantities of unburned carbon particles adversely affect the hardened properties of mortars and concretes (9090. Ha TH, Muralidharan S, Bae JH, Ha YC, Lee HG, Park KW, Kim DK. 2005. Effect of unburnt carbon on the corrosion performance of fly ash cement mortar. Constr. Build. Mater. 19(7):509–515. https://doi.org/10.1016/j.conbuildmat.2005.01.005.
, 9191. Lv B, Jiao F, Chen Z, Dong B, Fang C, Zhang C, Deng X. 2022. Separation of unburned carbon from coal fly ash: Pre-classification in liquid–solid fluidized beds and subsequent flotation. Process Saf. Environ. Prot. 165:408–419. https://doi.org/10.1016/j.psep.2022.07.031.
). Figure 15 (h-k) furthermore indicate that the increasing the amount of BA (30%), beyond a certain percentage increase the overall porosity in the matrix due to higher water absorption (Table 3) leading towards weak microstructural performance.

3.4. Mathematical relationships

Linear regression analysis has established a correlation between mechanical characteristics and durability characteristics (9292. Nassar RUD, Singh N, Varsha S, Sai AR, Sufyan-Ud-Din M. 2022. Strength, electrical resistivity and sulfate attack resistance of blended mortars produced with agriculture waste ashes. Case. Stud. Constr. Mater. 16:e00944. https://doi.org/10.1016/j.cscm.2022.e00944.
). Previous studies have reported R2 values of 0.7, indicating a strong correlation among these characteristics (9393. Simalti A, Singh AP. 2021. Comparative study on performance of manufactured steel fiber and shredded tire recycled steel fiber reinforced self-consolidating concrete. Constr. Build. Mater. 266(Part B):121102. https://doi.org/10.1016/j.conbuildmat.2020.121102.
, 9494. Mastali M, Dalvand A. 2016. Use of silica fume and recycled steel fibers in self-compacting concrete (SCC). Constr. Build. Mater. 125:196–209. https://doi.org/10.1016/j.conbuildmat.2016.08.046.
). 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 (5858. Rafieizonooz M, Mirza J, Salim MR, Hussin MW, Khankhaje E. 2016. Investigation of coal bottom ash and fly ash in concrete as replacement for sand and cement. Constr. Build Mater. 116:15–24. https://doi.org/10.1016/j.conbuildmat.2016.04.080.
, 6767. Majhi RK, Nayak AN. 2019. Properties of concrete incorporating coal fly ash and coal bottom ash. J. Inst. Eng. India Ser. A. 100:459–469. https://doi.org/10.1007/s40030-019-00374-y.
, 9595. Sukumar B, Nagamani K, Srinivasa Raghavan R. 2008. Evaluation of strength at early ages of self-compacting concrete with high volume fly ash. Constr. Build. Mater. 22(7):1394–1401. https://doi.org/10.1016/j.conbuildmat.2007.04.005.
, 9696. Singh RB, Debbarma S, Kumar N, Singh S. 2021. Hardened state behaviour of self-compacting concrete pavement mixes containing alternative aggregates and secondary binders. Constr. Build. Mater. 266(Part A):120624. https://doi.org/10.1016/j.conbuildmat.2020.120624.
). 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 (5858. Rafieizonooz M, Mirza J, Salim MR, Hussin MW, Khankhaje E. 2016. Investigation of coal bottom ash and fly ash in concrete as replacement for sand and cement. Constr. Build Mater. 116:15–24. https://doi.org/10.1016/j.conbuildmat.2016.04.080.
, 6767. Majhi RK, Nayak AN. 2019. Properties of concrete incorporating coal fly ash and coal bottom ash. J. Inst. Eng. India Ser. A. 100:459–469. https://doi.org/10.1007/s40030-019-00374-y.
, 9393. Simalti A, Singh AP. 2021. Comparative study on performance of manufactured steel fiber and shredded tire recycled steel fiber reinforced self-consolidating concrete. Constr. Build. Mater. 266(Part B):121102. https://doi.org/10.1016/j.conbuildmat.2020.121102.
). Figures 18 showed a significant correlation between compressive strength versus direct shear strength with R² values 0.9901 (9393. Simalti A, Singh AP. 2021. Comparative study on performance of manufactured steel fiber and shredded tire recycled steel fiber reinforced self-consolidating concrete. Constr. Build. Mater. 266(Part B):121102. https://doi.org/10.1016/j.conbuildmat.2020.121102.
, 9696. Singh RB, Debbarma S, Kumar N, Singh S. 2021. Hardened state behaviour of self-compacting concrete pavement mixes containing alternative aggregates and secondary binders. Constr. Build. Mater. 266(Part A):120624. https://doi.org/10.1016/j.conbuildmat.2020.120624.
, 9797. Sahmaran M, Yaman IO, Tokyay M. 2009. Transport and mechanical properties of self consolidating concrete with high volume fly ash. Cem. Concr. Compos. 31(2):99–106. https://doi.org/10.1016/j.cemconcomp.2008.12.003.
).

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.

3.5. Ecological evaluation of HVFYA-SCC mixes

There are several levers available in the literature to achieve the carbon neutrality such as carbon dioxide capture, utilization and storage (CCUS) (9898. Sanjuán MA, Argiz C, Mora P, Zaragoza A. 2020. Carbon dioxide uptake in the roadmap 2050 of the spanish cement industry. Energies. 13(13):3452. https://doi.org/10.3390/en13133452.
), carbon capture and sequestration (CCS) (9898. Sanjuán MA, Argiz C, Mora P, Zaragoza A. 2020. Carbon dioxide uptake in the roadmap 2050 of the spanish cement industry. Energies. 13(13):3452. https://doi.org/10.3390/en13133452.
), global warming potential (GWP) (9393. Simalti A, Singh AP. 2021. Comparative study on performance of manufactured steel fiber and shredded tire recycled steel fiber reinforced self-consolidating concrete. Constr. Build. Mater. 266(Part B):121102. https://doi.org/10.1016/j.conbuildmat.2020.121102.
) 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 (9999. Sharma R, Senthil K. 2023. An investigation on mechanical and microstructural properties of hybrid fiber reinforced concrete with manufactured sand and recycled coarse aggregate. J. Build. Eng. 69:106236. https://doi.org/10.1016/j.jobe.2023.106236.
). Various reliable sources were consulted to determine GWP, as presented in Table 6 (9393. Simalti A, Singh AP. 2021. Comparative study on performance of manufactured steel fiber and shredded tire recycled steel fiber reinforced self-consolidating concrete. Constr. Build. Mater. 266(Part B):121102. https://doi.org/10.1016/j.conbuildmat.2020.121102.
, 99-10399. Sharma R, Senthil K. 2023. An investigation on mechanical and microstructural properties of hybrid fiber reinforced concrete with manufactured sand and recycled coarse aggregate. J. Build. Eng. 69:106236. https://doi.org/10.1016/j.jobe.2023.106236.
100. Kurad R, Silvestre JD, De Brito J, Ahmed H. 2017. Effect of incorporation of high volume of recycled concrete aggregates and fly ash on the strength and global warming potential of concrete. J. Clean. Prod. 166:485–502. https://doi.org/10.1016/j.jclepro.2017.07.236.
101. Devi SC, Khan RA. 2020. Compressive strength and durability behavior of graphene oxide reinforced concrete composites containing recycled concrete aggregate. J. Build. Eng. 32:101800. https://doi.org/10.1016/j.jobe.2020.101800.
102. Mastali M, Dalvand A, Sattarifard AR, Illikainen M. 2018. Development of eco-efficient and cost-effective reinforced self-consolidation concretes with hybrid industrial/recycled steel fibers. Constr. Build. Mater. 166:214–226. https://doi.org/10.1016/j.conbuildmat.2018.01.147.
103. Pavlu T, Kocí V, Hájek P. 2019. Environmental assessment of two use cycles of recycled aggregate concrete. Sustain. 11(21):6185. https://doi.org/10.3390/su11216185.
). 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).