# Life Cycle Assessment of Substitutive Building Materials for Landfill Capping Systems in Vietnam
## 3 Results and Discussions
### LCA Analysis  
The LCA analysis involved comparing conventional mineral sealing layer mix 1 and alternative material layers (Supplementary Materials S1). The GWP impact results showed that all alternative mixes except mixes 6 and 7 had a lower impact than the conventional mix 1. The higher GWP impacts seen for mix 7, was greatly contributed to by the bentonite material flow, as the mix had 20% bentonite, along with the 65 km transport involved for ash. Mix 5 had the least GWP impact (107,984.38 kg CO\({}_{2}\) eq), with 50% fly ash having zero allocation for its production and the only impact was from the 65 km transport. The impact contribution was 62.7% from the transport process and 37.3% from clay extraction (Figure 5). The LCA analysis for the landfill capping layers highlighted that the greatest environmental footprint contribution resulted from material transportation processes. Thus, sourcing of materials closer to the site is indicated, or using low emission transport alternatives are needed to be considered. Similarly, mixes 2, 3 and 9 had a lower impact than the other alternatives and conventional mix 1, which could be directly associated with the reduced clay percentage in these mixes (Figure 6). The contribution analysis indicated that the mixes with an ash transport of 65 km had a transportation impact contribution of over 50% to 65%, while the mixes with regular materials with 30 km transport had a 37-46% impact contribution from transportation (Figure 5). This higher contribution from the transportation process remains a concern in similar other studies [87, 88]. Mix 2 and mix 3, using brick waste in its composition, had approx. 10% lesser GWP impact than the conventional mineral mix 1. Within mix 2, 30.1% of the GWP impact arose from clay, 8.63% from bentonite processes, and 15.6% from brick waste processes. Meanwhile the composition of mix 3 had 34.80% GWP impact from clay processes, 4.34% from bentonite, 0.89% from sand, and 13.5% from brick waste. \begin{table}
\begin{tabular}{l c c c c c} \hline  & **Clay** & **Bentonite** & **Milled Brick** & **Fly Ash** & **Rice Husk Ash** & **Sand** \\ \hline Mix 1 & 97\% & 3\% & - & - & - & - \\ Mix 2 & 47\% & 3\% & 50\% & - & - & - \\ Mix 3 & 54\% & 1.5\% & 43\% & - & - & 1.5\% \\ Mix 4 & 72\% & 1.5\% & 25\% & -  
Overall, mixes 8 and 9, containing rice husk ash at 40% and 20%, respectively, carried an approx. 40% lower GWP impact footprint than the conventional mineral sealing layer (mix 1), while mix 2 has a 50% reduction. In the mineral resource impact category, mix 6 with fly ash 80% had the least footprint, which was about 81.9% lower than mix 1. In general, all the alternative material-based mix layers possessed a lower mineral resource impact footprint than the conventional layer mix (Figure 7). Apart from mix 4 and mix 7 among the proposed alternative mix layers, other mixes on average had 40% lower land use impact than mix 1 (Figure 8). The higher percentage of 72% clay in mix 4 and a higher percentage of 20% bentonite in mix 7 along with ash transport 65 km contributed to this increased impact. Figure 5: Mineral sealing layer—Contribution analysis for GWP impact category. Figure 6: Global warming potential impact comparison between mineral sealing mix combinations. The impact categories of acidification and eutrophication for the alternative layer mixes remained lower than the conventional mix 1 (Figure 9). Mix 7 posed higher impacts than the other proposed alternatives across categories--global warming potential, stratospheric ozone depletion, fossil resource scarcity, and water consumption. This variation of mixes 6 and 7 was mainly contributed to by 20% bentonite and the 65 km ash transport in the mix recipe. The alternative mixes, except mix 6 and mix 7, contributed 25% less water consumption impact than the conventional mineral sealing mix 1.