## 5 Discussion and Future Research
### Implications of Each Scenario at the Macro Level  
The circularity assessment through these scenarios provides valuable insights into the macro-level or national scale, particularly since the construction sector accounts for 6% of the Spanish GDP, of which new-build residential construction represents 19% of its activities [109]. Each scenario illustrates a specific stage in the transition to a circular model, founded on the premise of implementing circularity strategies at the material level. In this study, the most circular scenario is that of S4, which incorporates RAC from the design stage. Thus, the comparative analysis between S1, which represents the current linear model based on extraction, use, and disposal, and S2, which illustrates the approach adopted by certain economic sectors to enhance circularity through strategies implemented at the EOL, reveals a 2.31% increase in the circularity performance of the structure (Figure 20b), with the SCI rising from 24.1% in S1 to 26.4% in S2 (Figure 21). Figure 20: Comparative analysis of ECI and SCI: (**a**) average increase in ECI of S2, S3, and S4 compared to S1; (**b**) increase in SCI of S2, S3, and S4 compared to S1 (Authors’ own). This modest enhancement in circular performance stems from the limited possibilities for resource reduction of S2, particularly concerning the optimisation of materials, the reuse of elements for similar or different purposes, and the maximisation of their usage as a structural system over time. At the macro scale, this scenario reveals restricted opportunities to achieve the absolute decoupling between GDP and DMC (Figure 2) within the construction sector, since it does not entail the reduction and maximised utilisation of resources at the highest level but inherently relies on the extraction of raw materials. This partial perspective on circularity also influences the CMR rate (Figure 3), which is diminished due to the adoption of less effective strategies and the loss of perspective in the recovery and integration of secondary raw materials in total consumption. This trend is mirrored in the UEPG report [35], which indicates that the RA production rate in Spain stands at approximately 2.4% (Figure 1). In order to increase circularity in the manufacturing process and maximise the use of resources, an increasing number of industries that form part of the construction sector are seeking to increase the circularity of materials through the use of a fraction of raw materials. Scenario 3 represents this perspective, leaving aside the cement industry since the inclusion of RA implies its consideration from the design phase. The evaluation of this scenario shows an increase in the circular performance of the structure of 2.81% (Figure 20b) concerning S1 by increasing the SCI from 24.09% to 26.9% (Figure 21). However, its increase concerning S2 is reduced to 0.5% by increasing its SCI from 26.4% in S2 to 26.9 in S3 (Figure 21). Although low, this improvement is not negligible given that the mass participation ratio of materials that include circularity strategies in this scenario represents only 6.67% of the total building mass (Table 1). The evaluation of this scenario on a larger scale demonstrates the very limited possibilities of increasing the circularity of the Spanish construction sector when, with representative reinforced concrete structures (Section 3), the adoption of strategies in concrete is not actively promoted. The evaluation of the findings in S3 promotes the assessment of S4, in which the use of RA in concrete is considered. Based on its results, the analysis shows an increase in circularity concerning S1 of 10.81% (Figure 20b), 8.0% more than S3 and 8.5% more than S2 (Figure 21). Although this increase is significant, circular performance remains low, with an SCI of 34.9%. These results reflect the extent to which the restrictive nature of design standards limits the potential for circularity in material strategies within residential building structures.