## 3 Results and Discussion
### Life-Cycle Assessment Results
The water consumption remained highest for the m-sand layer and least for the CDW layer among the alternatives. Among the alternative layers, CDW, EAF, and FA exhibited lower or negative land-use impacts. The GBF layer showed the highest impacts, similar to the normal scenario, despite the avoidance allocations in this scenario. Thus, the comparison of GBF slag and conventional river sand, again, was illustrated separately from the other alternatives (Figure 7). In comparison with the normal scenario, the GBF slag layer showed about \(50\%\) reduction across all impact categories in the avoidance scenario. Figure 6: Life-cycle impact analysis for subbase layers made from river sand, CDW, EAF, FA, or m-sand (avoidance scenario). In contrast to the normal scenario, the alternative mixes in the avoidance scenario exhibited far better energy demand in non-renewable fossil sources. The conventional sand layer and alternative EAF layer had only a marginal difference in this scenario in the non-renewable fossil category. Overall, across all energy source categories, the energy demand remained high for the conventional mix layer (Figure 8). Among the alternative mixes, the CDW layer utilized less energy, with avoidance benefits. The energy demand reduction was about 80% compared to the conventional layer in the non-renewable fossil category. The m-sand had the highest energy demand in the renewable water category, similar to the normal scenario. Figure 9 shows that the GBF slag layer utilized 50% less energy in the avoidance scenario compared with the normal scenario. At the same time, the energy demand of the GBF slag layer remained higher than that of the conventional layer in this scenario. Figure 8: Energy demand analysis for subbase layers made from river sand, CDW, EAF, FA, or m-sand (avoidance scenario). Figure 7: Life-cycle impact analysis for subbase layers made from river sand, CDW, EAF, FA, GBF, or m-sand (avoidance scenario). This study's results highlighted the better environmental footprints--especially in the global warming and mineral resource scarcity impact categories--of alternative materials in complete substitution. The LCA analysis results showed differences in the scenario analysis considering a normal scenario and an avoidance scenario. * The results for the road subbase layer indicated that construction waste and fly ash carried lower GWP, mineral resource scarcity, and other footprints in normal scenarios among the alternatives. Furthermore, on considering avoidance, the footprints of these materials were further reduced. The GWP impact reduced by 86.7% with construction waste, 13.2% with EAF slag, 55.8% with m-sand, and 47.9% with fly ash when considering the avoidance process benefits. With avoidance, except for GBF slag, all of the alternatives had a better impact footprint than the conventional subbase layer. * The overall LCA analysis for the road subbase layer highlighted that the majority of the footprint contribution was involved in the material transportation processes concerning the construction materials. Thus, sourcing of materials closer to the site or the use of low-emission transport alternatives needs to be considered. * The cumulative energy demand analysis highlighted the need to shift from fossil-based to renewable energy sources, which could benefit the environment. Although Vietnam does not yet have a relevant CDW material flow due to the fact that its infrastructure is still being built, the peak for CDW availability is expected by the Asian Development Bank in Vietnam in 2040 [85]. This will result in CDW becoming another key feedstock for road construction as a backfilling or building material, instead of being dumped into landfills or the natural environment [86]. [PERSON] et al., in 2016 [87], estimated that the total amount of CDW in Vietnam in 2020 amounted only to 6.3 million tons, and that it would reach 11 million tons in 2025. Moreover, only 1-2% of CDW was estimated to be recycled in Vietnam [40].