Introduction

The sustainability of concrete depends on the amount of CO2 and other GHGs emitted in the production and procurement of raw materials, mixing, casting, and curing. Due to globalization and increased population, industrial waste disposal is one of the most significant challenges humankind is facing. In this context, attempts are being made to reduce carbon emissions in concreting through the effective utilization of industrial by-products. Sustainable development goals (SDG) and standards have suggested using waste materials/by-products that reduce carbon emission and embodied energy.

Cement production is an energy-intensive process. As of now, 4400 million tons of cement are manufactured yearly worldwide. It is also anticipated that the number will rise to over 5500 million tons by 2050. About 8% of the world's CO2 emission is attributed to cement production. India stands at the second position in cement production and cement-related CO2 emissions. Cement production in India is estimated to rise to 3000 MT, emitting nearly 1.3 billion tons of CO2 by 2050 [1]. The CO2 emissions are found to reduce with SCMs in one of the binder phases. Approximately 13–22% of CO2 emissions were reduced by SCMs depending on the level of replacement used [2]. O'Brien et al. (2009) reported that the primary source of GHG emissions is the concrete industry. Seven percent of the global GHG emissions are from PC production. In addition, the cement industry releases gases such as SO2 and NOx that can cause environmental degradation and other associated effects [3, 4].

Many researchers have unanimously accepted that FA reduces GHG emissions when replaced with Portland cement [5]. The reduction in GHG emissions depends on the source and condition of raw materials, the type of supplementary cementitious material used, the percentage of replacement level, and the transportation distance. Industrial by-products, such as FA and slag, are widely used and accepted as partial replacements to OPC [6]. The effective use of improperly disposed of municipal and industrial by-products/wastes can reduce pollution and result in the sustainable use of natural resources [7]. Many experimental investigations are performed on using other wastes in concrete, such as palm oil fuel ash, rice hush ash, MSWA, incinerated bottom ash, agro-waste, and SSA, as a part of cementitious binder in concrete production [8,9,10,11,12,13,14,15,16,17,18,19,20,21,22].

Due to the ever-increasing energy demand, many thermal power plants were set up across the globe, resulting in the large-scale production of FA as a by-product. Therefore, the safe disposal of FA to prevent environmental pollution has become a global challenge. FA can be used as a valuable resource in concrete, greenhouse gas emissions, and embodied energy. HVFA is an approach to maximize the FA content in concrete and minimize OPC use for a similar level of mechanical properties. In contrast, Dunstan et al. (1992) referred to any concrete containing more than 40% of FA as HVFA concrete. Many experimental investigations have recommended 30–70% cement replacement by FA for concrete having 28 days strength of 40–50 MPa [3, 23,24,25,26,27,28,29,30,31,32,33].

Sivasundaram et al. (1990) observed the strength development of HVFA concrete over three years. Concrete gained strength of 70 MPa and modulus of elasticity of 47GPa with prolonged curing for two years [34]. Jiang and Malhotra (2000) recommended the use of a large amount of FA as binders (55%) in conjunction with the use of superplasticizers to achieve higher slumps of 100 mm and above since HVFAC is associated with a low W/B [32]. Bouzoubaa et al. (2000) found improvement in resistance to chloride ion penetration characteristics with HVFA blended cement. Further, it was also noticed that the performance of HVFA against chloride ion penetration was enhanced with an increase in the inter-grinding time of binder material [35].

SS is a by-product of the MWWTP. The estimated dry sludge production quantities annually are 8910, 6510, 3955, 2960, 650, 580, 550, and 370 thousand metric tons EU-27, USA, India, China, Iran, Turkey, Canada, and Brazil, respectively [36,37,38,39]. Presently, in India, out of the 62,000 MLD sewage generated, only 20,120 MLD goes into the treatment plant. The quantity of dry sludge generated in India is expected to increase many times due to the extensive installation of municipal sewage treatment plants under the Swachh Bharat Mission [31,32,33]. Due to large SS production, a proper disposal strategy is essential to manage dried sludge quantities. Various disposal strategies like agricultural manure, fillers in landscapes, gardening, and all the unused SS get dumped into the landfills. With the lack of land spaces and new environmental regulations, it is essential to explore new applications of SS [40, 41]. There is a need for efficient recycling, resource recalcination, and proper SS treatment [42]. Several researchers identified the presence of calcium, silica, and alumina phases in SS on analysis of its chemical composition. SS is found to possess properties similar to popular pozzolanic material used in concrete [43, 44].

Marisal et al. (2004) investigated the mechanical properties of concrete specimens containing treated plant sludge and recommended replacing levels up to 10% of binder content [45]. Baskar et al. (2006) have successfully replaced 9% of the binder with sludge obtained from the residue of the textile industry and wastewater in clay bricks. At a 9% replacement level, the brick samples satisfied the requirement of the BIS for compressive strength, weight loss, and shrinkage parameters [46]. Patel and Pandey (2009) reported that the sludge from the textile industry had a potential for reuse as construction materials [47]. Jamshidi et al. (2010) replaced cement with dry sludge at 0%, 5%, 10%, 20%, and 30% in concrete. The sizing and milling of dry sludge to a finer particle size can improve the mechanical properties of concrete any unreacted particles are left to act as a filler in concrete [48].

The dried sludge organic content limited to 13% can be used as an additive to mix. The increase in sludge by more than 5% was adversely affected workability [45,46,47]. Similar results have been found for both wet and dry wastewater sludge used in concrete.

The current study investigates the physical and chemical properties of FA and SS as a binder material. The fresh and hardened properties of concrete mixes at different levels of replacement of SS are reported. The higher the energy required to produce raw material, the higher the energy cost, resulting in higher CO2 emissions to the atmosphere and embodied energy. All these would result in higher CO2 emission and embodied energy, creating a higher negative impact on the environment. This study performs CE and EE of the binding material and concrete mixes at a different replacement level.

Experimental program

Materials

Binder material

OPC Forty-three Grade confirming IS 8112-2013 [49] is used for the study. Low calcium FA (Class F) was procured from a Raichur, Karnataka, India. Dried SS was collected from the dry sludge bed at the MWWTP at End Point, MAHE, Manipal, Karnataka, India, and dried for seven more days in sunlight to remove excess moisture.

After oven-drying at 105 °C for 24 h, the sludge was ground for 1 h in a ball mill. The ground residue was sieved through 150 and 75-micron meter IS standard sieves and collected separately. The particle size analysis of all the ingredients going into the binder system is illustrated in Fig. 1. The properties of various binder materials used in the present study are presented in Tables 1 and 2.

Fig. 1
figure 1

Particle size distribution of concrete ingredient

Table 1 Physical characteristics of raw materials
Table 2 Chemical composition of binder materials
Chemical analysis with X-ray fluorescence

Semi-chemical quantitative analysis of the oxides is performed, and the outcomes are tabulated in Table 2. Sewage sludge comprises SiO2, Al2O3, Fe2O3, CaO, Na2O, and P2O5, which is quite similar to FA. The proportions of SiO2 and Al2O3, which are the main reactive components responsible for pozzolanic reactions in the binder system (ASTM C125, 2007), are lower than FA. Sludge is composed primarily of quartz and calcite. The same is noted by Valls et al. (2004) [45]. Clay is absent in dry sludge, which signifies the absence of a stable binder phase on hydration. However, it can be used as a partial replacement for cement. The range in which various compounds are present in the sludge presented in previous research articles is also listed in Table 2. The ternary representation of SiO2-CaO-Al2O3-Fe2O3 concerning that of OPC, FA, and SS is presented in Fig. 2.

Fig. 2
figure 2

Ternary 3D plot of binder materials

X-ray powder diffraction (XRD) of SS

XRD spectra of SS are presented in Fig. 3. The crystalline phases of the SS mainly consist of quartz SiO2 4.1%, Akermanito 34.4%, Ca3SiO5 53.3%, and Cristobalite 8.1%.

Fig. 3
figure 3

XRD pattern of sewage sludge

Thermogravimetric analysis of sewage sludge

The result of TGA is presented in Fig. 4. The SS sample tested was found to have undergone thermal degradation in two phases. The first phase of primary degradation occurred at the temperature range of 100–500 °C, wherein the sludge sample was found to experience a high rate of mass loss. In the second phase, continuous decomposition of SS occurs at a high temperature of 500–900 °C with a comparatively lower mass-loss rate.

Fig. 4
figure 4

The TGA of raw SS

Fourier transform infrared spectroscopy of sewage sludge

FTIR analysis was carried out for SS using JASCO FTIR-6300 with a wavelength range of 400–4000 cm−1 results which are shown in Fig. 5. Inorganic bonded O–H groups with a wavenumber of 3250 cm−1 are observed. The broad peak at the 3600–4000 cm−1 region signifies the presence of O–H and N–H functional groups. Hence, alcohols, acids, amides, and amines are also noted. Multiple peaks indicate the presence of C–H groups in the 1042–2925 cm−1 region. The primary absorbance in FTIR spectra in the region 450–1050 cm−1 is due to the Si–O bond of silicate impurities and traces of clay minerals.

Fig. 5
figure 5

FTIR spectra of sewage sludge

Microstructure

The FESEM images of OPC, FA, and SS are shown in Fig. 6. From the FESEM image of SS, it is observed that the particle sizes appear to be more prominent than FA. SS appears crystalline in nature. It consists of the random orientation of solids with irregular shapes and sizes. Therefore, to attain higher reactivity, it is essential to grind the SS to a finer level.

Fig. 6
figure 6

Microstructure of sewage sludge, fly ash, and OPC

Aggregates

Natural river sand and gravel as fine and coarse aggregate in accordance with IS 383-1970 1970 (Reaffirmed 2011) are used for the present study. Table 3 and Fig. 1 show aggregates physical properties and sieve analysis.

Table 3 Properties of natural aggregates

Superplasticizer

High range water reducing agent Rofluid H1 (PCE base), with a specific gravity of 1.15 and pH of 4.5, was used in all mixes to enhance the workability of concrete. The chloride ion and alkaline percentages are ≤ 0.1 and 0.4, respectively.

Preparation of the paste

The mix proportions of various combinations of binder blends used for the study are listed in Table 4. The blended binder paste was carried out as per the norms stipulated in EN 196-3 [58].

Table 4 Mix designation for blended mixes

Mix proportioning

The mix proportions used for the current investigation are presented in Table 5. Sewage sludge was replaced at 5%, 10%, and 15% of the total binder content. Physical and mechanical properties of HVFA high strength concrete with three levels of sludge replacement were investigated through experimental procedures. Using the Department of Environment's Design (DOE) method, M50 concrete is designed with 50% cement and 50% FA as a binder. The w/b ratio of 0.3 is used for all the mixes. After casting, the specimens are de-molded after 24 h and then immersed in water for curing as per IS 10086-2008 [59].

Table 5 Mix proportion of concrete used in present study

Specimen casting and curing

OPC, FA, and SS were mixed thoroughly to obtain uniform binder mix. The aggregates are mixed with binders for 2 min to obtain a uniform dry mix. Uniform concrete mix was obtained by continuing the mixing for 2–3 min after water dispersion, and chemical admixture was poured. Later, concrete was cast into specific molds. The molded samples were kept in laboratory conditions for 24 ± 0.5 h and de-molded, later stored in a curing tank in the conventional method for the required durations [60,61,62].

Experimental procedure

Test on binder material

The setting time of the binder was determined as per the procedure prescribed in ASTM C191 [63]. Standard consistency of binder pastes is performed as per ASTM C187-16 [64]. SAI test was performed according to ASTM C618-05 [65] to study the pozzolanic activity of the binder mix. The fluidity of binder paste was measured (mini-slump flow) as per ASTM C1437 [66].

Tests on concrete

"Compressive strength test has been performed at 7, 14, 28, 56, and 90 days of the curing period using 150 mm cubic size, as per the Indian Standard Specifications IS:516-1959 [60]. The loading rate of 14 N/mm2/min was maintained using CTM of capacity 3000 kN. The split tensile strength was determined as per IS 5816-1999 [67] using specimen sizes of 150 mm diameter and 300 mm height at 7, 28,56, and 90 days. The rate of load application was within the range of 1.2–2.4 N/mm2/min. The flexural strength test was performed using the prism of size 100 × 100 × 500 mm as per IS 516-1959 [60]. Modulus of elasticity (MoE) has been conducted as per IS 516-1959 [60] on the cylinder specimen of size 150 mm diameter and 300 mm height after 28 days of curing. Deformation of the sample under compressive load was found using compressometer and linear variable differential transformer (LVDT) equipment. The ultrasonic pulse velocity (UPV) test was performed as per IS 13311-1-1992 [68] using a TICO Ultrasonic instrument supplied by PROCEQ SA, Switzerland. A water absorption test has been conducted on 150 mm cube specimens following the specifications prescribed in BS 1881-122-1983 [69]. Details of the tests and the corresponding codes referred are mentioned in Table 6 [62].

Table 6 Summary of experimental tests conducted

Results and discussion

Initial and final setting time

The IST and FST of pastes tested are presented in Table 7. IST and FST vary from 155 to 255 and 265 to 375, respectively. It is noticed that with an increase of SS, the setting time also increases for the pastes containing both 150 µm and 75 µm downsized dried sludge. However, the paste samples containing 75 µm downsized SS have exhibited acceptable setting times at 5% and 10% replacement levels. Mirza et al. (2002), Duran-Herrera et al. (2011), and Huang et al. (201) reported an increase in IST and FST of cement paste with the inclusion of SCMs [25, 72, 73].

Table 7 Setting time, consistency, and slump flow of binder paste

Standard consistency of binder

The standard consistencies of binder pastes studied at different replacement levels are shown in Table 7. The consistency value of FA blended cement paste at 50% cement replacement level is 34%, while the control sample consistency was 31%. A similar trend was observed by Marthong and Agrawal (2012) [74]. Replacement of SS resulted in an increase in the consistency values of the blended pastes due to the higher powder volume and porous and crystalline nature of SS. It is also noted from Table 7 that the consistency value is higher for binder paste with 75 µm downsize SS particle compared to 150 µm downsize at the same replacement level.

Fluidity (mini-slump flow) of binder

The fluidity of various paste compositions studied with and without sludge replacement is presented in Table 7. The slump flow values of the paste mixes were found to range from 151 to 195 mm compared to the slump value of 180 mm for OPC. It is observed that the fluidity of the OPC paste was found to have increased on replacing OPC with 50% FA. However, with the incorporation of SS into the binder, the fluidity is considerably reduced. The SS particles are porous and irregular in shape, hence, more susceptible to water absorption on particle surfaces [75]. Also, the size of SS was found to influence the fluidity.

Strength activity index

The SAI test was conducted to evaluate the pozzolanic activity of SS and presented in Fig. 7. According to ASTM C618-05 [65], the substitutive material is designated a pozzolan if it achieves a 75% of the strength gained by OPC mortar at 7 14, 28, 56, and 90 days, respectively, with 20% cement replacement. According to the results of the SAI, SS (75 µm) exhibits moderate pozzolanic activity. It can also be seen from the figure that the SAI of SS increased as the curing day advances. The presence and quantities of amorphous phases in the pozzolan contribute to pozzolanic when used as partial replacement to cement.

Fig. 7
figure 7

Strength activity index of supplementary cementitious materials

In comparison, SS has proven to possess lower SAI than FA. Finer grinding may be used for improving pozzolanic activity. In the present study, SS with a particle size of 75 µm possesses moderate pozzolanic activity, suitable to be used as SCM's.

Influence of sewage sludge on slump flow

The slump flow values of freshly mixed concrete mix are illustrated in Fig. 8. A higher slump value is observed in mix M2 because of a higher proportion of FA compared to the OPC (M1) mix. Sahmaran and Yaman (2007) also reported that OPC replacement with 50% FA increased the slump flow by 23.2% [76]. The increase in the percentage replacement of SS resulted in a decrease in slump value for the concrete in both 75 and 150 µm size particles. Similar results are observed by Jamshidi et al. (2011) [48], Ghada Mourtada et al. (2016) [74], and Ehab et al. (2019) [77].

Fig. 8
figure 8

Slump flow of the concrete mix

Concrete density

The 28-day concrete density was determined according to BS 1881: Part 114:1983 [71] (Method of determination of density of hardened concrete) [71] and presented in Fig. 9. The density of specimens increased with curing age. Continuous hydration and pozzolanic action from binder materials resulted in dense microstructure at a later age. Compared to the control sample, the concrete density began to drop in addition to 5% of SS. Based on the results obtained, it can be noted that the concrete density decreased with an increase in SS particle size. The same trend was observed by Amminudin et al. (2020) [56]. It is important to note that the addition of FA to the mix lowers the fresh concrete density. The lower specific gravity of FA and SS compared to OPC accounts for a decrease in density. The same trend is observed in studies reported earlier [3, 23, 78]. The deadweight of the structural element is reduced due to a reduction in the density of concrete. So, the use of SS in the binder system can be considered one of the advantages.

Fig. 9
figure 9

Density of concrete mix at 28 days

Compressive strength

The CS test was performed on concrete samples at 7, 14, 28, 56, and 90 curing days. The measurement of CS of the concrete sample with variable SS content is shown in Table 8. The replacement of 150 µm downsized SS at 5%, 10%, and 15% resulted in a decrease in 28 days strength by 24.12%, 25.54%, and 36.54%, respectively. Whereas 75 µm downsize contributed 1.4%, 11.17%, and 17.99% reduction for 5%, 10%, and 15% replacement levels, respectively. Jamshidi et al. (2011, 2012) [48, 79] observed that 5%, 10%, and 20% addition of dry sludge resulted in a decrease in strength by approximately 9%, 14.5%, and 28% in 28 days and 3.5%, 8%, and 20% in 90 days cured samples. It is also noted that for both the sizes, 75 µm and 150 µm sized SS, compressive strength at 90 days for 5% and 10% replacement levels is within the acceptable limits for M50 concrete. The relationship between CS and percentage replacement level is plotted, individual equations are presented in Figs. 10 and 11, and a strong relationship between percentage replacement and CS with R2 lying between 83.81 and 93.42%.

Table 8 CS, STS, and FS of mixes at different curing ages
Fig. 10
figure 10

Relationship between CS and percentage replacement level of 150 µm downsized SS

Fig. 11
figure 11

Relationship between CS and percentage replacement level of 75 µm downsized SS

Split tensile strength

The STS results of eight mixes are illustrated in Table 8. The 28 days lowest strength of 2.96 MPa is observed in mix 5 (M5). The replacement of 150 µm downsized SS at 5%, 10%, and 15% replacement levels resulted in a considerable decrease in strength. Whereas 75 µm downsized, SS concrete samples contributed reasonably good strength than 150 µm. The relationship between STS and percentage replacement level is plotted, and individual equations are presented in Figs. 12 and 13. A direct relationship equation is plotted considering 7, 28 56, and 90 days CS and STS and presented in Fig. 14. R2, a value of 0.809, indicates a correlation between them.

Fig. 12
figure 12

Relationship between STS and percentage replacement level of 150 µm downsized SS

Fig. 13
figure 13

Relationship between STS and percentage replacement level of 75 µm downsized SS

Fig. 14
figure 14

Relationship between CS and STS

Flexural strength

The FS experiment results at 7, 14, 28, 56, and 90 days are illustrated in Table 8. The 28 days lowest strength of 3.94 MPa is observed for the M5 mix. The replacement of 150 µm downsized SS at 5%, 10%, and 15% replacement levels resulted in a drastic decrease in strength. Whereas 75 µm downsized SS exhibited higher strength than 150 µm. The relationship between tensile strength and percentage replacement level is plotted, and individual equations are presented in Figs. 15 and 16. A direct relationship equation is plotted considering 7, 28, 56, and 90 days CS and STS and presented in Fig. 17. The R2 value of 0.873 is observed, indicating a good correlation between them.

Fig. 15
figure 15

Relationship between FS and percentage replacement level of 150 µm downsized SS

Fig. 16
figure 16

Relationship between FS and percentage replacement level of 75 µm downsized SS

Fig. 17
figure 17

Relationship between CS and FS

Modulus of elasticity (MoE)

The MoE affects reinforced concrete's safety, durability, density, and life span. The 28 days MoE of concrete specimens is calculated by applying a series of compressive stress cycles up to about 40% of the measured compressive strength and is presented in Fig. 18. The replacement of 150 µm downsizes SS decreased the modulus of elasticity, whereas it is similar to the control mix in the samples containing 75 µm downsize SS. The incorporation of SS led to a decrease in MoE due to the de-densification of pore structure. A linear degradation in the value of modulus of elasticity with an increase in SS content is observed. A linear relationship between CS and MoE at 28 days is plotted in Fig. 19. A good correlation is observed with the R2 value of 0.917.

Fig. 18
figure 18

MoE of mixes at 28 days

Fig. 19
figure 19

Relationship between CS and MoE

Influence of sewage sludge on quality aspect of concrete

Ultrasonic pulse velocity (UPV)

The UPV test results for the mix at 28 and 90 days and correlation between UPV and CS are presented in Fig. 20. The mixes result was between 3400 and 3700 m/s, which falls under the decent to the excellent category as per IS 13111 (Part 1). The linear regression analysis has been plotted (Fig. 21) between UPV and CS. A direct relationship was obtained as y (UPV)=2917.24 + 12.49 X (CS), with an R2 value of 0.8954 showing a good correlation.

Fig. 20
figure 20

Comparison between CS and UPV of the mix at 28 and 90 days

Fig. 21
figure 21

Linear regression between CS and UPV

Water absorption (WA)

WA of eight mixes investigated at 28, 56, and 90 days of curing is presented in Table 9. WA decreases with an increase in curing ages in all mix specimens. An increase in SS percentage increased water absorption. However, 75 µm downsized replacement of SS as binder material gives better results than 150 µm downsized SS particle.

Table 9 Water absorption in percentage at 28, 56, and 90 days

Assessment of environment impact and cost implication

Using a higher dosage of supplementary cementitious material in concrete minimizes environmental impact and increases compressive strength. Cradle-to-gate EE, CE, and COST are quantified for different binder combinations. The energy consumption and CE can vary depending upon the manufacturing process, raw material, and distance from the source. Therefore, representative data from the literature were used in this study.

Carbon dioxide emission

Global warming is exacerbated by urbanization and industrialization, which leads to the depletion of natural resources, prompting scholars worldwide to consider sustainable development. As a large user of natural resources and energy, the concrete industry has significantly increased GHG emissions. According to estimates, the global population is expected to reach ten billion by 2050, resulting in increased construction and development activities and a negative impact on the environment [78, 80].

The CO2 emission parameters were calculated in this study by calculating carbon emissions during the preparation of SCMs. According to previous studies, the carbon footprint of FA is low because it is a waste by-product of coal-burning power plants. Researchers in earlier studies state that carbon mission from FA is negligible because it is a waste by-product arising from the coal-burning power station. But in the current study, the value of 0.008 kg eq.CO2/kg is considered for FA, as per Hammond and Jones (2011) [80, 81].

Similarly, SS raw material contributes zero CO2 emission, as it is also a by-product in municipal wastewater treatment plants [5, 82]. However, the energy utilized to improve reactivity by drying, grinding, sieving, and transport is considered for calculating carbon emission for SS and FA. According to the UK Government, conversion factors for GHG report 2021 are considered while calculating carbon emissions. Table 10 represents the calculated CO2 emission factors for SS (150 µm) and (75 µm). The final carbon emission factors of ingredients used in concrete mixes are presented in Table 11.

Table 10 CO2 emission factors for sewage sludge
Table 11 Carbon emissions factors of raw materials

The CO2 emission of individual and total cementitious material per mix is illustrated in Figs. 22 and 23. Replacing OPC by increasing the amount of SCMs per unit volume of concrete resulted in reducing CO2 emission of cementitious material in mixes up to 57%. The amounts of CO2 released by each concrete mix depend upon the proportions of materials, concrete production, and raw material transport, as presented in Fig. 24. The CO2 emission factor was considered 0.008 kg CO2/kg for concrete production as Kin et al. (2016) [86]. The purpose of the CO2 emission analysis is not to achieve a mix with the lowest CO2. Achieving a mix with less CO2 emissions is also important, which shows acceptable mechanical properties. The results show that Mix M1 with 100% OPC has the highest emission rate of 601.24 kg CO2/m3, while the lowest value of 293.95 kg CO2/m3 and 295.17 kg CO2/m3 is observed in mix M5 and M8. When SCMs were incorporated, a reduction in CO2 emissions was observed. According to the current study result, the binder was the major contributor to CO2 emissions at rates ranging from 80 to 90% of the total emission of 1 m3, depending upon the replacement ratio of SCMs.

Fig. 22
figure 22

CO2 emission of total cementitious material per mix (per m3)

Fig. 23
figure 23

CO2 emission of total cementitious material per mix (per m3)

Fig. 24
figure 24

Total CO2 emission during concrete production (kg CO2/m3)

Eco-efficiency

Eco-efficiency is the ratio between 28-day mechanical strength and CO2 equivalent emissions of the concrete mixes. Figure 25 represents the concrete eco-efficiency of eight mixes and illustrates that the mix with alternative binder materials shows better efficiency than the OPC mix. The efficiency value observed (CS) at 28 days was 0.101 MPa/kg. CO2 m3 was in line with findings of Alnahhal et al. [86] and Stark et al. [87].

Fig. 25
figure 25

Concrete eco-efficiency (compressive, flexural, and tensile) strength/CO2 emissions

Concrete mixes with alternative binder material have shown better eco-efficiency than the control mix. The maximum eco-efficiency of 0.185 (CS), 00,158 (FS), and 0.0123 (STS) is noticed with 75 µm downsized SS at 5% replacement.

Embodied energy and cost of blended binder

The EE of each binder material is presented in Table 12. In the current study, while comparing, the only binder material is considered since fine, and coarse aggregate content is constant for all the mix. Figure 26 shows the embodied energy of binder material of different mixes. The embodied energy of SS at 150 µm and 75 µm is calculated using available data from the literature [10, 80, 88]. It can be observed that a decrease in cement content and an increase in supplementary cementitious material can significantly reduce the EE and CE.

Table 12 EE and material cost of ingredients
Fig. 26
figure 26

Embodied energy of binder material in each mix

Environmental impact and binder cost per unit CS of concrete

The environmental impact quantification and binder cost per unit CS for different binder materials are calculated. The EI, CI, and binder cost index (COST) are calculated based on Eqs. 1, 2, and 3 derived with the help of an earlier study carried out by Jing Yu et al. (2021) [81].

$${\text{EI}}_{i } \left( {\frac{Mj}{{{\text{m}}^{3} }}/{\text{MPa}}} \right) = \frac{{{\text{Embodied Energy of binder material required for}} 1 {\text{m}}^{3 } {\text{of concrete}}}}{{i - {\text{day compressive strength of standard concrete specimen}}}}$$
(1)
$${\text{CI}}_{i } \left( {\frac{{{\text{kg CO}}_{2} }}{{{\text{m}}^{3} }}/{\text{MPa}}} \right) = \frac{{{\text{Carbon Emission of binder material required for}} 1 {\text{m}}^{3 } {\text{of concrete}}}}{{i - {\text{day compressive strength of standard concrete specimen}}}}$$
(2)
$${\text{COST}}_{i } \left( {\frac{Rs }{{{\text{m}}^{3} }}/{\text{MPa}}} \right) = \frac{{{\text{Embodied Energy of binder material required for}} 1 {\text{m}}^{3 } {\text{of concrete}}}}{{i - {\text{day compressive strength of standard concrete specimen}}}}$$
(3)

where i denotes the curing time in days.

The calculation results on EIi, CIi, and COSTi for binder material per meter cube are shown in Figs. 27, 28, and 29 at 28, 56, and 90 days. EI value of 51.93, 49.2, and 48.69 (MJ/kg)/MPa is observed for OPC mix at 28, 56, and 90 days. There is a drastic reduction in the embodied energy for the other mixes with OPC replacement. The least embodied energy index value of 26.42, 25.07, and 24.27 (MJ/kg)/MPa is observed at mix 06. The mix 7 value is on par with mix 2, which has a 50% cement replacement with FA. A similar trend is observed for 56 and 90 days.

Fig. 27
figure 27

Comparison of EI, CI, and COST per unit CS of the mix at 28 days

Fig. 28
figure 28

Comparison of EI, CI, and COST per unit CS of the mix at 56 days

Fig. 29
figure 29

Comparison of EI, CI, and COST per unit CS of the mix at 90 days

The Carbon Emission Index value of mixes 2–8 is lesser than the control mix (M1) observed for 28, 56, and 90 days. The addition of SS resulted in the reduction of carbon emissions. At 90 days age, the trend of COST 90 is similar to COST 28 and COST 56. The COST 90 values of cement with different replacement levels of SCM are very close to each other due to significant strength development at a later stage. The mix with SS 150 µm at 5, 10, 15, and 75 µm at 10 and 15 replacement levels exhibited slightly lower CS than the control mix. But it has superior environmental and economic benefits by considering the environmental impact and material cost per unit strength.

Conclusion

The present study investigated the characteristics of SS, mechanical properties of concrete with different replacement levels along with carbon emissions, and embodied energy to develop sustainable and environmentally efficient concrete. A total of eight mixes with different levels of SS replacement as a binder material were cast and tested. The following main conclusions were drawn based on laboratory observations and findings.

  • The main mineral components of SS are silicon dioxide, calcium, iron, and aluminum compounds. Based on the oxide content in SS, it is suitable to replace the Portland cement content in standard concrete.

  • Mechanical characterizations such as CS, FS, and STP with 150 µm were observed with a reduction in strength, whereas the strength obtained at a 5% replacement level of 75 µm is on par with the control mix. There is no significant reduction in mechanical strength for 75 µm SS at 5% and 10% level at 90 days.

  • All of the mixes tested for UPV reported between 3400 and 3700 m/s, which falls into the decent to excellent range. A direct relationship between compressive strength and UPV was obtained as y (UPV) = 2917.24 + 12.49 X (CS), with an R2 value of 0.8954 showing a good correlation.

  • Partial replacement of SS as a binder material generally affects eco-efficiency, with values similar to or higher than the control mix. The advantages of utilizing SS as a partial substitute binder material lie in reducing CO2 emissions in making concrete and significantly reducing environmental problems caused by SS disposal.

  • Incorporating SS as a binder to the concrete has a lower environmental impact, embodied energy, CO2 emission, and cost per unit strength. But more than 10% replacement level resulted in reducing CS, FS, and STS by 11.17%, 6.23%, and 6.99%.

In the context of sustainable development, using SS as a binder material in concrete and these findings can help the efforts to reduce the carbon footprint and embodied energy in the construction industry. It can also reduce the burden and environmental effects of disposal of SS.