Life cycle assessment of low carbon concrete using recycled concrete aggregate and rice husk ash

Goal and scope definition

The objective of this study is to conduct a LCA of the production of conventional concrete and various concrete mixtures with both RCA and RHA as aggregate and cement replacements, respectively. This is to test whether the addition of RCA and RHA is more sustainable than conventional concrete alone across various environmental impact indicators. In terms of lifespan, the simultaneous addition of RCA and RHA can increase the predicted service life of concrete due to reduced porosity and improved resistance to chloride penetration, indicating that the alternatives used do not compromise concrete quality^[33]. The assessment was based on a functional unit of 1 m³ of concrete, which was also extended to study its implications for constructing a standard two-storey school building in the Philippines. Seven environmental impact categories were also included in the assessment—GWP, MRS, terrestrial acidification (TA), freshwater ecotoxicity (FE), WC, human carcinogenic toxicity (HCT), and CED—using SimaPro version 10.2.0.2. The first six impact categories were evaluated using the ReCiPe 2016 Midpoint (Hierarchist) assessment method.

Ten concrete mixtures, previously studied by Verbo et al.,^[30] were evaluated as different RCA and RHA replacement scenarios against conventional concrete (Base Case), as shown in Table 1. These were selected based on compliance with the 28-day compressive strength requirement (> 21 MPa) of the Department of Public Works and Highways (DPWH)^[34], and acceptable durability-related service-life performance in terms of electrical resistivity (ER) and chloride migration. As shown, all concrete mixtures satisfy the minimum compressive strength with measurements ranging from 25.70 to 46.74 MPa, despite varying RCA and RHA replacement levels. Post-conditioning bulk resistivities ranged from 7.52 to 18.75 kΩ cm, while chloride migration resistances ranged from 0.0398 × 10¹² to 0.1527 × 10¹² s/m². Concrete mixtures incorporating 10% RHA generally exhibit higher electrical resistivity and chloride migration resistances relative to mixtures without RHA, indicating improved durability performance and greater resistance against chloride ingress and ion transport. The mechanical and durability performance data served as the engineering performance basis for the present study. In particular we extended the analysis through an LCA framework to integrate environmental impacts, and sustainability performance indicators (SPI) that provide a better understanding of the tradeoffs between environmental burdens and structural and durability-related functionality associated with low-carbon concrete production in the Philippines.

System boundary analysis

Figure 1 shows the process flows for the production of conventional concrete and low-carbon concrete with RCA and RHA replacements. A cradle-to-gate approach was considered for the evaluation of system boundaries, which included processes from raw material extraction until the final production step before leaving the plant.

Figure 1. Cradle-to-gate system boundaries of process flows for conventional and low-carbon concrete production using recycled concrete aggregates and rice husk ash.

Life cycle inventory analysis

A life cycle inventory (LCI) analysis was performed following the steps outlined in ISO 14040^[31] and ISO 14044^[32] for all relevant energy and mass flow inputs and outputs, as well as data on environmental emissions. Foreground inventory data, including concrete mix proportions, transportation distances, RCA recovery, and RHA production, were based on Philippine-specific conditions. In contrast, background processes including cement production, electricity generation, fuel production, and material processing were represented using Ecoinvent Rest-of-World datasets where Philippine-specific inventories were unavailable. Although using non-local proxy datasets for cement manufacturing may introduce minor geographical bias, reflecting regional variations in kiln thermal efficiency and fuel usage, several methodologies were developed to maintain the robustness of the results. Transport logistics and electricity inputs are estimated based on the Philippine context, representing accurate data in terms of raw material supply and electricity mix for concrete crushing. While the database used for the burning of rice husk in a drum-type combustor is geographically specific to India, it exhibits a similar small-scale and less mature technology used in the agricultural regions in the Philippines. Moreover, because cement production largely dominates the environmental hotspot in concrete manufacturing, minor changes in secondary processes may statistically be insufficient to alter the results. Overall, the inventory provides adequate data quality for comparative LCA, with reasonable temporal representativeness for mature processes such as cement, sand, and gravel production. Table 3 presents the inventory analysis of materials and transport considered in the study.

Impact assessment

The impact assessment methods used in this research are ReCiPe 2016 Midpoint (Hierarchist) and CED. The impact categories evaluated under the ReCiPe method were GWP, TA, FE, MRS, HCT, and WC. GWP describes the effects of a product or service on climate change relative to the impact of carbon dioxide with the same quantity and time duration. CED measures the total energy inputs throughout a product’s life cycle, such as in the forms of electricity, oil, natural gas, and coal, among many others. MRS is associated with the product’s contribution to the resource consumption of nonrenewable resources, such as limestone and aggregates. TA describes the changes in soil chemical properties linked with the deposition of nutrients, primarily ammonia, sulfur oxides, and nitrogen oxides. Chemical emissions harming freshwater ecosystems are quantified using the impact category, FE. This is measured using kilograms of 1,4-dichlorobenzene (DCB) equivalent. Lastly, the effects of a product’s life cycle on water scarcity are measured through quantifying surface and groundwater consumption.

To complement environmental performance with structural and durability-related functionality, SPI were evaluated by relating GWP with concrete performance metrics. In contrast with the ESI that relates embodied carbon to compressive strength^[29], the proposed SPI framework expands the analysis to both structural capacity and durability characteristics, thereby providing a broader assessment of production-stage sustainability. GWP was selected because climate change mitigation remains the primary objective of low-carbon concrete systems, and embodied carbon is the most widely reported indicator in concrete LCA studies, facilitating comparison with previous work. Compressive strength was selected to represent structural performance, whereas post-conditioning bulk ER, and chloride migration coefficient (D_nssm) were selected to represent durability because these properties are widely recognized indicators of reinforcement corrosion resistance and long-term service-life performance under chloride exposure conditions. This approach enables systematic assessment of the tradeoffs between embodied carbon, structural performance, and long-term durability.

The structural sustainability performance indicator (SPI_S) is defined as:

where f_c represents the 28-day compressive strength. Similarly, durability-oriented sustainability indicators were evaluated using bulk electrical resistivity (SPI_ER) and chloride resistance (SPI_Cl) and were similarly defined as:

The reciprocal of the chloride migration coefficient was adopted so that lower chloride diffusivity, corresponding to greater resistance against chloride ingress, results in a lower SPI_Cl value. Lower SPI values indicate lower embodied carbon required to achieve a given level of structural or durability performance. This formulation enables a systems-level comparison of environmental burden and functional concrete performance beyond conventional embodied-carbon assessments alone.

Sensitivity and scenario analysis

To ensure the reliability and stability of results generated from the LCA, sensitivity analyses were performed. GWP was selected as the response variable because it represents the primary environmental indicator associated with low-carbon concrete production and serves as the basis for comparing the carbon reduction potential of the evaluated mixtures. Sensitivity was quantified as the percentage change in GWP relative to the base case scenario. Given the inherent uncertainties associated with geographical logistics and RHA combustion, transportation distances and RHA yield per combustion batch were varied. The method involves systematically varying one independent parameter at a time while keeping all other inventory data constant, thereby isolating the specific environmental impact of the targeted variable across the various concrete groups.

The first parameter subjected to sensitivity analysis was the transportation distance, as raw material sourcing locations, transportation routes, and regional supply chain conditions vary substantially across different construction projects within the Philippines, thereby contributing to geographical uncertainty. A variance of ±30% was applied to the baseline distance to rigorously capture the changing material supply chain network and the varying logistical environment of the Philippines. Second, to address the archipelagic geography within the country, a scenario-based sensitivity analysis on transportation mode was conducted. This evaluated the environmental impact of transitioning from the baseline road transport to a multimodal transportation logistics incorporating ferry transport.

Third, technological uncertainty associated with biomass combustion and ash recovery was evaluated by varying the RHA yield by ±10% per combustion batch to simulate variations in rice husk quality and efficiency fluctuation. This analysis was performed for all RHA-containing mixtures. Finally, technological and background dataset uncertainties in cement production were addressed by varying its manufacturing efficiency by ±30% relative to the baseline emission. This parameter captures operational variations in kiln thermal efficiency, fuel usage, and potential discrepancies introduced by relying on global background database.

Comparative environmental performance of concrete mixes across impact categories

The environmental performance of conventional concrete and low-carbon concrete mixtures incorporating RCA and RHA was evaluated using midpoint impact assessment methods under a cradle-to-gate system boundary. However, it should be noted that although foreground processes, including transportation distances, RHA production, and RCA recovery, were modeled using Philippine-specific conditions, portions of the background LCI were represented using Ecoinvent RoW datasets because equivalent Philippine inventories are currently unavailable. Consequently, the reported environmental impacts may be influenced by the differences between Philippine production systems and the generic technologies represented in the database. Variations in electricity generation mixes, cement manufacturing technologies, fuel sources, transportation efficiencies, and industrial process characteristics may affect the absolute magnitude of the calculated environmental impacts. For example, electricity-related emissions may differ because the Philippine electricity grid remains more dependent on fossil fuels than several regions represented in the RoW datasets, while cement manufacturing technologies may vary with respect to kiln efficiency, clinker substitution practices, and fuel utilization. Nevertheless, because all concrete mixtures were evaluated using the same background inventory framework and Philippine-specific foreground data, the results remain appropriate for comparative assessment, with consideration of the geographical limitations. The comparative assessment across different impact categories is shown in Figures 2-8.

Figure 2. Global warming potential and %GWP reduction of varying concrete mixes. Note: The vertical axis is intentionally truncated to improve visualization of the relative contributions of the life cycle inventory components.

Figure 3. Cumulative energy demand of various concrete mixes per cubic meter of concrete. Note: The vertical axis is intentionally truncated to improve visualization of the relative contributions of the life cycle inventory components.

Figure 4. Mineral resource scarcity of various concrete mixes per cubic meter of concrete. Note: The vertical axis is intentionally truncated to improve visualization of the relative contributions.

Figure 5. Water consumption of various concrete mixes per cubic meter of concrete.

Figure 6. Terrestrial acidification of various concrete mixes per cubic meter of concrete. Note: The vertical axis is intentionally truncated to improve visualization of the relative contributions of the life cycle inventory components.

Figure 7. Freshwater ecotoxicity of various concrete mixes per cubic meter of concrete. Note: The vertical axis is intentionally truncated to improve visualization of the relative contributions of the life cycle inventory components.

Figure 8. Human carcinogenic toxicity of various concrete mixes per cubic meter of concrete. Note: The vertical axis is intentionally truncated to improve visualization of the relative contributions of the life cycle inventory components.

Results showed that the incorporation of RHA and RCA generally reduced the environmental burdens associated with concrete production, although varying tradeoffs were observed depending on the impact category considered. Across nearly all impact categories, cement production remained the dominant contributor because of clinker calcination, fuel combustion, and energy-intensive processing requirements. Transportation, on the other hand, was identified as a secondary contributor, particularly for RHA and RCA due to additional recovery and distribution processes. Consistent with the findings of Nian et al.,^[28] these remained the dominant contributors to the environmental profile of recycled aggregate concrete systems. Moreover, while cradle-to-grave assessment can additionally account for carbon uptake during the use phase, the present cradle-to-gate study identifies material production and transportation as the principal intervention points for reducing embodied carbon during the production stage. This agreement indicates that reducing clinker demand and optimizing material logistics are robust carbon mitigation strategies across different lifecycle assessment boundaries. Consequently, environmental improvements were more strongly influenced by cement replacement through RHA incorporation than by RCA substitution alone, highlighting the critical role of cement decarbonization in reducing the life cycle impacts of concrete production.

These environmental benefits are partially influenced by the adoption of a zero-burden allocation approach, wherein the upstream impacts associated with the generation of construction-demolition waste and rice husks are assigned to their original product systems. This approach was selected to evaluate the environmental implications of recovering and utilizing waste materials after they have entered the waste management system. This is consistent with the allocation hierarchy described in ISO 14044 and widely adopted in open-loop recycling and circular economy assessments. Nonetheless, while the zero-burden assumption is consistent in circular economy and waste valorization studies, alternative allocation approaches such as mass allocation, economic allocation, and system expansion may attribute portions of the upstream environmental burdens to RCA and RHA. Under a mass allocation approach, portions of the upstream environmental burdens associated with concrete production or rice cultivation would redistribute upstream environmental burdens among the products and coproducts of the multifunctional production system according to their respective mass fractions. Similarly, economic allocation would assign burdens based on the market value of recycled products, whereas system expansion would account for the environmental credits associated with avoided virgin aggregate extraction and cement production. Consequently, mass and economic allocation would increase the absolute environmental impacts associated with RCA and RHA compared with the cut-off approach, whereas system expansion may instead offset part of these burdens through avoided virgin aggregate extraction and cement production. However, the contribution analysis shown in Figures 2-8 demonstrated that cement production consistently accounted for the largest proportion of GWP, CED, MRS, and several other midpoint categories, whereas RCA recovery, RHA processing, and transportation collectively represented comparatively smaller contributions. Thus, although the absolute environmental benefits associated with RCA and RHA may decrease under allocation-based approaches, the relative ranking of the evaluated mixtures is expected to remain unchanged due to the substantial contribution of cement. In addition, because RCA and RHA generally possess substantially lower economic value than virgin aggregates and Portland cement, only a relatively small fraction of the upstream burdens would be assigned under economic allocation. Hence, while alternative allocation-based approaches would influence the magnitude of the reported environmental benefits, these are unlikely to alter the principal conclusions of this study.

Endpoint damage assessment

The endpoint damage assessments provide a macro-level evaluation of the overall environmental consequences of concrete production by translating midpoint impacts into three principal endpoint categories - human health, ecosystem quality, and resource depletion. In contrast to midpoint indicators that evaluate isolated environmental mechanisms, these criteria evaluate impacts at the end of the cause-effect chain to provide a broader systems-level interpretation of environmental damage^[40]. The comparative analysis between conventional concrete and the RHA-10:RCA-100 mixture, shown in Figure 9, reveals distinct trade-offs, with the vertical axis representing the relative percentage contribution scaled relative to the maximum damaging mixture within each damage category.

Figure 9. Comparative endpoint damage contributions of base case scenario with RHA-10:RCA-100 concrete mix.

In the human health damage category, a major environmental trade-off is observed; the low-carbon concrete mixture acts as the maximum impact contributor at 100%, whereas the conventional concrete with no RCA and RHA replacements accounts for only approximately 32.4% of this total baseline damage. This significant difference is driven largely by RHA, which single-handedly accounts for roughly 71.2% of this mixture’s entire human health impact. While lowering cement content yields a slight reduction in cement-associated toxicity emissions, the emissions linked to localized agricultural biomass combustion during RHA processing introduce substantial damage within the human health endpoint domain.

On the contrary, the trend completely reverses within the ecosystems damage category, where the conventional mixture establishes the maximum damage threshold at 100%, while the LCC mixture accounts for about 95% of its impact. This 5% decrease is associated with both the addition of RHA and RCA, with the former resulting in the reduction of cement’s impact, while the latter eliminated the impact of gravel. The combined environmental burdens introduced by adding RHA and RCA were outweighed by removing the intensive upstream processes of cement and gravel processing.

Finally, the resources damage category indicates a relatively minor net environmental benefit when transitioning from the conventional to the low-carbon concrete mixture. The base case marks the maximum resource damage at 100%, while the RHA-10:RCA-100 mixture reduces this primary resource depletion indicator to roughly 97.5%. Across both concrete formulations, cement remains as the primary driver of resource scarcity, dictating more than 60% of the respective impacts due to the high mineral and fossil energy requirements of industrial manufacturing. Notably, transportation became a critical secondary resource hotspot, accounting for about 26% to 27% of the total resource damage in both concrete groups. This demonstrates that while substituting natural gravel with 100% RCA successfully eliminates the direct geological depletion of virgin aggregates, the fossil energy and diesel fuel required to transport raw materials create a significant environmental burden.

Infrastructure-scale avoided burden analysis

Although the environmental contribution of RCA appeared relatively minor across several midpoint impact categories on a per-functional-unit basis, its cumulative benefits become more apparent when extrapolated to larger concrete volumes. To illustrate this, the calculated cradle-to-gate GWP values were linearly extrapolated to a modeled two-storey building, as reported in Figure 10. Based on the Department of Public Works and Highways standard design, the modeled building contains approximately 201.23 m³ of concrete^[41]. This corresponds to approximately 133.33 metric tons of sand and 192.94 metric tons of gravel, comprising a total of approximately 326.26 metric tons of natural aggregates for the conventional concrete mixture. As RCA replacement increased from 0% to 100%, the quantity of natural aggregates decreased to 157.44 metric tons, primarily through the displacement of gravel extraction and processing, while the sand content increased to compensate for the lower density and higher porosity of RCA and to satisfy the target workability and compressive strength requirements. Correspondingly, the extrapolated net avoided GWP increased with increasing RCA replacement, ranging from approximately 410.99 kg CO₂-eq for the RHA-0:RCA-25 mixture, to 1,342.88 kg CO₂-eq for the RHA-0:RCA-100 mixture. These findings demonstrate that although environmental benefits of RCA appear modest on a per-functional-unit basis, their cumulative impact becomes increasingly significant when implemented in large-scale infrastructure applications.

Figure 10. Net avoided GWP of concrete mixes with varying replacement levels of RCA for a two-storey building-scale scenario.

Sensitivity of GWP to alternative inventory assumptions and transport scenarios

To evaluate the robustness of results, a comprehensive sensitivity analysis was conducted on two critical parameters: transportation distances varying by ±30% and RHA yield varied by ±10%. The analyses shown in Figures 11 and 12 specifically tracked fluctuations in GWP from the baseline across various concrete mixes. When evaluating the influence of transportation logistics, the GWP of all concrete mixtures demonstrated a low-to-moderate sensitivity.

Figure 11. Sensitivity of GWP to transportation distances varied at ±30% for different concrete mixtures.

Figure 12. Sensitivity of GWP to RHA yield varied at ±10% for different concrete mixtures.

In Figure 11, low-to-moderate sensitivity on the GWP of all concrete mixtures to transportation-distance perturbations was observed. A 30% reduction in transport distances reduced the GWP by 2.09% to 2.42%, translating to approximately 8.26 to 9.06 kg CO₂-eq. Conversely, extending transport distances by 30% increased the total GWP by 1.92% to 2.46%. The RHA-containing concrete mixtures exhibited the highest sensitivity to both transport distance reduction and extensions, indicating a slightly higher reliance on localized sourcing and shorter supply chains for RHA than for aggregates. Despite these variations, the relative environmental ranking of the evaluated concrete mixtures remained unchanged. Concrete mixtures with RCA and RHA consistently maintained lower GWP values than the conventional concrete baseline under both low-variance and high-variance scenarios. Notably, even under the worst-case transportation scenario of +30% distance, the GWP of the RHA-10:RCA-100 mixture remained lower than the best-case transport scenario (-30% distance) of the conventional concrete mixture. This demonstrates that the environmental advantages of the low-carbon mixtures are not solely dependent on favorable logistical conditions. This further supports that the environmental performance of the concrete mixtures is governed primarily by material production rather than transportation activities. This finding is consistent with the hotspot analysis, which identified cement manufacturing as the dominant contributor across most environmental impact categories.

In contrast, the overall GWP profile of the various RHA-containing concrete mixes proved to be highly insensitive to fluctuations in RHA yield during combustion, as shown in Figure 12. A 10% increase in efficiency resulted in a lower GWP, with an average of approximately 0.17% reduction from baseline. Meanwhile, a 10% reduction in efficiency showed approximately 0.24% increase in GWP. The limited sensitivity is consistent with the contribution analysis, which showed that RHA production contributed only 8.78 kg CO₂-eq compared with 324 kg CO₂-eq from cement production, corresponding to approximately 2.71% of the cement-related emissions. Consequently, a ±10% variance in RHA yield which induces a change of 0.17% up to 0.24% translates to a minor 0.25% change in total GWP. These confirm that moderate fluctuations in RHA production efficiency are insufficient to significantly alter the environmental performance or comparative ranking of the evaluated concrete mixtures.

The sensitivity analysis of cement manufacturing efficiency, shown in Figure 13, reveals that GWP across all concrete mixtures is highly responsive to variations, with roughly ±27% variance in GWP from the baseline. This relationship highlights the dominant role of cement production in the overall carbon footprint of concrete. Notably, mixtures incorporating 10% RHA exhibit slightly lower sensitivity compared to mixtures without RHA, demonstrating more resiliency to upstream cement production variability. In contrast to the high sensitivity observed for cement manufacturing efficiency, varying the recycling efficiency of RCA produces only a minor shift in GWP across all concrete mixtures. This limited sensitivity can be attributed to the relatively small contribution of RCA production to the overall carbon footprint compared to the more emission-intensive processes such as cement production.

Figure 13. Sensitivity of GWP to cement manufacturing efficiency varied at ±30% for different concrete mixtures.

Finally, Figure 14 evidently shows how the road-and-ferry transportation substantially increases the GWP of all concrete mixtures, reflecting the additional fuel consumption and emissions associated with ferry operations. Across both transportation scenarios, GWP consistently declines as RCA content increases and as RHA replaces a portion of cement, reflecting the reduced usage of natural aggregate and cement. The relative benefit of RHA and RCA incorporation appears more evident under the multimodal transportation, where the gap between RHA-0:RCA-0 and RHA-10:RCA-100 widens compared to the road-only case. This implies that the environmental advantages of using alternative materials become even more significant in transport-intensive regions.

Figure 14. Sensitivity of GWP to varying transportation modes for different concrete mixtures.

Integrated sustainability performance assessment

The calculated SPIs for various concrete performance metrics are shown in Figures 15-17.

Figure 15. Structural sustainability performance indicator (SPI_S) of concrete mixture at various RCA and RHA replacement levels.

Figure 16. Bulk electrical resistance sustainability performance indicator (SPI_ER) of concrete mixtures at various RCA and RHA replacement levels.

Figure 17. Chloride resistance sustainability performance indicator (SPI_Cl) of concrete mixtures at various RCA and RHA replacement levels.

In Figure 15 for compressive strength, the conventional concrete mixture exhibited the lowest structural carbon intensity at approximately 8.548 kg CO₂-eq/MPa, primarily due to its relatively high compressive strength of 46.74 MPa despite possessing the highest embodied carbon among all mixtures. Increasing RCA replacement generally increased SPI_S because the reduction in compressive strength exceeded the corresponding reduction in GWP. At full RCA replacement (RCA-100), the mixture exhibited the highest structural carbon intensity at approximately 15.3716 kg CO₂-eq/MPa due to the combined effects of lower compressive strength and only modest carbon reduction. Among the RHA-containing mixtures, the RHA-10 concrete mixture showed the lowest SPI_S at approximately 9.669 kg CO₂-eq/MPa, while the RHA-10:RCA-100 mixture achieved approximately 11.712 kg CO₂-eq/MPa. These results indicate that although conventional concrete remained favorable when evaluated solely on compressive strength per unit carbon emission, partial cement replacement through RHA improved the strength-carbon balance of several RCA-containing mixtures through pozzolanic strengthening and matrix densification mechanisms.

A contrasting trend was observed for durability-oriented sustainability indicators. From Figure 16, the SPI_ER demonstrated substantial improvements with RHA incorporation across all RCA replacement levels. Conventional concrete exhibited an SPI_ER of approximately 33.604 kg CO₂-eq/(kΩ cm), whereas RHA-containing mixtures ranged from 19.936 to 27.732 kg CO₂-eq/(kΩ cm). The lowest values were observed in the RHA-10 and RHA-10:RCA-25 concrete mixtures at 19.936 and 20.023 kg CO₂-eq/(kΩ cm), respectively. In contrast, the RCA-100 mixture exhibited the highest SPI_ER at approximately 52.533 kg CO₂-eq/(kΩ cm), while incorporating RHA at 10% replacement reduces this intensity to approximately 27.732 kg CO₂-eq/(kΩ cm). Similarly in Figure 17, the chloride migration resistance sustainability indicator (SPI_Cl) showed significant improvements with combined RCA and RHA incorporation. The conventional concrete mixture exhibited an SPI_Cl of approximately 6.89 × 10^-9 kg CO₂-eq/(s/m²), whereas RHA-10:RCA-100 achieved the lowest value at 2.42 × 10^-9 kg CO₂-eq/(s/m²). This corresponds to 65% reduction in chloride-related carbon emissions relative to conventional concrete. This can be attributed to the lower chloride migration coefficients observed in RHA-containing mixtures, indicating enhanced resistance against chloride ingress and potentially improved long-term durability performance.

The SPI results demonstrate that the preferred concrete mixture depends strongly on the selected functional performance criterion. Conventional concrete remained the most favorable mixture when studied strictly in terms of compressive strength per unit GWP because of its relatively established high mechanical strength. However, when durability-related service-life indicators were considered, the RHA-RCA mixtures exhibited substantially superior sustainability performance. In particular, the RHA-10:RCA-100 mixture simultaneously achieved the lowest GWP and the best chloride migration resistance SPI among all evaluated mixtures. These findings suggest that evaluating low-carbon concrete solely based on compressive strength may underestimate the sustainability benefits associated with enhanced durability, longer service life, reduced maintenance requirements, and circular material utilization.

Lastly, the observed differences between strength-based and durability-based SPIs also highlight an important consideration regarding the interpretation of environmental performance under the adopted functional unit. Considering that the assessment adopted a volumetric functional unit (1 m³ of concrete), this study enables a consistent comparison of production-stage environmental impacts among the evaluated mixtures. However, the improved bulk ER and reduced chloride migration observed for the RHA-containing mixtures indicate enhanced durability and the potential for extended service life. Consequently, if a performance-based functional unit, such as one year of service life or equivalent structural functionality, were adopted, the environmental advantages of the RHA-containing mixtures would likely become more pronounced because the production-stage impacts would be distributed over a longer service period. Further, although the proposed SPI provides a practical means of integrating production-stage environmental impacts with structural and durability performance, its interpretation is only limited as comparative production-stage sustainability metrics. It does not explicitly account for deterioration, maintenance, repair, and end-of-life processes. Because the SPIs are directly related to the measured engineering properties, their reliability also depends on the quality of the compressive strength, bulk ER, and chloride migration measurements. Therefore, reliable engineering measurements obtained using standardized testing procedures are essential for meaningful application of the SPIs. Future cradle-to-grave assessments should incorporate service-life prediction and performance-based functional units to provide a more comprehensive evaluation of the life cycle sustainability of RCA-RHA concrete systems.