## 3 Results and Discussion
### Life-Cycle Assessment Results
In the mineral scarcity impact category, the CDW and fly ash layers exhibited 71% lower impact than EAF slag and 19% lower impact than river sand. In general, among the alternatives, except for GBF slag and m-sand, the other alternative materials possessed very low levels of land use and water consumption impacts. The impacts generated from the GBF slag were seen as higher than those of all other materials (Figure 2). The higher ratio of GBF would be because steel processing is a very energy-intensive process using abiotic materials, and blast furnace slag is a residue of those processes. Large energy consumption has impacts on GHG generation and ozone formation, and it might also produce fine particulate matter. In parallel, steel manufacturing uses abiotic materials that lead to resource scarcity and higher water consumption, land consumption, and eutrophication. Therefore, the comparison of GBF slag and conventional river sand was kept separate to allow for a better understanding of the other alternatives (Figure 3). In a normal scenario without GBF slag, the EAF slag subbase layer had the highest energy demand across most categories (Figure 4). All other layer alternatives possessed lesser energy demand in the fossil-based category than the conventional river sand layer. Overall, across all energy source categories, the energy demand remained low for the fly ash mix layer. The water-based renewable energy share was highest for the m-sand layer. Figure 5 shows that the GBF slag has a high energy demand--about nine times higher than that of the conventional sand layer. This indicates that steel processing is an energy-intensive process. Figure 2: Life-cycle impact analysis for subbase layers made from river sand, CDW, EAF, FA, or m-sand (normal scenario). The avoidance scenario involves comparing the conventional subbase layer and alternative material subbase layers with the avoidance process to technosphere factors. In  
Figure 4: Energy demand analysis for subbase layers made from river sand, CDW, EAF, FA, or m-sand (normal scenario). Figure 5: Energy demand analysis for subbase layers made from river sand or GBF (normal scenario). Figure 3: Life-cycle impact analysis for subbase layers made from river sand, CDW, GBF, FA, or m-sand (normal scenario). this comparison (Figure 6), the conventional river sand layer had a higher GWP impact than the other materials. All other alternative materials, in general, had a lower GWP contribution than the river sand layer. Like the normal scenario, overall, GBF slag had approx. 7-8 times higher impact than all other materials. The comparison of GBF slag and conventional river sand, therefore, was kept separate to allow for a better understanding of the other alternatives. Among the alternative materials, CDW had the least impact. The contribution analysis in the avoidance scenario showed that among the alternative layers, except for GBF slag, all alternative materials had carbon contribution only from the transportation process. Their carbon contribution from material processing was negative or zero. The construction waste layer had a GWP reduction of about \(-66\%\), EAF slag about \(-37.6\%\), and m-sand about \(-16.8\%\) in the material processing area, as compared with the conventional river sand layer. The alternative materials' usage showed a negative GWP impact in the material processing stages. Over the other impact categories, the GBF slag layer had the highest impact across all of the alternative layers (Figure 5). The m-sand layer exhibited a higher water consumption impact than the GBF slag layer--almost \(62\%\) higher. In general, CDW, EAF, and FA showed a minimal environmental footprint in the avoidance scenario. In the ozone formation impact category, the CDW layer, EAF layer, and m-sand layer showed some negative reduction. Notably, the CDW and EAF slag layers showed negative reduction benefits in land-use impact categories, which were \(-16.8\%\) and \(-25.8\%\), respectively.