# Comparative \"from Cradle to Gate\" Life Cycle Assessments of Hot Mix Asphalt (HMA) Materials
## 4 Discussion  
The obtained results are interesting and they should be examined to highlight the difference in terms of environmental performances. For the _GWP_, the differences in the results between the two asphalt plants are mainly due to the use of different fuel sources in the manufacturing process. This result was already anticipated based on the differences previously discussed about the LCA of the HMA of both companies. Transport phase (A2) accounts for 22% and 20% of the total _GWP_, while internal transport included in the A2 phase resulted to be negligible, as it represents less than 1% of the total _GWP_ emissions. As observed in Table 12, the _GWP_ in phase A3 for Company 2 resulted to be 20% greater than that obtained for Company 1. The reason explaining this result lies in the fact that natural gas is the most environmentally friendly fossil fuel, as explained before. Indeed, natural gas is a much cleaner fuel source than crude oil and its derivatives, since its combustion produces 20%-30% less carbon dioxide (CO\({}_{2}\)) compared to other petroleum-based products [58; 59]. In terms of the Acidification Potential (_AP_) emissions, it is observed that more than 90% of the _AP_ emissions comes from processes related to raw materials (A1). The third party certified analysis of smokestacks emissions demonstrated strong differences between the two mix plants. Data in Table 12 show that the use of low-sulfur crude oil and gasoil instead of methane makes the A3 _AP_ emissions of Company 2 five times higher than the A3 emissions of Company 1. As stated previously, the use of methane instead of gasoil and low-sulfur crude oil explains this result, as confirmed by [PERSON] et al. [60]. Regarding the Ozone layer Depletion Potential (_ODP_) emission, the results show that more than 90% of the total emissions in both companies are related to raw materials and fuel processes (A1 stage). Similar to what was reported for the _AP_ emissions, the use of low-sulfur crude oil and gasoil instead of methane also has a significant impact on _ODP_ (stage A3 in Table 12). Besides, it was found that the item related to internal transport processes included in the A2 stage represents less than 2% of the total _ODP_ results. The results listed in Table 12 show that the differences in the contribution of the Eutrophication Potential (_EP_) emissions for the A1 phase between the two companies are not appreciable. In this \begin{table}
\begin{tabular}{c c c c c c c c c c c c c c c} \hline \hline  & & & \multicolumn{4}{c}{**Company 1**} & \multicolumn{4}{c}{**Company 2**} \\ \hline \multirow{2}{*}{**Phase**} & \multirow{2}{*}{_GWP_} & \multirow{2}{*}{_AP_} & \multirow{2}{*}{_ODP_} & \multirow{2}{*}{_EP_} & \multirow{2}{*}{_POCP_} & \multicolumn{2}{c}{_ADP_} & \multirow{2}{*}{_HTP_} & \multirow{2}{*}{_GWP_} & \multirow{2}{*}{_AP_} & \multirow{2}{*}{_ODP_} & \multirow{2}{*}{_EP_} & \multirow{2}{*}{_POCP_} & \multicolumn{2}{c}{_ADP_} \\  & & & & & & & & & & & & & & & _Fossil_ & **HTP** \\ \hline A1 & 43 & 95 & 87 & 90 & 93 & 72 & 70 & 39 & 90 & 92 & 90 & 93 & 72 & 82 \\ A2 & 22 & 3 & 7 & 7 & 6 & 28 & 1 & 20 & 0 & 4 & 3 & 6 & 28 & 0 \\ A3 & 35 & 2 & 7 & 3 & 1 & 0 & 29 & 42 & 10 & 4 & 7 & 1 & 0 & 18 \\ \hline \hline \end{tabular} Note: _GWP_: global warming potential; _AP_: acidification potential; _ODP_: formation potential of tropospheric ozone; _EP_: eutrophication potential; _POCP_: formation potential of tropospheric ozone; _ADP fossil_: abiotic depletion potential for fossil resources; _HTP_: human toxicity potential.  
\end{table} 
Table 12: Results of LCA for the two companies (values in percentage). case, the distances from the quarries to the plants, as well as the type of fuel sources used, explain the differences observed in the A2 and A3 results between the two companies (i.e., larger transport-related emissions and lower manufacturing-related emissions for Company 1).