Multi-scale performance, life-cycle and economic assessment of blended concrete using recycled coarse aggregates

Abstract

The construction industry confronts dual challenges of natural resource depletion and construction waste accumulation, necessitating sustainable alternatives to conventional concrete production. This investigation evaluated mechanical properties, microstructural characteristics, environmental impacts, and economic feasibility of concrete incorporating recycled coarse aggregate (RCA) at varying replacement levels (0%, 12.5%, 25%, 50%, 65%) with ground granulated blast furnace slag (GGBS) supplementation (20%, 25%). Mechanical testing revealed optimal performance at 12.5% RA replacement, achieving 55.43 MPa, compressive strength at 28 days, representing 13% improvement over control specimens. Higher replacement ratios demonstrated progressive strength deterioration, with 65% replacement yielding 48% reduction in compressive strength. Microstructural analysis through SEM-EDS confirmed enhanced interfacial transition zone densification in RA12.5%+GGBS mixes, correlating with superior mechanical performance. Life cycle assessment following ISO 14040:2006 demonstrated 27% reduction in carbon emissions (282.2 vs. 386.7 kg CO₂-eq/m³) for RA12.5%+GGBS25% configuration, with manufacturing processes contributing 46.5% of total environmental burden. Environmental impact categories showed consistent improvements, particularly resource depletion (28.2% reduction) and terrestrial acidification (24.2% reduction). Economic analysis revealed 30.6% lifecycle cost savings (₹13,550/m³) over 50-year service period despite modest 5.3% initial cost reduction, with 2.1-year payback period and favorable sensitivity to carbon pricing mechanisms (-3.8% per ₹100/ton CO₂). Sensitivity analysis identified cement price volatility as primary economic risk factor (± 6.8% impact). The convergence of optimal mechanical, environmental, and economic performance at 12.5% RCA with 20–25% GGBS substitution challenges conventional assumptions regarding linear sustainability benefits, establishing critical thresholds for sustainable concrete design in infrastructure applications.

Introduction

Concrete is widely regarded as one of the most important materials in construction industry^1,2,3,4,5. The production of ordinary portland cement (OPC), which is responsible for around 7–8% ofworldwide CO₂ emissions and is one of the most energy-intensive industrial processes^6,7,8. The waste produced by coal-based power plants and industries, including ash, sludge, and byproducts, contributes to landfill and air pollution, posing significant challenges for the energy and construction sectors in terms of proper utilization^9,10,11,12. A recent report by the Ministry of Power, Government of India, indicates that the total fly ash produced by thermal power plants in India is roughly 270.82 million tons. The residual fly ash may still offer significant landfill challenges and land degradation issues if not appropriately utilized or disposed of¹³. The Indian Minerals Yearbook indicates that global output of GGBFS will approximate 270 million tonnes from 2020 to 2025. According to a research by the Indian Bureau of Mines, India’s yearly production of GGBFS exceeds 17 million tons, primarily utilized in slag-based cements¹⁴. An estimated 150 metric tons (MT) of construction and demolition debris is produced in India every year, making it a significant contributor to the world’s C&D garbage and a growing environmental problem for many Indian cities^15,16.

Building and demolition debris is now utilized for landfilling in India. The Centre for Science and Environment (CSE) predicted that India recycles only 1% of its construction and demolition (C & D) waste created (CSE, 2020)¹⁷. In addition to the increased demand for aggregates, which accelerates the depletion of natural resources, the construction sector has grown significantly due to increased economic growth following urbanization and development and rehabilitation projects in the city. The countries like Germany, Japan, China and some other countries are doing research on recycling of the materials from the demolished waste as use of fine and coarse aggregates. To genuinely commit to sustainable development, there is an urgent need to prioritize eco-friendly and recyclable building materials, for the sustainable infrastructure development Numerous studies have been conducted on the performance of RA, and SCMs are as follows: Bachene Sara et al. (2023)⁵ investigates the RCS’s effect on compressive strength for replacement rates below 50%, but finds that strength values for RCS-treated mortars are quite similar to control mortars at this point. Additionally, a mortar containing 50% RCS has a 30% improvement in compressive strength, and the use of 20% GGBFS as a cement substitute improves long-term strength. Lusman Sulaiman et al. (2025)¹⁸ investigate sustainable concrete manufacturing Combining fresh and saltwater with 15%, 30%, 45%, 60%, 75%, and 90% processed recycled coarse aggregate (Tr-RCA). Increased concentrations of saltwater and Tr-RCA diminished workability while improving durability by reducing water absorption. The results indicate that processed RCA and saltwater may provide environmentally sustainable concrete with favorable mechanical and microstructural characteristics, hence encouraging the use of recycled materials and alternative water sources in sustainable building. V. V. S. Sarma et al. 2023¹⁹, examines sustainable concrete technology and provides a basis for constructing recycled aggregate concrete (RAC) for structural applications with recycled coarse aggregate, fly ash, and ground granulated blast-furnace slag (GGBS). The findings indicate that the concrete formulation including 40% recycled aggregate, 15% fly ash, and 15% GGBS as cement substitutes is optimal for improving the mechanical properties of concrete.

H.Panghal et al. (2024)²⁰ examines substituting natural aggregates with RCA at 0, 25, 50, 75, and 100%. Results show that the best combination at 25% RCA replacement (RCA 25) has stronger compressive (11.56%), flexural (3.06%), and split tensile (5.17%) strengths than control concrete. Additionally, RCA25 increases modulus of elasticity 8.91%. W Cheng et.al.(2025)²¹ At 28-day compressive strength of 36.0 MPa, splitting tensile strength of 2.76 MPa, and flexural strength of 5.0 MPa, is observed at recycled concrete (RC) with 60% weathered granite coarse aggregate (WGCA) maximized mechanical parameters. Rawaz Kurda et al. (2020)²². The findings demonstrate that, with a constant binder concentration, the use of RA has a detrimental impact on all mechanical characteristics of concrete. The mechanical performance of concrete is impacted by FA as well, with the exception of the modulus of elasticity. Si-Min Jian, Bo Wu(2021)²³, observed that concrete with 100% RCA substitution showed lower compressive strength compared to mixes with NCA or those with partial RCA replacement. Jiabin Wang et al. 2023²⁴ examines up to 20% FA or 20% SF in binary blends or 15% SF in ternary blends, the RAC showed better compressive and splitting tensile strength and gradually decreased strength with FA or SF. With increased MK replacement ratios, RAC with binary or ternary mixes became stronger.

The recycled coarse aggregate (RA) as a partial substitute for natural coarse aggregate in concrete has sustainability benefits, and life cycle assessment (LCA) is crucial are important and some of the reviews are discussed as follows: Roberto Cerchione et al. 2023²⁵ examines the environmental effect of concrete production utilizing just coarse natural aggregates (NA) and combinations with coarse recycled concrete aggregates (RCA) from fixed and mobile treatment facilities, replacing 30% and 100% of coarse NA by weight. The findings emphasize the need to develop circular economy ideas and practices in the building and demolition industry to minimize primary resource use, notably sand and gravel. Replacing NA with RA by weight may reduce construction and demolition waste disposal consequences. Bruno Estanqueiro et al. 2018²⁶ provide fresh life cycle assessment (LCA) data on coarse sediment generation from natural and recovered materials (mobile or stationary recycling facilities). After deliberately demolishing structures to enhance waste collection, reuse, and recycling, material reuse must be improved. These aggregates are better for the environment than natural aggregates for concrete since they require less land and produce fewer inorganic pollutants. However, fine recycled aggregates that are not transported to landfills may make coarse recycled aggregates greener. Weiqi Xing et al. 2022²⁷ prepared a consistent mix design framework, this study compares virgin aggregate concrete, recycled aggregate concrete, and CO₂ concrete and evaluates the environmental impact of 57 concrete products using life cycle assessment techniques. The global warming potential of concrete per unit volume varies between 278.35 and 524.44 kg CO₂ equivalent, demonstrating that different concrete mix designs can have markedly varied environmental impacts. The inclusion of supplementary cementitious ingredients in recycled aggregate concrete amplifies its ecological advantages. Table 1 illustrates the summary of the previous studies.

Table 1 Literature comparison studies

The significance of recycling coarse aggregate from construction and demolition (C&D) waste is primarily rooted in environmental sustainability and resource conservation, as validated by extensive research. Studies confirm that utilizing recycled coarse aggregate (RA) diverts massive amounts of C&D waste from landfills and reduces the consumption of virgin natural aggregates, thereby mitigating the ecological impact of quarrying and resource depletion. Despite the studies have synthesized the individual effects of RA and supplementary cementitious materials on mechanical performance or environmental impact in isolation, no study to date has simultaneously integrated experimental mechanical characterisation, ITZ chemistry quantification through Ca/Si ratio analysis, site-specific LCA inventory measurement, and lifecycle economic evaluation within a single controlled experimental system for the specific RA–GGBS combination investigated here. Furthermore, the non-linear relationship between RA replacement level and the convergence of mechanical, environmental, and economic performance — particularly the existence of an optimal performance threshold below maximum recycled content — remains insufficiently explored in the published literature. The present study addresses these through an original multi-scale experimental investigation conducted under the material and regional conditions of the Guntur–Telangana infrastructure context in India. Furthermore, existing LCA studies on SCMs for low-carbon concrete have predominantly adopted cradle-to-gate boundaries and desk-based inventory approaches within high-income country contexts (e.g., Australia, Europe), which are not directly transferable to the Indian construction sector due to fundamental differences in electricity grid carbon intensity, material cost structures, aggregate sourcing logistics, and regulatory frameworks. A cradle-to-grave assessment integrating site-specific primary inventory data within the Indian context, combined with original experimental and microstructural characterisation, represents a methodological advancement over existing cradle-to-gate and review-based studies in this domain.

Thus, the current study is focused on coming up with a multi-scale assessment system that will understand the synergistic effects of fly ash with recycled aggregates (RA), GGBS mix in terms of mechanical, microstructural, statistical analysis, sensitivity analysis, environmental and life cycle assessment (LCA) and economic considerations.

Although the application of recycled aggregate concrete and supplementary cementitious materials has been widely studied, contradictory results are to be found in terms of the optimal levels of replacement and their sustainability implication. Additionally, the majority of the literature concentrates on either mechanical performance or environmental evaluation alone without much scale cross-correlation. As such, the aims of this research are to:

• (i) measure mechanical performance of concrete at different levels of RCA replacement;

• (ii) test microstructural features and Ca/Si change in the ITZ;

• (iii) evaluate the environmental effects through life cycle assessment; and.

• (iv) examine the economic viability so as to detect a performance-sustainability level.

Experimental program

Physical properties of the materials

The study employed Ordinary Portland Cement (OPC) of 53 grade, in accordance with IS 12269:2015³⁸, having a specific gravity value of 3.15 and a density of 1.55 kg/cm³. Class C fly ash (FA) was sourced from the Vijayawada Thermal Power Station (VTPS) located in Ibrahimpatnam, Andhra Pradesh, exhibiting a specific gravity of 2.15. The ground granulated blast furnace slag (GGBS) was procured from Astra Chemicals located in Tamil Nadu. Locally available natural sand was collected which conforming the zone-II grading with a specific gravity of 2.37 respectively as per IS 383–2016³⁹. Natural coarse aggregate (NA) having the aggregate impact value of 15.74%, and specific gravity of 2.8 and bulk density of 1460 kg/m³ with fineness modulus of 7.3 and recycled coarse aggregate (RA) having the aggregate impact value of 15.56% is oberved as per IS 2386 − 1963⁴⁰, and specific gravity of 2.7 and bulk density of 1420 kg/m³ with fineness modulus of 7.23 in size range of 12.5 mm to 20 mm. The water absorption for the NA and RA was 1.27% and 3.36%. Figure 1 represents the grading of aggregates used for the study.

Fig. 1

Grading of aggregates used for the study

Preparation of recycled aggregate

Demolished concrete waste is collected from the concrete laboratory in the college as shown in Fig. 2a. After the collection of demolished concrete waste from laboratory sorting was done and unwanted materials were removed. Sorted materials are wetted for 2 days in the water as shown in Fig. 2b. Wetting of demolished collected material helps in removing dust, dirt and adhered mortar. After the wetting for 2 days in the water. Material is placed under the sun dried for approximately for a week for natural exposure. In this study energy method for drying the aggregates is compared with the artificial heat sources and this brings the aggregate to an optimum moisture level. This helps prevent over-saturation, ensuring that the aggregate do not absorb too much water during future concrete mixing. Then the material is divided into coarse aggregate and fine aggregate with the help of los angles abrasion (LAA) test apparatus. LAA machine rotates a drum containing the collected material and steel balls, As the drum rotates, due to mechanical impact of steel balls the collected recycled material causes to breaking of the adhered mortor. This process helps to separate the aggregate core from the old, weaker cement paste, making the recycled aggregate cleaner and reusable as shown in Fig. 2c. After the abrasion test the recycled aggregate are collected in tray as shown in Fig. 2d. After that recycled coarse aggregate (RA) attained were screened and sieved to get the desired size between 12.5 mm and 20 mm. The complete recycling process of C& D waste are represented in Fig. 2. The coarse aggregates were recycled and exposed to controlled mechanical treatment with a Los Angeles abrasion drum to partially remove loosely fixed mortar. It was done at definite revolutions with no steel charge as a surface treatment instead of a typical abrasion resistance test. This mechanical rubbing technique was implemented to enhance the quality of aggregates through minimization of weak mortar layers and water uptake.

Fig. 2

Preparation of RA ,(a) collection of C & D waste, (b) curing of C & D waste , (c) C & D waste sent to test and (d) collection of RA after the test.

Methodology

From the available literature review, an attempt is made to evaluate the effect of RA in concrete on the physical and mechanical performance of concrete Fig. 3 shows the experimental flow line of the study. In the study RA is replaced at 0%, 12.5%, 25%, 50% and 65% with coarse aggregate in Phase-I and in Phase-II the optimum mix of RA% the cement is replaced with GGBS (20,25%) to assess the mechanical properties (as compressive strength, split tensile strength, flexural strength) as per IS 516:2021⁴¹ and quality of concrete as per IS 13,311 (Part1):1992⁴² at different curing periods (7, 28 and 90 days). Apart to assess the impact of RA and GGBS in concrete matrix, microstructural studies like SEM, EDS are carried out after 28 days of curing. Statistical analysis, sensitivity analysis and life cycle assessment (LCA) as per ISO 14,040:2006⁴³ are carried out for the mixes for the reliability of the study to assess the ecofriendly concrete for sustainable infrastructure development. The entire research methodology is represented in an experimental process of graphical chart which are represented in Fig. 4.

Fig. 3

Shows the experimental flow line of the study.

Fig. 4

Experimental process of RA concrete.

Mix proportion and preparation

To determine the optimum recycled coarse aggregate (RA%) content, RA was replaced at 0%, 12.5%, 25%, 50%, and 65% in the mixes prepared as per IS:10262:2019⁴⁴. RA replacement levels (12.5%, 25%, 50%, and 65%) that had been selected were intended to represent the performance behavior of the low, moderate, and high substitution ranges as shown in Table 2. To explore the possibility of using limited incorporation of RA to enhance the effect of particle packing and internal curing without causing pronounced mechanical degradation, a 12.5% replacement level was subjected. The 25% and 50% are ranges that are usually explored on structural recycled aggregate concrete. The 65% percent was added to test its performance in high replacement conditions and to test mechanical environmental trade-offs of aggressive natural aggregate substitution.

Recycled coarse aggregates (RA) have a lower absorption capacity of water compared to their original. This was established before mix design. To ensure uniform effective water-binder ratio on all mixes, RA was pre-wetted to saturated surface-dry (SSD) state prior to mixing. The process reduced uncontrolled incorporation of water in the mix and even provided homogenous hydration condition across the replacement levels. In all the mixes were designed with fly ash fixed at 10% of the total cementitious content, ensuring consistency in binder composition across replacement levels. In Phase-I, the natural coarse aggregate (NA) was a replaced with recycled course aggregate (RA) at 0% is denoted as RA0% (control mix), while combination replacement of NA with 12.5% of RA is denoted as RA12.5%, similarly other mixes are designated as follows up to RA 65%. In Phase-II, from the optimum mix i.e., RA12.5%, the cement (C) is replaced with GGBS20% is denoted as (RA12.5% + GGBS20%) and GGBS25% is denoted as (RA12.5% + GGBS25%).

The preparation of the mixes is carried out in the pan mixture. In the pan mixture the dry components (aggregates, binders) are mixed for 3 min, followed by gradual addition of water with the overall time of 5 to 10 min as per IS: 12,119: 1987⁴⁵, until to achieve the homogeneous mix. All the mixing preparations are conducted at room temperature of 65 ± 5% thereafter the fresh concrete samples are tested for workability. Then the fresh concrete samples are poured into cubes (100 mm x 100 mm x 100 mm), cylinders (100 mm dia x 200 mm length) and beam moulds (100 mm x 100 mm x 500 mm) for each associated mix and kept for curing for 7, 28 and 90 days. The Fig. 5, represents the experimental preparation. Figure 5a, illustrate pan mixer and Fig. 5b shows preparation of samples.

Table 2 Details of the composition of mixes, Kg/Cu.m.

Fig. 5

Experimental preparation (a) pan mixer preparation, (b) preparation of samples.

Experimental program

The experiment program was done in two phases. The first stage involved the comparison of different RA replacement levels in the absence of SCM to determine the baseline mechanical behaviour. On these findings, it was determined that 12.5% RA was the break-even point which ensured a similar level of structural performance as the control mix. The second stage involved the introduction of GGBS at this determined level to examine whether the refinement and enhancements in sustainability was possible, without complexified performance loss to be introduced at higher levels of RA. This staged method enabled easier separation of RA effects and RA-GGBS interaction processes.

Experimental tests

An experimental testing program was conducted to evaluate the mechanical performance of concrete incorporating recycled coarse aggregates (RA) along with fly ash, GGBS as a partial replacement for cement. All samples from each series were cured at 27 ± 1 °C. Ten cubes, each measuring 100 × 100 × 100 mm, were produced for the study to test direct stress, Four beam specimens measuring 100 × 100 × 500 mm were created to assess bending strength at 7, 28 and 90 days of curing, and nine cylinders measuring 100 × 200 mm were fabricated to evaluate split tensile strength to assess mechanical properties were conducted as per IS: 516–2015³¹ at 7, 28, 90 days of curing.After 28 days of curing the microstructural assessment is carried out. Apart to asses the quality of concrete ultrasonic pulse velocity carried for cube samples as at 28, 90 days of curing ages. Testing was done through Ultrasonic Pulse Velocity (UPV) as per IS 13,311 (Part 1):1992⁴². The ultrasonic pulse velocity tester was [insert equipment model and manufacturer] having transducers with a frequency of [e.g., 54 kHz]. The method which was used was the direct transmission and the transducers were placed on the opposing sides of the specimen. The gel was coupled with a coupling gel that was used to get acoustic contact. To compute pulse velocity, we used:

$$\:V=\frac{L}{T}$$

(1)

where V is the pulse velocity (km/s), L is the path length (mm), and T is the measured transit time (µs).

SEM/EDS analysis

The crystalline structure of the sample mixtures is analyzed using microstructural analysis. Subsequent to the compressive strength test, a specimen was extracted from the inner core of the sample and analyzed using scanning electron microscopy (SEM). Scanning Electron Microscopy (SEM) generates images of a substance by sending a beam of electrons onto its surface. The TESCAN manufacture, with VEGA3 model were outfitted with operating voltage of 15Kv usage with solid state backscattered electron (SBHT) detector capabilities for the mixtures, and EDS analysis was performed at the same location to evaluate the elemental characteristics of the mixtures.

Sensitivity analysis

The analysis was performed to evaluate the influence of engineered mix design parameters such as Cement, Silica fume, Normal Sand, M-Sand, NA_total, RA_total, and RA percentage of total coarse aggregate for four output properties: compressive strength, split tensile strength, flexural strength, and ultrasonic pulse velocity (UPV). Inputs such as fly ash, water, and some coarse aggregate fractions were excluded, as they were constant across all mixes and provided no variation for statistical analysis. To reduce collinearity between similar variables, the total values for natural coarse aggregate (NA_total) and recycled coarse aggregate (RA_total) were computed, and the percentage contribution of RA to total coarse aggregate (RA_pct) was also derived. First, all varying input variables were standardized using the z-score transformation (Eq. 2). This calculation ensures comparability between variables with different units and scales.

$$\:z=\frac{x-\mu\:}{\sigma\:}$$

(2)

Where, is the observed input value, is the mean of the input, and is its standard deviation. This step ensures comparability between variables with different units and scales. Secondly, a multiple linear regression model was then fitted separately for each output by using Eq. 3. Where, is the output (e.g., compressive strength), is the intercept, are the standardized coefficients for each input , and is the residual error. Along with standardized coefficients (unitless, effect per one standard deviation), unstandardized coefficients (effect per one unit increase) were also calculated (Eq. 4). where is the residual sum of squares and is the total sum of squares. Finally, a one-way sensitivity analysis was carried out. Starting from the mean mix design, each input was varied by ± 10% while keeping others constant. where ŷ is the predicted value, and is the mean of feature (Eq. 5). This approach highlights which variables have the highest influence on each property.

$$\:y = \alpha \: + \sum\limits_{{i = 1}}^{n} {\beta _{i} z_{i} + \varepsilon }$$

(3)

$$\:{R}^{2}=1-\frac{{SS}_{res}}{{SS}_{tot}}$$

(4)

$$\:\text{Im} pact_{i} = \hat{y}\left( {x_{i} \times \:1.1} \right) - \hat{y}\left( {x_{i} \times \:0.99} \right)$$

(5)

Life cycle assessment

The LCA was conducted following a cradle-to-grave system boundary in accordance with ISO 14040:2006⁴³, as defined across five sequential stages: (i) raw material extraction, (ii) processing and transportation, (iii) concrete manufacturing, (iv) use phase, and (v) end-of-life disposal. The LCA system boundary flow diagram for each stage is illustrated schematically in Fig. 6. The allocation approach followed the cut-off system model from Ecoinvent 3.8, assigning zero upstream environmental burden to waste-derived GGBS and RA, consistent with their secondary material status.

Fig. 6

LCA system boundary flow diagram.

The functional unit is 1 m³ of M40-grade concrete over a 50-year service life. This service life is consistent with IS 456:2000⁴⁶ design provisions for reinforced concrete in moderate exposure conditions and aligns with analysis periods adopted in comparable LCA studies on recycled aggregate concrete^25,26,29. Indirect durability evidence from this study — UPV values exceeding 4,500 m/s (classified as ‘excellent’ per IS 13311 (Part 1):1992⁴² and SEM/EDS-confirmed ITZ densification in GGBS-modified mixes — provides material-level support for this assumption. Direct long-term validation through chloride penetration or carbonation depth testing is recommended for future work.

Environmental impact characterization followed the ReCiPe 2016 Midpoint (H) methodology across all 18 impact categories, computed as:

$$\text{Im} pact{\text{ }}category{\text{ }} = {\text{ }}\Sigma {\text{ }}\left( {Qi{\text{ }} \times {\text{ }}CF_{i} } \right)$$

(6)

where Q_i is the inventory quantity of substance i (kg, MJ, or m³) and CF_i is the corresponding characterization factor. Eight categories — climate change (GWP100), ozone depletion, terrestrial acidification, freshwater and marine eutrophication, human toxicity, ecotoxicity, and particulate matter formation — were selected for in-depth discussion on the basis of: (i) demonstrable sensitivity to mix design variation (> 5% change across replacement levels); (ii) consistency with impact categories reported in peer-reviewed concrete LCA studies^25,26,29; and (iii) direct relevance to Indian environmental policy frameworks, including the National Action Plan on Climate Change.

Foreground inventory data were collected from concrete production facilities within a 50 km radius of the study site. Material quantities per mix design are cement (369 kg/m³), fly ash (41 kg/m³), GGBS (0–92.25 kg/m³), aggregates (1261.53 kg/m³), and water (164 L/m³). Background data were drawn from the Ecoinvent 3.8 database. Energy consumption during RA processing — comprising crushing (8.2 kWh/ton), screening (4.1 kWh/ton), and washing (2.9 kWh/ton) — was measured on-site at 15.2 kWh/ton using calibrated power analyzers. The regional electricity grid emission factor (0.82 kg CO₂/kWh) was sourced from the Central Electricity Authority of India (CEA, 2021)¹³.

In transporting all material, it was through road freight using medium-duty trucks (10–12 tonne payload). The transport distances were confirmed by direct supplier contact: NCA at quarries (local) at 25 km; RA at registered waste processing facilities of C&D that are registered (15 km); and cement at the nearest bulk distribution depot at 85 km. The emission factor of LCA road freight was assumed to be 0.062 kg CO₂/tonne -km (Ecoinvent 3.8; lorry > 16 t, EURO5 specification). The calculations of impacts were run in the SimaPro 9.4. The uncertainty was measured using the Monte Carlo simulation (1,000 runs) where a variation of ± 10% was made on the key inventory parameters and findings are presented in 95% confidence.

Economic analysis

The analysis was done based on an economic assessment by following ISO 15686-5:2017⁴⁷, life cycle costing (LCC) with a 50-year duration of life analysis. All seven mix designs were calculated using four financial measures, which included net present value (NPV), internal rate of return (IRR), and benefit-cost ratio (BCR) and payback period. The formulation of the governing LCC is (Eq. 7).

$$LCC = C_{{initial}} + \sum {[Ct/(1 + r){}^{ \wedge }t] + C_{{end - of - life}} }$$

(7)

where C _initial is the upfront material and construction cost (₹/m³), C_t encompasses all time-dependent cash flows at year t — including maintenance, operational costs, and environmental compliance — r is the annual discount rate (5%), and t spans 1 to 50 years. The end-of-life charges include demolition (₹450/m³), landfill (₹150/ton) or recycling credits (₹75/ton), based on the disposal condition. Table 6 shows all the cost elements such as the material procurement, RA processing, transportation, labour, equipment operation, quality control, maintenance and disposal end of life.

Three regional suppliers were averaged to cover the period between January 2024 and March 2025 to average material unit costs because of market variation. RA processing charges were calculated using the existing rates of the registered C&D waste processing plants in Guntur-Telangana area: crushing (₹85/ton), screening (₹35/ton), and washing (₹25/ton). The transportation expenses remained calculated at ₹2.8/tonne km and the different regional logistics companies confirmed the cost based on the same distances covered by road freight, which was the same as the LCA inventory (25 km; 15 km; 85 km). Minor repairs were planned to be done after 10 years and major structural rehabilitation after 25 years, in accordance with M40-grade concrete service life planning as per IS 456:2000⁴⁶. The RA12.5%+GGBS mixes were considered to have durability-equivalent to a control mix as was evidenced by the similarity of UPV values and water absorption outcomes at this replacement level. The rate of cost inflation for all expenditures that were time-related was set at 4% per annum, which is in line with the long-term data on building cost inflation recorded by the reserve bank of India. Carbon tax scenarios of ₹50, ₹100, and ₹150/ton CO₂ were modelled as forward-looking sensitivity cases; no carbon tax is currently mandated in India. All key assumptions are consolidated in Table 3 for transparency and reproducibility.

Table 3 Key economic assumptions.

Results and discussions

Effect of RCA on workability of concrete

The workability of fresh concrete was assessed using a slump cone immediately after mixing as per IS 1199: 1959⁴⁸. The slump values for the mixtures are 80 mm, 85 mm, 74 mm, 56 mm, and 55 mm for RA0% to RA65%, and 80 mm for both RA12.5%+GGBS20% and RA12.5%+GGBS25% mixtures. The slump cone measurements of environmentally friendly concrete samples (EFC) with varying percentages of recycled aggregate (RA%) demonstrated superior slump cone values compared to the control mix. The test results revealed that the workability of EFC is significantly influenced by the partial substitution of cement with ground granulated blast-furnace slag (GGBS) and natural aggregate (NA) with recycled aggregate (RA).

Effect of RA% mixes on compressive strength

To investigate the mixes, cube samples of 100 × 100 × 100 mm were prepared and direct stress is obtained from the UTM testing machine. A plot shows effect of RA% compressive strength as shown in Fig. 7. The basic outcomes of the compression strength test evidently reveal the effect of the presence of recycled coarse aggregates (RA) to concrete performance. All the five mixes researched on in Phase-I, the mix with 12.5% RA replacement was able to show improved results in all curing periods compared to the control mix (RA0%). After 7 days of curing, RA12.5% mix was at a compressive strength of 34.98 MPa which was a bit better than the 33.43 Mpa of RA0%. This low growth of approximately 4.6% indicates that additions of a small percentage of RA have no adverse influences on the initial strength and even have some internal healing advantages because RA particles have greater water absorption. However, a significant decrease in compressive strength with increasing proportion of RA above 12.5% with RA25, RA50 and 65% representing decreasing values respectively than RA0%. The findings indicate that even though it is possible to incorporate RA in low levels without affecting the initial strength, increased replacement level has serious effects on the concrete integrity. This was the same trend at 28 days at Mix-2 registered 55.43 MPa which was approximately 13% higher than the control mix of 48.9 MPa. The gain shows that there is the possibility of more even hydration or greater interaction of the paste and aggregates when limited RA is used. On the other hand, as shows reduced significantly in performance at after 12.5%, which suggests that additional RA material creates decreases the overall bonding of the concrete. The 90-day strength data reinforced these observations. The RA12.5% mix achieved a peak of 57.25 MPa, outperforming RA0% by over 8%, while the other mixes remained significantly lower. This loss of compressive strength more than 12.5% RA may be explained by the adhered mortar prevalence which brings more porosity and inferior paste associations. The ITZ around the recycled aggregates will tend to have microcracks and the increased micro void content than the natural aggregates. The cumulative effect of numerous weak ITZ regions leads to decreased transfer of loads and earlier cracks initiation as the cumulative effects of RA content increases. As a result, the degradation of strength increases with increased replacement.

In Phase-II, for RA12.5% mix, the cement is replaced with GGBS at 20% and 25% shows better performance in the replacement of cement. At 7, 28and 90-daysshows a same pattern of improvement is observed compared with the control mix achieved 36.94 MP 58.86 MPa,57.85 MPa for (RA12.5%+GGBS20%) and 35.24 MPa, 53.68 MPa and 59.47 MPa (RA12.5%+GGBS25%) which is around 9.5%, 16.9% and 8.62% and 5.13% 8.90% and 11.09% respectively compare with the control mix (RA0%). The gain of strength is achieved due to incorporation of GGBS would lead the hydration and formation of denser matrix. These results highlight the potential of using 12.5% RA and with 20% and 25% GGBS to not only replace natural aggregates sustainably but also maintain or slightly improve compressive performance. Beyond that percentage, the compromised quality of recycled aggregates becomes too significant to ignore, affecting both strength and reliability. Md Habibur Rahman Sobuz et al. 2025²⁸ discovered that 45% modified reclaimed concrete aggregate (MRCA) with and 10% metakaolin had a compressive strength of 60.36 MPa, matching the control high-strength concrete. This mix uses 45% recycled aggregates and 10% metakaolin, reducing its environmental impact. However, Yergol et al. 2024⁴⁹ Composite concrete products including 60% recycled aggregate demonstrate a decrease in compressive strength at both 7 and 28 days of curing age. The addition of 10% silica fume and 45% recycled aggregate leads to improved compressive and split tensile strength compared to previous silica fume-based composite concrete products.

Durability tests were not directly carried out but the higher the content of RA the higher the permeability and drying shrinkage of the material is expected to be because of higher porosity and weaker ITZ regions. On the other hand, addition of GGBS is known to perfect pore structure and lessen permeability by extra development of C-S-H. Thus, although increased RA replacement can negatively influence the properties of durability, GGBS modification can be used to partially eliminate their effects. These implications, however, need to be verified through long-term durability evaluation.

Fig. 7

Effect of RA% on compressive strength of mixes.

Effect of RA% mixes on split tensile strength

The split tensile strength shows similar comparison with compressive strength, at 7 days, RA12.5% recorded a value of 2.97 MPa, slightly above RA0%, which stood at 2.85 MPa as shown in Fig. 8. While the difference might seem minor, it reinforces the idea that low RCA content does not hinder the concrete’s tensile capacity. As the replacement level increased, the tensile strength values dropped off more noticeably. At RA25%, 50% and RA65% recorded 2.46 MPa, 2.21 MPa, and 2.10 MPa respectively each one reflecting a clear loss in strength when compared with RA12.5%. At 28 days, RA12.5% again held the highest value at 3.55 MPa, which was nearly identical to RA0%. However, the higher RA mixes struggled, falling by 20% or more relative to RA12.5%. After 90 days, RA12.5% still led with 3.71 MPa, slightly better than RA0% is 3.66 MPa, confirming that a 12.5% RA replacement does not affect and may slightly enhance tensile strength. Mixes with more RA consistently underperformed, likely due to poor bonding and weak interfaces between the old mortar and fresh paste. These results make it clear that higher RA levels impact tensile strength. For the better performance the cement is replaced with GGBS at 20% and 25% shows better performance in the replacement of cement. At 7, 28and 90-days shows a same pattern of improvement is observed compared with the control mix achieved 3.2 MPa,3.83 MPa,3.8 MPa for (RA12.5%+GGBS20%) and 2.98 MPa, 3.86 MPa and 3.85 MPa (RA12.5%+GGBS25%) which is around 10.93%, 7.83% and 3.68% and 4.36%, 3.55% and 4.93% respectively compare with the control mix (RA0%). R.Yergol et al. 2024⁴⁹ observes the incorporation of 10% silica fume and 60% recycled aggregate results in a 5% decrease in both compressive strength and split strength relative to the intended mean strength. Apart from the study of Uma Shankar Biswal, Pasla Dinakar (2021)³⁶ discovered that 30% CFA on RAC performed poorly compared to OPC-only RAC, contrary to its behavior in VAC. The findings suggested replacing OPC with 50% GGBS for long-term CS and sustained RAC. The study’s cementitious material with the greatest STS was RAC with 50% GGBS as OPC substitution.

Fig. 8

Split tensile strength of RA% mixes.

Effect of RA% mixes on flexural strength

The flexural strength values, as presented in Fig. 9, highlight how varying levels of recycled coarse aggregate (RA) influence the concrete’s ability to withstand bending stresses. This is a marginal increase in early years, which implies that the minimal percentage of RA does not have a negative influence on flexural behavior. RA12.5% recorded the highest flexural strength of 6.3 MPa at 28 days, this was 3.3% greater than what was recorded at RA0 which was 6.1 MPa. The RA25% also reduced significantly including 4.85 MPa RA50% and RA65% were also less by 23, 26.8, and 27.5% respectively compared to RA12.5%. At 90 days, RA12.5% still performed best as it obtained 6.8 MPa, which was an improvement of 3% compared to RA0% (6.6 MPa). Comparatively, RA25% was 5.48 MPa, RA50% was 5.21 MPa and RA65% was 5.18 MPa. These values are equivalent of 19.4, 23.4 and 23.8% decrease in flexural strength compared to RA12.5%.

This reduction of strength suggests that the cement-recycled aggregate particle bond might not be a strong bond that can resist the cracking that can occur during tension at higher levels of RA. The decrease in flexural strength at increased replacement levels of recycled aggregate (RA) of 25 to 65%, is a pointer of early crack initiation and unstable crack propagation due to bending. The flexural failure is controlled by tensile stresses at extreme tension fiber, the existence of old mortar in RA has a high impact on cracking and probable weak interfaces between the old mortar and the new paste. Recycled aggregates are more porous with internal heterogeneity and microcracks as compared to natural aggregates and have several weak interfacial transition zones (ITZs) and higher content of microvoids. These weak zones serve as stress concentration areas, which favor initial cracking and under loading of the crack.

To ensure improved performance the cement is substituted with GGBS at 20% and 25% is found to be improved in the replacement of cement. At 28 and 90-days shows a same pattern of improvement is observed compared with the control mix achieved 6.5 MPa,6.8 MPa for (RA12.5%+GGBS20%) and 6.4 MPa, 6.9 MPa for (RA12.5%+GGBS25%) which is around 6.15%, 2.94% and 4.68%, 4.34% respectively compare with the control mix (RA0%). These findings, visualized clearly that while 12.5% RA maintains or slightly enhances flexural performance, further replacement consistently reduces strength, likely due to the increased presence of old adhered mortar, microcracks, and less effective stress distribution within the beam specimens. Conversely, the RA12.5% mix, especially the with GGBS (20–25%), demonstrates a better performance because of the refinement of pores and the densification of ITZ because of secondary C-S-H formation. This helps in increasing matrix integrity and preventing crack propagation as well as flexing strength.

Fig. 9

Flexural strength of RA% mixes.

Ultrasonic pulse velocity results

The result of the ultrasonic pulse velocity (UPV) test, as shown in Fig. 10, can be useful in the determination of the internal uniformity and density of different concrete mixes which have different proportions of RA. This is a non-destructive test that is used to check the quality of concrete depending on the rate at which the ultrasonic waves travel through the material. An increase in velocities normally signifies a concrete that is more dense and more homogenous with fewer voids or internal defects. With a curing period of 28 days, RA0% registered the greatest value of UPV at 5155 m/s, which confirmed high quality of concrete. RA12.5% had a slightly lower velocity of 4739 m/s, approximately 8.1% lower than RA0, but still, this value is within the range of the category of excellent according to the IS 13,311 (Part 1):1992⁴².

As RA% content increased, the velocity readings gradually declined. RA25%recorded 4475 m/s, while RA50% andRA65% dropped further to 4000 m/s and 3900 m/s, respectively. When compared to RA12.5%, these represent 5.6%, 15.6%, and 17.7% reductions in UPV. The decreasing trend can be attributed to the higher porosity and presence of micro voids within the recycled aggregate is observed in SEM images. These defects interrupt the continuity of the cement matrix and create discontinuous wave transmission paths, reducing ultrasonic pulse velocity.

At 90 days, the values followed a similar pattern. RA0% reached a UPV of 5214 m/s, and RA12.5% showed 4500 m/s, showing a 13.7% reduction from the control. The RA25%, RA50% and RA65% registered 4250 m/s, 4000 m/s, and 4000 m/s, translating to 5.5% and 11.1% reductions relative to RA12.5%. Even though the velocities dropped with increasing RA content, all mixes remained above the 3900 m/s threshold, indicating that the internal structure of the concrete was still within acceptable limits. As observed in Fig. 10 the consistency and compaction of the mixes decrease as RA percentage rises, but a 12.5% replacement level does not significantly affect the internal integrity of the concrete. In contrast, mixes with GGBS (20%, 25%) showed a denser matrix and improved ITZ quality due to secondary C–S–H formation, which enhanced internal compactness and supported relatively higher UPV values.

Fig. 10

Ultrasonic pulse velocity of RCA mixes.

Statistical analysis

The optimization of concrete mix designs, the calibration of strength models, and the reduction of destructive sampling all depend on statistical analysis of concrete strength. This study examines the reliability of the mixes through an analysis of mechanical properties, including compressive strength, split tensile strength, and flexural strength, as well as ultrasonic pulse velocity at various curing ages, utilizing linear regression analysis. The statistical correlation between compressive strength and split tensile strength is analyzed at 7 days, 28 days, and 90 days of curing, as illustrated in Fig. 11, with R² values of 0.99, 0.94, and 0.90, respectively. The statistical correlation between flexural strength and split tensile strength at 28 days and 90 days of curing is illustrated in Fig. 12, with R² values of 0.91 and 0.96, respectively. The statistical correlation between compressive strength and flexural strength at 28 days and 90 days of curing is illustrated in Fig. 13, with R² values of 0.89 and 0.85, respectively. The statistical correlation between compressive strength and ultrasonic pulse velocity is analyzed at 7 days, 28 days, and 90 days of curing, as illustrated in Fig. 14, achieving R² values of 0.93 for each time point. This regression plots demonstrates potential for non-destructive in-situ strength estimation without core extraction. The relationships can support quality control and structural evaluation. However, field application requires calibration for local materials and moisture conditions. Since UPV depends on aggregate type and porosity, the equations should be applied only within similar RA% replacement ranges and GGBS% and validated before use.

Fig. 11

LR-analysis of compressive strength vs. split tensile strength of mixes.

Fig. 12

LR-analysis of split strength vs. flexural strength of mixes.

Fig. 13

LR-analysis of compressive strength vs. flexural strength of mixes.

Fig. 14

LR-analysis of compressive strength vs. UPV of the mixes.

Sensitivity analysis

The regression and sensitivity analysis were carried out for all four outputs, namely compressive strength, split tensile strength, flexural strength, and ultrasonic pulse velocity (UPV). The results showed that cement content played the most important role in determining the strength related properties, with higher cement content consistently improving both compressive and flexural strength. For compressive strength the regression indicated a strong positive effect of cement, while RA content showed a negative effect. The sensitivity analysis confirmed that a 10% change in cement had the largest influence on predicted compressive strength. For split tensile strength cement again showed a positive effect, while RA content had a negative influence, and sensitivity analysis indicated that cement variation was the most critical factor. Flexural strength followed a similar trend with cement contributing positively and RA reducing the values, and the sensitivity analysis showed cement to be the most sensitive parameter. In the case of UPV the percentage of RA in total coarse aggregate showed a positive effect, while cement appeared as a negative factor, and the sensitivity analysis suggested that cement still dominated the influence on predicted.

Overall cement was the most significant contributor of mechanical performance in this data set and the proportion of RA was noted to decrease strength related properties across the proportional levels. The percentage of RA played a visible role on the ultrasonic pulse velocity, which means that the substitution of natural aggregate by recycled one influences the compactness of the mix. It is necessary to mention that the data comprises of only six mixes and the regression models thus fit in the data very well but might not represent the behavior in a wider mix of concrete mixes. The results should then be deemed to represent trends and more testing using more data points should be carried out to reinforce the findings and offer more accurate models. Table 4. The regression model was designed using the relatively few mix compositions; hence the large coefficient of determination is to be viewed in the context of the experimental data. The relationship suggested is only exploratory and can only be held in the range of parameters that have been tested. To come up with a generalized predictive model, more data values would be needed.

Table 4 Summary of regression coefficients and one-way sensitivity analysis results for all four output properties.

Microstructural analysis

SEM analysis

The microstructural analysis is an important instrument in concrete studies since it offers first-hand visualization of the interior architecture of the hardened matrix that cannot be completely guided by mechanical research. In this study, SEM was employed to determine these microstructural parameters of various concrete mixes. SEM technique can be used to visualize micro-level features at high magnification including the presence of voids, dry particles, crack initiation zones and the distribution of hydration products like calcium silicate hydrate (C-S-H) gel. It is an analysis that bridges the disparity between external mechanical action and internal material action, and supports test findings with visual microstructural data. As can be seen in Fig. 15, the SEM micrographs of Phase-I samples including mixes of 0, 12.5, 25, 50 and 65% replacement, there is a clear indication of deterioration of the microstructure with the increase of the RA content. The control mix (0% RA) has a comparatively compact hydration product and a low number of recognizable pores or natural fissures as indicated in Fig. 15a. Nonetheless, the higher the percentage of RA, the more irregular and porous the internal structure of the latter becomes. The hydration of the fly ash, microcracks, and loosely packed products are observed in the 25% RA and 50% RA mix, especially. On the SEM analysis at RA25% and RA50% demonstrations of Fig.15c and 15d, the flyash particles inside matrix like smooth and spherical images on SEM reveals the unhydrated particles in concrete matrix.

These defects arise primarily due to the adhered mortar present on recycled aggregates, which introduce old, brittle material into the mix and interrupt the bonding between new paste and aggregates. In contrast, the mix containing 12.5% RA shows a more refined and denser microstructure with fewer cracks and a denser packing of hydration products. The ITZ in this mix appears to be better developed, with fewer discontinuities, indicating good adhesion between recycled aggregates and the surrounding cement paste as shown in Fig. 15b. The 12.5% RA mix showed superior compressive, tensile, and flexural strength compared to mixes with higher RA content.

In Phase-II, for the optimum RA12.5% mix, cement is replaced with GGBS20% and GGBS25%, the SEM images for the mix are shown in Fig. 15e, for RA12.5%+GGBS20% and the SEM images with RA12.5%+GGBS25% are shown in Fig. 15f. From, Fig. 15e,f revealed a significant presence of dense and homogenous microstructure and indicating high degree of pozzolanic reactions and effective utilization of calcium hydroxide.

Unlike conventional concrete, RA contains adhered old mortar, higher porosity, and a weakened, heterogeneous ITZ. In this system, GGBS act as a supplementary cementitious material, it compensates for RA and induced deficiencies. Due to GGBS presence in the matrix leads to formation of denser hydration products develops with secondary C–S–H gel densifies the ITZ around the old mortar, reduces calcium hydroxide, and refines pore connectivity^50,51,52. Additionally, RA% higher absorption facilitates internal curing, and sustaining delayed GGBS hydration and CASH in the matrix which develops the improved performance results from coupled microstructural interactions primarily attributable to its well-established pozzolanic and latent hydraulic reactions^53,54.

EDS analysis

The elemental characterization of the mixes is carried out derived from a SEM image spectrum under consistent magnification and preparation conditions using EDS analysis to assess phases. In EDS analysis showing dominant Ca, Si, Al, Fe and O peaks, in the mixes, for performance of recycled coarse aggregate (RA) and GGBS in the concrete mixes are represented in the Fig. 16. From Fig. 16a-f shows, consistent hydration environment was established as the oxygen concentration was reasonably constant (56.21% to 68.80%) throughout all mixes. A gradual rise in silicon content (from 6.69% to 12.61% of the total) and a systematic drop in calcium concentration (from 9.72% to 23.54%) suggest that pozzolanic reactions. The results showed that mix containing with RA12.5% (optimum) shows low Ca/Si ratio about 1.8 compare to the RA0%, hence the mix with RA12.5% actively helps to maintain the hydration bond process in the matrix leads to improvement in the mechanical properties like compressive and tensile strength. However, the for the optimum mix (RA12.5%) the cement is replaced with GGBS at 20% and 25% shows the low Ca/Si ratio about 0.77 and 1.32 compare to the RA0%. In general, three mixes show Ca/Si ratio < 2 (RA12.5%, RA12.5%+GGBS20%, and RA12.5% + GGBS25%) which indicates dense concrete matrix. A relative decrease in Ca intensity with increased Si signal indicated secondary hydration. This augmentation in the concentrations of calcium, aluminum, and silicon consequently enhances the formation of calcium-silicate-hydrate (C–S–H) and minor Mg and Al peaks further suggest the formation of calcium-aluminate-silicate-hydrate (C-A-S-H) type phases, resulting in the development of a continuous layer that serves to agglomerate, thereby amplifying the binding phase within the concrete matrix^24,55,56. As per the Zhang et al. 2024²⁹ Larger Ca/Si ratios reduce strength, whereas lower ratios provide more stable C-S-H gel formations with better mechanical properties.

.

Fig. 15

SEM image of mixes. (a) RA0%, (b)RA12.5%, (c)RA25%, (d)RA50, (e) RA12.5%+GGBS20%, (f) RA12.5%+GGBS25%.

Fig. 16

EDs analysis of mixes. (a) RA0%, (b) RA12.5%, (c) RA25%, (d) RA50%, (e) RA12.5%+GGBS20%, (f) RA12.5%+GGBS25%.

Sustainability assessment

Life cycle assessment

Total GWP decreased from 386.7 kg CO₂-eq/m³ in the control mix (RA0%) to 282.2 kg CO₂-eq/m³ for RA12.5%+GGBS25%, representing a 27% reduction, as shown in Fig. 17. This corresponds to an absolute saving of 104.5 kg CO₂-eq/m³ — equivalent to 52–209 tonnes CO₂ avoided per typical infrastructure project consuming 500–2,000 m³ of concrete.

The reduction places this study at the upper boundary of the 5–18% GWP reduction range reported for RA concrete without SCMs (Cerchione et al., 2023²⁵; Estanqueiro et al., 2016²⁶), with the additional GGBS substitution contributing 60.4 kg CO₂-eq/m³ of the total saving. Contrary to the expectation that higher recycled content yields proportionally greater environmental benefit, RA65% achieved only 14.1% GWP reduction despite maximum waste incorporation. RA processing energy — measured on-site at 15.2 kWh/ton and fully incorporated into the inventory — increases progressively with replacement level, directly explaining this diminishing return. Hwang et al²⁹. similarly reported that CO₂ inventories for recycled coarse aggregates range from 0.5 to 7.9 times those of natural coarse aggregate, confirming that processing energy offsets must be explicitly accounted for in RA-based LCA studies.

Monte Carlo simulation (1,000 iterations, ± 10% variation on cement quantity, electricity consumption, and transport distances) yielded the following 95% confidence intervals: RA0% — 386.7 ± 19.3 kg CO₂-eq/m³; RA12.5%+GGBS25% — 282.2 ± 14.1 kg CO₂-eq/m³; RA65% — 331.8 ± 18.6 kg CO₂-eq/m³. The confidence intervals for the optimal mix and control do not overlap, confirming that the reported 27% GWP reduction is statistically robust. The wider uncertainty range for the control mix reflects its greater sensitivity to cement quantity variability, consistent with cement production being the dominant inventory contributor.

Manufacturing processes contributed 50.5% of total emissions in the control mix, rising marginally to 51.9% in RA12.5%+GGBS25%, as detailed in Table 5. Within this stage, cement production and clinker burning dominate at 138.5 kg CO₂-eq/m³ across all non-GGBS mixes - representing 70.8% of the manufacturing burden. GGBS substitution at 20% and 25% reduces this to 110.8 and 103.9 kg CO₂-eq/m³ respectively, making cement replacement the single most effective decarbonisation lever at the manufacturing stage. RA processing sub-processes — crushing, screening, and washing — contribute 10.2 kg CO₂-eq/m³ at 12.5% replacement, rising to 53.3 kg CO₂-eq/m³ at 65% replacement. At RA65%, RA processing alone accounts for 22.3% of the total manufacturing burden, compared to 5.0% at RA12.5% — a fourfold increase that quantitatively explains the diminishing GWP returns at higher replacement levels. Transportation emissions show a favourable inverse trend, declining from 28.3 kg CO₂-eq/m³ (RA0%) to 19.1 kg CO₂-eq/m³ (RA65%), attributable to the shorter haulage distance of demolition-sourced RA (15 km) relative to quarried NCA (25 km). Batching, casting, and curing contributions remain constant across all mixes (28.7 kg CO₂-eq/m³), confirming that these process stages offer no differentiation potential between mix designs.

The natural aggregate savings is proportional to the RA content, on a scale of 157.5 kg/m³ at RA12.5%, up to 819 kg/m³ at RA65%, as shown in Fig. 18. Nevertheless, there is an inverse relationship between energy saving with RA12.5% be 641 MJ/m³ and RA65 be 605 MJ/m³, with the advantage of material substitution partially compensated by RA processing inefficiencies. The use of GGBS-modified mixes resulted in significantly better energy conservation (915 MJ/m³) by lowering the amount of clinker necessary to make the mixture and in greater pozzolanic efficiency. There was low change in water consumption among all mixes (28.5–32.8 L/m³) meaning that mix design changes had negligible hydrology. The CO₂ cut per cubic metre decrease to 104.5 kg/m³ (RA12.5%+GGBS25%), which proves that the aggregate and binder substitution is the most effective in terms of environmental benefits, which are not possible when substituting with the RA.

The normalised impact assessment of eight categories of the environment, that are reported relative to the control mix indicated consistent performance improvement at RA12.5% but diminishing returns at higher replacement levels as represented by Fig. 19. RA12.5% achieved an average 12.5% reduction across all categories, while RA65% achieved only 7.8%, with ozone depletion approaching baseline values (97.2% of control). Resource depletion showed the greatest absolute sensitivity to sustainable modifications, improving from 87.5% (RA12.5%) to 71.8% (RA12.5%+GGBS25%), reflecting the synergistic reduction in both aggregate extraction and cement production burdens. Terrestrial acidification decreased from 85.3% to 75.8% with GGBS incorporation, driven by reduced SO₂ and NOx emissions associated with clinker burning.

Table 5 Manufacturing sub-process CO₂ contribution breakdown (kg CO₂-eq/m³).

Fig. 17

Life cycle carbon footprint analysis.

Fig. 18

Resource conservation and waste diversion.

Fig. 19

Normalized environmental impact assessment.

Economic analysis

Initial material and construction costs decreased by 5.8% (RA0% from ₹5350/m³ to 5039/m³ (RA65%)), a 65% virgin aggregate substitution caused, although initially, virgin aggregate substitution refer to Table 6 cost breakdown. This small saving can be attributed to the cost disaggregation: the gross natural aggregate saving of 821/m³ (between ₹1261 to ₹440/m³) is significantly reduced by RA procurement (₹410/m³), more labour (₹112/m³ - attributable to extra pre-wetting and handling), more equipment and energy escalation (₹98/m³ used to crush and screen and wash materials), and more quality control spending (₹43/m³). All these processing penalties absorb ₹253/m³ of the gross aggregate saving, with a net initial reduction of 311/m³. The cost of transportation dropped to ₹298/m³ (RA65%), of which 40 shorter haulage distance of the demolition-originated RA (15 km) compared to the 25 km distance of quarried NCA (25 km) at ₹2.8/tonne-km, which somewhat canceled out the penalties associated with processing. The cost of environmental compliance dropped by ₹150/m³ (RA0%) to 43/m³ (RA12.5%+ GGBS25%), which is the regulatory cost benefits of sustainable material utilization. It was found that the best mix, RA12.5%+GGBS25%, had the lowest initial cost of GGBS-containing mixes (₹5045/m³) due to the synergistic effect of the cost of cement and good transport logistics.

The differences in costs between mixes over the 50-year service period is significantly higher than that at the procurement level as shown in Fig. 20. RA12.5%+GGBS25% had 27.1% LCC reduction relative to the control, equivalent to ₹8005/m³ NPV savings - versus 4.7% LCC reduction of RA65%. Maintenance costs were the most important lifecycle performance determinant, with RA12.5%+GGBS25% ranging between ₹9200/m³ and RA65% ranging between 12,300/m³ − 3100/m³ difference directly proportional to the mechanical performance difference between the two. An increased replacement level is characterized by faster surface wear and crack vulnerability, and it has to be repaired more often. The modelled carbon prices of 50–150/ton CO₂, using pricing scenarios of dis-incentivation (RA0%) and incentivization (RA12.5 + GGBS25), showed a penalty to the less sustainable mixes as regulatory systems increase in severity, with an environmental compliance cost decreasing by 25%, i.e. 2500/m³ (RA0%) to 1000/m³ (RA12.5 + GGBS25).

Table 7 (financial performance indicators) shows NPV, IRR, BCR, and payback period of all mix designs. The IRR was between 8.7% (RA65%) and 19.4% (RA12.5%+GGBS25%), and the GGBS-adjusted mixes has always performed better at the same replacement level as the RCA-only mixes. The payback time increased to 5.2 years (RA65%), 2.1 years (RA12.5%+GGBS25%) respectively, which is dependent on the replacement ratio, which is inversely proportional to their replacement ratio, due to the rising maintenance requirement with higher RCA contents. All configurations had a positive value above unity, which demonstrates that the lifecycle returns were positive in the entire experimental space but RA12.5%+GGBS25% had the highest BCR (1.37) due to the combination of low initial cost, reduced maintenance spending and savings in environmental compliance.

Cement pricing was found as the most significant economic risk factor, producing a ± 6.290% LCC variation over a ± 20% mix design price change - the largest of all parameters investigated as demonstrated in Fig. 21. Secondary sensitivity (± 2.13.8% and ± 1.83.2) was assigned to aggregate and labour costs, and the tiniest effect was the variation of transportation cost (± 0.91.4%). Carbon taxation cases preferred the more sustainable mixes out of proportion: RA12.5% + GGBS25% would receive 4.2% LCC benefit per 0.01 increment CO₂, contrasted with 2.1% with RA12.5% alone, due to its significantly lower carbon intensity. These results support the assertion that the economic argument in favor of RA12.5%+GGBS25% gets increasingly more powerful with the implementation of carbon pricing systems, whereas the economic stance of high-RCA mixes is relatively unchanged with the change in regulations.

The findings of the cost model are in line with the published literature. Bostanci et al. (2018)³⁷ reported that high-RCA mixes have comparable or slightly higher initial costs compared to NAC attributed to overheads in processing- must have been directly related to the 5.8% small initial cost reduction in RA65% here. In his study, Paruthi et al. (2024)⁵¹ reported IRR of 1520% of GGBS-based sustainable concrete, as compared to the 19.4% IRR in this study of RA12.5%+GGBS25%.The integrated sustainability index (Fig. 22) combines technical, environmental and economic performance into a single composite score, RA12.5%+GGBS25% with the highest score (83.3) based on balanced sub-scores of 85 (technical), 88 (environmental) and 84 (economic). In the material costs and regional conditions of this paper, financial, technical and environmental performance is optimized at moderate RCA replacement with GGBS supplementation - a result that should be checked at varying regional cost structure and aggregate supply conditions.

Table 6 Economic analysis - cost breakdown (₹/m³).

Table 7 Financial performance indicators

Fig. 20

Life cycle cost analysis (50-year period).

Fig. 21

Economic sensitivity analysis.

Fig. 22

Integrated sustainability performance index.

Conclusions

This research was carried out to study effect of partially replaced with recycled coarse aggregate (RA) (0, 12.5%, 25%, 50% and 65%) and GGBS20% and GGBS25%, the following conclusions are drawn as follows:

• 1. The investigation revealed that substituting 12.5% of NA with RA offered more favorable results regarding strength and workability in comparison with replacement levels (RA0%). The combination of 12.5% recycled aggregate and 20% and 25% ground granulated blast-furnace slag demonstrated superior performance in compressive strength, as well as in split tensile and flexural strength tests, when compared to other mixtures containing higher percentages of recycled aggregate. All the concrete samples underwent the Ultrasonic Pulse Velocity (UPV) test, resulting in remarkably favorable outcomes. This confirms that the concrete mixes had good quality, were dense and had a uniform structure.

• 2. Statistical analysis models are developed for mixes demonstrated superior accuracy compared with other properties providing reliable tools for practical implementation and sensitivity analysis attributes high positive influence on the binders and RA12.5%on the output parameters.

• 3. The findings SEM/EDS, analysis observed that with RA12.5% (optium) shows best performance compare with control mix RA0% and shows better dense matrix. Apart the for the optimum RA12.5%+GGBS20%, RA12.5%+GGBS25% mixes shows dense homogenous concrete matrix and formation dense CSH gel which accumulates at the ITZ zones enhances the strength of the mixes and in EDS analysis shows low Ca/Si ratio of about 0.77 and 1.32 for the RA12.5%+GGBS20%, RA12.5%+GGBS25%. Hence it is evident that due to low Ca/Si ratio at GGBS20% and GGBS25% exhibits better compressive strength of 58.86 MPa and 53.68 MPa than RA0% respectively.

• 4. The RA12.5%+GGBS25% mix achieved the best environmental performance among all formulations, reducing GWP by 104.5 kg CO₂-eq/m³ (27%) relative to the control and yielding 915 MJ/m³ energy savings. Higher replacement levels (50–65%) produced diminishing environmental returns despite proportionally greater waste diversion (569.2–739.9 kg/m³ natural aggregate displaced), as RCA processing energy penalties progressively offset the benefit of reduced virgin material extraction. Manufacturing processes constituted the dominant environmental burden across all mixes (46.5–51.9% of total GWP), with cement production accounting for 70.8% of manufacturing-stage emissions. Within the tested experimental matrix and regional inventory conditions, cement replacement through GGBS is the more effective decarbonisation strategy than increasing RCA content beyond 12.5%, where mechanical performance and environmental returns both declines.

• 5. Lifecycle cost analysis over 50 years identified RA12.5%+GGBS25% as the economically optimal configuration, achieving 27.1% LCC reduction (NPV: ₹8,005/m³; IRR: 19.4%; BCR: 1.37; payback: 2.1 years) relative to the control. Despite a modest initial cost reduction of 5.8%, maintenance cost savings of ₹3,100/m³ and environmental compliance savings of ₹1,500/m³ over the service period are the primary drivers of lifecycle financial advantage. High RCA replacement (RA65%) yielded only 4.7% LCC savings and the lowest financial performance indicators (IRR: 8.7%; BCR: 1.05; payback: 5.2 years), confirming that maximising recycled content does not optimise economic performance. Cement price volatility (± 6.2–9.0% LCC impact for ± 20% price variation) was identified as the primary financial risk, while carbon taxation at ₹100/ton CO₂ provides an additional 4.2% LCC benefit to the optimal mix — strengthening its economic position as regulatory frameworks evolve. These findings are applicable under the material cost conditions and carbon pricing scenarios modelled for the Guntur–Telangana region and are subject to regional variability in material markets and regulatory contexts.

The optimal RCA replacement level of 12.5% identified in this study is specific to the material properties, mix proportions, and boundary conditions of the experimental programme. Higher replacement levels may be viable under different SCM combinations, aggregate quality, or curing regimes, and the results should not be interpreted as a universal performance ceiling.

Overall, this study shows that it is possible to make strong and durable concrete using recycled materials. This method not only supports environmental conservation by reducing construction waste but also encourages more sustainable practices in the construction industry.

Limitations of the study

Five limitations should be considered when interpreting and generalizing the findings of this study. First, the assumed 50-year service life was not validated through direct long-term durability testing; chloride penetration depth, carbonation resistance, and freeze-thaw performance data under sustained loading conditions were beyond the scope of this investigation and are recommended for future study. Second, all LCA inventory data and unit cost figures are region-specific, drawn from the Andhra Pradesh/Telangana construction context; their direct transferability to other geographic or regulatory environments may be limited. Third, durability equivalence between RA12.5%+GGBS mixes and the control mix in the economic model was assumed on the basis of UPV and water absorption proxies rather than experimentally validated long-term performance data. Fourth, the ReCiPe 2016 Midpoint (H) characterisation factors applied are global average values and do not incorporate region-specific endpoint weighting, which may affect the relative ranking of impact categories in localised assessments. Fifth, the economic model does not account for future aggregate scarcity scenarios or policy-mandated minimum recycled content requirements, both of which could substantially alter the relative cost competitiveness of higher RCA replacement levels.

Future scope of work

Several research directions are identified as priorities based on the limitations of this study. Long-term durability validation — encompassing chloride penetration, carbonation depth measurement, and cyclic loading performance — is needed to confirm the assumed 50-year service life for the optimal mixes through accelerated ageing protocols. The influence of RCA source variability (aggregate size distribution, original concrete grade, and degree of contamination) on the mechanical and environmental performance relationships established here warrants systematic investigation. Making the LCA area extend to include carbon accounting at the demolition stage and recycling rates of all end of life products would enhance the comprehensiveness of the cradle-to-grave evaluation. Economically, the LCC model is sensitive to the aggregate scarcity pricing and mandatory recycled content policies in changing Indian construction regulations, an analytical direction that is fruitful. Lastly, the combined multi-scale model used here, which involves mechanical characterisation, ITZ chemistry, LCA and LCC needs to be applied to ternary binder systems including fly ash and silica fume in addition to GGBS, in order to know whether or not additional synergistic benefits to the environmental and economic performance can be realised.