Enhancing the sustainability of high strength concrete in terms of embodied energy and carbon emission by incorporating sewage sludge and fly ash

This paper discusses the properties of dried sewage sludge (SS) and its influence on the microstructure development of HVFA concrete when used as a partial replacement of binder material. A detailed characterization of dried sludge samples collected from a sewage treatment plant is carried out using XRF, XRD, TGA, and FTIR techniques. HVFA concrete mix is designed for 50 MPa with 50% fly ash of the total binder content. Sludge is ground to a particle size of 150 µ and 75 µ and replaced at levels of 5%, 10%, and 15% of the total binder content. The strength activity index of the dried sludge sample is acceptable as per standards. Taking concrete mixes with HVFA as a reference, the fresh properties of binder paste and concrete with sewage sludge have been studied. Mechanical properties that define the applicability to various infrastructure projects are reported for all the studied mixes. EI, CI, COST per unit compressive strength for all mixes are also determined to comment on the environmental impact of the use of SS in concrete. The compressive strength of concrete specimens decreases with the increase in replacement level of SS. However, in comparison with OPC concrete, 75 µm SS at 5% replacement level concrete mechanical strength is within the acceptable limit for M50 concrete mix. The addition of 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%.


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 CO 2 emission is attributed to cement production. India stands at the second position in cement production and cement-related CO 2 emissions. Cement production in India is estimated to rise to 3000 MT, emitting nearly 1.3 billion tons of CO 2 by 2050 [1]. The CO 2 emissions are found to reduce with SCMs in one of the binder phases. Approximately 13-22% of CO 2 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 SO 2 and NO x 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][41][42][43][44][45][46][47][48][49][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 CO 2 emissions to the atmosphere and embodied energy. All these would result in higher CO 2 emission and embodied energy, creating a higher negative 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.
Chemical analysis with X-ray fluorescence Semi-chemical quantitative analysis of the oxides is performed, and the outcomes are tabulated in Table 2 [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 SiO 2 -CaO-Al 2 O 3 -Fe 2 O 3 concerning that of OPC, FA, and SS is presented in Fig. 2.

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   [50] high temperature of 500-900 °C with a comparatively lower mass-loss rate. 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.

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.

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.

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].

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].

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.

Initial and final setting time
The IST and FST of pastes tested are presented in Table 7

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.  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

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.

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

Split tensile strength
The STS results of eight mixes are illustrated in Table 8

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  Fig. 17. The R 2 value of 0.873 is observed, indicating a good correlation between them.

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 R 2 value of 0.917.

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)

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.

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 CO 2 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. CO 2 /kg is considered for FA, as per Hammond and Jones (2011) [80,81].   Similarly, SS raw material contributes zero CO 2 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 CO 2 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.

Fig. 21 Linear regression between CS and UPV
The CO 2 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 CO 2 emission of cementitious material in mixes up to 57%. The amounts of CO 2 released by each concrete mix depend upon the proportions of materials, concrete production, and raw material transport, as presented in Fig. 24. The CO 2 emission factor was considered 0.008 kg CO 2 /kg for concrete production as Kin et al. (2016) [86]. The purpose of the CO 2 emission analysis is not to achieve a mix with the lowest CO 2 . Achieving a mix with less CO 2 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 CO 2 /m 3 , while the lowest value of 293.95 kg CO 2 /m 3 and 295.17 kg CO 2 /m 3 is observed in mix M5 and M8. When SCMs were incorporated, a reduction in CO 2 emissions was observed. According to the current study result, the binder was the major contributor to CO 2 emissions at rates ranging from 80 to 90% of the total emission of 1 m 3 , depending upon the replacement ratio of SCMs.

Eco-efficiency
Eco-efficiency is the ratio between 28-day mechanical strength and CO 2 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. CO 2 m 3 was in line with findings of Alnahhal et al. [86] and Stark et al. [87]. 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.

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 R 2 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 CO 2 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, CO 2 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.
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