## 5 Conclusions  
In this study, the potential environmental impacts caused by sand and gravel (natural aggregates) as well as recycled, slag, bottom ash, and waste glass aggregates (recycled and by-product aggregates) were compared based on LCA. In addition, six environmental impacts were evaluated (ADP, GWP, ODP, AP, POCP, and EP) along with the environmental cost that was associated with the aggregates when used in concrete. The results of this study can be summarized as follows:  
1. The environmental costs of all the concrete mixture proportions investigated in this study were found to range from 5.88 to 8.79 USD/m\({}^{3}\), while that of the standard mixture was calculated to  
Figure 4: Environmental Cost and Environmental Impact Ratio in Comparison with Standard Mixture. Figure 3: Environmental Impact Ratio with Respect to Raw Materials in the Standard, F-W30, and F-R30 Mixtures. be 6.78 USD/m\({}^{3}\). High environmental costs were observed mainly from concrete mixtures that incorporated recycled and waste glass aggregates, which exhibited higher ODP and EP evaluation results than the standard mixture. 2. Concrete that had some of its aggregate contents replaced with slag aggregate as the fine aggregates or bottom ash aggregate as the coarse aggregates exhibited lower environmental impacts than the standard mixture concrete that incorporated only natural aggregate in all environmental impact categories. However, concrete that incorporated bottom ash aggregate as the fine aggregates demonstrated a higher environmental impact than the standard mixture concrete in all the environmental impact categories. 3. The ADP and POCP evaluation results for all the concrete mixture proportions investigated in this study were similar to those of the standard mixture and ranged from 84.46% to 107.98% of the results noted for the standard mixture. 4. The GWP, AP, and EP evaluation results for the concrete mixtures that had some of their aggregate replaced with waste glass aggregate were very high and ranged from 118.56% (GWP) to 220.61% (EP) of those of the standard mixture. However, their ODP evaluation results were slightly lower than that of the standard mixture. 5. The ODP evaluation results exhibited the highest correlation with recycled aggregate. The ODP evaluation results of the concrete mixtures that incorporated recycled aggregate represented about 120% of that of the standard mixture. 6. The environmental impact ratio of aggregate used in the standard mixture ranged from 0.48% (POCP) to 7.23% (EP). However, when some of the fine aggregate was replaced with recycled and waste glass aggregates, the environmental impact ratio of the aggregate increased in the range 1.00% (ADP of F-W30) to 57.95% (EP of F-W30). Based on the LCI DBs and evaluation range limits applied in this study, some of the recycled and by-product aggregates exhibited higher environmental impacts than natural aggregate in some or most of the environmental impact categories and their environmental costs were also higher. However, because they are fabricated using recycled resources, they are important in terms of conserving the limited amount of available natural resources and in terms of reducing the potential environmental impact and cost associated with waste materials in landfills. Therefore, LCA research that considers the wider environmental impact categories and evaluation ranges of various aggregates, as well as research on concrete and concrete structures that incorporate such aggregates, will be required in the future. It is also deemed necessary to seek methods to reduce various environmental loads, including ODP and EP, through further research concerning the development of recycled and by-product aggregates. Conceptualization, S.R. ; methodology, W.-J.P.; formal analysis, S.R. ; data curation, R.K.; writing--original draft preparation, S.R. ; writing--review and editing, W.-J.P. and H.B. ; supervision, W.-J.P.; project administration, H.B.